Influence of solubilizing agents (cyclodextrin or surfactant) on phenanthrene degradation by electro-Fenton process--study of soil washing recycling possibilities and environmental impact.

One of the aims in soil washing treatment is to reuse the extracting agent and to remove the pollutant in the meantime. Thus, electro-Fenton (EF) degradation of synthetic soil washing solutions heavily loaded with phenanthrene was suggested for the first time. Two solubilising agents hydroxypropyl-beta-cyclodextrin (HPCD) and Tween 80(®) (TW 80) were chosen as cyclodextrin (CD) and surfactant representatives, respectively. In order to reuse HPCD and to degrade the pollutant simultaneously, the following optimal parameters were determined: [Fe(2+)] = 0.05 mM (catalyst), I = 2000 mA, and natural solution pH (around 6), without any adjustment. Only 50% of TW 80 (still higher than the critical micelle concentration (CMC)) can be reused against 90% in the case of HPCD while phenanthrene is completely degraded in the meantime, after only 180 min of treatment. This can be explained by the ternary complex formation (Fe(2+)-HPCD-organic pollutant) (equilibrium constant K = 56 mM(-1)) that allows OH to directly degrade the contaminant. This confirms that Fe(2+) plays an important role as a catalyst since it can promote formation of hydroxyl radicals near the pollutant and minimize HPCD degradation. After 2 h of treatment, HPCD/phenanthrene solution got better biodegradability (BOD5/COD = 0.1) and lower toxicity (80% inhibition of luminescence of Vibrio fischeri bacteria) than TW 80/phenanthrene (BOD5/COD = 0.08; 99% inhibition of V. fischeri bacteria). According to these data, HPCD employed in this integrated (soil washing + EF degradation) approach gave promising results in order to be reused whereas the pollutant is degraded in the meanwhile.