Activation of Persulfate by temperature and iron filings for the treatment of synthetic olive mill wastewater: Optimization of conditions with a box–Behnken design

• Synthetic mixtures allow for a better understanding of variable effects in study. • Persulfate activation by temperature was studied using the Box-Behnken design. • Persulfate activation almost completely degraded the 3,4- Dihydroxybenzoic acid. • Ecotoxicological tests demonstrated that iron is beneficial. The Persulfate (PS, S 2 O 8 2- ) activation by temperature and iron filings (IF) was analysed following the degradation of a five phenolic acids mixture: 3,4- Dihydroxybenzoic (3,4-DHB), 4-Hydroxybenzoic (4-HB), 3,4-Dimethoxybenzoic (3,4-DMB), 3,4,5 Trimethoxybenzoic (3,4,5-TMB) and Trans-cinnamic acid (TCA). Due to the high number of variables ruling this process, the Box-Behnken design (BBD) was used to examine the effect of each experimental variable (initial pH, dosage of PS, and operating temperature) keeping the IF load constant (1 g/L). The model obtained predicted as optimal conditions an initial pH 3, a PS dosage of 1000 mg/L, and an operating temperature of 49 °C. The optimal conditions allowed the removal of about 98.87 %, 12.58 %, 13.95 %, 25.69 %, and 81.34 % for 3,4-DHB, 4-HB, 3,4-DMB, 3,4,5-TMB and TCA, respectively. Thus, this process proved to be efficient in the degradation of 3,4- DHB and less suitable for the degradation of 4-HB. A removal of 35.09 % was measured for chemical oxygen demand (COD) for the optimal conditions after 3 h.. IF reuse studies were also addressed and it was verified that this heterogeneous catalyst maintains its efficiency for at least for three reuse cycles.