Comparative Analysis of Photocatalytic and Electrochemical Degradation of 4-Ethylphenol in Saline Conditions

We evaluated electrochemical degradation (ECD) and photocatalytic degradation (PCD) technologies for saline water purification, with a focus on rate comparison and formation and degradation of chlorinated aromatic intermediates using the same non-chlorinated parent compound, 4-ethylphenol (4EP). At 15 mA.cm(-2), and in the absence of chloride (0.6 mol.L-1 NaNO3 was used as supporting electrolyte), ECD resulted in an apparent zero-order rate of 30 mu mol L-1.h(-1), whereas rates of similar to 300 mu mol L-1.h(-1) and similar to 3750 mu mol L-1.h(-1) were computed for low (0.03 mol.L-1) and high (0.6 mol.L-1) NaCl concentration, respectively. For PCD, initial rates of similar to 330 mu mol L-1.h(-1) and 205 mu mol L-1.h(-1) were found for low and high NaCl concentrations, at a photocatalyst (TiO2) concentration of 0.5 g.L-1, and illumination at lambda(max) approximate to 375 nm, with an intensity similar to 0.32 mW.cm(-2). In the chlorine mediated ECD approach, significant quantities of free chlorine (hypochlorite, Cl-2) and chlorinated hydrocarbons were formed in solution, while photocatalytic degradation did not show the formation of free chlorine, nor chlorine-containing intermediates, and resulted in better removal of non-purgeable hydrocarbons than ECD. The origin of the minimal formation of free chlorine and chlorinated compounds in photocatalytic degradation is discussed based on photoelectrochemical results and existing literature, and explained by a chloride-mediated surface-charge recombination mechanism.