# Polypyrrole/Magnetic/Tea Waste Composites for PO43− Ions Removal: Adsorption-Desorption, Kinetics, and Thermodynamics Studies

Authors: Fatima Saleem; Haq Nawaz Bhatti; Amina Khan; Lamia Ben Farhat; Zainab Mufarreh Elqahtani; Norah Alwadai; Munawar Iqbal
Journal: Adsorption Science & Technology
 (2022)

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## Abstract

The polypyrrole (PPY/TW) and magnetic (MG/TW) composite with tea waste (TW) was prepared and used as an adsorbent for PO43− ions removal from aqueous media. The composite were characterized with SEM and FTIR techniques. Batch study was conducted to investigate the effect of different reaction parameters on the adsorption of PO43− ions. The native TW, PPY/TW, and MG/TW showed the PO43− ions removal of 7.2, 7.3, and 7.9 (mg/g), respectively, using 0.05 g adsorbent dose and 10 mg/L initial concentration of PO43− ions at pH of 6, 10, and 3, respectively, and equilibrium was reached in 90 min. Kinetics and isotherm models were employed on the PO43− ions adsorption data and PO43− ions adsorption followed the pseudo-second order kinetics, intraparticle diffusion, and Langmuir isotherm models. Thermodynamics analysis reveals an exothermic process and spontaneous adsorption of PO43− ions on the composites. Results revealed that the magnetic and polypyrrole composites with tea waste have auspicious potential as an adsorbent and this class of the composites can be utilized for the removal of PO43− ions from the effluents.


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## Body

## 1. Introduction

In view of alarming environmental pollution situation worldwide [1, 2], the phosphorus is a one of major issues, which is commonly present in water reservoirs and involves variety of complex processes mainly eutrophication and nutrient enrichment. It is widely used in industries and agricultural sectors [3]. Food industries produces beverages and products that are potent source of phosphorus; however, only 16% of it is absorbed by the human body from their diet. The fertilizers use in agricultural sector led to the increase load of phosphorus in water reservoirs by runoff through farmlands, agricultural soils, and animal manures. The quantity of phosphorus higher than 0.02 mg/L is harmful because higher amount causes production of algal blooms which results in eutrophication [3–5]. So, it is important to maintain the quantity of phosphorus below its threshold amount.Different approaches are in practice to remove the phosphorus along with other pollutants from wastes, and among these approaches, adsorption is suitable and highly efficient technique because of its simplicity in operational design and adsorbent recycling abilities [6–9]. Adsorption, in accordance with wastewater treatment, is accommodation of pollutant on the surface of a solid adsorbent. The pollutant that adheres to the adsorbent is called adsorbate, and so the solid material is referred as adsorbent. The rate of attachment of pollutants on the solid surface is affected or controlled by different factors such as pH, particle size of solid adsorbent, contact time, temperature, and concentration of particular pollutant. In accordance with several advantages, biomasses proved their remarkable abilities as adsorbents to remove phosphorous from water [10–13].These biosorbents can be algal, fungal, and bacterial biomasses and waste materials (industrial, agricultural, and domestic). The biomasses possess several functional groups, i.e., carboxyl and hydroxyl (anionic) amine groups (cationic), which help them to adsorb different anionic/cationic species through different interactions such as ion exchange, chelation, and electrostatic interactions. The waste materials from domestic, agricultural, and industrial units attracted much interest because of cost effectiveness and freely available, is a one good choice, and can be utilize them for the treatment of wastewater [14–16]. Tea waste is a renewable source, green, and sustainable adsorbent for phosphorus removal from the polluted water to manage the waste through waste. Tea waste have different biological molecules (lignin, cellulose, carbohydrates, phenolic, aromatic carboxylate, phenolic-hydroxyl, and phenolic-oxyls) which are responsible for the adsorption of adsorbate. However, limited binding sites on the surface is one drawback these materials and the efficiency of adsorbents can be increased by modifications in biomasses, which leads to the enhanced adsorption capacity of the native adsorbent [17]. Previous studies revealed that when the biomass is coupled with other functional moieties, the adsorption capacities were enhanced many folds. During modification, the physicochemical properties of the composite materials are enhanced, which can adsorb the pollutant more effectively versus native/unmodified adsorbents [18–23].The polypyrene and magnetic composites with tea waste biomass were prepared in this study, and same were employed for the removal of PO43− ions from aqueous media: The adsorption was studied as a function of various process variable along with desorption of PO43− ions from the adsorbent. The equilibrium, kinetics, and thermodynamics analysis were undertaken to analyze the nature and mechanism of PO43− ions adsorption on to the composites.


## 2. Material and Methods

### 2.1. Reagents and Chemicals

The formaldehyde, sodium phosphate dibasic heptahydrate, H2SO4, HCl, NaOH FeCl3.6H2O, sodium sulfide, ammonium molybdate, and ammonia solution were of Sigma-Aldrich. The PO43− ions stock solution was prepared from sodium phosphate dibasic heptahydrate. The 1000 mg/L of phosphate ions stock solution was prepared, and then, it was further diluted to different concentrations in distilled water.


### 2.2. Preparation of Adsorbents

Tea waste (TW) was collected from tea stall in university cafeteria, UAF, FSD, Pak. The collected TW was washed with distilled water. Then, it was treated with 5 mL formaldehyde (HCOH) and 100 mL sulfuric acid (H2SO4) for 48 h at 120 rpm, dried in an oven, ground, and sieved through 300 μm sieve (OCT-4527-01). Magnetic composite of TW (MG/TW) was prepared by the co-precipitation method. Initially, 5 g biomass and 22.5 g of FeCl3.6H2O were added in a beaker, and the beaker was kept at 60 °C with stirring on hot plate. Then, ammonia was added dropwise into the solution. Ammonia was added until the basic pH (9-11) was achieved. Black powder of magnetic composite was filtered and dried. For pyrrole composite of TW (PPY/TW), 3 g of TW was taken into a 500 mL beaker and was soaked into the pyrrole solution (0.2 M) overnight. The next day, the beaker was put onto the magnetic stirrer, and 100 mL of FeCl3.6H2O (0.3 M) was added into it dropwise. After stirring, the black precipitates appeared at the bottom of beaker was collected and dried in oven at 60 °C.


### 2.3. Characterization

FTIR analysis was done in the range of 650-4000 cm-1 for Native TW, PPY/TW, and MG/TW using FTIR Spectrophotometer to appraise the functional groups on the surface of the composites. The SEM analysis was employed to evaluate the morphology of the prepared biocomposites.


### 2.4. Adsorption Study

The adsorption was designed to study the effect of different operating parameters, i.e., initial concentration of PO43− ions, adsorbent dose, pH, contact time, and temperature. To investigate the effect of one parameter, all the other parameters were kept constant [24]. In this research work, the range of parameters were 0.05-0.3 g adsorbent dose, 3-10 pH, 3-15 mg/L initial concentration of PO43− ions, 5-120 min contact time, and 27-62 °C. The PO43− ions contaminated water (50 mL) was taken in a conical flask and was mixed and stirred at a constant speed of 120 rpm in the orbital shaker. The initial and equilibrium concentration of PO43− ions was determined by spectrophotometer (UV-2800, USA) at 715 nm. Equatiom (1) is used to determine the PO43− ions removal efficiency:
(1)qe=Ci−Ce×VW.whereCi,qe,Ce,W, and V are the initial concentration, adsorption efficiency, equilibrium concentration, composite dose, and solution volume, respectively.


### 2.5. PO43− Ions Analysis

For the detection of residual concentration of PO43− ions, 0.5 mL of ammonium molybdate (NH4)2MoO4) and 3 mL of 0.25 N sulfuric acid (H2SO4) were added in the test tube. Then, 3 mL sample/filtrate (PO43− ions solution) was transferred to a test tube, and 1 mL of 0.002 M of sodium sulfide (Na2S.9H2O) was added, and let it rest for 20 min to develop color. The initial and equilibrium concentrations of PO43− ions were determined by Spectrophotometer (UV-2800, USA) at 715 nm.


### 2.6. Desorption Study

Desorption experiments were designed to investigate the stability and reusability of adsorbents. The capability of any adsorbent to desorb suggests the restoration of adsorbent. Reagents were used like NaOH and HCl for desorption of the PO43− ions from three adsorbents. Desorption efficiencies are calculated using the following equation:
(2)Desorption%=Cd−Vdqe×w,where Cd,Vd,w, and qe are the concentration of PO43− ions, volume of the solution, and mass of the composite and is the amount of PO43− ions adsorbed, respectively.


## 2.1. Reagents and Chemicals

The formaldehyde, sodium phosphate dibasic heptahydrate, H2SO4, HCl, NaOH FeCl3.6H2O, sodium sulfide, ammonium molybdate, and ammonia solution were of Sigma-Aldrich. The PO43− ions stock solution was prepared from sodium phosphate dibasic heptahydrate. The 1000 mg/L of phosphate ions stock solution was prepared, and then, it was further diluted to different concentrations in distilled water.


## 2.2. Preparation of Adsorbents

Tea waste (TW) was collected from tea stall in university cafeteria, UAF, FSD, Pak. The collected TW was washed with distilled water. Then, it was treated with 5 mL formaldehyde (HCOH) and 100 mL sulfuric acid (H2SO4) for 48 h at 120 rpm, dried in an oven, ground, and sieved through 300 μm sieve (OCT-4527-01). Magnetic composite of TW (MG/TW) was prepared by the co-precipitation method. Initially, 5 g biomass and 22.5 g of FeCl3.6H2O were added in a beaker, and the beaker was kept at 60 °C with stirring on hot plate. Then, ammonia was added dropwise into the solution. Ammonia was added until the basic pH (9-11) was achieved. Black powder of magnetic composite was filtered and dried. For pyrrole composite of TW (PPY/TW), 3 g of TW was taken into a 500 mL beaker and was soaked into the pyrrole solution (0.2 M) overnight. The next day, the beaker was put onto the magnetic stirrer, and 100 mL of FeCl3.6H2O (0.3 M) was added into it dropwise. After stirring, the black precipitates appeared at the bottom of beaker was collected and dried in oven at 60 °C.


## 2.3. Characterization

FTIR analysis was done in the range of 650-4000 cm-1 for Native TW, PPY/TW, and MG/TW using FTIR Spectrophotometer to appraise the functional groups on the surface of the composites. The SEM analysis was employed to evaluate the morphology of the prepared biocomposites.


## 2.4. Adsorption Study

The adsorption was designed to study the effect of different operating parameters, i.e., initial concentration of PO43− ions, adsorbent dose, pH, contact time, and temperature. To investigate the effect of one parameter, all the other parameters were kept constant [24]. In this research work, the range of parameters were 0.05-0.3 g adsorbent dose, 3-10 pH, 3-15 mg/L initial concentration of PO43− ions, 5-120 min contact time, and 27-62 °C. The PO43− ions contaminated water (50 mL) was taken in a conical flask and was mixed and stirred at a constant speed of 120 rpm in the orbital shaker. The initial and equilibrium concentration of PO43− ions was determined by spectrophotometer (UV-2800, USA) at 715 nm. Equatiom (1) is used to determine the PO43− ions removal efficiency:
(1)qe=Ci−Ce×VW.whereCi,qe,Ce,W, and V are the initial concentration, adsorption efficiency, equilibrium concentration, composite dose, and solution volume, respectively.


## 2.5. PO43− Ions Analysis

For the detection of residual concentration of PO43− ions, 0.5 mL of ammonium molybdate (NH4)2MoO4) and 3 mL of 0.25 N sulfuric acid (H2SO4) were added in the test tube. Then, 3 mL sample/filtrate (PO43− ions solution) was transferred to a test tube, and 1 mL of 0.002 M of sodium sulfide (Na2S.9H2O) was added, and let it rest for 20 min to develop color. The initial and equilibrium concentrations of PO43− ions were determined by Spectrophotometer (UV-2800, USA) at 715 nm.


## 2.6. Desorption Study

Desorption experiments were designed to investigate the stability and reusability of adsorbents. The capability of any adsorbent to desorb suggests the restoration of adsorbent. Reagents were used like NaOH and HCl for desorption of the PO43− ions from three adsorbents. Desorption efficiencies are calculated using the following equation:
(2)Desorption%=Cd−Vdqe×w,where Cd,Vd,w, and qe are the concentration of PO43− ions, volume of the solution, and mass of the composite and is the amount of PO43− ions adsorbed, respectively.


## 3. Results and Discussion

### 3.1. Characteristics of the Adsorbents

FTIR spectrum of native TW (Figure1) showed a broad peak at 3283 cm-1 which is due to the presence of bonded hydroxyl groups at the surface of the native TW. Peaks in the region of 2900-2700 cm-1 showed the stretching of C-H bond of alkane, which are part of the complex structure of biomass. The band at 1638 cm-1 is due to carboxyl group stretching (-COOH). The peaks below 1190 cm-1 to 1130 cm-1 showed secondary amine and from 1090-1020 cm-1 peaks showed the stretching of primary amine. The FTIR spectrum of PPY/TW (Figure 1) showed the presence of several peaks in 950-1225 cm-1 which confirmed in-plane bending of the aromatic C-H bond. As the structure of polypyrrole has ring structure, so these peaks are due to C-H bond in the ring of polypyrrole. The peaks in 1650-2000 cm-1 range appeared due to the aromatic structure. A peak in 2100-2140 cm-1 confirmed the presence of C ≡ C terminal alkyne monosubstituted. The broad peak at 3377 cm-1 in the FTIR spectrum of MG/TW (Figure 1) confirmed the presence of hydroxy group, H-bonded OH stretch. Alkenyl C=C stretch was observed in the peak range of 1620-1680 cm-1. The peak at 1351 cm-1 showed that the carboxylate functional group is present in the structure of magnetic composites. The surface morphology of the native TW, PPY/TW, and MG/TW is evident from the SEM images (Figure 2). The more surface area the more will be the adsorption efficiency of the adsorbent. The SEM of native TW showed a highly porous and rough surface. The pores will contribute to the surface area of the adsorbent which can favor effective interaction of PO43− ions with the active sites. SEM analysis of PPY/TW showed that the highly rough surface and SEM image of MG/TW showed that particles are in dispersed form and are of different shapes. The MG/TW showed the mean particle size of 21.872 μm.Figure 1
FTIR analysis (a) TW, (b) PPY/TW adsorbent, and (c) MG/TW adsorbent.
(a)(b)(c)Figure 2
Surface analysis, (a) TW, (b) PPY/TW, and (c) MG/TW adsorbents.
(a)(b)(c)


### 3.2. pHpzc (Point Zero Charge) Analysis

Point zero charge depict the charge on the surface of the biomass. If the pH of the point of zero charge is lower than optimum pH, then the biomass will possess a negative charge, and if pH of point zero charge is higher than the optimum pH, then the biomass have positive charge. As evident from the data (Figure3), the pH of point zero charge (pHpzc 6.5) is higher than the optimum pH (6) of TW; hence, it was found that there is positive charge on biomass surface which is in accordance with the adsorption of PO43− ions. As phosphates being negatively charged will adsorb favorably on positively charged biomass.Figure 3
Point of zero charge analysis.The biosorption capacities of 50, 64.6, and 52.5 (%) were achieved for TW, MG/TW, and PPY/TW composite at pH 6, 3, and 10, respectively (Figure4). For native tea waste, the adsorption capacity was achieved at pH 6 because in pH range 5-7, phosphorus exist in the form of H2PO4- and HPO4-2 and the surface of the adsorbent is positively charged due to protonation [25]. The PO43− ions adsorbed on the surface through ion exchange and electrostatic attraction. Pyrrole composites showed 5.25 mg/g removal capacity at pH 10. This may be due to the presence of another component on the surface of adsorbent where highly alkaline pH increases adsorption capacity for PO43− ions on the adsorbent surface because of metal ions present on biomass surface [26]. Hence, higher pH than 10 did not favor the adsorption process because addition of alkali reduces the activity of the adsorbent. The alkaline media may block the active sites and preclude them from the efficient adsorption [27]. The basicity results in repulsion of negatively charged ions, i.e., OH- and phosphate ion. At pH 3, the surface of magnetic composites is positively charged due to protonation and adsorbed phosphate ions as H3POH4.Figure 4
. pH effect on adsorption of PO43− ions.


### 3.3. Composite Dose Effect on PO43− Ions Adsorption

The amount of dose determines the adsorption capacity of biomass for adsorbate molecules. Dose effect was studied for tea waste and its composites in the range of 0.05, 0.1, 0.15, 0.2, and 0.3 g by keeping all the other parameters constant at optimum pH. The optimum dose for the adsorption of PO43− ions was determined 0.05 g/50 mL solution for native TW, PPY/TW, and MG/TW composites. Native TW, PPY/TW, and MG/TW showed percentage removal of 36, 43, and 44 (%), respectively. The dosage amount higher than 0.05 g depicted a decrease in adsorption capacity of the composites, which is due to the agglomeration of adsorbent which cover the available active sites. At higher dose, equilibrium is established, and all the active sites became saturated, and after getting this point, the PO43− ions started to release from the adsorbent back to the solution which decreases the removal efficiency (Figure 5).Figure 5
Effect of adsorbent dose on PO43− ions adsorption.


### 3.4. Contact Time Effect on PO43− Ions Adsorption

It was evident that contact time of 90 min showed the removal effectiveness of 43.79, 52.54, and 64.63 (%) for native TW, PPY/TW, and MG/TW, respectively (Figure6). After 90 min, no further change in efficiency was noted. At the start, the adsorption efficiency was increased with contact time because the active sites on adsorbent were available for PO43− ions and adsorption rate was fast. At equilibrium, all the active sites became saturated with adsorbate, and no further adsorption occurred because of the unavailability of active sites [28].Figure 6
Effect of contact time on PO43− ions adsorption.


### 3.5. Initial Concentration Effect in PO43− Ions Adsorption

To optimize the sorbate initial concentration, experiments were also conducted an optimized pH, contact time, and composite dose. Initial concentration was varied in 3-15 mg/L with 0.05 g dose of adsorbent for 90 min of contact time. The response showed a maximum removal efficiency of 43.79, 52.54, and 64.63 (%) for native TW, PPY/TW, and MG/TW, respectively, at initial concentration of 10 mg/L (Figure7). It is evident from the data that with increase in initial concentration of PO43− ions and removal efficiency did not increase due to saturation of active sites on adsorbents and unavailability of favorable sites for remaining PO43− ions at higher concentration [28].Figure 7
Effect of initial conc. of phosphorus on PO43− ions adsorption.


### 3.6. Temperature Effect on PO43− Ions Adsorption

Figure8 showed the adsorption capacity of native TW, PPY/TW, and MG/TW composites for the adsorption of PO43− ions as a function of temperatures (300.15-335.15 K) at optimized conditions. The decrease in removal capacity was observed because of the instability of adsorbents at higher temperatures, and this can deteriorate the structure of the adsorbents, which is in line with previous reports [29]; a decrease in adsorption capacity was observed at higher temperature, and the same was observed in the current study; the adsorption of PO43− ions was declined at higher temperature. The effect of adsorption temperature is different in the case of different adsorbents. The zirconium-modified zeolite amended with sediment revealed an increase in adsorption of phosphate at higher temperatures [30].Figure 8
Effect of temperature on PO43− ions adsorption.


### 3.7. Kinetics Studies

The adsorption kinetics enables to analyze the rate of adsorption process and mechanism of mass transfer from liquid phase to solid adsorbent. Different kinetic models have been utilized to appraise the PO43− ions adsorption rate. In adsorption of PO43− ions, three models were used including pseudo-first-order (PFO), pseudo-second-order (PSO) and intraparticle diffusion models [31–33]. The PFO explains the liquid-solid adsorption system and describes the relation between the quantity of sorbate adsorbed and time required for the same [34]. Equation (4) shows the straight line equation for the PSO.
(3)logqe−qm=logqe−K1.t2.303,Wherelogqe−qm is at y-axis, logqe is at x-axis, K1/2.303 is slope (K1=rateconstant (L/min)), and logqe is intercept. qe is the adsorption capacity at a specific time, and qm is the maximum adsorption capacity. The PSO describes the adsorption of adsorbate on the surface of sorbent as function of the square of active sites of the adsorbent [34]. Equation (5) shows the relation of PSO.
(4)tqt=1K2qe2+tqe,Where1/K2qe2 is intercept and 1/qe is the slope. K2 (g/mg/min) represents the second-order kinetic rate constant, qe (mg/g) is the maximum adsorption capacity at timet, and qm (mg/g) is the maximum adsorption capacity at equilibrium. For the value of coefficient of determination, R2 for the PSO obtained was 0.9916, 0.9212, and 0.9916 for native TW, PPY/TW, and MG/TW. Hence, the PO43− ions adsorption followed the PSO.Generally, there are different steps of the liquid-solid adsorption process. Mass transfer occurs in 4 steps: formation of layer of sorbate on the solid (external diffusion), diffusion of sorbate molecules in the pores of adsorbent (internal diffusion), transfer of sorbate to the active sites in the adsorbent, and development of interactions (adsorption, precipitation, and complexation) between sorbate molecules and macropores and micropores of the adsorbent [35]. Mechanism of adsorption can involve one step or multiple steps. As batch study involving high stirring during adsorption can undergo diffusion of film and intraparticle diffusion or sometimes both [34]. Intraparticle diffusion describes the relationship between uptake efficiency and t1/2. The intraparticle diffusion relation is depicted in the following equation:
(5)qt=Kpi.t1/2+Ci,where Kpi (mg/g min1/2) is the slope and known as the rate constant, Ci is the intercept and qt is the adsorption capacity at given time. The graph between t1/2 and adsorption capacity shows whether the adsorption is intraparticle only or both. The R2 values are above 0.90 for three adsorbents; it showed that the intraparticle diffusion of PO43− ions adsorption occur for native TW, PPY/TW, and MG/TW composites (Table 1).Table 1
Kinetic parameters for the adsorption of PO43− ions on the native and composite adsorbents.
Kinetic modelsParametersNative TWPPY/TWMG/TWPseudo-first-orderR20.57360.59880.6757K (min-1)0.020.0310.030Qe (cal.) (mg/g)1.82.703.33Qe (Exp.) (mg/g)4.35.26.4Pseudo-second-orderR20.99160.92120.9916K (min-1)0.0340.000430.02Qe (cal.) (mg/g)4.5377.56.75Qe (Exp.) (mg/g)4.35.26.4Intraparticle diffusion modelR20.95530.98010.976Kpi0.1440.1750.22Ci2.803.3553.988


### 3.8. Equilibrium Studies

Isotherms are basically related to the homogeneity and heterogeneity of adsorbents and enables to finds the possible interactions between sorbate molecules and sorbent. Isotherm plots relate adsorption capacity,qe, to the concentration of adsorbate at equilibrium [36]. These isotherms provide an overall understanding of mechanism pathways and design of adsorption systems. The following isotherms were applied to the adsorption data of PO43− ions on to the composites.


#### 3.8.1. Langmuir Isotherm

Langmuir isotherm is based on the postulation that the adsorption of adsorbate molecules on the surface of the sorbent forms monolayer which means that one molecule attaches at the particular active site. Energy of the adsorption process remains constant, and none of the molecules moves across the surface of the sorbent, and it represents that adsorption is homogenous [36]. Langmuir model is represented in the following equation:
(6)Ceqe=1qmb+Ceqm,where Ce/qe is y-axis, Ce (mg/L) is the concentration of adsorbate at time t, 1/qm is slope, qm is maximum adsorption capacity at equilibrium state, 1/qmb is intercept, and b is the Langmuir constant also represented as KL. Essential features of adsorption can be explained by a dimensionless constant of Langmuir constant called separation factor RL (Equation (8)):
(7)RL=11+bCo,where Co is initial concentration of adsorbate (mg/L). RL value indicates the adsorption is favorable or not. If its value range between 0 and 1, it means adsorption if favorable, and if greater than 1, then adsorption in unfavorable and adsorption is irreversible when the value is 0. The values of R2 indicate that adsorption data of PO43− ions followed Langmuir model and the adsorption of PO43− ions on the native TW, PPY/TW, and MG/TW is monolayer adsorption.


#### 3.8.2. Freundlich Isotherm

The adsorption that occurs at the heterogeneous surfaces is explained by Freundlich isotherm [37]. The linear form of Freundlich isotherm is depicted in the following equation:
(8)logqe=logKF+1nlogCe,where KF, 1/n, Ce, and qe are the adsorption capacity, intensity, PO43− ions concentration at equilibrium, and adsorption capacity at equilibrium, respectively. Adsorption intensity indicates the distribution of the energy and the heterogeneity of the adsorption. The following are the results obtained for adsorption of PO43− ions on adsorbents by applying Freundlich model. Adsorption data of PO43− ions of native TW followed Freundlich isotherm model (Table 2).Table 2
Equilibrium parameters for the adsorption of PO43− ions on the native and composite adsorbents.
ModelsParameterNative TWPPY/TWMG/TWLangmuir modelB0.2970.14790.137RL0.3130.4030.42R20.99810.90230.962Qm (cal.) (mg/g)4.556.767.29Qm (exp.) (mg/g)4.375.256.46FreundlichN5.131.922.40Kf2.982.013.20R20.91120.72640.7065TemkinA106.703.327.77B0.65531.70721.7034R20.91780.77470.7606Huskin-JuraA5.621.985.57B1.1730.890.861R20.70820.6310.4794Dubinin-Radushkevichβ (mol2KJ-2)8 × 10-86 × 10-73 × 10-7R20.95530.98010.9764Qm (cal.) (mg/g)4.295.826.91qm (Exp.) (mg/g)4.375.256.46E (KJ/mole)0.6250.8331.67


#### 3.8.3. Temkin Isotherm

The Linear form of Temkin isotherm model is shown in the following equation [36]:
(9)Qe=BlnA+BlnCe,where B=RT/b and A=equilibriumbindingconstant. In the straight-line equation, B is the slope, and BlnA is the intercept. The Temkin model explains the distribution of binding energies over active sites and revealed that heat of adsorption decreases with the increase of coverage of sites. We can calculate the values of the slope, intercept, and R2 by plotting graph between lnCe and qe. Value of R2 showed that adsorption of PO43− ions on native TW followed Temkin isotherm model, while PPY/TW and MG/TW did not follow the Temkin model.


#### 3.8.4. Harkin-Jura Isotherm

Multilayer adsorption on the adsorbent and heterogeneous pore distribution is explained by Harkin-Jura isotherm [37]. Linear form of this model is shown in the following equation:
(10)1qe2=BA−1AlogCe,where 1/qe2 (y-axis), logCe (x-axis), 1/A is the slope, and B/A is the intercept. The values of A and B can be calculated by plotting a graph between logCe and 1/qe2. The following is the graphical representation of the model for PO43− ions the adsorption on the TW, PPY/TW, and MG/TW composites. None of the adsorption data fitted well to the Harkin-Jura isotherm model.


#### 3.8.5. Dubinin-Radushkevich Isotherm

This model gives us an estimation about the free energy of porosity of adsorbent and heterogeneous surface of adsorbent [38]. Dubinin-Radushkevich isotherm model is represented in the following equation:
(11)lnqe=lnqm−βε2,where βis the slope and lnqm is intercept. ε can be calculated using Equation (13), and the values of E can be estimated using the following equation:
(12)ε=RTln1+1Ce,(13)E=12β1/2,where ϵ is Polanyi potential, β is Dubinin-Radushkevich constant, R is gas constant (8.31 Jmol−1 k−1), T is absolute temperature, and E is mean adsorption energy. Values of R2 are noted 0.9553, 0.9801, and 0.9764 for Native TW, PPY/TW, and MG/TW, respectively. All the adsorbents followed the Dubinin-Radushkevich isotherm which showed that this process is temperature dependent. The values of all equilibrium parameters are given in Table 2.


### 3.9. Thermodynamics Studies

Thermodynamics of adsorption gives us the estimation that how change in enthalpy (ΔH) and entropy (ΔS) determines the free energy content (ΔG) of the adsorption process at constant temperature and pressure [39]. According to thermodynamics, the PO43− ions will spontaneously adsorb on the surface of adsorbent if the change in free energy will be negative. Equations (14) and (15) utilize for the thermodynamics analysis:
(14)∆G=∆H−T∆S,(15)lnKd=−∆HRT+∆SR,where Kd is equilibrium rate constant, ΔH is change in enthalpy change, ΔS is entropy change, R is general gas constant (8.314 J/mole.K), and T is absolute temperature. The values of ΔH and ΔS were calculated from the plot of lnKd versus 1/T. Negative values of ∆G for native TW, PPY/TW, and MG/TW indicated that the adsorption of PO43− ions is exothermic and spontaneous. Change in entropy ΔS was positive for native TW and MG/TW. Table 3 showed the values of the thermodynamic parameters for PO43− ions adsorption on to native TW, PPY/TW, and MG/TW.Table 3
Thermodynamic parameters for the adsorption of PO43− ions on the native and composite adsorbents.
Temperature (K)300.15307.15314.15321.15328.15335.15Native TWΔG (kJ/mole)0.7− 1.06− 1.21− 2.10− 2.690.98ΔH (kJ/mole)6.66ΔS (J/mole.K)0.024PPY/TWΔG (kJ/mole)− 1.50− 1.56− 2.48− 2.16− 3.66− 2.01ΔH (kJ/mole)− 31.24ΔS (J/mole.K)− 0.096MG/TWΔG (kJ/mole)− 1.50− 1.57− 2.48− 2.16− 3.66− 2.01ΔH (kJ/mole)9.27ΔS (J/mole.K)0.036


### 3.10. Effect of Interfering Ions on Adsorption

Wastewater streams contain with different type of salts and their presence effect the adsorption capabilities of the adsorbents. Adsorption of PO43− ions was affected in the presence of the monovalent, divalent, and trivalent salts. The NaCl, CaCl2, and AlCl3.6H2O salts (0.025 g) were taken in 50 mL of 10 mg/L phosphate solution with 0.05 g adsorbent at optimized pH values and were run in an orbital shaker for 90 min at 120 rpm. The presence of salts decreases the adsorption capacity. The effect of AlCl3.6H2O on MG/TW was negligible, but NaCl and CaCl2 have significantly reduced the removal efficiency. It was because of blockage of active sites with the salt ions rather than the PO43− ions. It was clear from the data analysis that monovalent ions decreased the sorption capacity more than divalent and trivalent ions for all the adsorbents. The order of reducing removal efficiency can be represented as Na+ > Ca2+ > Al3+. The removal efficiency was reduced from 43 to 39 (%), 52 to 40 (%), and 64 to 56 (%) in the presence of monovalent ions for native TW, PPY/TW, and MG/TW, respectively (Figure 9).Figure 9
(a) Salt effect on the adsorption of PO43− ions and (b) Desorption study of PO43− ions.
(a)(b)


### 3.11. Desorption Study

Desorption experiments were designed to investigate the stability and reusability of adsorbents. The capability of any adsorbent to desorb suggests the recycling of the adsorbent, which made the adsorbent cost-effective. The desorption study of PO43− ions from native TW, MG/TW, and PPY/TW was studied in a 0.5-0.25 g range using reagent molarity (0.1-1.0 N). Native TW and MG/TW showed maximum adsorption with 0.1 N NaOH and minimum at 1 N NaOH solution, while PPY/TW showed maximum desorption in 0.1 N HCl and minimum with 1 N HCl solution (Figure 9). Percentage desorption obtained was 72.66, 61.03, and 94.43 (%) for native TW, MG/TW, and PPY/TW composites, respectively, using NaOH and HCl (0.1 N) solution. These findings revealed that the PPY/TW and MG/TW has promising efficiency for the adsorption of PO43− ions and their subsequent recovery. Hence, these composite can be employed for the adsorption of PO43− ions from the effluents since adsorption has various advantages versus other conventional techniques [40]. Previous findings also revealed that the composites offer higher adsorption efficacies versus their native counterpart for the inorganic ions removal from the effluents (Table 4), which could be employed for the removal of inorganic ions from the wastewater to avoid their negative impact.Table 4
Adsorption efficiency comparison of composites for different inorganic ions.
S. noAdsorbentAdsorbateRemoval efficiencyReferences1Graphene oxide and Fe3O4 compositeNO₃¯ and PO43¯ ions89 and 93 (%)[5]2Zeolite/geopolymer compositePO43− and NH4+ ions206 and 140 (mg/g)[41]3Mango stone biocompositePO43− ions95 mg/g[3]4g-C3N4 and acid-activated montmorillonite compositePO43− and Pb(II) ions5.06 and 182.7 (mg/g)[42]


## 3.1. Characteristics of the Adsorbents

FTIR spectrum of native TW (Figure1) showed a broad peak at 3283 cm-1 which is due to the presence of bonded hydroxyl groups at the surface of the native TW. Peaks in the region of 2900-2700 cm-1 showed the stretching of C-H bond of alkane, which are part of the complex structure of biomass. The band at 1638 cm-1 is due to carboxyl group stretching (-COOH). The peaks below 1190 cm-1 to 1130 cm-1 showed secondary amine and from 1090-1020 cm-1 peaks showed the stretching of primary amine. The FTIR spectrum of PPY/TW (Figure 1) showed the presence of several peaks in 950-1225 cm-1 which confirmed in-plane bending of the aromatic C-H bond. As the structure of polypyrrole has ring structure, so these peaks are due to C-H bond in the ring of polypyrrole. The peaks in 1650-2000 cm-1 range appeared due to the aromatic structure. A peak in 2100-2140 cm-1 confirmed the presence of C ≡ C terminal alkyne monosubstituted. The broad peak at 3377 cm-1 in the FTIR spectrum of MG/TW (Figure 1) confirmed the presence of hydroxy group, H-bonded OH stretch. Alkenyl C=C stretch was observed in the peak range of 1620-1680 cm-1. The peak at 1351 cm-1 showed that the carboxylate functional group is present in the structure of magnetic composites. The surface morphology of the native TW, PPY/TW, and MG/TW is evident from the SEM images (Figure 2). The more surface area the more will be the adsorption efficiency of the adsorbent. The SEM of native TW showed a highly porous and rough surface. The pores will contribute to the surface area of the adsorbent which can favor effective interaction of PO43− ions with the active sites. SEM analysis of PPY/TW showed that the highly rough surface and SEM image of MG/TW showed that particles are in dispersed form and are of different shapes. The MG/TW showed the mean particle size of 21.872 μm.Figure 1
FTIR analysis (a) TW, (b) PPY/TW adsorbent, and (c) MG/TW adsorbent.
(a)(b)(c)Figure 2
Surface analysis, (a) TW, (b) PPY/TW, and (c) MG/TW adsorbents.
(a)(b)(c)


## 3.2. pHpzc (Point Zero Charge) Analysis

Point zero charge depict the charge on the surface of the biomass. If the pH of the point of zero charge is lower than optimum pH, then the biomass will possess a negative charge, and if pH of point zero charge is higher than the optimum pH, then the biomass have positive charge. As evident from the data (Figure3), the pH of point zero charge (pHpzc 6.5) is higher than the optimum pH (6) of TW; hence, it was found that there is positive charge on biomass surface which is in accordance with the adsorption of PO43− ions. As phosphates being negatively charged will adsorb favorably on positively charged biomass.Figure 3
Point of zero charge analysis.The biosorption capacities of 50, 64.6, and 52.5 (%) were achieved for TW, MG/TW, and PPY/TW composite at pH 6, 3, and 10, respectively (Figure4). For native tea waste, the adsorption capacity was achieved at pH 6 because in pH range 5-7, phosphorus exist in the form of H2PO4- and HPO4-2 and the surface of the adsorbent is positively charged due to protonation [25]. The PO43− ions adsorbed on the surface through ion exchange and electrostatic attraction. Pyrrole composites showed 5.25 mg/g removal capacity at pH 10. This may be due to the presence of another component on the surface of adsorbent where highly alkaline pH increases adsorption capacity for PO43− ions on the adsorbent surface because of metal ions present on biomass surface [26]. Hence, higher pH than 10 did not favor the adsorption process because addition of alkali reduces the activity of the adsorbent. The alkaline media may block the active sites and preclude them from the efficient adsorption [27]. The basicity results in repulsion of negatively charged ions, i.e., OH- and phosphate ion. At pH 3, the surface of magnetic composites is positively charged due to protonation and adsorbed phosphate ions as H3POH4.Figure 4
. pH effect on adsorption of PO43− ions.


## 3.3. Composite Dose Effect on PO43− Ions Adsorption

The amount of dose determines the adsorption capacity of biomass for adsorbate molecules. Dose effect was studied for tea waste and its composites in the range of 0.05, 0.1, 0.15, 0.2, and 0.3 g by keeping all the other parameters constant at optimum pH. The optimum dose for the adsorption of PO43− ions was determined 0.05 g/50 mL solution for native TW, PPY/TW, and MG/TW composites. Native TW, PPY/TW, and MG/TW showed percentage removal of 36, 43, and 44 (%), respectively. The dosage amount higher than 0.05 g depicted a decrease in adsorption capacity of the composites, which is due to the agglomeration of adsorbent which cover the available active sites. At higher dose, equilibrium is established, and all the active sites became saturated, and after getting this point, the PO43− ions started to release from the adsorbent back to the solution which decreases the removal efficiency (Figure 5).Figure 5
Effect of adsorbent dose on PO43− ions adsorption.


## 3.4. Contact Time Effect on PO43− Ions Adsorption

It was evident that contact time of 90 min showed the removal effectiveness of 43.79, 52.54, and 64.63 (%) for native TW, PPY/TW, and MG/TW, respectively (Figure6). After 90 min, no further change in efficiency was noted. At the start, the adsorption efficiency was increased with contact time because the active sites on adsorbent were available for PO43− ions and adsorption rate was fast. At equilibrium, all the active sites became saturated with adsorbate, and no further adsorption occurred because of the unavailability of active sites [28].Figure 6
Effect of contact time on PO43− ions adsorption.


## 3.5. Initial Concentration Effect in PO43− Ions Adsorption

To optimize the sorbate initial concentration, experiments were also conducted an optimized pH, contact time, and composite dose. Initial concentration was varied in 3-15 mg/L with 0.05 g dose of adsorbent for 90 min of contact time. The response showed a maximum removal efficiency of 43.79, 52.54, and 64.63 (%) for native TW, PPY/TW, and MG/TW, respectively, at initial concentration of 10 mg/L (Figure7). It is evident from the data that with increase in initial concentration of PO43− ions and removal efficiency did not increase due to saturation of active sites on adsorbents and unavailability of favorable sites for remaining PO43− ions at higher concentration [28].Figure 7
Effect of initial conc. of phosphorus on PO43− ions adsorption.


## 3.6. Temperature Effect on PO43− Ions Adsorption

Figure8 showed the adsorption capacity of native TW, PPY/TW, and MG/TW composites for the adsorption of PO43− ions as a function of temperatures (300.15-335.15 K) at optimized conditions. The decrease in removal capacity was observed because of the instability of adsorbents at higher temperatures, and this can deteriorate the structure of the adsorbents, which is in line with previous reports [29]; a decrease in adsorption capacity was observed at higher temperature, and the same was observed in the current study; the adsorption of PO43− ions was declined at higher temperature. The effect of adsorption temperature is different in the case of different adsorbents. The zirconium-modified zeolite amended with sediment revealed an increase in adsorption of phosphate at higher temperatures [30].Figure 8
Effect of temperature on PO43− ions adsorption.


## 3.7. Kinetics Studies

The adsorption kinetics enables to analyze the rate of adsorption process and mechanism of mass transfer from liquid phase to solid adsorbent. Different kinetic models have been utilized to appraise the PO43− ions adsorption rate. In adsorption of PO43− ions, three models were used including pseudo-first-order (PFO), pseudo-second-order (PSO) and intraparticle diffusion models [31–33]. The PFO explains the liquid-solid adsorption system and describes the relation between the quantity of sorbate adsorbed and time required for the same [34]. Equation (4) shows the straight line equation for the PSO.
(3)logqe−qm=logqe−K1.t2.303,Wherelogqe−qm is at y-axis, logqe is at x-axis, K1/2.303 is slope (K1=rateconstant (L/min)), and logqe is intercept. qe is the adsorption capacity at a specific time, and qm is the maximum adsorption capacity. The PSO describes the adsorption of adsorbate on the surface of sorbent as function of the square of active sites of the adsorbent [34]. Equation (5) shows the relation of PSO.
(4)tqt=1K2qe2+tqe,Where1/K2qe2 is intercept and 1/qe is the slope. K2 (g/mg/min) represents the second-order kinetic rate constant, qe (mg/g) is the maximum adsorption capacity at timet, and qm (mg/g) is the maximum adsorption capacity at equilibrium. For the value of coefficient of determination, R2 for the PSO obtained was 0.9916, 0.9212, and 0.9916 for native TW, PPY/TW, and MG/TW. Hence, the PO43− ions adsorption followed the PSO.Generally, there are different steps of the liquid-solid adsorption process. Mass transfer occurs in 4 steps: formation of layer of sorbate on the solid (external diffusion), diffusion of sorbate molecules in the pores of adsorbent (internal diffusion), transfer of sorbate to the active sites in the adsorbent, and development of interactions (adsorption, precipitation, and complexation) between sorbate molecules and macropores and micropores of the adsorbent [35]. Mechanism of adsorption can involve one step or multiple steps. As batch study involving high stirring during adsorption can undergo diffusion of film and intraparticle diffusion or sometimes both [34]. Intraparticle diffusion describes the relationship between uptake efficiency and t1/2. The intraparticle diffusion relation is depicted in the following equation:
(5)qt=Kpi.t1/2+Ci,where Kpi (mg/g min1/2) is the slope and known as the rate constant, Ci is the intercept and qt is the adsorption capacity at given time. The graph between t1/2 and adsorption capacity shows whether the adsorption is intraparticle only or both. The R2 values are above 0.90 for three adsorbents; it showed that the intraparticle diffusion of PO43− ions adsorption occur for native TW, PPY/TW, and MG/TW composites (Table 1).Table 1
Kinetic parameters for the adsorption of PO43− ions on the native and composite adsorbents.
Kinetic modelsParametersNative TWPPY/TWMG/TWPseudo-first-orderR20.57360.59880.6757K (min-1)0.020.0310.030Qe (cal.) (mg/g)1.82.703.33Qe (Exp.) (mg/g)4.35.26.4Pseudo-second-orderR20.99160.92120.9916K (min-1)0.0340.000430.02Qe (cal.) (mg/g)4.5377.56.75Qe (Exp.) (mg/g)4.35.26.4Intraparticle diffusion modelR20.95530.98010.976Kpi0.1440.1750.22Ci2.803.3553.988


## 3.8. Equilibrium Studies

Isotherms are basically related to the homogeneity and heterogeneity of adsorbents and enables to finds the possible interactions between sorbate molecules and sorbent. Isotherm plots relate adsorption capacity,qe, to the concentration of adsorbate at equilibrium [36]. These isotherms provide an overall understanding of mechanism pathways and design of adsorption systems. The following isotherms were applied to the adsorption data of PO43− ions on to the composites.


### 3.8.1. Langmuir Isotherm

Langmuir isotherm is based on the postulation that the adsorption of adsorbate molecules on the surface of the sorbent forms monolayer which means that one molecule attaches at the particular active site. Energy of the adsorption process remains constant, and none of the molecules moves across the surface of the sorbent, and it represents that adsorption is homogenous [36]. Langmuir model is represented in the following equation:
(6)Ceqe=1qmb+Ceqm,where Ce/qe is y-axis, Ce (mg/L) is the concentration of adsorbate at time t, 1/qm is slope, qm is maximum adsorption capacity at equilibrium state, 1/qmb is intercept, and b is the Langmuir constant also represented as KL. Essential features of adsorption can be explained by a dimensionless constant of Langmuir constant called separation factor RL (Equation (8)):
(7)RL=11+bCo,where Co is initial concentration of adsorbate (mg/L). RL value indicates the adsorption is favorable or not. If its value range between 0 and 1, it means adsorption if favorable, and if greater than 1, then adsorption in unfavorable and adsorption is irreversible when the value is 0. The values of R2 indicate that adsorption data of PO43− ions followed Langmuir model and the adsorption of PO43− ions on the native TW, PPY/TW, and MG/TW is monolayer adsorption.


### 3.8.2. Freundlich Isotherm

The adsorption that occurs at the heterogeneous surfaces is explained by Freundlich isotherm [37]. The linear form of Freundlich isotherm is depicted in the following equation:
(8)logqe=logKF+1nlogCe,where KF, 1/n, Ce, and qe are the adsorption capacity, intensity, PO43− ions concentration at equilibrium, and adsorption capacity at equilibrium, respectively. Adsorption intensity indicates the distribution of the energy and the heterogeneity of the adsorption. The following are the results obtained for adsorption of PO43− ions on adsorbents by applying Freundlich model. Adsorption data of PO43− ions of native TW followed Freundlich isotherm model (Table 2).Table 2
Equilibrium parameters for the adsorption of PO43− ions on the native and composite adsorbents.
ModelsParameterNative TWPPY/TWMG/TWLangmuir modelB0.2970.14790.137RL0.3130.4030.42R20.99810.90230.962Qm (cal.) (mg/g)4.556.767.29Qm (exp.) (mg/g)4.375.256.46FreundlichN5.131.922.40Kf2.982.013.20R20.91120.72640.7065TemkinA106.703.327.77B0.65531.70721.7034R20.91780.77470.7606Huskin-JuraA5.621.985.57B1.1730.890.861R20.70820.6310.4794Dubinin-Radushkevichβ (mol2KJ-2)8 × 10-86 × 10-73 × 10-7R20.95530.98010.9764Qm (cal.) (mg/g)4.295.826.91qm (Exp.) (mg/g)4.375.256.46E (KJ/mole)0.6250.8331.67


### 3.8.3. Temkin Isotherm

The Linear form of Temkin isotherm model is shown in the following equation [36]:
(9)Qe=BlnA+BlnCe,where B=RT/b and A=equilibriumbindingconstant. In the straight-line equation, B is the slope, and BlnA is the intercept. The Temkin model explains the distribution of binding energies over active sites and revealed that heat of adsorption decreases with the increase of coverage of sites. We can calculate the values of the slope, intercept, and R2 by plotting graph between lnCe and qe. Value of R2 showed that adsorption of PO43− ions on native TW followed Temkin isotherm model, while PPY/TW and MG/TW did not follow the Temkin model.


### 3.8.4. Harkin-Jura Isotherm

Multilayer adsorption on the adsorbent and heterogeneous pore distribution is explained by Harkin-Jura isotherm [37]. Linear form of this model is shown in the following equation:
(10)1qe2=BA−1AlogCe,where 1/qe2 (y-axis), logCe (x-axis), 1/A is the slope, and B/A is the intercept. The values of A and B can be calculated by plotting a graph between logCe and 1/qe2. The following is the graphical representation of the model for PO43− ions the adsorption on the TW, PPY/TW, and MG/TW composites. None of the adsorption data fitted well to the Harkin-Jura isotherm model.


### 3.8.5. Dubinin-Radushkevich Isotherm

This model gives us an estimation about the free energy of porosity of adsorbent and heterogeneous surface of adsorbent [38]. Dubinin-Radushkevich isotherm model is represented in the following equation:
(11)lnqe=lnqm−βε2,where βis the slope and lnqm is intercept. ε can be calculated using Equation (13), and the values of E can be estimated using the following equation:
(12)ε=RTln1+1Ce,(13)E=12β1/2,where ϵ is Polanyi potential, β is Dubinin-Radushkevich constant, R is gas constant (8.31 Jmol−1 k−1), T is absolute temperature, and E is mean adsorption energy. Values of R2 are noted 0.9553, 0.9801, and 0.9764 for Native TW, PPY/TW, and MG/TW, respectively. All the adsorbents followed the Dubinin-Radushkevich isotherm which showed that this process is temperature dependent. The values of all equilibrium parameters are given in Table 2.


## 3.8.1. Langmuir Isotherm

Langmuir isotherm is based on the postulation that the adsorption of adsorbate molecules on the surface of the sorbent forms monolayer which means that one molecule attaches at the particular active site. Energy of the adsorption process remains constant, and none of the molecules moves across the surface of the sorbent, and it represents that adsorption is homogenous [36]. Langmuir model is represented in the following equation:
(6)Ceqe=1qmb+Ceqm,where Ce/qe is y-axis, Ce (mg/L) is the concentration of adsorbate at time t, 1/qm is slope, qm is maximum adsorption capacity at equilibrium state, 1/qmb is intercept, and b is the Langmuir constant also represented as KL. Essential features of adsorption can be explained by a dimensionless constant of Langmuir constant called separation factor RL (Equation (8)):
(7)RL=11+bCo,where Co is initial concentration of adsorbate (mg/L). RL value indicates the adsorption is favorable or not. If its value range between 0 and 1, it means adsorption if favorable, and if greater than 1, then adsorption in unfavorable and adsorption is irreversible when the value is 0. The values of R2 indicate that adsorption data of PO43− ions followed Langmuir model and the adsorption of PO43− ions on the native TW, PPY/TW, and MG/TW is monolayer adsorption.


## 3.8.2. Freundlich Isotherm

The adsorption that occurs at the heterogeneous surfaces is explained by Freundlich isotherm [37]. The linear form of Freundlich isotherm is depicted in the following equation:
(8)logqe=logKF+1nlogCe,where KF, 1/n, Ce, and qe are the adsorption capacity, intensity, PO43− ions concentration at equilibrium, and adsorption capacity at equilibrium, respectively. Adsorption intensity indicates the distribution of the energy and the heterogeneity of the adsorption. The following are the results obtained for adsorption of PO43− ions on adsorbents by applying Freundlich model. Adsorption data of PO43− ions of native TW followed Freundlich isotherm model (Table 2).Table 2
Equilibrium parameters for the adsorption of PO43− ions on the native and composite adsorbents.
ModelsParameterNative TWPPY/TWMG/TWLangmuir modelB0.2970.14790.137RL0.3130.4030.42R20.99810.90230.962Qm (cal.) (mg/g)4.556.767.29Qm (exp.) (mg/g)4.375.256.46FreundlichN5.131.922.40Kf2.982.013.20R20.91120.72640.7065TemkinA106.703.327.77B0.65531.70721.7034R20.91780.77470.7606Huskin-JuraA5.621.985.57B1.1730.890.861R20.70820.6310.4794Dubinin-Radushkevichβ (mol2KJ-2)8 × 10-86 × 10-73 × 10-7R20.95530.98010.9764Qm (cal.) (mg/g)4.295.826.91qm (Exp.) (mg/g)4.375.256.46E (KJ/mole)0.6250.8331.67


## 3.8.3. Temkin Isotherm

The Linear form of Temkin isotherm model is shown in the following equation [36]:
(9)Qe=BlnA+BlnCe,where B=RT/b and A=equilibriumbindingconstant. In the straight-line equation, B is the slope, and BlnA is the intercept. The Temkin model explains the distribution of binding energies over active sites and revealed that heat of adsorption decreases with the increase of coverage of sites. We can calculate the values of the slope, intercept, and R2 by plotting graph between lnCe and qe. Value of R2 showed that adsorption of PO43− ions on native TW followed Temkin isotherm model, while PPY/TW and MG/TW did not follow the Temkin model.


## 3.8.4. Harkin-Jura Isotherm

Multilayer adsorption on the adsorbent and heterogeneous pore distribution is explained by Harkin-Jura isotherm [37]. Linear form of this model is shown in the following equation:
(10)1qe2=BA−1AlogCe,where 1/qe2 (y-axis), logCe (x-axis), 1/A is the slope, and B/A is the intercept. The values of A and B can be calculated by plotting a graph between logCe and 1/qe2. The following is the graphical representation of the model for PO43− ions the adsorption on the TW, PPY/TW, and MG/TW composites. None of the adsorption data fitted well to the Harkin-Jura isotherm model.


## 3.8.5. Dubinin-Radushkevich Isotherm

This model gives us an estimation about the free energy of porosity of adsorbent and heterogeneous surface of adsorbent [38]. Dubinin-Radushkevich isotherm model is represented in the following equation:
(11)lnqe=lnqm−βε2,where βis the slope and lnqm is intercept. ε can be calculated using Equation (13), and the values of E can be estimated using the following equation:
(12)ε=RTln1+1Ce,(13)E=12β1/2,where ϵ is Polanyi potential, β is Dubinin-Radushkevich constant, R is gas constant (8.31 Jmol−1 k−1), T is absolute temperature, and E is mean adsorption energy. Values of R2 are noted 0.9553, 0.9801, and 0.9764 for Native TW, PPY/TW, and MG/TW, respectively. All the adsorbents followed the Dubinin-Radushkevich isotherm which showed that this process is temperature dependent. The values of all equilibrium parameters are given in Table 2.


## 3.9. Thermodynamics Studies

Thermodynamics of adsorption gives us the estimation that how change in enthalpy (ΔH) and entropy (ΔS) determines the free energy content (ΔG) of the adsorption process at constant temperature and pressure [39]. According to thermodynamics, the PO43− ions will spontaneously adsorb on the surface of adsorbent if the change in free energy will be negative. Equations (14) and (15) utilize for the thermodynamics analysis:
(14)∆G=∆H−T∆S,(15)lnKd=−∆HRT+∆SR,where Kd is equilibrium rate constant, ΔH is change in enthalpy change, ΔS is entropy change, R is general gas constant (8.314 J/mole.K), and T is absolute temperature. The values of ΔH and ΔS were calculated from the plot of lnKd versus 1/T. Negative values of ∆G for native TW, PPY/TW, and MG/TW indicated that the adsorption of PO43− ions is exothermic and spontaneous. Change in entropy ΔS was positive for native TW and MG/TW. Table 3 showed the values of the thermodynamic parameters for PO43− ions adsorption on to native TW, PPY/TW, and MG/TW.Table 3
Thermodynamic parameters for the adsorption of PO43− ions on the native and composite adsorbents.
Temperature (K)300.15307.15314.15321.15328.15335.15Native TWΔG (kJ/mole)0.7− 1.06− 1.21− 2.10− 2.690.98ΔH (kJ/mole)6.66ΔS (J/mole.K)0.024PPY/TWΔG (kJ/mole)− 1.50− 1.56− 2.48− 2.16− 3.66− 2.01ΔH (kJ/mole)− 31.24ΔS (J/mole.K)− 0.096MG/TWΔG (kJ/mole)− 1.50− 1.57− 2.48− 2.16− 3.66− 2.01ΔH (kJ/mole)9.27ΔS (J/mole.K)0.036


## 3.10. Effect of Interfering Ions on Adsorption

Wastewater streams contain with different type of salts and their presence effect the adsorption capabilities of the adsorbents. Adsorption of PO43− ions was affected in the presence of the monovalent, divalent, and trivalent salts. The NaCl, CaCl2, and AlCl3.6H2O salts (0.025 g) were taken in 50 mL of 10 mg/L phosphate solution with 0.05 g adsorbent at optimized pH values and were run in an orbital shaker for 90 min at 120 rpm. The presence of salts decreases the adsorption capacity. The effect of AlCl3.6H2O on MG/TW was negligible, but NaCl and CaCl2 have significantly reduced the removal efficiency. It was because of blockage of active sites with the salt ions rather than the PO43− ions. It was clear from the data analysis that monovalent ions decreased the sorption capacity more than divalent and trivalent ions for all the adsorbents. The order of reducing removal efficiency can be represented as Na+ > Ca2+ > Al3+. The removal efficiency was reduced from 43 to 39 (%), 52 to 40 (%), and 64 to 56 (%) in the presence of monovalent ions for native TW, PPY/TW, and MG/TW, respectively (Figure 9).Figure 9
(a) Salt effect on the adsorption of PO43− ions and (b) Desorption study of PO43− ions.
(a)(b)


## 3.11. Desorption Study

Desorption experiments were designed to investigate the stability and reusability of adsorbents. The capability of any adsorbent to desorb suggests the recycling of the adsorbent, which made the adsorbent cost-effective. The desorption study of PO43− ions from native TW, MG/TW, and PPY/TW was studied in a 0.5-0.25 g range using reagent molarity (0.1-1.0 N). Native TW and MG/TW showed maximum adsorption with 0.1 N NaOH and minimum at 1 N NaOH solution, while PPY/TW showed maximum desorption in 0.1 N HCl and minimum with 1 N HCl solution (Figure 9). Percentage desorption obtained was 72.66, 61.03, and 94.43 (%) for native TW, MG/TW, and PPY/TW composites, respectively, using NaOH and HCl (0.1 N) solution. These findings revealed that the PPY/TW and MG/TW has promising efficiency for the adsorption of PO43− ions and their subsequent recovery. Hence, these composite can be employed for the adsorption of PO43− ions from the effluents since adsorption has various advantages versus other conventional techniques [40]. Previous findings also revealed that the composites offer higher adsorption efficacies versus their native counterpart for the inorganic ions removal from the effluents (Table 4), which could be employed for the removal of inorganic ions from the wastewater to avoid their negative impact.Table 4
Adsorption efficiency comparison of composites for different inorganic ions.
S. noAdsorbentAdsorbateRemoval efficiencyReferences1Graphene oxide and Fe3O4 compositeNO₃¯ and PO43¯ ions89 and 93 (%)[5]2Zeolite/geopolymer compositePO43− and NH4+ ions206 and 140 (mg/g)[41]3Mango stone biocompositePO43− ions95 mg/g[3]4g-C3N4 and acid-activated montmorillonite compositePO43− and Pb(II) ions5.06 and 182.7 (mg/g)[42]


## 4. Conclusions

This research work illustrates the applicability of tea waste composite for the treatment of PO43− ions. The adsorption of PO43− ions was maximum for native TW, PPY/TW, and MG/TW at pH of 6, 10, and 3. Other optimum parameters were 0.05 g composite dose, 90 min contact time, and 10 mg/L initial PO43− ion concentration at 55 °C. The PSO and intraparticle diffusion models fitted well to the PO43− ions adsorption onto composites. Equilibrium isotherm fitted best for Langmuir isotherm that confirms the homogenous monolayer adsorption of PO43− ions. The Dubinin-Radushkevich isotherm confirms that the adsorption of PO43− ions onto the three adsorbents is temperature dependent. Thermodynamics proved exothermic and spontaneous nature of the adsorption. PO43− ions adsorption is affected in the presence of salts, and monovalent salts lower the uptake of PO43− ions more than the divalent and trivalent salts. Native TW and magnetic composites can be regenerated using NaOH as eluting agent. Hence, the composites have promising adsorption potential, which can be applied for the sequestration of PO43− ions from the effluents and also extendable for other inorganic ions commonly present in the effluents.



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*Source: 4071162-2022-09-06.xml*