Optimizing sludge dewatering with a combined conditioner of Fenton's reagent and cationic surfactant.

Enhancing sludge dewatering is of importance in reducing environmental burden and disposal costs. In this work, a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), was combined with Fenton's reagent for sludge dewatering. Results show that the Fenton-CTAB conditioning significantly promotes the sludge dewatering. Using combined techniques of response surface methodology and uniform design, dosages of Fe2+, H2O2, and CTAB for water content response were optimized to be 89, 276, and 233 mg/g dry solids (DS), respectively. The water content of sludge decreased from 79.0% to 66.8% under the optimal conditions. Compared with cationic polyacrylamide, the Fenton-CTAB system exhibited superior sludge dewatering performance. To gain insights into the mechanisms involved in sludge dewatering, the effects of Fenton-CTAB conditioning on the composition of extracellular polymeric substances (EPS) and the morphology of the sludge flocs were investigated. The decomposition of EPS into some dissolved organics and the release of proteins in tightly bound EPS facilitated the conversion of bound water to free water and further reduced the water content of sludge cake. After conditioning, morphology of sludge showed aggregation. Overall, the enhanced sludge dewatering by Fenton-CTAB treatment provides an efficient way for management of sewage sludge.

MGDA-assisted plant washing agent for improving the removal of Cd and Cu from farmland soils.

Utilizing plant extracts for soil washing is a promising and cost-efficient strategy to permanently remove toxic metals from farmland soils. However, the efficiency of current plant extracts in heavy metals removal is constrained by the need of excessively high liquid-solid ratios (> 10:1), which limits their practical application in the farmland soils. To address this challenge, we developed a novel binary washing agent consisting of Fructus mume residue extract (RPM) and methylglycinediacetate acid (MGDA). Through optimization experiments, we determined an optimal composition of 50 g L-1 RPM and 0.34 g L-1 MGDA (RPMG3), which exhibited a remarkable synergistic effect on the removal of Cd and Cu from two polluted farmland soils. Compared to the single use of RPM or MGDA, RPMG3 increased metal removal efficiency by 44.6% - 303.8% in the alkaline dryland soil and 55.8% - 141.9% in the slightly acidic paddy soil, at an optimum liquid-solid ratio of 2:1. The contents of soil Cd and Cu can be reduced to below the risk screening values via acid activation, metal ion exchange, and complexation of functional groups. Moreover, the potential ecological risks associated with Cd in the soils were significantly mitigated due to the decrease in exchangeable and reducible Cd fractions following RPMG3 washing. Additionally, RPM and RPMG3 washing led to enhancements in soil organic carbon and nutrient concentrations, as well as increased activities of soil enzymes including catalase, urease, and ß-glucosidase. Notably, RPMG3 washing exhibited the most pronounced promotion effect on wheat seed germination and growth. Taken together, the binary complex of RPMG3 demonstrates potential as an environmentally friendly green washing agent capable of in reducing heavy metals from farmland soil with great efficiency, even at low liquid-solid ratios.

Removal of Cd from contaminated farmland soil by washing with residues of traditional Chinese herbal medicine extracts.

Soil washing is one of the effective methods for permanent removal of heavy metals from farmland soil, and selection of washing agents determines heavy metal removal efficiency. However, there is still a lack of cost-efficient and eco-friendly washing agents. In this study, three residues of traditional Chinese herbal medicine (RTCHM) extracts: residues of Prunus mume (Sieb.) Sieb. et Zucc. (RPM), residues of Schisandra chinensis (Turcz.) Baill. (RSC), and residues of Crataegus pinnatifida Bunge (RCP), were tested for their potential of Cd removal. The variations in amounts and compositions of dissolved organic carbon (DOC) and citric acid were responsible for the difference in Cd removal efficiencies of RTCHM extracts. Fourier-transform infrared spectrophotometer (FTIR) analysis showed that hydroxyl, carboxyl, and amine were the main functional groups of RTCHM extracts to chelate with heavy metals. The optimum conditions for RTCHM extracts were 100 g L-1 concentration, solid-liquid ratio 1:10, pH 2.50, and contact time of 1 h, and the highest Cd removal efficiencies of RPM, RSC, and RCP extracts reached 35%, 11%, and 15%, respectively. The ecological risk of Cd decreased significantly due to the decrease of exchangeable and reducible Cd fractions. RTCHM extracts washing alleviated soil alkalinity and had little effect on soil cation exchange capacity. Meanwhile, the concentrations of soil organic matter and nitrogen were enhanced significantly by RPM extracts and the activities of soil catalase and urease were also improved. Overall, among the tested extracts, RPM extracts was a much more feasible and environment-friendly washing agent for the remediation of Cd-contaminated farmland soil.

Mechanisms for enhanced lignin humification with reduced organic matter loss by goethite in biogas residue composting.

In this study, effects of three iron (oxyhydr)oxides on the biogas residue composting, i.e., composting with goethite (CFe1), hematite (CFe2) or magnetite (CFe3), were investigated. Results showed that composting performance of CFe1 was much better than those of CFe2 and CFe3. Addition of goethite increased temperature of CFe1 and enhanced lignin humification. More than 31.49% of Fe(III) in goethite was reduced to amorphous Fe(II) during the composting, suggesting that goethite worked as electron acceptor for microbial metabolism and heat generation. The functional bacteria Chloroflexi and Actinobacteria, and genes encoding key enzymes (AA1 family), which play essential roles in humification of lignin, were enriched in CFe1. Besides, goethite reduced 10.96% organic matter (OM) loss probably by increasing the molecular size and aggregation of OM for its protection during the composting. This study shows that adding goethite is an efficient strategy to enhancing the humification of lignin-rich biowaste.