Evidence of precipitate formation and byproduct transfer to nonaqueous phase liquids as a result of persulfate exposure

Abstract

AbstractFor optimal treatment of a source zone using in situ chemical oxidation (ISCO), the oxidant needs to be delivered as close to the nonaqueous phase liquid (NAPL) as possible to enhance the rate of dissolution. However, the interaction between the oxidant and NAPL may result in physical and chemical changes that affect both treatment effectiveness and post‐treatment dissolution behavior from remnant NAPL. Due to a host of favorable attributes, persulfate is frequently selected as the preferred oxidant but little is known about changes that can occur when persulfate interacts with a NAPL. In this study, two series of static batch experiments were conducted. In the first series, NAPL collected from a former manufactured gas plant (FMGP) site was exposed to a persulfate solution for ~60 days, while in the second series, individual and a mixture of pure organic liquids were exposed to a persulfate solution for ~50 days. It was observed that dark‐brown precipitates were formed in almost all the experimental systems. Notably, a layer of precipitates accumulated along the NAPL/water interface in the FMGP NAPL system and inhibited dissolution. In some of the pure organic liquid systems, the aqueous phase appeared yellow and precipitates were formed. Precipitate formation was attributed to the polymerization of aromatic compounds initiated by sulfate and hydroxyl radicals. Although precipitate formation is detrimental to mass transfer, surfactant‐like byproducts which may also be produced could lead to an emulsified aqueous phase, thereby enhancing dissolution and altering NAPL/water surface tension. Several of the organic byproducts that were detected in the aqueous phase were also detected in the NAPL, suggesting that these byproducts which were generated in aqueous phase migrated into the NAPL. Depending on the concentration of these byproducts in the NAPL, they could impact important physical properties (e.g., viscosity and density) and affect NAPL composition, which can affect intra‐NAPL diffusion and post‐treatment dissolution behavior. Although additional investigation is required, the results from this study indicate that the impacts of byproducts generated when using persulfate in ISCO applications need to be considered.