An electrochemical approach to quantitative análisis of herbicides and to the study of their interactions with soils components

ANA VALERIA JUÁREZ ; JULIETA SOLEDAD RIVA; LIDIA MABEL YUDI

Herbicides and environment

INTECH

Lugar: Rijeka; Año: 2011; p. 229 - 252

Triazine herbicides have been extensively applied to pre and post-emergence weed control. Many studies were focused on ecological and health hazards of these compounds and their toxic effects are very well known. For this reason, the use of some triazine pesticides has been banned in some countries or their permitted levels in drinking water is very low, so that analytical procedures for quantitative determination of several triazines, as well as their degradation products, at low levels are often requested. In this sense, several techniques has been developed for this quantification, like HPLC, CG-MS, capillary electrophoresis, solid-phase microextraction coupling with GC, LC, ion mobility spectrometry and with HPLC, immunosensor and multibiosensor based on immobilized Photsystem II, micellar electrokinetic chromatography, tandem techniques cyclic voltammetry and differential-pulse polarography on solid electrodes, photosynthetic electron transport (PET) electrochemical biosensors, PET colorimetric detection, adsorptive stripping voltammetry in dispersed media.In the last years, the environmental pollution by pesticides has become in a serious problem especially in aquatic ecosystems, due to their heavy use in agriculture and to their persistence. The half-lives of herbicides vary from weeks to several months, and under environmental conditions are usually degraded to compounds with better water solubility. Indeed, the most important physicochemical properties of these pesticides and their degradations products are the solubility in water and the capacity to be retained by the organic matter of the soil. Triazine herbicides are stable in the soil for 3-12 months, so that, the use of agricultural chemicals requires knowledge of their stability and transformation in the environment as well as their influence on micro-organisms. These s-triazine herbicides and some of their degradation products are used by water and soil microbes as a source of energy (alkyl fragments) and nitrogen (amine fragments). For this reason, not only the development of new sensitive and selective analytic techniques for the determination of s-triazine herbicides and their metabolites in the environment, but also the recognition of their interactions with different elements, especially with heavy metals cations and organic compounds present in soils, are important problems in modern s-triazine chemistry. The study of complex formation or adsorption behaviour between herbicides and cations or organic molecules (as humic acids) contained in soils is an important topic because it determines pesticide mobility, its bioavailability and its effectiveness. Humic acids are widely distributed in soils and, under natural conditions, they have been found as colloid suspension in water forming a large part of the dissolved carbon. Organic pollutants, such as pesticides, can exist in dissolved state or in association with the colloidal phase, or adsorbed by the soil. The driving forces of this adsorption involve electrostatic interactions, hydrogen bond, charge transfer, acceptor – donor mechanisms, Van der Waals forces and hydrophobic interactions. These mechanisms operate simultaneously and their competition determines the retention of pesticides in soil and aquatic systems. Adsorption of the pollutant is strongly influenced by the properties of the solute and the composition of soil organic matter.  For this reason, the characteristics of soils affect the herbicide adsorption, transport and degradation processes and, in consequence, the fate of herbicides in the environment. Regarding to the interaction with inorganic species, Al(III) is a cation present in most soils, and several authors have studied its complexes with different herbicides in aqueous solutions or in complex model systems, which closely simulate those found in soils by using pure montmorillonite or montmorillonite covered by different amounts of OH-Al species (chlorite-like complexes) as adsorbents. Several methods were employed in these investigations: macroscopic and molecular scale techniques, potentiometric titration data combined with EXAFS, ATR-FTIR and NMR, as well as spectroscopic data.The present chapter is focused in the progress made in the s-triazine quantification as well as in the study of their interactions with organic and inorganic compounds of soils employing electrochemical methods applied at the interface between two immiscible electrolyte solutions (ITIES).The interface between two immiscible electrolyte solutions and the transport of different ions across it are an important branch of electrochemistry because of its importance in the examination of heterogeneous kinetics and potential analytical applications. This methodology is used as an appropriate electroanalytical technique for quantitative determination of organic ions. The possibility of working in an oil/water system overcomes problems such as the low solubilities of many organic compounds, like the case of s-triazines, in water. Moreover, the traces quantification of pesticides in different kind of samples requires pre-concentration techniques. In the past few years, new techniques were developed like liquid-liquid extraction, solid phase extraction, molecular imprinted polymers and carbon nanotubes, among others. These pre-concentration procedures were employed before the quantification of the pesticide, coupled to different techniques like GC-MS, capillary electrophoresis, non-aqueous capillary electrophoresis and micellar electrokinetic capillary chromatography. In this sense, the use of a combined procedure consisting in a previous pre-concentration stage, followed by square wave voltammetry at a water/1,2-dichloroethane interface has achieved to improve the detection limit for s-triazines quantification. The pre-concentration of the analyte in the organic phase is possible due to its high solubility and partition coefficient in this solvent. On the other hand, voltammetry at ITIES has proven to be a valuable tool to elucidate the stoichiometry of complex formation and to identify and evaluate successive complex formation at the interface. With the purpose of contributing to the knowledge of the interaction between Prometrine  and soils components, the complex formation of the herbicide PROM with Al(III) cation and humic acids, at the water / 1,2 – dichloroethane interface, has been studied and the results are presented in this chapter.