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In this study, red mud modified by manganese dioxide(MRM) was utilized as an adsorbent to effectively remove Cd 2+ from aqueous solution. The characteristics were analysed by SEM–EDS, XRD, BET, FTIR and XPS. Different factors that affected the Cd 2+ removal on MRM, such as dosage, initial pH, initial Cd 2+ concentration, were investigated using batch adsorption experiments. Simultaneously, the adsorption kinetics, adsorption isotherms and adsorption thermodynamics of Cd 2+ were also investigated using adsorption experiments data. The characterization results showed that MRM had a rougher, larger specific surface area and pore volume (38.91 m 2 g −1 , 0.02 cm 3 g −1 ) than RM (10.22 m 2 g −1 , 0.73 cm 3 g −1 ). The adsorption experiments found that the equilibrium adsorption capacity of MRM for Cd 2+ was significantly increased to 46.36 mg g −1 , which was almost three times that of RM. According to the fitting results, the pseudo-second-order kinetic model described the adsorption process better than the pseudo-first-order kinetic model. The Langmuir model fitted the adsorption isotherms well, indicating that the adsorption process was unimolecular layer adsorption and the maximum capacity was 103.59 mg g −1 . The thermodynamic parameters indicated that the adsorption process was heat-trapping and spontaneous. Finally, combined XPS and FTIR studies, it was speculated that the adsorption mechanisms should be electrostatic attachment, specific adsorption (i.e., Cd–O or hydroxyl binding) and ion exchange. Therefore, manganese dioxide modified red mud can be an effective and economical alternative to the removal of Cd 2+ in the wastewater treatment process.

Bisulfite (BS)-based advanced oxidation processes (AOPs) are attractive for pollutant degradation, but often depend on costly transition metals with leaching risks. Herein, we report a citric acid-modified red mud catalyst (RMAC) for efficient Congo Red (CR) removal. Citric acid acted Simultaneously as an acid activator and carbon template, enlarging the surface area from 31.10 to 116.40 m 2 g −1 (3.74-fold increase). Under optimal conditions (5 mM BS, pH = 5, 80 mg L −1 CR), RMAC3-800 achieved 98.8% CR removal with a pseudo-first-order rate constant of 0.1399 min −1 and retained > 80% efficiency after three reuse cycles. Radical scavenging and EPR analyses confirmed SO 4 •− (53.7%) and •OH (46.3%) as the dominant species, whereas XPS identified Fe 0 as the principal active site. GC-MS detected six intermediates, supporting the proposed oxidative cleavage and mineralization pathways of the degradation process. A preliminary bench-scale cost analysis estimated an operating cost of ~ 13.94 RMB m −3 (≈ 1.95 USD m −3 ), underscoring its economic feasibility. This study demonstrates a cost-effective, recyclable, and sustainable catalytic system for wastewater treatment and red mud valorization.

During tunnel construction in strongly developed karst terrain, water inrush hazards often occur due to the complex hydrogeological conditions, which require accurate prediction of water inflow. In this study, a dynamic modeling approach for water inflow prediction of karst tunnels using the conduit flow process (CFP) is developed that considers both karst duality and changing boundary conditions of the tunnel. The CFP model has a good agreement with field-observed hydraulic head after calibration, and the Nash–Sutcliffe model efficiency (NSE) for the CFP model is 97.3%. Numerical calculation of water inflow was conducted in a successive drilling scenario with permeability change of the surrounding rocks. Additionally, a modular three-dimensional finite-difference ground-water flow model (MODFLOW) has been applied to predict the water inflow, for comparison with the CFP model. The prediction results obtained from the CFP model are generally in close agreement with the field-observed results; the percentage errors were 13.3% and 5.4%, respectively. For the MODFLOW model, the percentage errors were 34.2% and 36.8%, respectively. The proposed CFP model is both closer to reality and more reasonable than the MODFLOW model in predictive analysis of water inflow into karst tunnels, reflecting the influence of karst conduits on the water inflow process.

Biomass energy has been used for decades in lieu of fossil fuels. However, biomass, such as wheat straw, typically contains a high concentration of alkali elements, which is likely to induce unfavorable conditions during combustion, such as slagging, agglomeration, and corrosion in the boiler. This study investigated the effects of leaching on the chemical compounds and sintering temperatures of wheat straw ash before and after leaching by tap water. Ash melting and sintering degree tests were conducted using hot-stage microscopy and a scanning electron microscope, respectively. The results show that the ash content in wheat straw decreased by 26.09% (from 4.14% to 3.06%) following leaching, as did the chlorine (Cl), sulfur (S), and nitrogen (N). Meanwhile, the ash-related elements such as potassium (K), magnesium (Mg), and silicon (Si) reduced after leaching too. Additionally, the higher heating value increased slightly, from 19.25 to 19.53 MJ/kg. At the same time, leaching improved the ash melting temperatures of wheat straw during combustion and minimized the ash sintering degree. Similar results were also shown in scanning electron microscope (SEM) images, which clearly indicated that the leached samples had a lighter sintering degree than the original samples. Overall, the leaching process had a positive effect on the ash sintering problems of wheat straw.

This paper explores the feasibility of flexible biogas production by co-digestion of food waste and sewage sludge based on experiments and mathematical modeling. First, laboratory-scale experiments were carried out in variable operating conditions in terms of organic loading rate and feeding frequency to the digester. It is demonstrated that biogas production can achieve rapid responses to arbitrary feedings through co-digestion, and the stability of the anaerobic digestion process is not affected by the overloading of substrates. Compared with the conventional continuous mode, the required biogas storage capacity in flexible feeding mode can be significantly reduced. The optimum employed feeding organic loading rate (OLR) is identified, and how to adjust the feeding scheme for flexible biogas production is also discussed. Finally, a simplified prediction model for flexible biogas production is proposed and verified by experimental data, which could be conveniently used for demand-oriented control. It is expected that this research could give some theoretical basis for the enhancement of biogas utilization efficiency, thus expanding the applications of bio-energy.

Owing to the intrinsic heterogeneity and anisotropy of karst systems, traditional hydrological exploration methods face significant challenges when investigating karst conduit networks. This study employs pyKasso for the stochastic simulation of karst conduit networks in Panzhou City, focusing on uncertainty analysis through local sensitivity analysis and Monte Carlo methods. The simulation process incorporates geological, topographic, and fracture data to create a realistic representation of the karst network. We found that the spatial configuration and characteristics of the karst network are significantly influenced by various input parameters such as fracture parameters, inlets, outlets, and cost ratios. We highlight the minimal influence of fracture densities and the substantial impact of the count of inlets and outlets on crucial network metrics. The results demonstrate the critical role of parameter sensitivity and variability in modeling the intricate karst systems, providing valuable insights for hydrogeological studies and the management of karst water.

Red-mud leachate from tailings ponds contains Cr(VI), which can pollute groundwater via infiltration through anti-seepage layers. This paper investigates leachate from a red-mud tailings pond in southwest China and the red clay in the surrounding area to simulate the adsorption of Cr(VI) onto clay at different pHs, using geochemical equilibrium software (Visual MINTEQ). We also performed dynamic adsorption testing of Cr(VI) on a clay anti-seepage layer. The dynamic adsorption behaviors and patterns in the dynamic column were predicted using the Thomas and Yoon–Nelson models. Visual MINTEQ predicted that Cr(VI) adsorption in red-mud leachate onto clay was 69.91%, increasing gradually with pH, i.e., adsorption increased under alkaline conditions. Cr(VI) concentration in the effluent was measured using the permeability test through a flexible permeameter when the adsorption saturation time reached 146 days. At a low seepage rate, Cr(VI) adsorption onto the clay anti-seepage layer took longer. Saturation adsorption capacity, q0, and adsorption rate constant, Kth, were determined using the Thomas model; the Yoon–Nelson model was used to determine when the effluent Cr(VI) concentration reached 50% of the initial concentration. The results provide parameters for the design and pollution prediction of the clay anti-seepage layer of red-mud tailings ponds.

This study analyzes the leaching behavior of elements from red mud (bauxite residue) at pH values ranging from 2 to 13. The leaching characteristics of metals and contaminated anions in five red mud samples produced by Bayer and combined processes were analyzed using the batch leaching technique following the US Environmental Protection Agency (USEPA) Method 1313. In addition, the geochemical model of MINTEQ 3.1 was used to identify the leaching mechanisms of metals. The results showed that Ca, Mg, and Ba follow the cationic leaching pattern. Al, As, and Cr show an amphoteric leaching pattern. The leaching of Cl− is unaffected by the pH. The maximum leaching concentration of the proprietary elements occurs under extremely acidic conditions (pH = 2), except for As. The leaching concentration of F− reaches 1.4–27.0 mg/L in natural pH conditions (i.e., no acid or base addition). At the same pH level, the leaching concentrations of Pb, As, Cr, and Cu are generally higher from red mud produced by the combined process than that those of red mud from the Bayer process. The leaching concentration of these elements is not strongly related to the total elemental concentration in the red mud. Geochemical modeling analysis indicates that the leaching of metal elements, including Al, Ca, Fe, Cr, Cu, Pb, Mg, Ba, and Mn, in red mud are controlled by solubility. The leaching of these elements depended on the dissolution/precipitation of their (hydr)oxides, carbonate, or sulfate solids.

A nationwide investigation was carried out to evaluate the geochemical characteristics and environmental impacts of red mud and leachates from the major alumina plants in China. The chemical and mineralogical compositions of red mud were investigated, and major, minor, and trace elements in the leachates were analyzed. The mineral and chemical compositions of red mud vary over refining processes (i.e., Bayer, sintering, and combined methods) and parental bauxites. The main minerals in the red mud are quartz, calcite, dolomite, hematite, hibschite, sodalite, anhydrite, cancrinite, and gibbsite. The major chemical compositions of red mud are Al, Fe, Si, Ca, Ti, and hydroxides. The associated red mud leachate is hyperalkaline (pH > 12), which can be toxic to aquatic life. The concentrations of Al, Cl−, F−, Na, NO32−, and SO42− in the leachate exceed the recommended groundwater quality standard of China by up to 6637 times. These ions are likely to increase the salinization of the soil and groundwater. The minor elements in red mud leachate include As, B, Ba, Cr, Cu, Fe, Ni, Mn, Mo, Ti, V, and Zn, and the trace elements in red mud leachate include Ag, Be, Cd, Co, Hg, Li, Pb, Sb, Se, Sr, and Tl. Some of these elements have the concentration up to 272 times higher than those of the groundwater quality standard and are toxic to the environment and human health. Therefore, scientific guidance is needed for red mud management, especially for the design of the containment system of the facilities.

In recent years, the ecological safety issues of red mud tailings ponds have been frequent, with problems such as the seepage damage of anti-seepage materials at the bottom of tailings ponds, failure of anti-seepage systems, and leakage of pollutants. In order to deeply analyze the influence of red mud (RM) leachate on the microstructure of the modified red clay (RC) anti-seepage layer, this article explores the influence characteristics of strong alkaline RM leachate on the microstructure of a modified RC anti-seepage layer under actual working conditions through a combination of permeability tests and microscopic characterization. The results showed that as the RM leachate permeation time increased, varying changes occurred in the permeability coefficient of the modified RC with different FA contents, among which the permeability coefficient of the modified RC with an 8% FA content showed a significant decreasing trend, reaching 5.98 × 10−11 m/s after stabilization. After permeation, numerous small pores were generated in the modified clay; furthermore, the small particles of the FA-modified clay were significantly reduced compared to pure clay. As the permeation time increased, the 8% FA-modified RC showed a phenomenon of first increasing and then decreasing in specific surface area, with a small change from 27.71 m2/g to 27.52 m2/g, indicating that this sample had high stability and the specific surface area was not significantly affected by permeation. This is mainly caused by the influence of gelling materials produced by the pozzolanic reaction and activation effect upon FA addition. The soil structure became more compact at the microscopic level with increasing FA content, resulting in particle aggregation, increased specific surface area, and narrowed small-pore size distribution. After 60 days of permeation, the single-shoulder peak of the 8% FA-modified RC was still the lowest at about 0.30 dV/dr. Compared to other samples, the pore size was smaller and less affected by the leachate. Overall, the microstructure of the 8% FA-modified RC was less affected by the leachate. This study provides an explanatory basis for the macroscopic mechanical phenomena by analyzing the influence of microstructure. It further provides a reference for studying the selection of anti-seepage materials.