Effectiveness of UV-based advanced oxidation processes for the remediation of hydrocarbon pollution in the groundwater: a laboratory investigation.

The effectiveness of advanced oxidation processes in a batch and a flow reactor was investigated for the remediation of hydrocarbon pollution in the groundwater underlying a petrochemical industrial site. The main organic contaminants present in the groundwater were MTBE, benzene, alkyl-benzenes and alkyl-naphthalenes. Experimental results with a batch reactor showed that for all the organic contaminants the removal efficiency order is UV/TiO2 approximately UV/H2O2>UV (medium-pressure) in a synthetic aqueous solution, compared to UV/H2O2>UV (medium-pressure)>UV/TiO2 for the real polluted groundwater. The much lower performance of UV/TiO2 with respect to UV/H2O2 was inferred to the matrix of the groundwater, i.e. the salt content, as well as the organic and particulate matter. In fact, it is likely that the salts and dissolved organic matter quench the superoxide anion O2(-) and hydroxyl radicals just formed at the surface of the TiO2 catalyst. MTBE was the hardest compound to remove with each of the investigated treatments. UV and UV/TiO2 treatments were not able to reach a residual concentration of 10 microg/L (set by Italian legislation) even after 180 min. As for the UV/H2O2 process, only the MTBE degradation rate resulted affected by the initial H2O2 concentration, while for other compounds a complete removal was obtained within 20 min even with the lowest H2O2 concentration used (0.13 g/L). Only after 120 min of treatment, with an initial H2O2 concentration of 0.13 g/L, did the residual MTBE concentration fall below the above reported maximum admissible concentration. Instead, by using an initial concentration of 2g/L a residual concentration lower than 5 microg/L was obtained after just 30 min of reaction. The UV/H2O2 process was also investigated with a flow reactor. Results showed that it was more efficient than the batch reactor for removing MTBE, in terms of reaction time and initial H2O2 concentration required. This is consistent with the higher power of the UV lamp and with the different geometry of the flow reactor, which has a much shorter optical path than the batch reactor. By-product characterisation was also performed showing that t-butyl-formate and low molecular weight organic acids are formed as intermediate and final by-products, respectively. Finally, a preliminary evaluation of the operational cost of the UV/H2O2 process showed a value of 1.7 euro/m3 under the optimised condition.