Kinetic studies and adsorptive removal of chromium Cr(VI) from contaminated water using green adsorbent prepared from agricultural waste, rice straw European Journal of Chemistry 13 (1) (2022) 78-90 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2022 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. https://dx.doi.org/10.5155/eurjchem.13.1.78-90.2189 European Journal of Chemistry View Journal Online View Article Online Kinetic studies and adsorptive removal of chromium Cr(VI) from contaminated water using green adsorbent prepared from agricultural waste, rice straw Izaz Ul Islam 1, Mushtaq Ahmad 1, Maqbool Ahmad 1, Shah Rukh 2 and Ihsan Ullah 1,* 1 Department of Chemistry, Government Post Graduate College Mardan, Higher Education Department, Khyber Pakhtunkhwa, 23200, Pakistan 2 Department of Chemistry, Government Girls Degree College Takht Bhai Mardan, Higher Education Department, Khyber Pakhtunkhwa, 23200, Pakistan * Corresponding author at: Department of Chemistry, Government Post Graduate College Mardan, Higher Education Department, Khyber Pakhtunkhwa, 23200, Pakistan. e-mail: ihsan.chem@tju.edu.cn (I. Ullah). 10.5155/eurjchem.13.1.78-90.2189 Received: 29 September 2021 Received in revised form: 11 November 2021 Accepted: 27 November 2021 Published online: 31 March 2022 Printed: 31 March 2022 Water pollution caused by heavy metals is of great concern because of rapid industrialization, lack of wastewater treatment, and inefficient removal of these metals from wastewater. The present project was designed to develop a green adsorbent from rice straw and to investigate it for the removal of chromium from chromium-contaminated water. Rice straw biochar was prepared and then modified with FeCl3·6H2O and FeSO4·7H2O to enhance its Cr removal efficiency. Modified and unmodified biochar were characterized by Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Fourier Transform Infrared Spectroscopy (FTIR). Batch sorption experimentations were performed to inquire about adsorption kinetics, isotherms, and Cr(VI) adsorption mechanism onto iron-modified rice straw biochar (FMRSB). The results specified that the apex adsorption capability of the adsorbent for chromium was 59 mg/g and the maximum removal efficacy was 90.9%. Three isotherm models, Sips, Freundlich, and Langmuir models were applied to the experimental data. Among them, the Sips isotherm model reveals the most excellent fitting with a maximum correlation coefficient (R2 = 0.996) that was adjusted to the experimental data. Regarding kinetic studies, the Pseudo second-order (PSO) exhibits the best fitting with a higher correlation coefficient (R2 = 0.996). The kinetic equilibrium data expressed that the adsorption of Cr(VI) on the FMRSB surface was chemisorption. The mechanism of adsorption of Cr(VI) on FMRSB was predominantly regulated by anionic adsorption through adsorption coupled reduction and electrostatic attraction. The present study demonstrated that the use of modified biochar prepared from agricultural wastes is an environmentally safe and cost-effective technique for the removal of toxic metals from polluted water. Pyrolysis Rice straw Chromium Adsorption Raw rice straw biochar Iron modified rice straw biochar Cite this: Eur. J. Chem. 2022, 13(1), 78-90 Journal website: www.eurjchem.com 1. Introduction The acquit of a large number of toxic substances i.e., heavy metals into the water bodies are a consequence of rapid industrialization and swift in commercial and anthropogenic activities. Due to these toxic substances, the biotic components of the environment are at high risk. Even at low concentrations, they are extremely toxic because of their accumulation in the human body and therefore very harmful to living organisms and human beings. Once the heavy metals are introduced into the food chain, they are biomagnified regularly and their concentration is increased to a harmful level. Zinc, arsenic, chromium, nickel, copper, mercury, lead, and cadmium are heavy metals that are frequently released by various industrial operations. 0.050, 0.006, 0.05, 0.25, 0.01, 0.08, 0.00003, and 0.2 are the maximum contaminant limit (MCL) for arsenic, lead, chromium, copper, cadmium, zinc, mercury and nickel, respect- tively. Chromium, among the heavy metals, is one of the most toxic, carcinogenic, and mutagenic metals that adversely affect all the biotic components of the ecosystem [1,2]. The toxicity of chromium is due to its non-degradable nature [3]. Chromium exists in multitudinous valencies from -2 to +6, however, Cr(III) and Cr(VI) forms of chromium are most profuse [4]. Chromium (VI) and its compounds are comparatively more toxic than trivalent and metallic chromium. In developed countries, the Cr(VI) concentration in industrial effluents is 0.5 ppm [5]. The chromium contamination of the water reservoir is because of the uncontrolled release of effluents from various industrial operations, such as electroplating, production of glass, metal coating, pigments, leather making, textile, and paint industries that are laden with a considerable amount of chromium waste [6,7]. According to USEPA and WHO, the acceptable Cr(VI) limit in drinkable and wastewater from various industrial operations is 0.05 and 0.5 mg/L, respectively [8]. Chromium entry into the human body occurs via eating, breathing, drinking, or contiguity of the skin with chromium and its compounds. The adverse consequences of chromium exposure include infection of the respiratory tract, cancer of the lungs, rashes on the skin, ABSTRACT RESEARCH ARTICLE KEYWORDS https://dx.doi.org/10.5155/eurjchem.13.1.78-90.2189 https://www.eurjchem.com/ https://dx.doi.org/10.5155/eurjchem.13.1.78-90.2189 mailto:ihsan.chem@tju.edu.cn http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.13.1.78-90.2189&domain=pdf&date_stamp=2022-03-31 Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 79 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 immune system suppression, hepatic diseases, and bleeding of the nose. Besides these various adverse effects like allergic, dermatitis, ulceration of mucous and skin membrane, allergic asthmatic reactions, nasal septum perforation, bronchial carcinomas, hepatocellular deficiency, renal oligo anuric deficiency, gastro-enteritis, carcinogenic and mutagenic effects have been reported [9-11]. The implementation of wastewater treatment policies and cost-effective water treatment schemes is the need of time to protect the human population, aquatic life, and environment. For the Cr(VI) removal from wastewater, several prevalent technologies have been used, including reverse osmosis, ion exchange, precipitation, adsorption, solvent extraction, electrolysis, and chemical reduction preci- pitation, etc. [12-14]. These prevalent technologies have high operational costs and result in the generation of secondary waste and the Cr(VI) is not removed but only transferred [12]. So the appetency is to introduce such a removal method that is low in operational cost, free from the generation of secondary waste, efficient in the Cr(VI) disposal from the wastewater. Currently, researchers are interested to use green adsorbents, which are cost-effective, and can be prepared from easily available materials, are eco-friendly, and have good efficiency. Modified biochar prepared from various biomasses is consi- dered the best adsorbent possessing all the above-mentioned qualities. Pyrolysis of biomass carried out in a finite oxygen environment results in a black compact carbon material called biochar. Biochar is prepared from various agricultural wastes like peanut hull, corncob and corn straw, rice straw, rice husk, walnut hull, date pits, banana straw, and banana peel [10,15- 17]. Biochar has been enormously used as an adsorbent in the disposal of numerous adulterations especially trace elements because of its perpetual porous texture and large aggregation of functional groups containing oxygen like carboxyl, hydroxyl, and amino groups [10,16]. Various studies pointed out that compared to primitive biochar, modified biochar exhibit large removal efficiency and adsorption capacity [18,19]. In addition to various modification processes modification of biochar with iron is encouraging due to its elevated effective solid-liquid separation and maximum adsorption capacity for the conta- minants [20]. Using various materials iron modified biochar has been synthesized such as raw corncob [16], peanut hull powder [19], Melia azedarach wood [20], willow residue [21], rice straw [22], and corn straw [23]. Rice is a core food for most of the population of the globe, particularly in South-Asia, and its production is widespread. Annual rice crop production generates an enormous quantity of rice straw forecast as ~8×1011 kg of which only 20% is used for serviceable purposes while the remaining is burned or unified in soil [24]. Various studies indicated that unlawful manage- ment of agricultural desolate like field scrapping and open burning, etc., evokes numerous health and environmental problems [25]. The composition of rice straw is, hemicellulose (19.7-35.7%), cellulose (32-38.6%), and lignin (13.5-22.3%) it also contains a significant amount of silica [24,26]. By observing the structure of the component of rice straw i.e. cellulose, hemicellulose, and lignin we can affirm rice straw as a suitable adsorbent for the treatment of Cr(VI) polluted wastewater. Thus, the immanent intent of this study is to fabricate biochar from rice straw and then modify it with FeCl3·6H2O and FeSO4·7 H2O. The iron-modified biochar will be investigated for the remediation of Cr(VI) from chromium-tainted water through adsorption. We expect that the modification of biochar with FeCl3·6H2O and FeSO4·7H2O will enhance the Cr removal efficiency of the adsorbent. The elemental, morphological, and functional group properties of biochar will be analysed by EDS, SEM, and FTIR. The Batch adsorption experiments will be accomplished to examine the behaviour of Cr(VI) adsorption on adsorbent at the different contact times, FMRSB dose, pH, and the adsorbate preliminary concentration. Equilibrium models and kinetic models of Cr(VI) adsorption onto FMRSB will also be inquired. The basic aims and objectives of this study are to fabricate an effective adsorbent from rice straw, evaluate the Cr(VI) adsorption, and reveal its mechanism via adsorption isotherms and adsorption kinetics. 2. Experimental 2.1. Materials The entire reagents and chemicals pre-owned in this research work were of the analytical grade. Potassium dichromate (K2Cr2O4) was purchased from Sigma Aldrich. FeSO4·7H2O, FeCl3·6H2O, NaOH, and HCl were purchased from BDH (Germany). For glassware washing and solution prepa- ration, deionized water was used. For biochar preparation, the raw materials used were rice straw. 2.2. Methods 2.2.1. Collection of rice straw and preparation of biochar The rice straw was collected from district Mardan, Khyber Pakhtunkhwa, Pakistan. The raw rice straw was washed with tap water several times followed by deionized water to eradicate dust. The chemical and additional adulterations cling to raw rice straw that is not removed by water treatment are subjected to acid-base treatment. Firstly, the rice straw was washed away with 0.1 M HCl (BDH) solution, subsequently washed with 0.1 M NaOH (BDH) solution. Rice straw washing was completed by a final rinse with distilled water. The Raw Rice Straw (RRS) was dried up at 74 °C for 48 h in a hot air oven (GRX-15A) to get a constant weight. The rice straw biochar was prepared from the pyrolysis of rice straw in a furnace by providing an oxygen-deficient environment at a maximum temperature of 550 °C for 1 h. To obtain the particles of the uniform diameter the biochar was grinded and sieved through 250 µm mesh. The biochar obtained was labelled as raw rice straw biochar (RRSB). 2.2.2. Preparation of iron modified rice straw biochar Iron-modified biochar was prepared as follows. Briefly, the suspension of RRSB was prepared by adding 10 g of RRSB in 500 mL of de-ionized water and stirred by using a magnetic stirrer for 1 h to prepare a stable suspension. The FeCl3 and FeSO4 solutions were prepared by adding 7.3 g of FeSO4·7H2O and 7.23 g of FeCl3·6H2O to 100 mL of distilled water in a separate beaker. The solutions of FeCl3 and FeSO4 were added to the suspension of RRSB. The suspension was agitated with help of a magnetic stirrer at 120 rpm. After the contact time of 1 h, using distilled water, the iron-modified rice straw biochar was rinsed to remove the organic residue until the solution pH reached 7. The granular powder of the modified biochar was filtered and then by using the hot air oven it was dried at a temperature of 75 °C for 24 h. The iron-modified rice straw biochar was labelled as FMRSB. 2.3. Characterization The RRSB and FMRSB were characterized for the surface morphology, elemental composition, and functional group analysis. The surface morphology and elemental composition of the biochar were examined using a scanning electron micros- cope (SEM), JSM5910 Model JEOL, Japan, and energy dispersive X-ray spectroscopy (EDS) U.K, JSM5910, INCA200/Oxford instruments). The functional groups were analysed using Fourier-Transform Infrared Spectroscopy (FT-IR). 80 Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 2.4. Batch adsorption 2.4.1. Batch adsorption experiment To investigate the adsorption potential of FMRSB for Cr(VI), the batch adsorption experiments were performed. Briefly, a stock solution of Cr(VI) (1000 mg/L) was prepared by dissolving 2.828 g of K2Cr2O7 in 1000 mL of deionized water. The stock solution was diluted to prepare the requisite concentration of Cr(VI) solution in the range of 5 to 45 mg/L. Then, the specific amount of FMRSB was added to 50 mL of 20 mg/L of Cr(VI) solution for each experiment. The mixture was agitated at 120 rpm at a temperature of 25±2 °C using a magnetic stirrer. The pH of the solution was adjusted using 0.1 M solutions of HCl and NaOH and then the mixture was filtered using filter paper. The remnant Cr(VI) concentration was determined with the help of an Atomic Absorption Spectro- meter (AAS) (Perkin Elmer, USA, Model AAS 700) at the end of each experiment. The Cr(VI) adsorption capacities of FMRSB at any time t, at equilibrium and per cent removal efficacy, were deliberated by using Equations (1-3), respectively. 𝑄𝑄𝑡𝑡 = (Co − 𝐶𝐶𝑡𝑡) 𝑀𝑀 × 𝑉𝑉 (1) 𝑄𝑄𝑒𝑒 = (Co − 𝐶𝐶𝑒𝑒) 𝑀𝑀 × 𝑉𝑉 (2) % Removal = (Co − Ce) C0 × 100 (3) where Co, Ce, and Ct signify the Cr(VI) solution concentration in mg/L at the initiation time, at equilibrium, and at any time t, respectively, V is the test sample (L) solution volume, and M represents the FMRSB mass added (g). 2.4.2. Effect of pH The pH of the medium significantly affects the adsorption of adsorbate from the medium. The effects of pH on the Cr(VI) adsorption were investigated in the pH range from 2 to 11 (2, 3, 5, 7, 9, and 11) by maintaining the various parameters constants like adsorbate initial concentration, FMRSB dosage, tempe- rature and time. The sample solution was stirred until it attained equilibrium. After equilibrium, the samples were collected and the pH effect was investigated. 2.4.3. FMRSB dose effect on adsorption The FMRSB dosage is a prominent factor that strongly affects the adsorption efficiency. FMRSB dose in the order of 0.02 to 0.07 g/L was employed to investigate the effects of the FMRSB dosage on the adsorption efficiency. During this process, a specific amount of FMRSB was added to 50 mL Cr(VI) solution at pH = 3 till the equilibrium was reached. An efficient dose was selected by investigating the removal efficiency (%), adsorption capacity at the various dosage of FMRSB. 2.4.4. Effect of contact time To investigate the effects of contact time on the adsorption process, the adsorption experiment was performed for different contact times, ranging from 30 to 240 min. The contact time at which the adsorbent shows maximum efficiency was chosen as the optimum contact time for batch sorption experiments. 2.4.5. Adsorption kinetics of chromium Various models have been proposed that explain the adsorption process. These models are very beneficial for scaling up the adsorption method. Adsorption kinetics play a key role in determining the nature and mechanism of adsorption. Two kinetics models pseudo-first-order and pseudo-second-order, were used to study the adsorption phenomena. Equations (4 and 5) describe the linear form of the corresponding model, respectively. 𝑙𝑙𝑙𝑙𝑙𝑙(𝑞𝑞𝑒𝑒 − 𝑞𝑞𝑡𝑡) = 𝑙𝑙𝑙𝑙𝑙𝑙𝑞𝑞𝑒𝑒 − ( 𝑘𝑘1 2.303 )𝑡𝑡 (4) 𝑡𝑡 𝑞𝑞𝑡𝑡 = 1 𝑘𝑘2𝑞𝑞𝑒𝑒2 + 𝑡𝑡 𝑞𝑞𝑒𝑒 (5) 2.4.6. Effect of preliminary concentration of Cr(VI) The consequences of the adsorbate preliminary concent- ration on the adsorption process were inquired in the order of 5 to 45 mg/L. The adsorption capacity and removal efficacy (%) was studied at various initial concentration of the adsorbate at a constant pH and specific amount of FMRSB dose at room temperature. 2.4.7. Adsorption isotherms Adsorption isotherms were used to explain the relationship between equilibrium adsorption capacity and equilibrium concentration. For this purpose, to experimental data, three isotherm models namely Sips, Freundlich, and Langmuir models were applied. These isotherm models are narrated by Equations (6-8), respectively. qe = qmaxkLce 1+(kLce) (6) 𝑞𝑞𝑒𝑒 = 𝑘𝑘𝑓𝑓𝐶𝐶𝑒𝑒𝑞𝑞 1 𝑛𝑛� (7) 𝑞𝑞𝑒𝑒 = 𝑞𝑞𝑚𝑚(𝑏𝑏𝐶𝐶𝑒𝑒) 1 𝑛𝑛 1+(𝑏𝑏𝐶𝐶𝑒𝑒) 1 𝑛𝑛 (8) 3. Results and discussion 3.1. Comparative analysis of the removal efficiencies of various adsorbents and selection of suitable adsorbent The comparative analysis of the removal efficiencies of unmodified and modified rice straw and rice husk was carried out for the selection of an effective adsorbent for the removal of Cr(VI). The removal efficiency of various adsorbents, e.g., raw rice straw, raw rice husk, iron-modified rice straw, and iron- modified rice husk, was studied under suitable similar conditions. A 70 mg/L of Cr(VI) solution was taken as a reference standard, in which no adsorbent was added and labelled as 1C. Four sample of solutions (50 mL each) containing 70 mg/L of Cr(VI) were taken for the comparative analysis and labelled as 1A, 1B, 1D, and 1E. According to the reported literature, the optimal conditions for adsorption (time 180 min, pH = 3, and 0.02 g of adsorbent dose) were set and removal efficiency was measured. 0.02 g of rice straw and rice husk were added to sample solution 1A and 1B, respectively. Similarly, 0.02 g of iron modified rice straw and rice husk was added to 1D and 1E, respectively. The pH was adjusted to 3 and the samples were stirred for 180 min. Then sample solutions were filtered and the filtrate was evaluated for the concent- ration of Cr(VI) by flame atomic absorption spectroscopy. The results were summarized in Table 1. The comparative analysis results exhibited that iron modified rice straw biochar is the most efficient adsorbent for the Cr(VI) with 94.22% removal efficiency. Hence, iron- modified rice straw biochar was selected as a productive adsorbent for the removal of Cr(VI) in this study. Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 81 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 Table 1. Results of comparative test analysis. Sample ID Adsorbent Mean (mg/L) Standard deviation % RSD Removal efficiency (%) 1A Raw rice straw 24.66 0.128 0.52 65.24 1B Raw rice husk 14.35 0.029 0.20 79.77 1C Reference standard 70.46 0.136 0.19 - 1D Modified rice straw 4.10 0.025 0.61 94.22 1E Modified rice husk 13.17 0.108 0.82 81.44 (a) (b) (c) (d) (e) (f) (g) Figure 1. SEM images of RRSB at different magnifications under the scanning electron microscope. 3.2. Morphological analysis of RRSB and FMRSB The morphological analysis of RRSB and FMRSB was performed to examine the surface morphology and changes that occurred in the surface topography of biochar after modifi- cation. The SEM images of RRSB (unmodified) and FMRSB (Iron modified) are given in Figures 1 and 2, respectively. These results indicated that the surfaces are composed of macro- porous structures. These results might be due to the formation of mineral aggregate during pyrolysis, which can block micropores and mesopores. Furthermore, FMRSB has a hetero- geneous surface morphology, because of iron oxide deposition on the modified biochar surface. The deposition of iron oxide particles was further confirmed from EDS results, where an increase in the per cent weight of iron has been observed. Similar surface morphology changes have been already reported by Hoang et al. and Bulut et al. [16,26]. 3.3. EDX elemental analysis of RRSB and FMRSB Figures 3a and 3b recapitulate the EDS spectrum of RRSB and FMRSB. The biochar samples were examined with EDS for elemental analysis. Figure 3a exhibits the per cent elemental composition which mainly consists of carbon (64.01%), oxygen (17.63%), and silicon (11.06%). Furthermore, the elemental percentage of iron in unmodified biochar is 0.27%, which is significantly very small and can be attributed to the memory effect in the instrument. Figure 3b exhibited the per cent elemental composition of iron-modified biochar which mainly consists of carbon (30.07%), oxygen (23.51%), silicon (7.5%), and iron (34.30%) by weight. The high elemental percentage of iron in FMRSB suggests that biochar has been successfully modified with iron. 3.4. FTIR analysis FTIR analysis (400-4000 cm-1) of unmodified and iron- modified biochar was performed to investigate the nature of various functional groups of biochar. The FTIR spectra of RRSB and FMRSB marked several clear and serrated bands in the respective wavelengths. The FTIR result exhibited that after modification shifts occurred in several characteristic bands of biochar, this is because of changes taking place in the nature of functional groups after modification with iron. The peaks at 3401 and 3341 cm-1 mark the presence of stretching vibration of the -OH group of the aromatic ring [27- 29]. The bands at 2913 cm-1 of RRSB and 2940 cm-1 of FMRSB correspond to the existence of aliphatic group (-CHn-) stretching [25]. Similarly, the peaks at 1755 cm-1 of RRSB and 1752 cm-1 of FMRSB suggest the presence of carbonyl and ketonic groups [15,25]. The band at 1585 cm-1 of RRSB and 1619 cm-1 of FMRSB corresponds to the stretch of the double bond C=C, due to the presence of aromatic rings (like benzene, phenol, etc.) [25,30]. The absorption peaks at 1422 cm-1 of RRSB and 1432 cm-1 of FMRSB show the existence of -COO and -CONH [13,31]. The bands present at the range of 1318 cm-1 for RRSB and at 1328 cm-1 for FMRSB denoted the existence of the C-O-C group, which is because of the carbohydrate derivative [27]. The absorption peak at 871 cm-1 for RRSB and 916 cm-1 for FMRSB confirmed the Si-O, Si - O- Si, and Si-OH bonds [15,21]. The absorption bands at 780 cm-1 for RRSB and at 794 cm-1 for FMRSB were due to the -NH2 group’s vibrations [13]. The broadband in the region of 560 to 767 cm-1 is due to the stretching vibration of the Fe-O group in FMRSB [16]. By comparing the FTIR spectra of RRSB and FMRSB the peak shifted from 3401, 2913, 1745, 1585, 1422, 1318, 1055, 872, 780 cm-1 to 3341, 2940, 1752, 1619, 1432, 1332, 1000, 916 and 794 cm-1, respectively. These changes in the position of the spectral bands and the appearance of the Fe-O band confirmed that RRSB has been successfully modified with iron oxide through electrostatic interaction with surface functional groups [13,15]. 82 Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 (a) (b) (c) (d) (e) (f) (g) Figure 2. SEM images of FMRSB at different magnifications under the scanning electron microscope. (a) (b) Figure 3. EDS results of RRSB (a) and FMRSB (b). 3.5. pH effect on the adsorption of Cr (VI) The pH of the solution has a significant effect on adsorption, as it not only influences the extent of ionization but also affects the adsorbent surface properties [20]. The pH of the solution is directly correlated to the adsorption capacity and affects it by remodelling the physiochemical characteristics (Binding to the surface, the process of diffusion, and surface charge) of both adsorbate and adsorbent [13]. The pH effects on the removal efficacy and adsorption of FMRSB were deliberated using 20 mg/L as initial concentration of Cr(VI), 0.02 g/50 mL FMRSB dose, and 180 min contact time at a temperature of 25±3 °C. The solution pH was altered from 2 to 11. The data obtained from the results of solution pH studies on the elimination of Cr(VI) is revealed in Figure 4. Figure 4 indicates that the removal efficacy and the adsorption capacity increased when pH increases from 2 to 3. At pH = 3 Cr(VI) removal efficacy and the adsorption capacity were achieved up to 90.9 % and 45.45 mg/g, respectively. The removal efficacy and adsorption capacity at pH = 2 are lesser than that of pH = 3, which may be because at low pH there is an excessive amount of H+ ions that result in the decrease of adsorption capacity by occupying the surface functional group of FMRSB [14]. At comparatively high pH values ranging from 3 to 5, the solution having an excess of H+ ions was neutralized by negative charge hydroxyl (OH-) ions and as a result, the dichromate ions successfully diffuse towards the adsorbent surface and are adsorbed [13]. As the pH of the solution is raised from 5 to 11 the adsorption capacity and removal efficiency of Cr(VI) decrease rapidly. Due to the rise in pH of solution, the concentration of OH- ions increase which combines with Cr(III) in the solution and results in the formation of Cr(OH)3 that hindered the movement of chromium ions towards the functional groups present on the FMRSB surface [16]. Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 83 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 Figure 4. pH effect on Cr(VI) adsorption onto FMRSB. Figure 5. FMRSB dose effect on the adsorption of Cr(VI) onto the surface of FMRSB. Due to the rise in pH, a significant amount of OH- ions are generated on the FMRSB surface, as an outcome, it becomes negatively charged which results in the origination of electro- static repulsion between FMRSB and Cr(VI) ions [32,33]. These outcomes similitude to prior analyses on the abstraction of Cr(VI) by using biochar under additional acidic circumstances [34-36]. In the pH range of 3 to 9 the Cr(VI) prevailing species are HCrO4-1, Cr2O7-2, CrO4-2 [21]. HCrO4-1, Cr2O7-2 are the dominant forms in solution at pH of 2 to 6, it converts to dichromate CrO4-2 at pH greater than 6. The adsorption free energy of CrO4-2 and HCrO4-1 ranges from -2.1 to -0.3 kcal/mol and -2.5 to -0.6 kcal/mol, respectively. Due to higher adsorption free energy, CrO4-2 adsorption is difficult than HCrO4-1 at the same concentration [16]. The results point out that the FMRSB maintain their maximum rate of removal in a broad range of pH range under acidic circumstances that show good flexibility to changes in pH. The flexibility to pH change might be ascribed to the creation of stable functional groups on the FMRSB surface as an outcome of the process of ageing [21]. Our experimental findings are parallel to previous studies [37-39]. 3.6. FMRSB dosage effect on adsorption The dosage of the adsorbent (FMRSB) is a prominent consideration that strongly influences the adsorption of Cr(VI) onto the FMRSB surface. The adsorbent dosage greatly influences the adsorption capacity by governing the acces- sibility and availability of active sites for adsorption [16,40]. The FMRSB dose effect on the removal efficacy and the capability of the adsorption of Cr(VI) was investigated under the subsequent conditions; 20 mg/L preliminary concentration of Cr(VI), optimum pH of 3, contact time of 180 min, and variable FMRSB prescribed amount (0.02, 0.03, 0.04, 0.05, 0.06, and 0.07 g/L). Figure 5 illustrated that with the raise of adsor- bent dosage from 0.02 g/50 mL of Cr(VI) solution to 0.05 g/50 mL, the removal efficiency increased continuously from 90.9 to 95.78%. This increase in the removal efficiency indicates a strong correlation between active sites on the FMRSB surface and Cr(VI) removal efficiency. Hence, it can be concluded that by rising the FMRSB prescribed amount, the availability of active sites increases which results in increased absorption and hence the Cr(VI) removal efficiency [36,37]. This can be observed that with the further rise of FMRSB dosage beyond 0.05 g/50 mL, the removal efficiency remained constant. On the other side if we notice the adsorption capacity it is maximum at the adsorbent dosage of 0.02 g/50 mL having the value of 45.45 mg/g. When we increase the adsorbent dose up to 0.07 g/50 mL the capability of the adsorption of FMRSB for Cr(VI) declined from 45.45 to 13.6 mg/g. The preliminary concentration of Cr(VI) is fixed in all experiments. Thus, when we increase the dosage of FMRSB, the ratio of metal ions to active sites decreases as a result, not all active sites participate in the metal ions adsorption, and their accessibility is also reduced due to the overlapping of FMRSB particles. Hence, the adsorption capability of Cr(VI) onto the FMRSB surface is reduced [13,40,41]. 84 Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 Figure 6. Adsorption contact time effect on the Cr(VI) adsorption onto the surface of FMRSB. Figure 7. The plot of the PFO model for Cr[VI] adsorption onto FMRSB. 3.7. Contact time effect on the adsorption In the batch adsorption method, the contact time influenced the adsorption [42]. The contact time effect was examined at various times keeping other conditions constant i.e. 20 mg/L preliminary concentration of Cr(VI), temperature 25±3 °C, FMRSB prescribed amount of 0.02 g/L, solution pH = 3, and agitation speed = 120 rpm. The contact time effect was investigated in the range of time of contact from 30 to 240 min. The data obtained from the study of the time of adsorption on removal efficacy and the adsorption capacity of Cr(VI) onto FMRSB is revealed in Figure 6. Figure 6 results indicated that the adsorption and removal efficiency increased promptly as the contact time increased from 30 to 120 min and slowed down gradually from 120 to 180 min of contact time. For any initial concentration, the optimal adsorption capacity and removal efficiency that was accomplished within about 180 min was 45.45 mg/g and 90.9 %, respectively. The adsorption rates become constant from 180 to 240 min of contact time. Adsorption kinetics to attain equilibrium passes through two phases, i.e., primary instant uptake pursued by the consequent gradual uptake [33]. In the beginning, the adsorption was fast, which maybe because of the availability of a large number of vacant sites [43]. With the passage of time, the number of the available active site gradually decrease as a result the rate of adsorption becomes slower, as contact time reach 180 min. The subsequent slow uptake is owing to the arousal of forces of repulsion between the molecules of solute on the bulk phase and solid, for the active sites that are remaining and available for the adsorption process on the adsorbent surface [44]. The adsorption time of 180 min was allocated as the equilibrium contact time in this study. This equilibrium contact time (180 min) has been previously reported in the literature using raw corn cob and corn straw as adsorbents [16,31]. 3.8. Kinetic models Chemical kinetic is considered as an important element that gives details about different pathways of reaction and adsorption mechanism. To study the mechanism of adsorption, various models have been proposed. Linear regression coefficient (R2), which explains the applicability of the corresponding model to the experimental data. Models having correlation coefficients (R2 value) close to 1 or equal to 1 can successfully explain the adsorbate adsorption kinetics [42,45,46]. 3.8.1. Pseudo first order kinetic model Pseudo first order (PFO) kinetic model was proposed by Lagergren and is also known Lagergren kinetic model [42,45]. This kinetic model is used to describe the relationship between the rate at which the active site of the adsorbent is engaged and the number of unoccupied active sites [46]. It is the first equation for sorption and is generally expressed by Equation (9). 𝑑𝑑𝑞𝑞 𝑑𝑑𝑡𝑡 = 𝑘𝑘1(𝑞𝑞𝑒𝑒 − 𝑞𝑞𝑡𝑡) (9) Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 85 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 Table 2. Calculated kinetic parameters of PFO and PSO kinetic models. Co qe [Exp] Pseudo first-order model (PFO) Pseudo second-order model (PSO) 20 mg/L 45.45 mg/g qe, cal = 87.5 mg/g qe, cal = 50.9 mg/g K1 = -0.00013 1/min K2 = 0.000706 g/mg.min R2 = 0.910 R2 = 0.996 Figure 8. The plot of PSO kinetic model of Cr(VI) adsorption onto FMRSB. where qe, denotes the equilibrium adsorption capacity (mg/g). In the above equation at time t, the adsorption capability (mg/g) is delineated by qt and k1, representing the PSO adsorption rate constant, (min-1). By rearranging the above equation, we can get another useful form. That is shown in Equation (10). 𝑙𝑙𝑙𝑙𝑙𝑙(𝑞𝑞𝑒𝑒 − 𝑞𝑞𝑡𝑡) = 𝑙𝑙𝑙𝑙𝑙𝑙𝑞𝑞𝑒𝑒 − ( 𝑘𝑘1 2.303 )𝑡𝑡 (10) The log qe is such a parameter that makes the PSO equation differ from the true first-order Equation [45]. Pseudo first- order kinetic plot for the experimental data is shown in Figure 7. 3.8.2. Pseudo second order kinetic model (PSO) The Pseudo second-order kinetic model is the most frequently used model to elucidate the kinetics and process of adsorption, especially for modern adsorbent materials [47]. According to the assumption of the PSO kinetic model, the chemical sorption governs the adsorption rate that is proportional to the second power of the active sides [48]. This model considered the adsorption on the surface of the adsorbent as a rate-limiting step involving chemisorption, in which the adsorbate from a solution is removed as an outcome of physicochemical interactions among the two phases [49]. This PSO kinetic is used to explain the reliance of the adsorption capability of the adsorbent on time [50]. The rate equation for the PSO is expressed by Equation (11). 𝑑𝑑𝑞𝑞 𝑑𝑑𝑡𝑡 = 𝑘𝑘2(𝑞𝑞𝑒𝑒 − 𝑞𝑞𝑡𝑡)2 (11) 𝑘𝑘2 = Rate constant of PSO adsorption. The above equation can be arranged into a more useful form into Equation (12). 1 𝑞𝑞𝑒𝑒−𝑞𝑞𝑡𝑡 = 1 𝑞𝑞𝑒𝑒 + 𝐾𝐾2𝑡𝑡 (12) Equation (13) represents the linear form of PSO, 𝑡𝑡 𝑞𝑞𝑡𝑡 = 1 𝑘𝑘2𝑞𝑞𝑒𝑒2 + 𝑡𝑡 𝑞𝑞𝑒𝑒 (13) Pseudo-second-order kinetic plot for the experimental data is shown in Figure 8. By comparing the various parameters indicated in Table 2 for PFO and PSO, we can observe that PSO shows good fitting to the experimental kinetic data than PFO. Thus, PSO can successfully explain the adsorption process of Cr(VI) having an excellent correlation coefficient (R2 = 0.996) and the calculated qe of PSO is close enough to the experimental data. These results and fitting of kinetic data to PSO revealed that Cr(VI) is chemisorbed on the surface of FMRSB [51,46]. 3.9. Preliminary concentration effect of Cr(VI) on the adsorption The preliminary concentration of adsorbate is a prominent factor that affects the adsorption rate by establishing a driving force that shifts the Cr(VI) present in an aqueous solution onto the surface of the absorbent [32]. The mobility, concentration, and charge of ions in an aqueous solution determine the adsorption rate. The Cr(VI) ions in solution prefer to be adsorbed at relatively positive charged centres on the surface and in pores of tiny particles of FMRSB [40]. The preliminary concentration effect of Cr(VI) on the adsorption capacity was investigated under the following conditions: Sorbate preliminary concentration in the array of 5 to 45 mg/g, time of contact 180 min, adsorbent dose of 0.02 g/50 mL, and a pH of 3. Figure 9c displays that as the prelimi- nary concentration of Cr(VI) was increased from 5 to 20 mg/L the capability of the adsorption upsurged rapidly from 10.25 to 45.45 mg/g. From 20 to 30 mg/L, the capability of the adsorption increased gradually from 53.85 to 58.85 mg/g with a further rise in the preliminary concentration of Cr(VI) from 30 to 45 mg/L the adsorption capacity approximately becomes constant. Figure 9c indicates that removal efficiency increase from 81.4 to 90.9% when the preliminary concentration of Cr(VI) increased from 5 to 20 mg/g, meanwhile with rising of Cr(VI) preliminary concentration from 20 to 45 mg/L the removal efficacy decreased up to 50.1%. Compared to 5 and 10 mg/L, 20 mg/L of preliminary Cr(VI) concentration has high removal efficiency. This is because the diffusion of anionic chromium species towards the cationic charged centres on the surface of the adsorbent was greater in 20 mg/L of preliminary Cr (VI) concentration than 5 and 10 mg/L, correspondingly [20]. This is illustrated in Figure 9a. 86 Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 (a) (b) (c) Figure 9. (a) Explain the rate of diffusion of chromate ions toward the active site of the adsorbent at various initial concentrations, (b) Explain the decline in removal efficiency of Cr(VI) having a preliminary concentration of 30 mg/L and 40 mg/L due to limited availability of active sites and weak electrostatic repulsion, (c) Effect of preliminary concentration on the adsorption of Cr (VI) onto FMBC. When the Cr(VI) preliminary concentration rises from 20 to 40 mg/L, the removal efficacy declined from 90.9 to 50.1%. This decrease in the removal efficiency is attributed to the limited availability of active sites on the surface of FMRSB and the arousal of weak electrostatic repulsion between incoming chromium species and already adsorbed chromium species on the adsorbent surface [16,20,42]. This is illustrated in Figure 9b. 3.10. Equilibrium model Adsorption isotherm plays a vital role in describing the interactions between adsorbate and the adsorbents. The relationship between the adsorption of adsorbate to the adsorbent and the adsorbate concentration in the solution can be successfully explained by the adsorption isotherm models when the liquid (solution containing metal ions) and solid phases are in equilibrium [45,52]. 3.10.1. Langmuir model The Langmuir isotherm model is used to determine the adsorbent adsorption capacity and describe that adsorption occurs by a monolayer on a homogeneous surface with no interaction of sorbed molecules on adjacent sites. According to the assumption of this model, the energy that is required for the adsorption onto the active sides present on the surface is always homogenous [45,52]. The expression of Langmuir adsorption isotherm is illustrated by Equation (14): 𝑞𝑞𝑒𝑒 = 𝑞𝑞𝑚𝑚𝑚𝑚𝑚𝑚𝑘𝑘𝐿𝐿𝑐𝑐𝑒𝑒 1+(𝑘𝑘𝐿𝐿𝑐𝑐𝑒𝑒) (14) 𝑞𝑞𝑒𝑒, denotes the equilibrium capability of the adsorption, mg/g. qm indicates the apex adsorption capability, the metal concentration in solution at equilibrium is denoted by Ce. KL is the Langmuir constant cognate to the energy of the adsorption. Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 87 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 Table 3. Illustrate various parameters of Langmuir, Freundlich, and Sips isotherm model. Qm(exp) (mg/g) Langmuir model Freundlich model Sips model 59 qm (mg/g) = 65.834 - qm (mg/g) = 59.89 KL = 0.905 (L/mg) KF = 20 (mg/g) b = 1.279 R2 = 0.963 R2 = 0.707 R2 = 0.996 - 1/n = 0.401 1/n = 1.38 Table 4. Comparative studies of the adsorption capability of rice straw with other adsorbents used in previous studies. Adsorbent Adsorption (mg/g) References Rice husk Corncob Melia azedarach wood Palm pressed fibre 379.00 25.94 45.45 15.00 [13] [16] [20] [55] Sugar can bagasse 13.4 0 [56] Corn stalk (raw carbon) 27.76 [57] Rice straw 45.45 Current studies Figure 10. Isotherm model plots for adsorption of Cr(VI) onto FMRSB. 3.10.2. Freundlich model The Freundlich isotherm model elucidates that multilayer adsorption occurs on a heterogeneous surface with the interaction between adsorbed molecules [21]. According to this model when the active sites of the adsorbent are completely occupied by the adsorbate the adsorption energy decreases exponentially and this process of adsorption is reversible [46]. It is represented in Equation (15): 𝑞𝑞𝑒𝑒 = 𝑘𝑘𝑓𝑓𝐶𝐶𝑒𝑒𝑞𝑞 1 𝑛𝑛� (15) 𝑘𝑘𝑓𝑓 = Freundlich constant also called Freundlich capacity. 𝑛𝑛, is the heterogeneity element and their reciprocal specifies the adsorption strength. 3.10.3. Sips model This model is the consortment of the Freundlich and Langmuir model, which defines the surface of the sorbent as homogeneous and the uptake of the metal ion is a communal process due to adsorbate interaction [16,21,45]. It is described in Equation (16): 𝑞𝑞𝑒𝑒 = 𝑞𝑞𝑚𝑚(𝑏𝑏𝐶𝐶𝑒𝑒) 1 𝑛𝑛 1+(𝑏𝑏𝐶𝐶𝑒𝑒) 1 𝑛𝑛 (16) By applying the above formulas to our experimental data, the correlation coefficient and their adsorption parameters were acquired accordingly that are shown in Table 3, respectively. This can be observed from the investigational statistics that the adsorption of Cr(VI) onto the surface of FMRSB has a good fit to the Sips model with a correlation coefficient R2 value of 0.996 as compared to Langmuir and Freundlich's model having an R2 value of 0.963 and 0.707, respectively. The best adsorp- tion of Cr(VI) onto FMRSB was explained by the Sips model with a qm value of 59.89 mg/g cognate as well with qm(exp) = 59 mg/g (Figure 10). This implies that the mechanism of adsorption of Cr(VI) was a monolayer on the homogeneous surface of the adsorbent. The adsorption capacity acquired through the Sips model is more genuine compared to that acquired from Freundlich and Langmuir models, respectively [16,53,54]. Table 4 illustrates the adsorption capacity of FMRSB that was used as an adsorbent in the present study and compares it with the adsorption capability of other adsorbents used in previous studies. This table shows that rice straw possess greater adsorption potential for the Cr(VI) removal as compared to other adsorbents, 3.11. Proposed adsorption mechanism for Cr(VI) Adsorption has befitted the leading mode in recent years for the Cr(VI) removal. Adsorption kinetics and adsorption isotherms have been considered as the primitive standpoint of research for the probation of adsorption mechanisms [57]. The Cr(VI) adsorption onto FMRSB entangles the following possible mechanisms; (I) Anionic adsorption through electrostatic attraction (II) Adsorption coupled reduction [16,45]. Anionic adsorption through the electrostatic attraction of Cr(VI) onto FMRSB played a significant part. The biochar surface at a pH of 3 bears a positive charge. This is because at low pH concent- ration of hydrogen ion is in abundance that causes the protonation of the functional group to confront on biochar that bestows their positive charge. Anionic chromium species (Chromate CrO42- /dichromate Cr2O72-) in solution will cling to the positive charge surface of the biochar through electrostatic attraction [45]. 88 Islam et al. / European Journal of Chemistry 13 (1) (2022) 78-90 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.1.78-90.2189 3Fe+2 + HCrO4-+ 7H+ → 3Fe+3 + 4H2O + Cr+3 (17) CrO4-2 + 8H+ + 3e- → Cr+3 + 4H2O (18) Cr3+ + 3OH- → Cr(OH)3 (19) Fe2+ + 2OH- → Fe(OH)2 (20) Fe3+ + 3OH- → Fe(OH)3 (21) Fe2+ + 2Cr3+ + 8OH- → FeCr2O4 + 4H2O (22) The adsorption of Cr(VI) onto FMRSB is also possible to occur through the adsorption coupled reduction mechanism. This mode of mechanism was first proposed for algae sargassum biomass by Volesky. During this mechanism, the corresponding ion of the interest is reduced as a result the anionic form is converted into the cationic form which is then adsorbed onto FMRSB. The EDS analysis denotes that FMRSB contains a significant concentration of Fe and Mn that are responsible for the reduction of Cr(VI) into Cr(III). Generally, the adsorption coupled mechanism consists of two steps: In the first step when the chromate ion (HCrO4-) approaches the surface of adsorbent Fe2+ already adsorbed on the surface of the adsorbent as a result of modification oxidized to Fe3+ and as a result, electrons are released that reduced the Cr(VI) to Cr(III). In a second step, the Cr(III) interacts with the surface functional groups and forms Cr(OH)3 and FeCr2O4, respectively. The reduction coupled mechanism not only removed the Cr(VI) from wastewater but convert it into chromite which is a stable form. Hence, the subordinate pollution of chromium in the environment is eluded and the acquired chromite will be accurately utilized [5,58]. 4. Conclusion Iron modified rice straw biochar was fabricated and investigated for the removal of Cr(VI) from chromium contaminated water. The SEM, FTIR, and EDS results exhibited that rice straw biochar was successfully modified with iron, which magnifies the Cr(VI) adsorption. This study concluded that FMRSB exhibits excellent Cr(VI) removal capability under acidic conditions. The batch adsorption experiments evince that a preliminary Cr(VI) concentration of 20 mg/L, FMRSB dose of 0.02 g/L, Initial solution pH = 3, and 180 min contact time are the optimum conditions to efficiently remove Cr(VI) from chromium contaminated water. The high adsorption capacity and removal efficiency achieved was 45.45 mg/g and 90.9%, respectively. The best fitting of the experimental equilibrium data to Sips model pointed to a monolayer adsorption mechanism for Cr(VI) removal via homogeneous adsorbent surface. In the kinetic model, the PSO exhibited that Cr(VI) chemisorbed on the surface of FMRSB. The cost-effective preparation of biochar, excellent adsorption capability of toxic metals and easy and environmentally safe operation make biochar a good choice for water treatment. The work can be extended for the removal of numerous toxic chemical species from water. Acknowledgments We are pleased to acknowledge the Centralized Resource Laboratory (CRL), the University of Peshawar, for providing laboratory facilities for the characterization of samples. Furthermore, we did not receive any financial funds to accomplish this project. Disclosure statement Conflict of interests: The authors declare that they have no conflict of interest. Ethical approval: All ethical guidelines have been adhered. Sample availability: Samples of the compounds are available from the author. CRediT authorship contribution statement Conceptualization: Ul Islam Izaz, Ullah Ihsan; Methodology: Ul Islam Izaz, Ahmad Mushtaq; Software: Ahmad Mushtaq, Ul Islam Izaz; Validation: Ullah Ihsan, Formal Analysis: Ul Islam Izaz, Ahmad Mushtaq; Investigation: Ul Islam Izaz, Ahmad Mushtaq; Resources: Ul Islam Izaz, Ahmad Maqbool; Data Curation: Ul Islam Izaz, Ahmad Mushtaq; Writing - Original Draft: Ul Islam Izaz, Writing - Review and Editing: Ullah Ihsan, Rukh Shah; Visualization: Ul Islam Izaz, Ahmad Mushtaq; Supervision: Ullah Ihsan, Rukh Shah; Project Administration: Ullah Ihsan, Ul Islam Izaz. ORCID and Email Izaz Ul Islam izazumislam@gmail.com https://orcid.org/0000-0003-2423-6539 Mushtaq Ahmad mushtaqchem112@gmail.com https://orcid.org/0000-0003-0481-3984 Maqbool Ahmad a.maqbool950@gmail.com https://orcid.org/0000-0002-1948-9200 Shah Rukh passions_rukh@yahoo.com https://orcid.org/0000-0002-3448-0765 Ihsan Ullah ihsan.chem@tju.edu.cn https://orcid.org/0000-0003-4708-4558 References [1]. Rodrigues, E.; Almeida, O.; Brasil, H.; Moraes, D.; dos Reis, M. A. L. Adsorption of Chromium (VI) on Hydrotalcite-Hydroxyapatite Material Doped with Carbon Nanotubes: Equilibrium, Kinetic and Thermodynamic Study. Appl. Clay Sci. 2019, 172, 57–64. [2]. Zewail, T. M.; Yousef, N. S. Chromium Ions (Cr6+ & Cr3+) Removal from Synthetic Wastewater by Electrocoagulation Using Vertical Expanded Fe Anode. J. Electroanal. Chem. (Lausanne Switz) 2014, 735, 123–128. [3]. Shobier, A. H.; El-Sadaawy, M. M.; El-Said, G. F. 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ACS Omega 2020, 5 (42), 27323–27331. [58]. Tripathi, A.; Dwivedi, A. K. Studies on recovery of chromium from tannery wastewater by reverse osmosis. J. Ind. Pollution Control 2013, 289 (1), 29–34. Copyright © 2022 by Authors. This work is published and licensed by Atlanta Publishing House LLC, Atlanta, GA, USA. The full terms of this license are available at http://www.eurjchem.com/index.php/eurjchem/pages/view/terms and incorporate the Creative Commons Attribution-Non Commercial (CC BY NC) (International, v4.0) License (http://creativecommons.org/licenses/by-nc/4.0). By accessing the work, you hereby accept the Terms. This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (http://www.eurjchem.com/index.php/eurjchem/pages/view/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). http://www.eurjchem.com/index.php/eurjchem/pages/view/terms http://creativecommons.org/licenses/by-nc/4.0 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms 1. Introduction 2. Experimental 2.1. Materials 2.2. Methods 2.2.1. Collection of rice straw and preparation of biochar 2.2.2. Preparation of iron modified rice straw biochar 2.3. Characterization 2.4. Batch adsorption 2.4.1. Batch adsorption experiment 2.4.2. Effect of pH 2.4.3. FMRSB dose effect on adsorption 2.4.4. Effect of contact time 2.4.5. Adsorption kinetics of chromium 2.4.6. Effect of preliminary concentration of Cr(VI) 2.4.7. Adsorption isotherms 3. Results and discussion 3.1. Comparative analysis of the removal efficiencies of various adsorbents and selection of suitable adsorbent 3.2. Morphological analysis of RRSB and FMRSB 3.3. EDX elemental analysis of RRSB and FMRSB 3.4. FTIR analysis 3.5. pH effect on the adsorption of Cr (VI) 3.6. FMRSB dosage effect on adsorption 3.7. Contact time effect on the adsorption 3.8. Kinetic models 3.8.1. Pseudo first order kinetic model 3.8.2. Pseudo second order kinetic model (PSO) 3.9. Preliminary concentration effect of Cr(VI) on the adsorption 3.10. Equilibrium model 3.10.1. Langmuir model 3.10.2. Freundlich model 3.10.3. Sips model 3.11. Proposed adsorption mechanism for Cr(VI) 4. Conclusion Acknowledgments Disclosure statement CRediT authorship contribution statement ORCID and Email References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField16: PrintField17: PrintField18: PrintField19: PrintField110: PrintField111: PrintField112: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: PrintField26: PrintField27: PrintField28: PrintField29: PrintField210: PrintField211: PrintField212: