Persulfate Interaction with Tropical Soils

Persulfate use as a chemical oxidant for remediation of contaminated sites has increased due to its higher in situ persistence and reactivity with environmentally relevant contaminants. The interaction of persulfate with soil materials can significantly influence treatment efficiency by either promoting activation or by decreasing its persistence. In this investigation, the interaction between persulfate and three soils (two highly weathered and clayey Oxisols, and a sandy Psamment soil) found in tropical environments was evaluated. A series of batch tests were conducted using a high (14 g L-1) and low (1 g L-1) persulfate concentration. Additional persulfate mass was added after 120 days into low persulfate concentration systems. Persulfate degradation generally followed a first-order kinetic model; however, the reaction rate coefficients varied with initial concentration and with the addition of persulfate at 120 days. A kinetic model that accounts for the presence of a finite mass of oxidizable material was shown to capture all experimental conditions. In all systems, the pH decreased significantly, causing favorable conditions for the dissolution of clay minerals such as kaolinite, as supported by mineralogical analyses. This dissolution resulted in an increase in exchangeable aluminum and iron, as well as conversion of iron associated with amorphous oxides to crystalline oxides. The increased availability of these species resulted in accelerated persulfate degradation. Based on the data set assembled, a conceptual model was developed that represents the interaction between persulfate and soils that have clay minerals containing iron that are susceptible to dissolution.