Hexavalent chromium contamination is a global environmental issue and usually reoccurs in alkaline reduced chromite ore processing residues (rCOPR). The oxidation of Cr(III) solids in rCOPR is one possible cause but as yet little studied. Herein, we investigated the oxidation of Cr(OH) 3 , a typical species of Cr(III) in rCOPR, at alkaline pH (9−11) with δ-MnO 2 under oxic/anoxic conditions. Results revealed three pathways for Cr(III) oxidation under oxic conditions: (1) oxidation by oxygen, (2) oxidation by δ-MnO 2 , and (3) catalytic oxidation by Mn(II). Oxidations in the latter two were efficient, and oxidation via Pathway 3 was continuous and increased dramatically with increasing pH. XANES data indicated feitknechtite (β-MnOOH) and hausmannite (Mn 3 O 4 ) were the reduction products and catalytic substances. Additionally, a kinetic model was established to describe the relative contributions of each pathway at a specific time. The simulation outcomes showed that Cr(VI) was mainly formed via Pathway 2 (>51%) over a short time frame (10 days), whereas in a longer-term (365 days), Pathway 3 predominated the oxidation (>78%) with an increasing proportion over time. These results suggest Cr(III) solids can be oxidized under alkaline oxic conditions even with a small amount of manganese oxides, providing new perspectives on Cr(VI) reoccurrence in rCOPR and emphasizing the environmental risks of Cr(III) solids in alkaline environments.

Through controlling the phase transformation and chromium species under hydrothermal condition, the Cr(VI) was extracted fully from hazardous Cr(VI)-containing gypsum sludge, with a very high efficiency of more than 99.5%. Scanning transmission electron microscopy, X-ray absorption fine structure, and density functional theory calculation results revealed that the dissolution-recrystallization of CaSO 4 •2H 2 O into CaSO 4 was the key factor to fully release the encapsulated Cr(VI). Moreover, the mineralizer (persulfate salt) provided H + and SO 4 2− ions, the former made an acidic condition to transform the released CrO 4 2− into the specie (Cr 2 O 7 2− ) with less similarity to SO 4 2− , which further prevented the recombination of the released Cr(VI) with gypsum; and the latter was essential to accelerate crystal growth of calcium sulfate so as to enhance Cr(VI) extraction. This work would provide an instructive guidance to fully extract heavy metals from hazardous solid wastes via the control of crystal transformation and the pollutant species.

Efficient and feasible pretreatment of lignocellulosic biomass waste is an important prerequisite step to promote subsequent enzymatic hydrolysis and enhance the economics of biofuels production. This study focuses on the pretreatment of wheat straw (WS) with triethylbenzyl ammonium chloride/lactic acid (TEBAC/LA)-based deep eutectic solvents to enhance biomass fractionation and lignin extraction. Effects of pretreatment time, temperature, and TEBAC/LA molar ratio on pretreatment were evaluated systematically. Results suggested that 89.06 ± 1.05% of cellulose and 71.00 ± 1.03% of xylan were hydrolyzed with enzyme loadings of 35 FPU cellulase and 82 CBU β-glucosidase (per gram of dry biomass) after pretreatment by TEBAC/LA (1:9) at 373 K for 10 h. A total monosaccharide yield of 0.550 g/g WS (91.27% of the theoretical yield) was achieved with 79.73 ± 0.93% of lignin removal. Furthermore, the 1H–13C two-dimensional heteronuclear single quantum correlation (2D-HSQC) NMR spectroscopy showed that the regenerated lignin (75.69 ± 1.32% purity) was mainly composed of the syringyl units and the guaiacyl units. Overall, the results in this study provide an effective and facile pretreatment method for lignocellulosic biomass waste to enhance enzymatic hydrolysis saccharification.

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Levulinic acid (LA) is a versatile platform chemical in the modern concept of the biorefinery and can be used to synthesize a broad range of desirable chemicals and fuel additives. Unfortunately, because LA released from biomass hydrolysate is accompanied by formic acid (FA) and 5-hydroxymethylfurfural (5-HMF), it is also important to investigate the binary and ternary adsorption equilibrium, as well as competitive dynamic fixed-bed column adsorption from the viewpoint of industrial application. Batch adsorption experiments showed that the affinity of SY-01 resin toward FA–LA–5-HMF were in the order of 5-HMF > LA > FA under noncompetitive and competitive systems. The highest adsorption capacity were 7.54 mg/g wet resin for FA, 103.51 mg/g wet resin for LA, and 107.73 mg/g wet resin for 5-HMF. Interestingly, the presence of FA has a synergistic effect on the adsorption of LA and 5-HMF onto SY-01 resin in a binary- or ternary-mixtures system, leading to a slight increase in adsorption uptakes. Furthermore, a mathematical model based on the general rate model coupled with the noncompetitive single-component and competitive multicomponent Langmuir isotherm was successfully developed to simulate the breakthrough curves of FA–LA–5-HMF from single, binary, as well as ternary-component mixtures. The proposed methodology for fixed-bed column multicomponent competitive adsorption model can be successfully implemented to completely design the separation unit of LA from aqueous solution or biomass hydrolysate. Furthermore, it also has the potential to expand the application to the actual biomass hydrolysate, saving a lot of manpower and material resources.