Abstract
In this research work, the adsorption of two bipyridinium herbicides (i. e., Diquat and Paraquat) on natural soil and on model soil surfaces has been studied at different water chemistries commonly found in the environment (e.g., pH, supporting electrolyte, and presence of humic or fulvic acids). The experimental work was carried out in the laboratory, using experimental batches of clean quartz sand, silanized quartz sand and sandy soil as a model of agricultural topsoil where herbicides are commonly used and can be adsorbed. The concentrations reached at the equilibrium were analyzed by UV-Visible Spectroscopy for the supernatant fraction of the samples. The concentrations were fitted using adsorption isotherms to determine the adsorption mechanisms (i.e., chemisorption or physisorption) at the interface. In general terms, we have encountered that the nature of the soil matrix plays an important role on the study of pollutant adsorption. In experiments carried out on silica sand, the most abundant component of the natural soil matrix, no significant sorption was observed (<1.5 mg/g) for any of the herbicides. Yet, in experiments carried out on the presence of clay and natural organic matter (i.e., fulvic and humic acids), the adsorption of both herbicides is much higher, likely due to the chemical structure of the molecules that might facilitate the complexation with both herbicides. This investigation improves our understanding of the role that soil granular components play on the absorption of two commonly used herbicides and adequately predict their fate in natural aquatic environments.
Highlights
The global demand for food crops is expected to increase between 59 to 98% by 2050 and farmers have responded by increasing the productivity through the extensive use of agrochemicals (Valin et al, 2014)
Part of the cleaned quartz sand was subjected to anhydrous silanization with aminosilane following the protocol proposed by Chen et al (2001) to create a highly hydrophobic surface often encountered in agricultural settings (Hallett et al, 2011)
While poor adsorption of toxic agrochemicals on the soil will contribute to leaching into aquifers and direct surface water contamination (Teutli-Sequeira et al, 2020); high binding affinity for components of the soil matrix might indirectly contribute to groundwater pollution
Summary
The global demand for food crops is expected to increase between 59 to 98% by 2050 and farmers have responded by increasing the productivity through the extensive use of agrochemicals (e.g., fertilizers, pesticides, and herbicides) (Valin et al, 2014). Agrochemicals deposit on the topsoil and are carried to surface water bodies by overland flow and/or subsurface waterflow contaminating groundwater aquifers (Azizullah et al, 2011; Evans et al, 2018). In the specific case of herbicides, two of the most commonly used in developing countries due to their non-selective characteristics and low-cost are: Diquat (DQ) and Paraquat (PQ) (Akhavein and Linscott, 1968; Pateiro-Moure et al, 2010). Both compounds are included in the priority. Promotion of Chemical Safety, 1992; Barcelo, 1993; Fortenberry et al, 2016)
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