**1. Introduction**

470 Pesticides in the Modern World - Risks and Benefits

Santos J.L., Aparicio M., Callejón M., Alonso E. Occurrence of pharmaceutically active

compounds during 1-year period in wastewaters from tour wastewater treatment plants in Seville (Spain). Journal of Hazardous Materials 164, 1509-1516 (2009).

> The extensive use of herbicides in agriculture and their potentially toxic effects have promoted studies investigating the physical, chemical and biological processes that determine the mobility, bioavailability and degradation of these compounds in soils (Blasioli et al., 2011). Knowledge of these processes will enable prediction of the transport and fate of herbicides in soils and aquatic systems, and thus enable measures to be taken to limit their environmental impact.

> Retention is considered the main cause of the deactivation of herbicides in soils, and is important from the point of view of inhibiting the toxic properties of herbicides and of restricting their transport into aquatic systems (Jones & Bryan, 1980). Although not unique, adsorption reactions (i.e. accumulation of chemical species at the solid-solution interface) are the main cause of the retention of organic contaminants in soils, and their extent will depend on the physicochemical properties of both the adsorbent (soil) and the adsorbate (herbicide).

> The chemical characteristics of organic compounds are largely responsible for their behaviour in soil, and the differences in adsorption of different herbicides in the same soil are attributed to their distinct chemical properties. Although herbicides are very diverse, two groups can be distinguished in order to interpret their interactions with soil components: those involving chemical forces and those involving physical forces. The first group comprises ionic or ionizable hydrophilic compounds, while the second group comprises non polar hydrophobic compounds.

> Bipyridinium cations, such as paraquat (1,1'–dimethyl–4,4'–bipyridinium ion), are the best known members of the ionizable herbicides as they have been extensively used in agriculture and are consequently widely distributed in soils and waters. Paraquat (PQ) is applied as a dichloride or dibromide salt, which when dissolved in water releases the organic cation PQ2+, which can be adsorbed on the soil surface, either by replacing inorganic cations or by an ionic interaction mechanism with negatively charged sites on the soil surface, in which the electrostatic effect will be determinant (Narine & Guy, 1982). PQ adsorbs on humic substances and the degree of adsorption increases as the pH increases, as

Interactions Between Ionic Pesticides and Model Systems for Soil Fractions 473

Various chemical complexation models have been developed to describe adsorption data at the solid-solution interface. The major advantage of the complexation models over empirical approaches is that the former have the potential to be predictive and are applicable to more than one experimental condition. However, this potential has yet to be fulfilled in a general way. The most significant benefit provided by the models is the consideration of charge on both the adsorbate and the adsorbent surface, and the insight provided into the stoichiometry and reactivity of adsorbed species. These models have been widely applied to the reference materials selected as models of soil components, but have scarcely been used

Humic substances constitute the largest fraction of natural organic matter, and therefore may be used as reference materials to elucidate the mechanism of interaction between ionic herbicides and SOM (Hesketh et al., 1996). The classical view (Stevenson, 1994) states that humic substances are polydisperse mixtures, and that their chemical behaviour is generally determined by two types of functional groups: carboxylic and phenolic groups. Most studies involving cation binding to humic sustances have been carried out in solution (Milne et al., 2003), and the information obtained for dissolved humic substances has been used to predict the binding properties of humic substances in soils (Tipping, 2002). However, some authors question whether the parameters calculated from laboratory experiments carried out with dissolved natural organic matter can be used to provide an accurate description of the properties of the solid (soil) organic matter (Merdy et al., 2006). In order to simulate more realistic conditions, peat soils with an organic matter content higher than 90%, can be used as reference materials to study the binding properties of humic substances in the solid phase.

Iron mineral oxides and (hydr)oxides are common mineral compounds, particularly in soils. Goethite (α-FeOOH) is the most widely used reference material for (oxy)hydroxides as it is the most common and stable crystalline iron oxide under the conditions existing in soil. This mineral has a relatively high surface area, is very reactive (Cornell and Schwertmann, 1996) and therefore plays an important role in the migration of cations and anions in natural environments. Its properties have been studied in great detail in recent years, and there is therefore abundant information about its structure, chemical properties and adsorption, which facilitate the study of its interactions with ionic herbicides (Cornell & Schwertmann, 1996; Antelo et al., 2005; Brigante et al., 2010). Ionic pesticides may interact with reactive sites on the mineral surface via electrostatic interactions, cation exchange reactions and surface binding

In soils and aquatic systems, mineral oxides are often found associated with natural organic matter and their mutual interaction will change the chemical and adsorptive properties of both the organic and the mineral fraction. Because soil colloids are multicomponent associations, to achieve a more realistic view of the behaviour of cationic and anionic pesticides in natural systems, the adsorption reactions that take place in these combined systems must be analysed. Therefore, once the adsorption of ionic herbicides on

reactions, so that a high degree of adsorption might take place on mineral surfaces.

for mixtures of these components or for soils.

**2. Model systems for the soil fractions** 

**2.1 Organic matter** 

**2.2 Goethite** 

**2.3 Humic acid-coated goethite** 

a result of the progressive ionization of the acid groups in the organic matter (Iglesias et al., 2009; Brigante et al., 2010). PQ shows affinity for iron oxides and clay minerals (Clausen & Fabricius, 2001; Seki & Yurdakoç, 2005; Pateiro-Moure et al., 2010). Studies of PQ adsorption to clays in the pH range 5.0-7.0 reveal surface concentrations of between 0.18 and 4 μmol⋅m-2, depending on the type of clay. The higher adsorption of PQ on clays is explained by the ability of the planar PQ molecules to enter the lattice layers (Bromilow, 2003) and the extent of the PQ adsorption on soils is determined by the amount and type of clay minerals present in the soils (Roberts et al., 2002).

Phenoxyalkanoic acids (such as MCPA) and their derivatives constitute a large group of herbicides that are used extensively in agriculture. This type of organic pesticide has a carboxyl group that is mainly responsible for the interactions with soil components (Tunega et al., 2004). At pH > 4 the acid group (pKa = 3.10) will be ionized giving rise to significant adsorption of the anionic form of the pesticide on mineral oxides. It is known that the adsorption of pesticides of this type on goethite is strongly dependent on pH and ionic strength, and has been suggested to be due to the favourable interaction between the sorbing anions and the positively charged surface of the iron oxides (Celis et al., 1999; Spadoto & Hornsby, 2003). It is also known that the MCPA molecule has a phenolic lipophilic structure that makes this herbicide a model substance for the study of adsorption-desorption mechanisms in soils, which may involve both polar and hydrophobic interactions.

The soil surface, which comprises organic and inorganic components, is the primary site for chemical accumulation of organic herbicides, and its reactivity depends on the number and type of accessible functional groups. In addition to their specific reactivity, the reactivity of any functional group also depends on the proximity of other functional groups, so that the ionization of an acid group is enhanced by a nearby withdrawing substituent. Another factor to be taken into account is the accessibility of the functional groups to the adsorbates, as the association between different soil minerals and natural organic matter may make some functional groups inaccessible.

Oxides and hydroxides of iron and aluminium are particularly important inorganic soil components as although they are not major components, they are responsible for many chemical processes that take place in the soil because of their high specific surface and reactivity. For example, iron oxides contribute to the variable charge of soil and are known to play an important role in the adsorption of anions and cations (Cornell & Schwermann, 1996).

The organic matter on the soil surface constitutes the main source of the variable charge of surface horizons. Humic substances are the main source of pH-dependent changes in the soil organic matter (SOM), and the functional groups that generate the charge are mainly carboxylic groups. The ionized fraction of the acid groups increases as the pH increases and the negatively charged groups formed will interact electrostatically with the cations present in the environment.

Adsorption reactions in soils have historically been described by empirical adsorption isotherm equations. An adsorption isotherm is a plot of the concentration adsorbed on to a solid surface against the concentration in aqueous solution, for different total concentrations of a chemical species. These equations are popular because they are simple to use and the adjustable parameters are easily estimated, although they do not provide information about the chemical binding mechanisms (Sposito, 1982). Although adsorption isotherm equations are excellent for describing adsorption experiments, it must be taken into account that they are numerical relationships used to fit data, so that the equation parameters obtained are valid only for the chemical conditions under which the experiments are conducted.

Various chemical complexation models have been developed to describe adsorption data at the solid-solution interface. The major advantage of the complexation models over empirical approaches is that the former have the potential to be predictive and are applicable to more than one experimental condition. However, this potential has yet to be fulfilled in a general way. The most significant benefit provided by the models is the consideration of charge on both the adsorbate and the adsorbent surface, and the insight provided into the stoichiometry and reactivity of adsorbed species. These models have been widely applied to the reference materials selected as models of soil components, but have scarcely been used for mixtures of these components or for soils.
