Bioavailability of heavy metals in soil: a review of tools, models, and regulatory applications

Bioavailability plays a pivotal role in determining the environmental and health risks associated with heavy metals in soils. While total metal concentrations are often measured, they do not necessarily reflect the fraction that is accessible to biota or capable of inducing toxic effects. The bioavailable fraction is influenced by a multitude of soil characteristics, including pH, organic matter content, redox conditions, and microbial activity. Over the past two decades, considerable research efforts have been devoted to developing reliable, reproducible, and cost-effective methods for assessing the bioavailability of metals in various soil contexts. This review provides a comprehensive synthesis of the most widely used chemical extractants, including CaCl₂, diethylenetriaminepentaacetic acid (DTPA), and ethylenediaminetetraacetic acid (EDTA), as well as sequential extraction procedures such as the BCR method. Their strengths, limitations, and selection criteria are discussed in detail. Additionally, biological assays, including plant uptake studies, microbial bioassays, and enzymatic activity evaluations, are evaluated for their utility in capturing ecologically relevant endpoints. Recent advances in molecular microbiology, including high-throughput sequencing and metagenomic analyses, offer novel insights into microbial responses to metal stress and are increasingly integrated into bioavailability assessment frameworks. The review also explores integrative modeling approaches that combine empirical data with theoretical frameworks, such as biotic ligand models and geochemical speciation models. These models are instrumental in translating laboratory findings to field-scale predictions, and have been adapted for use in both aquatic and terrestrial environments. Case studies from Europe, North America, and China are presented to illustrate the diversity of approaches and highlight region-specific practices. These examples underscore the need for harmonization and standardization in bioavailability assessment protocols. The regulatory landscape is examined, revealing varying degrees of recognition and incorporation of bioavailability concepts across jurisdictions. The European Union, for example, increasingly mandates bioavailability-informed assessments under frameworks such as the REACH regulation, whereas other regions are at different stages of implementation. Ultimately, the review provides a decision-making framework for selecting suitable tools based on site-specific conditions, assessment objectives, and regulatory requirements. The proposed decision tree integrates chemical, biological, and modeling approaches to support holistic risk assessments. Future directions are discussed, emphasizing the importance of cross-disciplinary collaboration, field validation, and the development of bioavailability-based guidance values.