Effect of pH, ionic strength, and background electrolytes on Cr(VI) and total chromium removal by acorn shell of Quercus crassipes Humb. & Bonpl

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.

Amino-functionalized manganese oxide for effective hexavalent chromium adsorption

This study explores the use of functionalized manganese oxide (K-MnO2-NH2) for the removal of hexavalent chromium (Cr(VI)) ions, a highly toxic heavy metal contaminant, from wastewater. The synthesis of K-MnO2-NH2 was achieved through a two-step process, followed by comprehensive characterization using various analytical techniques, which confirmed the material's formation as a pure phase. The K-MnO2-NH2 exhibited exceptional chromium removal efficiency, achieving up to 90% (4.53 mg/g) of Cr(VI) removal at pH 2. This high efficiency is attributed to the incorporation of amine groups via functionalization with 3-aminopropyltriethoxysilane (APTES), which introduces active sites with a strong affinity for Cr(VI) ions. Kinetics studies indicated that a chemical reaction governs the adsorption process, while thermodynamic data suggested it to be exothermic, and thermodynamic data reveal the process to be exothermic. The Freundlich isotherm best described the adsorption behavior. The Cr(VI) adsorption capacity of K-MnO2-NH2 was determined to be 45.17 mg/g. K-MnO2-NH2 effectively removed Cr(VI) from industrial wastewater, achieving a removal efficiency of around 41% (25.5 mg/g) and visible discoloration showing excellent reusability, maintaining over 80% removal efficiency after five cycles without requiring regeneration. This innovative approach highlights the potential of K-MnO2-NH2 as a sustainable and effective solution for Cr(VI) removal in environmental remediation and water purification.

A systematic review on adsorptive removal of hexavalent chromium from aqueous solutions: Recent advances

The contamination of natural resources by hexavalent chromium (Cr(VI)) originating from natural and anthropogenic activities is a serious environmental concern. Although many articles on chromium remediation have been published, a comprehensive understanding of the mechanisms involved in remediation with different sorbents is not yet available. In this systematic review, the performance and applicability of several adsorptive materials for Cr(VI) removal from aqueous media are discussed, along with a detailed analysis of the mechanisms involved. Statistical analysis is applied to compare the efficacies of different adsorbents, while a similar approach is used to determine the effects of sorbent properties and experimental conditions on the adsorption capacity. A detailed analysis of the factors involved in fixed-bed column studies is also presented. A suitable desorption approach to the regeneration of the spent adsorbent and its adsorption performance in reuse is also examined. Among the different sorbents, nanoparticles and mineral-doped biochar were found to be the most effective sorbents, while the adsorption was higher at low pH (~4.0) than that at intermediate pH (6-8). Contrary to our expectation, adsorption was high for sorbents with low specific surface areas, suggesting that the adsorption of Cr(VI) is largely influenced by the chemical properties of the sorbents. The optimum adsorption in fixed-bed column systems is obtained at a lower Cr(VI) ion concentration, a lower influent flow rate, and a higher bed height. Since most of the studies reviewed herein were merely experimental and utilized ideal conditions with the presence of a single contaminant, i.e. Cr(VI) in water, further studies on adsorption dynamics with the presence of other interfering ions are suggested. This review is promising for the further development of Cr(VI) removal strategies and closes the research gaps pertaining to their challenges.

Nano-ZrO2/TiO2 Impregnated Orange Wood Sawdust and Peach Stone Shell Adsorbents for Cr (VI) Removal

Background:

In this work, Cr (VI) adsorption on nano-ZrO2๏TiO2 impregnated orange wood sawdust (Zr๏Ti/OWS) and nano-ZrO2๏TiO2 impregnated peach stone shell (Zr๏Ti/PSS) was investigated by applying different adsorption parameters such as Cr (VI) concentrations, contact time, adsorbent dose, and pH for all adsorbents.

Methods:

The adsorbents were characterized by SEM and FT-IR. The equilibrium status was achieved after 120 min of contact time and optimum pH value around 2 were determined for Cr (VI) adsorption. Adsorption data in the equilibrium is well-assembled by the Langmuir model during the adsorption process.

Results:

Langmuir isotherm model showed a maximum adsorption value of OWS: 21.65 mg/g and Zr๏Ti/OWS: 27.25 mg/g. The same isotherm displayed a maximum adsorption value of PSS: 17.64 mg/g, and Zr๏Ti/PSS: 31.15 mg/g. Pseudo-second-order kinetic models (R2=0.99) were found to be the best models for describing the Cr (VI) adsorption reactions.

Conclusıon:

Thermodynamic parameters such as changes in ΔG°, ΔH°, and ΔS° have been estimated, and the process was found to be spontaneous.

Effect of Ionic Strength and Coexisting Ions on the Biosorption of Divalent Nickel by the Acorn Shell of the Oak Quercus crassipes Humb. & Bonpl

This study investigated the effect of ionic strength and background electrolytes on the biosorption of Ni2+ from aqueous solutions by the acorn shell of Quercus crassipes Humb. & Bonpl. (QCS). A NaCl ionic strength of 0.2 mM was established to have no effect on the Ni2+ biosorption and the biosorption capacity of the heavy metal decreased as the ionic strength increased from 2 to 2000 mM. The background electrolytes (KCl, NaNO3, Na2SO4, CaCl2, MgSO4, and MgCl2) had no adverse effects on the biosorption of Ni2+ at a concentration of 0.2 mM. However, at background electrolyte concentrations of 2 and 20 mM, divalent cations (Ca2+ and Mg2+) had greater negative effects on the biosorption of Ni2+ compared to the monovalent cations (Na+ and K+). Additionally, the SO42− and Cl− anions affected the biosorption of Ni2+. The fractional power, Elovich, and pseudo-second order models represented the kinetic processes of the biosorption of Ni2+ adequately. The results show that QCS can be a promising and low-cost biosorbent for removing Ni2+ ions from aqueous solutions containing various types of impurities with different concentrations.

Characteristics of natural biopolymers and their derivative as sorbents for chromium adsorption: a review

Abstract

Chromium is widely used in industry, and improper disposal of wastewater and industrial residues containing excessive chromium can contaminate water and soil, endangering both environmental and human health. Natural biopolymers and their derivatives have been investigated for removal of chromium (Cr) from wastewater. Cellulose, lignin, tannin, chitin, chitosan, and polypeptides are abundant in nature, and have high potential as adsorbents due to their easy access, low cost, and the recyclability of the captured heavy metals. In order to improve their mechanical strength, recyclability, specific surface area, binding site number, and adsorption rate as adsorbents, native materials have also been modified. This review discusses the source of chromium contamination and the main species of interest, as well as their toxicity. The structures of the aforementioned biopolymers were analyzed, and the adsorption mechanism of chromium and the main influencing factors on this process are discussed. The modification methods of various adsorbents and their adsorption effects on chromium are also detailed, and the developmental direction of research on the use of biopolymer adsorption remediation to control chromium contamination is discussed.

Graphical abstract

Hexavalent chromium removal and total chromium biosorption from aqueous solution by Quercus crassipes acorn shell in a continuous up-flow fixed-bed column: Influencing parameters, kinetics, and mechanism

Continuous fixed-bed column studies were carried out, utilizing acorn shell from Quercus crassipes Humb. & Bonpl. (QCS), in order to remove total chromium and Cr(VI) from aqueous solution. Effects of various fixed-bed column parameters such as influent solution pH, influent flow rate, QCS bed height, and influent Cr(VI) concentration were investigated. Results from the fixed-bed column experiments demonstrate that total chromium biosorption and Cr(VI) removal by QCS depend strongly on the pH of influent solution. The highest capacities for Cr(VI) removal and total chromium biosorption are about 181.56 and 110.35 mg g-1 and are achieved at influent solution pH of 1.0 and 2.0, respectively. Besides this, total chromium biosorption capacities increased from 104.25 to 116.14 mg g-1, 109.07 to 117.44 mg g-1, and 85.02 to 129.87 mg g-1, as bed height, inlet flow rate, and influent Cr(VI) concentration increased from 1.7 to 6.5 cm, 0.25 to 1 mL min-1, and 50 to 400 mg L-1, respectively. The dose-response model defines the entire breakthrough curve for total chromium biosorption onto QCS, under all experimental conditions. X-ray photoelectron spectroscopy (XPS) and biosorption kinetic studies revealed that QCS is able to remove toxic Cr(VI) from acidic liquid solution by means of a complex mechanism that involves the binding of Cr(VI) oxyanions to positively charged groups present at the QCS surface, after which the Cr(VI) species are reduced to Cr(III) by adjacent electron donor groups, and the generated Cr(III) ions then become partially bound to the QCS biomass and partially released into the liquid phase. Results show that QCS can be employed as an easily accessible, abundant, eco-friendly, and inexpensive biosorbent for the removal of total chromium and Cr(VI) from Cr(VI) solutions, in continuous operation.

Effect of pH on hexavalent and total chromium removal from aqueous solutions by avocado shell using batch and continuous systems

Solution pH appears to be the most important regulator of the biosorptive removal of chromium ions from aqueous solutions. This work presents a kinetic study of the effects of solution pH on Cr(VI) and total chromium removal from aqueous solution by Hass avocado shell (HAS) in batch and continuous packed bed column systems. Different Cr(VI) and total chromium removal performances of HAS were obtained in pH-shift batch, pH-controlled batch, and continuous systems. These results emphasize the great importance of determining the most appropriate pH for Cr(VI) and total chromium removal, considering the operational mode of the proposed large-scale treatment system. Total chromium biosorption batch kinetics was well described by the Elovich model, whereas in the continuous system, the fitness of the kinetic models to the experimental data was pH dependent. X-ray photoelectron spectroscopy and kinetic studies clearly indicated that the reaction mechanism of Cr(VI) with HAS was the reductive biotransformation of Cr(VI) to Cr(III), which was partially released to the aqueous solution and partially biosorbed onto HAS.

Chromium Biosorption from Cr(VI) Aqueous Solutions by Cupressus lusitanica Bark: Kinetics, Equilibrium and Thermodynamic Studies

The present study investigated the kinetics, equilibrium and thermodynamics of chromium (Cr) ion biosorption from Cr(VI) aqueous solutions by Cupressus lusitanica bark (CLB). CLB total Cr biosorption capacity strongly depended on operating variables such as initial Cr(VI) concentration and contact time: as these variables rose, total Cr biosorption capacity increased significantly. Total Cr biosorption rate also increased with rising solution temperature. The pseudo-second-order model described the total Cr biosorption kinetic data best. Langmuir´s model fitted the experimental equilibrium biosorption data of total Cr best and predicted a maximum total Cr biosorption capacity of 305.4 mg g(-1). Total Cr biosorption by CLB is an endothermic and non-spontaneous process as indicated by the thermodynamic parameters. Results from the present kinetic, equilibrium and thermodynamic studies suggest that CLB biosorbs Cr ions from Cr(VI) aqueous solutions predominantly by a chemical sorption phenomenon. Low cost, availability, renewable nature, and effective total Cr biosorption make CLB a highly attractive and efficient method to remediate Cr(VI)-contaminated water and wastewater.