Persistence of persulfate in karst groundwater and its influencing factors

Persulfate (S ₂ O ₈ ^2- , PS) is a new type of oxidant used for in situ chemical oxidation (ISCO) during the remediation of contaminated groundwater. 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 (1) In static water, the half-lives of 1, 8, and 20 g/L of PS in limestone media were 102, 185, and 202 d, respectively, and 19, 34, and 51 d in lime soil media, respectively. As the injection concentration increased, the persistence of PS also increased. The half-life range of PS in limestone column and conduit was 0.05 ~ 0.13 d and 0.36 ~ 1.70 d, respectively, indicating that PS exhibited poor persistence under flowing karst water conditions. (2) The pH remained at neutral to slightly alkaline levels in limestone media, which buffered the acidizing effect of high PS concentration. When PS concentrations were 8 and 20 g/L, the organic matter content in lime soils decreased from an initial 45.57 g/Kg to 35.07 and 24.63 g/Kg, respectively. The rich organic matters in lime soils greatly consumed PS with a rapid degradation rate constants of 20.31 and 13.47d ^-1 , respectively. (3) The decomposition of PS led to obvious hydro-chemical responses under static groundwater conditions. The pH values dropped 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 media. When the limestone media were present, PS injection at the concentration of 20 g/L stimulated the dissolution of carbonate, producing Ca ²⁺ concentration 8–10 times higher than the background value. However, the hydro-chemical changes remained relatively stable under flowing karst water conditions.