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Adsorption of glyphosate on goethite (alpha-FeOOH): Surface complexation modeling combining spectroscopic and adsorption data

Journal article
Authors Caroline Jonsson
P. Persson
S. Sjoberg
J. S. Loring
Published in Environmental Science & Technology
Volume 42
Issue 7
Pages 2464-2469
ISSN 0013-936X
Publication year 2008
Published at Department of Chemistry
Pages 2464-2469
Language en
Keywords SOLID-SOLUTION INTERFACE, COMPETITIVE ADSORPTION, AGED GAMMA-AL2O3, ION, ADSORPTION, WATER, ACID, (HYDR)OXIDES, PHOSPHATE, COADSORPTION, PHOSPHONATES
Subject categories Surface and colloid chemistry, Coordination chemistry, Solution chemistry

Abstract

N-(phosphonomethyl)glycine (glyphosate, PMG) is the most widely used herbicide, and its adsorption onto soil minerals plays a significant role in its mobility and rate of degradation. In this work, we present the results of the first serious effort to find a realistic surface complexation model that fits both adsorption and total proton concentration data for PMG on the common soil mineral, goethite. Special attention was focused on making sure that the final model was in good semiquantitative agreement with previously reported X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopic measurements. Electrostatic effects were accounted for using the Basic Stern model, and the charges of the PMG-containing surface complexes were assumed to be distributed across the O- and beta-planes. The reactions for the protonation of the goethite surface were described using the 1 pK model, We optimized on the intrinsic formation constants and the charge distributions of the complexes, as well as the initial total proton concentration (l = 0.1 M Na(NO3), 25.0 degrees C), and the following model was obtained. FeOH0.5- + H3L reversible arrow FeHL1.5- +H++H2O Log10 beta = 4.70 +/- 0.08, Q(0) = -0.18 +/- 0.02 FeOH0.5- + H3L reversible arrow FeL2.5- + 2H(+) + H2O Log(10) beta = -3.9 +/- 0. 1, Q(0) = -0.7 +/- 0.1 Here, beta is the intrinsic formation constant, Q(0) is the charge at the 0-plane, and the errors are reported as one standard deviation. The charge distributions of the complexes are rationalized by considering intramolecular hydrogen bonding between the protons of the amine group and both the phosphonate and carboxylate groups.

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