Spatial decoupling and stoichiometric design of zinc speciation during field-scale calcium polysulfide remediation in a contaminated aquifer.

While in situ chemical stabilization using calcium polysulfide (CPS) is increasingly applied for heavy metal immobilization, its field-scale mechanisms remain poorly understood. This study evaluated the spatiotemporal geochemical mechanisms of Zn speciation during a field-scale CPS application in a highly contaminated aquifer. Although groundwater monitoring demonstrated effective attenuation of dissolved Zn, solid-phase analyses revealed a deviation from anticipated behavior: the immobilized Zn predominantly formed as Zn-hydroxides (44-75% of total solid-phase Zn) rather than thermodynamically stable Zn-sulfides. Integrated hydrogeochemical modeling and independent batch experiments demonstrated that this phase shift is governed by the local stoichiometric Zntot/Stot ratio. Simulations showed that under S-excess conditions (Zntot/Stot<1), Zn predominantly precipitated as sulfides, whereas under Zn-excess conditions (Zntot/Stot≥1), Zn-hydroxides emerged up to 55%. Batch experiments corroborated this trend, showing that as the Zntot/Stot ratio increased from 0.20 to 2.04, the Zn-hydroxide fraction increased from 25% to 71%. To explain these observations, this study proposes a conceptual model based on the spatial decoupling of reactive transport. In dynamic aquifers, reactive sulfide is rapidly attenuated by hydrodynamic dispersion and competitive sinks (e.g., native Fe/Mn oxides), whereas the alkaline buffering capacity of the CPS plume persists further along the flow path. This spatial decoupling creates a CPS-limited zone (CLZ) where Zn-hydroxide precipitation is thermodynamically favored, thereby localizing the sulfide-dominant near-injection zone (NIZ). These findings emphasize that successful field-scale CPS applications require both stoichiometric optimization and spatial injection network design.