Effects of Dissolved Carboxylates and Carbonates on the Adsorption Properties of Thiuram Disulfate Pesticides PANAGIOTA STATHI, † KONSTANTINOS C. CHRISTOFORIDIS, † ATHANASIOS TSIPIS, ‡ DIMITRA G. HELA, § AND YIANNIS DELIGIANNAKIS* ,† Laboratory of Physical Chemistry, Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 30100 Agrinio, Greece, Section of Inorganic and Analytical Chemistry, Department of Chemistry, University of Ioannina, Panepistimioupoli Douroutis, 45110 Ioannina, Greece, and Department of Farm Organization and Management, University of Ioannina, Seferi 2, 30100 Agrinio, Greece The adsorption of thiram and disulfiram onto R-Al 2 O 3 and montmorillonite clay has been studied in the presence of small carboxylate anions, bicarbonate, formate, and oxalate. At natural concentrations, HCO 3 - enhances dramatically the adsorption of both pesticides on R-Al 2 O 3 and clay. An analogous significant enhancement of pesticide adsorption is also observed in the presence of formate and oxalate. Density functional theory calculations demonstrate that in solution a stable molecular complex between one molecule of thiram and one molecule of HCO 3 - is formed with interaction energy -35.6 kcal/mol. In addition, two H 2 O molecules further stabilize it by an interaction energy of -3.6 kcal/mol. This clustering [thiram- HCO 3 - -2H 2 O] leads to a change of the electronic structure and the ultraviolet-visible spectrum of thiram that is observed experimentally. Surface complexation modeling shows that the molecular cluster [thiram-HCO 3 - - 2H 2 O], which bears a total net charge of -1, is responsible for the observed enhanced adsorption on the charged surface of alumina and clay at pH below their points of zero surface charge. The results reveal a novel pervasive role of carboxylate anions and particularly HCO 3 - on the adsorption of dithiocarbamate pesticides in natural waters. Introduction Thiram (C6H12N2S4, tetramethyl thiuram disulfide) and di- sulfiram (C10H20N2S4, tetraethyl thiuram disulfide) are among the thiuram disulfides as animal repellents to protect fruit trees and ornamentals from damage by rabbits, rodents, and deer or as rubber vulcanization accelerators (1). The half-life for thiram in soil has been reported to be between 2 and 3 weeks (2). Both thiram and disulfiram are hydrophobic neutral molecules that are strongly adsorbed on soils (1, 3, 4), making their elimination from the environ- ment difficult. However, the physicochemical parameters for the adsorption of thiram and disulfiram in soils have not been evaluated. Strong adsorption of thiram on lignin has been reported (5). The carboxyl groups of lignin were postulated (5) to play an important role in the observed strong adsorption of thiram; however, no further evidence was provided. The adsorption mechanisms of organic molecules and pesticides on soils are quite diverse (2). Appropriate studies on soil constituents allow the identification of the controlling factors (6-12) and mechanisms (8, 11-12). Among the soil constituents, clay minerals (2, 4, 7) and mineral oxides (2, 5, 6) strongly influence the fate of pesticides in soils due to their large surface area and abundance in agricultural soils. In this context, in the present work we have studied the adsorption properties of thiram and disulfiram on aluminum oxide and montmorillonite clay. Carbon dioxide is ubiquitous in natural environments and is present either as gaseous CO2 or dissolved in water, i.e., as CO2(aq) and carbonate species HCO3 - and CO3 2-. A typical range of carbonate species in groundwater is expected to be in the range of 0.5-10 mM (13, 14). Atmospheric CO2 contributes more than 60% of the carbonate present in river water, while 31% comes from carbonate minerals and 7% from oxidation of organic carbon in sediments (15). The ubiquity of CO2 and carbonates therefore makes it necessary to include these compounds in studies of adsorption and mobility of chemical compounds in the natural environment. However, dissolved species, i.e., CO2, HCO3 - , and CO3 2- , have mainly been neglected or specifically avoided in adsorption studies, and their possible impact on parameter estimation of adsorption phenomena anions has not been fully ap- preciated (14, 16). In the present work the role of dissolved carbonates on the adsorption of thiuram disulfides has been examined in detail. The objectives of the present work were (a) to investigate the adsorption mechanism of thiram and disulfiram on alumina and clay and (b) to elucidate the role of small carboxylates, particularly HCO 3 - , on the adsorption of these pesticides. The observed phenomena were interpreted by the combined use of quantum chemical calculations and macroscopic surface complexation modeling (17). Experimental Procedures Materials. Aluminum oxide R-Al2O3 was obtained from Sigma-Aldrich (Lot No. 0.5716CU-414). The clay used was a synthetic sodium-saturated montmorillonite, Kunipia F (KUN) [Na0.87[Al3.12 Fe(III)0.20Mg0.61Ti0.01](Si7.90Al0.10)O20(OH)4], purchased from Kunimine Industries Co. This is a very high purity montmorillonite and was used as the model clay in order to eliminate potential interferences from spurious contaminants often occurring in natural montmorillonite (18). The surface charge and protonation constants were determined by mass titration and potentiometric titration. Analytical quantitative determination of thiram and disul- firam in aqueous solution was done with HPLC (19, 20). Light absorption spectra were recorded in the ultraviolet-visible (UV-vis) wavelengths (190-800 nm). Full experimental details are provided in the Supporting Information. Adsorption Experiments. For the adsorption studies, batch samples were prepared by spiking pesticides from a stock solution in methanol with appropriate volumes, typically 20 mL, of Milli-Q water. This resulted in methanol concentrations below 1%. Adsorbent suspensions, e.g., either * Corresponding author phone: +003 26410 39516; fax: +003 26410 39576; e-mail: ideligia@cc.uoi.gr. † Department of Environmental and Natural Resources Manage- ment. ‡ Department of Chemistry. § Department of Farm Organization and Management. Environ. Sci. Technol. 2006, 40, 221-227 10.1021/es051451s CCC: $33.50  2006 American Chemical Society VOL. 40, NO. 1, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 221 Published on Web 11/22/2005