Competitive sorption between glyphosate and inorganic phosphate on clay minerals and low organic matter soils

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1 Journal of Radioanalytical and Nuclear Chemistry, Vol. 249, No. 2 (2001) Competitive sorption between glyphosate and inorganic phosphate on clay minerals and low organic matter soils H. M. Dion, 1,3 * J. B. Harsh, 2,3 ** H. H. Hill Jr. 1,3+ 1 Department of Chemistry, Washington State University, USA 2 Department of Crop and Soil Sciences, Washington State University, USA 3 Center for Multiphase Environmental Research, Washington State University, Pullman, WA 99164, USA (Received December 13, 2000) Inorganic phosphate may influence the adsorption of glyphosate to soil surface sites. It has been postulated that glyphosate sorption is dominated by the phosphoric acid moiety, therefore, inorganic phosphate could compete with glyphosate for surface sorption sites. We examine sorption of glyphosate in low organic carbon systems where clay minerals dominate the available adsorption sites using 32 P-labeled phosphate and 14 C-labeled glyphosate to track sorption. We found glyphosate sorption strongly dependent on phosphate additions. Isotherms were generally of the L type, which is consistent with a limited number of surface sites. Most sorption on whole soils could be accounted for by sorption observed on model clays of the same mineral type as found in the soils. Introduction Glyphosate [N-(phosphonomethyl)glycine] is a nonselective, broad-spectrum, post-emergent herbicide used in a variety of agricultural and domestic settings. It is already one of the most commonly applied herbicides and its use is likely to increase in the coming years due to the increased demand for Round-up Ready crops. 3 It has been determined that the phosphate moiety of glyphosate is responsible for its strong adsorption to soils and that soil phosphate capacity is related directly to glyphosate adsorption. 4,9,10 Competitive effects of phosphate on glyphosate adsorption have yet to be quantified which is of increasing importance due to the inception of Round-up Ready crops. It is estimated that 57% of all soybeans grown in the United States are Round-up Ready (USDA 1999). Knowledge of the fate and transport of pesticides applied to soils has become increasingly important as we seek to protect groundwater from toxic chemicals and crops from persistence. The residence time for a specific pesticide depends upon soil and analyte characteristics, such as, texture, cation exchange capacity, ph, organic carbon content, degradation or transformation rate, and analyte chemical properties. One of the factors that is particularly important with regard to more polar compounds, such as weak acids and bases, is the clay content of the soil. The objective of this study was to quantify the competitive chemical adsorption phenomenon between glyphosate and ortho-phosphate using dual-tracer batch equilibrium experiments. Studies were conducted on several whole soils (kaolinitic, illitic, smectitic) before and after removing organic matter and pure clay minerals. Pure minerals were examined in order to develop a comparison between sorption on high clay subsoils and clay minerals. Chemicals Experimental Unlabeled and 14 C-labeled glyphosate, 98.7% pure, were obtained from Monsanto (St. Louis, MO). Potassium phosphate dibasic was used in addition to diammonium hydrogen phosphate to achieve desired phosphate concentrations (96% pure, J. T. Baker). All compounds were used as delivered. Neutron activation The 32 P was prepared at Washington State University s Nuclear Radiation Center by neutron activation using the 31 P(n,γ) 32 P reaction. Diammonium hydrogen phosphate [(NH 4 ) 2 HPO 4 ] was purchased from J. T. Baker (99.6% pure) and used as received. A known amount of dry ammonium phosphate was triple sealed in polyethylene vials and irradiated in the WSU Triga III fueled reactor for 6 hours, which equals a thermal neutron fluence of ~10 16 cm 2. A cool-down time of 12 hours was used to allow for short-lived activation products to decay. After 12 hours the sample was counted on a germanium detector, this revealed trace quantities of 24 Na (T 1/2 = 14.5 h); however, this impurity does not interfere with competitive sorption because the 24 Na was completely decayed before the experiments were begun. Ammonium phosphate was quantitatively dissolved in ultra-pure water. The solution ph was * h_dion@wsu.edu ** harsh@wsu.edu + hhhill@wsu.edu /2001/USD Akadémiai Kiadó, Budapest 2001 Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht

2 Table 1. Soil physical and chemical characteristics Soil Organic carbon, % ph Sand, % Silt, % Clay, % Illitic ± ± ± ± 2.01 Kaolinitic ± ± ± ± 2.55 Smectitic ± ± ± ± 1.70 Soils Three soils were collected and air-dried for the experiments. X-ray diffraction analysis was conducted on the three soils to determine the underlying mineralogy (Philips X-ray diffractometer). Oriented samples of the three test soils were used and treatments included airdried, heated to 300 and 550 C saturated with both potassium and magnesium, and glycerol solvated. The three soils used were an illitic Palouse silt loam; an unmapped weathered kaolinite soil obtained from a roadcut in Latah County, Idaho; and a Sharpsburg silty clay loam dominated by smectites obtained from Union County, Iowa. Each of the three soils was characterized for texture (pipet method, modified by WSU Pedology and Quaternary Studies Laboratory), organic carbon content, 7 and ph (soil saturation extract). Results of the soil characterization are contained in Table 1. Clay minerals Pure clay minerals were selected for comparison to the clay fraction in the selected soils. Illite and kaolinite clay minerals were obtained from Washington State University s mineral collection and were pulverized prior to obtain a mean particle size between 25 and 2 µm. Beidellite clay (Bid-1) was purchased from the Clay Mineral Repository and pulverized prior to use (University of Missouri, St. Louis, MO). Competitive sorption experiments The procedure for the sorption experiments was conducted using a soil:solution ratio of 1:10 m/m. One gram of soil or clay mineral was placed in a sterilized 50 ml polyethylene centrifuge tube and 10 ml of solution was added. The 32 P was added as a spike to a matrix solution of potassium phosphate dibasic to reach the desired initial concentration, 0.005, 0.05, and 0.1 mol/l. A mixture of unlabeled and carbon 14-labeled glyphosate was added at 7.75E-3 (3.82E+3 dpm), 7.78E- 1 (2.785E+3 dpm), 1.48 (4.35E+3 dpm), and 2.96 mmol/l (8.68E+3 dpm) with initial activity shown in brackets. All glyphosate and phosphate spike additions were less than ml in total added volume. Blanks carried out through the same sample treatments showed that degradation due to microbiological or chemical pathways or loss to equipment was negligible. Sampling time was determined elsewhere by conducting timed experiments during which samples were taken at 1, 5, 12, 24, 48, 72 h, 1, 2, 4, and 6 weeks. After 48 hours, glyphosate sorption from solution was negligible, for all soils tested, as determined by linear regression analysis on the data points from 48 to 72 hours. 1 Suspensions were placed on an end-over-end mixer until sampling. At 48 hours, the suspensions were centrifuged at 10,000 rpm for 15 minutes with the supernatant solution retained for analysis. In addition to the competitive sorption experiments, glyphosate sorption was examined without added phosphate. 1 Liquid scintillation counting Two ml of supernatant solution was added to 12 ml of scintillation cocktail (ScintiVerse II, Fisher Chemical, Fair Lawn, NJ) and counted by LSC (Model 1900TR LSC, Packard/Canberra, Meriden, CT). Because 32 P is a high-energy beta-emitter, crossover calculations were necessary to deconvolute the 14 C spectra from the 32 P spectra. Energy window A was set between 0 and 156 kev and window B was set between 156 and 2,000 kev; this allowed for minimal 14 C contributions to the activity in window B. Although, there was a significant contribution to the activity in region A from the 32 P, this overlap was corrected by determining contribution to the counts in region A due to 32 P. In addition, it was found that a certain ratio of 32 P counts were being detected in both region A and region B; this was corrected by calculating a cross talk value which allowed determinations to be made based on separate samples containing only the initial activities of 14 C and 32 P respectively: Ratio of counts in region A versus cpmb blank counts in region B = (1) cpma blank Cross talk calculation = corrected cpmb ratio A B (2) cpm 32 P = countsb blank (3) cpm 14 C = (countsa blank) crosstalk (4) 386

3 Sorbed glyphosate and phosphate were determined through back calculations using the initial aqueous concentration and the final aqueous concentration. Results accurate prediction of total phosphate capacity it would be necessary to determine sorbed concentrations much higher than those studied in this experiment: y = ax ( + bx) 1 Sorption experiments where y glyphosate or phosphate sorbed (mmol. kg 1 ), x glyphosate or phosphate concentration (mmol. l 1 ), a empirical fitting parameter, b empirical fitting parameter. Adsorption isotherms from the soils and clay minerals were plotted using the 48-hour equilibrium concentrations. Previous studies have shown sorption past 48 hours is inconsequential to the total amount sorbed. 1 Initial phosphate concentrations were 5.3, 50.5, and mmol/l in order to represent concentrations normally added to agricultural soils and glyphosate concentrations ranged from 7.75E-3 to 2.96 mmol/l. Isotherms depicting glyphosate sorption with varying levels of phosphate are shown in Fig. 1. As the initial level of phosphate increases, sorption of glyphosate decreases. For all systems, addition of as little as 5.3 mmol/l of phosphate causes a significant decrease in sorption of glyphosate. Also, as initial levels of glyphosate increased, the effect of the phosphate tended to have less of an impact on glyphosate sorption. Isotherms were characterized as being of the Langmuir type, indicating a decrease in the slope of the sorption isotherm, consistent with sorption on material with a limited number of sorption sites. It is also consistent with the hypothesis that glyphosate sorption will decrease in the presence of ortho-phosphate assuming that they compete for the same surface sites. Table 2 contains the Langmuir curves and regression analysis for the phosphate and glyphosate sorption isotherms. Langmuir curves are used to estimate the total sorption due to surface site saturation [Eq. (5)]. Results indicate that glyphosate sorption should be much greater than phosphate sorption; however, only three points were used to generate the phosphate curve and underestimation is possible. In order to make a more Nevertheless, the Langmuir a parameter, which is related to the strength of interaction of the sorbate with the sorbent, indicates that glyphosate is more strongly sorbed than phosphate to both smectitic and illitic minerals and soils. The results for kaolinite and the weathered soil containing kaolinite indicate that phosphate and glyphosate are on a more equal footing in competition for sorption sites. The Langmuir parameters do not differ significantly for phosphate sorption among the minerals and soils; therefore, the low a parameter for glyphosate on kaolinite may indicate a fundamental difference in sorption mechanism on this mineral. Figure 2 qualitatively shows the relationship between glyphosate sorption as a function of phosphate concentration. This relationship was investigated further by plotting the equilibrium sorbed glyphosate concentration (initial concentration 2.96 mmol/l) versus the equilibrium sorbed phosphate concentration (mmol/l) at each phosphate concentration. This plot was examined using linear regression analysis (Table 3): (5) y = mx + b (6) where y glyphosate sorbed (mmol. kg 1 ), x phosphate sorbed (mmol. kg 1 ), m slope of regression line, b y-intercept. Table 2. Langmuir fit for phosphate and glyphosate sorption isotherms Phosphate Glyphosate Soil K b r 2 K b r 2 Illite soil E E Illite mineral E E Kaolinite soil E E Kaolinite mineral E E Smectite soil E E

4 Fig. 1. Glyphosate sorption on selected soils and clay minerals; a illitic soil, b illite mineral, c kaolinitic soil 388

5 Fig. 1. Glyphosate sorption on selected soils and clay minerals; d kaolinite, e smectitic soil Table 3. Linear regression analysis of glyphosate sorption (initial concentration of 2.96 mmol/l) versus phosphate sorption (initial concentrations of 5.3, 50.5, and mmol/l) Soil m b r 2 surfaces in the order smectitic soil > kaolinitic soil > kaolinite mineral. This preference for glyphosate on the soils and minerals studied may be due to glyphosate s multiple reactive functional groups, which could form bidentate or tridentate complexes with the mineral surface. These results agree with the Langmuir K values which show glyphosate sorbed far more strongly than phosphate. Illite soil Illite mineral Kaolinite soil Kaolinite mineral Smectite soil Sorption of glyphosate and phosphate on the soils are If sorption was preferential for either of the analytes then the slope of the regression line would be either less than one indicating preference for glyphosate or greater than one indicating phosphate preference. In all of the systems, there appears to be preferential adsorption of glyphosate to the surface sites because the slope of each of the systems studied was less than one. The greatest preference was on the illitic soil and illite mineral with slopes of and 19.66, respectively, with the other similar to sorption on the clay minerals chosen as analogs. This supports the idea that the predominant clay minerals in a soil system may be the most important factor for predicting glyphosate/phosphate sorption in low organic carbon systems. Competition between phosphate and glyphosate reinforces other studies that implicate the phosphate moiety in glyphosate sorption to soils. A quantitative analysis leads to the conclusion that glyphosate is preferentially adsorbed over phosphate with the exception of the kaolinitic soil. 389

6 * H. D. was partially supported by the National Science Foundation s Integrative Graduate Education and Research Training Grant to Washington State University under Grant # In addition, we would like to acknowledge Pat FUERST for loaning of radio labeled 14 C glyphosate and WSU s Nuclear Radiation Center for reactor time in preparation of the 32 P used in these experiments. References 1. H. M. DION, J. B. HARSH, H. H. HILL, Jr., Soil Sci. Soc. Am. J., to be published. 2. Farmer Reported Genetically Enhanced Varieties, 1999 Crop Production Report Economic Research Service USDA, p J. E. FRANZ, M. K. MAO, J. A. SIKORSKI, Glyphosate: A Unique Global Herbicide. ACS Monograph 189. American Chemical Society, Washington, DC R. J. HANCE, Pestic. Sci., 7 (1976) L. M. LAVKULICH, J. H. WIENS, Soil Sci. Soc. Amer. Proc., 34 (1970) R. H. LOEPPERT, W. P. INSKEEP, in: Methods of Soil Analysis Part 3-Chemical Methods, D. L. SPARKS (Ed.), SSSA Book Series:5. Soil Sci. Soc. Am., Inc. and Amer. Soc. Agronomy, Inc. Madison, WI. 1996, Chapter 23: Iron. 7. D. W. NELSON, L. E. SOMMERS, in: Methods of Soil Analysis Part 3-Chemical Methods, J. M. BIGHAM (Ed.), SSSA Book Series:5. Soil Sci. Soc. Am., Inc. and Amer. Soc. Agronomy, Inc. Madison, WI. 1996, Chapter 34: Total Carbon, Organic Carbon, and Organic Matter. 8. Particle Size Analysis. Pedology and Quaternary Studies Laboratory, Department of Crop and Soil Science, WSU. Rev. 4/ A. PICCOLO, G. CELANO, M. ARIENZO, A. MIRABELLA, J. Environ. Sci. Health, B29 (1994) P. SPRANKLE, W. F. MEGGITT, D. PENNER, Weed Sci., 23 (1975) M. E. SUMNER, W. P. MILLER, in: Methods of Soil Analysis Part 3-Chemical Methods, J. M. BIGHAM (Ed.), SSSA Book Series:5. Soil Sci. Soc. Am. Inc. Amer. Soc. Agronomy, Inc. Madison, WI. 1996, Cation Exchange Capacity and Exchange Coefficients, p Fig. 2. Sorption of glyphosate on selected soils and clay minerals with and without the presence of phosphate; a illitic soil and illite mineral, b kaolinitic soil and kaolinite mineral, c smectitic soil and beidellite mineral 390

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Journal of Radioanalytical and Nuclear Chemistry, Vol. 253, No. 1 (2002) 115 120 Thermodynamic parameters of Cs + sorption on natural clays T. Shahwan, H. N. Erten* Department of Chemistry, Bilkent University,