Highly efficient adsorption of Cr(VI) from aqueous solution by Fe3+ impregnated biochar

Biochar (BC) has been widely used as a low-cost adsorbent for the removal of contaminants from water and soil. However, the adsorption ability of BC towards heavy metal oxyanions (e.g., Cr(VI)) is relatively low due to the negatively charged surface of BC. In this study, pristine BC was impregnated with Fe³⁺ to improve its Cr(VI) adsorption capability. Fe³⁺-impregnated BC (Fe³⁺-BC) was successfully synthesized by a simple impregnation method and used for the removal of Cr(VI) from aqueous solution. Various factors affecting the adsorption, such as impregnation ratio, pH, adsorbent dosage, contact time, temperature, and the presence of humic acid, were investigated in detail. Results showed that Fe³⁺-BC had strong adsorption ability to Cr(VI) with a maximum adsorption capacity of 197.80 mg/g, which were not only significantly higher than that of the pristine BC, but also were superior to many previously reported adsorbents. It was favorable for Cr(VI) adsorption under the condition of acidic and high temperature. The adsorption data obeyed Sips and Langmuir isotherms and the kinetic data were well described by the pseudo-first-order kinetic model. The results herein revealed that the Fe³⁺-impregnated BC had a good potential as a highly efficient material for adsorption of Cr(VI) from aqueous solution.     

Adsorption of Cr(VI) using activated neem leaves: kinetic studies

In the present study, adsorbent is prepared from neem leaves and used for Cr(VI) removal from aqueous solutions. Neem leaves are activated by giving heat treatment and with the use of concentrated hydrochloric acid (36.5 wt%). The activated neem leaves are further treated with 100 mmol of copper solution. Batch adsorption studies demonstrate that the adsorbent prepared from neem leaves has a significant capacity for adsorption of Cr(VI) from aqueous solution. The parameters investigated in this study include pH, contact time, initial Cr(VI) concentration and adsorbent dosage. The adsorption of Cr(VI) is found to be maximum (99%) at low values of pH in the range of 1-3. A small amount of the neem leaves adsorbent (10 g/l) could remove as much as 99% of Cr(VI) from a solution of initial concentration 50 mg/l. The adsorption process of Cr(VI) is tested with Langmuir isotherm model. Application of the Langmuir isotherm to the system yielded maximum adsorption capacity of 62.97 mg/g. The dimensionless equilibrium parameter, R _L, signifies a favorable adsorption of Cr(VI) on neem leaves adsorbent and is found to be between 0.0155 and 0.888 (0<R _L<1). The adsorption process follows second order kinetics and the corresponding rate constant is found to be 0.00137 g/(mg) (min).     

Engineered biochar modified with iron as a new adsorbent for treatment of water contaminated by selenium

The purpose of this study was to develop an efficient method of biochar modification for effective removal of Se(VI) ions from water. Commercially available biochar produced from wheat straw was impregnated by Fe(NO₃)₃ (0.8, 4 and 10% w/v) and pyrolyzed at 200 °C. Optimum pH, adsorption kinetics, and Se(VI) adsorption isotherms were determined for the studied biochars. The modification significantly increased biochar’s ability for Se(VI) adsorption. The biochar modified with 10% Fe(NO₃)₃ has the highest adsorption effectiveness. The experimentally determined maximum adsorption capacity for the biochar modified with 10% Fe(NO₃)₃ was 14.3 mg g⁻¹ for pH 5, which was the optimum pH value. X-Ray Photoelectron Spectroscopy (XPS) and Photoacoustic Fourier Transform Infrared Spectroscopy (FTIR-PAS) investigation confirmed the presence of iron oxides/hydroxides on the surface of the modified biochar. The modification also resulted in the formation of oxygen containing functional groups. The study proved that the proposed modification can be efficient in increasing the biochar effectiveness in removing Se(VI) from water.     

Removal of trace Cr(VI) from water using chitosan-iron nanowires in porous anodic alumina

Chitosan-iron nanowires in porous anodic alumina (PAA) have been successfully prepared under ambient conditions as an adsorbent. The adsorbent was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and N₂-BET surface area. The results showed that PAA can disperse and protect Fe⁰ nanorods from oxidation. The adsorption characteristics of trace Cr(VI) onto adsorbent have been examined at different initial Cr(VI) concentrations with pH 5. Batch adsorption studies show that the removal percentage of adsorbent for the removal of trace Cr(VI) is strongly dependent on the initial Cr(VI) concentrations. Langmuir and Freundlich isotherm models were used to analyze the experiment data. The adsorption of trace Cr(VI) by adsorbent is well modeled by the Langmuir isotherm and the maximum adsorption capacity of Cr(VI) is calculated as 123.95 mg/g which is very closed to the experiment results. Intraparticle diffusion study shows that the intraparticle diffusion of adsorbent is not the sole rate-controlling step. The negative value of Gibbs free energy change, ΔG^o, indicated that the process of Cr(VI) onto adsorbent was spontaneous. This work has demonstrated that chitosan-iron nanowires in porous anodic alumina as an adsorbent has promising potential for heavy metal removal at trace level.     

Removal of Cr (VI) from aqueous solution using magnetic biochar synthesized by a single step method

Magnetic biochar, as an adsorbent, was synthesized by a single step method, where iron salt was directly mixed with pinewood sawdust by chemical co-precipitation and subsequently pyrolyzed at 700°C for Cr (VI) removal from aqueous solution. The effects of some important parameters including adsorbent dosage (0.4–2.8?g/L), pH (1–10) of the solution, contact time (0–1440 minutes), initial concentration (30–120?mg/L), and temperature (20–40°C) were investigated in batch experiments. Both pre- and post-adsorbents were characterized by SEM-EDX and XPS to investigate the adsorption mechanism. The maximum adsorption capacity of the tested magnetic biochar under the certain experimental conditions determined as optimal was 42.7?mg/g for Cr (VI). The adsorption data were proved to be suitable for the pseudo-second order model for kinetics and the Langmuir model for isotherms with correlation R²?=?0.9996 andR²?>?0.9980, respectively, after fitting with four kinetic models (pseudo-first order, pseudo-second order, W-M model, and Elovich) and three isotherm models (Langmuir, Freundlich, and Temkin). The characteristic analyses further verified that the efficient particle was a mixture of iron oxides in essence, and it had a strong effect on the spontaneous and endothermic adsorption process.     

Facial Synthesis of Adsorbent from Hemicelluloses for Cr(VI) Adsorption

It is challenging work to develop a low-cost, efficient, and environmentally friendly Cr(VI) adsorbent for waste water treatment. In this paper, we used hemicelluloses from chemical fiber factory waste as the raw material, and prepared two kinds of carbon materials by the green hydrothermal method as adsorbent for Cr(VI). The results showed that hemicelluloses hydrothermally treated with citric acid (HTC) presented spherical shapes, and hemicelluloses hydrothermally treated with ammonia solution (HTC-NH₂) provided spongy structures. The adsorption capacity of the samples can be obtained by the Langmuir model, and the adsorption kinetics could be described by the pseudo-second-order model at pH 1.0. The maximum adsorption capacity of HTC-NH₂ in the Langmuir model is 74.60 mg/g, much higher than that of HTC (61.25 mg/g). The green hydrothermal treatment of biomass with ammonia solution will provide a simple and feasible way to prepare adsorbent for Cr(VI) in waste water treatment.     

Kinetics of chromium(VI) adsorption from model solutions on iron oxide

Adsorption of Cr(VI) on Fe₂O₃ from model solutions with various Cr(VI) concentrations was studied. The adsorption capacity was determined, the constants of chromium(VI) adsorption on iron(III) oxide for the pseudo-second-order model were calculated, and the diffusion coefficients for the process were evaluated.     

Efficiency of a novel nitrogen-doped Fe3O4 impregnated biochar (N/Fe3O4@BC) for arsenic (III and V) removal from aqueous solution: Insight into mechanistic understanding and reusability potential

Worldwide, arsenic contamination has become a matter of extreme importance owing to its potential toxic, carcinogenic and mutagenic impact on human health and the environment. The magnetite-loaded biochar has received increasing attention for the removal of arsenic (As) in contaminated water and soil. The present study reports a facile synthesis, characterization and adsorption characteristics of a novel magnetite impregnated nitrogen-doped hybrid biochar (N/Fe₃O₄@BC) for efficient arsenate, As(V) and arsenite, As(III) removal from aqueous environment. The as-synthesized material (N/Fe₃O₄@BC) characterization via XRD, BET, FTIR, SEM/EDS clearly revealed magnetite (Fe₃O₄) impregnation onto biochar matrix. Furthermore, the adsorbent (N/Fe₃O₄@BC) selectivity results showed that such a combination plays an important role in targeted molecule removal from aqueous environments and compensates for the reduced surface area. The maximum monolayer adsorption (Q_max) of developed adsorbent (N/Fe₃O₄@BC) (18.15 mg/g and 9.87 mg/g) was significantly higher than that of pristine biochar (BC) (9.89 & 8.12 mg/g) and magnetite nano-particles (MNPs) [7.38 & 8.56 mg/g] for both As(III) and As(V), respectively. Isotherm and kinetic data were well fitted by Langmuir (R² = 0.993) and Pseudo first order model (R² = 0.992) thereby indicating physico-chemical sorption as a rate-limiting step. The co-anions (PO₄^3-) effect was more significant for both As(III) and As (V) removal owing to similar outer electronic structure. Mechanistic insights (pH and FTIR spectra) further demonstrated the remarkable contribution of surface groups (OH^–, –NH₂ and –COOH), electrostatic attraction (via H- bonds), surface complexation and ion exchange followed by external mass transfer diffusion and As(III) oxidation into As(V) by (N/Fe₃O₄@BC) reactive oxygen species. Moreover, successful desorption was achieved at varying rates up to 7th regeneration cycle thereby showing (N/Fe₃O₄@BC) potential practical application. Thus, this work provides a novel insight for the fabrication of novel magnetic biochar for As removal from contaminated water in natural, engineering and environmental settings.     

Adsorption-reduction performance of tea waste and rice husk biochars for Cr(VI) elimination from wastewater

In the current study tea waste and rice husk biochars were used for the elimination of Cr(VI) from wastewater with the objectives to study the effect of pH (3–10), shaking time (0.016–24 h), sorbent dose (0.1–1.3 g L⁻¹) and initial concentration of Cr(VI) (10–250 mg L⁻¹). The Cr(VI) sorption was studied under various factors in which solution pH played a main role and at pH 5.2, maximum 99.3% and 96.8% Cr(VI) were removed by tea waste biochar (TWB) and rice husk biochar (RHB), respectively. In comparison, 197.5 mg g⁻¹ and 195.24 mg g⁻¹ Cr(VI) were sorbed by TWB and RHB, respectively with 120 mg L⁻¹ initial Cr(VI) concentration. In contact time study, after 2 h, equilibrium was achieved for both biochars which indicated that the Cr(VI) elimination from aqueous medium is a fast process. Kinetic and isotherm modeling data showed that pseudo-second order model and Langmuir (monolayer sorption) models provided the best fit for sorption of Cr(VI) onto both biochars. The –OH, COO– and –NH₂ functional groups were involved in the sorption of Cr(VI) onto biochars according to FTIR. Biochars produced from both biomass effectively removed Cr(VI) from polluted water, however in comparison sorption capacity of TWB was slightly higher than RHB. It was concluded that TWB and RHB could provide a cost-effective and viable option for elimination of Cr(VI) from wastewater.     

Performance of graphene Oxide/SiO2 Nanocomposite-based: Antibacterial Activity,dye and heavy metal removal

Nanocomposite materials have been successfully applied to remediation of organic and inorganic contaminants from polluted water. The present study investigates the synthesis, characterizations, and adsorptive performances of graphene oxide/SiO₂ nanocomposite-based adsorbent. Graphene oxide/SiO₂ was used for the adsorption of methylene blue (MB) and Cr (VI) ion from wastewater. Furthermore, the antibacterial activity performance of synthesized nanocomposite was studied. The adsorption consideration has been performed by various adsorption parameters in our laboratory. X-ray crystallography (XRD), Scanning electron microscope (SEM), Energy Dispersive X-ray Analysis (EDX) and, thermal gravimetric analysis (TGA) methods were applied in the characterization, morphological structure, crystallinity, and thermal stability of graphene oxide/SiO₂. Maximum capacities of adsorption of graphene oxide/SiO₂-based adsorbent had been evaluated by the Langmuir isotherm model for MB and Cr (VI) ion as 555.50 and 181.81 mg/g, respectively. Generally, adsorption experiments revealed that the performances of graphene oxide/SiO₂ nanocomposite for all adsorbents have been found in the order MB > Cr (VI). Furthermore, antibacterial activity study against gram-positive and gram-negative bacteria showed and proved that graphene oxide/SiO₂ composite showed a remarkable ability to kill bacteria.     

Speciation of Cr(III) and Cr(VI) in Environmental Samples after Solid Phase Extraction on Amberlite XAD–2000

A method for speciation of Cr(III) and Cr(VI) in real samples has been developed. Cr(VI) has been separated from Cr(III) and preconcentrated as its pyrrolidinedithiocarbamate (APDC) complex by using a column containing Amberlite XAD–2000 resin and determined by FAAS. Total chromium has also been determined by FAAS after conversion of Cr(III) to Cr(VI) by oxidation with KMnO₄. Cr(III) has been calculated by subtracting Cr(VI) from the total. The effect of pH, flow‐rate, adsorption and batch capacity and effect of various metal cations and salt anions on the sorption onto the resin were investigated. The adsorption is quantitative in the pH range of 1.5–2.5, and Cr(VI) ion was desorbed by using H₂SO₄ in acetone. The recovery of Cr(VI) was 97 ± 4 at a 95% confidence level. The highest preconcentration factor was 80 for a 200 mL sample volume. The adsorption and batch capacity of sorbent were 7.4 and 8.0 mg g^?1 Cr(VI), respectively, and loading half time was 5.0 min. The detection limit of Cr(VI) is 0.6 μg/L. The procedure has been applied to the determination and speciation of chromium in stream water, tap water, mineral spring water and spring water. Also, the proposed method was applied to total chromium preconcentration in microwave digested moss and rock samples with satisfactory results. The developed method was validated with CRM‐TMDW‐500 (Certified Reference Material Trace Metals in Drinking Water) and BCR‐CRM 144R s (Certified Reference Material Sewage Sludge, Domestic Origin) and the results obtained were in good agreement with the certified values. The relative standard deviations were below 6%.     

Removal of Chromium Ions by Fe3O4‐containing Polypyrrole Matrix and Environmental Applications

In this work, the PPy/Fe₃O₄@TiO₂ composite was synthesized and characterized by X‐ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and magnetic measurements (using a vibrating sample magnetometer). The adsorption performance of PPy/Fe₃O₄@TiO₂ composite for Cr(VI) ions was evaluated by UV irradiation. The effects of pH, adsorbent dose, contact time, and the initial concentration on the adsorption performance of Cr(VI) onto PPy/Fe₃O₄@TiO₂ were investigated. The maximum adsorption capacity of Cr(VI) upon doped PPy/Fe₃O₄@TiO₂ is 85.30 mg/g at room temperature. The total adsorption process likely follows the Langmuir model and pseudo‐second‐order kinetics. Our study suggests that the PPy/Fe₃O₄@TiO₂ composite can be efficiently used for the adsorption of Cr(VI) ions.     

Removal of chromium(VI) from wastewater using Sorel's cement

Summary Synthetic Sorel's cement [3Mg(OH)₂ ^. MgCl₂ ^. 8H₂O], is used as a new adsorbent material for removal of chromium(VI) ion from wastewater effluents. Parameters including contact time, adsorbent dosage and pH are examined and optimized. The equilibrium data are fitted very well to the Langmuir and Freundlich isotherms rather than linear. The adsorption isotherm indicates that the monolayer coverage is 21.4 mg Cr(VI) ion per g of Sorel's cement. The adsorbent is considered as a better replacement technology for removal of Cr(VI) ion from aqueous solutions due to its low cost, good efficiency, fast kinetics, and simple preparation. It offers remarkable efficiency for Cr(VI) removal from wastewater compared with many other natural and synthetic adsorbents.     

Performance and mechanisms of NaOH and ball-milling co-modified biochar for enhanced the removal of Cd2+ in synthetic water: A combined experimental and DFT study

A multifunction magnetic carbonaceous adsorbent (MBM/OH/BC) was synthesized by facile NaOH and ball-milling co-modified biochar, and applied for removing Cd²⁺ in water. The results showed MBM/OH/BC exhibited excellent adsorption capacity (183.59 mg/g for Cd²⁺), short equilibrium time (60 min) and good reusability (the declining efficiency <20% after four adsorption and desorption cycles) compared with pristine biochar. Meanwhile, the modification mechanisms of NaOH and ball-milling to biochar were explored by a series of characterizations (XRD, SEM-EDS, BET, XPS and FTIR, et al). The results indicated the remarkable adsorption performance of MBM/OH/BC was mainly attributed to the co-modification significantly increased the specific surface area, mineral content and cation exchange capacity of biochar, thereby further improving the precipitation, cation exchange and complexation with Cd²⁺. In addition, the results of adsorption mechanisms showed that the joint contribution proportion to the total adsorption capacity of precipitation, cation exchange and complexation was up to 90%, indicating the three mechanisms were the primary adsorption mechanism. To sum up, NaOH and ball-milling co-modification was an effective strategy to increase the adsorption capacity of the pristine biochar for heavy metals from water.     

Enhanced Adsorption of Hexavalent Chromium onto Magnetic Calcium Ferrite Nanoparticles: Kinetic,Isotherm, and Neural Network Modeling

Calcium ferrite nanoparticles with super-paramagnetic behavior were synthesized via simple chemical precipitation method for effective removal of hexavalent chromium from aqueous media. The properties of synthesized nanoparticles were studied by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET), and vibrating sample magnetometer (VSM) measurements. The ferrite nanoparticles have shown polycrystalline nature and high BET specific surface area (229.83 m²/g) with active functional groups on the surface. The adsorption process follows second-order kinetics with the involvement of intra-particle diffusion and adsorption capacity as much as 124.11 mg/g was determined from the Langmuir isotherm. The thermodynamic analysis revealed that the adsorption process was feasible, spontaneous, and exothermic in nature. A three-layer feed-forward back-propagation artificial neural network (ANN) model was employed to predict the removal (%) of Cr(VI) ions as output. Optimal ANN network (4:8:1) shows the minimum mean squared error (MSE) of 0.00161 and maximum coefficient of determination (R²) of 0.984. The adsorption process is mostly influenced by solution pH and followed by adsorbent dosage, initial Cr(VI) concentration, and contact time as illustrated by sensitivity analysis. With small size and high surface area, biocompatibility, ecofriendly nature, easy magnetic separation, and enhanced adsorption capacity towards Cr(VI), calcium ferrite nanoparticles will find its potential application in wastewater remediation.     

Influence of the composition of double oxyhydrates MxMn1?xO2·nH2O (Mz+ = Zr, Al, Sn, Ti) on the sorption activity and cation mobility

Adsorption of Cr(VI) on Fe₂O₃ from model solutions with various Cr(VI) concentrations was studied. The adsorption capacity was determined, the constants of chromium(VI) adsorption on iron(III) oxide for the pseudo-second-order model were calculated, and the diffusion coefficients for the process were evaluated. Original Russian Text ? V.N. Belyakov, T.V. Yatsenko, T.V. Mal’tseva, A.V. Pal’chik, 2009, published in Zhurnal Prikladnoi Khimii, 2009, Vol. 82, No. 5, pp. 753–756.     

Elucidating dominant factors of PO43–, Cd2+ and nitrobenzene removal by biochar: A comparative investigation based on distinguishable biochars

Biochars produced from crab shell (CSB), oak sawdust (OB), Jerusalem artichoke tuber (JAB) and sorghum grain (SB) displayed distinguishable adsorption-related characteristics, such as specific surface area (SSA), ash content and acidic oxygen-containing functional groups (AFGs), which linked to the biochar adsorption mechanisms of most pollutants. Herein, PO₄^3–, Cd²⁺, and nitrobenzene (NB) were employed for adsorption by these biochars to elucidate the dominant factors for the adsorption. Adsorption performance of the three pollutants onto these four biochars varied considerably, as exemplified by the excellent adsorption of PO₄^3– and Cd²⁺ onto CSB (225.3 and 116.0 mg/g, respectively) as compared with onto the other three biochars (4.2–37.1 mg/g for PO₄^3– and 9.7–41.0 mg/g for Cd²⁺). OB displayed the best adsorption of NB (72.0 mg/g), followed by SB (39.5 mg/g), JAB (31.1 mg/g), and CSB (23.6 mg/g). The kinetics and isotherm adsorption assessments couple with material characterization suggested that the sorption of selected pollutants on biochars was attributed to the multiple mechanisms involved, including coprecipitation, chemical bonds, cation exchange, physical absorption, and complexation. Further path analysis suggested that AFGs and ash content in biochars were more important than SSA with regards to pollutant removal, especially, with ash playing a crucial role in the removal of Cd²⁺ and PO₄^3–, and AFGs being mainly responsible for NB adsorption. These findings might offer guidance on the preparation or modification of biochar with a targeted function for pollutant removal through an understanding the dominant factors.     

Optimization of Parameters for Cr(VI) Adsorption on Used Black Tea Leaves

Dynamic characteristics of Cr(VI) sorption on used black tea leaves (UBTLs) as a low-cost adsorbent are studied. Batch experiments were conducted to evaluate the effects of Cr(VI) concentration, solution pH and temperature on the removal process. Both of adsorption and reduction, involved in the process, are affected by the processing parameters. The adsorption kinetics is described successfully using pseudo-second order rate equation and the rate constant decreases with increasing the initial concentration of Cr(VI) up to 150 mg/L (for 0.1 g/L UBTLs) then becomes slow. Experimental and calculated kinetic data for equilibrium are well expressed by Langmuir isotherm. The solution pH has a profound effect on the adsorption rate. The rate constant increases linearly with an increase in temperature, and the low value of activation energy of adsorption, 16.3 kJ/mol, indicates that Cr(VI) is easily adsorbed on UBTLs. The maximum Cr(VI) adsorptive conditions, with a minimum reduction, were achieved from the dynamics of operational parameters: the initial Cr(VI) concentration < 150 mg/L (for 0.1 g/L UBTLs); the initial solution pH = 1.54–2.00 and the processing temperature < 50 ^∘C, for the possibility of its practical application.     

Separation and spectrophotometric determination of very low levels of Cr(VI) in water samples by novel pyridine-functionalized mesoporous silica

A modified SBA-15 mesoporous silica was developed, as an adsorbent, for the removal of Cr(VI) ions from natural-water samples. The effects of experimental parameters, including pH of solution, sample and eluent flow rate, the eluent composition, the eluent volume, and the effect of coexisting ions on the separation and determination of Cr(VI), were investigated. It was shown that Cr(VI) was selectively adsorbed from aqueous solution at pH 3, but Cr(III) could be adsorbed from solution at alkaline pH range. The retained Cr(VI) was eluted with 0.5?mol?L^?1 KCl solution in 0.1?mol?L^?1 Na₂CO₃ subsequently. Under the optimum conditions, the modified mesoporous silica (py-SBA-15) with a high pore diameter exhibited an adsorption capacity of 136?mg?g^?1 and a lower limit of detection than 2.3?µg?L^?1 by using diphenylcarbazide as a chromophorous reagent for the determination of Cr(VI) ions. A preconcentration factor as high as 200 was calculated for Cr(VI). The loaded py-SBA-15 can be reactivated with recovery of more than 98.5% over at least eight cycles. The relative standard deviation (RSD) for Cr(VI) ion recovery was less than 1.8%. Validation of the outlined method was performed by analysing a certified reference material (BCR 544). The proposed method was applied to determine Cr(VI) value in natural and waste water samples successfully.     

Preparation of FeS/iron oxide composite and flax stem‐based porous carbon and its Cr(VI) removal ability

Using flax stem and ferrous sulfate, a composite porous carbon material was prepared by means of high‐temperature roasting and a one‐step process in a muffle furnace. The samples were characterized using X‐Ray diffraction (XRD) and Scanning electron microscopy (SEM), and the effects of ferrous sulfate concentration, carbonization temperature, and pH values of Cr(VI) aqueous solution on the removal performance of Cr(VI) were studied. XRD and SEM analysis showed that the prepared samples were amorphous porous carbon loaded with FeS/Fe₂O₃/Fe₃O₄. High FeSO₄ impregnation concentration, high carbonization temperature, and a low pH value of Cr(VI) aqueous solution were beneficial for Cr(VI) removal. When pH = 2, the amount of Cr(VI) removal was 99.93 mg/g by the sample obtained from 1 g flax powder impregnated in 4.5 mmol FeSO₄/40 mL H₂O solution and calcined for 2 hr at 800°C.