On the controversial effect of sodium sulphate as supporting electrolyte on electrocoagulation process: A review

Abstract

One of the important factors for electrocoagulation (EC) process is the conductivity of the solution to be treated. Essential mechanisms in EC are charge neutralisation by metal (Fe/Al) hydroxocations and aggregation by van der Waals forces since fl occulation of pollutants with metal is caused by their adsorption on metal hydroxide aggregates. In order to enhance the EC process efficiency in organic wastewater effluents, sodium sulphate (Na₂ SO₄) has been used as a supporting electrolyte (SE) to increase the electric current diffusion in the EC cell. However, literature has reported controversial effect of sodium sulphate on EC process. This review tries to understand the sulphate effect on the EC reactions. Na₂SO₄ has been found less efficient than NaCl as SE in EC process for the removal of humic substances, oil-in-water emulsions, and fluoride. However, for unskimmed milk sample and cutting oil emulsion sulphate anions were found to be quite harmful both for electrical consumption and EC efficiency. These results may be related to the facts that in the sulphate media the 70% of the aluminium is in the form of Al(OH)_3(am) and in the chloride media this percentage is around 40% and the adsorption of chloride or sulphate ions onto the surface of the Al(OH)_3(am) can reduce the adsorption efficiency. Consequently, metal cations must be well distributed in the wastewater before the metal hydroxide formation to decrease the negative effect of SE in EC process.

Keywords:

• Electrocoagulation (EC)
• Supporting electrolyte (SE)
• Sodium sulphate
• Coexisting anions
• Chemical coagulation (CC)