The role of particle size on the conductive diamond electrochemical oxidation of soil-washing effluent polluted with atrazine

• Surfactant soil washing of synthetic soils polluted with atrazine generated a complex waste.
• CDEO favors complete waste mineralization produced from soil-washing with surfactants.
• Treatment by CDEO of an emulsion combines micro-drop size reduction and mineralization.
• SO42 − is released from SDS and transformed into S2O82 − improving oxidation rate.
• Oxidations are occurring on the micelles, then surfactant decays faster than pesticide.

Soil washing using surfactant solutions is a very effective and widely used technique for the remediation of polluted soil. However, it is not a finalistic technology and once the pollutant is transferred to the washing-solution, the resulting aqueous waste has to be treated. In this work, the feasibility of Conductive-Diamond Electrochemical-Oxidation (CDEO) to treat the effluent of a surfactant-aided soil washing process was studied and some important mechanistic aspects were pointed out. Atrazine was selected as the model pesticide and sodium dodecyl sulfate (anionic surfactant) was used as washing agent. Besides COD, TOC, and pollutants concentration, the changes of z-potential and particle size were monitored during electrolysis in order to clarify the degradation mechanisms involved in the electrochemical treatment of emulsions. The results show that CDEO enables a complete reduction in the organic load of the waste. However, the process efficiency seems to be influenced by the size of particles present in the reaction media, which decreases continuously during the treatment. Steric hindrance of the large micelles (mean size 1100 um) seems to prevent direct oxidation of micelles on the anodic surface and hence only mediated processes can explain the results obtained. In this case, it may be assumed that peroxocarbonate (from carbonate salts added as supporting electrolyte) and peroxosulfate ions (formed from the oxidation of sulfate ions released from SDS molecule) are the main species expected according to the washing fluid composition.