Perspectives on Research Challenges of Microplastic–Heavy Metal Complexes: Interfacial Mechanisms, Environmental Behaviors and Co-Toxicity Risks

Microplastic–heavy metal (MP-HM) complexes markedly modify HM migration, transformation and bioavailability in the environment, threatening ecosystem stability and public health. Many studies have examined the interfacial adsorption and toxicity of single-factor MP-HM contamination, yet existing reviews mostly rely on phenomenological descriptions, lacking systematic synthesis and critical discussion of core mechanisms, model constraints and field-scale evolutionary discrepancies. The long-term stability, interfacial reconstruction, and interconversion between free and MP-bound HMs under in situ dynamic conditions remain poorly understood. To fill this gap, this review clarifies divergent adsorption mechanisms governed by MP polymer properties and HM speciation, and identifies the fundamental limitations of conventional static models for dynamic environmental systems. We further analyze decoupling mechanisms underlying synergistic/antagonistic multi-factor interactions in complex matrices and explain inconsistencies across experimental observations. Accordingly, priority research directions are proposed, including in situ dynamic monitoring, coupled model refinement, and speciation-dependent risk assessment. This review provides new insights into the interfacial evolution of MP-HM co-contamination and supports targeted environmental risk mitigation strategies.