Role of competitive adsorption between microplastics and dissolved organic matter in triggering abrupt changes in soil free heavy metal dynamics

With the intensification of global plastic pollution, elucidating the ecological risks of combined microplastics (MPs) and heavy metals (HMs) has become increasingly urgent. This study investigated the mechanisms by which competitive adsorption between MPs and dissolved organic matter (DOM) can induce nonlinear, abrupt shifts in free-state HMs concentrations. The MPs degradation, DOM evolution, and their effects on HMs transformation were examined through controlled microcosm experiments using typical MPs—polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC)—and HMs (Cr, As) in red soil. The results revealed polymer-specific and HMs-dependent degradation. Cr(VI) accelerated PS degradation, thereby releasing intermediates that competed with humic substances for adsorption sites. As humic content fell below a critical threshold of 40%, free Cr(VI) abruptly increased by 52.7%. Moreover, As(III) promoted PE fragmentation, thereby stimulating microbial activity and increasing the proportion of mobile DOM components. Once the biological index (BIX) exceeded 1.8, As(III) mobility increased, thereby reducing its residual level by 29.7%. PVC gradually reduced Cr(VI) to Cr(III) and stabilized DOM, thereby mitigating further pollution. This study introduces two early-warning indicators (humus content below 40% and BIX above 1.8) for predicting abrupt HMs changes. These findings provide both theoretical and practical strategies for managing co-contaminated soils. • MPs-DOM competitive adsorption triggers abrupt heavy metal dynamics. • PS degradation depletes humus (1.8), enhancing As(III) mobility. • Identified thresholds enable proactive risk management for co-contaminated soils.