Efficient oxidation of high levels of soil-sorbed phenanthrene by microwave-activated persulfate: implication for in situ subsurface remediation engineering

In situ chemical oxidation using activated persulfate is increasingly applied for source destruction of hydrophobic organic contaminants (HOCs), e.g., PAHs, the target group contaminants of this study. However, the low aqueous solubility and strong sequestration of HOCs by soil aggregates are major hurdles for the oxidation of HOCs in the subsurface. This study aimed to investigate the potential of using microwave-activated persulfate to tackle the above issues and accelerate the remediation of HOC-contaminated sites. Soil sorbed with high levels of phenanthrene, a model compound for PAHs, was used in the study. The effects of microwave, reaction temperature, reaction time, oxidant dosage, and soil to water ratio on the destruction of phenanthrene, persulfate consumption, and slurry pH were studied. Furthermore, the mechanisms of the microwave-activated process were discussed by comparing the degradation results using microwave-activated persulfate with those using conventional heat-activated persulfate, and potential microwave-specific effects were explored. This study shows that microwave-activated persulfate is highly effective toward the destruction of high levels of soil-sorbed phenanthrene. The results indicate that higher slurry temperature is critical to initiate a positive feedback loop that induces rapid and efficient destruction of phenanthrene. Furthermore, potential mechanisms of the positive feedback loop were discussed. Though persulfate is mainly activated by the thermal effect of microwave, non-thermal microwave-specific effects were preliminary observed, which induce additional activation of persulfate and further enhance the destruction efficiency of phenanthrene. In addition, how to integrate an in situ microwave soil heating system with the traditional persulfate remediation process for subsurface engineering was discussed to provide guidance for potential applications in field scales. Overall, the feasibility of using microwave heating to accelerate persulfate remediation of subsurface organic contaminations was established.