Degradation of the commercial Paraquat herbicide by UVA-LED Photo-Electrooxidation utilizing a BDD-Fe system: Multiple response optimization

Abstract

The UVA-LED photo-electrooxidation process was studied using Fe and boron-doped diamond electrodes and a UVA LED lamp for the degradation of gramoxone (paraquat, PQ), a highly toxic herbicide widely used in agriculture. The factors of hydraulic retention time (HRT), electric current (j) and the concentration of MgSO₄ as a supporting electrolyte were evaluated regarding the response variables percentage removal of chemical oxygen demand (COD) and PQ concentration using the desirability function through a Box‒Behnken experimental design for maximum response of both variables. A degradation efficiency of 47.8% of PQ was achieved, as well as a removal of 78.1% of COD and 71.6% of total organic carbon (TOC) for a commercial herbicide concentration of 60.4 mg/L PQ, electric current of 0.84 A, and electrolytic support of 6.3 g/L. After 172.3 min of hydraulic retention time, the electrical consumption was 42.5 kW/m³. To calculate the kinetic constants, the first order, second order and Behnajady-Modirshahla-Ghanbery (BMG) kinetic models were evaluated. In the two response variables (PQ and COD), the model that showed the best fit was the BMG. The HRT/electrical current interaction had a significant effect on PQ removal, and this interaction improved the herbicide removal to 77% after 300 min of HRT, and 0.9 A and 6.3 g/L MgSO₄ generated a consumption of 114.7 kW/m³. Excess electrical consumption is related to the complexity of the water, and the UVA-LED photoelectrooxidation reactor (LPEr) process (BDD-Fe/UVA-LED) may be suitable for the removal of contaminants in waters with low complexity or as a polishing stage in wastewater treatment.