Photolytic degradation of acephate, glyphosate and malathion

Abstract

A photolytic cell system suitable for the treatment of wastewater streams containing three pesticides, i) acephate, ii) glyphosate and iii) malathion is reported. The system is capable of destroying these three organic compounds, commonly present in wastewater streams originating from agrochemical industries in Malaysia, and can lead to complete mineralisation under the optimum conditions. The system is based on an advanced oxidation process and involves the production of hydroxyl free radicals in the presence of a UV source. The performance of the system, for the three pesticides, was optimised by investigating the effects of i) UV source, ii) pH of the solution, iii) initial concentration of the substrate, iv) addition of oxidants, v) hydrogen peroxide (H2O2) in the presence and absence of single and mixed metal ions. To monitor the degradation efficiency of the system, the residual concentrations of these organic compounds and metal ions were analysed using five analytical techniques i) total organic carbon (TOC), ii) high performance liquid chromatography (HPLC), iii) ion chromatography (IC), iv) UV/Visible spectroscopy (UV/Vis), and v) atomic absorption spectroscopy (AAS). The data show that the developed photolytic cell system is capable of achieving complete mineralisation of the three pesticides with the use of both 400 W and 600 W UV lamps. However, the 400W UV lamp was used, for economic reasons, to optimise the system for the remaining factors. Changes in the pH of aqueous solutions influenced the degradation efficiency and a complete degradation of the three pesticides was achieved at their self-pH values ranging from 5.0-5.5. The degradation of acephate increased and malathion decreased at their higher initial concentrations whereas no significant effect related to concentration was observed for glyphosate. Results show that the degradation followed a first order kinetics and the degradation rates were: malathion > acephate > glyphosate. The addition of 30 mg/L of H2O2 enhanced the degradation of the pesticides and after 5 hours irradiation these were 95.7%, 91.5% and 81.3% for malathion, acephate and glyphosate respectively. The presence of metal ions was observed to affect degradation (Table 1). With 5.0 mg/L of Fe(II) the degradation of all three pesticides increased, and in all cases acephate removal was improved. Removal of both malathion and glyphosate was negatively affected by copper, an effect that work with mixtures indicated was stronger than the positive effect of iron. The addition of H2O2, in the presence of single metal ions, increases the degradation. However, the addition of H2O2, in the presence of mixed metal ions, has no significant effect on the degradation of glyphosate and malathion. The effect of mixed metal ions on the three pesticides and the effect of Zn(II) ions on acephate and malathion are reported for the first time in this thesis. The developed photolytic cell system can be used for the treatment of wastewater streams originating from point sources, for example, agrochemical industries, under the optimum conditions. The synergistic combination of the developed system with the existing standard technologies is also proposed for the treatment of surface water at water treatment facilities in Malaysia. The application of the developed system can also be extended, with minimum modifications, for the treatment of wastewater streams originating from different manufacturing industries in Malaysia, for example, textile, paper/pulp, printing, coke, petroleum, paint, solvent, pharmaceuticals and wood-preserving chemicals. All these industries produce wastewater streams containing low concentrations of organic pollutants and heavy metal ions. Table 1. Effect of metals on removal Fe Cu Zn mix Malathion + - o - Acephate + + + + Glyphosate + - o