Enhanced removal of glyphosate from aqueous solution by nano-CaO2/AS composite: Oxidation and precipitation

Abstract

The massive use and accumulation of glyphosate will cause a series of ecological and environmental problems, so there is an urgent need for effective treatment and recovery methods of glyphosate. In this research, based on the properties of CaO2 that can provide both reactive oxygen and Ca2+, nano-CaO2 was selected as the oxidant and precipitant. Meanwhile, an iron-containing alkaline soil (AS) was used as a dispersant for nano-CaO2 as well as a catalyst to reinforce the advanced oxidation. The nano-CaO2/AS composite with high reactivity was successfully prepared by a simple precipitation method. Due to the activation of OH• by the iron-containing substances, the addition of AS greatly enhanced the removal efficiency of glyphosate in the advanced oxidation system. The influences of composite dosage, initial pH value, coexisting substances, and initial glyphosate concentration on the degradation of glyphosate were investigated. The results indicated that acidic conditions were favorable for the advanced reaction process, and common coexisting substances had rarely adverse impacts on the removal of glyphosate. In addition, possible degradation mechanism of glyphosate was proposed based on the free radical quenching experiments and characterization analyses. The removal of glyphosate by nano-CaO2/AS composite was a synergistic action of advanced oxidation and precipitation, and OH• and O- 2• were the main active radicals involved in the reaction system. Most of glyphosate can be oxidized to inorganic anions. The HPO2- 4decomposed by glyphosate combined with Ca2+ dissolved from nano-CaO2 in solution to produce the precipitation of CaHPO4·2H2O. The solid precipitation was rich in available phosphorus and can be recycled as a phosphorus fertilizer, which could promote the plant growth. Therefore, the nano-CaO2/AS composite is a promising material for glyphosate wastewater treatment and phosphorus recovery.