Sequential use of Fenton and electro-Fenton process for the oxidation of an effluent-containing hypophosphite and phosphite

Abstract

The efficiency of both chemical and electrochemical Fenton processes to treat wastewater effluents from the manufacture of phosphorus-based flame retardants was investigated. Experimental tests were performed on solutions ranging from 600 to 2000 mg l⁻¹ in terms of reduced phosphorus. Different concentrations and ratios of hydrogen peroxide and ferrous ions were tested, and the best conditions were found for both the processes. For the electrochemical treatment, where a gas diffusion cathode was adopted, the effect of carbon coating and current density was investigated, identifying the hydrophilic carbon coating, named Vulcan, as the best cathode material and the value of 100 A m⁻² as the optimal current density. The results show that both the Fenton processes provide a powerful conversion to phosphate, with oxidation yields higher than 99%. However, a more cost-friendly sequential treatment, combining a preliminary chemical step to a final electro-oxidation, was proposed to reach a residual concentration of non-oxidized phosphorus less than 10 mg l⁻¹, in compliance with the Italian regulation limits for industrial wastewater disposal. A preliminary economic evaluation is provided.

Keywords:

• Fenton’s oxidation
• Electro-Fenton
• Flame retardant
• Real wastewater
• Hypophosphite
• Phosphite

Acknowledgments

This study was partially supported by the Italian Ministry of University and Research. The assistance of “Industrie De Nora S.p.A.” in providing the electrode materials is gratefully acknowledged.