Chromium desalinization using novel chitosan functionalized iron oxide-biochar composites: Analysis, synthesis, characterization and adsorption performance

In the study


Introduction
Chromium, a versatile and widely used metal, has found its way into various industrial processes, from steel production to electroplating.However, industrial waste often contains chromium heavy metal ions as a byproduct of these activities.These ions, primarily in the form of hexavalent chromium (Cr (VI), pose significant toxicity to aquatic life and are harmful to human health.Understanding the sources, characteristics, and potential hazards of chromium heavy metal ions from industrial waste is crucial for effectively managing and remedying these pollutants.Effective disposal of chromium from wastewater is essential to protect the environment, and human health, and ensure compliance with global regulations while promoting sustainable water management.Ecological elimination [1], chemical precipitation [2], redox [3] and adsorption [4] are only some of the methods that have been used for Cr (VI) removal so far.Adsorption is considered the best and most widely used approach for eliminating toxic contaminants due to its inexpensive, effortless, and highly influential capability.Experiments have Chromium desalinization by chitosan-Fe2O3-biochar composite Anurag Choudhary et al used a wide range of adsorbing materials, including biochar, chitosan, and iron oxide, to remove Cr (VI).The peanut (Arachis hypogaea L.) is an ancient crop that is widely cultivated in tropical and subtropical climates.India holds the position of being the second-largest producer of peanuts globally, following China.Peanut shells are the remainder of the initial processing of peanuts [5].The peanut shell is a substantial byproduct of the peanut industries, constituting around 25% to 30% of the legume's total weight.These shells are discarded as such as waste after the removal of groundnut seed from its pod at the last phase of peanut processing.These are the abundant agro-industrial waste product which has a very slow degradation rate under natural conditions [6].The peanut shell is a lignocellulosic material mainly composed of cellulose (44.8%), hemicelluloses (5.8%), and lignin (36.1%), which are complex organic polymers.To successfully manage and control this extremely troublesome detrimental species, it may be unique and enticing to convert shells into biochar and utilize them in eliminating cationic toxicants.Biochar produced from waste biomass finds varied applications in diversified realms.In agriculture, it stimulates soil fertility, water retention, and nutrient efficiency.As a stable carbon sink, biochar contributes to climate change mitigation by sequestering carbon in the soil for many years.It also acts as a water filtration medium, removing contaminants from wastewater.Its adsorption properties make it useful for environmental remediation efforts, and it can even be used as a renewable energy source [7].Additionally, recent research has shown that biochar may be utilized as a thrifty sorbent to remove a variety of toxins from water.The biochar possesses several desirable characteristics, including a substantial specific surface area, a porous structure, an abundance of surface functional groups, and mineral components.These properties render biochar a suitable adsorbent for the removal of pollutants from aqueous solutions.Enhancements and treatment methods applied to biochar customization result in improved adsorption capabilities compared to its pristine form.By modifying, its surface properties can be tailored to target specific contaminants, making it even more effective for adsorption purposes.Consequently, a range of techniques, including chemical functionalization, physical activation, impregnation, coating, and surface oxidization, have been employed to enhance the efficacy of these materials in the context of environmental remediation [8].Chitosan, a biopolymer derived from chitin, has gained remarkable attention and intensively studied for its potential application in heavy metal removal due to its unique properties such as non-toxic nature, abundant, remarkable adsorption capacity, biocompatibility, and budget-friendly substance [9].The existence of abundantly functioning groups, specifically amine and hydroxyl, in the polymer's scaffold facilitates active receptor sites to adsorb metal ions more proficiently.Chitosan possesses a unique structure that includes high-functioning groups, such as amino (-NH 2 ) and hydroxyl (-OH) groups, in its backbone.These functional groups serve as active sites for adsorption.The amino groups can act as Lewis bases, capable of forming coordination complexes with metal ions through ion exchange or chelation processes.The hydroxyl groups, on the other hand, contribute to the adsorption through electrostatic interactions, hydrogen bonding, and surface complexation [10].Although chitosan-modified biochar has a higher adsorption capacity, it is necessary to perform time-consuming post-treatment activities such as sedimentation, centrifugation, and filtering to collect the solid particles that were filtered out of the treated water.Since it takes a long time for the lowdensity particles to completely settle, the recovery of these tiny particles from the huge industrial waste treatment tank becomes difficult.Therefore, the introduction of magnetic characteristics to the biochar-based adsorbent may further alleviate the challenging recovery of the biochar-based adsorbents from the water bodies.Magnetic impregnation of the biochar would increase the composite's reusability after modification.Many recent investigations have synthesized flexible composite adsorbents by combining magnetic particles and chitosan with biomass.As a bonus, the introduction of magnetic species may attract additional metal ions owing to its superior conductivity, making magnetic separation a win-win technique for recovering adsorbent from an aqueous medium.Magnetic biochar and chitosan, when combined, provide an excellent composite for cleaning up secondary pollution by removing a wide range of metals [11].The current research on the utilization of chitosan for surface modification of biochars to improve their ability to apprehend heavy metals is still limited.This combination shows enormous potential as it effectively utilizes the perks of biochar and chitosan, resulting in significant benefits for binding Cr (VI).Fe 2 O 3 particles are very useful in the treatment of wastewater due to their accessible cost and ease of separation.They have been used to effectively remove heavy metals and organic pollutants [12].

Anal. Methods Environ. Chem. J. 7 (1) (2024) 30-48
Biomass adsorbents are widely applied as secret weapons for the sustainable adsorption of environmental toxicants.The green sorbents prepared by biomass are invaluable tools for unlocking the power of waste by reducing the cost of remediation techniques for safe water.This research explores the efficacy of removing Cr (VI) using crosslinked chitosan hydrogel immobilized with magnetic biochars created from peanut shells by pyrolytic technique.Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) analysis, X-ray photoelectron spectroscopy (XPS), and (Brunauer-Emmett-Teller) BET analysis were all collectively used to characterize the generated material.The adsorption property of chitosan-modified magnetic biochar for Cr (VI) in aqueous solution was also studied by examining the effects of pH, kinetics, and sorption isotherms.

Materials
Chitosan powder (85 % acetylation degree) was purchased from Alfa Aesar suppliers in, the USA.(Lot No A2041604).Peanut shells were collected from locally available fields to make pristine biochar.Ferric chloride hexahydrate, Glutaraldehyde (50 % V/V), acetic acid, Sodium hydroxide, potassium dichromate and 1,5 diphenyl carbazide (DPC) were purchased from Fisher Scientific, India.The solution was prepared using double-distilled water, and all of the chemicals utilized in the experiment were of the analytical reagent grade.

Incorporation procedure of Fe 2 O 3 -biochar composite
The collection of peanut shells and synthesis of biochar pyrolytically was referred to in our previously published literature [13], herein, slight modifications were applied to obtain a magnetic biochar composite.According to the standard procedure, the dried peanut shells were crushed into micron-sized particles in a domestic mixer grinder before being sieved via a sieved shaker machine to produce a 70-80 mesh screen.The next step included soaking 25 grams of totally dried and powdered biomass in 80 mL of a 2M FeCl 3 solution.For 0.5 hours, the solution was continuously stirred using a magnetic stirrer at room temperature.After that, the mixture was aged for an additional 0.5 hours at 70 0 C to speed up hydrolysis and Fe 3+ precipitation.After that, the pre-treated biomass sample was placed in an alumina crucible and heated in a vacuum-applied muffle furnace at temperatures of 250, 350, 450, and 550 °C with rises of 10 °C per minute and retention times of 10 minutes at each level while N 2 was continuously pumped in at a rate of 20 mL min -1 .The sample was kept at the same temperature after reaching 550 °C for two hours in an oxygen-free atmosphere.After finishing the process, the furnace's temperature naturally dropped to room temperature.The dried material was then ground and sieved to produce magnetic biochar that had a particle size of less than 0.5 mm.The dedicator held the prepared material.The finished product is known as a PSB/γ-Fe 2 O 3 composite (yield 38.5%).

Preparation of chitosan functionalized Fe 2 O 3 -biochar composite
Chitosan powder (8.0 g) was first dissolved in 2% (v/v) acetic acid (500 mL) to obtain a homogenized viscous chitosan solution under continuous stirring for 2 h at 50 °C.Afterwards, 4 g of PSB/γ-Fe 2 O 3 composite was added to 50 ml of chitosan solution and the compounds were reacted in the ultrasonic bath for 30 min at 40 °C.10 mL glutaraldehyde (25 % in H 2 O) was then injected dropwise very slowly into the reaction system.After 30 min, keeping at 40 °C, NaOH solution (0.1 M) was added dropwise into the mixtures until the pH value reached 9. The solution was then allowed to stand as such in a Petri dish and undisturbed for the next 24 hours.A soft cross-linked three-dimensional network structure of chitosancoated biochar magnetic hydrogel was formed and the solution turned light yellow to brownish yellow.The prepared chitosan hydrogel was then thoroughly washed with milli-Q water several times to remove any unreacted monomer.The final product was named CH@PSB-γ-Fe 2 O 3 composite.The detailed preparation process is shown in Figure 1.

Characterization methods
Standard spectroscopic methods were leveraged to explore the structural peculiarities of the synthesized material.Fourier-transform infrared spectroscopy (FTIR) (Perkin Elmer) with a working range of 4000-400 cm -1 provides verified existence of abundant functional groups and some morphological alteration resulting from the incorporation of chitosan onto the surface of magnetic biochar.Exterior aesthetics and microstructure were analyzed by Scanning electron microscope (Carl Zeiss, Germany, model: Zeiss Gemini 300).The crystalline structure was examined by an X-ray diffractometer (Panalytical XPERT-PRO, UK) with 2ϴ range from 5 0 to 98 0 at a wavelength of 1.54059 A 0 .Energy dispersive X-ray spectrum (EDS) was obtained for elemental compositions as a crucial tool for unravelling materials' composition and structural characteristics, shedding light on their chemical makeup and potential interactions.XPS spectra (Scienta omicron, Germany,) furnished with Al K α X-ray source (1486.7 eV) in ultra-high vacuum were used to determine the surface composition, relative abundance of these compositions and surface chemical structure of the prepared adsorbent.The point of zero charges (PZC) is the location where the total amount of positive and negative charges on the surface of a substance is equalized.The surface exterior gets negatively charged above the PZC and positively charged below it.The surface area and pore size distributions of PSB/γ-Fe 2 O 3 and CH@PSB/γ-Fe 2 O 3 were calculated using the Barrett-Joyner-Halenda (BJH) model and the Brunauer-Emmett-Teller (BET) equation, respectively (quantachrome V5.21 adsorption analyzer).The samples were degassed for 5.3 hours at 200 °C and weighed around 0.0442 mg.To get pore size distributions and BET surface area, BET and BJH interpretations were then applied using the N 2 adsorption-desorption isotherm at 77.35 K.

Chromium Adsorption study through batch system
Chromium (VI) reference solutions (100 mL) of varied quantities were imparted with a predetermined amount of adsorbent in Erlenmeyer flasks of 250 mL for every single batch experiment.Adsorption activity was then assessed at a predetermined contact time at room temperature using an orbital agitator with a 140 rpm shaking speed.Once the solutions had reached equilibrium, they were all collected from the flask, traversed through a micron filter, and the quantity of remanent chromium content in the mixture was quantified using a spectrophotometer (Perkin Elmer, Lambda 850).In the experimental method, calibration standards were meticulously prepared by determining absorbance peak by scanning through wavelength range from 190 to 800 nm for known samples of chromium ranging from 0.2 to 8.0 mg L -1 .DPC (Diphenyl Carbazide) method was used to determine the absorbance peak calorimetrically by the reaction of 1,5 Diphenyl Carbazide with dissolved hexavalent chromium solution to produce red-violet complex in an acidic medium (pH= 2.0 ± 0.5) [14].The Maximum absorbances were measured at 543 nm against the reagent blank, Hence, this wavelength was selected to determine the absorption peak for equilibrium chromium content during all the experiments.The procedure was carried out in standard 10 mm quartz cuvettes in triplicate.The average absorbance was calculated, and a calibration curve was plotted.Validation of the analytical method was performed by finding the linearity range, detection limit (LOD), quantification limit (LOQ), relative standard Fig. 1.Schematic presentation for synthesizing CH@PSB/ γ-Fe 2 O 3 deviation (%RSD) and recovery percentage to verify the absence of interference as per recommendations of IUPAC [15].Batch systems were set in a way to evaluate the impact of contact durations (5, 10, 20, 30, 60, 90, and 120 min), pH (ranging from 2-12), the effect of coexisting ions, ionic strength and reusability of the prepared material.Sorption results for hexavalent chromium onto CH@PSB/γ-Fe 2 O 3 were expressed in terms of dismissal efficacy (%) and uptake capacity (mg g -1 ) using Equations (1 and 2).
(Eq.1) (mg g -1 ) (Eq.2) In the given formula, m represents adsorbent mass (gram), C i and C t signify the initial and residual chromium concentrations (mg L -1 ) observed at time t (min), and V stands for volume (L) in a batch study.

Adsorption isotherm
Adsorption isotherms reflect the correlation between the concentration of a substance in either a gas or liquid phase and the quantity becomes adsorbed onto a solid surface when equilibrium is reached.The Langmuir and Freundlich isotherm models were utilized for calculating removal capacity.These models aid in comprehending the equilibrium connection between the quantity of chromium adsorbed onto the adsorbent at a certain temperature and the initial concentration of chromium in the solution.Isotherm equilibriums were studied for the varied concentrations of 100 mL chromium solution tested for 150 minutes stirring at ambient temperature with an optimum pH level of 4 by introducing 0.5 g adsorbent.The equilibrium concentration of chromium was determined spectrophotometrically.The Langmuir isotherm is often used to describe monolayer adsorption on a homogenous surface.It was adopted for adsorption to occur at specific sites on the adsorbent surface, and each site can adsorb only one sorbate ion [16].The Freundlich isotherm is an empirical model focused on adsorption likely to occur on heterogeneous surfaces with varying adsorption energies [17,18].
Linear from for Langmuir and Freundlich isotherm equations were employed to describe the equilibrium adsorption parameters using Equations (3) and ( 4), respectively: Where q e is the maximum adsorption capacity (mg g -1 ) of the prepared adsorbent, C e is the equilibrium concentration of chromium (VI) (mg L -1 ).K L and K f are Langmuir and Freundlich constants, respectively.

Adsorption kinetic study
The field of kinetic investigation pertains to examining the temporal evolution of chemical reactions or processes.The study utilized various kinetic frameworks to analyze the mechanisms of mass transfer and chemical reactions associated with adsorption.Adsorption occurs when an adsorbate molecule interacts with an active site on the surface of an adsorbent molecule.The study employed to estimate pseudo-first and pseudo-second-order models.Through examining adsorption kinetics, valuable insights associated with the efficacy, ideal circumstances, and fundamental mechanisms of the aforementioned process were explored [19].To comprehend the process of sorption on CH@PSB/ γ-Fe 2 O 3 , the experimental data was subjected to fitting using Lagergren's pseudo-first and pseudo-second order kinetics [20].The Concentration of collected Chromium (VI) mixed with adsorbent material was determined spectrophotometrically at different time intervals of 2-120 min.The adsorption capacities provided at different times served as input for pseudofirst and pseudo-second order kinetics expressed as Equations ( 5) and ( 6), respectively: (Eq.5) (Eq.6) Where, and are adsorbed quantity (mg g -1 ) of chromium (VI) at equilibrium and at time t, respectively.K 1 (min -1 ) and K 2 (g mg -1 min -1 ) are adsorption rate constants for pseudo-first and pseudosecond kinetics.Traditional linear plots will be obtained for both the mechanism and to predict the reliability of the study, A linear regression algorithm was applied for optimization analysis.

Linear Regression analysis
The most frequent statistical technique for evaluating the model's validity is the linear regression coefficient.It statically measures the average functional relationship between variables.Also, it measures the degree of dependence of one variable on the other(s).It can be represented as blow Equations 7 and 8.
(Eq.7) (Fig. 8a) (Fig. 8b) (Fig. 8c) Where, q i and q model, i have experimentally obtained data and isotherm model prediction, respectively.For ith data point where n in the number of experimental data point.

Regeneration of adsorbent material
To evaluate the reusability of the synthesized adsorbent, six repeated cycles of sorption-desorption assays were tested.100 mg of adsorbent was added to 100 ml Cr (VI) (100 mg L -1 ) for agitating at 140 RPM at 27 o C for 12 hrs.After each agitation, adsorbent particles were separated from the suspension by a micron filter and washed thoroughly with desorption reagent (0.1 HNO 3 ) followed by double deionized water and then dried at 80 o C. The regenerated adsorbent was reused for the next cycle of the sorption process.

Influence of co-existing ions
The adsorption effectiveness may be considerably impacted by the presence of other ions in the solution.Since the ultimate goal is the treatment of industrial effluents, which will most probably contain other ions (like Cl -, NO 3 -, SO 4 2-, Na + , Mg 2+ etc.) along with heavy metal ions, an efficient adsorbent is expected to retain its high adsorption capacity even in the presence of other ions.For determining the effect of the presence of other ions, the adsorption efficiency with the prepared adsorbent was checked at two different concentrations (1 ppm and 10 ppm) of NaCl, and NaNO 3 .Na 2 SO 4 , Na 2 (PO 4 ) 3 along with chromium ions.113), (024), (116), and (214) crystal planes respectively, are the characteristics for Fe 2 O 3 deposition on the PSB surface [21].However, no apparent differences were observed for PSB/γ-Fe 2 O 3 and CH@PSB/ γ-Fe 2 O 3 (Fig. 2c).The resulting XRD patterns were analyzed with peak shifting and lower peak intensity suggesting reduced crystallinity in CH@PSB/γ-Fe 2 O 3 due to interaction of chitosan polymer.The weaker diffraction signals compared to the well-ordered crystalline for PSB/γ-Fe 2 O 3 composite are due to the coating of chitosan on the Fe 2 O 3 -biochar surface.Chitosan typically contains amorphous regions, which might contribute to low intense diffraction peaks in the diffractograms.These amorphous regions could mask or dampen the intensity of the crystalline peaks from the magnetic biochar.Additionally, the peak shifts indicate alterations in the lattice parameters or crystallographic arrangement caused by the introduction of chitosan.These results suggest that chitosan was successfully immobilized with PSB/γ-Fe 2 O 3 surface.FT-IR spectra for the pristine chitosan, PSB/γ-Fe 2 O 3, and CH@PSB/ γ-Fe 2 O 3 are shown in Chromium desalinization by chitosan-Fe2O3-biochar composite Anurag Choudhary et al Figure 3.The absorption band at 3200∼3600 cm -1 is attributed to the -OH and N-H groups stretching vibration present in chitosan (Fig. 3a) but the intensity of these bands reduced after coating due to interaction of these functional groups with biochar surface (Fig. 3c).The band at 2931 and 2850 cm -1 (Fig. 3c) was assigned for starching vibration of aliphatic CH bond present in magnetic biochar [22].The appearance of characteristic peaks around 1650-1550 cm -1 , attributed to the amide I and II bands corresponding to N-H bending and C-N stretching vibrations and the presence of a band at around 1027 cm -1 related to the C-O stretching vibration are strong evidence of successful chitosan coating on the magnetic biochar surface.These peaks are specific to chitosan's polysaccharide structure and are not typically observed in the FTIR spectra of magnetic biochar.The peak at 1365 cm -1 is often attributed to the bending vibration of the methyl (CH 3 ) groups.It indicates the presence of methyl groups in the composite, which could be associated with either the chitosan component or the magnetic biochar.The peak at 1407 cm -1 might be due to the presence of stretching vibration of the C-O bond in the -COOH group.In addition, a peak at 1653 cm -1 is related to the amide I band, which arises from the C=O stretching vibration of the amide (CONH) bonds in chitosan.It confirms the presence of chitosan in the coated biochar sample.The peak at 1605cm -1 in Fe 2 O 3 -biochar composite (Fig. 3b) was attributed to vibrations involving metal-oxygen bonds which disappeared after reacting with chitosan suggesting a significant change in the composition or structure of the material due to the chemical interaction with composite and surface coverage.The detection of the aforementioned peaks confirms the successful incorporation of chitosan onto the biochar surface, validating the formation of the chitosan-magnetic biochar composite.Additionally, the disappearance of peaks at 1064 and 1150 cm -1 in chitosan spectra (Fig. 3a) reveals that coating onto the magnetic biochar surface suggests significant This indicates the presence of Fe 3 O 4 particles in the prepared sample [23].Notably, the functional groups of CH@PSB/ γ-Fe 2 O 3 were different from those of pristine chitosan and magnetic biochar, which impacted its subsequent adsorption ability.Magnetic biochar made from ferric chloride has distinctive characteristics in its surface morphology.As shown in Figure 4 (b), it generally has a porous structure with a variety of pore sizes and internal mesopore distributions, which increases its surface area and adsorption capacity.The breakdown of lignocellulosic material at high temperatures was linked to the production of a sequence of cracks on the surface of biochar, which caused volatile chemicals to evaporate from the newly created pores.Some shimmering developed on the surface of PSB/ γ-Fe 2 O 3 , indicating the existence of thick surface layering particles, which are composed of magnetic nanoparticles inserted during manufacturing.These magnificent particles might be incorporated into the porous surface of the biochar or scattered throughout it.Due to the carbonization process and the development of cracks and fissures, the surface seems uneven and rough.Figure 4 (d, e, and f) shows that chitosan coating on magnetic biochar may result in a reduction in pore volume or quantity when compared to uncoated magnetic biochar.This is mainly because the magnetic biochar's surface develops a layer of chitosan coating, which partly fills in the pores already present and decreases the volume of those pores.However, CH@PSB/γ-Fe 2 O 3 had a higher sorption capacity than PSB/γ-Fe 2 O 3 , indicating that the adsorption process was predominantly dependent on the functional groups rather than the surface pore structure.The results for BET surface analysis are shown in Table 1 the results were compared with BET analysis of pristine biochar made from peanut shells with Fe 2 O 3 and crosslinked chitosan-loaded samples showed a significant reduction in surface area as well as pore volume and pore size.The reduction is due to the aggregation of Fe 2 O 3 nanoparticles and chitosan on to surface.While the reduction in surface area, pore size, and volume might seem unfavourable, the addition of chitosan onto the magnetic biochar surface might enhance specific functionalities of the material's behavior for targeted adsorption.

Characterization of Prepared Composite
With the intent of deeper insight into the surface chemical and elemental composition of the prepared adsorbent, XPS survey spectra were recorded and analyzed according to   6c] finds two deconvoluted peaks with binding energies at 531.59 (eV) and 529.24 (eV) corresponding to C=O and C-O functional groups, respectively.The high-resolution XPS spectra of Fe 2p of CH@PSB/γ-Fe 2 O 3 exhibited four peaks: 723.77 eV could be assigned for Fe 2P 1/2 ; 709.71 for Fe 2P 3/2 ; and peaks corresponding to binding energies at 718.02 and 713.71 may be associated with shake-up or satellite peaks which are characteristics of Fe in Fe 2 O 3 .The XPS results of chitosan-coated magnetic biochar indicated the potential appearance of the iron oxides, reduced iron species, interacting with chitosan polymer.Figure 7a illustrates the zero point charge of CH@PSB/γ-Fe 2 O 3 is located at 4.5 pH.As a result of the protonation of the amine group in chitosan, the adsorbent's surface acquires a positive charge below this pH, which is favorable for the adsorption of negatively charged entities.

Adsorption studies for chromium 3.2.1.Consequences of changing solution pH
The adsorption of chromium by CH@PSB/γ-Fe 2 O 3 can be significantly influenced by the pH of the solution.When pH is less than 6.8, HCrO 4 -becomes major speciation but at pH more than 7.2, Cr (VI) ions can form negatively charged oxyanions such as chromate (CrO 4 2-) and dichromate(Cr 2 O 7 2-).The point of zero charge for CH@PSB/γ-Fe 2 O 3 was located at 4.5 pH (Fig. 7a).Adsorption studies are typically conducted at varied ranges of pH to determine the optimal conditions for efficient chromium removal.Sorption capacity for Cr (VI) notably declined from 85.2 mg g -1 to 24.6 mg g -1 with the change in pH value from 2 to 8 as depicted in Figure 7b.Sorption performance for the produced composites was found to be boosted at lower pH value, mainly due to enhanced electrostatic attraction created by protonated amino functional groups on chitosan.At higher pH, the negatively charged surface of adsorbent material impeded chromium sorption due to electrostatic repulsion among (CrO 4 2-) and OH -. Figure 8 illustrates the process of chromium adsorption by sorbent material.

3.2.2.Influence of Ionic Strength on Adsorption
The sorption behaviour of chromium by CH@PSB/γ-Fe 2 O 3 was investigated in the presence of a varied concentration of NaNO 3 and results were exhibited as decreasing adsorption capacity from 83.6 to 30.1 mg L -1 (Fig. 9).With increasing NaNO 3 strength from 0.05 M to 2M maintaining pH at 4.0, suggesting supersession in Cr (VI) sorption at elevated ionic strength.This was primarily due to the competitive sorption of nitrate ions onto CH@PSB/γ-Fe 2 O 3 .

Adsorption Equilibrium Analysis
The adsorption capacity for the prepared material was evaluated using the two most universally adopted Langmuir and Freundlich isotherm models.The pertinent reports are provided in Table 2 and Figure 10 (a and b) are illustrated for validity of the models.The linear fitting of the observed data based on origin pro 8.5 software revealed that the performance of CH@PSB/γ-Fe 2 O 3 was better fitted by the Langmuir equation with a correlation factor (R 2 ) of 0.9998.These findings demonstrated the monolayer adsorption process and Equation 3 was used to determine the maximum adsorption capacity (Q max ) as 14.285 mg g -1 .

Adsorption kinetics
Sorption rate is another consideration for studying adsorption capacity associated with contact duration.The sorption kinetic was investigated for 0 to 90 min of contact time.As shown in Figure 11 sorption occurred rapidly for an initial 20 min of contact and then this trend slowly grew until it reached a plateau.Sorption equilibrium attained in just 40 min.
The elevated abundance of adsorption surfaces may be assigned for the speedy progress in the initial adsorption rate of Cr (VI) [24].Adsorption rates accordingly decreased as the number of accessible adsorption sites decreased over time.As a result, 40 minutes was shown to be an ideal contact time for the adsorption process.The sorption kinetics for pseudo-first and pseudo-second-order models were applied for chromium adsorption onto CH@PSB/γ-Fe 2 O 3 and corresponding results were evaluated in Figure 12 (a and b) and Table 2.The experimental findings of calculated parameters revealed that the data were best signified by the pseudo-second-order model (Table 2), with a correlation coefficient (R 2 ) of 0.9994 with an equilibrium sorption capacity of 83.33 mg g -1 .Kinetics consideration is suggested for surface adsorption and chemisorption mechanism in the ratecontrolling step.

Reutilization of the adsorbent
Six consecutive iterations of sorption-desorption were tested for synthesized adsorbent to execute reusability.As demonstrated in Figure 13, removal efficiency gradually declined with every next cycle, and it dropped from 85 % to 26 % in the sixth round.The material has excellent reusability for up to four cycles.Overall, the results of these recycled trials provide compelling evidence that the prepared CH@PSB/γ-Fe 2 O 3 demonstrated excellent reusability and holds significant promise for practical applications.

Influence of coexisting ions on removal efficiency for real water samples
Drinking water can contain multiple ions as water sources often come into contact with geological formations, industrial activities, or other environmental factors.The synchronous presence of other ions can significantly influence the removal of chromium during water treatment.The interactions between these coexisting ions and chromium can affect the adsorption capacity, speciation, and overall efficiency of the removal process.Experiments were therefore carried out to learn how coexisting ions affect chromium removal in the presence of four ions such as Cl -, SO 4 2-, NO 3 -, and PO 4 2-(for 1 ppm or 1 mgL -1 and 10 ppm concentrations).The experimental observations (Fig. 14) suggesting for consistent efficiency at approximately 80 % chromium removal by CH@PSB/γ-Fe 2 O 3 at 1 ppm solution.Except for PO 4 2-removal efficiency by coexisting ions was only minimally hampered.It is because of competitive interactions of anionic species with functional groups on modified chitosan.However, there is a weaker interaction for Cl -, SO 4 2-, NO 3 -but due to the higher affinity of PO 4 2-ion with iron oxide its effect is high.In addition, as the concentration of coexisting ions increased to 10 ppm, this influence was higher.

Conclusion
In this study, a new adsorbent material was fabricated by immobilizing chitosan-coated iron oxide biochar for the effective desalinization of chromium ions from aqueous solution.Various spectroscopic studies confirmed the synthesis of a composite material comprising Fe 2 O 3 -biochar coated further with crosslinked chitosan.This hybrid material combines the magnetic properties of Fe 2 O 3 with the porous structure of biochar, enhanced by the presence of chitosan.The successive coating of chitosan onto the Fe 2 O 3 -biochar surface adds a layer of complexity to the material's composition and properties, potentially impacting its surface area, porosity, and adsorption capabilities.This synthesized material holds promise for applications in the adsorption of chromium ions and the spectroscopic method used in the determination of chromium showed a LOD of 0.65 μg L -1 with a recovery percentage of more than 92 %.The optimized pH for the highest removal capacity (0.4 ± 84.9 mg g -1 ) is in the range of 3-5 pH.Additionally, there is a slight decrease in capacity with an increase in ionic strength.Langmuir isotherm equation fits well with adsorption data and the obtained removal capacity for CH@PSB/γ-Fe 2 O 3 exhibits 14.285 mg g -1 in batch system.The kinetic study suggests that pseudo-second-order is the best description of chromium adsorption onto CH@ PSB/γ-Fe 2 O 3 suggesting a chemisorption pathway.The a slight decrease in removal efficiency in the presence of some other anions except for PO 4 2-ion.A reusability assay indicates for excellent utility of the prepared material for toxicant removal and more than 50 % sorption capacity is retained for up to four cycles.In conclusion, the synthesized biosorbent is a novel green material for effective holds significant potential for wide-ranging applications in the field of Cr (VI) removal.

Acknowledge
Sardar Singh Poonia want to express his sincere gratitude towards CSIR (award no:09/098(0142)/2019-EMR-I) for providing support and financial aids in research.He would like to give special thanks to Head, Department of Chemistry for their exceptional support in providing the necessary resources.

Table 1 .
BET and BJH analysis results for prepared samples