The Remediation of Hexavalent Chromium Using Orange Peel Biosorbent from Industrial Effluents

Muhammad Kashif Channa ¹, Muhammad Waris ^2,*, Sultana Rahman ³, Ali Bahar Shahani ⁴

1. National Centre of Excellence in Analytical Chemistry, University of Sindh Jamshoro Sindh, Jamshoro, Pakistan

2. Department of Environmental Science, Aror University of Art, Architecture, Design and Heritage, Sukkur Sindh Pakistan

3. Department of Chemistry, Quaid-I-Azam University, Islamabad, Pakistan

4. Department of chemistry, Shah Abdul Latif University, Khairpur, Pakistan

* Correspondence: Muhammad Waris

Academic Editor: Mohammad Boshir Ahmed

Special Issue: Advanced Materials and Technologies for Pollutants Removal and Environmental Remediation

Received: February 28, 2024 | Accepted: December 12, 2024 | Published: February 09, 2025

Adv Environ Eng Res 2025, Volume 6, Issue 1, doi:10.21926/aeer.2501011

Recommended citation: Channa MK, Waris M, Rahman S, Shahani AB. The Remediation of Hexavalent Chromium Using Orange Peel Biosorbent from Industrial Effluents. Adv Environ Eng Res 2025; 6(1): 011; doi:10.21926/aeer.2501011.

© 2025 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

Natural water often contains chromium in the oxidation states of trivalent (Cr^III) and hexavalent (Cr^VI). Chromium in the oxidation states of trivalent (Cr^III) and hexavalent (Cr^VI) is frequently found in natural water. These can vary greatly from one another because of their unique biological, geochemical, and toxicological characteristics [1]. Cr^III compounds are sparingly soluble in water, while Cr^VI compounds are readily soluble to enhance the levels of Cr^VI in water sources. Humans can be exposed to Cr, when eating food, drinking water and inhaling air [2]. The average daily intake of total Cr from air, water, and food is estimated to be 0.01-0.03 mg, 2.0 mg L^-1, and 60 mg, respectively. Trivalent chromium (Cr^III) is an essential mineral for maintaining optimal glucose tolerance, lipid and protein metabolism, and other human metabolic functions [3]. The most hazardous type of Cr, however, is Cr^VI, due to it has strong oxidation potential, which allows it to permeate biological cell membranes and damage macromolecules, proteins, and DNA, it has some carcinogenic consequences [4,5]. The Cr^VI is damaging to the lungs, liver, and kidneys and inhibiting the cell's enzymatic uptake of sulphur. Chronic ulceration, dermatitis, a corrosive response in the nasal septum, and local lung damage are the main toxic consequences of Cr^VI [6].

The metallurgical industry primarily uses the Cr^VI compounds for chrome plating and chrome alloy. In the chemical industry, it serves as an oxidizing agent in the form of various Cr compounds. Different compounds of Cr are also used for tanning leather. As a result, about 40% of used Cr is discharged in the effluents as Cr^VI [7]. The compounds Cr^VI were also used in textile dyeing, ceramics, glass and photography. About 90% of chromium produced from chrome ore was used in metallurgical industries for steel, alloy, and nonferrous alloy production [8]. Whereas the remaining extracted chrome was used for the rest of the industrial activities. The untreated industrial and municipal wastes have created multiple environmental problems for mankind. Effluent discharge from the industries is a potential source of aquatic environmental pollution [9]. Industrial effluents can be characterized by different physical properties like dark colour, pH, suspended solids (SS), chemical oxygen demand (COD) and high biochemical oxygen demand (BOD). Elevated level of COD indicates the recalcitrance of discharged chemicals [10]. The chemicals present in water bodies may lead to bioaccumulation in both plant and animal’s systems [10,11]. Therefore, the gradual monitoring and assessment of Cr and its different forms in industrial effluents is very important [6,12]. Ion exchange characteristics can be found in various natural and manmade goods. The most significant ion exchangers are, by far, the organic resins. In order to remove Cr^VI from synthetic wastewater, anion exchange resins are utilized. High chemical and mechanical stability, high ion exchange capacity, and high ion exchange rate are the key benefits of employing these resins [13]. Physical-chemical techniques such as ion exchange, precipitation, oxidation, and reverse osmosis have been used to remove Cr^VI ions from industrial effluents [13,14]. By reducing the Cr^VI at an acidic pH and employing the proper lowering agents, the Cr^VI has traditionally been removed from solutions [5,15]. However, such technologies are expensive, and a large amount of sludge disposal may cause environmental problems. The key factors for implementing an effective remediation method are the cost and availability of adsorbent as well as the convenience of the treatment process.

Using raw orange peel, the adsorptions of numerous metal ions, including Pb(II), Ni(II), Zn(II), Cu(II), and Co(II), were investigated. When the orange peel is treated with nitric acid, the adsorption of these metals is 7.75, 6.01, 5.25, 3.65, and 1.82 mg/g, respectively [16]. The current study set out to use the natural biosorbent "orange peel" to remove Cr^VI from industrial effluents. Utilizing orange peels with modifications to increase removal capacity will promote the process of treating industrial wastewater.

2. Materials and Methods

2.1 Sampling and Pretreatment

2.1.1 Industrial Effluent

Industrial effluents from several businesses in Jamshoro and Hyderabad were sampled at random. Based on their characteristics, the industries are grouped into four groups: the plastic industry (PI), the chemical industry (CI), the textile and apparel industry (TFI), and the food sector (FI). Each of the two or three industries that were selected for each category was represented by four samples (n = 40). In 1 L polyethylene plastic bottles, samples of industrial wastewater were collected. Half a portion was used for the BOD test using 500 ml size microbiological examination bottles, and the remaining portion was used for physical examination and speciation of chromium. The four quality indicators, such as pH, total dissolved solids (TDS), biochemical oxygen demand (BOD), and electric conductivity (EC), were then assessed for each sample.

2.1.2 Biosorbent Collection and Pre-Treatment

The orange fruits were bought from a market in Hyderabad, Sindh, Pakistan, and the peel and pulp were separated. Following a series of washes with tap, distilled, and deionized water, the separated peels were allowed to air dry for a week. The peel was then pulverized and heated for two hours in an electric oven at 343 K. For the preparation of orange peel biosorbent (OPB), the dried material was ground and sieved in an electrical sieve shaker of the Ro-Tap type, and the US mesh No. 200 was used to collect the particles. Particles smaller than 100 µm were produced by sieving for the bio-sorption process. For future use, the sieved OPB was kept in a vacuum desiccator.

2.1.3 Chemicals and Reagents

The experiment used deionized water from an ELGA laboratory water system (Bucks, UK) with a resistance of 0.05 µs cm^-1. Analytical reagent-grade HNO₃ (purity 65%, Sp.gr: 1.41) and H₂O₂ (30%, Sp.gr: 1.11) were purchased from Merck (Darmstadt, Germany). Potassium dichromate (K₂Cr₂O₇; Sigma Chemical, MO, USA) (0.28 g) was dissolved in 1.0 L of 1.5% HNO₃ to prepare the 1000 mg L^-1 stock solution of Cr^VI. The certified standard solution was diluted to create the Cr standard solutions (1000 mg L^-1), Fluka (Buchs, Switzerland). The proper amount of 0.1 mol L^-1 HCl/NaOH solutions was added to adjust the solution's pH from 2 to 9. A pH meter was used to calibrate the pH of the solution. Acetone (Merck) was utilized directly without further purification. A minimal amount of acetone was used to dissolve 0.25 g of 1,5-diphenylcarbazide (Merk) and dilute it to 100 mL. The suitable dilution of concentrated sulfuric acid prepared sulfuric acid of the desired concentration. Before use, all glass and polyethylene bottles underwent a thorough cleaning, an overnight soak in 5 mol L^-1 HNO₃, and a thorough rinsing with distilled and deionized water.

2.2 Apparatus

A pH meter was used to determine the samples' pH levels (781-pH meter, Metrohm). WIROWKA Laboratoryjna type WE-1, nr-6933 centrifuge (MechanikaPhecyzyjna, Poland) was used for centrifugation with speed ranging between 0-6000 rpm and timer 0-60 min. The samples were shaken using a flask electric shaker (Gallenkamp, England) 220/60 HZ. An agate ball mixer mill (MM-2000 Haan, Germany) ground the dry samples. For the quantitative examination of total Cr, inorganic Cr, and their species, a Perkin Elmer atomic absorption spectrometer, Model A Analyst 700 USA, with a deuterium lamp back corrector, pyro-coated graphite tubes with an integrated platform, and autosampler 800 is utilized [17]. The measuring condition of micro sample injection system-flame atomic absorption spectrometry (MIS-FAAS) and graphite furnace atomic absorption spectrometry (GFAAS) for Cr^VI and total Cr were given in Table 1. The biosorption capability of the biosorbent was evaluated using an FTIR spectrometer and a scanning electron microscope (SEM) (JEOL model JSM-6380 and Thermo Nicolet Analytical Instruments, Madison, WI).

Table 1 Measurement conditions for Cr by atomic absorption spectrometry (Hitachi 180-50 Japan).

2.3 Determination of Cr^VI

The final volume was kept at 100 mL by adding 3.0 mL of 0.25% diphenylcarbazide after mixing 1.0 mL of a 10 mg L^-1 solution of Cr^VI with 3.0 mL of 0.5 mol L^-1 H₂SO₄. 2 mL min^-1 of flow rate was kept up as the sample solution was placed onto the OPB-containing column. The absorbance of the resultant solution from the column demonstrated that the reddish violet complex was adsorbed onto the column. A subsequent step involved eluting the adsorbed compound with 15.0 mL of 2.5 M H₂SO₄ in acetone. To remove interference from organic matrices, the final solutions were evaporated to dryness at 80°C. The residues were diluted with 1.0 ml of 0.2 M HNO₃ solution. The concentration of Cr^VI was determined by MIS-FAAS.

2.3.1 Determination of Total Chromium

For total Cr, 100 mL of industrial effluent samples were filtered by Whatman No. 42 filter paper and pre-concentrated >25 mL at 70°C on an electric hot plate. Then, the resulting solution was kept at 4°C till further analysis, as described in previous work [18]. Analytical blanks were prepared similarly to samples but without the addition of a sample in order to determine the detection limit.

2.3.2 Remediation of Cr Species

Batch equilibrium sorption experiments were carried out in 250 ml conical flasks with 100 ml of solution of Cr species with 0.001 g of biomass for 2 h. These experiments were done by varying concentrations of adsorbate in solution and then changing the adsorbent dose (0.05, 0.10, 0.25, and 0.50 g L^-1). Cr^VI different concentrations in solutions were 100, 200, 300, 400 and 500 µg L^-1. After optimizing pH, the concentration of adsorbate, and biosorbent dose, further experiments were done by varying shaking time (5, 10, 20, 30, 40, 50, and 60 min) and temperature (298, 318, 333, 348, and 363 K). The solutions were separated from the biomass by filtering the solutions via the Whatman No. 42 filter paper. MIS-FAAS determined the initial and final Cr^VI concentrations in each flask. Finally, the approach was used on actual field samples of industrial effluents from Hyderabad and Jamshoro, Pakistan, at these optimal circumstances. At the appropriate conditions, each experiment was carried out three times.

OPB sample pellets were prepared for FTIR spectroscopic analysis by combining 1:3 OPB and potassium bromide (KBr), which were then pressed into a 13-mm in diameter for 5 minutes. OPB sample pellets were prepared for FTIR spectroscopic analysis by combining 1:3 OPB and potassium bromide (KBr), which were then pressed into a 13-mm diameter for 5 minutes. Before and after Cr^VI loading, from 4000 to 400 cm^-1 of OPB, the pellets were scanned. OPB unloaded and loaded with Using an electric oven at 105°C for one hour, Cr^VI was fully dried, and samples were placed on the holders or stubs using double-sided conductive tapes for scanning electron microscopy (SEM) analysis. To eliminate the charge effect, the samples were coated in graphite. SEM analysis of the coated samples was performed using various accelerating voltages and magnifications.

2.3.3 Biosorption Capacity

Equation (1) was used to determine the number of sorbates that were adsorbed (in µg or mg) per unit mass of biosorbent (q_e):

\[ q_e=\frac{(C_i-C_e)}{m}V \tag{1} \]

Where "m" is the dry biomass of peel employed as a biosorbent, "q_e" is the biosorption capacity, C_i and C_e are the initial and final concentrations of the sorbate (mg L^-1), and V is the volume of the solution (L).

3. Results and Discussion

Figure 1 a, b, c provides the findings of the physico-chemical characteristics of the composite industrial effluent samples from four distinct industries, including pH, TDS, EC, and BOD. FI, PI, CI, and TFI effluent samples were found to have pH values between 7.00 and 8.60, 6.60 to 6.80, 6.70 to 7.90, and 6.10 to 6.60, respectively. Because various chemicals are used in the cleaning, washing, and polymerization/dying processes in the PI and TFI industries, their pH values are slightly acidic [10]. The pH of the effluent was mildly acidic (5.96 ± 0.09), though higher values (7.16 ± 0.02) were noted. According to Pei et al. [5], the alkaline pH of natural water may affect fish, causing harm to their outside surfaces like their gills, eyes, and skin, as well as making it difficult for them to get rid of metabolic waste. Additionally, the elevated pH of industrial effluents may make other compounds more harmful [5,19].

Figure 1 Physico-chemical parameters of the industrial effluent samples from distinct industries.

TDS levels in effluent samples from FI, PI, CI, and TFI ranged from 951 to 1820, 1348 to 1382, 1345 to 1385, and 450 to 1322, respectively. While the EC of the FI, PI, CI, and TFI effluent samples under study ranged from 0.87 to 4.01, 3.0 to 3.6, 3.00 to 3.60, and 0.4 to 5.3 mS cm^-1, respectively. While the EC of the FI, PI, CI, and TFI effluent samples under study ranged from 7.87 to 8.01, 8.0 to 8.6, 6.00 to 7.60, and 8.4 to 9.3 mS cm^-1, respectively. Based on the concentration, degree of ion dissociation, and temperature, the EC gives information about the dissociated and dissolved chemicals [20]. The relative EC ranges for the effluents of FI, PI, CI, and TFI were 0.87-4.01, 3.00-3.61, 2.88-3.72, and 0.40-5.30 mS cm^-1. It was discovered that the EC of the industrial effluent types under study exceeded the US-allowed EPA's limits (1.00 mS cm^-1). It is due to the spilling of oil, grease, ammonia, sulphide, dyes, and waste effluents from TFI, which may result in a decline in the quality of drinking water [5].

The mean BOD values were determined to be 475 ± 61.4, 575 ± 7.10, 565 ± 7.10, and 580 ± 20.0 mg L^-1 in the effluent samples from FI, PI, CI, and TFI, respectively. This might be due to different chemicals used in textile industries for mercerizing, bleaching/scouring, and yawing which include sodium chlorite, sodium sulphite, NaCl, sodium bisulphate, sodium bicarbonate, formic acid, H₂SO₄, vegetable tannins, syntans, resins, sodium formate, polyurethane, fat emulsions, dyes, binders, waxes, pigments, lacquers/formaldehyde, NaOH, sodium phosphate sodium hypochlorite, acids, H₂O₂, surfactants, Cl₂, and NaSiO₃ [21,22].

The total Cr contents in effluents of FI, PI, CI and TFI varied in the range of 30.0-38.0, 23.0-27.0, 35.0-95.0 and 40.0-180 mg L^-1, respectively. All of the analyzed industries' effluent samples had total Cr concentrations that exceeded the US EPA threshold of 0.10 mg L^-1 for total Cr. The total Cr contents showed significantly positive correlation with pH, TDS and EC in effluent samples of FI (r > 0.60; p < 0.20), whilst strongly negative correlation was observed between total Cr and BOD (r= -0.61; p = 0.20). The total Cr contents in effluent samples of PI were reversely correlated with pH and TDS (r > -0.90; p > 0.05), whereas, it has a significant correlation with EC and BOD (r > 0.80; p < 0.10). However, total Cr levels have a positive correlation with pH and TDS (r > 0.60; p < 0.20) and a negative correlation with EC and BOD (r > -0.60; p > 0.20).

3.1 Removal of Cr^IV from Industrial Effluents by Orange Peel Biosorbent

3.1.1 FTIR Study of OPB

FTIR spectroscopy is one of the most crucial methods for determining the main functional groups contained in compounds [21,23]. By comparing the FTIR spectra of OPB before and after Cr^VI adsorption, it was feasible to determine the interactions between the functional groups of the orange peel biosorbent (OPB) and Cr^VI ions. The unloaded OPB's resulting spectra revealed a broad and powerful peak at 3100-3600 cm^-1 caused by the overlap of stretching vibrations for -OH and -NH₂ (Figure 2). The peak at 2924 cm^-1 indicates the methyl and methoxy groups' -CH stretching vibration. The peak at 2924 cm^-1 indicates the methyl and methoxy groups' -CH stretching vibration. A peak at 1741 cm^-1 is due to carboxylic acid/esters stretching in the -C=O direction. A peak for the -NH₂ bending vibration can be found at 1641 cm^-1. The -CH₃ symmetrical deformation mode (scissoring) in the amide group is characterized by a peak at 1381 cm^-1. The -C-N vibration is indicated by the band at 1246 cm^-1. The stretching vibrations of carboxylic acids, alcohols, ether, and/or N-H deformation of amines can be attributed to the peaks between 1300 and 1000 cm^-1 [24].

Figure 2 FTIR spectra of unloaded OPB (black line) and Cr^VI loaded OPB (red line).

Peaks in the FTIR spectra of Cr^VI-loaded OPB are deformed and shifted (Figure 1). At wave number 3446 cm^-1, which was changed to wave number 3410.8 cm^-1, a significant difference was seen in the stretching vibration of OH and NH₂. Additionally, the -NH₂ bending vibration at 1643.7 cm^-1 wave number was moved to higher frequencies at 1643.9 cm^-1 (Figure 2). The C-N functional group's FTIR peak moved from 1242 to 1271. In contrast, the peak of -CO was moved from 1069.8 to 1101.8 cm^-1 by the adsorption of Cr^VI. Consequently, using FT-IR spectrum analysis, it can be concluded that interactions between biomasses' amine and -CO functional groups allow Cr^VI to bind to OPB (the peel of Citrus aurantium).

3.1.2 SEM Study OPB

Scanning electron microscopy (SEM) was used to examine the surface morphologies of OPB with Cr^VI that had been loaded and unloaded and the resulting micrograph is given in Figure 3 a, b. The micrograph (Figure 3a) indicates cavities, pores, and more rough surfaces with several large clusters of mesopores spotted. Due to the high amount of biosorption-ready surface area, this may be the cause of the high biosorption capacity. After Cr^VI loading, the change in morphology of OPB that Cr^VI is bio-sorbed onto the OPB (Figure 3b).

Figure 3 SEM micrographs of (a) unloaded OPB and (b) Cr^VI loaded OPB.

3.2 Optimization of Different Parameters for the Removal of Cr^VI

3.2.1 Effect of pH

Due to the importance of pH during the biosorption experiment, the impact of pH on the biosorption of Cr^VI on OPB was also examined [19,25]. The interaction between the analyte of interest and the binding sites on the sorbent surface in an aqueous solution was significantly impacted by the pH variation. Between pH 2 and 9, at a given concentration of Cr^VI (0.25 mg L^-1), we looked at the effects of pH, contact time (20 minutes), biosorbent dosage (0.01 g L^-1), and temperature on the binding of Cr^VI on the surface of OPB (298 K) as shown in Figure 4. In the pH range of 2 to 7, it was discovered that the biosorption was more than 94%. This is because, Cr^VI exists in dichromate, form either in K₂Cr₂O₇ or anionic Cr₂O₇^-2 at pH 2.0 to 7.0. However, the maximum biosorption (q_e max = 47.0-48.0 mg g^-1) was achieved at pH 5.0-7.0. Thus, pH 7.0 was selected for further experiments.

Figure 4 Effect of pH of the biosorption of Cr^VI by OPB at Cr^VI concentration 0.25 mg L^-1; contact time 20 minutes; adsorbent dosage 0.1 g L^-1 and temperature 298 K.

3.2.2 Effect of Concentration

The effect of Cr^VI concentration on the investigation of the quantitative biosorption, with all parameters at constant values from 0.05-1.0 mg L^-1 (Figure 5). According to the results, the % biosorption of Cr^VI was steady >0.25 mg L^-1, but after that point, a gradual decline in the % biosorption was observed (Figure 5). This might be due to the limited active sites of the biosorbent for the removal of Cr^VI. As a result, 0.25 mg L^-1 of Cr^VI concentration was selected as the quantitative concentration for additional tests.

Figure 5 Effect of Cr^VI concentration (mg L^-1) by OPB at contact time 20 minutes; adsorbent dosage 0.1 g L^-1; temperature 298 K and pH 7.0.

3.2.3 Effect of Dosage

When the Cr^VI concentration was 0.25 mg L^-1, the contact time was 20 minutes, the temperature was 298 K, and the pH was 7.0, the impact of biosorbent (OPB) dosage on the % of Cr^VI biosorption (Figure 6). Gradually increasing the OPB dosage from 0.05 to 0.10 g L^-1 (Figure 6) has increased in the % biosorption (50.0 to 95.5%). With the increased dosage of OPB, there was no discernible increase in % biosorption. As a result, the dosage of 0.10 g L^-1 OPB has been chosen for the quantitative elimination of Cr^VI.

Figure 6 Effect of dosage of biosorbent for the removal of Cr^VI by OPB at Cr^VI concentration 0.25 mg L^-1; contact time 20 minutes; temperature 298 K and pH 7.0.

3.2.4 Effect of Contact Time and Temperature

The effect of contact time for the quantitative % biosorption of Cr^VI has been checked from 5.0 to 60 min (Figure 7). At the initial stage, the biosorption rate of Cr^VI was very rapid as over 80% of Cr^VI was absorbed at both 5.0 and 10 min for OPB. Later on, near the 20-minute equilibrium point, it started to slow down the percent of biosorption. Following that, there was no discernible change in the % of Cr^VI biosorption. In order to determine the quantitative % biosorption under equilibrium conditions, 20 min contact duration has been chosen. Similar research has been done on the impact of temperature for the quantitative % biosorption of Cr^VI from 298 to 363 K at the optimal values of other parameters (Figure 7). According to reports, when the temperature rises, the % of Cr^VI absorbed by the OPB will gradually decline (Figure 7). Cr^VI elimination from complicated aqueous solution at ambient temperature is thus best accomplished by the proposed OPB (298-303 K).

Figure 7 Effect of contact time for the removal of Cr^VI by OPB at Cr^VI concentration 0.25 mg L^-1 adsorbent dosage 0.01 g L^-1 temperature 298 K and pH 7.0.

3.3 Isothermal Study

The Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models have all been used to describe the biosorption of Cr^VI at equilibrium conditions. The Langmuir theory's fundamental premise is that sorption occurs at particular homogenous spots within the sorbent [26,27]. The outcomes of a plot of Ce/qe vs. Ce shown as a straight line with a slope of 1/Q and an intercept of 1/Qb are shown in Table 2. The biosorption of Cr^VI ions onto biosorbent material was highly linear in the Langmuir model. The magnitude of Q was found to be 0.298-0.308 mmol g^-1 (47.0-48.0 mg g^-1) at 298 K. The value of b was noted from 4.70 to 6.30 × 10^-2 L mol^-1. Cr^VI strongly bonds to the biosorbent material at 298 K is also implied by a high value for "b".

Table 2 Langmiur, Freundlich and D-R characteristic constants for Cr^VI biosorption onto OPB.

A dimensionless characteristic parameter (R_L) has been estimated from the value of b. The R_L was found in the range of 0.124 to 0.987 by using the relationship:

\[ R_L=\frac{1}{1+bC_i} \tag{2} \]

Where, b is the Langmuir constant and Ci is the initial concentration. The calculated values of R_L have predicted the favorable biosorption of Cr^VI ions onto OPB at 298 K. The R_L between 0 and 1, demonstrated that the biosorption process was quite favorable at 298 K.

The intercept and slope of the linear plot of lnqe vs. lnCe are used to calculate the Freundlich constants Cm and 1/n, respectively. The 1/n ratio was in the range of 0 and 1, indicating that the conditions for the biosorption of the understudy biosorbent material were good (Table 2). However, as the R² value was found to be >0.99, the relationship between the quantities of biosorbed Cr^VI ions and their equilibrium concentration in the solution was determined to be consistent with the Freundlich model.

The potential biosorption process mechanism can be hypothesized. One mole of solute was found to have a free energy of transfer (E) of 8.60 to 9.00 kJ mol^-1 from the solution to the surface of the biosorbent material using the equation E = 1/√-2β. The obtained E values demonstrated that the biosorption process involves chemical ion exchange, whereas E 8 kJ mol^-1 demonstrated that physical sorption is also involved [28,29]. Therefore, the sorption of Cr^VI onto OPB may be a combination of physical processes according to biosorption energy. All three isotherms provided strong correlation coefficients and a good match to the experimental data (Table 2). The ability of all three isotherms to be used for the biosorption of arsenic demonstrates the viability of heterogeneous energy distribution of active sites on the OPB surface as well as monolayer sorption.

3.4 Kinetic Study

The workings of the adsorbate-biosorbent system and its response time can be better understood by using kinetic models. The pseudo-first-order and pseudo-second-order models were assessed in order to understand the experimental findings in order to determine the kinetic configuration that governs the biosorption phenomena [28,30]. The results of the experiments were also tested using the pseudo-second-order kinetic model. With chemical sorption acting as the rate-controlling step, this model is more likely to predict the kinetic behavior of biosorption [31]. The pseudo-second-order rate constant, k₂, and q_e, was calculated using the slope and intercept plots of t/qt vs. t (Table 3). The estimated and observed q_e values coincide, and the plots exhibit good linearity with an R² of better than 0.96.

Table 3 Kinetic parameters obtained from pseudo-first-order and pseudo-second-order for Cr^VI biosorption onto OPB.

3.5 Thermodynamic Study

The thermodynamic characteristics of the biosorption of Cr^VI onto OPB were calculated using equations 1 and 2, including enthalpy change (ΔH°), Gibbs free energy change (ΔG°), and entropy change (ΔS°). The Gibbs free energy measures how spontaneously the biosorption process occurs, and a higher negative value suggests more advantageous sorption from an energetic standpoint [31]. At all study temperatures, the value of ΔG° was negative. The ΔG° values are unchanged until 318 K, after which they slightly decline (Table 4). Enthalpy, or heat, of biosorption for physical biosorption ranges from 0.5 to 5 kcal mol^-1 [25]. Additionally, the positive value of So (Table 4) denotes the likelihood of advantageous biosorption.

Table 4 Thermodynamic parameters for Cr^VI biosorption onto OPB.

3.6 Desorption and Regeneration Studies

HCl, HNO₃, and NaOH were used in varying quantities to desorb the biosorbed Cr^VI ions from the OPB. (Table 5). It was shown that 1.0 mol L^-1 HCL desorbed more than >93% of Cr^VI, whereas 1.0 mol L^-1 HNO₃ and NaOH recovered roughly 91.4 and 89.5% of Cr^VI, respectively. For the following studies, biosorbed Cr^VI on OPB was dissolved in 10 mL of 1.0 mol L^-1 HCl. After ten studies, it was found that the biosorbent material's capacity was essentially constant (deviation of 1-2%); hence, many biosorbent material uses were deemed sufficient.

Table 5 Influence of different eluents of the % desorption of Cr^VI from OPB.

3.7 Application on Industrial Effluents

This study has shown that OPB has the potential to remove Cr^VI from the industrial effluents of several Hyderabadi and Jamshoro enterprises. The OPB's most alluring feature is that it can be collected as waste in significant amounts from juice factories and retailers around the nation. Through a series of tests, the OPB proved that at pH 7, >10.0 mg L^-1 of Cr^VI that was present in the industrial effluents of various industries could be removed (Table 6). It might, therefore, be suggested for the effective removal of Cr^VI from the effluent of various industries.

Table 6 Analytical data of Cr^VI concentration in effluent samples of different industries before and after biosorption.

4. Conclusion

It has been determined that the levels of EC, TDS, and BOD in effluent samples from four industries were much higher than the US EPA legal limit. Industrial wastewater samples from the textile industry exhibited increased levels of total Cr that were 400-1800 times greater than the amount allowed by US EPA regulation. In effluent samples from FI, CI, and TFI, the amounts of total Cr were shown to have a significant linear association coefficient with pH, TDS, EC, and BOD (p < 0.01). 1.0 mol L^-1 HCl desorbed the Cr^VI from OPB than 1.0 mol L^-1 HNO₃ or NaOH. The interactions between Cr^VI and the functional groups on the OPB surface were studied using FTIR and SEM. The biosorption of Cr^VI onto OPB was demonstrated by a kinetic examination of the equilibrium data, which followed the pseudo-second-order kinetic model well. Based on these findings, it can be said that OPB, which has a high biosorption capability, is a viable alternative for removing Cr^VI from aqueous solutions and is also readily available and environmentally acceptable.

Author Contributions

Muhammad Waris: Conceptualization, writing – original draft, writing – review and editing. Muhammad Kashif Channa: Software, writing – review and editing. Sultana Rahman: Methodology, writing – review and editing. Ali Bahar Shahani: Conceptualization, review and editing. All authors have read and approved the published version of the manuscript.

Competing Interests

The authors have declared that no competing interests exist.