Single versus multi-metal sulfide systems: The role of cysteine and complex environmental conditions

The presence of dissolved organic matter (DOM) can impact metal sulfide (MeS) precipitation and mobility. Thiol containing ligands such as cysteine have been shown to be effective capping agents in single metal MeS studies, allowing NPs to persist in oxic environments. In this study, both single (Cd or Zn) and multi-MeS (Cu, Pb, Cd, Zn, and As) nanoparticle (NP) formation was characterized to understand the impact of the thiol cysteine (CYS) on early stage (3 h) MeS NP behavior. Short duration single metal batch experiments, in the absence and presence of CYS, confirmed that MeS species readily formed solids with limited dissolved fraction; however, multi-metal systems exhibited divergent behavior reflecting a wider range of NP sizes and an increased dissolved concentration. Multi-metal batch experiments revealed that metals were generally sequestered into MeS solids in accordance with MeS solubility products (i.e., from least to most soluble: Cu > Pb ~ Cd > Zn). CYS concentrations in excess of sulfide (10:1 CYS:S ratio) stabilized MeS within the Small NP size fraction (3.2 nm < d < 43 nm) and limited Pb, Cd, and Zn dissolution compared to molar ratios of 1:1. In the combined presence of CYS and Ca²⁺, multi-MeS particle aggregation increased substantially compared to monovalent systems. Dissolution increased for Pb and Zn as a function of matrix ionic strength whereas dissolved Cu trends changed as a function of cation valence state (e.g., Na⁺ vs. Ca²⁺). Most noteworthy, single-metal Zn and Cd batch experiments demonstrated that single-metal studies can overestimate MeS NP resistance to oxidative dissolution compared to multi-metal counterparts. Thus, caution should be taken when broadly applying mechanisms and rates elucidated from single-metal systems.