Among patients receiving opioid prescriptions for pain, higher opioid doses were associated with increased risk of opioid overdose death.

The adsorption and desorption mechanisms of natural organic matter (NOM) on mineral surfaces are not completely understood because of the heterogeneity and complexity of NOM and adsorbent surfaces. This study was undertaken to elucidate the interaction mechanisms between NOM and iron oxide surfaces and to develop a predictive model for NOM adsorption and desorption. Results indicated that ligand exchange between carboxyl/ hydroxyl functional groups of NOM and iron oxide surfaces was the dominant interaction mechanism, especially under acidic or slightly acidic pH conditions. This conclusion was supported by the measurements of heat of adsorption (microcalorimetry), FTIR and I3C NMR analysis, and competitive adsorption between NOM and some specifically adsorbed anions. A modified Langmuir model was proposed in which a surface excess-dependent affinity parameter was defined to account for a decreasing adsorption affinity with surface coverage due to the heterogeneity of NOM and adsorbent surfaces. With three adjustable parameters, the model is capable of describing a variety of adsorption isotherms. A hysteresis coefficient, h, was used to describe the hysteretic effect of adsorption reactions that, at h = 0, the reaction is completely reversible, whereas at h = 1, the reaction is completely irreversible. Fitted values of h for NOM desorption on iron oxide surfaces ranged from 0.72 to 0.92, suggesting that the adsorbed NOM was very difficult to be desorbed at a given pH and ionic composition. Our results imply that a better mechanistic understanding of the interaction between NOM and oxide surfaces is needed to improve our predictive capabilities in NOM transport and cotransport of contaminants associated with NOM or iron oxides.

John E McCarthy (1) is group leader for biological chemistry at the Environmental Sciences Division of Oak Ridge National Laboratory, which he joined in 1980 and where he has been directing a research project on colloid-facilitated transport for the Department of Energy since 1985. His research interests are in the fate and transport of contaminants and in the biological aspects of exposure to contaminants. Mc-Carthy holds a B. S. from Fordham University and a Ph.D. from the University of Rhode Island. John M. Zachara (r) is a technical group leader in the Geochemistry Section of Battelle ' s Pacific Northwest Laboratory, which he joined in 1979. His research interests include surface and colloid chemical reactions that control the migration of organic and inorganic contaminants in groundwaters and soils. Zuchara obtained his4.S. at Bucknell University and his Ph. D. from Washington State University.

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The subsurface transport of inorganic and organic contaminants may be strongly related to the movement of dissolved organic carbon (DOC) through a soil profile. A variety of soil chemical and hydrologic factors control the mobility of the DOC, which may enhance or impede the transport of the associated contaminants. In this study, the sources of DOC adsorption on two proposed waste‐site soils are defined, and the chemical mechanisms operative during the adsorption process are specified. Adsorption isotherms for the two soils determined at constant pH, ionic strength (I), and temperature indicated that DOC adsorption increased with increasing soil profile depth. Different adsorption capacities were exhibited by the two soils, however, which was related to their contrasting indigenous organic matter contents and mineralogies. The adsorption of DOC by the soils was not a function of solution I (I = 0.001 to 0.1 mol L−1 using NaCl); however, DOC adsorption was dependent on solution pH, with maximum adsorption occurring at ≃4.5. Competitive ion‐exchange studies using Na2SO4 as an ionic‐strength adjuster suggested that a portion of the DOC was electrostatically bound to the soil via anion exchange. By using thermodynamic principles, the predominant mechanism of DOC retention by the soil was found to be physical adsorption driven by favorable entropy changes. This is supported by preferential adsorption of the hydrophobic organic solutes to the soil relative to the hydrophilic organic solutes.