Removal Of Chromium Research Articles

Optimizing chromium removal from synthetic wastewater via electrocoagulation process with a natural coagulant (blended of eggshell powder and lime) using response surface methodology

The presence of chromium (Cr) in synthetic wastewater has become a serious environmental issue. Therefore, main aim of this work was to investigate Cr removal from synthetic wastewater via electrocoagulation (EC) with a natural coagulant using aluminum electrodes. The central composite design (CCD) of the response surface methodology (RSM) method was used to optimized the operating variables of solution pH (5–9), initial Cr concentration (225–475 mgL-1), reaction time (30–40 min), and applied current (0.35–0.55 A). The ANOVA results clearly shows that the quadratic model (p < 0.0001) was sufficient to the best predicting of the removal performance of Cr (R2 = 0.9994 for electrode distance of 0.5 cm and 0.9924 for 1 cm). The maximum removal (99.836 % for electrode distance of 0.5 cm, and 98.175 % for 1 cm) of Cr was achieved with optimized conditions of solution pH 7.053, initial Cr concentration 337.795 mgL−1, reaction time 37.148 min, and applied current of 0.505 A. From this finding, it was proved that the EC process assisted with natural coagulant is an efficient, and cost-effective method for the removal of Cr from synthetic wastewater.

Metal-organic-framework and walnut shell biochar composites for lead and hexavalent chromium removal from aqueous environments

Extensive research in recent years has explored the realm of porous carbon composites for various applications, including electrochemistry, structural materials, environmental remediation, and more. In particular, the fabrication of porous carbon composites using a metal-organic framework (MOF) and biochar (BC) for aqueous remediation is a fairly new avenue of research. In this study, a MOF-BC composite was synthesized with unmodified and chemically modified BCs using solvothermal synthesis. The composites were used as adsorbents to remediate heavy metals, such as lead (II) and chromium (VI), from aqueous environments. It was verified that the MOF was homogeneously deposited onto the BC's surface using various material characterization techniques. Lead and chromium adsorption studies revealed a high adsorption capacity with greater than 99% removal for lead and ∼65% for chromium, respectively. Impressively, for lead, the highest observed experimental adsorption capacity of the MOF-chemically modified BC composite was 535 mg/g, compared to 240 mg/g for pristine BC. Meanwhile, the adsorption capacity of the same MOF-BC composite for chromium ions was low at 18 mg/g, compared to 80 mg/g for the chemically modified BC. The MOF-BC had a rapid adsorption rate, achieving equilibrium at only 150 min of reaction time for lead ions. MOF-BCs have higher adsorption for cationic lead through physisorption and ion-exchange mechanisms, whereas, for anionic chromium, removal is dominated only by physisorption mechanisms. The outcomes and methodological developments attained in this study offer a novel and compelling approach for synthesizing MOF-BC composites for aqueous remediation applications.

Optimizing Hexavalent Chromium Removal in Italy

Optimizing Hexavalent Chromium Removal in Italy

Addressing mathematical discrepancies in the adsorption model of Hashem et al. (2024): A technical commentary

This letter provides a critical technical analysis of the application of the Freundlich isotherm in the study by Hashem et al. (2024), entitled "Chromium adsorption capacity from tannery wastewater on thermally activated adsorbent derived from kitchen waste biomass." While the study presents an innovative approach by utilizing pumpkin peel as an adsorbent for chromium removal, a critical issue was identified in the formulation and application of the Freundlich isotherm. The authors presented an incorrect mathematical modification of the isotherm, which significantly impacts the validity of the adsorption parameters derived, such as KF and 1/n, and ultimately affects the conclusions drawn about the system's adsorption capacity. Additionally, the assertion that the adsorption process can continue indefinitely is fundamentally flawed based on the principles of adsorption thermodynamics. This letter aims to rectify these discrepancies by revisiting the correct formulation of the Freundlich isotherm and highlighting the importance of accurate mathematical modeling in ensuring scientifically sound and replicable results. Despite these issues, the study remains a valuable contribution to the field, and the suggested revisions would further strengthen its impact.

Porous biochar-driven innovative Cr(VI) reduction and resource recovery

Catalyst-mediated Cr(VI) reduction face significant challenges in by-product ion residues and catalyst cost. The development of low-cost catalysts and catalytic technologies with near-zero impurity introduction is therefore of great practical importance. This study developed a pristine biochar-mediated nanoconfinement strategy that facilitated the chemical reaction between oxalic acid (OA) and Cr(VI) with a stoichiometric ratio of 3.3:1. The method achieved the simultaneous Cr(VI) reduction and OA removal, and completed the chromium removal process with near-zero impurity introduction after a precipitation process. While traditional catalyst-mediated reduction relied on mechanisms of functional group or active sites, this study took a novel path by adopting a focus on pore size. The structure–activity relationship, and experimental design demonstrated the predominant role of porous properties in the reaction, thus proposing a nanoconfinement Cr(VI) reduction mechanism. Continuous flow results revealed a treatment capacity of 1750 mL/h for the complete removal of Cr(VI) from Cr electroplating rinse water. The effluent, precipitated with CaO, had 0.06 mg/L of total Cr and 0.56 mg/L of OA, while the precipitate, recovered as CaCr2O4, demonstrated a total Cr recovery efficiency of 90.4%. Compared to industrial Cr(VI) chemical reduction, this study minimizes impurity introduction to the catalytic system. This study explores an effective strategy for Cr(VI) reduction and Cr resource recovery using low-cost pristine biochar, and provides navigation for the removal and resource utilization of other heavy metals or radionuclides.

Development of free flowing granular hybrid functionalized clay sorbent filters for chromium removal from waste water

Chromium (VI) contamination in water resources at industrial sites poses significant environmental and health risks. Conventional sorbents often suffer from limitations such as clogging, back pressure, poor kinetics, and challenges in coupling with flow systems and regeneration. This study focuses on developing, characterizing, and applying free-flowing granular (FFG) amidoxime-functionalized montmorillonite (Mt) clay sorbents (AO-Mt-H-C) to enhance chromium (VI) removal from contaminated waste water. Batch adsorption experiments demonstrated that AO-Mt-H-C outperformed montmorillonite clay modified with quaternary ammonium alone (Mt-H-C), achieving a Cr (VI) removal efficiency of 50.6 mg g−1 compared to 44.3 mg g−1, due to the synergistic interactions of quaternary ammonium and amidoxime functionalities. To address the limitations of conventional sorbents, such as clogging and poor kinetics, an FFG filter column was developed and tested, showing effective Cr (VI) removal across various water sources with minimal interference in drinking and groundwater. In industrial wastewater with higher matrix loads, over 99 % removal efficiency was achieved with a 25 % increase in sorbent dosage. Desorption and reusability assessments confirmed the AO-Mt-H-C's effectiveness across three cycles. Mathematical modelling using Thomas and Yoon–Nelson equations supported the design of customized treatment systems. Scalability was demonstrated by treating 1.6 m3 of textile industrial effluents (100 mg L−1 Cr (VI)) with quantitative removal efficiency (>99.5 %). This study illustrates that the hybrid functionalization and free-flowing granulation of clay sorbent materials significantly enhance Cr (VI) removal, providing valuable insights for advanced water pollution mitigation and environmental sustainability.

Hydrochar produced from mixed feedstocks as efficient adsorbent for selenium and chromium removal from acidic wastewater

Selenium (Se) and chromium (Cr) in water might be harmful because of their high toxicity in the aquatic environment. Efficient removal of their hexavalent forms (Se(VI) and Cr(VI)) from extreme acidic wastewater is still challenging. Therefore, several new hydrochars suited for efficient removal of trace Se(VI) and Cr(VI) from acidic wastewater were fabricated from biomass through a hydrothermal method. Most functional groups were retained in the hydrochar, which gave it an excellent adsorption capacity (43.2 mg·g−1 and 132 mg·g−1 for Se(VI) and Cr(VI), respectively). After one batch treatment at exceptionally low pH and a hydrochar dose of 0.2 g·L−1, Se(VI) and Cr(VI) could be removed from 1 mg·L−1 to 6 μg·L−1 and 15 μg·L−1, respectively. The mechanism of adsorption at various pH was elucidated by Scanning Electron Microscopy, Thermogravimetry, Fourier Transform Infrared Spectrometer, X-Ray Photoelectron Spectroscopy and density functional theory (DFT) calculation. The adsorption mechanism for Se(VI) in acidic solution involved the binding of SeO42− with protonated amino groups (−NH3+) of Mix hydrochar. In the pH range of 6.5 to 7, Cr(VI) combined with −NH3+ functional groups primarily in the form of CrO42−. When the pH fell below 6.5, Cr(VI) bound with both −NH3+ and carbonyl groups (−C=O) in the form of HCrO4−. Fast kinetics were observed during the removal process of Se(VI) and Cr(VI). The hydrochar adsorbent showed good stability in acidic environments and during many adsorption cycles. These findings provide theoretical basis for design of adsorption materials for acidic wastewater treatment.

Removal of chromium and copper from synthetic water samples using passion fruit peels

In this work, modified passion fruit husk was used as biomass for the removal of chromium and copper present in the waters. PH, contact time, and particle size were considered experimental factors. Stirring speed, dosage, and temperature were considered constants. The results showed a 69% decrease in chromium using a pH 4, 90 min, and 300 μm composition. For copper, 74.4% was removed at pH 4, with 120 min and 300 μm, based on initial concentrations of 30 ppm for both metals. The results showed that passion fruit peel is a potential effective removal agent for chromium and copper in synthetic waters. This supports its possible application in the purification of metal-contaminated water, which could contribute to environmental protection.

Anthill clay activated Ocimum gratissimum extract for effective adsorption of methylene blue and chromium (VI) ion from wastewater: Insights into the adsorption isotherms, kinetics, thermodynamics, and mechanisms

The removal of methylene blue (MB) dye and hexavalent chromium (Cr(Vl)) ions from water bodies have necessitated the search for a promising adsorbent material with high adsorptive performance, facile operation, and economical. Here, a novel adsorbent was prepared by activating anthill clay (AC) with Ocimum gratissimum leaf extract (OG@AC) by ultrasonic strategy method to eliminate MB and Cr(Vl) from the aqueous environment; however, the activation strategy serves as an important factor that influenced the adsorptive performance towards MB and Cr(Vl). Furthermore, a high BET surface area of 120.48 m2/g was measured for OG@AC which is attributed majorly to the green activation strategy. The SEM micrograph of OG@AC was seen to have displayed a leaf-like plate formed by quartz and kaolin. In the batch experimental study, the MB and Cr(Vl) adsorption capacities towards OG@AC were determined to be 227.52 and 143.45 mg/g, respectively. Also, the adsorption kinetics and isotherms models results revealed that the adsorption process of OG@AC towards MB and Cr(Vl) was best described by pseudo-first-order and Freundlich models, suggesting that the adsorption process involves a multilayer physisorption. Moreso, thermodynamic results revealed the adsorption process is feasible, spontaneous, and endothermic in nature. The OG@AC exhibited an outstanding regeneration ability and stability towards MB and Cr(Vl) by maintaining a removal efficiency of 79.11 % and 81.20 % even at the tenth successive adsorption-desorption cycles. Overall, this research sheds more insight on how anthill clay is activated using a feasible design and green chemical (Ocimum gratissimum leaf extract) to enhance its adsorptive performance towards MB and Cr(Vl), which can broaden its wide application in the real world.

Electrokinetic Remediation of Chromium-Contaminated Puliyantangal Lake Sediments in India and the Effect of Bone Dust Amendment in Chromium Removal

Electrokinetic Remediation of Chromium-Contaminated Puliyantangal Lake Sediments in India and the Effect of Bone Dust Amendment in Chromium Removal

Constructed wetland coupled microbial fuel cell (CW-MFC) with Phragmites australis planted for hexavalent chromium removal and electricity generation

Hexavalent chromium (Cr(VI)) in aqueous solution is characterized by strong mobility, high solubility, and significant biological toxicity. In this study, four constructed wetland combined with microbial fuel cell (CW-MFC) systems were established to explore the changes in electricity generation, water purification, and microbial community structure under the influence of plant, bioelectrochemical, and Cr(VI) stress. The planted CW-MFC (total Cr: 97.5 %, Cr(VI): 99.2 %) and non-planted CW-MFC (total Cr: 95.3 %, Cr(VI): 97.3 %) showed significantly higher Cr removal efficiency than open circuit CW-MFC (total Cr: 86.9 %, Cr(VI): 84.8 %). The presence of plants and Cr(VI) increased the average voltage and power density of CW-MFC. Additionally, plants, bioelectrochemistry, and Cr(VI) improved COD removal and significantly enhanced the abundance and diversity of microorganisms in the system during long-term domestication. The Cr(VI)-reducing bacterium Exiguobacterium was least abundant in non-Cr(VI) containing CW-MFC system. The abundance of the electrochemically active bacteria (EAB) Geobacter increased under bioelectrochemistry and Cr(VI).

Selective removal of chromium and chloride by flow electrode capacitive deionization (FCDI) with carrier-facilitated ion exchange membrane

In confronting the widespread challenge of chromium (Cr(VI)) pollution in industrial effluents, this study pioneers the integration of flow electrode capacitive deionization (FCDI) with carrier-facilitated ion exchange membrane (IEM) for selective heavy metal removal, which offers a marked enhancement in selectivity and energy efficiency over conventional treatments. Our comprehensive study demonstrates that the synergistic system significantly optimizes Cr(VI) elimination from low-concentration effluents, achieving an unprecedented selectivity coefficient of 20.6 under optimal operational parameters. The system performance is critically influenced by the system’s operational parameters, including current density and ion concentration, with lower current densities favoring reduced energy consumption and higher selectivity without compromising removal efficiency. The pronounced permeability coefficient suggested that the Cr(VI) transport in the membrane occurs via continuous pathways, rather than fixed-site jumping mechanism. Furthermore, this innovative configuration exhibits remarkable adaptability to fluctuating Cr(VI) and Cl− concentrations, ensuring consistent high-performance removal. Comparative analysis with traditional electrodialysis (ED) systems illustrates the potential of our proposed method in delivering significant energy savings and operational benefits. This investigation not only broadens the horizon of membrane separation technology but also underscores the potential of FCDI-based processes in fostering the development of more sustainable water treatment and hazardous material remediation strategies.

Surface modified novel synthesis of Spirulina assisted mesoporous TiO2@CTAB nanocomposite employed for efficient removal of chromium (VI) in wastewater

In this study, an investigation of the adsorption efficiency of a novel Spirulina based TiO2@CTAB nanoparticle is carried out against one of the highly toxic heavy metal Cr (VI). The co-precipitation method was used to prepare the nanoparticles which were analyzed by various techniques including XRD, UV–Vis spectroscopy, Raman Spectroscopy, FT-IR, BET, FESEM with EDX, Zeta Potential, and Particle Size Analyzer. The pore diameter of the TiO2@CTAB nanocomposite was found to be 3.425 nm with a total surface area of 125.709 m2/g. The pHZPC and XPS studies were used to describe the mechanism of adsorption of Cr (VI) by TiO2@CTAB. For synthesized material, the Langmuir adsorption isotherm model exhibits improved adsorption than the Freundlich adsorption isotherm model, having a removal capacity of 127.551 mg/g. The adsorption of CR (VI) on the surface of TiO2@CTAB follows pseudo-second-order kinetics having an R2 value of 0.99983. Furthermore, the prepared mesoporous TiO2@CTAB adsorbent has high reusability without considerable degradation and may avoid the production of secondary pollutants to a large extent.

Characteristics of soil pore structure response to electric field strength and their effects on Cr(VI) removal from a historically chromium-contaminated soil

The characteristics of soil pore structure response to electric field strength and their effects on the migration of hexavalent chromium during electrokinetic remediation remain unclear. This paper investigates the effects of soil actual voltage and pore redistribution on hexavalent chromium removal under various voltage gradients during electrokinetic remediation. The electric field across the soil was the dominant factor that affected the soil properties, pore structure, and transportation of hexavalent chromium. The removal efficiency of dissolved hexavalent chromium was significantly enhanced from 36.1 % to 80.5 % as the theoretical voltage gradients increased from 0.5 V·cm−1 to 3 V·cm−1 due to the highest voltage proportion (52.7 %) of the soil chamber at 3 V·cm−1, while it only increased to 82.0 % at 4 V·cm−1. The optimal actual electric field strength of 1.00–1.50 V·cm−1 across the soil matrix facilitated the efficient electromigration of hexavalent chromium. The electric field force exerted a direct (0.70) or indirect action intensity (1.55) on soil pore structure by altering soil chemical properties. Soil mesopores and ultra-micropores transformed into micropores through collapsing, shrinking, ion migration, weakening electrostatic repulsion, and enhancing adhesive bonding among particles with increasing electric field. The distribution and structure of soil pores were more homogenous under optimal electric field conditions, thereby improving the migration path of hexavalent chromium. Soil mesopores facilitated hexavalent chromium removal and the formed micropores were opposite. These findings have significant scientific and practical implications for applying the in-situ electrokinetic techniques in the remediation of metal-contaminated soils.

Amine-functionalized cellulose-silica composites for the remediation of hexavalent chromium (Cr IV) in contaminated water

Adsorption method for Hexavalent chromium (Cr(VI) removal from domestic and industrial wastewater is widely desirable due to public health concern of the heavy metal. The purpose of this study was to investigate the evaluation of Cr(VI) adsorption using a novel adsorbent: amine-functionalized cellulose-silica composite derived from banana fibers. We employed the in-situ sol–gel method to create cellulose-silica silane functionalized composites, analyzing them through different characterization techniques such as Attenuated total reflectance- Fourier transform infrared spectroscopy (ATR-FTIR), X-ray Diffraction Analysis (XRD), Thermo gravimetric analysis (TGA)/differential thermogravimetric (DTG), Brunauer–Emmett–Teller (BET), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) techniques. ATR-FTIR depicted key organic constituents in raw banana pseudo stem fibers (BF) and the formation of SiO bonds in BCSiO2 composite, and further enhanced by the grafting of N-[3-(trimethoxysilyl)propyl ethylenediamine (DAPTMS) onto the BC-SiO2 surface in BC-SiO2-DAPTMS. XRD and TGA/DTG analyses revealed changes in crystallinity and thermal stability, while BET analysis showcased altered surface area and pore characteristics in BC-SiO2-DAPTMS (2 %). SEM and TEM imaging provided visual evidence of structural modifications and improved dispersion in BC-SiO2-DAPTMS composites. The impact of composite weight, contact time, and pH on Cr(VI) removal rates was examined, revealing optimal performance at slightly acidic pH 4 value (80.7 %) and enhanced efficiency with increased contact time of 65 min (86.66 %), composite weight of 1 g (82.62 %), and initial concentration was for 0.8 mg/l (80 %). The kinetics and isotherms analyzed using pseudo second order (PSO) and pseudo first order (PFO) models, highlight the composite’s efficiency. The Freundlich model was found to better fit the adsorption isotherm data, while the PSO model described the kinetics more accurately. These insights contribute to optimizing the BC-SiO2-DAPTMS (2 %) composite for efficient Cr(VI) ion removal in water treatment applications.

BaTiO3 Nanoparticles for highly efficient piezocatalytic reduction of toxic hexavalent chromium: Synthesis, optimization, and kinetic study

Drinking water contamination with heavy metals from industrial activities is an important issue. In particular, the hexavalent chromium ions Cr (VI) which are classified as group (A) carcinogens. Our research reports an effective removal of hexavalent chromium from water through the use of BaTiO3 (BTO) as a piezocatalyst. We meticulously prepared pure tetragonal BTO by optimizing the synthesis parameters. The optimized BTO catalyst was utilized for the removal of Cr(VI) under applied mechanical force and removed up to 96 % of Cr(VI) from the solution. Our kinetic study revealed that the reaction follows a pseudo-first order. Thermodynamic studies indicated that the reaction is most spontaneous at temperatures of 20 °C and concentrations of 20 ppm. Our proposed mechanism for removing Cr(VI) by BTO piezocatalysts suggests that only 15 % of chromium was removed by adsorption, while the rest was removed by reduction through the piezocatalytic process.

Fungi species identified from polluted environment for chromium sequestration and solid state fermentation on tannery shaving waste

Accumulation of hexavalent chromium in the environment affects our ecosystem. Physico-chemical treatment did not solve the problem, due to high requirement of operational costs & energy, insolubility of Cr (VI) at any pH & sludge accumulation. The study aim is screening and evaluating fungi for chromium biosorption for mycoremediation. Soil and effluent sample were collected. PDA media was used for fungi purification. Fungal chromium tolerance evaluation and optimization was conducted at pH (4, 7, 10), T0 (25, 27.5, 35 °C), Cr concentration (5000–25000 ppm) using Full Factorial experimental design. Spore count, biomass production, Cr tolerance index, mycelia growth were evaluated. Flame Absorptive Spectrophotometer was used for quantification of fungal chromium removal. Fungi species were identified by MALDI-TOF. Fungi growth & degradation of tannery shaving waste was evaluated. The result revealed 6 potential fungi, T. longibrachiatum, T, koningii, A. niger, G. candidum, P. rubens, T. orientale were identified. Optimium mean biomass & spore count were 0.514 ± 0.051& 1.39 ± 0.038 at pH4 and 27.5 °C respectively. MTL test indicate that Trichoderma species, A. niger & P. rubens survive up to 10,000 ppm, 15, 000 ppm & 20,000 ppm respectively. FAAS analysis of fungi chromium removal from aqueous media result indicate that A. niger (99%). T. longibrachiatum (85%) and P. ruben (84.6%). These 6 fungi are efficient in tannery shaving waste degradation but A. niger was superior in spore count 15.1 Log/mL and 7 g substrate weight loss. In conclusion three fungi have high chromium tolerance and removal capacity and shaving waste degradation promising for mycoremediation.

Sustainable Removal of Cr(VI) from Wastewater Using Green Composites of Zero-Valent Iron and Natural Clays

Composites for efficient removal of hexavalent chromium Cr(VI) from industrial wastewater were obtained by deposition of nano-zero-valent iron (nZVI), synthesized by environmentally friendly synthesis using oak leaf extract, on inexpensive, natural, readily available and cheap natural raw materials, sepiolite (SEP) or kaolinite/illite (KUb) clay, as support. nZVI particles were deposited from the FeCl3 solution of different concentrations, with the same volume ratio extract/FeCl3 solution (3:1), and with different masses of SEP or KUb. Physico–chemical characterization (SEM/EDS, FTIR, BET, determination of point of zero charge) of the composites and nZVI was performed. The results of SEM and BET analyses suggested more homogeneous deposition of nZVI onto SEP than onto KUb, which ensures greater availability of the nZVI surface for Cr(VI) anions. Therefore, the higher Cr(VI) removal at all investigated initial pH values (pHi) of the solution (3, 4 and 5) was achieved with the SEP composites. The adsorption results indicated that the elimination of Cr(VI) was achieved via the combined effect of reduction and adsorption. The removal of total chromium at pHi = 3 was approximately the same as that of Cr(VI) removal for the KUb composites, but lower for the SEP composites, indicating lower removal of Cr(III) compared to the reduced Cr(VI). The SEP/nZVI composite with the highest removal efficiency was applied for Cr(VI) removal from real wastewater at pHi = 3 and pHi = 5. The results demonstrated the high Cr(VI) removal capacity, validated the assumption that a good dispersion of nZVI particles is beneficial for Cr(VI) removal and showed that the produced green composites can be efficient materials for the removal of Cr(VI) from wastewater.

Enhancing hexavalent chromium removal from textile effluent with low-cost adsorbent: simulation and a techno-economic study

AbstractThis paper simulated hexavalent chromium (Cr(VI)) adsorption using cocoa pod husk biosorbent in a fixed bed column using Aspen Adsorption. This study was designed to show the effectiveness of computational methods in designing, optimising and evaluating the scaled-up adsorption process using low-cost adsorbents. To the best of our knowledge, the economic analysis of Cr(VI) removal using biosorbent adsorption columns with the assistance of Aspen Adsorption and response-surface methodology (RSM) has not been performed previously. Design Expert and RSM were used to optimise and describe the effect of flow rate and initial concentration on breakthrough and saturation times. The breakthrough time was improved by a higher bed height (2.0 m), a wider diameter (2.0 m), and lowering the flow rate (0.010 L/s). The initial concentration had no effect (1.00 mol/L). The predicted breakthrough and saturation time were 29,360 s and 313,351 s, respectively. Two scenarios were economically compared over 20 years. Scenario 1 (1-day breakthrough time) costs $746,585 and Scenario 2 (4-week breakthrough time) costs $1,538,319. This is because Scenario 2 used a taller, wider column which required a greater amount of adsorbent, and 387,873 m3 of water were processed, respectively. Processed water was dependent on the flow rate and breakthrough time. It was concluded that cocoa pod husk could be an efficient adsorbent and the adsorption process can be successfully simulated and optimised. The use of alternative low-cost adsorbents should be encouraged. The economic study showed that simulation and RSM data could successfully be used for economic analysis. Graphical abstract

Effect of straw decomposition on hexavalent chromium removal by straw: Significant roles of surface potential and dissolved organic matter

Mobility and bioavailability of hexavalent chromium (Cr(VI)) in agricultural soils are affected by interactions between Cr(VI) and returned crop straws. However, the effect of straw decomposition on Cr(VI) removal and underlying mechanisms remain unclear. In this study, Cr(VI) removal by pristine and decomposed rice/rape straws was investigated by batch experiments and a series of spectroscopies. The results showed that straw decomposition inhibited Cr(VI) removal, regardless of straw types. However, the potential mechanisms of the inhibition were distinct for the two straws. For the rice straw, a lower zeta potential after decomposition suppressed Cr(VI) sorption and subsequent reduction. In addition, less Cr(VI) was reduced by the decomposed rice straw-derived dissolved organic matter (DOM) than the pristine one. In contrast, for the rape straw, due to the increased zeta potential after decomposition, the decreased Cr(VI) removal was mainly ascribed to less Cr(VI) reduction by the rape straw-derived DOM. These results emphasized the significant roles of straw surface potential and DOM in Cr(VI) removal, depending on straw types and decomposition, which facilitate the fundamental understanding of Cr(VI) removal by straws and are helpful for predicting the environmental risk of Cr and rational straw return in Cr(VI)-contaminated fields.