Recent advances in CO2 reduction with renewable reductants under hydrothermal conditions: towards efficient and net carbon benefit CO2 conversion.

The ever-growing atmospheric CO2 concentration threatening the environmental sustainability of humankind makes the reduction of CO2 to chemicals or fuels an ideal solution. Two priorities are anticipated for the conversion technology, high efficiency and net carbon benefit, to ensure the mitigation of the CO2 problem both promptly and sustainably. Until now, catalytic hydrogenation or solar/electro-chemical CO2 conversion have achieved CO2 reduction promisingly while, to some extent, compromising to fulfill the two rules, and thus alternative approaches for CO2 reduction are necessary. Natural geochemical processes as abiotic CO2 reductions give hints for efficient CO2 reduction by building hydrothermal reaction systems, and this type of reaction atmosphere provides room for introducing renewable substances as reductants, which offers the possibility to achieve CO2 reduction with net carbon benefit. While the progress in CO2 reduction has been abundantly summarized, reviews on hydrothermal CO2 reduction are relatively scarce and, more importantly, few have focused on CO2 reduction with renewable reductants with the consideration of both scale of efficiency and sustainability. This review provides a fundamental and critical review of metal, biomass and polymer waste as reducing agents for hydrothermal CO2 reduction. Various products including formic acid, methanol, methane and multi-carbon chemicals can be formed, and effects of operational parameters such as temperature, batch holding time, pH value and water filing as well as detailed reaction mechanisms are illustrated. Particularly, the critical roles of high temperature and pressure water as reaction promotor and catalyst in hydrothermal CO2 conversion are discussed at the mechanistic level. More importantly, this review compares hydrothermal CO2 reduction with other methods such as catalytic hydrogenation and photo/electrocatalysis, evaluating their efficiency and potential for net carbon benefit. The aim of this review is to promote the understanding of CO2 activation under a hydrothermal environment and provide insights into the efficient and sustainable strategy of hydrothermal CO2 conversion for future fundamental research and industrial applications.

Recent advancements in azo dye decolorization in bio-electrochemical systems (BESs): Insights into decolorization mechanism and practical application.

Recent advances in bio-electrochemical systems (BESs) for azo dye removal are gaining momentum due to having electrode biocarrier and electro-active bacteria that could stimulate decolorization via extracellular electron transfer. Enhanced decolorization performance is observed in most laboratory studies, indicating the great potential of BESs as an alternative to the traditional biological processes or serving as a pre-/post-processing unit to improve the performance of biological processes. It is proven more competitive in environmental friendly than physicochemical methods. While, the successful application of BESs to azo dye-containing wastewater remediation requires a deeper evaluation of its performance, mechanism and typical attributes, and a comprehensive potential evaluation of BESs practical application in terms of economic analysis and technical optimizations. This review is organized to address BESs as a practical option for azo dye removal by analyzing the decolorization mechanisms and involved functional microorganisms, followed by the comparisons of device configurations, operational conditions, and economic evaluation. It further highlights the current hurdles and prospects for the abatement of azo dyes via BES related techniques.

Mutual effect between electrochemically active bacteria (EAB) and azo dye in bio-electrochemical system (BES).

Herein, the mutual effect between azo dye and the performance of electrochemically active bacteria (EAB) is investigated in detail, which is crucial to understand and control the bio-electrochemical systems (BESs) operation for azo dye containing wastewater treatment. EAB is enriched at controlled potential of -0.2 V vs Ag/AgCl in single-chamber BESs. Over 95% azo dye (alizarin yellow R (AYR)) was decolorized regardless of the initial AYR concentration ranging from 30 to 120 mg/L within 24 h. The fastest decolorization rate was obtained at AYR initial concentration of 70 mg/L, which was 4.25 times greater in the closed circuit BESs than that in the open circuit one. 16S rRNA gene based microbial community analysis showed that Geobacter was dominant in EAB with relative abundance increased from 77.98% (0 mg/L AYR) to 92.22% (70 mg/L AYR), indicating that azo dye selectively boosts the growth of exoelectrogens in electrode biofilm communities. Under electricity stimulation, extracellular process can be mutually conducted by azo dye compounds, which is favorable for accelerating reaction rate and avoiding of significant toxic effect on EAB.