Degradation of Glyphosate in Agriculture Wastewaters Through Photocatalysis Combined with Microalgae Treatment

Abstract

The continuous increase in food demand is leading to disproportionate and indiscriminate use of herbicides in agriculture. Glyphosate (N-(phosphonomethyl) glycine) is one of the most widely used active substances in these products, with the capacity to prevent the growth of weeds and to kill unwanted plants nonselectively in farmlands. However, the potential effects of glyphosatebased herbicides on non-target organisms are mostly unknown. Still, in 2015 the International Agency for Research on Cancer (IARC) classified this compound as probably carcinogenic to humans (including it in group 2A of the list of cancer agents). Nowadays, it is not uncommon to find glyphosate traces in groundwaters and, sometimes, even in drinking water and food products. Photocatalysis is one of the promising technologies for novel applications in wastewater treatments, showing good results in terms of oxidation of pollutants; nevertheless, in some cases, intermediate compounds could be released in groundwaters. In this study, photocatalysis and microalgae treatments were combined in photobioreactors to study glyphosate degradation. An intensive experimental campaign was carried out in different conditions of pollutant concentration in the presence of heterogeneous photocatalysts, such as TiO2, and using the Chlorella microalgal specie. The biological treatment enhances the previous partial oxidation achieved in the photoreactor, degrading the intermediates and optimizing herbicide mineralization. A robust and quick analysis method by liquid chromatography was implemented to measure the glyphosate and its degradation intermediates in the synthetic wastewater before and after the combined treatment. In addition, final degradation products, such as phosphate, were also analyzed. After a 3-hour photocatalytic treatment, a glyphosate abatement of about 67% was achieved, accompanied by the concomitant production of phosphate, nitrate and ammonium that are effectively consumed by microalgae. Results show that the combined treatment has a synergetic effect on glyphosate degradation and increases the mineralization efficiency, achieving global removal efficiencies higher than single-separated processes. This study, hence, demonstrates that photocatalysis and microalgae combined treatment is an eco-friendly method for the treatment of glyphosate-polluted wastes.