Some aspects of the adsorption of glyphosate and its degradation products on montmorillonite

Abstract

The most worldwide used herbicide is glyphosate, phosphonomethylglycine (PMG). Consequently, a significant amount of PMG, its metabolites (sarcosine, SAR, and aminomethylphosphonic acid, AMPA) and the degradation product, methylphosphonic acid (MPA), reaches the soil, which acts as final sink. Because clays are one of the most reactive components of soils, expansive clays such as montmorillonite (Mt) are used to retain agriculture contaminants with some success. In this work, as a preliminary step for the evaluation of the risk that PMG, SAR, AMPA, and MPA occurrence could have on the environment, their adsorption on Mt surface was performed. The adsorption process was analyzed at constant adsorbate concentrations and two pH values to take into account the different protonation states of the amino group. DTA, XRD, zeta potential measurements, and XPS were used to identify the interactions or association mechanisms with the clay surface, the entry of adsorbates into the Mt interlayer, and electric charge changes on the Mt surface, and evaluate the acid-base surface complex constants, respectively. The interlayer thickness in acid media indicated that adsorbates are able to enter the interlayer in planar form. Besides, for the Mt-PMG sample, some PMG molecules could be also inserted as a bilayer or with a tilt angle of 52.4° in the interlayer. However, in alkaline media, the interlayer thickness indicated that the adsorbate arrangement differed from that of acidic media where PMG and MPA could have more than one orientation. The surface complex deprotonation constants were determined for the =NH ⇆ =NH+H process, being 3.0, 5.0, and 7.3 for PMG, AMPA, and SAR, respectively.

Interaction between glyphosate and montmorillonite in the presence of artificial seawater

“…Carneiro et al (2011) found a pH pzc value of 1.42 for unmodified montmorillonite. Flores et al (2018) and Khoury et al (2010) also found a similar result for natural montmorillonite pH pzc (2.7).…”

“…3). Because interlayers of montmorillonite are negatively charged, glyphosate enters more easily in the montmorillonite interlayers when its concentration is high and its molecule is positively charged (Khoury et al, 2010;Ren et al, 2014;Flores et al, 2018). Thus, in the present work we can affirm that sorption process of glyphosate onto montmorillonite was through adsorption, meaning glyphosate adsorbed onto the surface of the mineral.…”

“…This small variation in basal montmorillonite spacing demonstrates that glyphosate did not enter in the montmorillonite interlayer. However, Khoury et al (2010) and Flores et al (2018) using the following conditions pH 2.0 and 30 mmol L À1 of glyphosate and pH 2.0 and 8.7 mmol L À1 of glyphosate, respectively, observed that glyphosate enters in the montmorillonite interlayers. The entrance of glyphosate in the montmorillonite interlayers was dependent on its concentration as well as net charge of it (Khoury et al, 2010).…”

“…pH, surface area and amount of metals are among of the several aspects that are important in the glyphosate adsorption onto clay minerals. The adsorption of glyphosate onto clays minerals decreased when the pH increased (Mcconnell and Hossner, 1985;Glass, 1987;da Cruz et al, 2007;Damonte et al, 2007;Khoury et al, 2010;Flores et al, 2018). Probably because of the increase of negative charges onto minerals and in the glyphosate molecule.…”

Technological Evaluation of Cubati/Brazil Montmorillonite / Avaliação Tecnológica Da Montmorillonita De Cubati/Brasil

“…The adsorption of potentially toxic metals [7][8][9][10], herbicides [11][12][13][14][15] and dyes [16][17][18] has often been found in literature, since clay minerals consist of a low cost and high availability raw material, especially if compared to the usual adsorbent. Currently, some studies have focused on the adsorption of drugs present in water sources [19][20][21][22][23][24][25][26][27][28][29], Brazilian Journal of Development, Curitiba, v.7, n.1, p.5203-5222 jan. 2021 due to the harmful effects on flora, fauna and, consequently, on population.…”