Chemical speciation and transformation of heavy metals in soil and groundwater: Implications for ecotoxicology and remediation

Abstract

Soil and groundwater heavy metal pollution has become an urgent environmental problem because heavy metals such as arsenic, cadmium, lead, and mercury are recalcitrant, bio-hazardous substances that accumulate in the ecosystem, leading to risks to human health. This review focuses on the chemical characteristics, speciation forms, and transformations of heavy metals in soil and aqueous systems, with special concern for the migration, mobilization, and toxic effects of these contaminants. It focuses on new progress in the analytical methodology for molecular dynamics simulations, spectroscopic experiments, and machine learning applications, which have deepened our understanding of metal action at the molecular level. New removal technologies, such as bioremediation, phytoremediation, and the use of nanomaterials, are also evaluated in terms of their environmental exposure and remedial effects, as well as their associated costs. Additionally, an overview is provided of the complex interplay between heavy metals and environmental parameters, such as pH and redox states, as well as organic matter, that affect their solubility and toxicity. The paper concludes by highlighting research gaps, which underscore the need for further studies on the long-term impacts of heavy metal pollution and the development of integrated management options to protect the integrity of ecosystems and human well-being.