Composition characteristics and metal binding capacity of dissolved organic matter in the agriculture-dominated Tuhai River

Abstract

An essential part of river ecosystems, dissolved organic matter (DOM) has an enormous effect on the transport and variability of heavy metals. Nonetheless, our understanding of how the DOM affects heavy metal behavior in agricultural rivers remains limited. In this research, the DOM characteristics and effects on heavy metals (Cu²⁺, Zn²⁺, and As³⁺) were analyzed in the Liaocheng section of the Tuhui River (LCTH) during High, Normal, and Low-water periods. EEM-PARAFAC identified three fluorescent components: C1 are microbial humic-like substances, C2 are terrestrial humic-like substances, and C3 are protein-like substances. Humic-like substances were dominant in all water periods, accounting for approximately 70% of the total DOM. It is interesting to note that the ratio of microbial-derived components to terrestrial-derived components, (C1 + C3)/C2 in Chiping District (3.30 ± 1.18) was significantly higher than that in Shen County (2.40 ± 0.57). On this basis, to further substantiate the binding capacity of various types of DOM with Cu²⁺, samples from Lao Nanzhen (LNZ) and Li Fengtao (LFT) sites were selected for fluorescence titration experiments. The results showed that the samples from both sites were humic-like substances preferentially complexed with Cu²⁺. The complexation of terrestrial humic substances (C2) with Cu²⁺ was significantly higher in LFT (logK_a=3.65±0.08) than in LNZ (logK_a=3.16±0.06). Structural equation modeling (SEM) revealed that humic-like substances in the LCTH had a greater effect on different heavy metals than protein-like substances. The findings elucidate how DOM affects heavy metals, contributing to efforts aimed at controlling heavy metal pollution in river systems.

Keywords

agriculture / dissolved organic matter (DOM) / heavy metals / structural equation modeling (SEM)

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