Waterworks Archive

The influence of nonionic surfactants on adsorption hydroxylation and transport of atrazine in soil

V.P. (Bill) Evangelou
Department of Agronomy
University of Kentucky

Weed control in crops with herbicides has become an environmental concern because of their ground- and surface-water contamination potential. Atrazine (2-chloro-4-isopropylamino-6-ethylamino-s-triazine), a preemergence herbicide, is widely used throughout the world and is directly applied to soil. The movement of atrazine in soil occurs mainly through its solution phase.

   Much research using pure atrazine has been carried out on atrazine adsorption by soil and/or soil clays. Field use of atrazine, however, involves formulations that contain additive material (e.g., nonionic surfactants) to enhance atrazine’s solubility, phytotoxicity and wetting properties. It was hypothesized that the presence of a nonionic surfactant would have a large influence on the sorption/desorption behavior of atrazine in soil. For this reason, several experiments were carried out to determine 1) how a nonionic surfactant reacts with atrazine and clay minerals, 2) the influence of the nonionic surfactant on hydroxylation (inactivation) of atrazine, and 3) the influence of the nonionic surfactant on mobility of atrazine. For this study, the non-ionic surfactant Brij 35 [C12H25(OCH2CH2)23OH] (a well-characterized polyether and aliphatic hydrocarbon chain surfactant) was chosen. The actual nonionic surfactants used by herbicide manufacturers are closely guarded trade secrets.

   In surfactant solutions, surface tension, osmotic pressure and density changes occur in a narrow concentration range called the critical micelle concentration (CMC). The CMC values of most nonionic surfactants are much smaller in comparison to those of ionic surfactants. Because of this, nonionic surfactants are widely used in agrochemicals.

   Calcium or sodium exchanged montmorillonite or illite clay suspensions (1% w/v) were mixed with two different concentrations of Brij 35 surfactant (0.1% and 10%) in the presence and absence of 0.1% atrazine. These samples were air dried and analyzed using infrared (FT-IR) spectroscopy. The FT-IR spectra of the control sample (homoionic clay with no surfactant added) was compared with the spectra of Brij 35 treated homoionic clay sample. Figure 1 shows major FT-IR bands in the 1300 to 1700 cm-1 region of Ca-montmorillonite.

   H2O deformation frequency changed from 1621 without Brij 35 to 1640 cm-1 with Brij 35. Furthermore, Brij 35 decreased the intensity of the H2O deformation band, suggesting water displacement from the clay interlayer by Brij 35. Increasing the Brij 35 concentration to 2% replaced most of the water from the clay interlayer, and the Brij 35 became more directly coordinated to the exchangeable cation.

   X-ray diffraction of the Ca-clay samples showed that the d-spacing in the presence of two different concentrations of Brij-35 changed from 15.6 D to 17.5 D when treated with 0.1% Brij 35, and 19.5 D when treated with 10% Brij 35. This could be due to greater interlayer surfactant adsorption. Based on the dimensions of the alkyl chain (4.0 D) and the silicate layer (9.6 D), if Brij-35 was lying horizontally between the silicate layers of montmorillonite in a monolayer distribution, the d-spacing would be approximately 13 D. Hence, the d-spacing values of 15.6 D for Ca2+ saturated montmorillonite should correspond to at least a monolayer distribution of Brij 35 in the interlayer with alkyl chains parallel to the silicate-oxygen sheet.

   The surfactant Brij 35 could enter the interlayer of montmorillonite by replacing the water coordinated to cations. The amount of surfactant entering the interlayer of montmorillonite increased with increasing surfactant concentration. As Brij 35 entered the interlayer, the hydrophobicity of clay interlayer was enhanced. Increasing the clay interlayer hydrophobicity allowed atrazine to enter the clay interlayer as well.

   Many studies have confirmed that atrazine (bio-active form) is hydrolyzed to hydroxyatrazine (bio-inactive form) This transformation represents chemical degradation, since atrazine is relatively resistant to microbial degradation. Atrazine hydrolysis occurs under acid, neutral, or basic conditions. Hydrolysis is slow under neutral conditions, but it increases with either increasing alkalinity or acidity.

   The relative rates of hydroxylation of atrazine with Ca-montmorillonite in the presence of Brij 35 were studied by observing the increase of the C=O FT-IR band as a function time (Figure 2). It is clearly shown that the presence of Brij 35 significantly increased the rate of atrazine hydroxylation in Ca-montmorillonite.

   Atrazine hydroxylation enhancement resulted from the interaction of Brij 35 with atrazine followed by the interaction of atrazine with the clay surface. The protonated atrazine keto form was not observed in Na- or Ca-montmorillonite when in the presence of Brij 35 suggesting an OH driven atrazine hydroxylation process. In essence, Brij 35 increased the apparent level of alkalinity on the clay surfaces.

   Atrazine mobility in soil clays was evaluated using breakthrough studies.

   The Cl-breakthrough curve was slightly shifted to the right of an ideal conservative solute because of its potential positive charges on its edges. This curve is almost symmetrical with minor tailing in the desorption portion. Atrazine in the absence of Brij 35 reached maximum peak near C/Co = 1. This demonstrated the absence of atrazine diffusion, since no interlayers are present in illite. A slow atrazine desorption was also observed in this system as demonstrated by the tailing. The decrease in the peak concentration when in the presence of Brij 35 demonstrated irreversible adsorption. One possible explanation for the decrease in maximum peak concentration in the presence of Brij 35 is a relatively strong Ca2+-Brij 35 interaction. About 38% of the atrazine was recovered in the effluent when atrazine was applied with Brij 35, while 73% was recovered in the absence of Brij 35. Suggesting that Brij 35 significantly reduced atrazine mobility in Ca-illite. In the case of Ca-montmorillonite, in the presence of Brij 35, 25% of the atrazine present in the column was recovered during desorption in comparison to 39% in the absence of Brij 35.

   Clay mineralogy and the presence of a surfactant, Brij 35, influence the potential of atrazine to hydrolyze as well as mobilize in a clay profile. Incubation studies showed that clay adsorbed atrazine near neutral pH remained relatively stable. However, in the presence of Brij 35, under the same clay conditions, atrazine rapidly hydrolyzed (inactivated). Miscible displacement studies showed that the components controlling atrazine distribution in clays were diffusion controlling components. Also, mobility of the herbicide relative to the mobility of water was influenced by Brij 35 as well as the mineralogy of the clay and surface cation composition of clay.

   Field data show that mobility of atrazine to surface- and/or groundwater takes place mostly during rainfall events immediately after field application of atrazine. The quantity of atrazine to be transferred to the ground- and/or surface water would depend on the magnitude of atrazine’s partitioning coefficient between the solid and soil solution phases as well as kinetics in attaining the partitioning equilibrium state. This study has shown that partition coefficient and kinetics would be affected by the surfactant used in the formulation. In most, if not all previous studies, this component (surfactant) has not been taken into consideration. Field studies are needed to elucidate the role of surfactants on mobility and hydrolysis of atrazine.

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Last modified: November 1998

Copyright ©1998 Kentucky Water Resources Research Institute,
University of Kentucky