Raymond Jianxiong Zeng

Sludge-conditioning strategies are commonly optimized for filtration performance, with less attention paid to the rheological and textural properties of concentrated sludge and their relevance to low-temperature drying. Here, thermally modified drinking-water sludge (HDWS) was evaluated as a waste-derived mineral conditioner for filtration dewatering and low-temperature drying. The specific resistance to filtration (SRF) was lowest at 30% DS, whereas the 60% DS treatment reduced the moisture content to 35.66% after drying at 60 °C for 30 min, indicating that the optimum filtration condition did not deliver the best drying performance. Increasing the HDWS dosage from 0 to 60% DS reduced the Jenike shear stress from approximately 565 to 490 Pa and the apparent yield stress from approximately 670 to 380 Pa, while also decreasing adhesiveness and cohesiveness. At higher dosages, the sludge retained relatively high small-strain stiffness but exhibited lower nonlinear elastic and viscous resistance. These responses were consistent with weakened macroscopic bonding and altered deformation-dependent energy dissipation after HDWS addition. A possible contribution from mineral-particle contacts is suggested, although the underlying microstructural mechanism was not directly resolved. The 60% DS treatment shortened the time required to reach 20% water content to approximately two-thirds of that for raw sludge but reduced the cake calorific value from approximately 9.6 to 4.3 kJ g⁻¹. HDWS, therefore, exhibited process-specific trade-offs, and its dosage should be selected according to the targeted unit operation and downstream management route rather than regarded as a single overall optimum.

Electrochemical CO2 reduction (CO2RR) under near-neutral conditions offers enhanced operational stability and compatibility with reactive carbon capture, yet downstream separation of dissolved products from supporting alkali cations remains a major barrier to practical deployment. Here we report a dual-cation-exchange-membrane solid electrolyte reactor (SER) that enables simultaneous organic acid separation, alkali cation recovery, and CO2 regeneration from neutral CO2RR effluents through highly efficient ion transport. Compared with state-of-the-art flow-electrode capacitive deionization systems, the SER exhibits substantially enhanced ion-transport performance without requiring flow electrodes or supporting electrolytes. Organic acids are selectively retained in the separation chamber with efficiencies of up to 94.7%, whereas more than 95% of alkali cations are recovered in the cathode chamber. Mechanistic analyses reveal that proton-assisted conductivity enhancement governs cation transport, whereas organic acid crossover remains limited and is primarily associated with electro-osmotic drag accompanying cation migration through the membrane. Integration of the SER with a practical reactive-capture electrolyzer demonstrates stable continuous coupling of carbon capture, electrochemical conversion, product separation and electrolyte recovery. These findings establish solid electrolyte reactors as an energy-efficient platform for electrochemical separations and highlight their potential for scalable downstream processing in CO2 electrovalorization systems.

The mobility of nickel (Ni), chromium (Cr), and lead (Pb) in basalt-derived paddy soils under redox-fluctuating conditions governs their accumulation in rice. However, the influence of redox-induced changes in soil components, particularly iron (Fe) (oxyhydr)oxides and organic matter (OM), on the mobility of these metals remains poorly understood. This study revealed that Fe redox cycling serves as the fundamental driving force controlling the release and immobilization of Ni, Cr, and Pb. Fe(III) reduction and OM complexation under anoxic conditions increased metal mobility, while Fe(II) oxidation and OM stabilization under oxic conditions reduced metal mobility. A kinetic model revealed that Ni and Pb mobility was predominantly controlled by Fe cycling, with Pb showing the strongest coupling (81.5-93.5% influence). For Cr, although Fe cycling remained the underlying driver, a greater proportion (82.6-83.6%) was associated with OM interactions. These findings advance risk assessment and inform redox-based remediation strategies for contaminated paddy soils.

Streaming potential generation through water flow in porous media represents an ancient and ubiquitous geophysical phenomenon. For centuries, this process has been regarded as merely a process of charge redistribution without involving any redox reactions. By using NO₃^- reduction as a model reaction, this study demonstrated for the first time that water flow drives abiotic denitrification associated with streaming potential generation. The nitrate (NO₃^-) reduction rate reached 10.6 μmol·L^-1·d^-1, which is comparable to that of FeS-driven chemical NO₃^- reduction (75.0-380 μmol·L^-1·d^-1) and significantly higher than that of photochemistry-driven NO₃^- reduction (0.1-1 μmol·L^-1·d^-1). Through monitoring of nitrogenous products and ¹⁵NO₃^- isotopic experiments, we showed that NO₃^- was selectively reduced to nitrogen (99%) via nitrite and nitrous oxide, confirming a denitrification process. Electron paramagnetic resonance (ESR) spectroscopy using DMPO as the probe detected the generation of hydrogen radicals (H•), which served as the reducing force for NO₃^- reduction. Using TEMPO as the electron probe, linear electron production was observed with an electron efficiency of 6.3% for NO₃^- denitrification. Moreover, H₂¹⁸O isotope experiments demonstrated that water oxidation is the ultimate electron source for NO₃^- reduction, indicating a chemical-free NO₃^- reduction process. An electric field strength of approximately 10⁶ V/cm was detected using surface-enhanced Raman scattering, providing evidence of a strong interfacial electric field (IEF)-induced electron transfer process during water flow. This work reveals ubiquitous but long-overlooked redox reactions associated with streaming potentials. Water-flow-driven denitrification also highlights a newly identified abiotic NO₃^- elimination pathway, suggesting potent chemical-free NO₃^- remediation strategies.

Medium-chain fatty acids (MCFA) are scarcely produced when waste activated sludge (WAS) is used as the sole substrate, limiting their practical development. Production typically requires external electron donors following chemical or physical pretreatments. This study demonstrates that an enriched MCFA-producing consortium (MPC) can initiate MCFA production from WAS without any external electron donors. In the WAS group, the COD conversion ratios from WAS to SCFA (short-chain fatty acids) and MCFA were 6.0% and 0%, respectively. After dosing MPC in WAS, these ratios increased to 20.7% and 4.6%, respectively. These results were attributed to the higher hydrolytic activity in MPC compared to WAS; for instance, the relative activity of alginate lyase in WAS was only 2.8% of that in MPC. Bacteroides (33.1%) was identified as the key bacterium in MPC due to its role in secreting multihydrolases and intracellular enzymes to degrade uronic acids. Conversion of the hydrolysates to MCFA occurred predominantly via the fatty acid biosynthesis (FAB) pathway, as evidenced by the comprehensive identification of FAB-related genes and active enzymes via multiomics analysis. The initiating mechanism is thus attributed to the high activity of both multihydrolases and the FAB pathway without external electron donors. Overall, this study demonstrates the oriented microbial method, coupling the hydrolytic efficiency of FAB-related bacteria and the favorable thermodynamics of the reverse β-oxidation (RBO) pathway in the future, to support the recovery of MCFA from WAS.

The hydrogen bond confers the structural recalcitrance of insoluble exopolysaccharides, but its role in the anaerobic fermentation of waste activated sludge (WAS) is seldom reported. As a typical high-molecular-weight exopolysaccharide with poor solubility, chondroitin sulfate (CS) was used to elucidate the role of hydrogen bonds in WAS fermentation. The chondroitin sulfate-degrading consortium (CSDC) was enriched for 180 days, but the methane production was limited (∼18%). The methane production was significantly improved by 88.2%, and extracellular enzyme activity increased by 11.8% after the fast acidic pretreatment of CS. The disruption of the hydrogen bond was confirmed by combining 41.6% reduction of particle size, 46.7% decrease of surface height, blue shifts in the O-H vibrational spectra, and 38.1% decline of interfacial free energy. The molecular dynamics simulation further showed that the hydrogen bond number after the fast acidic pretreatment was reduced from 51.4 ± 2.7 to 44.9 ± 2.1. Co-occurrence network analysis identified Thermovirga as a keystone taxon to facilitate microbial cooperation. The metagenomics and metaproteomics analyses revealed that Proteiniphilum and Mariniphaga served as key producers of extracellular CS-degrading enzymes, primarily CS lyase (EC 4.2.2.5). Above all, these findings demonstrate that the hydrogen bond represents a non-negligible structural constraint on the degradability of insoluble exopolysaccharides.