COMPOUND SPECIFIC ISOTOPE ANALYSIS TO INVESTIGATE SOURCES AND DEGRADATION OF GLYPHOSATE

Abstract

Compound specific isotope analysis (CSIA) is now a well-established routine approach to monitor the natural attenuation of traditional groundwater organic contaminants. However, assessment of micro-pollutant degradation in natural systems still remains a challenge. To advance the application of CSIA for pesticide degradation, my thesis focused on glyphosate (Roundup), a widely used post-emerging, non-selective herbicide that is effective against weeds. Glyphosate and its main degradation product AMPA (aminomethyl phosphonic acid) are frequently detected in surface and groundwater. First, a Compound-specific 15N/14N analysis of glyphosate and AMPA by a two-step derivatization in combination with GC/IRMS was developed. The dual element plot (δ13 C verses δ15 N isotope) was capable of distinguishing the different commercial glyphosate products and abiotic degradation of glyphosate. Thereafter, a bacterial strain was isolated from a vineyard field site that showed the ability to utilize glyphosate as phosphorus source and was identified as Ochrobactrum sp. FrEM. The evidence of sarcosine confirmed an alternative degradation route - (C-P cleavage) sarcosine pathway. However, only a small carbon isotope fractionation was observed ƐC = -4‰ ± 0.5‰ which suggests that the intrinsic isotope fractionation may have been masked. The dual isotopic analysis for ∆ δ13C and δ15N slope (4.6 ± 0.03, 0.4 ± 0.03) was able to distinguish abiotic and biotic degradation pathways. Finally, to apply our approach in the field, a method to enrich glyphosate and AMPA from water with activated alumina was developed. The activated alumina that was used had a high adsorption capacity and surface density of 85 mg/g and 2 µmol/m2 . Glyphosate was better adsorbed at pH value that was lower than the PZC (point of zero charge) of alumina surface. Consequently, desorption of glyphosate was favored when the alumina surface charge was negatively charged. There is a promising potential to use activated alumina for the enrichment of glyphosate and AMPA from water. This opens new prospects to identify the source of AMPA and investigate its fate in groundwater.