Dredged eutrophic sediments typically exhibit high water content, strong compressibility, and poor permeability, which severely hinder efficient dewatering and subsequent disposal. Although calcium silicate hydrate (CSH) has been increasingly applied for in-situ phosphorus immobilization, its influence on subsequent sediment dewatering remains unclear. This study investigated whether CSH-induced in-situ phosphorus immobilization could improve dewatering by promoting structural reorganization. Sediments conditioned with CSH-containing systems were compared with inert solids, and dewatering performance, pore topology, hydraulic transport, and cake stability were systematically evaluated. The results suggested that CSH conditioning was associated with dispersed, low-crystallinity interfacial deposition at particle surfaces and contacts, rather than bulk pore filling. Correspondingly, CSH-containing systems markedly reduced capillary suction time and filtration resistance, while producing a more favorable pore architecture after deep dewatering: effective porosity increased from 11.05% in CK to 45.11-50.77% in the representative SS-CSH and CFA-CSH systems, and tortuosity decreased from 1.90 to 1.52-1.60. PNM/VBS analysis further showed that permeability increased from 1.12/0.068 mm2 in CK to 8.83-11.56/1.53-1.90 mm2. Together with improved resistance to compression-induced collapse, these results indicate that dewatering enhancement was associated with the coupled regulation of pore connectivity, transport efficiency, and structural stability. Overall, CSH-mediated in-situ phosphorus immobilization can function not only as a chemical stabilization strategy but also as a structural conditioning step for downstream sediment dewatering.

To elucidate the hydraulic conductivity and polyethylene (PE) migration behavior of polymer-modified geosynthetic clay liners (GCLs) under dry-wet cycling, this study investigated xanthan gum (XG)-modified GCLs (XG GCLs) with XG loadings ranging from 3% to 12%. Flexible-wall permeameters were used to perform six dry-wet cycles with simulated landfill leachate (SLL). Multiscale analyses were conducted, including quantitative crack image processing, residual polymer determination, and SEM-EDS microstructural characterization. The results showed that successive dry-wet cycles gradually expanded surface cracks, reduced residual XG loadings, and caused a stepwise increase in hydraulic conductivity. The combined evidence indicated that the coupled effects of flow path enlargement and polymer elution led to the deterioration of XG GCL’s barrier performance. PE was not detected in the effluents of uncycled XG GCLs during the stable period; however, under the present experimental conditions, PE became consistently detectable in all XG GCL effluents after three dry-wet cycles, with PE concentrations increasing progressively as the number of cycles increased. SEM-EDS observations revealed that PE was mainly distributed within the XG hydrogel regions lining the walls of preferential flow paths. Under the combined influence of crack expansion and accelerated polymer elution induced by dry-wet cycling, PE was entrained by the XG hydrogel and transported through the XG GCL. The findings reveal the risks of barrier performance deterioration and PE breakthrough under dry-wet cycling, underscoring the need for risk-informed design, long-term monitoring, and maintenance when using XG-modified GCLs in landfill barrier systems.

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 m2 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 SO4•− (53.7%) and •OH (46.3%) as the dominant species, whereas XPS identified Fe0 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.

Internal phosphorus (P) release from sediments remains a major obstacle to long-term eutrophication control in shallow urban waters. This study synthesizes a calcium silicate hydrate (CFA-CSH) from carbide slag and fly ash and evaluates its performance in regulating sediment P cycling. Structural analyses show a tobermorite-dominated phase with hierarchical porosity and abundant Ca-Si-OH sites, enabling rapid P capture through surface complexation and Ca-P mineral formation. Incubation experiments demonstrate that CFA-CSH effectively suppresses dissolved P and diffusive fluxes across the sediment-water interface and maintains stable immobilization capacity under both undisturbed and mildly bioturbated conditions. The stabilization arises from a coupled mechanism in which sustained Ca²⁺ release promotes hydroxyapatite-like mineral formation while the modified geochemical environment favors microbial assemblages that support P retention. The combined capping-injection application further establishes complementary barriers that limit resuspension and enhance long-term stability. Overall, waste-derived CFA-CSH offers a robust and scalable pathway for sediment P immobilization and provides a practical strategy for integrating industrial solid-waste utilization with eutrophication mitigation.

The release of endogenous phosphorus (P) from sediments poses a significant challenge to eutrophic urban landscape water bodies, and traditional capping materials often carry ecological risks and high costs. This study proposes a green alternative by converting steel slag into calcium silicate hydrate (SS-CSH) through alkalihydrothermal activation. The optimized SS-CSH exhibits a high P adsorption capacity of 81.31 mg/g, achieved through a dual mechanism involving Ca2+-driven hydroxyapatite crystallization and surface complexation. In a simulated urban lake system, applying SS-CSH as a capping agent resulted in an 85 % reduction in sediment P flux by establishing a stable geochemical barrier. The addition treatment reduced exchangeable P by 89.5 % in sediments through in situ Ca-P precipitation. The combined capping-addition mode is most effective in reducing soluble reactive P and dissolved total P concentrations in the upper water layer, as it blocks surface diffusion and fixes deep mobile P, thereby enhancing the uniformity and long-term effectiveness of P fixation. Microbial analysis indicates that SS-CSH blocks the activation of reducible P and induces organic P fixation by synergistically inhibiting the activity of sulfate-reducing bacteria (e.g., Desulfobacterota) and anaerobic bacteria (e.g., Proteobacteria), while enriching organic P mineralizing bacteria (e.g., Flavobacterium), thereby reducing the risk of P release. In summary, the SS-CSH strategy based on industrial solid waste recycling demonstrates significant economic benefits and environmental compatibility, providing a feasible solution for P management in sediments of urban landscape water bodies.

During long-term storage, phosphogypsum generates substantial phosphorus-rich leachate, consequently causing environmental pollution and resource depletion. This highlights the necessity for efficient phosphorus recovery. Steel slag, known for its high metal oxide content, demonstrates significant potential for phosphorus adsorption, providing a cost-effective and efficient alternative for phosphorus recovery. This study employed mechanically activated steel slag (MASS) to couple the adsorption and crystallization processes for phosphorus recovery from phosphogypsum leachate. The adsorption properties of MASS on phosphate and the mechanisms by which it enhances Ca-P crystallization were also investigated. The findings indicated that mechanical activation significantly altered the crystal structure and chemical bonds of MASS, activated various metal oxides, and facilitated phosphorus recovery. A phosphate removal efficiency of 99.95 % was achieved with an optimal activation time of 30 min. Research has demonstrated that MASS, serving as a source of Ca2+ and alkali, promotes phosphate removal through adsorption and crystallization. Pseudo-second-order kinetics were observed during the adsorption-control phase, indicating a chemisorption-controlled mechanism. In the surface-induced crystallization phase, the incorporation of MASS significantly enhanced the crystallization efficiency of hydroxyapatite. Compared with homogeneous crystallization without MASS, induced crystallization produced larger crystals and achieved a higher phosphorus removal efficiency. The crystallization process is dependent on pH, with optimal phosphorus removal occurring at a pH of 8. In summary, the coupled process demonstrated improved phos- phorus removal efficiency, better control over crystal size, and enhanced pH buffering capacity, making it a promising method for recovering phosphorus from phosphogypsum leachate.

Understanding how soil characteristics influence remediation is critical for optimizing treatment strategies. This study investigates how soil particle size influences the efficiency of UV-assisted electrokinetic remediation (UV-EKR) for Cr(VI)-contaminated soils. The primary objectives are to evaluate the effects of particle size on electrokinetic transport and photochemical reactivity. Four soil fractions (< 150 μm, 150–335 μm, 335–1000 μm, 1000–2000 μm) were prepared by dry sieving. UV-EKR experiments were conducted to assess electrokinetic behavior, photoreactive interface dynamics, and Cr(VI) removal under controlled conditions. Electroosmotic flow, electrolyte, soil resistivity and pH, and Cr(VI) concentration profiles were monitored, and multivariate analysis was used to relate spatial heterogeneity to electrokinetic reaction gradients. Fine-grained soil (SG4, < 150 μm) exhibited enhanced photochemical precipitation of Fe/Cr (oxy)hydroxides in the anode region, leading to pore blockage, reduced electroosmotic flow, and increased resistivity. While finer soils (UV-EKR3–4) exhibited higher electrokinetic responsiveness, facilitating Cr(VI) removal in cathode-proximal zones (S4–S5) via alkaline desorption and elevated current density, coarse soils (SG1) maintained more stable transport and resisted pore clogging, favoring contaminant removal in acidic, dissolution-dominated anodic regions (S1–S2). Despite these contrasting behaviors, overall removal efficiencies remained comparable across soils (36.98–41.90

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.

This study investigated the influences of polymer types (Super Absorbent Polymer, SAP and Polyanionic Cellulose, PAC), preparation methods (dry-mixed, dry-sprinkled, or wet-mixed), initial polymer loadings (1.0%, 2.5%, 5.0%, 10.0%, and 15.0%), and bauxite liquors (synthetic bauxite liquor, BLS and actual bauxite liquor, BLA) on the polymer elution and the hydraulic conductivity of polymer-bentonite geosynthetic clay liners (PB GCLs). Hydraulic conductivity, total organic carbon analysis, viscosity, scanning electron microscopy, Fourier transforms infrared spectroscopy, and X-ray diffraction tests were combined to investigate the mechanisms controlling the polymer elution and hydraulic conductivity of PB GCLs. The results demonstrated that PB GCLs with high initial polymer loading (10.0%, 15.0%) or prepared using the dry-sprinkled method had low hydraulic conductivities (< 1.0 × 10−10 m/s) and polymer eluting rates (< 10.0 mg/d). PAC-enhanced GCLs had a higher polymer eluting rate than SAP-enhanced GCLs due to the complete water solubility of linear polymer (PAC). When PB GCLs were permeated by BLS (higher ionic strength than BLA), polymer hydrogels had lower viscosity because of coiled or contracted conformation, resulting in a higher polymer eluting rate (> 50.0 mg/d) and hydraulic conductivity (> 1.0 × 10−11 m/s).

Nitrogen (N) and phosphorus (P) are primary pollutants contributing to water quality degradation and eutrophication but are also indispensable resources for agricultural production. In this study, we synthesized porous MgO/Mg(OH)(2)-modified 2-modified starch gels (SC-M/SC-OM) to facilitate the simultaneous removal of N and P from wastewater and address the challenges associated with separating and recycling powdered materials. By immobilizing magnesium powder within the matrix of starch gel, with the addition of 1.7 % w/w of MgO and 2.4 % w/w of Mg(OH)(2), the adsorbent can be efficiently recovered from the liquid phase, minimizing material loss. Due to their inherent pH-regulating capabilities, the modified starch gels exhibited excellent removal performance over a wide pH range. At the appropriate adsorbent dose, the maximum recovery of SC-M was 70.072 mg/g P and 25.517 mg/g N, whereas SC-OM was marginally inferior to SC-M. The pseudo-first kinetic model more accurately represented the adsorption of the two materials, showing that N and P concentrations mainly affected the adsorption efficiency, which benefits nutrient recovery at higher concentrations. Starch gels function as carriers for powdered magnesium and contribute to the synergistic removal of N and P, particularly enhancing P removal. SC-OM primarily facilitates the recovery of N and P by forming struvite. At the same time, the adsorption during the hydration process of MgO also plays a crucial role in the recovery with SC-M. Finally, the pot experiment confirmed the feasibility of using the recovered SC as a sustained release fertilizer. This study reveals the significant potential of SC-M/SC-OM for environmentally friendly nutrient removal and recovery, providing a promising approach for wastewater treatment and nutrient recycling.

This study investigated the influences of polymer types, polymer loadings, and preparation methods on the hydraulic conductivity of polymer-enhanced bentonite geosynthetic clay liners (EB GCLs) to coal gangue leachate (CL). Sodium carboxymethylcellulose (CMC) and xanthan gum (XG) were selected to modify the sodium-bentonite GCLs (Na-B GCLs), with a range of polymer contents from 5% to 15%. The EB GCLs were prepared by wet-mixed, dry-mixed, and dry-sprinkled methods. Detection of effluent in the hydraulic conductivity test by inductively coupled plasma mass spectrometry (ICP) and analysis of EB GCLs’ morphologies by scanning electron microscopy (SEM) were conducted to explore the mechanisms controlling the hydraulic conductivity of EB GCLs. The hydraulic conductivity decreased with increasing swelling of the bentonite when the EB GCL had a low polymer loading (5%). However, no significant correlation was observed between the swell index and the hydraulic conductivity when the EB GCL had a high polymer loading (10% and 15%). Moreover, the polymer type and preparation method had little effect on the hydraulic conductivity of the EB GCLs when polymer loading increased to 15%. The ICP results indicated a decrease in ionic strength and an increase in RMD of the CL by cation scavenging, leading to an increase in the possibility of osmotic swelling. Additionally, SEM images showed that the clogging of pore spaces by the polymer was the main reason for the decreased hydraulic conductivity of EB GCLs.

The struvite method has been widely used to recover N and P from wastewater. How-ever, the drawbacks of alkali consumption and small crystallization are not negligible. Therefore, alkaline porous carrier materials are greatly desired to enhance struvite crystalline precipitation and maintain suitable pH values. In this study, using red mud (RM) as a carrier, MgO was loaded onto RM by co-precipitation method to prepare an N and P recovery material (MgO-RM). A response surface methodology based on the Box-Behnken design was used to explore the effects of the factors on N and P recovery. The multi-objective optimization of the recovery process was carried out using the desirability function approach to achieve an economically feasible recovery. Characterizations, including SEM-EDS, BET, FTIR, XRD, and XPS, were carried out to explore the recovery mechanism. The results demonstrated that the nano-sized MgO was well deposited on the RM surface, resulting in a larger specific surface area and greater reactivity of the MgO-RM. Using RM as a carrier significantly increased the struvite crystal size, and the MgO-RM could maintain the pH value of the solution in a suitable range for struvite growth. Under optimal conditions (dosage = 3.5 g/L, N/P = 1.8, and pH value = 3.4), the maximum N and P recovery capacity by MgO-RM was 57.23 mg/g and 128.05 mg/g. The recovery process may involve coupled reactions between physical adsorption, ion exchange, coordination exchange, and chemical precipitation. Struvite produced by chemical precipitation is the main recovery mechanism.(c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Recently, phosphorus recovery as vivianite (Fe3(PO4)2.8H2O) from anaerobic digestion of waste-activated sludge (WAS) has attracted increasing attention because of its high recovery efficiency and economic value. However, the relatively small crystal size restricts the separation and recovery of vivianite. In this study, magnetic biochar (MB) was prepared by the impregnation-pyrolysis method and selected as an additive to enhance vivianite crystallization during WAS anaerobic digestion. The effects of MB iron loading and dosage on the phosphorus recovery efficiency and methane yield were investigated through batch experiments, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to analyze the recovered precipitates. The results showed that the phosphorus recovery efficiency reached 45% and the methane yield increased by 27% when the mass ratio of biomass to FeCl3.6H2O (mB:mFe) was 2:1 and the dosage of MB was 1 g/gVS. SEM images displayed that the vivianite produced by MB as the seed crystal (102 mu m) was larger than those without addition of MB (32 mu m), and vivianite synthesized with MB could be easily separated from the sludge by a magnet. The microbial analysis found the functional bacteria related to Fe3+ reduction and methane production were further enhanced and enriched by the MB. In summary, this research confirms the feasibility of recovering phosphorus in the form of vivianite from waste-activated sludge using magnetic biochar as the seed crystal, which is conducive to its subsequent separation and utilization.

Bisulfate (BS) has recently attracted increasing attention as a green alternative to persulfates (PS) in sulfate radicals based advanced oxidation processes (SR-AOPs). Moreover, facile fabrication of zero-valent iron (ZVI) biochar composites and disposal of red mud remain challenges. Herein, ZVI biochar was synthesized from red mud (RM) and ginkgo leaves (GLs) by one-step pyrolysis and employed as an effective activator of BS for acid orange 7 (AO7) removal. Characterizations including SEM, BET, XRD and FTIR revealed that porous structure and Fe-0 particles of ZVI biochar were gradually developed with the increasing pyrolysis temperature from 400-800 degrees C. The ZVI biochar synthesized at 800 degrees C with RM: GLs mass ratio of 2:1 (RMBC2-800) showed optimal performance as that 95.3% of AO7 (60 mg/L) was removed with 2.5 mM of BS and 0.5 g/L of RMBC2800 under an initial pH of 6 within 60 min. Mechanism studies showed that SO4 center dot(-), OH center dot, O-2 center dot(-) and 1O2 were the active oxygen species in the catalytic reaction. Both Fe-0 and the ketonic functional groups (C==O) contributed to the high catalytic activity of RMBC2-800. Based on GC-MS analysis, seven degradation intermediates were identified and a possible degradation pathway was proposed. Furthermore, RMBC2-800 could be easily recycled by magnetic separation after wastewater treatment. This study not only proposed a highly efficient ZVI biochar/BS system in organic wastewater treatment, but also guided for effective utilization of red mud to realize the strategy of ``treat waste by waste''.

This study investigated the improvement performances and mechanisms of anaerobically digested sludge（ADS） dewaterability and the transformation behavior of heavy metals（HMs, including As, Cu, Cd, Cr, Pb, Zn, and Ni） by joint conditioning of indigenous Fe 2+ -activated peroxydisulfate（PDS） oxidation and walnut shell biochar（WS-BC）. Experimental results showed that the joint conditioning was the most effective when the initial pH was 6, the concentration of PDS was 6.2 g/L, RS-BC particle size and dosage were 125-150 μm and 25.6 g/L, respectively. Under this condition, the moisture content（MC） after low-pressure filtration was 70.3%, the capillary suction time（CST） was 114.7 s, and the CST reduction（CST R ） was as high as 81.3%, which implied an excellent improvement of sludge dewatering. For the improvement mechanism, indigenous Fe 2+ is released upon the interaction between PDS and iron mineral within ADS, rather than the dissolution of iron bound to organic matter. Then the decomposition of extracellular polymeric substances（EPS） by the PDS activation products, ·SO■ and ·OH, the bound water within EPS was released as free water, which is easily removed. The content of protein and polysaccharide in EPS decreased dramatically after adding WS-BC, and the adsorption of hydrophilic maters by WS-BC achieved further dewatering performance. ·SO■ was the major contributor to enhancing ADS dewaterability. Additionally, the micromorphology results showed that WS-BC established a rigid skeleton structure for water drainage, which ensures the mechanical dewatering performance after conditioning. After sequential extraction of HMs from conditioned sludge cake via Tessier’s method, indigenous Fe 2+ -activated PDS oxidation can improve immobilization of As, Cu, Cd, Cr, Pb, Zn and Ni, which were transformed into residual fractions mostly. Toxicity Characteristic Leaching Procedure（TCLP） results showed that the leaching toxicity reduced dramatically. Therefore, the joint application of indigenous Fe 2+ -activated PDS oxidation and WS-BC can be a feasible way to improve ADS dewaterability and reduce HMs risk during ADS dewatering. BCR results demonstrated that the joint pretreating method proposed in this study has superior environmental and economic ef?ciency.

Although anaerobic digestion is a promising way for valorization of waste activated sludge, the process severely deteriorates the sludge dewaterability. Herein, a novel strategy of iron and nitrogen co-doped biochar (Fe-N-BC) activated peroxydisulfate (PDS) was developed to improve anaerobically digested sludge (ADS) dewaterability. Results showed that Capillary suction time (CST) and moisture content (MC) of ADS after conditioned with FeN2-BC-800/PDS system dropped to 43.4 s and 66.8% when the initial pH was 6.0, dose of Fe-N2-BC-800 was 0.6 g/g VS, dose of PDS was 0.9 mmol/g VS, and reaction time was 30 min, respectively. Quantitative analysis indicated that the heterogeneous oxidation initiated by Fe-N2-BC-800 primarily contributed to the enhancement of ADS dewaterability. Mechanistic results revealed that (& BULL;SO 4 , & BULL;OH, and 1O2were generated as reactive oxygen species (ROS) through multiple active sites of Fe-N2-BC-800 during catalytic reaction. These highly ROS efficiently decomposed extracellular polymeric substances (EPS) and disintegrated ADS cells, thereby releasing bound and intracellular water into free water. Meanwhile, Fe-N2-BC-800/PDS significantly reduced hydrophilic biopolymers, improved hydrophobicity of ADS through adjusting protein secondary structure and enriching hydrophobic groups in sludge pellets. Furthermore, Fe-N2-BC-800 produced rigid and porous drainage routes in ADS as skeleton builder, inducing enhancement of ADS dewaterability combing with the strong oxidation. The proposed Fe-N2-BC-800/PDS treatment offers a novel and promising technology for high-efficiency enhancement of ADS dewaterability, providing a valuable reference for ADS deep dehydration.

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.

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.

In this study, red mud (RM) was used as a support for LaFeO3 to prepare LaFeO3-RM via the ultrasonic-assisted sol-gel method for the removal of methylene blue (MB) assisted with bisulfite (BS) in the aqueous solution. Characterization by scanning electron microscopy and the Brunauer-Emmett-Teller method indicated that LaFeO3-RM exhibited a large surface area and porous structure with a higher pore volume (i.e. 10 times) compared with the bulk LaFeO3. The XRD, XPS and FTIR results revealed that the support of porous RM not only dispersed LaFeO3 particles but also increased Fe oxidation capability, oxygen-containing functional groups and chemically adsorbed oxygen (from 44.3% to 90.3%) of LaFeO3-RM, which improved the catalytic performance in structure and chemical composition. MB was removed through the synergistic effect of adsorption and catalysis, with MB molecules first absorbed on the surface and then degraded. The removal efficiency was 88.19% in the LaFeO3-RM/BS system under neutral conditions but only 27.09% in the LaFeO3/BS system. The pseudo-first-order kinetic constant of LaFeO3-RM was six times higher than that of LaFeO3. Fe(III) in LaFeO3-RM played a key role in the activation of BS to produce SO 4 ⋅ - by the redox cycle of Fe(III)/Fe(II). Dissolved oxygen was an essential factor for the generation of SO 4 ⋅ - . This work provides both a new approach for using porous industrial waste to improve the catalytic performance of LaFeO3 and guidance for resource utilization of RM in wastewater treatment.