Persistence of persulfate in karst groundwater and its influencing factors

Abstract

Persulfate (S₂O₈²⁻, PS) is a new type of oxidant used for in situ chemical oxidation (ISCO) during the remediation of groundwater polluted by petroleum hydrocarbons. However, PS may be consumed by nontarget matters in aquifers, decreasing its persistence and remediation effect. To better understand the persistence of PS in widely distributed karst aquifers, microcosm, column, and conduit experiments were carried out in this study to simulate karst caves, fracture zones, and conduit environments under static or flow water conditions. Karst aquifer matters, including limestone and lime soil, and a novel carbonate rock conduit model were employed. PS decomposition at different concentrations, influencing factors, and hydro-chemical responses were discussed. The results of the study indicate that the half-lives of 1, 8, and 20 g/L of PS in limestone media were 102, 185, and 202 d, respectively, and correspondingly 19, 34, and 51 d in lime soil media under static water conditions. As the injection concentration increased, the persistence of PS also increased. The half-life ranges of PS in limestone column and conduit were 0.05–0.13 d and 0.36–1.70 d, respectively, indicating that PS exhibited poor persistence under flowing karst water conditions. The pH remained at neutral to slightly alkaline levels in limestone media, which buffered the acidizing effect of high PS concentrations. At PS concentrations of 8 and 20 g/L, the organic matter content in lime soils decreased from 45.57 g/Kg to 35.07 and 24.63 g/Kg, respectively. The rich organic matters in lime soils greatly consumed PS with decay rate constants of 20.31 and 13.47 d⁻¹, respectively. The decomposition of PS under static groundwater conditions led to obvious hydro-chemical responses. The pH decreased to a minimum of 1.4 and the dissolved oxygen concentration increased from 8.5 to 17.3 mg/L in the absence of solid particles. In the presence of limestone media, PS injection at a concentration of 20 g/L stimulated carbonate dissolution, resulting in a Ca²⁺ concentration 8–10 times higher than the background value. However, the hydro-chemical changes remained relatively stable under flowing karst water conditions.