Soil adsorption and clay mineralogy
Strong correlations between the sorption of nonpolar compounds and soil organic matter content has led to the near ubiquitous use of KOC as soil partitioning coefficient (Section 3.2.3, 5.1). A growing body of evidence is, however, emerging to indicate that interactions with soil clay minerals can make an important contribution to the sorption of a range of polar and aromatic compounds. In particular, expandable 2:1 silicate clays (e.g. montmorillonite and vermiculite) are of significance because of their large specific surface area and big cation exchange capacity (Green, 1974). These minerals are generally found in less heavily weathered soils. Kaolinite, a 1:1 non-expanding clay which can carry a net positive charge at low pHs, is often present in weathered soils. The contrasting surface charge properties of these soils can result in differences in the sorption behaviour of pesticide compounds.
The use of simplified xenobiotic-clay-water systems has provided useful insights into the nature of the sorption processes that take place. In soils, clay minerals are intimately associated with organic matter and hence findings in model systems cannot be directly extrapolated to whole soil systems. Polar and aromatic compounds can interact with clay minerals through a variety of mechanisms. Cationic herbicides such as paraquat and diquat are completely ionised in water. They readily replace inorganic cations from the surface of clays resulting in extensive sorption, especially to expandable clays (Weber and Weed, 1968). This mechanism is important for weak organic bases such as the herbicide s-triazine (1,3,5 triazine) (Celis et al, 1998). These compounds are protonated at pHs below their pKa resulting sorption to clay surfaces. It is important to note that the acidity at the clay surface is often higher than in the bulk solution; that can result in higher sorption than would be predicted from the pH of the bulk solution. The type of exchangeable cation present can also have a significant influence on the extent and mechanism of sorption observed. Some cations are more easily displaced by competing organic compounds and cations with relatively small hydration spheres such as K+, and can facilitate hydrophobic interactions with the uncharged siloxane surface of the clay (Li, 2004). Hydrophobic interactions were suggested to contribute to the sorption of dicamba, atrazine, simazine and prosulfuron (Zhao et al., 1996; Cox et al, 2000; Sheng et al., 2001; Hyun and Lee, 2004). There is also evidence that a prevalence of trivalent cations such as Al3+ on clay surfaces can enhance the polarisation of water molecules facilitating increased sorption as a result of hydrogen bonding. Hydrogen bonding mediated sorption has been proposed for primisulfuron and 2,4-dichlorophenoxyacetic acid (2,4-D) among others (Hermosin and Cornejo, 1993; Pusino et al, 2004). In the case of aromatic compounds, it is hypothesised that the presence of delocalised pi electron clouds can produce electron rich, donating (-ve) or electron deficient, accepting (+ve) aromatic ring structures, respectively. Such compounds can then interact with surfaces of the opposite nature such as polarised and charged mineral surfaces (Keiluweit and Kleber, 2009)
Interactions of a pesticide (agrochemical) with clay minerals can have a significant influence on their agronomic performance and environmental impact. Some of the sorption mechanisms outlined above can result in irreversible sorption to clays as observed in the case of nicosulfuron (Ukrainczyk and Rashid, 1995). This could result in reduced availability of compounds in some soil types which could reduce the duration of effect of soil applied compounds and would result reduced mobility in soil than would be predicted from an estimated KOC value.