Experimental, statistical and financial analysis of the treatment of organic contaminants in naphthenic spent caustic soda using electrocoagulation process modified by carbon nanotubes

This study aims for the treatment of organic contaminants in naphthenic spent caustic soda from petroleum refinery by electrocoagulation (EC) process, in which graphite cathodes are modified by carbon nanotubes (CNTs). Studied variables include electrode configuration, anode materials (iron/aluminum), perforated anode effect, pH, EC time, current density, type and volume of sludge, and electrode mass loss. Eventually, the total operating cost is estimated to assess the commercial practicability of the process. Cyclic voltammetry methodology (CVM) is used to investigate the effect of coated graphite electrodes with carbon nanotubes, using wet chemical coating with/without bonder, and CVD methods shows that the CVD method has the best performance. The central composite design (CCD) method and analysis of variance (ANOVA) are used to evaluate and optimize the variables and response, COD removal, and to determine significance and adequacy of mathematical models proposed by response surface methodology (RSM). The estimated optimal condition is 8, 120 min, and 18.750 mA cm−2 for pH, EC time, and current density respectively, in which the reactor successfully remove 76% of COD. The results show the quadratic model is adequately significant for the response and in coated samples not only hydrogen releasing happens sooner, but also by consuming lower energy, the higher current intensity will produce which is shown the importance of CNTs on the cathode electrode. Overall, financial and energy consumption analyses authenticated that this process is perfectly satisfactory as a commercial method to pretreat industrial wastewaters such as petroleum refinery plants in the future.