Remediation of phenanthrene & cadmium co-contaminated soil by using a combined process including soil washing and electrocoagulation

In the present work, the application of a combined process including soil washing by using the non-ionic surfactant Tween 80 and EDTA and electrocoagulation in remediation of phenanthrene (PHE) and cadmium (Cd) contaminated soils was investigated. In order to examine the effect of operational parameters on the efficacy of the process, Response Surface Methodology under Box-Behnken design was applied in both stages. Tween 80 solution and EDTA with concentrations of 1000-3000 and 1000-2000 mg L-1 respectively, at liquid/soil (L/S) ratio of 10, 20, and 30 v/w in a time interval of 2-24 h, were applied to remove PHE and Cd simultaneously from the co-contaminated soil. PHE and Cd extraction efficiency were mostly influenced by Tween 80 and EDTA concentration, respectively (P-value< 0.0001). L/S ratio also improved the removal efficiency of the contaminants significantly (P-value< 0.0001). Soil washing process under the optimal operational conditions of the surfactant concentration $$ \cong $$ approximately equal to 3000 mg L-1, EDTA concentration $$ \cong $$ approximately equal to 2000 mg L-1, L/S ratio $$ \cong $$ approximately equal to 30 v/w and washing time = 2 h resulted in the removal efficiency of 59.284 +/- 4.347% and 74.35 +/- 3.632% for PHE and Cd, respectively. Electrocoagulation of the adjusted synthetic effluent based on the optimal operational conditions of soil washing phase was carried out at pH values of 3-11, with a voltage of 10-30 v, and in reaction time of 45-90 min. The results demonstrated that the removal efficiency of both contaminants was mostly enhanced by increasing pH (P-value< 0.0001). The factor of voltage had also a positive significant effect on the responses (P-value< 0.0001). The presented optimum conditions for electrocoagulation including pH of 11, a voltage of 30 v, and reaction time of 45 min provided a removal efficiency of 94.85 +/- 1.715 and 100% for PHE and Cd respectively.