UV/H2O2 oxidation: kinetics of glyphosate in water. Application to commercial formulations

S. NEDER; A. NEGRO; A. CASSANO; C.S. ZALAZAR

San Diego

Conferencia; The17th International Conference on Advanced Oxidation Technologies for Treatment of Water, Air and Soil (AOTs-17); 2011

Al-Ekabi

Glyphosate is the active ingredient in herbicides and it is the most widely used in the world. The extensive increase in soy production, in many cases employing glyphosate-resistant soy, has been accompanied by an increase in the use of this weed suppressor. In Argentina, for example, glyphosate consumption increased from 1 million liters in 1991 to 180 million in 2007. The disposal of pesticide wastewaters including equipment rinsates as well as products derived from fumigations carried out during the harvest, such as used empty plastic weed killers containers, including their washing waters, constitute an unsolved problem in many countries. In some cases, disposal legislation is not accomplished with the pretext that there is a lack of availability of on-site, small scale, simple remediation technologies. Starting from a simplified sequence representing the glyphosate oxidative degradation employing hydrogen peroxide and UVC radiation, a mathematical model to interpret its reaction kinetics was derived. This representation must include all the required significant variables for an ulterior reactor design and scale-up. The obtained description was validated with a complete set of experiments. The photodegradation of glyphosate was carried out in a cylindrical reactor with two parallel, flat windows made of quartz. Each window was irradiated with a tubular, germicidal lamp (253.7 nm) placed at the focal axis of a parabolic reflector. 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. Hydrogen peroxide was analyzed with spectrophotometric methods at 350 nm. Total organic carbon (TOC) was also analyzed. Experiments were carried out changing the following variables: (i) initial glyphosate concentrations, (ii) initial hydrogen peroxide concentrations, and (iii) incident radiation on the windows of radiation entrance (or, according to IUPAC, the photon fluence rate, Ep,0) measured with potassium ferrioxalate actinometry. In order to be used with the stated purpose, the work was aimed at obtaining a result that must be independent of the shape, size and configuration of the laboratory reactor. Consequently, not only concentration dependences were examined but also a complete radiation model was included as part of the modeling. The kinetics parameters were obtained by comparing simulation concentrations obtained with the model with the experimental values gathered in the laboratory reactor, employing a multiparameter non-linear regression analysis. In addition, the potential of the H2O2/UV process for treating the wastewater coming from the more complex commercial formulations of glyphosate was studied. These second set of data were compared with the experiments made using glyphosate alone and it was found that the most significant kinetic constants were about 50% lower.