Abstract
By means of a molecular-level theory we investigate glyphosate adsorption from aqueous solutions to surface-grafted poly(allylamine) layers. Our molecular model of glyphosate and the polymeric material includes description of size, shape, conformational freedom, and state of protonation of both components. The composition of the bulk solution (pH, salt concentration and glyphosate concentration) plays a critical role to determine adsorption. Adsorption is a non-monotonic function of the solution pH, which can be explained in terms of the pH-dependent protonation behavior of both adsorbate and adsorbent material. Lowering the solution salinity is an efficient way to enhance glyphosate adsorption. This is because glyphosate and salt anions compete for adsorption to the polymer layer. In this competition, glyphosate deprotonation, to increase its negative charge upon entering the polymer layer, plays an critical role to favor its adsorption under a variety of solution conditions. This deprotonation is the result of the higher pH that establishes inside the polymer. Our results show that such pH increase can be controlled, while achieving significant glyphosate adsorption, through varying the grafting density of the material. This result is important since glyphosate degradation by microbial activity is pH-dependent. These polymeric systems are excellent candidates for the development functional materials that combine glyphosate sequestration and in situ biodegradation.
Highlights
Glyphosate (N-[phosphonomethyl] glycine) is the top-selling, most extensively applied pesticide worldwide (Woodburn 2000; Benbrook 2016)
The theory applied in this work allows for calculating the local densities of all chemical species that are consistent with thermodynamic equilibrium of the system
Glyphosate adsorption is driven by electrostatic attractions with the adsorbent material; the considerations that we have presented to describe how the behavior depends on solution composition can be applied to those systems
Summary
Glyphosate (N-[phosphonomethyl] glycine) is the top-selling, most extensively applied pesticide worldwide (Woodburn 2000; Benbrook 2016). Glyphosate has been found in soils, groundwaters, and surface waters including its bottom sediments (Ronco et al 2016; Rendon-von Osten and Dzul-Caamal 2017; Castro Berman et al 2018). The Lujan River in Argentina contains more than 3000 μg/kg of glyphosate (Primost et al 2017; Castro Berman et al 2018). In soils, it accumulates at a rate of 1 mg glyphosate/kg soil every five spraying events (Primost et al 2017), which can lead to an important degree of environmental pollution. Development of means for glyphosate removal must become an essential issue of present-day technology
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