In this study, mulberry tree stems were used as raw material to prepare magnetic modified biochar, Fe-BC-500, using the co-precipitation method. The structure of Fe-BC-500 was systematically characterized and tested for arsenic (As) adsorption in batch experiments by varying parameters such as solution pH (3–11), the concentrations of co-existing anions (2–20 mg/L), and ionic strength (0–0.5 mol/L NaNO₃). The results indicate that Fe-BC-500 exhibited optimal adsorption capacity at pH 4 and an initial As(V) concentration of 20 mg/L. The influence of co-existing anions on As(V) adsorption followed the order PO₄³⁻ > SO4²⁻ > NO₃⁻. Kinetic analysis showed that adsorption of Fe-BC-500 on As(V) followed a pseudo-second-order kinetic model, with a correlation coefficient of 1.00, indicating chemical adsorption. The Langmuir model accurately described the isothermal adsorption results, indicating monolayer adsorption. Mechanistic analysis showed that As(V) was fixed on the Fe-BC-500 surface through complexation reactions, demonstrating adsorption specificity. This study provides a theoretical basis and highlights the application potential of magnetically modified biochar for removing As(V) from water.

This study used red mud (RM), steel slag (SS), and desulfurization slag (DS) as raw materials to construct three catalytic oxidation systems, namely RM-DCDA/H₂O₂, SS-DS/H₂O₂, and SS-DS/peroxymonosulfate (PMS), to promote the utilization of industrial solid waste and enhance the treatment of recalcitrant antibiotic wastewater. The ciprofloxacin (CIP) degradation performances, influencing factors, and preliminary reaction mechanisms of these systems were investigated. RM formed an Fe₃N/C composite structure after acidification and dicyandiamide-assisted calcination. The Fe₃N active phase, coexistence of Fe²⁺/Fe³⁺, and N-doped C structure facilitated H₂O₂ activation and electron transfer. The SS-DS catalyst exhibited a rough and porous structure and contained Fe, Ca, and S species, which could provide reactive sites for H₂O₂ and PMS activation, following acid modification and urea-assisted calcination. Under the necessary reaction conditions, the CIP degradation efficiencies of the RM-DCDA/H₂O₂, SS-DS/H₂O₂, and SS-DS/PMS systems reached 94.60%, 92.58%, and 95.17%, respectively. These results indicate that RM- and SS-derived materials can be used for CIP oxidative degradation; however, the values should not be interpreted as a strict comparison of the intrinsic catalytic activity because the operating conditions differed among the systems. Parametric experiments showed that the catalyst dosage, oxidant concentration, and initial pH influenced the degradation efficiency. The H₂O₂-based systems were more suitable under acidic conditions, whereas the SS-DS/PMS system showed wider pH adaptability. Coexisting anion and humic acid experiments indicated that the systems were tolerant to natural organic matter, whereas HCO₃⁻ and HPO₄²⁻ inhibited degradation. CIP was further oxidized in total organic C and recycling experiments; however, it was difficult to completely mineralize it within a short reaction time, and the catalyst retained relatively high activity after repeated use. Radical quenching experiments suggested that ·OH and ·O₂⁻ participated in the degradation reactions in the RM-DCDA/H₂O₂ and SS-DS/H₂O₂ systems. In the SS-DS/PMS system, comparative quenching experiments revealed that non-radical singlet oxygen (¹O₂) was the dominant reactive species, while SO₄⁻· and ·OH contributed only marginally. In conclusion, RM and SS-DS can be used as low-cost raw materials to prepare industrial solid-waste-derived catalysts for the oxidative degradation of CIP, thereby providing a reference for industrial solid waste valorization and antibiotic wastewater treatment.

This study investigated the effects of calcium (Ca²⁺) and iron (II) Fe²⁺ concentrations (0–500 mg/L) on the heterogeneous nucleation and crystallization behavior of struvite (MgNH₄PO₄·6H₂O) through controlled batch precipitation experiments. Struvite formed under different Ca²⁺ and Fe²⁺ concentrations were systematically characterized using XRD, SEM, FTIR, and XPS, while real-time pH and redox potential (Eh) monitoring was employed to elucidate reaction dynamics and thermodynamic speciation and saturation indices were calculated, and classical nucleation theory (CNT) was applied to interpret nucleation behavior. The results show that Ca²⁺ primarily suppresses struvite formation through bulk-phase competition with Mg²⁺ for phosphate, diverting phosphate into Ca–P phases and progressively reducing struvite supersaturation, which leads to decreased crystallinity and distorted Crystal habit. In contrast, Fe²⁺ does not form detectable crystalline Fe-P phases but inhibits struvite crystallization mainly through surface-mediated processes. Surface analyses indicate that Fe-bearing species adsorb onto struvite surfaces and promote amorphous Fe-P deposition, increasing interfacial resistance to nucleation and growth. CNT analysis further reveals that Ca²⁺ inhibition is governed by reduced thermodynamic driving force, whereas Fe²⁺ inhibition is dominated by surface-related kinetic barriers. This study provides mechanistic insight into ion-specific interference during struvite crystallization and offers guidance for optimizing phosphorus recovery in ion-rich wastewater systems.

Coupled nitrogen (N) and phosphorus (P) cycling in calcareous karst soils, and how it responds to long-term land use and restoration, remains poorly quantified. We compared seven plots on a karst slope platform: three development types (degraded disturbed land, pasture, and fruit orchard) and four restoration types (planted evergreen forest, planted deciduous forest, evergreen–deciduous mixed forest, and naturally restored forest). Gross N transformation rates were measured by ¹⁵N isotope dilution, alongside soil properties, available P, microbial biomass N, and enzyme activities. Conservation plots generally had higher soil organic carbon, total N, microbial biomass N, and enzyme activity than development plots; soil organic carbon peaked under naturally restored forest (57.5 g·kg⁻¹) and was lowest in disturbed land (47.9 g·kg⁻¹). Naturally restored forest also showed the highest gross nitrification and total N mineralization, whereas disturbed land had the weakest ammonification, with negative gross rates pointing to N immobilization. Available P (up to 15.9 mg·kg⁻¹) tracked alkaline phosphatase activity, organic carbon, and total N rather than total P. Across the alkaline range (pH 7.2–7.8), random forest models ranked ammonium and enzyme activity, not pH, as the main predictors of N mineralization. Long-term land use and restoration were thus associated with consistent differences in karst soil N and P supply.

Municipal sludge anaerobic digestion (AD) is often constrained by limited substrate accessibility due to extracellular polymeric substances (EPSs), microbial cell walls, and compact floc structures. This review integrates pretreatment technologies and EPS characteristics through a five-dimensional conceptual framework: cell disruption, EPS solubilization, particle-size reduction, soluble chemical oxygen demand (SCOD) release, and enhanced biodegradability. The framework connects structural changes with substrate accessibility and digestion performance. Physical, chemical, and biological pretreatment methods are critically compared using study-specific evidence and descriptive numerical ranges rather than pooled estimates. Thermal hydrolysis promotes cell disruption and organic matter solubilization, while other methods offer complementary mechanisms with distinct operational constraints. However, increased SCOD does not necessarily translate into improved biodegradability or methane recovery. Similarly, improvements in laboratory-scale biochemical methane potential (BMP) cannot be directly extrapolated to full-scale digesters. Pretreatment effectiveness should therefore be evaluated by considering biodegradability, net methane gain, energy and chemical inputs, and downstream environmental impacts. This framework supports the interpretation of heterogeneous evidence and the selection of context-specific pretreatment strategies.