Figure 6 - uploaded by Donald Gamble
Content may be subject to copyright.
Source publication
Thirty years ago, it was universally assumed that it would never be possible to do rigorously quantitative chemistry with such complicated natural mixtures as agricultural soils, aquatic sediments, natural waters, or even their components. One of the practical implications of that assumption was that the physical chemistry interactions of metal ion...
Citations
... In contrast to the extractable and mineralized fractions, the nonextractable fraction had low variability in each CR considered. Herbicide stabilization as bound residues is considered to be a kinetically slow process (Gamble et al., 1994). Th ese researchers, working with atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4- diamine], argued that nonextractable herbicide residue formation was due to intraparticle soil diff usion. ...
The environmental fate of glyphosate [N-(phosphonomethyl)glycine] was studied in six crop residue (CR) types, three from maize (Zea mays L.) (M1, M2, and M3) and three from soybean [Glycine max (L.) Mem] (S1, S2, and 53). Glyphosate adsorption was characterized through isotherms. The glyphosate distribution in CRs was characterized through the balance of (14)C-glyphosate radioactivity among the mineralized fraction, the extractable fractions (water and NH(4)OH), and the nonextractable fraction. Crop residues were characterized by elemental composition, organic C, total N, and biochemical parameters (soluble fraction, cellulose, hemicellulose, and lignin). Total microbial activity (TMA) was also assessed. Limited and reversible glyphosate adsorption on soybean and maize CRs was determined. The sorption coefficient K(f) index range for maize CR was 1.5 to 8.3 L kg(-1) and 2.6 to 7.4 L kg(-1) for soybean CR. Organic C and hemicellulose partially explained adsorption variability. The addition of mineralized and nonextractable fractions of the initial (14)C-glyphosate applied on the CRs averaged 56%; however, differences were detected between soybean and maize CRs. Mineralization and nonextractable residues were 30.7 +/- 11 and 32.5 +/- 6% (soybean CR) and 44.3 +/- 12 and 17 +/- 7% (maize CR), respectively. We hypothesized that glyphosate molecules could be used initially by microorganisms as a labile C source. High variability in (14)C-glyphosate mineralization was observed in all crop residues, suggesting that the magnitude of the glyphosate mineralization process would be regulated by accessibility and the lability of other carbonate sources.
The chemistry of humic materials has been under world wide investigation for about a 100 years. The pace of the research continues
to increase. During at least the last 30 years, evidence has been accumulating in the literature that the principles of classical
chemistry as they are understood for monomeric reagents can to some extent be adapted to humic polyelectrolyte mixtures. Published
demonstrations of predictive chemical calculations for humic materials have resulted from this. There is a need for the predictive
capability to be improved, and applied to environmental remediation and regulatory practice.
The interaction of the pesticides, chlordimeform and lindane and the herbicides paraquat, 2,4-dichlorophenoxyacetic acid and atrazine with humic substances (humic and fulvic acids) has been studied. Binding isotherms were measured by equilibrium dialysis and used to derive the Gibbs energies of interaction of the biocide ligands with the humic substances. Detection of binding at very low ligand concentrations (50 μM) was demonstrated by ultracentrifugation. Microcalorimetry was used to measure the enthalpies of interaction as a function of ligand concentration. The data were interpreted using a Langmuir-type model to obtain the enthalpies of interaction at saturation and the association constants. Both equilibrium dialysis and microcalorimetry gave comparable specific Gibbs energies (Δg, Jg−1) of interaction for those systems (paraquat and chlordimeform) where a complete thermodynamic analysis was possible. The specific Gibbs energies of binding of paraquat and chlordimeform to aquatic fulvic acid were of the order of −5 Jg−1 and an order of magnitude larger than for binding to peat humic acid.
Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is retained against leaching losses in soils principally by sorption to organic matter, but the mechanism of sorption has been a matter of controversy. Conflicting evidence exists for proton transfer, electron transfer, and hydrophobic interactions between atrazine and soil humus, but no data are conclusive. In this paper we add to the database by investigating the role of (i) hydroxyatrazine (6-hydroxy-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and (ii) hydrophobicity in the sorption of atrazine by Brazilian soil humic substances. We demonstrate, apparently for the first time, that hydroxyatrazine readily forms electron-transfer complexes with humic substances. These complexes probably are the cause of the well-known strong adsorption by humic acids and they may be the undetected cause of apparent electron-transfer complexes between soil organic matter and atrazine, whose transformation to the hydroxy form is facile. We also present evidence that supports the important contribution of hydrophobic interactions to the pH-dependent sorption of atrazine by humic substances.
