Contaminant sorption by soils and sediments is characterized as a multiple reaction phenomenon. The approach is predicated on the observation that most natural soils and sediments are intrinsically heterogeneous even at the microscopic scale; that is, variable in composition and structure at both interparticle and intraparticle scales. Heterogeneity is demonstrated for a number of soils which, on the basis of conventional macroscopic properties, would be considered "homogeneous". That such heterogeneities are reflected in sorption reactions which differ between soils and between different fractions of soil is also demonstrated. A composite model, the distributed reactivity model (DRM), is introduced to characterize intrinsic heterogeneities in the properties and behaviors of soils and sediments and to capture the resulting nonlinearities of sorption isotherms. Finally, the significance of particlescale heterogeneity and distributed reactivity is illustrated by using measured parameters and DRM calculations to characterize differences in the relative sorption behavior of soils comprising different mass fractions of differently reactive components.

The behavior, transport and ultimate fate of contaminants in subsurface environments may be affected significantly by their participation in sorption reactions and related phenomena. The degree to which the resulting effects can be quantified and predicted depends upon the extent to which certain fundamental aspects of sorption are understood, and upon the accuracy with which these phenomena can be characterized and modeled in complex subsurface systems. Current levels of understanding of the reactions and processes comprising sorption phenomena are discussed in this paper, as are the forms and utilities of different models used to describe them. Emphasis is placed on concept development, on the translation of these concepts into functional models for characterizing sorption rates and equilibria, and on the application of these concepts and models for explaining contaminant behavior in subsurface systems. Examples are provided to illustrate the impacts of sorption phenomena on contaminant transport.

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In this study, we comprehensively evaluate chloride- and ionic-strength-mediated changes in the physical morphology, dissolution, and bacterial toxicity of silver nanoparticles (AgNPs), which are one of the most-used nanomaterials. The findings isolate the impact of ionic strength from that of chloride concentration. As ionic strength increases, AgNP aggregation likewise increases (such that the hydrodynamic radius [HR] increases), fractal dimension (Df) strongly decreases (providing increased available surface relative to suspensions with higher Df), and the release of Ag(aq) increases. With increased Ag(+) in solution, Escherichia coli demonstrates reduced tolerance to AgNP exposure (i.e., toxicity increases) under higher ionic strength conditions. As chloride concentration increases, aggregates are formed (HR increases) but are dominated by AgCl(0)(s) bridging of AgNPs; relatedly, Df increases. Furthermore, AgNP dissolution strongly increases under increased chloride conditions, but the dominant, theoretical, equilibrium aqueous silver species shift to negatively charged AgClx((x-1)-) species, which appear to be less toxic to E. coli. Thus, E. coli demonstrates increased tolerance to AgNP exposure under higher chloride conditions (i.e., toxicity decreases). Expression measurements of katE, a gene involved in catalase production to alleviate oxidative stress, support oxidative stress in E. coli as a result of Ag(+) exposure. Overall, our work indicates that the environmental impacts of AgNPs must be evaluated under relevant water chemistry conditions.

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Produced water contains large amounts of various hazardous organic compounds such as benzene, toluene, ethylbenzene, and xylenes (BTEX). With increasing regulations governing disposal of this water, low-cost treatment options are necessary.This study evaluated the effectiveness of surfactant-modified zeolite (SMZ) for removal of BTEX from produced water. The long-term effectiveness of SMZ for BTEX removal was investigated along with how sorption properties change with long-term use. The results from these investigations showed that SMZ successfully removes BTEX from produced water, and that SMZ can be regenerated via air-sparging without loss of sorption capacity. The BTEX compounds break through laboratory columns in order of decreasing water solubility and of increasing K ow . The most soluble compound, benzene, began to elute from the column at 8 pore volumes (PV), while the least soluble compounds, ethylbenzene and xylenes, began to elute at 50 PV. After treating 4500 pore volumes of water in the column system over 10 sorption/regeneration cycles, no significant reduction in sorption capacity of the SMZ for BTEX was observed. The mean Laboratory columns were upscaled to create a field-scale SMZ treatment system.The field-scale system was tested at a produced water treatment facility near Wamsutter, Wyoming. We observed greater sorption of BTEX in field columns tests than predicted from laboratory column studies. In the field column, initial benzene breakthrough occurred at 10 PV and toluene breakthrough began at 15 PV, and no breakthrough of ethylbenzene or xylenes occurred throughout the 80 PV experiment. These results, along with the low cost of SMZ, indicate that SMZ has a potential role in a cost-effective produced water treatment system.ii ACKNOWLEDGEMENTS

Treatment of nontraditional source waters (e.g., produced water, municipal and industrial wastewaters, agricultural runoff) offers exciting opportunities to expand water and energy resources via water reuse and resource recovery. While conventional polymer membranes perform water/ion separations well, they do not provide solute-specific separation, a key component for these treatment opportunities. Herein, we discuss the selectivity limitations plaguing all conventional membranes, which include poor removal of small, neutral solutes and insufficient discrimination between ions of the same valence. Moreover, we present synthetic approaches for solute-tailored selectivity including the incorporation of single-digit nanopores and solute-selective ligands into membranes. Recent progress in these areas highlights the need for fundamental studies to rationally design membranes with selective moieties achieving desired separations.