Changes in the complexation behavior of cadmium with dissolved organic matter in the Huaihe River basin: Environmental drivers, regional differences and socio-economic impacts

The mobility of Cd (II) in aquatic ecosystems is significantly influenced by its complexation with dissolved organic matter (DOM), but the impact of multidimensional factors on this complexation process remains poorly understood. This study examined Cd (II) binding of DOM from four distinct regions-agricultural (AgriDOM), industrial (IndDOM), wetland (WetDOM), and estuarine (EstDOM)-in a pollution-entwined, agro-saturated river. Analysis was performed using fluorescence excitation-emission matrix spectroscopy (EEM), principal component coefficients, parallel factor analysis (PARAFAC), moving-window two-dimensional correlation spectroscopy (MW2DCOS), and structural equation modeling (SEM). Five components were extracted from the titrants through EEM-PARAFAC (fluorescence excitation-emission matrix spectroscopy-parallel factor analyses), i.e., tryptophan-like substances (C1), photochemical products of terrestrial organic matter (C2), microbial humic-like component (C3), fulvic-like substance (C4), humic-like components (C5). C1 originated from wastewater and is significantly influenced by biological activity. C2 exhibited sensitivity to Cd (II) in regions with active photodegradation (AgriDOM, WetDOM). Agricultural activities mobilized C3, enhancing its complexation with Cd (II). Increased runoff from irrigation and drainage promoted a higher binding affinity of C4 for Cd (II). C5, derived from wastewater, declined in estuarine and high-salinity environments. In AgriDOM and IndDOM, Cd (II) preferentially associated with C5 and C4, reducing its mobility and facilitating its stabilization in soil. Within WetDOM and EstDOM, Cd (II) primarily bound to C1, markedly enhancing its mobility. The socio-economic condition of a region influences water environmental factors, thereby affecting the complexation behavior of Cd (II) with DOM. Agricultural activities reduce Cd (II) mobility, while industrial growth enhances its mobility through elevated GDP-P. The Huaihe River basin's industrial transformation toward greener, low-carbon practices has reduced pollution, demonstrating a negative correlation between economic growth and pollution levels, aided by improved environmental infrastructure. This study may provide a new perspective and framework for understanding and addressing Cd (II) contamination remediation.