The immobilization and transformation of arsenic at the mineral-organic interface in soil environments ultimately depend on the soil components and their interactions. Herein, the effect of humic acid (HA) and a typical bacterium (a Gram-positive Bacillus subtilis) coating on the adsorption of arsenate As(V) to kaolinite (Kao) mineral was investigated. The As(V) adsorption reaction kinetics, isotherms, thermodynamics, and mechanism on the clay mineral-organic composites of kaolinite-Bacillus subtilis (Kao-B.s) and humic acid-kaolinite-Bacillus subtilis (Kao-HA-B.s) were investigated. The As(V) adsorption on the composites was better fitted to pseudo-second-order kinetics and the Freundlich model. The adsorption capacity of As(V) followed the order of Kao-HA-B.s > Kao-B.s > B.s > Kao-HA > Kao. The positive ΔH (31.44, 5.87 kJ mol⁻¹) and ΔG (0.10–0.96 kJ mol⁻¹) values confirmed that the adsorption of As(V) by all composites was nonspontaneous and endothermic in character at room temperature. The FT-IR, XRD, and thermodynamic results revealed that the adsorption mechanism of As(V) on the kaolinite–organic interfaces could be attributed to the electrostatic forces between the terminal aluminum or silanol groups of kaolinite and As(V) and the complexation between HA, bacteria, and As(V), which formed an inner-sphere complex and surface complex, respectively. The experimental results showed that the adsorption of As(V) on the Kao-HA or Kao-bacteria system was accompanied by significant additive interactions, while the ternary Kao-HA-bacteria system had a significant inhibitory effect on As(V) binding at a higher HA content due to the shielding effect, with the promotion effect shown at a lower concentration for dispersion effect for HA on the kaolinite particles.

Sewage sludge, particularly paper mill sludge, is carbonaceous by nature. It can be employed as a heterogeneous catalyst for Fenton-like reactions to attain high catalytic activity and long-term stability. In this paper, the Co/Fe bimetallic composite catalyst was prepared by taking paper mill sludge as the carrier via impregnation and calcination. This catalyst was employed to catalyze peroxymonosulfate (PMS), to produce SO4•−, which has strong oxidizing power. This study focused on determining the optimal preparation conditions, characterization of the catalyst, and investigating the effects of various inorganic anions on the degradation of Rhodamine B (RhB) in the Co/Fe-PMS system. The optimal catalyst preparation condition was found to be a 0.5 mol/L impregnation concentration, with the best ratio of Co(NO3)2 to FeSO4 solution being 7:3. The presence of Co and Fe in the catalyst was determined through FTIR and XRD analyses. The catalyst is a porous material with magnetic properties and is capable of degrading more than 95% of RhB within two minutes. It was found that H₂PO₄⁻, Cl⁻, and HCO₃⁻ had negative impacts on the degradation of RhB. Low concentrations of NO₃⁻ had a positive effect, whereas high concentrations of NO₃⁻ had a negative effect. This study presents a novel method for preparing highly efficient Fenton-like catalysts and offers an effective approach for sludge utilization.

The fate of heavy metals in agricultural soils is governed by organo-mineral–microbial interactions, which predictive models often fail to capture. The competitive sorption mechanisms of Cd(II) and Cu(II) on montmorillonite/kaolinite composites (Mont/Kao) functionalized with humic acid (HA) and Pseudomonas putida (P. p), a model system representative of contaminated agricultural soils, were investigated. Batch experiments, XRD, FTIR, and thermodynamic analysis reveal that metal retention is a non-additive function of competing interfacial processes. Bacterial biomass dominated sorption, accounting for >50% of total metal uptake, with capacity ranked as: P. p > Mont/Kao-P. p > Mont/Kao-HA-P. p > Mont/Kao-HA > Mont/Kao. Humic acid exerts a dual, concentration-dependent role: Low levels enhanced adsorption via mineral dispersion, while high levels induced surface masking, suppressing bacterial binding sites. Competition was highly asymmetric: Cd(II) reduced Cu(II) maximum adsorption capacity by 75.5% in the Mont/Kao-HA system by preferentially occupying montmorillonite interlayer sites, whereas Cu(II) inhibited Cd(II) below pH 6. Single-metal sorption was characterized by positive ΔS° (32.96–58.89 J·mol⁻¹·K⁻¹), indicative of inner-sphere complexation, while negative ΔS° under competitive conditions signals a transition to outer-sphere complexation. This work provides mechanistic insights into site masking, competitive displacement, and ternary cation bridging controlling metal immobilization in organo-mineral assemblages.