Electro-Fenton process for treatment of hydroxybenzoic acids solutions using boron-doped diamond/carbon-felt cells: application to olive mill wastewater

The present study aims to remove hydroxybenzoic acids from olive mill wastewater by using the electro-Fenton process with boron-doped diamond/carbon-felt cells. Firstly, the oxidation behavior of a series of model hydroxybenzoic acids (salicylic, protocatechic, and gallic acids) was investigated by evaluating the effect of some operating parameters on the removal of the benzoic acid parent molecule. The degradation efficiency increased to 72.5% within 60 min, by increasing Fe2+ concentration (0.1–0.3 mM) and applied current (100–300 mA) and by decreasing benzoic acid concentration (0.2–1 mM). Secondly, the kinetics study of hydroxybenzoic acids oxidation at optimal conditions ([Fe2+] = 0.3 mM; I = 300 mA; C0 = 0.2 mM) showed an increase in oxidation rate with the increasing number of HO– substituents on the benzene ring: salicylic acid < protocatechic acid < gallic acid, and the degradation and mineralization efficiencies were equal or greater than 50% after only 30 min of electrolysis. Moreover, the formation of various colored Fe3+ complexes with studied hydroxybenzoic acids or their by-products was confirmed by using the spectrophotometric method, such as purple [Fe3+—salicylic acid], green–blue [Fe3+—protocatechic acid], and blue [Fe3+—gallic acid] complexes. Finally, the olive mill wastewater, biologically pretreated by a multi-soil-layering system, was electro-treated under previous optimum conditions. The mineralization efficiency, energy consumption, and electrical cost per m3 were 80%, 14.4 kWh/m3, and 15.4 MAD/m3, respectively, after 6 h of electrolysis.