Asami Suzuki Mashio

The extent to which the metal-binding functionality of dissolved organic matter (DOM) can be inferred from its conventional optical properties remains largely unknown. Optical measurements of dissolved organic matter (DOM), including chromophoric (CDOM) and fluorescent (FDOM) properties, are commonly used as proxies for humic substances, yet it remains unclear whether these approaches also reflect the same metal-binding fraction quantified by recently developed electrochemical techniques. In this study, electroactive humic substances (eHS), CDOM, FDOM, and dissolved trace metals were measured across a humic-rich tropical terrestrial-marine continuum, and the results were compared with a previous dataset from a less humified estuarine system. Principal component analysis revealed that eHS was decoupled from conventional optical indices, including the absorption coefficient at 254 nm (a254) and the humification index (HIX), but was closely associated with dissolved manganese and cobalt. Despite the contrasting environmental settings, a similar separation between eHS and optical DOM properties was observed, indicating this decoupling is a general feature rather than a site-specific phenomenon. These findings show that optical proxies alone do not adequately characterize the fraction of humic substances responsible for trace metal complexation and transport, whereas electrochemical measurements selectively quantify a chemically distinct, metal-reactive fraction of the DOM pool. Integrating optical and electrochemical approaches therefore provides complementary perspectives on DOM composition and functionality, offering a more complete framework for improving our understanding of trace metal-DOM interactions in aquatic environments.

The environmental fate of biodegradable aminopolycarboxylate chelating agents is strongly influenced by metal complexation; however, the combined effects of central metal ions and ligand structure on degradation behavior remain insufficiently understood. In this study, the biodegradation and photodegradation of the Mg, Ca, Zn, and Cu complexes of four biodegradable chelating agents (EDDS, HIDS, MGDA, and GLDA) were investigated under activated sludge and UV-lamp irradiation conditions. Biodegradation tests revealed metal-dependent behavior: complexes with the alkaline earth metals Mg and Ca were readily degraded, whereas complexes with the transition metals Zn and Cu, except for the EDDS complexes, exhibited high persistence. A qualitative inverse relationship was observed between biodegradability and metal–ligand stability. In contrast, photodegradation exhibited a distinct metal-specific pattern: Cu complexes underwent photodegradation, whereas Mg, Ca, and Zn complexes showed limited photodegradation regardless of ligand identity. These results demonstrate that the degradability of metal–chelate complexes cannot be predicted solely from the behavior of the corresponding free ligands and highlight the importance of considering metal complexation when evaluating the environmental persistence of biodegradable chelating agents in metal-containing aquatic systems.

The extent to which the metal-binding functionality of dissolved organic matter (DOM) can be inferred from its conventional optical properties remains largely unknown. Optical measurements of dissolved organic matter (DOM), including chromophoric (CDOM) and fluorescent (FDOM) properties, are commonly used as proxies for humic substances, yet it remains unclear whether these approaches also reflect the same metal-binding fraction quantified by recently developed electrochemical techniques. In this study, electroactive humic substances (eHS), CDOM, FDOM, and dissolved trace metals were measured across a humic-rich tropical terrestrial-marine continuum, and the results were compared with a previous dataset from a less humified estuarine system. Principal component analysis revealed that eHS was decoupled from conventional optical indices, including the absorption coefficient at 254 nm (a254) and the humification index (HIX), but was closely associated with dissolved manganese and cobalt. Despite the contrasting environmental settings, a similar separation between eHS and optical DOM properties was observed, indicating this decoupling is a general feature rather than a site-specific phenomenon. These findings show that optical proxies alone do not adequately characterize the fraction of humic substances responsible for trace metal complexation and transport, whereas electrochemical measurements selectively quantify a chemically distinct, metal-reactive fraction of the DOM pool. Integrating optical and electrochemical approaches therefore provides complementary perspectives on DOM composition and functionality, offering a more complete framework for improving our understanding of trace metal-DOM interactions in aquatic environments.

The extent to which the metal-binding functionality of dissolved organic matter (DOM) can be inferred from its conventional optical properties remains largely unknown. Optical measurements of dissolved organic matter (DOM), including chromophoric (CDOM) and fluorescent (FDOM) properties, are commonly used as proxies for humic substances, yet it remains unclear whether these approaches also reflect the same metal-binding fraction quantified by recently developed electrochemical techniques. In this study, electroactive humic substances (eHS), CDOM, FDOM, and dissolved trace metals were measured across a humic-rich tropical terrestrial-marine continuum, and the results were compared with a previous dataset from a less humified estuarine system. Principal component analysis revealed that eHS was decoupled from conventional optical indices, including the absorption coefficient at 254 nm (a254) and the humification index (HIX), but was closely associated with dissolved manganese and cobalt. Despite the contrasting environmental settings, a similar separation between eHS and optical DOM properties was observed, indicating this decoupling is a general feature rather than a site-specific phenomenon. These findings show that optical proxies alone do not adequately characterize the fraction of humic substances responsible for trace metal complexation and transport, whereas electrochemical measurements selectively quantify a chemically distinct, metal-reactive fraction of the DOM pool. Integrating optical and electrochemical approaches therefore provides complementary perspectives on DOM composition and functionality, offering a more complete framework for improving our understanding of trace metal-DOM interactions in aquatic environments.

The effective separation of critical metals (CMs) from organic solvent waste streams represents a formidable challenge in industrial environmental management, primarily because conventional aqueous-phase separation technologies, such as ion exchange and precipitation, struggle with the unique physicochemical properties of organic media. In this study, we introduce a highly effective dithiocarbamate-functionalized cellulose sorbent (DPE6) as a solvent-tolerant platform designed to overcome these limitations in the capture of CMs (Ni^II and Pb^II) from polar organic solvents. DPE6 demonstrated exceptionally rapid sorption kinetics, achieving equilibrium within 10 min, and exhibited superior maximum sorption capacities for Ni^II (968 and 976 μmol g^–1) and Pb^II (1217 and 1128 μmol g^–1) in pure methanol (MeOH) and N,N-dimethylformamide (DMF), respectively. Advanced spectroscopic characterization, utilizing Fourier-transform infrared (FT-IR), X-ray photoelectron (XPS), and X-ray absorption spectroscopy (XAS), provided conclusive evidence that metal uptake is driven by robust surface complexation with sulfur-active sites. To validate practical utility, DPE6 was applied to the recovery of Pb from organic effluents derived from the recycling of spent perovskite solar cells. The sorbent achieved quantitative extraction efficiencies of 91–99%, markedly outperforming commercial resins which failed in strongly solvating media. These findings position functionalized cellulose as a transformative solution for nonaqueous separations, offering a sustainable pathway for critical material recovery in advanced manufacturing sectors.

Freshwater salinization is an emerging environmental stressor that significantly influences aquatic contaminant cycling; however, its impact on phytoplankton-mediated arsenic (As) biotransformation remains poorly understood. This study investigates how salinity-driven physiological responses regulate arsenate (As(V)) uptake and speciation in three freshwater phytoplankton species (Scenedesmus acutus, Pediastrum duplex, and Staurastrum paradoxum) across a controlled salinity gradient (0–10‰). Cellular responses were evaluated through growth kinetics, photosynthetic efficiency (Y(II)), intracellular electrolyte composition, and antioxidant (thiol and pigment) biosynthesis. Furthermore, single-cell inductively coupled plasma mass spectrometry (SC-ICP-MS) and speciation analyses were employed to quantify intracellular As metabolism. The results reveal that in the salt-sensitive species (P. duplex and S. paradoxum), elevated salinity induced substantial inorganic ion influx and subsequent generation of reactive oxygen species (ROS). This stress activated glutathione (GSH)-based antioxidant defenses but concurrently suppressed As methylation, indicating a metabolic trade-off between oxidative stress mitigation and As biotransformation. Conversely, the salt-tolerant S. acutus maintained tight ionic homeostasis and upregulated carotenoid-based photoprotection. This adaptation preserved photosynthetic efficiency and facilitated the sustained conversion of inorganic As into less toxic, higher-order organic species despite osmotic stress. The findings demonstrate that species-specific osmotic and redox regulation mechanisms dictate As cycling under saline stress, highlighting a critical link between progressive freshwater salinization and altered contaminant bioaccumulation in aquatic ecosystems.