Copper binding heterogeneity of pH-fractionated soil dissolved organic matter (DOM) revealed by multi-spectroscopy and two-dimensional correlation analysis

Soil dissolved organic matter (DOM) plays a pivotal role in governing the environmental behavior and fate of metal ions. The inherent heterogeneity of DOM, however, leads to complex and poorly understood interactions with heavy metals. In this study, soil DOM was fractionated into four pH-dependent fractions via PPL solid-phase extraction combined with sequential acid elution. The binding interactions of these fractions with Cu(II) were systematically investigated through and, coupled with two-dimensional correlation (2D-COS) analysis. The results indicated that low-pH fractions exhibited enhanced aromaticity and more pronounced spectral responses. Regardless of the DOM fraction, 2D-COS analysis revealed that short-wavelength chromophores (< 230 nm) preferentially coordinated with Cu(II), and the binding sequence of fluorescent components followed protein-like > fulvic-like > humic-like components. Moreover, functional group analysis showed that carboxyl C = O groups dominated the binding response, followed by aliphatic C-OH or phenolic O-H, while pH-based fractionation of DOM primarily influenced binding accessibility rather than binding mechanism. Meanwhile, the parallel factor analysis (PARAFAC) results revealed that protein-like substances exhibited higher conditional stability constants, whereas humic-like fractions provided more accessible binding sites among the pH-dependent DOM fractions. Collectively, these findings illuminated that the heterogeneous complexation mechanisms between DOM fractions and Cu(II), and provide a basis for assessing environmental geochemical risks of heavy metals.