Accelerating effects of humin on sulfide-mediated azo dye reduction

A new strategy for treating Pb2+ and Zn2+ pollution with industrial waste derivatives Humin

Metal pollution caused by industrial waste accumulation is a long-term and far-reaching problem. Humin (HM), as a highly condensed organic component insoluble in alkaline or water solution, is often discarded as humic acid industrial waste. However, the abundant active functional groups in HM reported by some researches make it possible for HM to remove metals. In this study, a waste reuse strategy was proposed to reduce the pressure of industrial metal pollution on the environment. HM was obtained from lignite waste residue. Scanning electron microscopy, energy spectrum and Fourier infrared spectroscopy, combined with the adsorption models were employed to reveal the mechanism of HM adsorption. The results showed that HM had multiple adsorption mechanism and high biological stability. The adsorption capacity of HM to Zn²⁺ and Pb²⁺ were 194.88 mg/g and 289.59 mg/g respectively. HM adsorbed Zn²⁺ mainly by physical multilayer adsorption. And the adsorption of Pb²⁺ by HM was mainly a monolayer chemical reaction, which depended on its active functional groups and the exchange of valence electrons. Notably, HM could simultaneously remove Pb²⁺ and Zn²⁺ and almost did not affect its original adsorption capacity to single ions. These results will provide a new strategy for the treatment of metal pollution in the future and alleviate the pressure of multiple metal pollution of the environment.

The degradation of dissolved organic matter in black and odorous water by humic substance-mediated Fe(II)/Fe(III) cycle under redox fluctuation

In nature, the hydroxyl radical (•OH) is produced during the anaerobic-aerobic transition when groundwater level fluctuates. In addition, the •OH is also detected in iron-bearing clay minerals and iron oxides during the redox process. Goethite is one of the most stable iron oxides involved in biogeochemical cycles. In this study, the coexisting humic acid (HA) enhanced the generation of Fe(II) during the iron reduction process and accelerated the generation of •OH in the redox process of goethite. The organic contaminants in black and odorous water were decomposed by constructing an iron-reducing bacteria-HA-Fe(II)/Fe(III) reaction system under anaerobic-aerobic alternation. The results demonstrated that in the anaerobic stage, HA could promote the reduction and dissolution of goethite through the complexation effect and electron shuttle mechanism, as well as significantly strengthening the iron reduction process in water. Under aerobic conditions, Fe(II) in the reaction system would activate O₂ to generate •O₂^-. The •OH, formed by Fe (II) and •O₂^- via Fenton reaction and Haber-Weiss mechanism, oxidized dissolved organic matter (DOM) in water. The characterization of DOM by three-dimensional fluorescence spectroscopy (3DEEM) indicated that after four redox fluctuations, the organic contaminants in water samples were effectively degraded. Generally, this study provides new approaches and insights into the biogeochemical cycling of Fe and C elements and water pollution remediation at the anoxic-anoxic interface.

Biogenic sulfide for azo dye decolorization from textile dyeing wastewater

Azo dye is the most versatile class of dyes used in the textile industry. Although the sulfidogenic process shows superiority in the removal of azo dye, the role of biogenic sulfide produced by sulfate-reducing bacteria (SRB) in the decolorization of azo dye is unclear. This study explored the mechanism of biogenic sulfide for removal of a model azo dye (Direct Red 81 (DR 81)) through biotic and abiotic batch tests with analysis of intermediates of the azo dye degradation. The results showed that biogenic sulfide produced from sulfate reduction directly cleaved two groups of azo bond (-NN-), thereby achieving decolorization. Moreover, the decolorization rate was enhanced by nearly 3-fold (up to 42 ± 1 mg/L-hr; removal efficiency > 99%) by adding an external carbon source or elevating the initial azo dye concentration. This study showed that biogenic sulfide plays an essential role in azo dye decolorization and provides a new avenue for the potential application of biogenic sulfide from the sulfidogenic system for the treatment of azo dye-laden wastewater.

Humin: No longer inactive natural organic matter

The discovery of the function of humin (HM), an insoluble fraction of humic substances (HSs), as an extracellular electron mediator (EEM) in 2012 has provided insight into the role of HM in nature and its potential for in situ bioremediation of pollutants. The EEM function is thought to enable the energy network of various microorganisms using HM. Recently, a number of studies on the application of HM as EEM in anaerobic microbial cultures have been conducted. Even so, there is a need for developing a holistic view of HM EEM function. In this paper, we summarize all the available information on the properties of HM EEM function, its applications, possible redox-active structures, and the interaction between HM and microbial cells. We also suggest scopes for future HM research.

Biochar Derived from Agricultural Wastes as a Means of Facilitating the Degradation of Azo Dyes by Sulfides

Dyes are common contaminants, some of which are teratogenic, carcinogenic, and causative of ecological damage, and dye wastewater often contains toxic sulfides. Biochar has been widely used for the adsorption and catalysis degradation of pollutants, including dyes and sulfides, due to its abundant surface functional groups and large specific surface area. Therefore, the simultaneous treatment of dyes and sulfides with biochar may be a feasible, effective, and novel solution. This study sought to utilize low-cost, environmentally friendly, and widely sourced biochar materials from agricultural wastes such as corn stalk, rice chaff, and bean stalk to promote the reduction of dyes by sulfides. Through the action of different biochars, sulfides can rapidly decompose and transform oxidizing dyes. The RCB800 (rice chaff biochar material prepared at 800 °C) was observed to have the best effect, with a degradation rate of 96.6% in 40 min and 100% in 50 min for methyl orange. This series of materials are highly adaptable to temperature and pH, and the concentration of sulfides has a significant effect on degradation rates. Compared with commercial carbon materials, biochars are similar in terms of their catalytic mechanism and are more economical. Scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption and desorption characterization results indicated that biochar contains more pores, including mesopores, and a sufficient specific surface area, both of which are conducive to the combination of sulfides and dyes with biochar. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy showed that there are oxygen-containing functional groups (examples include quinones and carboxyl groups) on the surface of biochar that promote the reaction of sulfide and dye. The formation of active polysulfides also potentially plays an important role in the degradation reaction. This article outlines a new method for improving the degradation efficiency of azo dyes and sulfides via biochar materials derived from widely sourced agricultural wastes.