Solubility product (K_sp) and pH jointly control heavy metal fraction transformation in sulfidized sediments

Sulfidation of sediments-a ubiquitous natural process-critically governs heavy metal (HM) fate in aquatic systems. This study demonstrates that oxidized sediments transition from pollution source to sink upon sulfidation, mediated by metal sulfide K_sp. Due to its higher K_sp than FeS, Mn^2 + remains mobile during sulfidation, whereas Zn²⁺ and Ni²⁺ (with lower K_sp) are re-immobilized as sulfides in later stages. Sequential extraction revealed pH-dependent speciation shifts: acidic/neutral conditions transformed reducible fractions into acid-soluble and oxidizable species, while high pH alkaline conditions (e.g., >8.2) favored conversion to acid-soluble fractions with suppressed oxidizable fraction formation. In terms of mechanism, pH controls metal sulfide precipitation-promoting it under acidic/neutral conditions but inhibiting it under alkaline conditions due to enhanced S(-II) oxidation to SO₄²⁻ under the thermodynamically favorable conditions at high pH. These findings identify alkaline sulfidation as an ecologically high-risk scenario where heavy metals (HMs) persist in mobile (acid-soluble) forms rather than stabilizing as sulfides. Given increasing anthropogenic SO₄²⁻ loading in surface waters, this risk may be mitigated by a two-stage strategy: initially sequestering released metals with reactive amendments (e.g., zeolite), followed by in situ sulfidation to stabilize them as sulfides. Proactive management of metal-contaminated alkaline sediments informed by these mechanisms is essential to mitigate toxicity.