# Effective Approach of Activated Jordanian Bentonite by Sodium Ions for Total Phenolic Compounds Removal from Olive Mill Wastewater

Authors: Khansaa Al-Essa; Ethar M. Al-Essa
Journal: Journal of Chemistry
 (2021)

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## Abstract

Olive mill wastewater (OMW) is nowadays considered a serious environmental problem, especially within the Mediterranean region. With this in mind, water shortages are also a very serious and prevalent concern in third world countries. The aim of this study is to investigate the feasibility of using Jordanian bentonite, a simple and natural clay, as a possible adsorbent to decrease the negative characteristics of raw OMW, as an approach to the development of a methodology that addresses the OMW problem without affecting freshwater resources. The purified bentonite was activated by sodium ions at room temperature. FTIR, XRD, TGA, and BET surface area measurements were performed. OMW was contacted with both purified and activated bentonite in the batch technique to figure out the optimum parameters for the adsorption process. Physiochemical parameters of OMW were measured before and after treatment. The maximum adsorptionqm was found as 8.81 mg/g at 323 K for the total phenolic compounds. The Langmuir and Freundlich models were utilized to describe the equilibrium isotherms and both models fit well. The parameters of thermodynamic show that the adsorption process was feasible, spontaneous, and endothermic in nature. These promising results along with the sodium activation of bentonite significantly improve bentonite’s adsorption capacity.


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## Body

## 1. Introduction

Bentonite can potentially be utilized as an effective adsorbent towards wastewater treatment due to its low cost and eco-friendly properties. Primarily, this is due to bentonite’s high surface area and chemical stability. Bentonite, after some modifications, displayed greater stability in comparison to other minerals studied in relation to the adsorption/desorption process [1]. Enhancing the adsorption capacity of bentonite achieved by different activation methods which include acid activation [2, 3], organic activation [4, 5], nanobentonite composite [6, 7], polymer, and surfactant bentonite composite [8, 9], along with the combinations of bentonite-zeolite with bentonite-alum and bentonite-limestone [10] was examined. Bentonite is one of the most desirable adsorbents for the removal of several pollutants from wastewater [6]. Some researchers have also recommended the use of bentonite as an adsorbent for removing organic compounds [11, 12], oil content [13], and inorganic pollutants (such as heavy metal ions) [14–17], in addition to drug components [18, 19]. However, it was noted in the literature that treatment of olive mill wastewater (OMW) with a low-cost modified bentonite as being poorly discussed [20] and that more studies are needed. Jordanian Bentonite has significant physical, chemical, nontoxicity, and unique adsorption properties, along with its high specific surface area and cation exchange capacity (53–85 mEq/100 g) [21]. Some studies focused on the adsorption capacity of Jordanian bentonite toward some heavy metal ions, such as Ni(II), Co(II) [22], Th(IV), U(VI) [23], and Pb(II) [24], but the activation of Jordanian bentonite and its adsorption capacity for removing pollutants from OMW has not been studied in detail. Therefore, an adsorption isotherms study of total phenolic compounds (TPC) removal, along with the effects of different parameters on adsorption efficiency, needs to be conducted. Phenolic compounds have attracted great interest, with them being widely utilized in industrial processes to obtain petrochemical products, dyes, rubber, plastics, textiles, paper, foams, emulsifiers, and detergents [25]. They are also produced by different industries such as the drug industry and the olive oil production [26]. These compounds have been extensively discharged into wastewaters [27]. Such wastewater containing these kinds of pollutants is of concern due to its potential severe hazard and threat to human health, aquatic life, and groundwater [28]. Some of the phenolic compounds have high toxicity and nonbiodegradable substances and are persistent in the environment [29]. Dangerously, conventional biological processes do not have the ability to remove all phenolic contaminants that are present in industrial wastewater. The structures of the eleven phenols considered priority pollutants by the US Environmental Protection Agency (EPA) are shown in Figure 1. These highly toxic compounds for both humans and the environment as a whole may cause inflammation in the digestive system, an increase in blood pressure, and a reduction of blood ability to carry oxygen to tissues and organs [30, 31]. Hence, phenolic compounds should be removed entirely from wastewater before being discharged into the environment [32].Figure 1
Phenolic compounds structures considered priority contaminants by US EPA [29].Adsorption technology in particular has been utilized for the removal of organic compounds from wastewater [33]. Olive oil is an important industry sector, being the main component of the Mediterranean food [34]. Its seasonal production is accompanied with large quantities of OMW in a relatively short period. OMW is a dangerous environmental problem, with its high pollutant load, high turbidity, low pH, high salinity, and high organic content including phenolic compounds, organic acids, and polysaccharides, which are nonbiodegradable. OMW’s observable consequence includes the discoloring of both surface and groundwater, as well as affecting the soil quality and harming plant and aquatic organisms [35].Mediterranean countries, specifically Jordan, face a serious problem in managing OMW. OMW is mostly generated from small mills with outdated technologies for oil extraction. These mills usually have limited financial resources and are located far from each other. Due to their location, it is difficult to establish a central treatment location with the disposal facilities. Ergo, there is a need for effective technologies suitable for small scale olive mills that can decrease the environmental impact of OMW. Some methods have been implemented to treat OMW with varying degrees of success [36], eventhough a major drawback concerning some techniques is their high capital cost and low toxicity reduction. To manage the OMW sustainably, continuous research on the application of Jordanian bentonite has been conducted [1, 2]. Further activation methods will be introduced as an alternative low cost and environmentally safe adsorbent. The activated Jordanian bentonite (AJB) may offer the ability to reutilize OMW for irrigation and industrial purposes as a sustainable approach to reduce the odor and contamination of soil, surface, and groundwater.In this study, OMW collected from different olive oil mills in Jerash city will be characterized. The Jordanian Ca-bentonite is activated by sodium ions to produce an efficient adsorbent with improved adsorption capacity toward pollutants in OMW. AJB will be examined as an adsorbent for the removal of TPC from OMW, investigated for the effect of the adsorbent dosage as well as the initial concentration and temperature on adsorption efficiency. AJB will also be examined to establish the adsorption isotherms and simulate the experimental data with the Langmuir and Freundlich adsorption models and estimate the thermodynamic parameters of the TPC adsorption onto AJB adsorbent.It is hoped that the findings of this study cover the information gap missing from the existing scientific literature concerning sodium AJB and OMW treatment.


## 2. Materials and Methods

The natural Jordanian Ca-Bentonite clay used in this study was collected from the airport region, Al Azraq. The sample was crushed to particle size >250μm using a ball mill instrument. All chemicals used are of analytical grade.


### 2.1. Purification of Raw Bentonite

The purification steps were followed in detail as mentioned in the previous study [1]. The raw bentonite sample was dispersed in distilled water at 22°C, and the clay fraction was recovered by centrifugation at 700 rpm for 4.0 min. This process was repeated four times, in order to guarantee obtaining samples in a pure form, free from quartz, carbonates, calcites, iron hydroxide, and organic metals. The suspension was collected, following evaporation of water at 35°C on a hot plate. The samples were dried in an oven at 60°C, ground and sieved using a 63 μm mesh, and stored in tightly closed plastic bottles for use in the experiments.


### 2.2. Sodium Activation of the Purified Bentonite

The purified bentonite (PB) was prepared for sodium activation: 17 g ± 0.01 g of the purified sample was weighed into a flask and 250 mL of 1.0 m NaCl (Puriss) was added. The resulting suspension was stirred at room temperature for 48 h. When the mixing process was completed, the resulting slurry was filtered by a Büchner funnel; then, it was washed with deionized water several times until it was released from Cl−1 ions against a 5% AgNO3 (Puriss) solution. After drying the sample at 70°C for 24 h, it was regrinded to reach 300 μm particle size and stored in tightly closed plastic bottles to be used in adsorption studies.The following reaction had occurred:(1)Ca−Bent+2Na+⟶2Na−ΒentAJB+Ca2+.FTIR spectroscopy (Thermo Nicolet NEXUS 670 Spectrophotometer), XRD (Philips X pert pro), TGA (NETZCH STA 409 PG/PC Thermal Analyzer), and BET surface area analysis (Gemini VII from micromeritics) were handled for characterization of both PB and AJB.


### 2.3. Handling of Olive Mill Wastewater Samples

OMW was obtained from three different olive oil mills located at Jerash city,;the preservation, pretreatment, and physicochemical characterization of the OMW samples were operated as executed in the previous work [2]. Fresh OMW was fully characterized before and after treatment. Total dissolved solid (TDS) was measured using a Crison PL-700AL meter. Alkalinity, total of chlorine, phosphate, nitrate, and chemical oxygen demand (COD) concentrations were determined using A COD and multiparameter bench meter, PN HI83099-02. Sodium (Na+) and potassium (K+) ions concentrations were determined using a flame photometer (Corning 400). TPC were evaluated by UV-VIS spectrophotometer (Varian Cary 100) using the Folin–Ciocalteu method. Briefly, 2.5 mL of 0.2 N of Folin–Ciocalteu reagent was mixed with 0.5 mL of the OMW sample. The mixture was kept in the dark for 5 min. Then, 2 mL of a sodium carbonate solution (75 g/L) was added; the reaction was left in the dark at 25°C for 1 h and then centrifuged at 8000 rpm for 5 min. The absorbance of the supernatant was read at λ = 765 nm. Gallic acid was used as a standard for the calibration of the method. TPC were expressed as gallic acid equivalents in gram per liter (g GAE/L residue) [37].


### 2.4. Adsorption Experiments

Batch adsorption experiments were performed. The effect of AJB dosage, the initial concentration of TPC, and the solution temperature, on the percentage removal, were studied.The concentration of adsorbate retained in the adsorbent phase (q, mg g−1) was calculated from the following equation [38]:(2)q=Ci−Cem∗V,where q is the adsorbent phase concentration after equilibrium (mg adsorbate/g adsorbent), Ci and Ce are the initial and final (equilibrium) concentrations, respectively, of adsorbate in solution (mg/L), V is the solution volume (L), and m is the adsorbent mass (g). Percentage (%) removal of adsorbate was calculated using the following equation [39]:(3)%removal=Ci−CeCi∗100.


## 2.1. Purification of Raw Bentonite

The purification steps were followed in detail as mentioned in the previous study [1]. The raw bentonite sample was dispersed in distilled water at 22°C, and the clay fraction was recovered by centrifugation at 700 rpm for 4.0 min. This process was repeated four times, in order to guarantee obtaining samples in a pure form, free from quartz, carbonates, calcites, iron hydroxide, and organic metals. The suspension was collected, following evaporation of water at 35°C on a hot plate. The samples were dried in an oven at 60°C, ground and sieved using a 63 μm mesh, and stored in tightly closed plastic bottles for use in the experiments.


## 2.2. Sodium Activation of the Purified Bentonite

The purified bentonite (PB) was prepared for sodium activation: 17 g ± 0.01 g of the purified sample was weighed into a flask and 250 mL of 1.0 m NaCl (Puriss) was added. The resulting suspension was stirred at room temperature for 48 h. When the mixing process was completed, the resulting slurry was filtered by a Büchner funnel; then, it was washed with deionized water several times until it was released from Cl−1 ions against a 5% AgNO3 (Puriss) solution. After drying the sample at 70°C for 24 h, it was regrinded to reach 300 μm particle size and stored in tightly closed plastic bottles to be used in adsorption studies.The following reaction had occurred:(1)Ca−Bent+2Na+⟶2Na−ΒentAJB+Ca2+.FTIR spectroscopy (Thermo Nicolet NEXUS 670 Spectrophotometer), XRD (Philips X pert pro), TGA (NETZCH STA 409 PG/PC Thermal Analyzer), and BET surface area analysis (Gemini VII from micromeritics) were handled for characterization of both PB and AJB.


## 2.3. Handling of Olive Mill Wastewater Samples

OMW was obtained from three different olive oil mills located at Jerash city,;the preservation, pretreatment, and physicochemical characterization of the OMW samples were operated as executed in the previous work [2]. Fresh OMW was fully characterized before and after treatment. Total dissolved solid (TDS) was measured using a Crison PL-700AL meter. Alkalinity, total of chlorine, phosphate, nitrate, and chemical oxygen demand (COD) concentrations were determined using A COD and multiparameter bench meter, PN HI83099-02. Sodium (Na+) and potassium (K+) ions concentrations were determined using a flame photometer (Corning 400). TPC were evaluated by UV-VIS spectrophotometer (Varian Cary 100) using the Folin–Ciocalteu method. Briefly, 2.5 mL of 0.2 N of Folin–Ciocalteu reagent was mixed with 0.5 mL of the OMW sample. The mixture was kept in the dark for 5 min. Then, 2 mL of a sodium carbonate solution (75 g/L) was added; the reaction was left in the dark at 25°C for 1 h and then centrifuged at 8000 rpm for 5 min. The absorbance of the supernatant was read at λ = 765 nm. Gallic acid was used as a standard for the calibration of the method. TPC were expressed as gallic acid equivalents in gram per liter (g GAE/L residue) [37].


## 2.4. Adsorption Experiments

Batch adsorption experiments were performed. The effect of AJB dosage, the initial concentration of TPC, and the solution temperature, on the percentage removal, were studied.The concentration of adsorbate retained in the adsorbent phase (q, mg g−1) was calculated from the following equation [38]:(2)q=Ci−Cem∗V,where q is the adsorbent phase concentration after equilibrium (mg adsorbate/g adsorbent), Ci and Ce are the initial and final (equilibrium) concentrations, respectively, of adsorbate in solution (mg/L), V is the solution volume (L), and m is the adsorbent mass (g). Percentage (%) removal of adsorbate was calculated using the following equation [39]:(3)%removal=Ci−CeCi∗100.


## 3. Results and Discussion

### 3.1. Characterization of PB and AJB Adsorbents

#### 3.1.1. FTIR Spectra

The FTIR analysis of the PB and AJB was utilized to determine the functional groups on the surface of bentonite responsible for adsorption and to explore the effect of sodium activation on its chemical composition, which is shown in Figure2.Figure 2
FTIR spectra of PB and AJB.From the PB spectrum, absorption bands resulting from bending vibrations of Si-O groups are found in the 550–400 cm–1 region. The bands due to Si-O-Al and Si-O-Si deformations in the spectra occur near 530 and 460 cm–1, respectively [40]. The spectrum also contains a band at 706, 800, and 2355 cm–1 which is all attributed to quartz [41, 42]. A broad complex band near 1030 cm–1 is related to stretching vibrations of Si O groups [43]. On the other hand, the peak at 1641 cm–1 is for H-O-H bending, and the stretching vibration of OH appears at around 3451 cm−1. The adsorption band at 3616 cm–1 in the spectrum is assigned to stretching vibrations of the structural OH groups of dioctahedral bentonite.After sodium activation, the most significant change was a decrease in the intensity of the band of the Si-O stretching region at 1030 cm–1. This means that upon the activation process, there is a possibility of the formation of three-dimensional networks of amorphous silica, which may expose more adsorption sites, which may cause damage to the tetrahedral layer. The intensity of bending and stretching bands characterized the octahedral sheet for Al-Al-OH at a 1641 cm–1 decrease, which indicates the destruction of the octahedral layer. Additionally, a sharp decrease in the absorption band attributed to the OH vibration at 3616 cm−1 is due to the removal of the octahedral cations, thus causing the loss of water and hydroxyl groups coordinated to them [44]. The decrease in the characteristic band at 3451 cm–1 represents the fundamental stretching vibrations of different -OH groups present in Fe-OH-Al, Al-OH-Al, and Mg-OH-Al in the octahedral layer [45] which confirms the disfiguration of the layer. Furthermore, regarding quartz bands, the disappearance of the band at 2355 cm–1 is noted as well as a decrease in the intensities of bands at 706, 800 cm−1, which offers a strong indication that the activation process improves the purity of the bentonite.During the activation process, most band positions did not change, thus indicating that the basic bentonite structure did not interrupt.


#### 3.1.2. X-Ray Diffractograms

The X-ray diffraction patterns of PB and AJB samples are illustrated in Figure3. Patterns of PB montmorillonite were the main mineral. However, minor amounts of quartz, kaolinite, gypsum, and cristobalite were also identified [46].Figure 3
X-ray diffraction patterns of the PB and AJB.As seen from the AJB patterns, the XRD results indicate that changes in the structure of PB are induced by adding NaCl. Quartz is not present in the AJB sample; the peaks of quartz are diminished. The main montmorillonite peaks are present. Two distinct diffraction lines belonging to crystalline NaCl (35 and 55 Å) are seen, thus proving the accumulation of crystalline NaCl [47]. X-ray results of activated bentonite show there is a shift in the position of a few peaks (for example, 31.2–26.6 Å). After the addition of NaCl, the Na+ ions are absorbed to the surface of the montmorillonite crystal grains to form a hydrated shell [48]. This is an indication of the dissolution of the tetrahedral and octahedral sheets and subsequent release of the structural cations, that is, these cations have been eliminated from the octahedral positions, thereby exchanging with Na+ ions. Moreover, the interlamellar spacing between crystal grains is compressed [49]. The chemical composition and inner structure of AJB are changed. Distinctly, XRD analysis provides good evidence that the adsorptive power of AJB has increased.Clearly, the activation process causes a decrease in peak intensity. This mostly occurs in the case of montmorillonite, which means a reduction in its content and also the disappearing of the quartz impurity content. Furthermore, the peaks of PB patterns have relative symmetry, but the peaks of AJB patterns are moving to more dissymmetry. Also, the appearance of splitting in some peaks, which is an indication of phase transformation to a lower symmetry or partial distortion of its structure, indicates that small distortions can often be observed with peak broadening. These characteristics apparently indicate that bentonite is well activated by Na+ ions.The reduction in intensity and increase in the width of peaks at 24.1 Å indicates that the crystallinity of the bentonite is considerably affected by activation, and thus, the bentonite crystalline structure is decomposing, which means that the activation process is accompanied by the appearance of an amorphous phase, as confirmed by IR results.


#### 3.1.3. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was undertaken to investigate the effect of activation of bentonite with NaCl. Figure4 shows a distinct endotherm with a maximum between 30°C and 200°C that corresponds to the release of physically adsorbed water and the dehydration process. Dehydroxylation was executed with more and more favorable temperatures, ranging from 325 to 720°C. The decomposition of chemically bound water (OH-) is detected by an endothermic change with a maximum at 350°C. The dissociation of accumulated NaCl crystals is confirmed at 800°C [50]. The total mass losses within the temperature interval of 30–200°C of PB is much more than ABJ. Beyond this temperature, there are higher mass losses for AJB, and this is attributed to the dissociation of crystalline NaCl at 800°C. This finding confirms those of the XRD results that show a partial deterioration of the AJB microstructure. The results obtained confirm the formation of NaCl crystals in the AJB, where the accumulated NaCl crystals have a destructive effect mainly on the bentonite microstructure and a secondary effect on its increase in permeability. This is enhancing its adsorption capacity toward phenolic compounds, heavy metal ions, and other pollutants. Furthermore, the TGA result is in clear agreement with the FTIR and XRD studies, which indicate consecutive changes of the bentonite sheet upon the activation process.Figure 4
TGA curves of PB and AJB.


#### 3.1.4. BET Surface Area

The results of the BET analysis show that the surface area increases with increasing Na on the surface of bentonite. The surface area increases from 66.2 m2 g−1 for PB to 249.6 m2 g−1 for AJB. The improved surface area indicates the number of active sites increases on the surface of the adsorbent, which improves the enhancement in adsorption efficiency [51].In studying the adsorption and swelling properties of the clay-water system, one type of swelling, commonly known as the interlayer or interlamellar swelling, involves the expansion of the crystal lattice itself, as found in montmorillonite [52]. It is known that the swelling properties of bentonite depend on the type of exchangeable cations, whether Na+ or Ca2+. Calcium ions have a higher charge and smaller diameter than sodium ions, and as a result, they tend to interact more strongly with the aluminosilicate platelets, thus making them less disposed to swelling. Sodium ions hydrate more readily causes the bentonite to swell more, so any effort to increase the concentration of Na+ ions will improve the swelling properties of bentonite [53]. This is perfectly consistent with the obtained XRD result. However, improved swelling of bentonite after activation is expected to yield an increase in pore size and thus an increase in adsorption capacity. As a result, it improves the removal of phenolic compounds and other pollutants.


### 3.2. Characterization of OMW

The characteristics of the studied crude and treated OMW are summarized in Table1. The analysis of the OMW shows that it is composed mainly of organic compounds and inorganic compounds (mineral salts). Among the different mineral salts present in OMW, potassium has the highest concentration. Other cations and anions are also found in OMW at lower concentrations. Also, the high phenol content of OMW contributes to the high soluble COD of OMW. Total dissolved solids content is also high, as well as the phosphate content being significant. All the above parameters must be taken into consideration in the design of a well-integrated treatment process for OMW.Table 1
Main characteristics of the OMW sample untreated and treated with PB and AJB.
ParametersUntreated OMWLiterature ranges valuesReferenceTreated OMW with PBTreated OMW with AJB% removal using PB% removal using AJBSodium (Na+), mg L−1297.9200–570[54, 55]186.4119.437.459.9Potassium (K+), mg L−16366.3639–10800[56, 57]4075.12606.335.959.1TPC, g GAE/L1.340.26–10.7[56, 58]0.850.4436.667.2Alkalinity (CaCO3), mg L−120003150–9070[59]150050025.075.0Total chlorine, mg L−12033.3–142.7[60]15725.065.0Phosphate (PO43−), mg L−1412031.8–1820[61]46023088.894.4Nitrate (NO3−-N), mg L−1360350–390[62]23012036.166.7COD, mg L−1120001900–220000[63, 64]7030118241.490.2TDS, mg L−1347005900–103200[65]131401189962.165.7It is worth noting here that the parameter values are in good agreement with those reported in the literature. Clearly, using PB for OMW treatment is essential due to significant differences between its properties. Furthermore, it is noticed that the percentage removal increases vigorously and continuously throughout the use of AJB adsorbent, and this is a good indication that simple activation of bentonite can result in a good result on its adsorption behavior. Therefore, AJB adsorbent provides a valuable solution for the treatment and recyclability of OMW.


### 3.3. Adsorption Experiments

#### 3.3.1. TPC Removal

Both PB and AJB were tested for the removal of TPC, and batch experiments were performed, where several parameters were tested, to determine the adsorption effectiveness. Different adsorbent dosages (0.1, 0.5, and 1.0 g) were mixed with 10 mL of OMW with different initial TPC concentrations (1215.16, 1340.64, 1442.00, and 1563.43 mg/L) at different temperatures (293, 303, 313, and 323 K). OMW and adsorbent were stirred in Erlenmeyer flasks continuously for 3 h. After shaking, samples were filtered using a 0.45μm microfilter and then analyzed by a UV-VIS spectrophotometer at λ = 765 nm to measure the concentration of TPC. Finally, all analytical methods were applied at least in triplicate.The adsorption amount of TPC using AJB is larger than the unactivated one, as shown in Figure5. The proposed explanation of the enhanced adsorption involves bentonite having a large specific surface area and pore spaces, and it is based on the diffuse double layer theory, which predicted that double layer thickness decreases with increasing pore-solution concentration [66]. Moreover, sodium chloride dissociates into Na+ and Cl− in an aqueous solution. There is a strong electrostatic field around the anions and cations, and thus, an oriented array of water molecules is formed around these ions. The existence of ions enhances the combining powers between water molecules and phenolic compounds. On the other side, the hydraulic conductivity of the bentonite increases as the void ratio increases. Also, at a given void ratio, the hydraulic conductivity of the bentonite increases, as the ionic strength increases. This trend for increasing hydraulic conductivity may result from the influence of the permanent on effective porosity (the pore space available for conductive flow), which concludes that increasing the concentration of salts, such as NaCl, leads to an increase in permeability in bentonite due to a decrease in interparticle repulsion among negatively charged plates [67]. As a result of the activation of bentonite by NaCl, adsorption capacity increases towards TPC.Figure 5
Comparison of the percentage removal of TPC by PB and AJB. Mass of adsorbent = 1.0 g, initial TPC concentration, 1340.64 mg/L, 10 mL of OMW, and contact time = 3 h.Similar behavior was reported for TPC adsorption by hydrochloric acid-activated bentonite. Other than that, AJB by sodium ions provides a higher adsorption capacity [2]. It is thought that the presence of NaCl crystals has an effect on its increase in permeability, which leads to an increase in the cation exchange capacity [68]. Furthermore, the possible mechanism of the adsorption process of TPC onto AJB likely to be ionic interactions between phenolate ions and sodium on the surfaces of the prepared adsorbent Na-Bent (AJB) and sodium phenoxide (C6H5ONa) will be formed [69]. Sodium atoms presented on AJB play an essential role in enhancing the absorption capacity via the electrostatic attraction [70].(1) Effect of Adsorbent Dosage on Adsorption of TPC. The experimental data regarding the effect of adsorbent dosage on the percentage removal of TPC by AJB is shown in Figure 6. A series of batch experiments were carried out by contacting different amounts of AJB with 10 mL of OMW, with a constant initial TPC concentration of 1340.64 mg/L. The contact time was made for 3 h and at different four temperatures 293, 303, 313, and 323 K.Figure 6
Effect of adsorbent dosage on the percentage removal of TPC at different temperatures.The results show that increasing the dosage of AJB increased in the percentage removal of TPC. However, the adsorption capacity of AJB increases, and this could be due to an increase in the surface area and the availability of more active sites on the surface of AJB [71]. Moreover, the adsorption capacity for AJB is greater than PB at constant temperature and dosage, which indicates that the bentonite is well activated by NaCl. Therefore, to reduce the bentonite dosage, it is necessary to modify it into superior Na-bentonite.By comparison of these obtained results, with those in the previously reported work, the percentage removal of TPC at the same adsorbent dosage and all temperatures is higher while using AJB activated by sodium ions rather than by acid activation of bentonite [2].(2) Effect of Initial TPC Concentration. The initial TPC concentration is a very important factor to be explored in adsorption studies, as most contaminated OMW usually presents different concentrations of TPC. The effect of initial TPC concentration on the adsorption capacity and percentage removal is shown in Figure 7. The operating conditions for the batch experiments were 1.0 g of AJB per 10 mL of OMW, and the contact time was 3 h at 303 K.Figure 7
Effect of initial TPC concentration on the adsorption capacity and the percentage removal of TPC.First, an increase in adsorption capacity with an increase in initial TPC concentration was observed. This may be explained by the presence of more TPC in the solution available for binding onto the active sites of the AJB. Consequently, the adsorption reached a saturation value. Indeed, the initial TPC concentration provides an important driving force to overcome all mass transfer resistance. Hence, a higher initial concentration of TPC tends to enhance the adsorption capacity. A similar phenomenon was observed for the adsorption of phenol onto organobentonite [72]. Meanwhile, the percentage of removal decreased gradually with an increase in the initial TPC concentration. This decrease occurs because all adsorbents have a limited number of active sites, and at higher concentrations, the active sites become saturated [73]. Besides, the % removal of TPC was calculated concerning the initial concentration (Ci) of adsorbate in solution as clarified in the equation (3); therefore, as (Ci) increased in the denominator, the % removal will be decreased.The following flow chart shows steps and quality controls for producing reliable and comparable information of assaying the concentration of TPC using AJB adsorbent for wastewater treatment as shown in Figure8.Figure 8
Protocol for OMW treatment and assaying for TPC concentration using AJB adsorbent.


#### 3.3.2. Adsorption Isotherms

The adsorption isotherms for TPC removal by AJB were investigated using different initial concentrations at the adsorbent mass of 1.0 g at 293, 303, 313, and 323 K and for a period of 3 h. Later, the data obtained were fitted to the Langmuir and Freundlich isotherms.The Langmuir isotherm assumed that the monolayer adsorption of adsorbate onto a homogeneous adsorbent surface takes place with a single coating layer on this surface [74]. Moreover, there is no lateral interaction between the adsorbed molecules. The linear form of the Langmuir isotherm model can be expressed as [75, 76](4)Ceqe=1KLqm+Ceqm,where qe is the equilibrium adsorption capacity (mg/g), Ce is the equilibrium concentration of TPC (mg/L), qm is a maximum adsorption capacity (mg/g), and KL is the adsorption equilibrium constant (L/mg).The linear form of the Langmuir isotherm is shown in Figure9. The correlation coefficients, R2 >0.99 at all temperatures, indicate that the adsorption was a good fit for this model.Figure 9
Langmuir plot for the adsorption of TPC onto AJB. Adsorbent dosage = 1.0 g 10 mL of OMW, pH = 6, contact time = 3 h.The maximum adsorptionqm for TPC onto AJB equals to 8.81 mg/g. The adsorption process can be evaluated to see whether it is favorable by the use of a Langmuir dimensionless separation factor RL defined as [77](5)RL=11+KLCo,where Co (mg/L) is the initial TPC concentration in solution. If the value of RL is less than 1.0, the adsorption is considered to be favorable, but it is unfavorable if RL is greater than 1.0. The calculated RL values at different concentrations fall within the range of 0.098–0.122 (Table 2), thus indicating a favorable adsorption process.Table 2
Calculated values of separation factorRL for the adsorption of TPC onto AJB at 303 K.
Co1215.161340.641442.001563.43RL0.12240.11230.10520.0978The Freundlich isotherm is based on multilayer adsorption on heterogeneous surfaces [78]. The linear form of Freundlich can be represented as(6)logqe=logKF+1nlogCe,where KF and n are the Freundlich adsorption constants showing the adsorption capacity (mg/g) and intensity, respectively, which can be determined by the linear plot of log qe versus log Ce.The adsorption Freundlich isotherm obtained for TPC onto AJB is shown in Figure10, and the isotherm parameters for both models are presented in Table 3.Figure 10
Freundlich plot for the adsorption of TPC onto AJB. Adsorbent dosage = 1.0 g 10 mL of OMW, pH = 6, contact time = 3 h.Table 3
Langmuir and Freundlich isotherm parameters for the adsorption of TPC onto AJB.
T (K)Langmuir isothermFreundlich isothermqmKLR2nKFR22937.81680.00510.99814.84811.57740.97543038.02010.00590.99855.29581.87100.97533138.06400.00690.99885.96042.22220.98603238.81280.00910.99856.76482.89050.9576The obtained values ofn exhibited intense change at higher temperatures. All n values were greater than one, indicating favorable adsorption of TPC [79]. Also, the high correlation coefficient values (R2) of the Langmuir and Freundlich models indicate that the experimental data are well fitted by both models.Comparison of AJB with other adsorbents is necessary. Table4 provides the adsorption capacities for TPC on different types of adsorbents as reported in the literature. It is thought that those adsorbent’s properties have a significant effect on its efficiency, and experiment conditions may cause different results. Based on that, it seems that the AJB for TPC removal is a promising effective adsorbent. Results in Table 4 show that AJB has a high adsorption capacity compared to a wide range of adsorbents. On the other hand, the adsorption capacity of macroporous resins XAD-16 is higher than that of prepared AJB.Table 4
Comparison of adsorption capacities of different adsorbents for TPC reported in the literature.
AdsorbentAdsorption capacityqm (mg/g)ReferencePolydimethylsiloxane/oxMWCNTs4.39[80]Olive pomace11.40[81]HyAlFe-Mt10.2HyAlFe15.7[82]Silver and titanium oxides-doped activated carbon11.0168[83]Macroporous resins XAD1616[84]Nonionic styrene-divinylbenzene resin (XAD1)3.4Nonionic styrene-divinylbenzene resin (XAD2)7.6Strong basic anion exchange resin (IRA1)2.1[85]Strong basic anion exchange resin (IRA2)0.9XAD16HP2.30SP7002.59[86]AJB8.81Present studyThe activation by sodium ions has the potential to produce a high surface BET area for AJB (the surface area increases from 66.2 to 249.6 m2 g−1) characterized by high uptake capacity for organic compounds compared to other adsorbents. However, very few studies reportedly tested the adsorption of TPC onto the bentonite produced via this activation method. Among this method, the sodium ions activation method will gain more attention from researchers. This has been attributed to fact that such method is fast, ease of operational conditions, and characterized by cost savings over the conventional techniques [87].


#### 3.3.3. Thermodynamic Studies

(1) Effect of Temperature on Adsorption of TPC. In order to determine the effect of temperature on the adsorption of TPC onto AJB, experiments were run with four different values: 293, 303, 313, and 323 K. From the curves of Figure 11, we can notice that the percentage of efficient removal of TPC increases with temperature, which indicates that the adsorption process is endothermic. This could be due to increasing the mobility of the TPC, thus gaining more kinetic energy to diffuse from the bulk phase to the solid phase, with an increase in solution temperature. Furthermore, there is an increase in the number of surface active sites for the adsorption with increasing temperature, as a result of the dissociation of some of the surface components onto AJB [88, 89]. On the other hand, KL values are directly proportional to the temperature, as shown in Figure 11. KL is the Langmuir equilibrium constant related to the affinity of binding sites and energy of sorption, equation (4). Again, TPC has a good affinity to the AJB surface and increases gradually with increasing temperature. Similarly, at constant initial TPC concentration (1340.64 mg/L) and fixed dosage of both adsorbents (1 g), as clearly observed in Figure 11, the percentage removal of TPC using PB adsorbent increases with increasing temperature.Figure 11
Effect of solution temperature on the percentage removal of TPC and the Langmuir equilibrium constantKL using AJB.The temperature has an obvious effect on sodium modification of the bentonite, and Na+ in the diffusion layer present has a trend of moving to bentonite surfaces. The migration of Na+ is increased, which increases the content of Na + on the bentonite surface. Therefore, the reaction velocity between Na+ and Ca2+ is growing [90].Comparing the results of percentage removal of TPC obtained by using AJB, with those in the previously reported study, AJB presents a good adsorption capacity rather than the acid-activated Jordanian bentonite [2]. Hence, AJB utilization for the removal of TPC is very promising.To evaluate the feasibility of the adsorption process, thermodynamic parameters, where ΔG° is the Gibbs free energy (kJ/mol), ΔH° is the standard enthalpy (J/mol), and ΔS° is the standard entropy (J/mol K), were calculated from the curve relating the distribution coefficient (K) as a function of temperature, using the following equations [91]:(7)ΔG°=−RTlnK,lnK=ΔS°R−ΔH°RT,where R is the gas constant (8.3145 J.mol−1.K−1), and T is the temperature in Kelvin. The values of ΔH° and ΔS° were determined from the slope and intercept values of the straight line of plotting ln K versus 1/T, respectively. According to the data presented in Table 5, the spontaneity of the adsorption process is established by a decrease in ΔG° values, in addition to spontaneity increases as the temperature of the solution increases, which means that, as the adsorption process becomes more favorable, the negative values of ΔG° indicate that the adsorption of the TPC is spontaneous and favorable [92]. The positive value of ΔH° shows that the adsorption process is endothermic in nature [93]. This is following increasing adsorption equilibrium with increasing temperature. The positive value of ΔS° reflects an increase in the randomness at the interface between AJB and the phenolic solution during the adsorption process. This suggests that some structural changes occur on the adsorbent, in addition to the adsorbate, due to the exchange of the phenolic compounds with more mobile ions present on the AJB, which would cause an increase in the entropy during the adsorption process [94].Table 5
Thermodynamic parameters of adsorption of TPC onto AJB.
T (K)ΔG°29326.2230313.90313−5.22323−28.21ΔH° = 559.47ΔS° = 1.81Moreover, the ΔH° value was 559.47 kJ/mol, thus indicating that the adsorption of TPC onto AJB involved chemical adsorption. Value of enthalpy within the range 2.1–20.9 kJ/mol indicates physical sorption, and value over 20.9 kJ/mol indicates chemisorption [95]. Therefore, the process is irreversible [96]. Thus, the adsorption mechanism involves valences forces and chemical bond [75, 97].According to the results obtained from this work, by using effectively-prepared AJB adsorbent, it is favorable in terms of both economic and environmental features, and it could improve adsorption for the removal of hazardous materials, such as phenolic compounds from industrial effluents such as OMW. Furthermore, this study is expected to be of great benefit because all these processes are interrelated; especially given that there are no previous studies on Jordanian bentonite that considered all these processes.


## 3.1. Characterization of PB and AJB Adsorbents

### 3.1.1. FTIR Spectra

The FTIR analysis of the PB and AJB was utilized to determine the functional groups on the surface of bentonite responsible for adsorption and to explore the effect of sodium activation on its chemical composition, which is shown in Figure2.Figure 2
FTIR spectra of PB and AJB.From the PB spectrum, absorption bands resulting from bending vibrations of Si-O groups are found in the 550–400 cm–1 region. The bands due to Si-O-Al and Si-O-Si deformations in the spectra occur near 530 and 460 cm–1, respectively [40]. The spectrum also contains a band at 706, 800, and 2355 cm–1 which is all attributed to quartz [41, 42]. A broad complex band near 1030 cm–1 is related to stretching vibrations of Si O groups [43]. On the other hand, the peak at 1641 cm–1 is for H-O-H bending, and the stretching vibration of OH appears at around 3451 cm−1. The adsorption band at 3616 cm–1 in the spectrum is assigned to stretching vibrations of the structural OH groups of dioctahedral bentonite.After sodium activation, the most significant change was a decrease in the intensity of the band of the Si-O stretching region at 1030 cm–1. This means that upon the activation process, there is a possibility of the formation of three-dimensional networks of amorphous silica, which may expose more adsorption sites, which may cause damage to the tetrahedral layer. The intensity of bending and stretching bands characterized the octahedral sheet for Al-Al-OH at a 1641 cm–1 decrease, which indicates the destruction of the octahedral layer. Additionally, a sharp decrease in the absorption band attributed to the OH vibration at 3616 cm−1 is due to the removal of the octahedral cations, thus causing the loss of water and hydroxyl groups coordinated to them [44]. The decrease in the characteristic band at 3451 cm–1 represents the fundamental stretching vibrations of different -OH groups present in Fe-OH-Al, Al-OH-Al, and Mg-OH-Al in the octahedral layer [45] which confirms the disfiguration of the layer. Furthermore, regarding quartz bands, the disappearance of the band at 2355 cm–1 is noted as well as a decrease in the intensities of bands at 706, 800 cm−1, which offers a strong indication that the activation process improves the purity of the bentonite.During the activation process, most band positions did not change, thus indicating that the basic bentonite structure did not interrupt.


### 3.1.2. X-Ray Diffractograms

The X-ray diffraction patterns of PB and AJB samples are illustrated in Figure3. Patterns of PB montmorillonite were the main mineral. However, minor amounts of quartz, kaolinite, gypsum, and cristobalite were also identified [46].Figure 3
X-ray diffraction patterns of the PB and AJB.As seen from the AJB patterns, the XRD results indicate that changes in the structure of PB are induced by adding NaCl. Quartz is not present in the AJB sample; the peaks of quartz are diminished. The main montmorillonite peaks are present. Two distinct diffraction lines belonging to crystalline NaCl (35 and 55 Å) are seen, thus proving the accumulation of crystalline NaCl [47]. X-ray results of activated bentonite show there is a shift in the position of a few peaks (for example, 31.2–26.6 Å). After the addition of NaCl, the Na+ ions are absorbed to the surface of the montmorillonite crystal grains to form a hydrated shell [48]. This is an indication of the dissolution of the tetrahedral and octahedral sheets and subsequent release of the structural cations, that is, these cations have been eliminated from the octahedral positions, thereby exchanging with Na+ ions. Moreover, the interlamellar spacing between crystal grains is compressed [49]. The chemical composition and inner structure of AJB are changed. Distinctly, XRD analysis provides good evidence that the adsorptive power of AJB has increased.Clearly, the activation process causes a decrease in peak intensity. This mostly occurs in the case of montmorillonite, which means a reduction in its content and also the disappearing of the quartz impurity content. Furthermore, the peaks of PB patterns have relative symmetry, but the peaks of AJB patterns are moving to more dissymmetry. Also, the appearance of splitting in some peaks, which is an indication of phase transformation to a lower symmetry or partial distortion of its structure, indicates that small distortions can often be observed with peak broadening. These characteristics apparently indicate that bentonite is well activated by Na+ ions.The reduction in intensity and increase in the width of peaks at 24.1 Å indicates that the crystallinity of the bentonite is considerably affected by activation, and thus, the bentonite crystalline structure is decomposing, which means that the activation process is accompanied by the appearance of an amorphous phase, as confirmed by IR results.


### 3.1.3. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was undertaken to investigate the effect of activation of bentonite with NaCl. Figure4 shows a distinct endotherm with a maximum between 30°C and 200°C that corresponds to the release of physically adsorbed water and the dehydration process. Dehydroxylation was executed with more and more favorable temperatures, ranging from 325 to 720°C. The decomposition of chemically bound water (OH-) is detected by an endothermic change with a maximum at 350°C. The dissociation of accumulated NaCl crystals is confirmed at 800°C [50]. The total mass losses within the temperature interval of 30–200°C of PB is much more than ABJ. Beyond this temperature, there are higher mass losses for AJB, and this is attributed to the dissociation of crystalline NaCl at 800°C. This finding confirms those of the XRD results that show a partial deterioration of the AJB microstructure. The results obtained confirm the formation of NaCl crystals in the AJB, where the accumulated NaCl crystals have a destructive effect mainly on the bentonite microstructure and a secondary effect on its increase in permeability. This is enhancing its adsorption capacity toward phenolic compounds, heavy metal ions, and other pollutants. Furthermore, the TGA result is in clear agreement with the FTIR and XRD studies, which indicate consecutive changes of the bentonite sheet upon the activation process.Figure 4
TGA curves of PB and AJB.


### 3.1.4. BET Surface Area

The results of the BET analysis show that the surface area increases with increasing Na on the surface of bentonite. The surface area increases from 66.2 m2 g−1 for PB to 249.6 m2 g−1 for AJB. The improved surface area indicates the number of active sites increases on the surface of the adsorbent, which improves the enhancement in adsorption efficiency [51].In studying the adsorption and swelling properties of the clay-water system, one type of swelling, commonly known as the interlayer or interlamellar swelling, involves the expansion of the crystal lattice itself, as found in montmorillonite [52]. It is known that the swelling properties of bentonite depend on the type of exchangeable cations, whether Na+ or Ca2+. Calcium ions have a higher charge and smaller diameter than sodium ions, and as a result, they tend to interact more strongly with the aluminosilicate platelets, thus making them less disposed to swelling. Sodium ions hydrate more readily causes the bentonite to swell more, so any effort to increase the concentration of Na+ ions will improve the swelling properties of bentonite [53]. This is perfectly consistent with the obtained XRD result. However, improved swelling of bentonite after activation is expected to yield an increase in pore size and thus an increase in adsorption capacity. As a result, it improves the removal of phenolic compounds and other pollutants.


## 3.1.1. FTIR Spectra

The FTIR analysis of the PB and AJB was utilized to determine the functional groups on the surface of bentonite responsible for adsorption and to explore the effect of sodium activation on its chemical composition, which is shown in Figure2.Figure 2
FTIR spectra of PB and AJB.From the PB spectrum, absorption bands resulting from bending vibrations of Si-O groups are found in the 550–400 cm–1 region. The bands due to Si-O-Al and Si-O-Si deformations in the spectra occur near 530 and 460 cm–1, respectively [40]. The spectrum also contains a band at 706, 800, and 2355 cm–1 which is all attributed to quartz [41, 42]. A broad complex band near 1030 cm–1 is related to stretching vibrations of Si O groups [43]. On the other hand, the peak at 1641 cm–1 is for H-O-H bending, and the stretching vibration of OH appears at around 3451 cm−1. The adsorption band at 3616 cm–1 in the spectrum is assigned to stretching vibrations of the structural OH groups of dioctahedral bentonite.After sodium activation, the most significant change was a decrease in the intensity of the band of the Si-O stretching region at 1030 cm–1. This means that upon the activation process, there is a possibility of the formation of three-dimensional networks of amorphous silica, which may expose more adsorption sites, which may cause damage to the tetrahedral layer. The intensity of bending and stretching bands characterized the octahedral sheet for Al-Al-OH at a 1641 cm–1 decrease, which indicates the destruction of the octahedral layer. Additionally, a sharp decrease in the absorption band attributed to the OH vibration at 3616 cm−1 is due to the removal of the octahedral cations, thus causing the loss of water and hydroxyl groups coordinated to them [44]. The decrease in the characteristic band at 3451 cm–1 represents the fundamental stretching vibrations of different -OH groups present in Fe-OH-Al, Al-OH-Al, and Mg-OH-Al in the octahedral layer [45] which confirms the disfiguration of the layer. Furthermore, regarding quartz bands, the disappearance of the band at 2355 cm–1 is noted as well as a decrease in the intensities of bands at 706, 800 cm−1, which offers a strong indication that the activation process improves the purity of the bentonite.During the activation process, most band positions did not change, thus indicating that the basic bentonite structure did not interrupt.


## 3.1.2. X-Ray Diffractograms

The X-ray diffraction patterns of PB and AJB samples are illustrated in Figure3. Patterns of PB montmorillonite were the main mineral. However, minor amounts of quartz, kaolinite, gypsum, and cristobalite were also identified [46].Figure 3
X-ray diffraction patterns of the PB and AJB.As seen from the AJB patterns, the XRD results indicate that changes in the structure of PB are induced by adding NaCl. Quartz is not present in the AJB sample; the peaks of quartz are diminished. The main montmorillonite peaks are present. Two distinct diffraction lines belonging to crystalline NaCl (35 and 55 Å) are seen, thus proving the accumulation of crystalline NaCl [47]. X-ray results of activated bentonite show there is a shift in the position of a few peaks (for example, 31.2–26.6 Å). After the addition of NaCl, the Na+ ions are absorbed to the surface of the montmorillonite crystal grains to form a hydrated shell [48]. This is an indication of the dissolution of the tetrahedral and octahedral sheets and subsequent release of the structural cations, that is, these cations have been eliminated from the octahedral positions, thereby exchanging with Na+ ions. Moreover, the interlamellar spacing between crystal grains is compressed [49]. The chemical composition and inner structure of AJB are changed. Distinctly, XRD analysis provides good evidence that the adsorptive power of AJB has increased.Clearly, the activation process causes a decrease in peak intensity. This mostly occurs in the case of montmorillonite, which means a reduction in its content and also the disappearing of the quartz impurity content. Furthermore, the peaks of PB patterns have relative symmetry, but the peaks of AJB patterns are moving to more dissymmetry. Also, the appearance of splitting in some peaks, which is an indication of phase transformation to a lower symmetry or partial distortion of its structure, indicates that small distortions can often be observed with peak broadening. These characteristics apparently indicate that bentonite is well activated by Na+ ions.The reduction in intensity and increase in the width of peaks at 24.1 Å indicates that the crystallinity of the bentonite is considerably affected by activation, and thus, the bentonite crystalline structure is decomposing, which means that the activation process is accompanied by the appearance of an amorphous phase, as confirmed by IR results.


## 3.1.3. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was undertaken to investigate the effect of activation of bentonite with NaCl. Figure4 shows a distinct endotherm with a maximum between 30°C and 200°C that corresponds to the release of physically adsorbed water and the dehydration process. Dehydroxylation was executed with more and more favorable temperatures, ranging from 325 to 720°C. The decomposition of chemically bound water (OH-) is detected by an endothermic change with a maximum at 350°C. The dissociation of accumulated NaCl crystals is confirmed at 800°C [50]. The total mass losses within the temperature interval of 30–200°C of PB is much more than ABJ. Beyond this temperature, there are higher mass losses for AJB, and this is attributed to the dissociation of crystalline NaCl at 800°C. This finding confirms those of the XRD results that show a partial deterioration of the AJB microstructure. The results obtained confirm the formation of NaCl crystals in the AJB, where the accumulated NaCl crystals have a destructive effect mainly on the bentonite microstructure and a secondary effect on its increase in permeability. This is enhancing its adsorption capacity toward phenolic compounds, heavy metal ions, and other pollutants. Furthermore, the TGA result is in clear agreement with the FTIR and XRD studies, which indicate consecutive changes of the bentonite sheet upon the activation process.Figure 4
TGA curves of PB and AJB.


## 3.1.4. BET Surface Area

The results of the BET analysis show that the surface area increases with increasing Na on the surface of bentonite. The surface area increases from 66.2 m2 g−1 for PB to 249.6 m2 g−1 for AJB. The improved surface area indicates the number of active sites increases on the surface of the adsorbent, which improves the enhancement in adsorption efficiency [51].In studying the adsorption and swelling properties of the clay-water system, one type of swelling, commonly known as the interlayer or interlamellar swelling, involves the expansion of the crystal lattice itself, as found in montmorillonite [52]. It is known that the swelling properties of bentonite depend on the type of exchangeable cations, whether Na+ or Ca2+. Calcium ions have a higher charge and smaller diameter than sodium ions, and as a result, they tend to interact more strongly with the aluminosilicate platelets, thus making them less disposed to swelling. Sodium ions hydrate more readily causes the bentonite to swell more, so any effort to increase the concentration of Na+ ions will improve the swelling properties of bentonite [53]. This is perfectly consistent with the obtained XRD result. However, improved swelling of bentonite after activation is expected to yield an increase in pore size and thus an increase in adsorption capacity. As a result, it improves the removal of phenolic compounds and other pollutants.


## 3.2. Characterization of OMW

The characteristics of the studied crude and treated OMW are summarized in Table1. The analysis of the OMW shows that it is composed mainly of organic compounds and inorganic compounds (mineral salts). Among the different mineral salts present in OMW, potassium has the highest concentration. Other cations and anions are also found in OMW at lower concentrations. Also, the high phenol content of OMW contributes to the high soluble COD of OMW. Total dissolved solids content is also high, as well as the phosphate content being significant. All the above parameters must be taken into consideration in the design of a well-integrated treatment process for OMW.Table 1
Main characteristics of the OMW sample untreated and treated with PB and AJB.
ParametersUntreated OMWLiterature ranges valuesReferenceTreated OMW with PBTreated OMW with AJB% removal using PB% removal using AJBSodium (Na+), mg L−1297.9200–570[54, 55]186.4119.437.459.9Potassium (K+), mg L−16366.3639–10800[56, 57]4075.12606.335.959.1TPC, g GAE/L1.340.26–10.7[56, 58]0.850.4436.667.2Alkalinity (CaCO3), mg L−120003150–9070[59]150050025.075.0Total chlorine, mg L−12033.3–142.7[60]15725.065.0Phosphate (PO43−), mg L−1412031.8–1820[61]46023088.894.4Nitrate (NO3−-N), mg L−1360350–390[62]23012036.166.7COD, mg L−1120001900–220000[63, 64]7030118241.490.2TDS, mg L−1347005900–103200[65]131401189962.165.7It is worth noting here that the parameter values are in good agreement with those reported in the literature. Clearly, using PB for OMW treatment is essential due to significant differences between its properties. Furthermore, it is noticed that the percentage removal increases vigorously and continuously throughout the use of AJB adsorbent, and this is a good indication that simple activation of bentonite can result in a good result on its adsorption behavior. Therefore, AJB adsorbent provides a valuable solution for the treatment and recyclability of OMW.


## 3.3. Adsorption Experiments

### 3.3.1. TPC Removal

Both PB and AJB were tested for the removal of TPC, and batch experiments were performed, where several parameters were tested, to determine the adsorption effectiveness. Different adsorbent dosages (0.1, 0.5, and 1.0 g) were mixed with 10 mL of OMW with different initial TPC concentrations (1215.16, 1340.64, 1442.00, and 1563.43 mg/L) at different temperatures (293, 303, 313, and 323 K). OMW and adsorbent were stirred in Erlenmeyer flasks continuously for 3 h. After shaking, samples were filtered using a 0.45μm microfilter and then analyzed by a UV-VIS spectrophotometer at λ = 765 nm to measure the concentration of TPC. Finally, all analytical methods were applied at least in triplicate.The adsorption amount of TPC using AJB is larger than the unactivated one, as shown in Figure5. The proposed explanation of the enhanced adsorption involves bentonite having a large specific surface area and pore spaces, and it is based on the diffuse double layer theory, which predicted that double layer thickness decreases with increasing pore-solution concentration [66]. Moreover, sodium chloride dissociates into Na+ and Cl− in an aqueous solution. There is a strong electrostatic field around the anions and cations, and thus, an oriented array of water molecules is formed around these ions. The existence of ions enhances the combining powers between water molecules and phenolic compounds. On the other side, the hydraulic conductivity of the bentonite increases as the void ratio increases. Also, at a given void ratio, the hydraulic conductivity of the bentonite increases, as the ionic strength increases. This trend for increasing hydraulic conductivity may result from the influence of the permanent on effective porosity (the pore space available for conductive flow), which concludes that increasing the concentration of salts, such as NaCl, leads to an increase in permeability in bentonite due to a decrease in interparticle repulsion among negatively charged plates [67]. As a result of the activation of bentonite by NaCl, adsorption capacity increases towards TPC.Figure 5
Comparison of the percentage removal of TPC by PB and AJB. Mass of adsorbent = 1.0 g, initial TPC concentration, 1340.64 mg/L, 10 mL of OMW, and contact time = 3 h.Similar behavior was reported for TPC adsorption by hydrochloric acid-activated bentonite. Other than that, AJB by sodium ions provides a higher adsorption capacity [2]. It is thought that the presence of NaCl crystals has an effect on its increase in permeability, which leads to an increase in the cation exchange capacity [68]. Furthermore, the possible mechanism of the adsorption process of TPC onto AJB likely to be ionic interactions between phenolate ions and sodium on the surfaces of the prepared adsorbent Na-Bent (AJB) and sodium phenoxide (C6H5ONa) will be formed [69]. Sodium atoms presented on AJB play an essential role in enhancing the absorption capacity via the electrostatic attraction [70].(1) Effect of Adsorbent Dosage on Adsorption of TPC. The experimental data regarding the effect of adsorbent dosage on the percentage removal of TPC by AJB is shown in Figure 6. A series of batch experiments were carried out by contacting different amounts of AJB with 10 mL of OMW, with a constant initial TPC concentration of 1340.64 mg/L. The contact time was made for 3 h and at different four temperatures 293, 303, 313, and 323 K.Figure 6
Effect of adsorbent dosage on the percentage removal of TPC at different temperatures.The results show that increasing the dosage of AJB increased in the percentage removal of TPC. However, the adsorption capacity of AJB increases, and this could be due to an increase in the surface area and the availability of more active sites on the surface of AJB [71]. Moreover, the adsorption capacity for AJB is greater than PB at constant temperature and dosage, which indicates that the bentonite is well activated by NaCl. Therefore, to reduce the bentonite dosage, it is necessary to modify it into superior Na-bentonite.By comparison of these obtained results, with those in the previously reported work, the percentage removal of TPC at the same adsorbent dosage and all temperatures is higher while using AJB activated by sodium ions rather than by acid activation of bentonite [2].(2) Effect of Initial TPC Concentration. The initial TPC concentration is a very important factor to be explored in adsorption studies, as most contaminated OMW usually presents different concentrations of TPC. The effect of initial TPC concentration on the adsorption capacity and percentage removal is shown in Figure 7. The operating conditions for the batch experiments were 1.0 g of AJB per 10 mL of OMW, and the contact time was 3 h at 303 K.Figure 7
Effect of initial TPC concentration on the adsorption capacity and the percentage removal of TPC.First, an increase in adsorption capacity with an increase in initial TPC concentration was observed. This may be explained by the presence of more TPC in the solution available for binding onto the active sites of the AJB. Consequently, the adsorption reached a saturation value. Indeed, the initial TPC concentration provides an important driving force to overcome all mass transfer resistance. Hence, a higher initial concentration of TPC tends to enhance the adsorption capacity. A similar phenomenon was observed for the adsorption of phenol onto organobentonite [72]. Meanwhile, the percentage of removal decreased gradually with an increase in the initial TPC concentration. This decrease occurs because all adsorbents have a limited number of active sites, and at higher concentrations, the active sites become saturated [73]. Besides, the % removal of TPC was calculated concerning the initial concentration (Ci) of adsorbate in solution as clarified in the equation (3); therefore, as (Ci) increased in the denominator, the % removal will be decreased.The following flow chart shows steps and quality controls for producing reliable and comparable information of assaying the concentration of TPC using AJB adsorbent for wastewater treatment as shown in Figure8.Figure 8
Protocol for OMW treatment and assaying for TPC concentration using AJB adsorbent.


### 3.3.2. Adsorption Isotherms

The adsorption isotherms for TPC removal by AJB were investigated using different initial concentrations at the adsorbent mass of 1.0 g at 293, 303, 313, and 323 K and for a period of 3 h. Later, the data obtained were fitted to the Langmuir and Freundlich isotherms.The Langmuir isotherm assumed that the monolayer adsorption of adsorbate onto a homogeneous adsorbent surface takes place with a single coating layer on this surface [74]. Moreover, there is no lateral interaction between the adsorbed molecules. The linear form of the Langmuir isotherm model can be expressed as [75, 76](4)Ceqe=1KLqm+Ceqm,where qe is the equilibrium adsorption capacity (mg/g), Ce is the equilibrium concentration of TPC (mg/L), qm is a maximum adsorption capacity (mg/g), and KL is the adsorption equilibrium constant (L/mg).The linear form of the Langmuir isotherm is shown in Figure9. The correlation coefficients, R2 >0.99 at all temperatures, indicate that the adsorption was a good fit for this model.Figure 9
Langmuir plot for the adsorption of TPC onto AJB. Adsorbent dosage = 1.0 g 10 mL of OMW, pH = 6, contact time = 3 h.The maximum adsorptionqm for TPC onto AJB equals to 8.81 mg/g. The adsorption process can be evaluated to see whether it is favorable by the use of a Langmuir dimensionless separation factor RL defined as [77](5)RL=11+KLCo,where Co (mg/L) is the initial TPC concentration in solution. If the value of RL is less than 1.0, the adsorption is considered to be favorable, but it is unfavorable if RL is greater than 1.0. The calculated RL values at different concentrations fall within the range of 0.098–0.122 (Table 2), thus indicating a favorable adsorption process.Table 2
Calculated values of separation factorRL for the adsorption of TPC onto AJB at 303 K.
Co1215.161340.641442.001563.43RL0.12240.11230.10520.0978The Freundlich isotherm is based on multilayer adsorption on heterogeneous surfaces [78]. The linear form of Freundlich can be represented as(6)logqe=logKF+1nlogCe,where KF and n are the Freundlich adsorption constants showing the adsorption capacity (mg/g) and intensity, respectively, which can be determined by the linear plot of log qe versus log Ce.The adsorption Freundlich isotherm obtained for TPC onto AJB is shown in Figure10, and the isotherm parameters for both models are presented in Table 3.Figure 10
Freundlich plot for the adsorption of TPC onto AJB. Adsorbent dosage = 1.0 g 10 mL of OMW, pH = 6, contact time = 3 h.Table 3
Langmuir and Freundlich isotherm parameters for the adsorption of TPC onto AJB.
T (K)Langmuir isothermFreundlich isothermqmKLR2nKFR22937.81680.00510.99814.84811.57740.97543038.02010.00590.99855.29581.87100.97533138.06400.00690.99885.96042.22220.98603238.81280.00910.99856.76482.89050.9576The obtained values ofn exhibited intense change at higher temperatures. All n values were greater than one, indicating favorable adsorption of TPC [79]. Also, the high correlation coefficient values (R2) of the Langmuir and Freundlich models indicate that the experimental data are well fitted by both models.Comparison of AJB with other adsorbents is necessary. Table4 provides the adsorption capacities for TPC on different types of adsorbents as reported in the literature. It is thought that those adsorbent’s properties have a significant effect on its efficiency, and experiment conditions may cause different results. Based on that, it seems that the AJB for TPC removal is a promising effective adsorbent. Results in Table 4 show that AJB has a high adsorption capacity compared to a wide range of adsorbents. On the other hand, the adsorption capacity of macroporous resins XAD-16 is higher than that of prepared AJB.Table 4
Comparison of adsorption capacities of different adsorbents for TPC reported in the literature.
AdsorbentAdsorption capacityqm (mg/g)ReferencePolydimethylsiloxane/oxMWCNTs4.39[80]Olive pomace11.40[81]HyAlFe-Mt10.2HyAlFe15.7[82]Silver and titanium oxides-doped activated carbon11.0168[83]Macroporous resins XAD1616[84]Nonionic styrene-divinylbenzene resin (XAD1)3.4Nonionic styrene-divinylbenzene resin (XAD2)7.6Strong basic anion exchange resin (IRA1)2.1[85]Strong basic anion exchange resin (IRA2)0.9XAD16HP2.30SP7002.59[86]AJB8.81Present studyThe activation by sodium ions has the potential to produce a high surface BET area for AJB (the surface area increases from 66.2 to 249.6 m2 g−1) characterized by high uptake capacity for organic compounds compared to other adsorbents. However, very few studies reportedly tested the adsorption of TPC onto the bentonite produced via this activation method. Among this method, the sodium ions activation method will gain more attention from researchers. This has been attributed to fact that such method is fast, ease of operational conditions, and characterized by cost savings over the conventional techniques [87].


### 3.3.3. Thermodynamic Studies

(1) Effect of Temperature on Adsorption of TPC. In order to determine the effect of temperature on the adsorption of TPC onto AJB, experiments were run with four different values: 293, 303, 313, and 323 K. From the curves of Figure 11, we can notice that the percentage of efficient removal of TPC increases with temperature, which indicates that the adsorption process is endothermic. This could be due to increasing the mobility of the TPC, thus gaining more kinetic energy to diffuse from the bulk phase to the solid phase, with an increase in solution temperature. Furthermore, there is an increase in the number of surface active sites for the adsorption with increasing temperature, as a result of the dissociation of some of the surface components onto AJB [88, 89]. On the other hand, KL values are directly proportional to the temperature, as shown in Figure 11. KL is the Langmuir equilibrium constant related to the affinity of binding sites and energy of sorption, equation (4). Again, TPC has a good affinity to the AJB surface and increases gradually with increasing temperature. Similarly, at constant initial TPC concentration (1340.64 mg/L) and fixed dosage of both adsorbents (1 g), as clearly observed in Figure 11, the percentage removal of TPC using PB adsorbent increases with increasing temperature.Figure 11
Effect of solution temperature on the percentage removal of TPC and the Langmuir equilibrium constantKL using AJB.The temperature has an obvious effect on sodium modification of the bentonite, and Na+ in the diffusion layer present has a trend of moving to bentonite surfaces. The migration of Na+ is increased, which increases the content of Na + on the bentonite surface. Therefore, the reaction velocity between Na+ and Ca2+ is growing [90].Comparing the results of percentage removal of TPC obtained by using AJB, with those in the previously reported study, AJB presents a good adsorption capacity rather than the acid-activated Jordanian bentonite [2]. Hence, AJB utilization for the removal of TPC is very promising.To evaluate the feasibility of the adsorption process, thermodynamic parameters, where ΔG° is the Gibbs free energy (kJ/mol), ΔH° is the standard enthalpy (J/mol), and ΔS° is the standard entropy (J/mol K), were calculated from the curve relating the distribution coefficient (K) as a function of temperature, using the following equations [91]:(7)ΔG°=−RTlnK,lnK=ΔS°R−ΔH°RT,where R is the gas constant (8.3145 J.mol−1.K−1), and T is the temperature in Kelvin. The values of ΔH° and ΔS° were determined from the slope and intercept values of the straight line of plotting ln K versus 1/T, respectively. According to the data presented in Table 5, the spontaneity of the adsorption process is established by a decrease in ΔG° values, in addition to spontaneity increases as the temperature of the solution increases, which means that, as the adsorption process becomes more favorable, the negative values of ΔG° indicate that the adsorption of the TPC is spontaneous and favorable [92]. The positive value of ΔH° shows that the adsorption process is endothermic in nature [93]. This is following increasing adsorption equilibrium with increasing temperature. The positive value of ΔS° reflects an increase in the randomness at the interface between AJB and the phenolic solution during the adsorption process. This suggests that some structural changes occur on the adsorbent, in addition to the adsorbate, due to the exchange of the phenolic compounds with more mobile ions present on the AJB, which would cause an increase in the entropy during the adsorption process [94].Table 5
Thermodynamic parameters of adsorption of TPC onto AJB.
T (K)ΔG°29326.2230313.90313−5.22323−28.21ΔH° = 559.47ΔS° = 1.81Moreover, the ΔH° value was 559.47 kJ/mol, thus indicating that the adsorption of TPC onto AJB involved chemical adsorption. Value of enthalpy within the range 2.1–20.9 kJ/mol indicates physical sorption, and value over 20.9 kJ/mol indicates chemisorption [95]. Therefore, the process is irreversible [96]. Thus, the adsorption mechanism involves valences forces and chemical bond [75, 97].According to the results obtained from this work, by using effectively-prepared AJB adsorbent, it is favorable in terms of both economic and environmental features, and it could improve adsorption for the removal of hazardous materials, such as phenolic compounds from industrial effluents such as OMW. Furthermore, this study is expected to be of great benefit because all these processes are interrelated; especially given that there are no previous studies on Jordanian bentonite that considered all these processes.


## 3.3.1. TPC Removal

Both PB and AJB were tested for the removal of TPC, and batch experiments were performed, where several parameters were tested, to determine the adsorption effectiveness. Different adsorbent dosages (0.1, 0.5, and 1.0 g) were mixed with 10 mL of OMW with different initial TPC concentrations (1215.16, 1340.64, 1442.00, and 1563.43 mg/L) at different temperatures (293, 303, 313, and 323 K). OMW and adsorbent were stirred in Erlenmeyer flasks continuously for 3 h. After shaking, samples were filtered using a 0.45μm microfilter and then analyzed by a UV-VIS spectrophotometer at λ = 765 nm to measure the concentration of TPC. Finally, all analytical methods were applied at least in triplicate.The adsorption amount of TPC using AJB is larger than the unactivated one, as shown in Figure5. The proposed explanation of the enhanced adsorption involves bentonite having a large specific surface area and pore spaces, and it is based on the diffuse double layer theory, which predicted that double layer thickness decreases with increasing pore-solution concentration [66]. Moreover, sodium chloride dissociates into Na+ and Cl− in an aqueous solution. There is a strong electrostatic field around the anions and cations, and thus, an oriented array of water molecules is formed around these ions. The existence of ions enhances the combining powers between water molecules and phenolic compounds. On the other side, the hydraulic conductivity of the bentonite increases as the void ratio increases. Also, at a given void ratio, the hydraulic conductivity of the bentonite increases, as the ionic strength increases. This trend for increasing hydraulic conductivity may result from the influence of the permanent on effective porosity (the pore space available for conductive flow), which concludes that increasing the concentration of salts, such as NaCl, leads to an increase in permeability in bentonite due to a decrease in interparticle repulsion among negatively charged plates [67]. As a result of the activation of bentonite by NaCl, adsorption capacity increases towards TPC.Figure 5
Comparison of the percentage removal of TPC by PB and AJB. Mass of adsorbent = 1.0 g, initial TPC concentration, 1340.64 mg/L, 10 mL of OMW, and contact time = 3 h.Similar behavior was reported for TPC adsorption by hydrochloric acid-activated bentonite. Other than that, AJB by sodium ions provides a higher adsorption capacity [2]. It is thought that the presence of NaCl crystals has an effect on its increase in permeability, which leads to an increase in the cation exchange capacity [68]. Furthermore, the possible mechanism of the adsorption process of TPC onto AJB likely to be ionic interactions between phenolate ions and sodium on the surfaces of the prepared adsorbent Na-Bent (AJB) and sodium phenoxide (C6H5ONa) will be formed [69]. Sodium atoms presented on AJB play an essential role in enhancing the absorption capacity via the electrostatic attraction [70].(1) Effect of Adsorbent Dosage on Adsorption of TPC. The experimental data regarding the effect of adsorbent dosage on the percentage removal of TPC by AJB is shown in Figure 6. A series of batch experiments were carried out by contacting different amounts of AJB with 10 mL of OMW, with a constant initial TPC concentration of 1340.64 mg/L. The contact time was made for 3 h and at different four temperatures 293, 303, 313, and 323 K.Figure 6
Effect of adsorbent dosage on the percentage removal of TPC at different temperatures.The results show that increasing the dosage of AJB increased in the percentage removal of TPC. However, the adsorption capacity of AJB increases, and this could be due to an increase in the surface area and the availability of more active sites on the surface of AJB [71]. Moreover, the adsorption capacity for AJB is greater than PB at constant temperature and dosage, which indicates that the bentonite is well activated by NaCl. Therefore, to reduce the bentonite dosage, it is necessary to modify it into superior Na-bentonite.By comparison of these obtained results, with those in the previously reported work, the percentage removal of TPC at the same adsorbent dosage and all temperatures is higher while using AJB activated by sodium ions rather than by acid activation of bentonite [2].(2) Effect of Initial TPC Concentration. The initial TPC concentration is a very important factor to be explored in adsorption studies, as most contaminated OMW usually presents different concentrations of TPC. The effect of initial TPC concentration on the adsorption capacity and percentage removal is shown in Figure 7. The operating conditions for the batch experiments were 1.0 g of AJB per 10 mL of OMW, and the contact time was 3 h at 303 K.Figure 7
Effect of initial TPC concentration on the adsorption capacity and the percentage removal of TPC.First, an increase in adsorption capacity with an increase in initial TPC concentration was observed. This may be explained by the presence of more TPC in the solution available for binding onto the active sites of the AJB. Consequently, the adsorption reached a saturation value. Indeed, the initial TPC concentration provides an important driving force to overcome all mass transfer resistance. Hence, a higher initial concentration of TPC tends to enhance the adsorption capacity. A similar phenomenon was observed for the adsorption of phenol onto organobentonite [72]. Meanwhile, the percentage of removal decreased gradually with an increase in the initial TPC concentration. This decrease occurs because all adsorbents have a limited number of active sites, and at higher concentrations, the active sites become saturated [73]. Besides, the % removal of TPC was calculated concerning the initial concentration (Ci) of adsorbate in solution as clarified in the equation (3); therefore, as (Ci) increased in the denominator, the % removal will be decreased.The following flow chart shows steps and quality controls for producing reliable and comparable information of assaying the concentration of TPC using AJB adsorbent for wastewater treatment as shown in Figure8.Figure 8
Protocol for OMW treatment and assaying for TPC concentration using AJB adsorbent.


## 3.3.2. Adsorption Isotherms

The adsorption isotherms for TPC removal by AJB were investigated using different initial concentrations at the adsorbent mass of 1.0 g at 293, 303, 313, and 323 K and for a period of 3 h. Later, the data obtained were fitted to the Langmuir and Freundlich isotherms.The Langmuir isotherm assumed that the monolayer adsorption of adsorbate onto a homogeneous adsorbent surface takes place with a single coating layer on this surface [74]. Moreover, there is no lateral interaction between the adsorbed molecules. The linear form of the Langmuir isotherm model can be expressed as [75, 76](4)Ceqe=1KLqm+Ceqm,where qe is the equilibrium adsorption capacity (mg/g), Ce is the equilibrium concentration of TPC (mg/L), qm is a maximum adsorption capacity (mg/g), and KL is the adsorption equilibrium constant (L/mg).The linear form of the Langmuir isotherm is shown in Figure9. The correlation coefficients, R2 >0.99 at all temperatures, indicate that the adsorption was a good fit for this model.Figure 9
Langmuir plot for the adsorption of TPC onto AJB. Adsorbent dosage = 1.0 g 10 mL of OMW, pH = 6, contact time = 3 h.The maximum adsorptionqm for TPC onto AJB equals to 8.81 mg/g. The adsorption process can be evaluated to see whether it is favorable by the use of a Langmuir dimensionless separation factor RL defined as [77](5)RL=11+KLCo,where Co (mg/L) is the initial TPC concentration in solution. If the value of RL is less than 1.0, the adsorption is considered to be favorable, but it is unfavorable if RL is greater than 1.0. The calculated RL values at different concentrations fall within the range of 0.098–0.122 (Table 2), thus indicating a favorable adsorption process.Table 2
Calculated values of separation factorRL for the adsorption of TPC onto AJB at 303 K.
Co1215.161340.641442.001563.43RL0.12240.11230.10520.0978The Freundlich isotherm is based on multilayer adsorption on heterogeneous surfaces [78]. The linear form of Freundlich can be represented as(6)logqe=logKF+1nlogCe,where KF and n are the Freundlich adsorption constants showing the adsorption capacity (mg/g) and intensity, respectively, which can be determined by the linear plot of log qe versus log Ce.The adsorption Freundlich isotherm obtained for TPC onto AJB is shown in Figure10, and the isotherm parameters for both models are presented in Table 3.Figure 10
Freundlich plot for the adsorption of TPC onto AJB. Adsorbent dosage = 1.0 g 10 mL of OMW, pH = 6, contact time = 3 h.Table 3
Langmuir and Freundlich isotherm parameters for the adsorption of TPC onto AJB.
T (K)Langmuir isothermFreundlich isothermqmKLR2nKFR22937.81680.00510.99814.84811.57740.97543038.02010.00590.99855.29581.87100.97533138.06400.00690.99885.96042.22220.98603238.81280.00910.99856.76482.89050.9576The obtained values ofn exhibited intense change at higher temperatures. All n values were greater than one, indicating favorable adsorption of TPC [79]. Also, the high correlation coefficient values (R2) of the Langmuir and Freundlich models indicate that the experimental data are well fitted by both models.Comparison of AJB with other adsorbents is necessary. Table4 provides the adsorption capacities for TPC on different types of adsorbents as reported in the literature. It is thought that those adsorbent’s properties have a significant effect on its efficiency, and experiment conditions may cause different results. Based on that, it seems that the AJB for TPC removal is a promising effective adsorbent. Results in Table 4 show that AJB has a high adsorption capacity compared to a wide range of adsorbents. On the other hand, the adsorption capacity of macroporous resins XAD-16 is higher than that of prepared AJB.Table 4
Comparison of adsorption capacities of different adsorbents for TPC reported in the literature.
AdsorbentAdsorption capacityqm (mg/g)ReferencePolydimethylsiloxane/oxMWCNTs4.39[80]Olive pomace11.40[81]HyAlFe-Mt10.2HyAlFe15.7[82]Silver and titanium oxides-doped activated carbon11.0168[83]Macroporous resins XAD1616[84]Nonionic styrene-divinylbenzene resin (XAD1)3.4Nonionic styrene-divinylbenzene resin (XAD2)7.6Strong basic anion exchange resin (IRA1)2.1[85]Strong basic anion exchange resin (IRA2)0.9XAD16HP2.30SP7002.59[86]AJB8.81Present studyThe activation by sodium ions has the potential to produce a high surface BET area for AJB (the surface area increases from 66.2 to 249.6 m2 g−1) characterized by high uptake capacity for organic compounds compared to other adsorbents. However, very few studies reportedly tested the adsorption of TPC onto the bentonite produced via this activation method. Among this method, the sodium ions activation method will gain more attention from researchers. This has been attributed to fact that such method is fast, ease of operational conditions, and characterized by cost savings over the conventional techniques [87].


## 3.3.3. Thermodynamic Studies

(1) Effect of Temperature on Adsorption of TPC. In order to determine the effect of temperature on the adsorption of TPC onto AJB, experiments were run with four different values: 293, 303, 313, and 323 K. From the curves of Figure 11, we can notice that the percentage of efficient removal of TPC increases with temperature, which indicates that the adsorption process is endothermic. This could be due to increasing the mobility of the TPC, thus gaining more kinetic energy to diffuse from the bulk phase to the solid phase, with an increase in solution temperature. Furthermore, there is an increase in the number of surface active sites for the adsorption with increasing temperature, as a result of the dissociation of some of the surface components onto AJB [88, 89]. On the other hand, KL values are directly proportional to the temperature, as shown in Figure 11. KL is the Langmuir equilibrium constant related to the affinity of binding sites and energy of sorption, equation (4). Again, TPC has a good affinity to the AJB surface and increases gradually with increasing temperature. Similarly, at constant initial TPC concentration (1340.64 mg/L) and fixed dosage of both adsorbents (1 g), as clearly observed in Figure 11, the percentage removal of TPC using PB adsorbent increases with increasing temperature.Figure 11
Effect of solution temperature on the percentage removal of TPC and the Langmuir equilibrium constantKL using AJB.The temperature has an obvious effect on sodium modification of the bentonite, and Na+ in the diffusion layer present has a trend of moving to bentonite surfaces. The migration of Na+ is increased, which increases the content of Na + on the bentonite surface. Therefore, the reaction velocity between Na+ and Ca2+ is growing [90].Comparing the results of percentage removal of TPC obtained by using AJB, with those in the previously reported study, AJB presents a good adsorption capacity rather than the acid-activated Jordanian bentonite [2]. Hence, AJB utilization for the removal of TPC is very promising.To evaluate the feasibility of the adsorption process, thermodynamic parameters, where ΔG° is the Gibbs free energy (kJ/mol), ΔH° is the standard enthalpy (J/mol), and ΔS° is the standard entropy (J/mol K), were calculated from the curve relating the distribution coefficient (K) as a function of temperature, using the following equations [91]:(7)ΔG°=−RTlnK,lnK=ΔS°R−ΔH°RT,where R is the gas constant (8.3145 J.mol−1.K−1), and T is the temperature in Kelvin. The values of ΔH° and ΔS° were determined from the slope and intercept values of the straight line of plotting ln K versus 1/T, respectively. According to the data presented in Table 5, the spontaneity of the adsorption process is established by a decrease in ΔG° values, in addition to spontaneity increases as the temperature of the solution increases, which means that, as the adsorption process becomes more favorable, the negative values of ΔG° indicate that the adsorption of the TPC is spontaneous and favorable [92]. The positive value of ΔH° shows that the adsorption process is endothermic in nature [93]. This is following increasing adsorption equilibrium with increasing temperature. The positive value of ΔS° reflects an increase in the randomness at the interface between AJB and the phenolic solution during the adsorption process. This suggests that some structural changes occur on the adsorbent, in addition to the adsorbate, due to the exchange of the phenolic compounds with more mobile ions present on the AJB, which would cause an increase in the entropy during the adsorption process [94].Table 5
Thermodynamic parameters of adsorption of TPC onto AJB.
T (K)ΔG°29326.2230313.90313−5.22323−28.21ΔH° = 559.47ΔS° = 1.81Moreover, the ΔH° value was 559.47 kJ/mol, thus indicating that the adsorption of TPC onto AJB involved chemical adsorption. Value of enthalpy within the range 2.1–20.9 kJ/mol indicates physical sorption, and value over 20.9 kJ/mol indicates chemisorption [95]. Therefore, the process is irreversible [96]. Thus, the adsorption mechanism involves valences forces and chemical bond [75, 97].According to the results obtained from this work, by using effectively-prepared AJB adsorbent, it is favorable in terms of both economic and environmental features, and it could improve adsorption for the removal of hazardous materials, such as phenolic compounds from industrial effluents such as OMW. Furthermore, this study is expected to be of great benefit because all these processes are interrelated; especially given that there are no previous studies on Jordanian bentonite that considered all these processes.


## 4. Conclusion

The management of produced OMW is a particularly unsolved problem, especially in Jordan, due to its high content of phenolic compounds, their physicochemical composition, and the implicit toxic merits. More effective, simple, available, low cost, and environmentally friendly methods are needed mainly in developing countries. The current study presents a successful method for the activation of Jordanian bentonite using sodium chloride. The AJB has been examined as an adsorbent for OMW treatment. The physicochemical analysis appears to show a good performance for AJB, which can help reduce environmental damage, prevent groundwater contamination, and provide an alternative approach for olive mills, in regards to safe utilization of OMW. Moreover, AJB shows great potential for the removal of phenolic compounds pollutants. Results show the percentage of removal for phenolic compounds can be considered as satisfactory. Improvement of the adsorption efficiency is achieved by optimized different parameters, which were as follows: the adsorbent dosage was 1.0 g, initial TPC concentration was 1215.16 mg/L, and the temperature of the solution was 323 K. On the other hand, its equilibrium adsorption was well fitted to the Langmuir and Freundlich models. Thermodynamics studies have confirmed that the adsorption process was spontaneous and endothermic in nature.Notably, the AJB shows a substantially higher adsorption capacity compared to PB. This study provides a feasible method for utilizing activated bentonite for application in wastewater treatment.



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