Monitoring biochemical oxygen demand (BOD) decay offers critical insights into the aerobic biodegradation of dissolved organic matter (DOM). Focusing on the accumulated metabolites generated during DOM biodegradation, this study introduces novel fluorescence parameters that facilitate the rapid monitoring of BOD decay throughout the entire DOM degradation process until BOD is depleted. BOD decay during degradation of four synthetic DOM samples was first investigated (initial BOD < 10 mg/L) and showed a S-shaped decay pattern with excellent goodness of fit (R² > 0.93; p < 0.001). In contrast, critical spectral signals intensified also in the S-shaped pattern, of which the transition phase converged temporally with BOD decay (time difference < 0.7 d), demonstrating that chromophore-bearing metabolites accumulated synchronously with BOD decay. Pearson analysis further corroborated the highly significant correlations between BOD and spectral signals derived from metabolites throughout the process of DOM degradation. According to Pearson analysis, three metabolite-derived fluorescence parameters were yielded based on three newly specified fluorescence regions related to metabolites (mean r ≈ −0.70; p < 0.05). With a real water sample, we confirmed that these metabolite-derived fluorescence parameters outperformed traditional fluorescence parameters in tracking BOD decay, with a higher R² in multiple linear regression (R² > 0.8; p < 0.001). The findings present a promising approach for rapid tracking and early warning of BOD decay during DOM degradation, potentially contributing to water quality management.

Iron-based nanomaterials (Fe-NPs) have been extensively studied for heavy metal immobilization, yet knowledge of their post-treatment and long-term stability remains limited. Here, we systematically compared the remobilization of cadmium (Cd) from three widely used Fe-NPs, namely, nano–zero-valent iron (nZVI), sulfidated nano–zero-valent iron (S-nZVI), and pyrite nanoparticles (nFeS). Under both oxic and anoxic aging, solution pH strongly controlled Cd(II) speciation and the corrosion behavior of Fe-NPs. Acidic conditions (pH 4) induced substantial Fe-NP dissolution and enhanced Cd release, whereas alkaline conditions (pH 8) greatly suppressed both dissolution and Cd mobilization. During oxic aging, dissolved oxygen significantly accelerated the oxidative corrosion of Fe-NPs, thereby promoting secondary Cd release. The Cd release from nZVI became dramatically higher (up to 50.14%, even at pH 8), which sharply contrasted with the minimal release from S-nZVI (0.37%) and nFeS (0.03%). Elevated concentrations of Na⁺ and Ca²⁺ substantially reduced the stability of spent Fe-NPs, while CO₃^2– buffered the system and helped maintain lower dissolved Cd levels. Furthermore, mechanistic investigation, supported by X-ray diffractometer, X-ray photoelectron spectroscopy, and transmission electron microscopy analyses, revealed that nZVI partially reduced Cd(II) to Cd(0), which subsequently underwent reoxidation under oxic conditions, whereas S-nZVI and nFeS stabilized CdS by forming persistent CdS phases that effectively impeded its release. This study elucidates key factors that govern Cd remobilization and provides a theoretical basis for the long-term application of Fe-NPs in heavy-metal treatment.

The rapid spread of antibiotic resistance genes (ARGs) via plasmid-driven conjugation in pathogenic microbes poses a pressing challenge to global health. Quorum sensing (QS) is pivotal in modulating processes such as biofilm development and the release of virulence determinants, which in turn affect ARG transmission. In this research, an interspecies conjugation model was constructed using Escherichia coli DH5α and Pseudomonas aeruginosa PAO1 as model strains to explore the impact of cinnamaldehyde, a naturally occurring quorum sensing inhibitor (QSI), on the conjugative transfer of antibiotic resistance genes (ARGs) under sub-inhibitory concentrations (sub-MICs). The results revealed that cinnamaldehyde at sub-MICs markedly suppressed transfer frequency without hindering bacterial proliferation. This inhibition of conjugation was largely linked to the suppression of biofilm formation and extracellular polymeric substance (EPS) production, downregulation of QS-related genes rhlI and rhlR, and reduced secretion of virulence factor rhamnolipid, thereby further restricting biofilm-associated ARG dissemination. These mechanisms are all under the governance of the QS system. These findings suggest that cinnamaldehyde, as a QSI, holds promising potential for controlling the spread of bacterial resistance and provides a novel strategy for regulating horizontal gene transfer of ARGs.