The comprehensive investigation on removal mechanism of Cr(VI) by humic acid-Fe(II) system structured on V, Ti-bearing magnetite surface

Mn-Doped Spinel for Removing Cr(VI) from Aqueous Solutions: Adsorption Characteristics and Mechanisms

In this study, the manganese (Mn) was doped in the MnFe₂O₄ crystal by the solid-phase synthesis method. Under the optimum conditions (pH = 3), the max removal rate and adsorption quantity of Cr(VI) on MnFe₂O₄ adsorbent obtain under pH = 3 were 92.54% and 5.813 mg/g, respectively. The DFT calculation results indicated that the adsorption energy (E_ads) between HCrO₄^- and MnFe₂O₄ is -215.2 KJ/mol. The Cr(VI) is mainly adsorbed on the Mn atoms via chemical bonds in the form of HCrO₄^-. The adsorption of Mn on the MnFe₂O₄ surface belonged to chemisorption and conformed to the Pseudo-second-order equation. The mechanism investigation indicated that the Mn in MnFe₂O₄ has an excellent enhancement effect on the Cr(VI) removal process. The roles of Mn in the Cr(VI) removal process included two parts, providing adsorbing sites and being reductant. Firstly, the Cr(VI) is adsorbed onto the MnFe₂O₄ via chemisorption. The Mn in MnFe₂O₄ can form ionic bonds with the O atoms of HCrO₄^-/CrO₄^2-, thus providing the firm adsorbing sites for the Cr(VI). Subsequently, the dissolved Mn(II) can reduce Cr(VI) to Cr(III). The disproportionation of oxidized Mn(III) produced Mn(II), causing Mn(II) to continue to participate in the Cr(VI) reduction. Finally, the reduced Cr(III) is deposited on the MnFe₂O₄ surface in the form of Cr(OH)₃ colloids, which can be separated by magnetic separation.

Hexavalent Chromium Removal from Industrial Wastewater by Adsorption and Reduction onto Cationic Cellulose Nanocrystals

Cationic cellulose nanocrystals (CCNC) are lignocellulosic bio-nanomaterials that present large, specific areas rich with active surface cationic groups. This study shows the adsorption removal of hexavalent chromium (Cr(VI)) from industrial wastewaters by the CCNC. The CCNC were synthetized through periodate oxidation and Girard's reagent-T cationization. The high value of CCNCs cationic groups and anionic demand reveal probable nanocrystal-Cr(VI) attraction. Adsorption was performed with synthetic Cr(VI) water at different pH, dosage, Cr(VI) concentration and temperature. Fast removal of Cr(VI) was found while operating at pH 3 and 100 mg·L^-1 of dosage. Nevertheless, a first slower complete removal of chromium was achieved by a lower CCNC dosage (40 mg·L^-1). Cr(VI) was fully converted by CCNC into less-toxic trivalent species, kept mainly attached to the material surface. The maximum adsorption capacity was 44 mg·g^-1. Two mechanisms were found for low chromium concentrations (Pseudo-first and pseudo-second kinetic models and continuous growth multi-step intraparticle) and for high concentrations (Elovich model and sequential fast growth-plateau-slow growth intraparticle steps). The Sips model was the best-fitting isotherm. Isotherm thermodynamic analysis indicated a dominant physical sorption. The Arrhenius equation revealed an activation energy between physical and chemical adsorption. CCNC application at selected conditions in industrial wastewater achieved a legal discharge limit of 40 min.

Pyrolysis temperature and feedstock affected Cr(VI) removal capacity of sulfidated zerovalent iron: Importance of surface area and electrical conductivity

Herein, feedstock (pinewood, rice straw, and dairy manure) and pyrolysis temperature (300, 500, and 700 °C) were selected as the influencing factors of properties of biochar (BC) to identify the contribution of biochar's matrix on Cr(VI) removal by BC-supported sulfidated zero-valent iron (S-ZVI/BC). Results showed that higher temperature was more conducible to improve the electrochemical properties and specific surface areas of composites. Raman spectra of S-ZVI supported by pinewood-derived BC (S-ZVI/PBC) showed the I_D/I_G ratio increased from 0.639 to 0.975 for the composites prepared at 300-700 °C, indicating the increased structural defects and resulting in the greatest Cr(VI) removal (35.81 mg g^-1) and reduction (30.21 mg g^-1) amounts of S-ZVI/PBC700. Besides, S-ZVI/PBC exhibited greater electrochemical reactivity and surface area than S-ZVI harbored by BC from dairy manure and rice straw. Additionally, Pearson correlation analysis revealed that Cr(VI) removal was significantly positively correlated to surface area (R² = 0.90) and negatively correlated to Tafel corrosive potential (R² = 0.88). Both desorption experiment and XPS spectra of spent sorbents showed that reduction predominated the detoxifying mechanism of Cr(VI) followed by adsorption (due to corrosively-generated iron oxides and BC) and precipitation (Cr₂S₃). This suggested that biochar with greater specific surface area and electrical conductivity is more favorable to immobilize S-ZVI with respect to Cr(VI) removal.

Mechanism and multi-step kinetic modelling of Cr(VI) adsorption, reduction and complexation by humic acid, humin and kerogen from different sources

Humin (HM) and kerogen (KG) are widespread in soils and sediments, which have strong retention effects on the migration and transformation of Cr(VI) in subsurface environment. Previous studies mainly focused on the interaction between Cr(VI) and soluble organic matter, such as humic acid (HA); however, the adsorption and reduction mechanism for Cr(VI) by insoluble HM and KG are still unclear, the processes of which might be quite different from HA due to their different sources and humification degrees. Consequently, in this study, HA, HM and KG extracted from different sources were used to explore the adsorption, reduction and complexation mechanisms of Cr(VI) in soils and sediments, based on which a multi-step kinetic model of Cr(VI) was carried out. According to the results, the retention of Cr(VI) by humus was found to obey a coupling mechanism of "adsorption-reduction-complexation", where Cr(VI) adsorption was by complexation with carboxylic groups by ligand exchange. The phenolic and hydroxylic groups were determined to be the main electron donor for Cr(VI) reduction. Notably, the Cr(III) produced was found to be adsorbed on the surface of humus by complexation on phenolic and hydroxylic groups, and the excesses were released into the liquid phase after the saturation of complexation sites. Based on the revealed mechanism, a multi-step kinetic model for simultaneously describing Cr(VI) adsorption and reduction and behaviour of Cr(III) was proposed producing a better fitting performance (R² ≥ 0.984) than the first-order and second-order kinetic models (R² ≤ 0.84 and 0.87, respectively) and hence could provide more factual understanding of Cr(VI) transformation in soils and sediments enriched in various types of humus.