Sludge-derived iron-carbon material enhancing the removal of refractory organics in landfill leachate: Characteristics optimization, removal mechanism, and molecular-level investigation

Reuse of sludge from photo-Fenton process and further application as a catalyst in electrochemical processes for the treatment of landfill leachate

To evaluate the potential of sludge from solar photo-Fenton to be transformed into a catalyst for the heterogeneous electro-Fenton (EF) process, some parameters were investigated, including (i) the impact of heat treatment on catalytic activity through structural and compositional analysis of the iron species (100, 600 and 750 °C); (ii) analysis of the effect of pH and current density (j) on the decolorization and decarbonization efficiency; and (iii) the generation of high-value organic acids. The calcination of sludge to 600 and 750 °C presents changes in the multiferroic structures but does not enhance its catalytic activity, given the similarity of the treatment results of landfill leachate with the catalyst with and without calcination. The treatment of leachate via heterogeneous EF using sludge dried at 100 °C and under pH 3 achieves better results than the treatment without pH adjustment. At the evaluated j values (30, 60 and 90 mA cm-2), Chemical Oxygen Demand removal efficiencies of 47 %, 48 % and 50 % were respectively found, and Total Organic Carbon abatements of 36 %, 36 % and 42 % were reached. Despite the low effluent mineralization, high value-added products (carboxylic acids) are formed during 240 min of EF, with the possibility of selective electroconversion into acetic and formic acid. Regarding decolorization, a successful result was found with the j of 90 mA cm-2, which achieved almost 90 % color removal. Finally, a final treatment strategy is proposed using the sludge only dried to 100 °C, followed by heterogenous EF with acidification to pH 3 and j of 60 mA cm-2.

Weighted comprehensive risk assessment and pollution analysis of long-term printing and dyeing sludge landfills in an industrialized Chinese city

Sludge landfilling is widely used in China, accounting for approximately 65% of total sludge disposal, due to its simplicity and cost-effectiveness. However, with increasing land scarcity and stricter environmental regulations, the Chinese government has emphasized reducing sludge landfilling. Despite these efforts, sludge historically disposed of in landfills continues to pose risks, including heavy metal leaching and contamination of groundwater and soil. Comprehensive risk assessments for multi-contaminant industrial sludge in long-term landfills remain limited. This study conducted a risk survey and pollution assessment at a long-term sludge landfill in a typical industrial city in China. The sludge at the site was found to be mildly alkaline (pH 7.30-8.29) and had a low calorific value (1.37E+03-1.61E+03 kcal/kg), with significant dewatering challenges (1.23 × 1015 s2/g). The overall pollution load was relatively low, with a regional Pollution Load Index (PLI) of 0.65. However, due to extreme pollutants, the Nemerow Pollution Index (NPI) reached 9.49, indicating severe pollution. The single-factor index assessment indicates high pollution levels for Zn, Sb, BaP, and total petroleum hydrocarbons (TPHs). Ecological risk is primarily associated with Hg, Sb, BaP, InP, and TPHs. However, the weighted comprehensive risk assessment method proposed in the study identifies Zn, Pb, Sb, and TPHs as priority pollutants, with the main pollution hotspots being SZ4, SZ5, and SZ9. Some pollutants, such as Ni, antibiotics, polychlorinated biphenyls, and certain polycyclic aromatic hydrocarbons, were not detected in the sludge. Through principal component analysis and positive matrix factorization analysis, multiple sources of pollution at the landfill were identified, including the dyeing industry, electronics manufacturing, traffic emissions, fuel combustion, and urban runoff. This analysis highlights heavy metal pollution (Cr, Hg, Pb) and organic pollutants (PAHs, TPHs, DBA) resulting from industrial and urban activities. The results offer key insights to guide future sludge treatment and disposal at this landfill.

Overcoming metals redox rate limitations in spinel oxide-driven Fenton-like reactions via synergistic heteroatom doping and carbon anchoring for efficient micropollutant removal

The transition metals redox rate limitations of spinel oxides during Fenton-like reactions hinder its efficient and sustainable treatment of actual wastewater. Herein, we propose to optimize the electronic structure of Co-Mn spinel oxide (CM) via sulfur doping and carbon matrix anchoring synergistically, enhancing the radicals-nonradicals Fenton-like processes for efficient water decontamination. Activating peroxymonosulfate (PMS) with optimised spinel oxide (CMSAC) achieved near-complete removal of ofloxacin (10 mg/L) within 6 min, showing 8.4 times higher efficiency than CM group. Significantly higher yields of SO4·- and high-valent metal species in CMSAC/PMS system provided exceptional resistance to co-existing anions, enabling efficient removal of various emerging contaminants in high salinity leachate. Specifically, sulfur coordination and carbon anchoring-induced oxygen vacancy synergistically improved the electronic structure and electron transfer efficiency of CMSAC, thus forming highly reactive Co sites and significantly reducing the energy barrier for Co(IV)=O generation. The reductive sulfur species facilitated the conversion of Co(III) to Co(II), thereby maintaining the stability of the catalytic activity of CMSAC. This work developed a synergistic optimization strategy to overcome the metals redox rate limitations of spinel oxides in Fenton-like reactions, providing deep mechanistic insights for designing Fenton-like catalysts suitable for practical applications.

Nitrogen-doped activated carbon-based steel slag composite material as an accelerant for enhancing the resilience of flexible biogas production process against shock loads: Performance, mechanism and modified ADM1 modeling

Anaerobic digestion for flexible biogas production can lead to digestion inhibition under high shock loads. While steel slag addition has shown promise in enhancing system buffering, its limitations necessitate innovation. This study synthesized the nitrogen-doped activated carbon composite from steel slag to mitigate intermediate product accumulation during flexible biogas production. Material characterization preceded experiments introducing the composite into anaerobic digestion systems, evaluating its impact on methane production efficiency under hydraulic and concentration sudden shocks. Mechanistic insights were derived from microbial community and metagenomic analyses, facilitating the construction of the modified Anaerobic Digestion Model No. 1 (ADM1) to quantitatively assess the material's effects. Results indicate superior resistance to concentration shocks with substantial increment of methane production rate up to 33.45% compared with control group, which is mediated by direct interspecies electron transfer, though diminishing with increasing shock intensity. This study contributes theoretical foundations for stable flexible biogas production and offers an effective predictive tool for conductor material reinforcement processes.

A Fenton-based approach at neutral and un-conditioned pH for recalcitrant COD removal in tannery wastewater: Experimental test and sludge characterization

The combination of raw animal skin manufacturing processes involves the use of large amounts of chemicals, resulting in the generation of complex and highly polluted tannery wastewater. In this context, the high concentration of chloride in tannery wastewater represents a crucial bottleneck. Indeed, sodium chloride, commonly used in tannery industry to prevent skin rot, increases the concentration of chlorides up to 50 %. At the same time, most of the advanced oxidation processes usually employed in tannery wastewater treatment to remove recalcitrant COD involve the use of conditioning agents, thus increasing the overall concentration of chlorides in the treated effluent. The aim of this study was to evaluate the electrochemical peroxidation process (ECP) efficiency in the treatment of tannery wastewater without changing pH, to improve Fenton technology by avoiding the use of chemicals. The influence of different electric currents on COD and color removal was investigated. The characterization of the produced sludge was conducted through FTIR, SEM and XRD analysis, exploring the morphology and composition of precipitate, depending on the applied current. Although an electrical current of 750 mA yields the highest COD and color removal efficiency (69.7 % and 97.8 %, respectively), 500 mA can be considered the best compromise because of energy consumptions. Iron oxides and hydroxides were generated during the ECP process, playing the role of coagulants through the absorption of organic and inorganic contaminants. The consumption of energy increased as a function of time and applied current; however, cost analysis showed that the electrodes contributed the most to the total cost of the process. In authors' knowledge, the application of ECP process as a tertiary treatment for the removal of recalcitrant COD in tannery wastewater represents a novelty in the literature and the results obtained can be considered as the basis for scaling up the process in future research.

Leachate derived humic-like substances drive the variation in microbial communities in landfill-affected groundwater

Landfills are commonly used for waste disposal in many countries, and pose a significant threat of groundwater contamination. Dissolved organic matter (DOM) plays a crucial role as a carbon and energy source, supporting the growth and activity of microorganisms. However, the changes in the DOM signature and microbial community composition in landfill-affected groundwater and their bidirectional relationships remain inadequately explored. Herein, we showed that DOM originating from more recent landfills mainly comprises microbially produced substances resembling tryptophan and tyrosine. Conversely, DOM originating from older landfills predominantly comprises fulvic-like and humic-like compounds. Leachate leakage increases microbial diversity and richness and facilitates the transfer of foreign bacteria from landfills to groundwater, thereby increasing the vulnerability of the microbial ecosystem in groundwater. Deterministic processes dominated the assembly of the groundwater microbial community, while stochastic processes accounted for an increased proportion of the microbial community in the old landfills. The dominant phyla observed in groundwater were Proteobacteria, Bacteroidota, and Actinobacteriota, and humic-like substances play a crucial role in driving the variation in microbial communities in landfill-affected groundwater. Predictions using PICRUSt2 suggested significant associations between various metabolic pathways and microbial communities, with the Kyoto Encyclopedia of Genes and Genomes pathway "Metabolism" being the most predominant. The findings contribute to advancing our understanding of the transformation of DOM and its interplay with microbial communities and can serve as a scientific reference for decision-making regarding groundwater pollution monitoring and remediation.

The role of microstructure of extracellular proteins in dewaterability of alkaline pretreatment sludge during bioleaching

Alkaline pre-treatment is known to enhance the acid production efficiency of sludge but adversely affects its dewatering performance. In this study, the improvement of sludge dewaterability by a novel bioleaching system with inoculating domesticated acidified sludge (AS) and its underlying mechanism were investigated. The results showed that although the addition of Fe2+ and the reduction of pH improved the dewatering performance of sludge, their effects were inferior to that of AS + Fe. The addition of AS and Fe2+ significantly reduced the specific resistance to filtration and capillary suction time of the sludge by 98.6 % and 95.5 %, respectively. This improvement in dewatering performance was achieved through the combined actions of bio-acidification, bio-oxidation, and bio-flocculation. Remarkably, under alkaline pH, microorganisms in AS remained active, leading to the formation of iron-based bioflocculants, along with a rapid pH decrease. These bioflocculants, in combination with protein (PN) in tightly bound extracellular polymeric substances (TB-EPS) through amide bonding, transformed TB-EPS from extractable to non-extractable form, reducing PN content from 12.1 mg g-1DS to 5.09 mg g-1DS and altering the protein's secondary structure. Consequently, the gel-like TB-EPS matrix effectively broke down, releasing cellular water and significantly enhancing sludge dewaterability.