Glyphosate (N-(phosphonomethyl)glycine), the most commonly used herbicide worldwide, degrades relatively rapidly in soils under most conditions, presumably by microbial processes. The most frequently detected degradation product in soil and water is AMPA (aminomethylphosphonic acid). We report the first evidence for an abiotic pathway of glyphosate and AMPA degradation under environmentally realistic conditions. Both glyphosate and AMPA degraded at 20 degrees C in dilute aqueous suspensions of birnessite, a manganese oxide common in soils, as evidenced by the accumulation of orthophosphate in solution over a period of several days. It is concluded that the abiotic degradation involved C-P bond cleavage at the Mn oxide surface, although evidence for C-N bond cleavage in the case of glyphosate and sarcosine, a likely degradation product of glyphosate, was found. The degradation of glyphosate was faster than that of AMPA, and higher temperature (50 degrees C) resulted in faster degradation of both glyphosate and AMPA. The addition of sulfate to the solution had no marked effect on the reaction rate, although Cu2+ addition inhibited degradation. As this metal ion complexes strongly with glyphosate, the inhibition can be attributed to the ability of Cu2+ to limit glyphosate coordination to reactive oxidation sites at the Mn oxide surface. Using a similar experimental design, we were unable to detect glyphosate degradation in an equimolar solution of MnCl2 (0.5 mM). However, we demonstrated that the oxidation of Mn2+ is enhanced both in solution and on an inert surface, in the presence of glyphosate (4:1 Mn-glyphosate molar ratio). This result suggests that the oxidative breakdown of glyphosate in the presence of Mn2+ may ultimately occur following the spontaneous oxygen-mediated oxidation of manganese.

Anoxic conditions in Zn‐ and Cd‐contaminated soils and sediments result in the formation of highly insoluble metal sulfides. Little is known, however, about the ability of mixed Zn–Cd sulfides, the most likely solid phases to form in these environments, to persist and limit Zn and Cd solubility under intermittently oxidizing conditions that are common with fluctuating water tables in wet soils. We therefore conducted laboratory experiments to measure the zero‐order rate constants for oxidative dissolution of synthetic solid solutions of Zn–Cd sulfides under aerated conditions, and the release of SO42−, Zn2+, and Cd2+ into solution. It was found that ultrafine synthetic metal sulfide particles with moderately high Zn/Cd mole ratios (≥20) were relatively stable under fully aerated conditions, and oxidized at much slower rates than sulfides with low Zn/Cd ratios. At the same time, high‐Zn sulfides were very efficient at retaining Cd in insoluble form even at very high solid‐phase Cd concentrations (>10000 mg kg−1) and low pH (<5), preferentially releasing Zn into solution. The preferential retention of Cd in sulfide particles with high Zn/Cd ratios was indicated by the persistence of much higher Zn/Cd mole ratios in solution than in the solid as the sulfide suspensions were equilibrated with atmospheric O2 In contrast, Cd‐rich Zn–Cd sulfides were readily oxidized under aerated conditions with the release of high concentrations of both Zn and Cd into solution. We concluded that colloidal Zn–Cd sulfide solid solutions high in Cd would be unlikely to limit the solubility of Cd or Zn to levels that would be nontoxic to soil biota or plants, whereas metal sulfides with high Zn/Cd ratios may be sufficiently persistent to retain Cd in an insoluble form in intermittently aerobic as well as anaerobic heavy‐metal‐contaminated wetlands.

Glyphosate, the most widely used herbicide in agriculture, has a strong tendency to sorb on minerals by bonding with surface metals through its metal‐coordinating functional groups. However, this same chemical process can potentially mobilize sorbed trace metals by chelation and sorbed anions such as phosphate by displacement. The tendency for glyphosate to mobilize Cu and other elements was tested in soil leaching experiments by applying glyphosate alone or complexed with Cu to mineral and organic soil columns and measuring the concentrations of these elements in the leachates by inductively coupled plasma (ICP) spectrophotometry. Complexation with glyphosate inhibited Cu sorption on the soil columns, rendering this metal more mobile in both the mineral and organic soil. Glyphosate mobility was also enhanced when applied to the organic soil as the Cu complex, but this effect was not detected with the mineral soil. Glyphosate was adsorbed more completely on the organic soil than the mineral soil, suggesting a metal bridging mechanism to explain sorption of this anionic molecule. The application of a commercial Roundup spray to long‐contaminated soils containing elevated concentrations of heavy metals and phosphate resulted in significant increases in the leaching of Cu, Zn, Al, Ni, P, Si, and As. No significant increases in elemental leaching were detected in mineral and organic soils with normal background concentrations of heavy metals and P. Batch equilibration studies confirmed the glyphosate‐enhanced dissolution of Cu in Cu‐contaminated soils, but reductions of free Cu2+ activity were measured only in uncontaminated soils. The results indicate that several elements, particularly Cu, Al, and P, could be mobilized within the thin surface layer of soils receiving a high effective rate of glyphosate during herbicide application.

Sorption of glyphosate and competitive desorption of phosphate in soils has been measured in past studies largely under batch equilibration conditions, which maximize the potential for retention of these anions. The experiments reported here were conducted by applying dilute Roundup \ solutions to soil columns, followed by leaching with water and collection of initial leachate. Soils representing a wide range of physical and chemical properties, including total and soluble P, were included in the study. The concentrations of total dissolved P and orthophosphate (PO 4 3j ) in column leachates were measured using inductively coupled plasma spectrometry and colorimetry, respectively. Leachate glyphosate was estimated as the difference between total P and PO 4 3j . Column leaching experiments with Roundup showed nearly complete glyphosate retention on the coarse-textured soils initially tested, from 85% to 95% sorption, despite the fact that higher than agronomic rates, 7.4 to 14.8 mg kg j1 of glyphosate, were applied. However, a second identical application 5 days later on one of these soils decreased the retention efficiency to a range of 63% to 73%. The Roundup treatments did not increase dissolved PO 4 3j in leachates from these soils. A forest soil with low P status and a dairy farm soil with high P status were subjected to the same column leaching tests. The leachates from both soils treated with Roundup contained elevated total P compared with controls, shown to be predominantly PO 4 3j . Column leaching experiments with much higher glyphosate application rates showed that glyphosate can displace PO 4 3j from sorption sites on both organic and mineral soils. The experimental results suggest that glyphosate sorption does not necessarily result in PO 4 3j dissolution and that there is only limited competition for sorption sites between glyphosate and PO 4 3j . Strong glyphosate sorption on high-organic matter soils indicates bonding of this anion by a metal bridge to organic functional groups. (Soil Science 2007;172:17-26)

Long-term changes in the solubility and bioavailability of heavy metals in soils, accumulated as the result of waste application, cannot be predicted without knowledge of the nature of metal retention in these soils. To test the theory that Fe-and Al-rich mineral phases in sewage sludge-amended soils can act as long-term sinks for heavy metals, soils were sampled from two field sites, each with a similar history of multiyear application of either high-Fe, high-Al, or high-Ca sludge (¨25 years earlier) but with different textural characteristics. These soils were amended with Cd in the form of CdCl 2 in the laboratory to determine Cd solubility as a function of total added Cd over the range of 0 to 20 mg/kg. The slopes of these linear solubility functions, used to determine the strength of Cd sorption, revealed that the high-Fe and high-Al sludge amendments did not improve the soils' affinity for Cd at either site. The high-Al sludge treatment decreased the affinity for Cd in the more coarse-textured soil. The high-Ca sludge amendment, conversely, increased the affinity for Cd, probably a result of the higher soil pH that has persisted since the sludge application. The results suggest that sludge Fe and Al may not be effective in binding Cd in all soils of humidtemperate climates. (Soil Science 2006;171:21-28)