We performed batch sorption experiments that showed a significantly enhanced removal of inorganic oxyanions from aqueous solution by clinoptilolite-dominated zeolite modified by the quaternary amine hexadecyltrimethylammonium (HDTMA). Since HDTMA is too large to enter into the internal portion of the zeolite, sorption of the amine only occurred on the zeolite's external exchange sites. HDTMA was exchanged with extrastructural cations of the zeolite up to the external cation-exchange capacity. The HDTMA-modified surface was stable when exposed to extremes in pH and ionic strength and to organic solvents. While the natural zeolite had no affinity for the oxyanions, the HDTMA-modified zeolite showed significant removal of chromate, selenate, and sulfate from 0.005 M CaCL aqueous solution. Sorption data for each anion were well-described by the Langmuir isotherm equation. We found that anion sorption was highest when the zeolite was modified such that HDTMA satisfied 100% of its external cation-exchange capacity. The mechanism of anion retention appears to be the formation of an HDTMAanion precipitate on the zeolite surface.

We determined the effect of selected counterions (Cl-, Br-, and HSO4 -) on the sorption of the cationic surfactant hexadecyltrimethylammonuium (HDTMA) on clinoptilolite zeolite and on the subsequent sorption of chromate by HDTMA−zeolite. The HDTMA sorption on the zeolite, as characterized by the Langmuir sorption maximum, followed the trend HDTMA-Br > HDTMA-Cl > HDTMA-HSO4 (208, 151, and 132 mmol/kg, respectively). The same counterion trend was observed for HDTMA sorption on KGA-1 kaolinite. Measurement of counterion sorption indicated that HDTMA-Br and HDTMA-Cl formed complete bilayers on the zeolite, whereas HDTMA-HSO4 showed less than full bilayer formation. Competitive sorption between HDTMA-Br and HDTMA-Cl on the zeolite also showed a preference for the Br- counterion. The counterion stabilization of HDTMA admicelles on the zeolite surface follows the same trends as the counterion stabilization of micelles in solution. Chromate sorption was also strongly influenced by the HDTMA−zeolite counterion, with chromate sorption maxima decreasing in the order HDTMA-HSO4 > HDTMA-Cl > HDTMA-Br (28, 16, and 11 mmol/kg, respectively). The sorption of chromate and other divalent anions on HDTMA−zeolite results from a combination of entropic, Coulombic, and hydrophobic effects, all of which are functions of the initial HDTMA counterion. In the design of surfactant-modified clays and zeolites for environmental applications, the strong influence of the surfactant counterion must be considered.

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The sorption of nonpolar hydrophobic organic compounds by soil organic matter has long been attributed to a partitioning mechanism, with the sorption coefficient proportional to the fractional organic carbon content of the soil. However, deviations from this linear proportionality have been observed and reported in the literature by many authors. In our study a natural zeolite was modified with a cationic surfactant to achieve different fractional organic carbon contents and different surfactant molecule configurations on the surface. The sorption of perchloroethylene (PCE) by the surfactant-modified zeolite (SMZ) was found to be dependent on the bound surfactant molecule configuration as well as on the fractional organic carbon content. Below monolayer coverage by the surfactant, the PCE sorption coefficient on SMZ was proportional to the fractional organic carbon content. Above monolayer coverage, increasing fractional organic carbon content resulted in minimal further increase in the PCE sorption coefficient. The change in PCE sorption behavior was attributed to the structural differences between sorbed surfactant monolayers and bilayers. The surfactant surface configuration has a significant impact on the effective volume and density of the bound organic phase that is responsible for partitioning nonpolar organic compounds. The ratio of the organic carbon-based distribution coefficient (K oc) for the monolayer versus that for the bilayer systems was 1.7, similar to the estimated bilayer to monolayer hydrocarbon density of 1.3. Our results reinforce the notion that the structure of natural organic matter as well as its quantity controls the sorption of nonpolar organics to soils and sediments.

Abstract. Modeling water flow in macroporous field soils near saturation has been a major challenge in vadose zone hydrology. Using in situ and laboratory measurements, we developed new piecewise-continuous soil water retention and hydraulic conductivity functions to describe preferential flow in tile drains under a flood-irrigated agricultural field in Las Nutrias, New Mexico. After incorporation into a two-dimensional numerical flow code, CHAIN_2D, the performance of the new piecewise-continuous hydraulic functions was compared with that of the unimodal van Genuchten-Mualem model and with measured tile-flow data at the field site during a number of irrigation events. Model parameters were collected/estimated by site characterization (e.g., soil texture, surface/ subsurface saturated/unsaturated soil hydraulic property measurements), as well as by local and regional-scale hydrologic monitoring (including the use of groundwater monitoring wells, piezometers, and different surface-irrigation and subsurface-drainage measurement systems). Comparison of numerical simulation results with the observed tile flow indicated that the new piecewise-continuous hydraulic functions generally predicted preferential flow in the tile drain reasonably well following all irrigation events at the field site. Also, the new bimodal soil water retention and hydraulic conductivity functions performed better than the unimodal van Genuchten-Mualem functions in terms of describing the observed flow regime at the field site.

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

We determined the sorption of ionizable organic solutes on a natural zeolite modified with hexadecyltrimethylammonium (HDTMA), a cationic surfactant. The sorption of benzene and its ionizable analogues phenol and aniline by surfactant-modified zeolite (SMZ), prepared at different HDTMA surface coverages, was affected by solution pH. All of the sorption isotherms were linear and could be described by a distribution coefficient (K d ). At neutral pH, the K d values of benzene, phenol, and aniline on SMZ increased with HDTMA loading up to monolayer coverage of 100 mmol/kg. Beyond monolayer coverage, further increases in HDTMA loading did not increase the K d values of the solutes at pH 7.0, where all species exist primarily in their neutral forms. The K d values were consistent with the relative octanol-water partition coefficients of the three compounds, indicating that sorption of the neutral species was primarily by partitioning into the bound HDTMA organic pseudophase. Phenol sorption by SMZ treated to bilayer coverage increased as the pH, and hence fraction of anionic phenolate, increased. The counterion balance indicated that the increased retention of phenol was due partially to anion exchange of phenolate for bromide, the same mechanism responsible for sorption of inorganic anions by SMZ. In contrast, decreases in pH resulted in reduced aniline sorption due to a lower concentration of the neutral species and repulsion of the positively charged anilinium from SMZ treated to bilayer coverage. The results demonstrate that sorption of target species can be maximized by tailoring the HDTMA surface coverage to account for species and solution characteristics.

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