Jinyuan Jiang

The environmental impacts of treated acid mine drainage on receiving river systems remain insufficiently understood. This study investigated four typical closed coal mines in northern Sichuan Province, China, by analyzing heavy metals, sulfate, pH, UV-Vis spectroscopy, and dissolved organic matter (DOM) characteristics at 24 sampling sites along the receiving reaches. Parallel factor analysis (PARAFAC) and two-dimensional correlation spectroscopy (2D-COS) were employed to examine the longitudinal response sequence of DOM components. Results showed that pollutant concentrations generally increased immediately after the inflow of treated acid mine drainage and then progressively attenuated downstream, although the dominant pollution factors varied significantly among the reaches. DOM composition exhibited spatial heterogeneity, with protein-like components dominating three reaches and humic-like components prevailing in one reach. Based on the co-variation characteristics of DOM and heavy metals along the river course, four response patterns were identified: rapid-recovery, slow-recovery, disturbance–oscillation recovery, and delayed-recovery patterns. The 2D-COS analysis validated the rationality of these four patterns and revealed differences in the sensitivity of various DOM components to longitudinal disturbances. This study provides a scientific basis for the environmental impact assessment of mine water from remediated closed coal mines.

Biochar is a porous carbon material that can effectively remove NO_x from flue gas. The influence of K element and active sites on the microscopic mechanism of NO₂ gas adsorption and reduction reaction remains elusive. Through density functional theory (DFT), two NO₂ molecules are selected on reasonable biochar models to calculate the reaction pathway. The reaction pathways involve the sequential adsorption of two NO₂ molecules at different active sites for reduction reaction, and desorption of two NO molecules and one CO₂ molecule. Through the energy barrier difference of the reaction path and interaction region indicator (IRI) analysis of important molecular structures, it was found that K atom promotes the adsorption reduction reaction in three reaction processes: the breakage of N–O bond, the desorption of NO molecule, and the dissociation of CO₂ molecule. Based on the different numbers of active sites in the four reactant models, it can be concluded that the promotion range of the K atom for the adsorption and reduction process of NO₂ molecules is 0.6157 nm. Thermodynamic and kinetic analyses indicate that the addition of K enhances the upper limit and the maximum reaction rate of the reaction pathways. This study provides certain theoretical guidance for preparing biochar to regulate NO₂ emissions. Graphical Abstract

Biomass reburning is an efficient and low-cost way to control nitric oxide (NO), and the abundant potassium (K) element in biomass affects the heterogeneous reaction between NO and biochar. Due to the incomplete simulation of the NO heterogeneous reduction reaction pathway at the molecular level and the unclear catalytic effect of K element in biochar, further research is needed on the possible next reaction and the influencing mechanism of the element. After the products of the existing reaction pathways are referenced, two reasonably simplified biochar structural models are selected as the basic reactants to study the microscopic mechanism for further NO heterogeneous reduction on the biochar surface before and after doping with the K atom based on density functional theory. In studying the two further NO heterogeneous reduction reaction pathways, we find that the carbon monoxide (CO) molecule fragment protrudes from the surface of biochar models with the desorption of N-2 at the TS4 transition state, and the two edge types of biochar product models obtained by simulation calculation are Klein edge and ac56 edge observed in the experiment. In studying the catalytic effect of potassium in biochar, we find that the presence of K increases the heat release of adsorption of NO molecules, reduces the energy barrier of the rate-determining step in the nitrogen (N-2) generation and desorption process (by 50.88 and 69.97%), and hinders the CO molecule from desorbing from the biochar model surface. Thermodynamic and kinetic analyses also confirm its influence. The study proves that the heterogeneous reduction reaction of four NO molecules on the surface of biochar completes the whole reaction process and provides a basic theoretical basis for the emission of nitrogen oxides (NOx ) during biomass reburning.

Water pollution treatment technology assessment methods can be used to guide the selection of scientific and reasonable water pollution treatment technologies. At present, China has not yet established a standardized methodological system to scientifically evaluate these technologies, which makes it difficult to effectively screen water pollution treatment technologies suitable for specific watersheds or regions and restricts the sustainable development of local economy and society. In this study, an MDCA framework for water pollution treatment technologies was developed using a sustainable assessment approach. The framework begins with the definition of water pollution treatment technologies’ decision-making problems and then proceeds through the following: select potential water pollution treatment technologies; identify indicators; decision making; indicator scoring; indicator weighting; select appropriate assessment model; uncertainty analysis; and other steps to ultimately determine preferred options. To demonstrate the validity and applicability of the framework, typical urban wastewater treatment technologies were selected for case validation. The results showed that the comprehensive assessment results obtained by the multidimensional assessment model based on the ideal point method and weighted method were basically consistent. SBR and TAS can be used as recommended technologies for urban sewage treatment in the study area. However, these two technologies also have shortcomings, such as the unsatisfactory economic benefit of SBR, and the high sludge production and poor resistance to hydraulic shock loading of TAS. Among the six alternative technologies, CWS had the worst environmental benefit, mainly due to the low ammonia removal rate. A2/O has the worst economic and technical performance, mainly due to high investment and operation cost, relatively complex operation management, and poor resistance to hydraulic shock load. The method established in this study can not only select the technology, but also identify the shortcomings of the technology, therefore realizing the systematization and standardization.