Glyphosate and AMPA removal from water by solar induced processes using low Fe(iii) or Fe(ii) concentrations

Abstract

A solar photo-Fenton-like (SPF-like) process is explored for the removal of 1 mg L⁻¹ glyphosate and its main degradation by-product, aminomethylphosphonic acid (AMPA), from Milli-Q water by means of low Fe(III) concentrations (0.6–2 mg L⁻¹) at pH = 2.8 and variable H₂O₂ concentrations at the laboratory scale. The research is focused on glyphosate and AMPA oxidation, which present similar toxicity patterns. A 1 mg L⁻¹ glyphosate solution requires 5–6 h of the SPF-like process to be degraded when a minute Fe(III) concentration (0.6 mg L⁻¹) is used in acidic water. Glyphosate abatement time is diminished to 2 h when the Fe(III) concentration is increased to 2 mg L⁻¹. At pH levels above 2.8, the herbicide is partially adsorbed onto the colloids of iron oxy(hydroxide) compounds. AMPA requires a higher oxidative power than glyphosate to be degraded, and more than 6 h of solar treatment are needed, using 10 mg L⁻¹ h⁻¹ H₂O₂ and 2 mg L⁻¹ Fe(III) at acidic pH. Solar photo-Fenton (SPF), using 4 mg L⁻¹ Fe(II) and 10 mg L⁻¹ h⁻¹ H₂O₂ at pH = 2.8, must be applied to achieve practically total AMPA removal in 6 h of irradiation. SPF-like and SPF treatments led to 70% and 80% mineralization, respectively, under the best operational conditions. This work demonstrates that SPF-like and SPF at low Fe(III) or Fe(II) concentrations are effective treatments for the removal of glyphosate from water at acidic pH. Continuous addition of H₂O₂ is required for AMPA abatement.