Governing the fate of heavy metals: A review on complexation with compost-derived dissolved organic matter

Composting is widely recognized as a sustainable approach to managing organic waste, transforming it into valuable soil amendments. Yet heavy metals (HMs) present in compost can undermine its safety. Dissolved organic matter (DOM), the most reactive component, plays an ambivalent role by firmly binding HMs and reducing toxicity on one hand or forming soluble complexes that increase mobility and environmental risk on the other. This review synthesizes current knowledge of that dual role by examining how compost-derived DOM complexes with HMs. It tracks the evolution of analytical methods, elucidating chemical coordination mechanisms, including functional group chemistry, HSAB theory, and chelation, and identifying key determinants from feedstock origin and DOM maturity to environmental factors such as pH that govern whether metals are held in place or mobilized. This review reveals that the environmental outcome is not arbitrary but is tightly linked to the intrinsic properties of DOM. Mature, aromatic, high-molecular-weight DOM tends toward immobilization, while immature, labile, low-molecular-weight DOM heightens the mobilization risk. Looking ahead, the field must grapple with the complexity of multiple metals and soil contexts, shift towards engineering composts to produce DOM tailored for immobilization, develop predictive models for field outcomes, and promote standardization of DOM metrics so findings are comparable.