Examination the effects of Zn2+ and Cu2+ on the Pb2+ binding with dissolved organic matter using absorbance spectroscopy

Dissolved organic matter (DOM) strongly influences the geochemical mobility of metal ions, yet little is known about the mechanism of lead (Pb) binding to DOM in the presence of copper (Cu) and zinc (Zn). This research used different types of ultraviolet–visible (UV–vis) spectroscopy analysis technologies, including differential log-transformed spectra coupled with spectral slope calculation, and two-dimensional correlation spectroscopy (2D-COS) analysis to investigate the binding characteristics between Pb and DOM in the presence of Zn and Cu, respectively. The results indicate that the addition both of Zn 2+or Cu 2+can reduce the binding of DOM to Pb 2+, with the effect of Cu 2+being the most obvious. Specifically, significant changes were evident in the differential log-transformed absorbance at higher Zn2+/Cu2+ concentrations, including modified spectral shapes, absorption wavelength fluctuation, and differential absorption intensity changes. The differential log-transformed absorbance spectral slopes (∆S350-400) exhibited a positive correlation with the predicted NICA-Donnan model value in a wavelength range of 350–400 nm. This suggested that ∆S350-400 served as an indicator for assessing the complexation characteristics of Pb and DOM binding in the presence of Zn2+ and Cu2+, respectively. The 2D-COS spectral results showed that the presence of Zn2+/Cu2+ changed the binding order of the DOM functional groups in response to Pb2+. This study revealed that Zn2+ and Cu2+ competed with Pb2+ for complex sites on the DOM surface in different ways, altering the ability of Pb2+ to bind with DOM. The results suggest that using differential log-transformed spectroscopy combined with ∆S350-400 and 2D-COS analyses is a promising approach to understanding the mechanisms of Zn2+/Cu2+ effects on Pb-DOM interactions, which may provide new insights into the prevention, control, and remediation of environmental Pb pollution.