Simultaneous adsorption and reduction of hexavalent chromium on biochar-supported nanoscale zero-valent iron (nZVI) in aqueous solution

Adsorption of Hexavalent Chromium Using Activated Carbon Produced from Sargassum ssp.: Comparison between Lab Experiments and Molecular Dynamics Simulations

Adsorption is one of the most successful physicochemical approaches for removing heavy metal contaminants from polluted water. The use of residual biomass for the production of adsorbents has attracted a lot of attention due to its cheap price and environmentally friendly approach. The transformation of Sargassum-an invasive brown macroalga-into activated carbon (AC) via phosphoric acid thermochemical activation was explored in an effort to increase the value of Sargassum seaweed biomass. Several techniques (nitrogen adsorption, pHPZC, Boehm titration, FTIR and XPS) were used to characterize the physicochemical properties of the activated carbons. The SAC600 3/1 was predominantly microporous and mesoporous (39.6% and 60.4%, respectively) and revealed a high specific surface area (1695 m2·g-1). To serve as a comparison element, a commercial reference activated carbon with a large specific surface area (1900 m2·g-1) was also investigated. The influence of several parameters on the adsorption capacity of AC was studied: solution pH, solution temperature, contact time and Cr(VI) concentration. The best adsorption capacities were found at very acid (pH 2) solution pH and at lower temperatures. The adsorption kinetics of SAC600 3/1 fitted well a pseudo-second-order type 1 model and the adsorption isotherm was better described by a Jovanovic-Freundlich isotherm model. Molecular dynamics (MD) simulations confirmed the experimental results and determined that hydroxyl and carboxylate groups are the most influential functional groups in the adsorption process of chromium anions. MD simulations also showed that the addition of MgCl2 to the activated carbon surface before adsorption experiments, slightly increases the adsorption of HCrO4- and CrO42- anions. Finally, this theoretical study was experimentally validated obtaining an increase of 5.6% in chromium uptake.

Removal of hexavalent chromium via biochar-based adsorbents: State-of-the-art, challenges, and future perspectives

Chromium originates from geogenic and extensive anthropogenic activities and significantly impacts natural ecosystems and human health. Various methods have been applied to remove hexavalent chromium (Cr(VI)) from aquatic environmental matrices, including adsorption via different adsorbents, which is considered to be the most common and low-cost approach. Biochar materials have been recognized as renewable carbon sorbents, pyrolyzed from various biomass at different temperatures under limited/no oxygen conditions for heavy metals remediation. This review summarizes the sources, chemical speciation & toxicity of Cr(VI) ions, and raw and modified biochar applications for Cr(VI) remediation from various contaminated matrices. Mechanistic understanding of Cr(VI) adsorption using different biochar-based materials through batch and saturated column adsorption experiments is documented. Electrostatic interaction and ion exchange dominate the Cr(VI) adsorption onto the biochar materials in acidic pH media. Cr(VI) ions tend to break down as HCrO₄^-, CrO₄^2-, and Cr₂O₇^2- ions in aqueous solutions. At low pH (∼1-4), the availability of HCrO₄^- ions attributes the electrostatic forces of attraction due to the available functional groups such as -NH₄⁺, -COOH, and -OH₂⁺, which encourages higher adsorption of Cr(VI). Equilibrium isotherm, kinetic, and thermodynamic models help to understand Cr(VI)-biochar interactions and their adsorption mechanism. The adsorption studies of Cr(VI) are summarized through the fixed-bed saturated column experiments and Cr-contaminated real groundwater analysis using biochar-based sorbents for practical applicability. This review highlights the significant challenges in biochar-based material applications as green, renewable, and cost-effective adsorbents for the remediation of Cr(VI). Further recommendations and future scope for the implications of advanced novel biochar materials for Cr(VI) removal and other heavy metals are elegantly discussed.

Biochar-supported nZVI for the removal of Cr(VI) from soil and water: Advances in experimental research and engineering applications

Over the past decade, biochar-supported nZVI composites (nZVI/biochar) have been developed and applied to treat various pollutants due to their excellent physical and chemical properties, especially in the field of chromium (VI) removal. This paper reviewed the factors influencing the preparation and experiments of nZVI/biochar composites, optimization methods, column experimental studies and the mechanism of Cr(VI) removal. The results showed that the difference in raw materials and preparation temperature led to the difference in functional groups and electron transfer capabilities of nZVI/biochar materials. In the experimental process, pH and test temperature can affect the surface chemical properties of materials and involve the electron transfer efficiency. Elemental doping and microbial coupling can effectively improve the performance of nZVI/biochar composites. In conclusion, biochar can stabilize nZVI and enhance electron transfer in nZVI/biochar materials, enabling the composite materials to remove Cr(VI) efficiently. The study of column experiments provides a theoretical basis for applying nZVI/biochar composites in engineering. Finally, the future work prospects of nZVI/biochar composites for heavy metal removal are introduced, and the main challenges and further research directions are proposed.

Efficient Removal of Hexavalent Chromium from an Aquatic System Using Nanoscale Zero-Valent Iron Supported by Ramie Biochar

In this study, ramie biochar (RBC) was used to activate nano zero-valent iron (nZVI) to enhance hexavalent chromium (Cr(VI)) removal. The best results were obtained at a pyrolysis temperature of 600 °C, a biochar particle size of < 150 μm, and an iron to carbon ratio = 1:1. Under the optimal conditions, the removal of Cr(VI) by RBC600-nZVI (98.69%) was much greater than that of RBC600 (12.42%) and nZVI (58.26%). Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) revealed that the reaction mechanism at the Fe and Cr interface was a multiple interaction mechanism with reduction dominated, adsorption, and co-precipitation simultaneously. The enhanced performance of RBC600-nZVI resulted from the effective dispersion of nZVI on the surface of RBC600, therefore increasing the adsorption activity sites. At the same time, RBC600 and nZVI exerted a synergistic influence on the composite structure, which jointly promoted the reduction reaction of Cr(VI) and removed more Cr(VI). This study shows that RBC-nZVI is a potentially valuable remediation material that not only provides a new idea for the utilization of ramie waste, but also effectively overcomes the limitations of nZVI, thus, achieving efficient and rapid remediation of Cr(VI).

A novel preparation of S-nZVI and its high efficient removal of Cr(VI) in aqueous solution

The chitosan-stabilized biochar supported S-nZVI (CS@BC/S-nZVI) composite with low aggregation and superior antioxidation were successfully synthesized by liquid-phase reduction method for the outstanding removal of Cr(VI) from wastewater and characterized by SEM, BET, FTIR, XRD, and XPS. The optimized synthesis parameters of CS@BC/S-nZVI were determined as a 0.14 molar ratio of S/Fe and a 0.25 mass ratio of BC/Fe. The CS@BC/S-nZVI possessed a specific surface area of 199.246 m²/g and an average pore size and pore volume of 1.186 nm and 0.272 cc/g. The CS@BC/S-nZVI could remain reductive activity after Cr(VI) removal and present a remarkable tolerance to the coexisting ions during Cr(VI) removal. The adsorption data were fitted well by the pseudo-second order model and the Langmuir model. The removal of Cr(VI) by CS@BC/S-nZVI was an exothermic process with prominent Cr(VI) removal capacities of 244.07 mg/g at 120 min and 221.84 mg/g at 15 min at 25 ℃. Further mechanism analysis proved that the binding of Cr(VI) to CS@BC/S-nZVI was mainly a synergistic effect of reduction and electrostatic attraction. Overall, these findings shed new light on the research of a novel S-nZVI compound and revealed the potential practical application of CS@BC/S-nZVI in the future heavy metal removal from wastewater.

Simultaneous reduction and adsorption of arsenite anions by green synthesis of iron nanoparticles using pomegranate peel extract

PURPOSE

Arsenic is a toxic metalloid that is present in the environment as arsenate and arsenite anions. Exposure to arsenic anions caused skin problems, degenerative diseases, kidney, liver, and lung cancer. The synthesized iron nanoparticles (NPs) were examined as a green low-cost adsorbent for the removal of arsenite anions from aqueous solution via batch adsorption procedure.

METHODS

Iron NPs were prepared in a single step by the reaction of Fe+3 0.01 M solution with a fresh aqueous solution of 2% w/v pomegranate peel extract (PPE) as both reducing and capping agents. The physicochemical properties of peel were investigated by some experiments and functional groups were determined by the FT-IR spectrum. The electrochemical behavior of PPE was studied using cyclic voltammetry on a glassy carbon electrode as produced a cathodic peak at range 120-400 mV. The progress of nZVI production was monitored by a decrease of 372 nm wavelength UV-Vis spectra of PPE. The 27 adsorption experiments were carried out as a function of solution pH, initial arsenite concentration, mass adsorbent, and contact time according to DOE.

RESULTS

The rapid rate of adsorption was observed at 20-60 min, indicating that the principal mechanism dominating the sorption process was reduction and chemical adsorption. The arsenite removal efficiency was found to be dependent on the solution pH, adsorbent dose, and initial concentration, respectively.

CONCLUSION

The experimental data show the ability of the synthesized iron NPs to remove arsenate from solution in both synthetic and polluted natural water. The thermodynamic study suggested the spontaneous and endothermic nature of adsorption of arsenite by green synthesized iron NPs. The iron NPs synthesized with PPE increased the removal of arsenite with an increase in the active surface, indicating some chemical interactions between the adsorbent and oxoanions.