Semester of Graduation

Summer 2018


Master of Science (MS)


School of Plant, Environmental, and Soil Sciences

Document Type



Glyphosate [N-(phosphonomethyl) glycine)] (GPS) is currently the most commonly used herbicide worldwide, and is generally considered as immobile in soils. However, numerous reports of the environmental occurrence of the herbicide coupled with recent evidence of human toxicity require further investigation as to the behavior of GPS in the soil environment. The objectives of this study were to quantify GPS sorption and mobility in two Louisiana agricultural soils with varying physiochemical properties; Commerce silt loam and Sharkey clay. Results of batch sorption studies indicated a high affinity of both soils for solvated GPS, with 24-hour Freundlich partitioning coefficients of 158 and 396 L kg-1 for the Commerce and Sharkey soils, respectively. Sorption by the Commerce soil was likely facilitated by the relatively high amounts of amorphous Fe and Al oxides, whereas the high cation exchange capacity of the Sharkey soil likely allowed for complexation with surface exchangeable polyvalent cations. A two-site multi-reaction model incorporating time-dependent reversible and irreversible reactions provided an adequate description of the measured data.

The results of miscible displacement studies indicated that the mobility of GPS is highly limited in both soils, with only 2-3% of the applied herbicide mass recovered in the effluent solution. Similar to the batch study, a two-site multi-reaction transport model (MRTM) consisting of kinetic reversible and irreversible reactions provided a good description of the breakthrough data from both soils, and outperformed linear modeling approaches using CXTFIT.

Competitive batch experiments, where sorption of both GPS and phosphate was considered, suggested that competition between the two solutes for reactive sites in both soils is significant, with phosphate being preferentially sorbed. GPS mobility in the Commerce soil remained highly limited when applied in conjunction with and succeeding phosphate pulses. However, a phosphate pulse introduced to the column following a GPS pulse resulted in an additional 4% of the applied mass of the herbicide being recovered in a secondary breakthrough. These findings are significant, as they emphasize the importance of the timing of herbicide and fertilizer applications on the behavior and ultimate fate of GPS in soils.



Committee Chair

Magdi Selim