Degradation of glyphosate in water by the UV/H2O2 process

C.S. ZALAZAR; A. MANASSERO; M.L. MARIANI; A.C. NEGRO; R.J. BRANDI; A.E. CASSANO

Santa Fe, Argentina

Workshop; 2º Workshop Latinoamericano sobre Residuos de Plaguicidas, Alimentos y Medio ambiente.; 2009

Facultad de Ingeniería Química, UNL.

Glyphosate (N-phosphonomethyl glycine) is a non-selective, post-emergence, broad-spectrum organophosphate herbicide, which is the one most used in the world. Argentina is the second largest world producer of glyphosate-resistant (GR) soy. This production has been accompanied by an increase in the use of the herbicide (over 160 million L between 2004 and 2005). The United Nations estimates that of all pesticides used in agriculture, less than 1 % actually reaches the crops. The other part ends up contaminating the soil, the air and, mainly, the water. Advanced Oxidation Processes (AOPs) seem to be a suitable option to decrease glyphosate concentration in water within acceptable limits. AOPs are based on the generation of very reactive non- selective oxidizing species, particularly the hydroxyl radical (.OH). The combination of H2O2 and UV radiation could be a convenient process for the treatment of this type of pollutants. In this work the effects of H2O2 initial concentration, initial pH and irradiation rates in the glyphosate degradation were studied. In addition to this the main reaction intermediates were identified with the intention of proposing a plausible reaction pathway. The photodegradation of glyphosate was carried out in a cylindrical reactor irradiated with a tubular, germicidal lamp (l = 253.7 nm). The small reactor operates inside the loop of a bath recycling system. Glyphosate was analyzed by ion chromatography with a suppressed conductivity detector employing an Ion Pac AS4A-SC analytical column and a solution of Na2CO3 (9 mM) and NaOH (4 mM) as eluent [1]. Hydrogen peroxide was analyzed with spectrophotometric methods at 350 nm. Total organic carbon (TOC) was also analyzed in order to compare the degradation rate of glyphosate with the total mineralization rate to have better information concerning possible reaction intermediates. The best conditions of Glyphosate degradation for an initial concentration of 50 mg/L are pH 7 and hydrogen peroxide concentrations from 75 to 200 mg/L. A possible mechanism of the glyphosate decomposition reaction was proposed taking into account the bibliographic information and the obtained experimental results [2]. This is the first step to work in the derivation of a complete, validated, reliable reaction kinetics model.