# Efficient Sequestration of Cr(VI) from Aqueous Solution Using Biosorbent Derived fromArundo donax Stem

Authors: Krishna Prasad Bhattarai; Bishnu Datt Pant; Rajan Rai; Ram Lochan Aryal; Hari Paudyal; Surendra Kumar Gautam; Kedar Nath Ghimire; Megh Raj Pokhrel; Bhoj Raj Poudel
Journal: Journal of Chemistry
 (2022)

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## Abstract

The potential of a biosorbent derived fromArundo donax stem, a readily available agricultural product, was examined to remove Cr(VI) from water. Various techniques such as XRD, FTIR, SEM, and EDX were used for the characterization of the prepared adsorbent. The optimal pH for Cr(VI) biosorption was found to be 2.0. The experimental data best suits the Langmuir isotherm model and pseudosecond-order kinetics. The maximum biosorption capacity (qmax) of the investigated biosorbent for Cr(VI) was evaluated to be 76.92 mg/g by the Langmuir model. From the results of the Cr(VI) biosorption using charred Arundo donax stem powder (CADSP), it can be a novel, cost-efficient, and effective material for removing Cr(VI) from water and wastewater.


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## Body

## 1. Introduction

Humans and other living species require water to survive. Various water sources, including springs, streams, canals, reservoirs, wells, and rivers, are polluted because of human activity. Major contaminants of water include suspended matter, waste and chemical contamination, direct discharge of effluents, and oxygen depletion. Heavy metals and metalloids are among the most common chemical contaminants, and because of their toxicity and carcinogenicity, they constitute a significant source of water contamination [1].Because heavy metals can be found in significant concentrations in industrial wastewater, it is cleaned before being disposed of to prevent water pollution. Heavy metals are present in high proportion in the wastewater released into the receiving water body, which will have major negative effects on the ecosystem and cause a variety of diseases and problems [2]. Industrial growth, urbanization, and anthropogenic pollution are examples of human activities that contribute to heavy metals occurrence and accumulation in the surroundings. Heavy metal ions such as Cu(II), Cr(VI)/Cr(III), Cd(II), As(III)/As(V), Pb(II), Hg(II), Ni(II), and Zn(II) are commonly detected in contaminated water [3].Chromium is a highly harmful heavy metal that can be found in a variety of industrial and polluted effluents. Lung cancer, ulcers, nasal septum perforation, nervous system, and renal damage are all caused by chromium (VI) [4]. Cr(VI) compounds can cause skin disorders when they come into contact with the skin. Because of its oxidizing capacity and the creation of free-radicals during the conversion of Cr(VI) to Cr(III) [5]. Numerous industrial processes, such as metal polishing, iron and steel production, and chemical manufacture, emit heavy metals into the atmosphere [6]. In nature, mainly Cr(III) and Chromium (VI) are persistent enough to be detected; however, they can be found in a range of oxidation states. The hexavalent type is more harmful than the trivalent one, which is linked to cancer and mutation [7]. Cr(VI) is commonly excreted as oxyanions like chromate and dichromate ions in wastewater, and the latter is the most harmful. Due to its acute lethal effects on human health, the WHO set a maximum exposure limit for Cr(VI) in drinkable water of 0.05 mg/L [8].A variety of physicochemical techniques have been established for removing toxic metals or non-metals from water. Precipitation, ion exchange, electrochemical deposition, resin chelation, solid-phase extraction, reverse osmosis, and coagulation are some of the methods used [9]. These procedures, however, have drawbacks, including insufficient removal of metals; large reagents and energy demands; and the production of toxic waste [10].Biosorption is a cutting-edge method that is both effective and cost-effective. As a result, metals that were before impossible to remove have become simple and efficient to remove. Biosorption has several important advantages over other traditional treatment methods, including minimal price, high performance, reduction of chemical and biological waste, rejuvenation of biosorbent through desorption operations, and the potential for recovery of metals [11]. Certain agricultural raw waste products have gained popularity in recent years such as cocoa shell [12], potato peel [13], rice husk [14], olive stone [15], garlic peel [16], sawdust [17], banana peel [18], Carica papaya seed [19], Anabaena and Vetiveria [20], coconut husk [21], sweet lime peels [22], date tree leaves [23], and sugarcane bagasse [24] were studied for their efficiency in eliminating metals from effluents.Arundo donax, which is frequently found as bio-waste and has several functional groups on its surface and does not release soluble contaminants in treated water, makes it the ideal option for this study. Arundo donax stem powder offers unique morphological and physiological features, as well as tolerance to high amounts of heavy metals and unfavorable environments when used as a bioadsorbent. Its principal components are pectin, lignin, hemicellulose, and cellulose, and it has structural features that can be used to bind metals. It’s a substance with a lot of promise that’s commonly employed in environmental protection [25]. According to the best of our knowledge, there has been no study that has been reported in the literature of CADSP that investigates the adsorptive removal of Cr (VI). As a result, CADSP has been suggested for use as an innovative adsorbent in the process of removing Cr (VI) from the aqueous solution. The crucial objective of this research was to look at the biosorption capacity of CADSP for Cr (VI) ions from water. FTIR, XRD, SEM, and EDS characterization techniques were used to characterize adsorbents. The biosorption effectiveness was also studied for contact time, pH, and initial Cr (VI) concentration.


## 2. Materials and Methods

### 2.1. Reagents

Reagents of analytical grade were utilized, and they were not purified further. In a 0.1 N HNO3 solution, a 1000 mg/L potassium dichromate stock solution was prepared. Dilutions were made in 0.1 N nitric acid to make working solutions. The UV-vis spectrophotometer was used to determine the initial and equilibrium concentrations of Cr(VI) in the solution.


### 2.2. Preparation of Adsorbent fromArundo donax Stem

Arundo donax stems were collected from several areas throughout Kirtipur, Kathmandu, Nepal. They were cleaned with water and then exposed to the sun for five days to dry entirely. Using a mechanical grinder, the dried Arundo donax stems were broken into small pieces and powdered. After that, the powder was sieved at 212 μm to obtain a precise particle size. This Arundo donax stem powder was regarded as raw Arundo donax stem powder (RADSP).At room temperature, RADSP was treated with concentrated sulphuric acid. A high cellulose concentration in the RADSP is possible, and acid modification may well be able to produce an environment that is favorable to opening the ring. In addition to this, it produces a significant extent of microporosity and expands the surface area of the material [26]. A complete reaction was ensured by mixing concentrated H2SO4 in a 1 : 3 (W/V) ratio with RADSP and letting the mixture sit for 24 hrs. Washing with distilled water till the pH was neutrally eliminated the extra acid. After drying for 6 hours at 80°C in a hot air oven, it was sieved to a 212 μm particle size. The charred Arundo donax stem (CADSP) adsorbent was finely pulverized and sealed in an airtight container.


### 2.3. Characterization Techniques

The surface functional group of the sample was characterized using FTIR spectroscopy on an IR AFFINITY-1Shimadzu (Shimadzu, Kyoto, Japan) spectrometer. SEM images of CADSP and Cr (VI) adsorbed CADSP were taken on the JEOL-JSM-6700F instrument (Jeol Ltd., Tokyo, Japan) to identify the morphology and elemental composition. The crystallinity of the biosorbent was determined by X-ray diffraction analysis using an X-ray diffractometer (Rigaku Co., Japan).


### 2.4. Batch Biosorption and Kinetics Study

In a 25 mL reagent container, 25 mg of raw and charredA. donax stem powder was treated with 25 mL of Cr(VI) with a known starting concentration. At room temperature, the mixture was stirred for 24 hours on a mechanical shaker. The filtrates from the solution’s filtering were used to measure the residual metal ion concentration. To acquire equilibrium data, batch biosorption tests were conducted at room temperature with various pH, initial Cr(VI) concentrations, and contact times (10 minutes to 360 minutes). All the trials were done in triplicate and the average was calculated.The following equations were used to determine the capacity of biosorption at equilibriumqe (mg/g) and biosorption efficiency.(1)qemg/g=C0−CeM×V,(2)Biosorption efficiency%=C0−CeC0×100,where C0 represents the starting concentration (mg/L), Ce represents the Cr(VI) equilibrium time concentrations (mg/L), M represents the biosorbent mass (g), and V represents the volume of water in litre [16, 27].


#### 2.4.1. Isotherm Study

Monolayer biosorption on a biosorbent’s surface is represented using the Langmuir isotherm model. As seen below, it is expressed both nonlinearly and linearly by the following equations, respectively [28].(3)qe=qm⋅b⋅Ce1+b⋅Ce,(4)Ceqe=1qmb+Ceqm,where, the Langmuir constant is b (L/mg), while the maximum adsorption capacity is qm (mg/g).The Freundlich isotherm, which is articulated utilizing nonlinear equations, determines multilayer biosorption on a heterogeneous surface [29],(5)qe=KFCe1/n,(6)logqe=logKF+1nlogCe.Biosorption capacity is represented by the Freundlich constantKF ((mg/g) (L/mg)1/n), while biosorption intensity is represented by the Freundlich exponent n.


#### 2.4.2. Biosorption Kinetics

The biosorption kinetics were determined by pseudo-1st-order and pseudo-2nd-order models, that revealed the regulating mechanism of Cr(VI) biosorption on the biosorbent. Equation (7) expresses the pseudofirst order linear equation as [30],(7)logqe−qt=logqe−k12.303t.k1 (mg/g·min) is the pseudo first order rate constant, and qt (mg/g) is the sorption capacity at time t (mg/g). Equation (8) represents the pseudo-2nd-order linear equation as,(8)tqt=1k2qe2+1qet,where k2 (g/mg·min) represents the rate constant of the pseudo-2nd-order model [27].


#### 2.4.3. Thermodynamic Studies

The calculation of the change in thermodynamic parameters namely Gibbs free energy (∆Gº), entropy (ΔSº), as well as enthalpy (ΔHº) was done using experimental data and isotherm constants at the temperatures of 298 K, 308 K, and 318 K. The ∆G0 values were calculated employing the following equation:(9)△G0=−RTlnKC,where R is the universal gas constant with the value of 8.314 J·mol−1·K−1, T is the temperature in Kelvin and KC is the dimensionless thermodynamic equilibrium constant. The Langmuir constant (b) calculated from the experiment was employed to determine the value of KC. When a biosorption experiment is conducted in water and b has units of L/g, the KC may easily be obtained as a dimensionless parameter by applying the following equation [31–33]:(10)KC=b×55.5×1000×ΜW,where Mw denotes the molar mass of chromium (51.99 g·mol−1), and the factor 55.5 represents the number of moles of pure water in one litre of solution (∼1,000 g/L divided by 18 g/L).Also, ∆G0 is associated with ∆H0 and ∆S0 at a given temperature using the given equation:(11)△G°=△H°−Τ△S°.From equations (9) and (11), the Van’t Hoff equation is obtained [34].(12)lnKC=−△H°RT+△S°R.


#### 2.4.4. Desorption Studies

The biosorption-desorption cycle was used to test the efficacy of adsorbents that would be employed for biosorption several times. 50 mL of a known concentration and pH-optimized metal ion solution was treated with 50 mg CADSP in reagent bottles and shaken for 24 hours. The equilibrium concentration was then obtained to compute the amount of adsorbed. The residue was then combined with 50 mL of 1 N NaOH solution in the same reagent bottle, agitated for 24 hours before being filtered, and the residue was kept aside for further desorption. Equation (13) expresses the percent desorption (% D) of Cr(VI).(13)%D=AdAa×100,where Aa denotes the amount of Cr(VI) ions adsorbed in mg/g and Ad denotes the desorbed amount in mg/g [35, 36].


## 2.1. Reagents

Reagents of analytical grade were utilized, and they were not purified further. In a 0.1 N HNO3 solution, a 1000 mg/L potassium dichromate stock solution was prepared. Dilutions were made in 0.1 N nitric acid to make working solutions. The UV-vis spectrophotometer was used to determine the initial and equilibrium concentrations of Cr(VI) in the solution.


## 2.2. Preparation of Adsorbent fromArundo donax Stem

Arundo donax stems were collected from several areas throughout Kirtipur, Kathmandu, Nepal. They were cleaned with water and then exposed to the sun for five days to dry entirely. Using a mechanical grinder, the dried Arundo donax stems were broken into small pieces and powdered. After that, the powder was sieved at 212 μm to obtain a precise particle size. This Arundo donax stem powder was regarded as raw Arundo donax stem powder (RADSP).At room temperature, RADSP was treated with concentrated sulphuric acid. A high cellulose concentration in the RADSP is possible, and acid modification may well be able to produce an environment that is favorable to opening the ring. In addition to this, it produces a significant extent of microporosity and expands the surface area of the material [26]. A complete reaction was ensured by mixing concentrated H2SO4 in a 1 : 3 (W/V) ratio with RADSP and letting the mixture sit for 24 hrs. Washing with distilled water till the pH was neutrally eliminated the extra acid. After drying for 6 hours at 80°C in a hot air oven, it was sieved to a 212 μm particle size. The charred Arundo donax stem (CADSP) adsorbent was finely pulverized and sealed in an airtight container.


## 2.3. Characterization Techniques

The surface functional group of the sample was characterized using FTIR spectroscopy on an IR AFFINITY-1Shimadzu (Shimadzu, Kyoto, Japan) spectrometer. SEM images of CADSP and Cr (VI) adsorbed CADSP were taken on the JEOL-JSM-6700F instrument (Jeol Ltd., Tokyo, Japan) to identify the morphology and elemental composition. The crystallinity of the biosorbent was determined by X-ray diffraction analysis using an X-ray diffractometer (Rigaku Co., Japan).


## 2.4. Batch Biosorption and Kinetics Study

In a 25 mL reagent container, 25 mg of raw and charredA. donax stem powder was treated with 25 mL of Cr(VI) with a known starting concentration. At room temperature, the mixture was stirred for 24 hours on a mechanical shaker. The filtrates from the solution’s filtering were used to measure the residual metal ion concentration. To acquire equilibrium data, batch biosorption tests were conducted at room temperature with various pH, initial Cr(VI) concentrations, and contact times (10 minutes to 360 minutes). All the trials were done in triplicate and the average was calculated.The following equations were used to determine the capacity of biosorption at equilibriumqe (mg/g) and biosorption efficiency.(1)qemg/g=C0−CeM×V,(2)Biosorption efficiency%=C0−CeC0×100,where C0 represents the starting concentration (mg/L), Ce represents the Cr(VI) equilibrium time concentrations (mg/L), M represents the biosorbent mass (g), and V represents the volume of water in litre [16, 27].


### 2.4.1. Isotherm Study

Monolayer biosorption on a biosorbent’s surface is represented using the Langmuir isotherm model. As seen below, it is expressed both nonlinearly and linearly by the following equations, respectively [28].(3)qe=qm⋅b⋅Ce1+b⋅Ce,(4)Ceqe=1qmb+Ceqm,where, the Langmuir constant is b (L/mg), while the maximum adsorption capacity is qm (mg/g).The Freundlich isotherm, which is articulated utilizing nonlinear equations, determines multilayer biosorption on a heterogeneous surface [29],(5)qe=KFCe1/n,(6)logqe=logKF+1nlogCe.Biosorption capacity is represented by the Freundlich constantKF ((mg/g) (L/mg)1/n), while biosorption intensity is represented by the Freundlich exponent n.


### 2.4.2. Biosorption Kinetics

The biosorption kinetics were determined by pseudo-1st-order and pseudo-2nd-order models, that revealed the regulating mechanism of Cr(VI) biosorption on the biosorbent. Equation (7) expresses the pseudofirst order linear equation as [30],(7)logqe−qt=logqe−k12.303t.k1 (mg/g·min) is the pseudo first order rate constant, and qt (mg/g) is the sorption capacity at time t (mg/g). Equation (8) represents the pseudo-2nd-order linear equation as,(8)tqt=1k2qe2+1qet,where k2 (g/mg·min) represents the rate constant of the pseudo-2nd-order model [27].


### 2.4.3. Thermodynamic Studies

The calculation of the change in thermodynamic parameters namely Gibbs free energy (∆Gº), entropy (ΔSº), as well as enthalpy (ΔHº) was done using experimental data and isotherm constants at the temperatures of 298 K, 308 K, and 318 K. The ∆G0 values were calculated employing the following equation:(9)△G0=−RTlnKC,where R is the universal gas constant with the value of 8.314 J·mol−1·K−1, T is the temperature in Kelvin and KC is the dimensionless thermodynamic equilibrium constant. The Langmuir constant (b) calculated from the experiment was employed to determine the value of KC. When a biosorption experiment is conducted in water and b has units of L/g, the KC may easily be obtained as a dimensionless parameter by applying the following equation [31–33]:(10)KC=b×55.5×1000×ΜW,where Mw denotes the molar mass of chromium (51.99 g·mol−1), and the factor 55.5 represents the number of moles of pure water in one litre of solution (∼1,000 g/L divided by 18 g/L).Also, ∆G0 is associated with ∆H0 and ∆S0 at a given temperature using the given equation:(11)△G°=△H°−Τ△S°.From equations (9) and (11), the Van’t Hoff equation is obtained [34].(12)lnKC=−△H°RT+△S°R.


### 2.4.4. Desorption Studies

The biosorption-desorption cycle was used to test the efficacy of adsorbents that would be employed for biosorption several times. 50 mL of a known concentration and pH-optimized metal ion solution was treated with 50 mg CADSP in reagent bottles and shaken for 24 hours. The equilibrium concentration was then obtained to compute the amount of adsorbed. The residue was then combined with 50 mL of 1 N NaOH solution in the same reagent bottle, agitated for 24 hours before being filtered, and the residue was kept aside for further desorption. Equation (13) expresses the percent desorption (% D) of Cr(VI).(13)%D=AdAa×100,where Aa denotes the amount of Cr(VI) ions adsorbed in mg/g and Ad denotes the desorbed amount in mg/g [35, 36].


## 2.4.1. Isotherm Study

Monolayer biosorption on a biosorbent’s surface is represented using the Langmuir isotherm model. As seen below, it is expressed both nonlinearly and linearly by the following equations, respectively [28].(3)qe=qm⋅b⋅Ce1+b⋅Ce,(4)Ceqe=1qmb+Ceqm,where, the Langmuir constant is b (L/mg), while the maximum adsorption capacity is qm (mg/g).The Freundlich isotherm, which is articulated utilizing nonlinear equations, determines multilayer biosorption on a heterogeneous surface [29],(5)qe=KFCe1/n,(6)logqe=logKF+1nlogCe.Biosorption capacity is represented by the Freundlich constantKF ((mg/g) (L/mg)1/n), while biosorption intensity is represented by the Freundlich exponent n.


## 2.4.2. Biosorption Kinetics

The biosorption kinetics were determined by pseudo-1st-order and pseudo-2nd-order models, that revealed the regulating mechanism of Cr(VI) biosorption on the biosorbent. Equation (7) expresses the pseudofirst order linear equation as [30],(7)logqe−qt=logqe−k12.303t.k1 (mg/g·min) is the pseudo first order rate constant, and qt (mg/g) is the sorption capacity at time t (mg/g). Equation (8) represents the pseudo-2nd-order linear equation as,(8)tqt=1k2qe2+1qet,where k2 (g/mg·min) represents the rate constant of the pseudo-2nd-order model [27].


## 2.4.3. Thermodynamic Studies

The calculation of the change in thermodynamic parameters namely Gibbs free energy (∆Gº), entropy (ΔSº), as well as enthalpy (ΔHº) was done using experimental data and isotherm constants at the temperatures of 298 K, 308 K, and 318 K. The ∆G0 values were calculated employing the following equation:(9)△G0=−RTlnKC,where R is the universal gas constant with the value of 8.314 J·mol−1·K−1, T is the temperature in Kelvin and KC is the dimensionless thermodynamic equilibrium constant. The Langmuir constant (b) calculated from the experiment was employed to determine the value of KC. When a biosorption experiment is conducted in water and b has units of L/g, the KC may easily be obtained as a dimensionless parameter by applying the following equation [31–33]:(10)KC=b×55.5×1000×ΜW,where Mw denotes the molar mass of chromium (51.99 g·mol−1), and the factor 55.5 represents the number of moles of pure water in one litre of solution (∼1,000 g/L divided by 18 g/L).Also, ∆G0 is associated with ∆H0 and ∆S0 at a given temperature using the given equation:(11)△G°=△H°−Τ△S°.From equations (9) and (11), the Van’t Hoff equation is obtained [34].(12)lnKC=−△H°RT+△S°R.


## 2.4.4. Desorption Studies

The biosorption-desorption cycle was used to test the efficacy of adsorbents that would be employed for biosorption several times. 50 mL of a known concentration and pH-optimized metal ion solution was treated with 50 mg CADSP in reagent bottles and shaken for 24 hours. The equilibrium concentration was then obtained to compute the amount of adsorbed. The residue was then combined with 50 mL of 1 N NaOH solution in the same reagent bottle, agitated for 24 hours before being filtered, and the residue was kept aside for further desorption. Equation (13) expresses the percent desorption (% D) of Cr(VI).(13)%D=AdAa×100,where Aa denotes the amount of Cr(VI) ions adsorbed in mg/g and Ad denotes the desorbed amount in mg/g [35, 36].


## 3. Results and Discussion

### 3.1. Adsorbent characterization

In Figure1 demonstrates the mass titration results obtained with three different NaCl concentrations, and the point of zero charges (pHPZC) of CADSP was determined to be 4.0. When the pH of the solution is less than pHPZC, the electrostatic interaction between the anions and the positively charged surface of the biosorbent causes the surface to become positively charged, promoting the biosorption of anionic substances, whereas when the solution’s pH is higher than pHPZC, cationic substances biosorption is advantageous. The results show that the biosorption of Cr(VI) anionic species is more suitable at pH lower than pHPZC 4.0. Titrations with three different concentrations of NaCl, namely 0.01 M, 0.05 M, and 0.1 M with CADSP adsorbent, suggest that anionic Cr(VI) biosorption is more favorable at pH lower than pHPZC 4.0.Figure 1
Determination of point zero charge using 0.01 M, 0.05 M, and 0.1 M NaCl.A broad band at 3398.57 cm−1 in the FTIR spectra of RADSP shows the presence of (-OH) groups in biomass. This peak shift becomes stronger in the case of CADSP owing to the ring-opening and the considerable amount of -OH present (Figure 2). The vibration of C-H stretching is responsible for the peak at 2954.94 cm−1 in RADSP. In RADSP, a peak at 1705.07 cm−1 is allocated to the carbonyl group’s carbonyl stretching vibration [35, 37]. The spectra of the CADSP revealed a change in the intensity and frequency range of -OH and C=O stretching. In the case of CADSP compared to RADSP, there is also the formation of new peaks and the disappearance of a few peaks. In Cr(VI) adsorbed CADSP, the -OH peak has completely vanished in contrast to CADSP, this is most likely because -OH groups have a role in the biosorption of Cr(VI). Furthermore, the steep peaks that existed in CALS have vanished, and a switch in the intensity of existing peaks revealed the involvement of chemical reactions, which result in the breakdown of chemical bonds. All the foregoing characteristics point to a successful chemical conversion of RADSP to CADSP, with high chromium biosorption on the CADSP.Figure 2
FTIR spectrum of RADSP and CADSP.The XRD pattern was used to determine if the adsorbent was crystalline or amorphous and Figure3 depicts the XRD pattern of CADSP and Cr(VI) adsorbed CADSP. The week and dispersed peaks at 2θ are equal to 22° in CADSP, which is the distinctive peak of cellulose due to the presence of an amorphous structure, which suggests CADSP is made from cellulose with amorphous areas [38]. Furthermore, little shift in the peak position in Cr(VI) adsorbed CADSP without any sharp peaks suggests that the adsorbent was amorphous in nature throughout the process.Figure 3
XRD of CADSP and Cr(VI) adsorbed CADSP.The surface properties of RADSP, CADSP, and Cr(VI) adsorbed CADSP were investigated using SEM. Compared to RADSP (Figure4(a)), the surface of CADSP was found to be more porous (Figure 4(b)) with various shapes, sizes, and irregular rough surfaces indicating that the hydrolysis process occurred on the surface of RADSP due to the reaction of conc. H2SO4. The rough shapes developed by the H2SO4 having pores and cavities might be because of the breakage of the polymeric network of raw Arundo donax and its transformation to microscopic, high-surface-area particles. The chromium Cr(VI) ion biosorption reduced the roughness of the surface in Cr-CADSP having more shining particles (Figure 4(c)) [39]. EDX confirm the biosorption of Cr(VI) at 0.57 keV, 5.36 keV, and 5.82 keV (Figure 5). The primary components of A. donax are cellulose, hemicelluloses, and lignin, which are represented by C, O, and Si, respectively, with 64.4, 33.23, and 0.03 wt percent. The amount of Cr adsorbed in CADSP was found to be 3.33 wt% which revealed that a large amount of Cr(VI) had been deposited onto the A. donax biomass throughout the biosorption process.Figure 4
SEM image of (a) RADSP, (b) CADSP, and (c) Cr(VI)-CADS.
(a)(b)(c)Figure 5
(a) EDX electron and layered images and (b) EDS atomic percentage of the Cr(VI)-loaded CADSP.
(a)(b)


### 3.2. Effect of the pH

The chemical properties of metal ions, as well as the ionic states of binding sites contained in the adsorbent examined in aqueous solutions, are all substantially influenced by the pH value [40]. Figure 6 displays that pH 2 is ideal for the maximal biosorption and that as pH rises, Cr(VI) biosorption declines. The percentage removal was dramatically raised from 27.56 percent for RADSP to 90.01 percent for CADSP at optimum pH, demonstrating the major difference between the two adsorbents. Cr(VI) exists in several oxyanion forms, including Cr2O72−, HCr2O7−, CrO42−, and HCrO4− [35, 41, 42].Figure 6
Influence of pH on Cr(VI) biosorption onto RADSP & CADSP.CrO42− ions are the predominant type of ions in a basic media with a pH greater than 6. The ions HCrO4− and Cr2O72− are in equilibrium in an acidic solution with a pH of 1 to 6, but HCrO4− is the main species [43, 44].Because the predominant form of Cr(VI) at pH 2 is HCrO4−, and the solution includes a considerable number of H+ ions at this pH, exterior protonation of biosorbent results in the creation of a positively charged adsorbent. These protonated interfaces of sorbents react with Cr(VI) anion by an electrostatic attraction phenomenon due to which biosorption is in favor i.e, HCrO4− being adsorbed onto biosorbent surface [45]. At pH 1, despite the existence of an acidic medium, there are still additionally available protons that can help to reduce C(VI) to Cr (III). Because of competitiveness among protons, Cr(III) species, and active sites on the biosorbent’s surface, metal cations react with positively charged biosorbent’s surface via electrostatic repulsion, so biosorption is less favorable than that of pH 2. When pH rises, the protonation of the adsorbent surface decreases, resulting in a decrease in percent biosorption. Dissociation of the acidic functional group also increased the complexation between Cr(III) (obtained from Cr(VI) reduction) and an anionic functional group, lowering the specific biosorption of Cr(VI) [46]. Because raw RADSP showed a low potentiality compared to CADSP, further study was primarily focused on CADSP.


### 3.3. The Influence of Contact Time for Biosorption of Cr(VI)

By changing the contact time while maintaining the other variables constant, the contact time effect for chromium biosorption on CADSP was examined (Figure7). The presence of significant biosorption sites causes the adsorption rate of Cr(VI) to increase at initially [45, 47]. As time passed, Cr(VI) ion gradually occupied the biosorption sites, and the rate of Cr(VI) biosorption gradually increased until 210 minutes. After that, there is no significant rise in Cr(VI) biosorption rate, and it reaches equilibrium after 240 minutes.Figure 7
The influence of the agitation time for Cr(VI) biosorption on CADSP.


### 3.4. Batch Biosorption Isotherm Studies

The adsorption ’isotherm’s fluctuation in initial Cr(VI) concentration was assessed for its adsorption capacity. Figure8 shows the Cr (VI) adsorption isotherms onto CADSP at various temperatures between 298 and 318 K. The findings show that the adsorption capacity increased fast when the arsenate ion concentration rose from low to high, then rose gradually until reaching saturation (plateau) at high concentrations. The high adsorption rate in the early stage may be explained by the growing availability of vacant sorption sites of biosorbent. There are no longer any locations accessible for further adsorption since the adsorption capacity has achieved a plateau at a high concentration region. Additionally, at the examined range of temperatures, the adsorption capacity of CADSP rises with temperature, indicating an endothermic process of adsorption.Figure 8
Biosorption isotherm for Cr (VI) onto CADSP at different temperatures (298-318 K).The best-fit isotherm model for Cr(VI) biosorption onto CADSP was determined by analyzing the experimental data using the well-known Langmuir and Freundlich isotherm equations. Table1 lists the calculated values for the Langmuir and Freundlich parameters along with the corresponding R2 values. It was observed that the correlation coefficient (R2) values for the Langmuir isotherm were found to be greater than that of Freundlich isotherm. As a result, we may conclude that the biosorption isotherms most closely match the linear Langmuir isotherm model. It signifies a monolayer biosorption and a uniform distribution of reactive sites on the surface of the biosorbent [48]. The linearized Langmuir and Freundlich isotherm plots at room temperature of 298 K are shown in Figure 9.Table 1
Isotherm parameter determined for Cr(VI) biosorption onto CADSP at pH 2.
Isotherm modelParameter298 K308 K318 KLangmuirqm (mg/g)74.2477.6381.493b (L/mg)0.6500.9421.531R20.9990.9990.999FreundlichKF (mg/g)·(L/mg)1/n28.7532.0536.89n3.843.974.38R20.8340.8140.801Figure 9
(a) Langmuir, and (b) Freundlich plots for Cr(VI) biosorption using CADSP at 298 K.
(a)(b)


### 3.5. Batch Kinetic Studies

The kinetics of Cr(VI) biosorption on CADSP at optimal pH 2 was investigated using pseudo-1st-order and pseudo-2nd-order models (Figure 10). The plot analysis revealed that the pseudo-2nd-order model had a higher R2 than the pseudo-1st-order. Additionally, the biosorption of Cr(VI) onto CADSP followed pseudo-2nd-order kinetics, as evidenced by the close match between experimental and theoretical findings of qe (Table 2) [45, 49].Figure 10
(a) Pseudo-1st-order, and (b) pseudo-second-order kinetic plots for the biosorption of Cr(VI) onto CADSP.
(a)(b)Table 2
Kinetic parameters determined for the Cr(VI) biosorption onto CADSP.
Adsorbentqexp (mg/g)Pseudo-first orderPseudo-second orderK1 (min−1)qe (mg/gm)R2K2 (mg/g/min)qe (mg/gm)R2CADSP29.7520.01514.5210.9750.002231.2500.998Theqmax of Cr(VI) utilizing CADSP is compared to the qmax of many other adsorbents that have been found in the literature (Table 3). The Cr(VI) biosorption capability of CADSP is superior among the described adsorbents, implying that CADSP might be a viable choice for removing Cr(VI) from water samples.Table 3
Comparing the biosorption capacities of several biosorbents for Cr(VI) with CADSP.
S.N.BiosorbentpHqm (mg/g)Reference1Arthrobacter viscosus biomass2.020.37[50]2Waste newspaper3.059.88[51]3Coconut husk2.029.0[21]4Cactus fruit2.018.5[52]5Carbon slurry2.015.24[53]6Giant reed3.018.80[54]7Nutshell powder2.072.12[55]8Coconut coir pith2.076.3[56]9Human hair19.85[57]10Fish scale518.34[58]11Charred betel nut waste21.51 mmol/g[59]12Modified Arundo donax stem2.076.92Present study


### 3.6. Comparison of Cr (VI) Biosorption Capacities of Various Adsorbents with CADSP

As shown in Table3, the maximum bisorption capacity (qm) of CADSP for Cr (VI) estimated using the Langmuir model at room temperature (298 K) was 76.92 mg/g. This value is higher than that of many materials reported in prior studies. It implies that CADSP may work well and hold promise as a biosorbent for the removal of Cr (VI) from polluted water.


### 3.7. Thermodynamic Studies

The thermodynamic parameters were assessed from the biosorption isotherm investigations of Cr (VI) on CADSP at temperatures 298-318 K. The values of ∆H0 and ∆S0 were derived from the van’t Hoff plot of ln KC vs. 1/T as displayed in Figure 11, whereas the value of ΔG° was determined directly from Equation (9). A straight line with R2 = 0.982 results from the plot. The values of the assessed thermodynamic parameters are shown in Table 4. All the examined temperatures (298-318 K) had negative ΔG° values, indicating that this adsorption process was possible, thermodynamically advantageous, and spontaneous [34, 41]. A process of endothermic sorption is suggested by the positive value of ΔH°. The increased randomness of Cr (VI) ions on the surface of CADSP is also shown by the positive value of ∆S0.Figure 11
Van’t Hoff plot of lnKC as a function of 1/T.Table 4
Thermodynamic parameters of CADSP and Cr(VI) system.
T (K)b (L/mol)∆G0 (kJ/mol)∆H0 (J/mol)∆S0 (J/mol/K)2980.6507−35.8033.43232.163080.9422−37.953181.5312−40.45


### 3.8. Desorption Analysis of CADSP

NaOH solution of different concentrations was used as an eluent for the desorption study. Figure12(a) shows the effect of eluent concentration on CADSP. As pH study revealed that the biosorption is effective at lower pH, it indicated that an alkaline solution could be an effective eluent. As a result, NaOH solution can be used to desorb Cr(VI) from exhausted biosorbent. The experiment showed that the desorption percentage enhanced from 18.45% to 100% with the increase of NaOH concentration from 0.05 to 1.00 M. When the concentration of the NaOH solution is increased to a high level, there is a possibility that the adsorbed Cr(VI) ions will be replaced by the hydroxyl ions. In addition, a larger concentration of NaOH may have also played a part in the destruction of the CADSP’s chemical integrity. To reduce the cost, the reusability of the adsorbent is significant. Because HCrO4− is the main form of Cr(VI) at optimal pH 2, a considerable quantity of H+ ions may be exposed in the solution. Because of the protonation of the surface, the adsorbent develops a positively charged surface. Because of the electrostatic attraction between the protonated surfaces of adsorbents and HCrO4−, it adsorbs onto the surface of CADSP [45].Figure 12
(a) Desorption percentage of Cr(VI) vs. concentration of NaOH, and (b) fluctuation in the adsorption capacity of CADSP during the subsequent cycle of adsorption and desorption.
(a)(b)The regeneration studies of the prepared CADSP were assessed by subjecting four series of adsorption/desorption cycles (Figure12(b)). Cr(VI) sorption capacity of CADSP was decreased by only 15.15% after four cycles, suggesting that the CADSP can be reused. Hence, the adsorbent showed the potential to be reused for another cycle without disturbing the chemical stability of the adsorbent. Hence, the effective adsorption capacity with good Cr(VI) ions recovery and reusability is shown by the CALS.


### 3.9. Cr(VI) Biosorption/Desorption Mechanism

The surface area of the adsorbents is increased by the ring-opening of the RADSP cellulose units during the charring process, which also increases the active sites on the surface of the adsorbents. Scheme1 depicts the chemical pathway for the synthesis of CADSP from RADSP.Scheme 1
Mechanism of charring of RADSP to give CADSP [26, 60].In Scheme2 illustrates the mechanism of Cr(VI) biosorption on CADSP. Since HCrO4− is the main form of Cr(VI) at optimal pH 2, the solution at this pH contains a significant number of H+ ions. The process of surface protonation causes the biosorbent to have a positively charged surface. As a result, HCrO4− is electrostatically attracted to the protonated surfaces of the biosorbents, which causes it to bind to the surface of CADSP. Additionally, Cr(VI) might be adsorbed onto protonated sites of -OH within the CADSP by an ion-exchange mechanism. Additionally, an ion-exchange process may allow Cr(VI) to be adsorbed onto protonated sites of - OH inside the CADSP [61].Scheme 2
Mechanism of Cr(VI) biosorption on CADSP.NaOH solution can be used to regenerate biosorbent and recover Cr(VI) ions that have been consumed by the adsorbent. According to Scheme3, hydroxyl ions present in solution exhibit ligand replacement reaction with the sorbed oxyanions of Cr(VI) [45, 62].Scheme 3
Mechanism of desorption of Cr(VI).


## 3.1. Adsorbent characterization

In Figure1 demonstrates the mass titration results obtained with three different NaCl concentrations, and the point of zero charges (pHPZC) of CADSP was determined to be 4.0. When the pH of the solution is less than pHPZC, the electrostatic interaction between the anions and the positively charged surface of the biosorbent causes the surface to become positively charged, promoting the biosorption of anionic substances, whereas when the solution’s pH is higher than pHPZC, cationic substances biosorption is advantageous. The results show that the biosorption of Cr(VI) anionic species is more suitable at pH lower than pHPZC 4.0. Titrations with three different concentrations of NaCl, namely 0.01 M, 0.05 M, and 0.1 M with CADSP adsorbent, suggest that anionic Cr(VI) biosorption is more favorable at pH lower than pHPZC 4.0.Figure 1
Determination of point zero charge using 0.01 M, 0.05 M, and 0.1 M NaCl.A broad band at 3398.57 cm−1 in the FTIR spectra of RADSP shows the presence of (-OH) groups in biomass. This peak shift becomes stronger in the case of CADSP owing to the ring-opening and the considerable amount of -OH present (Figure 2). The vibration of C-H stretching is responsible for the peak at 2954.94 cm−1 in RADSP. In RADSP, a peak at 1705.07 cm−1 is allocated to the carbonyl group’s carbonyl stretching vibration [35, 37]. The spectra of the CADSP revealed a change in the intensity and frequency range of -OH and C=O stretching. In the case of CADSP compared to RADSP, there is also the formation of new peaks and the disappearance of a few peaks. In Cr(VI) adsorbed CADSP, the -OH peak has completely vanished in contrast to CADSP, this is most likely because -OH groups have a role in the biosorption of Cr(VI). Furthermore, the steep peaks that existed in CALS have vanished, and a switch in the intensity of existing peaks revealed the involvement of chemical reactions, which result in the breakdown of chemical bonds. All the foregoing characteristics point to a successful chemical conversion of RADSP to CADSP, with high chromium biosorption on the CADSP.Figure 2
FTIR spectrum of RADSP and CADSP.The XRD pattern was used to determine if the adsorbent was crystalline or amorphous and Figure3 depicts the XRD pattern of CADSP and Cr(VI) adsorbed CADSP. The week and dispersed peaks at 2θ are equal to 22° in CADSP, which is the distinctive peak of cellulose due to the presence of an amorphous structure, which suggests CADSP is made from cellulose with amorphous areas [38]. Furthermore, little shift in the peak position in Cr(VI) adsorbed CADSP without any sharp peaks suggests that the adsorbent was amorphous in nature throughout the process.Figure 3
XRD of CADSP and Cr(VI) adsorbed CADSP.The surface properties of RADSP, CADSP, and Cr(VI) adsorbed CADSP were investigated using SEM. Compared to RADSP (Figure4(a)), the surface of CADSP was found to be more porous (Figure 4(b)) with various shapes, sizes, and irregular rough surfaces indicating that the hydrolysis process occurred on the surface of RADSP due to the reaction of conc. H2SO4. The rough shapes developed by the H2SO4 having pores and cavities might be because of the breakage of the polymeric network of raw Arundo donax and its transformation to microscopic, high-surface-area particles. The chromium Cr(VI) ion biosorption reduced the roughness of the surface in Cr-CADSP having more shining particles (Figure 4(c)) [39]. EDX confirm the biosorption of Cr(VI) at 0.57 keV, 5.36 keV, and 5.82 keV (Figure 5). The primary components of A. donax are cellulose, hemicelluloses, and lignin, which are represented by C, O, and Si, respectively, with 64.4, 33.23, and 0.03 wt percent. The amount of Cr adsorbed in CADSP was found to be 3.33 wt% which revealed that a large amount of Cr(VI) had been deposited onto the A. donax biomass throughout the biosorption process.Figure 4
SEM image of (a) RADSP, (b) CADSP, and (c) Cr(VI)-CADS.
(a)(b)(c)Figure 5
(a) EDX electron and layered images and (b) EDS atomic percentage of the Cr(VI)-loaded CADSP.
(a)(b)


## 3.2. Effect of the pH

The chemical properties of metal ions, as well as the ionic states of binding sites contained in the adsorbent examined in aqueous solutions, are all substantially influenced by the pH value [40]. Figure 6 displays that pH 2 is ideal for the maximal biosorption and that as pH rises, Cr(VI) biosorption declines. The percentage removal was dramatically raised from 27.56 percent for RADSP to 90.01 percent for CADSP at optimum pH, demonstrating the major difference between the two adsorbents. Cr(VI) exists in several oxyanion forms, including Cr2O72−, HCr2O7−, CrO42−, and HCrO4− [35, 41, 42].Figure 6
Influence of pH on Cr(VI) biosorption onto RADSP & CADSP.CrO42− ions are the predominant type of ions in a basic media with a pH greater than 6. The ions HCrO4− and Cr2O72− are in equilibrium in an acidic solution with a pH of 1 to 6, but HCrO4− is the main species [43, 44].Because the predominant form of Cr(VI) at pH 2 is HCrO4−, and the solution includes a considerable number of H+ ions at this pH, exterior protonation of biosorbent results in the creation of a positively charged adsorbent. These protonated interfaces of sorbents react with Cr(VI) anion by an electrostatic attraction phenomenon due to which biosorption is in favor i.e, HCrO4− being adsorbed onto biosorbent surface [45]. At pH 1, despite the existence of an acidic medium, there are still additionally available protons that can help to reduce C(VI) to Cr (III). Because of competitiveness among protons, Cr(III) species, and active sites on the biosorbent’s surface, metal cations react with positively charged biosorbent’s surface via electrostatic repulsion, so biosorption is less favorable than that of pH 2. When pH rises, the protonation of the adsorbent surface decreases, resulting in a decrease in percent biosorption. Dissociation of the acidic functional group also increased the complexation between Cr(III) (obtained from Cr(VI) reduction) and an anionic functional group, lowering the specific biosorption of Cr(VI) [46]. Because raw RADSP showed a low potentiality compared to CADSP, further study was primarily focused on CADSP.


## 3.3. The Influence of Contact Time for Biosorption of Cr(VI)

By changing the contact time while maintaining the other variables constant, the contact time effect for chromium biosorption on CADSP was examined (Figure7). The presence of significant biosorption sites causes the adsorption rate of Cr(VI) to increase at initially [45, 47]. As time passed, Cr(VI) ion gradually occupied the biosorption sites, and the rate of Cr(VI) biosorption gradually increased until 210 minutes. After that, there is no significant rise in Cr(VI) biosorption rate, and it reaches equilibrium after 240 minutes.Figure 7
The influence of the agitation time for Cr(VI) biosorption on CADSP.


## 3.4. Batch Biosorption Isotherm Studies

The adsorption ’isotherm’s fluctuation in initial Cr(VI) concentration was assessed for its adsorption capacity. Figure8 shows the Cr (VI) adsorption isotherms onto CADSP at various temperatures between 298 and 318 K. The findings show that the adsorption capacity increased fast when the arsenate ion concentration rose from low to high, then rose gradually until reaching saturation (plateau) at high concentrations. The high adsorption rate in the early stage may be explained by the growing availability of vacant sorption sites of biosorbent. There are no longer any locations accessible for further adsorption since the adsorption capacity has achieved a plateau at a high concentration region. Additionally, at the examined range of temperatures, the adsorption capacity of CADSP rises with temperature, indicating an endothermic process of adsorption.Figure 8
Biosorption isotherm for Cr (VI) onto CADSP at different temperatures (298-318 K).The best-fit isotherm model for Cr(VI) biosorption onto CADSP was determined by analyzing the experimental data using the well-known Langmuir and Freundlich isotherm equations. Table1 lists the calculated values for the Langmuir and Freundlich parameters along with the corresponding R2 values. It was observed that the correlation coefficient (R2) values for the Langmuir isotherm were found to be greater than that of Freundlich isotherm. As a result, we may conclude that the biosorption isotherms most closely match the linear Langmuir isotherm model. It signifies a monolayer biosorption and a uniform distribution of reactive sites on the surface of the biosorbent [48]. The linearized Langmuir and Freundlich isotherm plots at room temperature of 298 K are shown in Figure 9.Table 1
Isotherm parameter determined for Cr(VI) biosorption onto CADSP at pH 2.
Isotherm modelParameter298 K308 K318 KLangmuirqm (mg/g)74.2477.6381.493b (L/mg)0.6500.9421.531R20.9990.9990.999FreundlichKF (mg/g)·(L/mg)1/n28.7532.0536.89n3.843.974.38R20.8340.8140.801Figure 9
(a) Langmuir, and (b) Freundlich plots for Cr(VI) biosorption using CADSP at 298 K.
(a)(b)


## 3.5. Batch Kinetic Studies

The kinetics of Cr(VI) biosorption on CADSP at optimal pH 2 was investigated using pseudo-1st-order and pseudo-2nd-order models (Figure 10). The plot analysis revealed that the pseudo-2nd-order model had a higher R2 than the pseudo-1st-order. Additionally, the biosorption of Cr(VI) onto CADSP followed pseudo-2nd-order kinetics, as evidenced by the close match between experimental and theoretical findings of qe (Table 2) [45, 49].Figure 10
(a) Pseudo-1st-order, and (b) pseudo-second-order kinetic plots for the biosorption of Cr(VI) onto CADSP.
(a)(b)Table 2
Kinetic parameters determined for the Cr(VI) biosorption onto CADSP.
Adsorbentqexp (mg/g)Pseudo-first orderPseudo-second orderK1 (min−1)qe (mg/gm)R2K2 (mg/g/min)qe (mg/gm)R2CADSP29.7520.01514.5210.9750.002231.2500.998Theqmax of Cr(VI) utilizing CADSP is compared to the qmax of many other adsorbents that have been found in the literature (Table 3). The Cr(VI) biosorption capability of CADSP is superior among the described adsorbents, implying that CADSP might be a viable choice for removing Cr(VI) from water samples.Table 3
Comparing the biosorption capacities of several biosorbents for Cr(VI) with CADSP.
S.N.BiosorbentpHqm (mg/g)Reference1Arthrobacter viscosus biomass2.020.37[50]2Waste newspaper3.059.88[51]3Coconut husk2.029.0[21]4Cactus fruit2.018.5[52]5Carbon slurry2.015.24[53]6Giant reed3.018.80[54]7Nutshell powder2.072.12[55]8Coconut coir pith2.076.3[56]9Human hair19.85[57]10Fish scale518.34[58]11Charred betel nut waste21.51 mmol/g[59]12Modified Arundo donax stem2.076.92Present study


## 3.6. Comparison of Cr (VI) Biosorption Capacities of Various Adsorbents with CADSP

As shown in Table3, the maximum bisorption capacity (qm) of CADSP for Cr (VI) estimated using the Langmuir model at room temperature (298 K) was 76.92 mg/g. This value is higher than that of many materials reported in prior studies. It implies that CADSP may work well and hold promise as a biosorbent for the removal of Cr (VI) from polluted water.


## 3.7. Thermodynamic Studies

The thermodynamic parameters were assessed from the biosorption isotherm investigations of Cr (VI) on CADSP at temperatures 298-318 K. The values of ∆H0 and ∆S0 were derived from the van’t Hoff plot of ln KC vs. 1/T as displayed in Figure 11, whereas the value of ΔG° was determined directly from Equation (9). A straight line with R2 = 0.982 results from the plot. The values of the assessed thermodynamic parameters are shown in Table 4. All the examined temperatures (298-318 K) had negative ΔG° values, indicating that this adsorption process was possible, thermodynamically advantageous, and spontaneous [34, 41]. A process of endothermic sorption is suggested by the positive value of ΔH°. The increased randomness of Cr (VI) ions on the surface of CADSP is also shown by the positive value of ∆S0.Figure 11
Van’t Hoff plot of lnKC as a function of 1/T.Table 4
Thermodynamic parameters of CADSP and Cr(VI) system.
T (K)b (L/mol)∆G0 (kJ/mol)∆H0 (J/mol)∆S0 (J/mol/K)2980.6507−35.8033.43232.163080.9422−37.953181.5312−40.45


## 3.8. Desorption Analysis of CADSP

NaOH solution of different concentrations was used as an eluent for the desorption study. Figure12(a) shows the effect of eluent concentration on CADSP. As pH study revealed that the biosorption is effective at lower pH, it indicated that an alkaline solution could be an effective eluent. As a result, NaOH solution can be used to desorb Cr(VI) from exhausted biosorbent. The experiment showed that the desorption percentage enhanced from 18.45% to 100% with the increase of NaOH concentration from 0.05 to 1.00 M. When the concentration of the NaOH solution is increased to a high level, there is a possibility that the adsorbed Cr(VI) ions will be replaced by the hydroxyl ions. In addition, a larger concentration of NaOH may have also played a part in the destruction of the CADSP’s chemical integrity. To reduce the cost, the reusability of the adsorbent is significant. Because HCrO4− is the main form of Cr(VI) at optimal pH 2, a considerable quantity of H+ ions may be exposed in the solution. Because of the protonation of the surface, the adsorbent develops a positively charged surface. Because of the electrostatic attraction between the protonated surfaces of adsorbents and HCrO4−, it adsorbs onto the surface of CADSP [45].Figure 12
(a) Desorption percentage of Cr(VI) vs. concentration of NaOH, and (b) fluctuation in the adsorption capacity of CADSP during the subsequent cycle of adsorption and desorption.
(a)(b)The regeneration studies of the prepared CADSP were assessed by subjecting four series of adsorption/desorption cycles (Figure12(b)). Cr(VI) sorption capacity of CADSP was decreased by only 15.15% after four cycles, suggesting that the CADSP can be reused. Hence, the adsorbent showed the potential to be reused for another cycle without disturbing the chemical stability of the adsorbent. Hence, the effective adsorption capacity with good Cr(VI) ions recovery and reusability is shown by the CALS.


## 3.9. Cr(VI) Biosorption/Desorption Mechanism

The surface area of the adsorbents is increased by the ring-opening of the RADSP cellulose units during the charring process, which also increases the active sites on the surface of the adsorbents. Scheme1 depicts the chemical pathway for the synthesis of CADSP from RADSP.Scheme 1
Mechanism of charring of RADSP to give CADSP [26, 60].In Scheme2 illustrates the mechanism of Cr(VI) biosorption on CADSP. Since HCrO4− is the main form of Cr(VI) at optimal pH 2, the solution at this pH contains a significant number of H+ ions. The process of surface protonation causes the biosorbent to have a positively charged surface. As a result, HCrO4− is electrostatically attracted to the protonated surfaces of the biosorbents, which causes it to bind to the surface of CADSP. Additionally, Cr(VI) might be adsorbed onto protonated sites of -OH within the CADSP by an ion-exchange mechanism. Additionally, an ion-exchange process may allow Cr(VI) to be adsorbed onto protonated sites of - OH inside the CADSP [61].Scheme 2
Mechanism of Cr(VI) biosorption on CADSP.NaOH solution can be used to regenerate biosorbent and recover Cr(VI) ions that have been consumed by the adsorbent. According to Scheme3, hydroxyl ions present in solution exhibit ligand replacement reaction with the sorbed oxyanions of Cr(VI) [45, 62].Scheme 3
Mechanism of desorption of Cr(VI).


## 4. Conclusions

The biosorption study of a biosorbent made fromA. donax stem powder revealed that CADSP can effectively remove Cr(VI) from water by the process of biosorption. XRD examination revealed that the adsorbent’s crystallinity is amorphous, while SEM and EDX revealed the rough and porous surfaces are present in CADSP, which are filled in Cr-adsorbed CADSP. The pHPZC of CADSP was determined to be 4.0 and the adsorbent showed the best biosorption at a pH value of 2. With a maximal biosorption capacity (qm) of 76.92 mg/g at optimal pH, Cr(VI) was fitted into the Langmuir biosorption isotherm and pseudo-second-order kinetic models. The desorption of Cr(VI) from Cr-adsorbed CADSP was carried out with varying doses of NaOH as the eluent. According to the findings of this study, A. donax stem powder can be used to make an effective and inexpensive biosorbent for eliminating Cr(VI) from water and wastewater.



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