From redox chemistry to biological uptake: Mechanistic controls on the environmental behavior of essential and toxic trace elements

Background Redox-sensitive trace elements such as iron (Fe), manganese (Mn), selenium (Se), and chromium (Cr) exhibit multiple oxidation states that strongly control their environmental mobility, bioavailability, and toxicity. Variations in redox conditions across natural and engineered systems drive dynamic transformations that regulate element cycling and exposure pathways. Objectives This review aims to provide a mechanistic synthesis of how redox transformations govern speciation, transport, and biological uptake of Fe, Mn, Se, and Cr across environmental compartments, while linking analytical constraints with environmental risk assessment. Key findings Under reducing conditions, dissolved Fe(II) and Mn(II) can reach 0.1–100 mg L⁻¹, whereas oxidized Fe(III) and Mn(IV) form low-solubility oxides with high sorption capacity. Selenium oxyanions typically occur at