Robert S. Bowman

Sorption of chromate and other inorganic anions by organo-zeolite

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 zeolites 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

Applications of surfactant-modified zeolites to environmental remediation

Abstract This paper summarizes recent work on environmental applications of surfactant-modified zeolite (SMZ), particularly for the removal of contaminants from water. SMZ was tested as a sorbent, and as a component of combined sorption/degradation media. Use of SMZ as a sorbent in a sub-surface permeable barrier was evaluated. A 15-week pilot test showed barrier retardation factors of 44 and 39 for chromate and PCE, respectively, similar to the values predicted from laboratory experiments. SMZ was also tested as a sorbent for organics present in oilfield wastewaters. Laboratory column experiments showed that major fuel components were effectively removed by SMZ and that the SMZ could be regenerated by air sparging. Finally, laboratory and field experiments showed that SMZ removed 99–100% of bacteria and viruses present in sewage effluent. The effectiveness of SMZ in combined sorption/degradation media was also evaluated. A permeable barrier pilot test with SMZ/zero-valent-iron pellets showed that chromate was totally removed from contaminated groundwater while PCE concentrations were reduced by two orders of magnitude. Finally, laboratory microcosm experiments showed that toluene-degrading organisms could be effectively cultured on SMZ.

Retention of inorganic oxyanions by organo-kaolinite.

A natural kaolinite (KGa-1b) was treated with the surfactant hexadecyltrimethylammonium bromide (HDTMA-Br) to a level twice that of the cation exchange capacity (CEC). Sorption of nitrate, arsenate, and chromate by the resultant organo-kaolinite was then quantified. Sorption of each oxyanion was well-described by the Langmuir isotherm. Sorption of nitrate was the greatest, with a Langmuir sorption maximum of 24 mmol/kg, although chromate showed the highest sorption affinity of 20 L/kg. Sorption of nitrate, arsenate, and chromate on organo-kaolinite was at least two orders of magnitude greater than their sorption on unmodified kaolinite. Desorption of the bromide counterion indicated that each of the oxyanions was retained by ion exchange on an HDTMA bilayer formed on the organo-kaolinite. Chromate sorption on the organo-kaolinite was unaffected by solution pH in the range 5-9, but decreased at pH 11 due to competition of OH- for anion exchange sites. Similarly, chromate exchange by organo-kaolinite was reduced in the presence of high background levels of chloride. Chromate was effectively retained when flowing through a packed bed of organo-kaolinite: after an input of more than 40 pore volumes, the effluent concentration of chromate was less than 10% of the input concentration, and 90% of the original HDTMA remained on the organo-kaolinite. The results demonstrate that properly prepared organoclays can remove oxyanions, as well as nonpolar organics, from contaminated waters.

New piecewise-continuous hydraulic functions for modeling preferential flow in an intermittent-flood-irrigated field

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.

SORPTION OF OXYANIONS BY SURFACTANT-MODIFIED ZEOLITE

ABSTRACT Zeolite has high internal and external surface areas and high internal and external cation exchange capacities suitable for surface modification by cationic surfactants. When the initial surfactant concentration is less than the critical micelle concentration, the sorted surfactant molecules primarily form a monolayer. Limited chromate sorption indicates that patchy bilayer may also form. When the initial surfactant concentration is greater than the critical micelle concentration and enough surfactant exists in the system, the sorbed surfactant molecules form bilayers, producing maximum chromate sorption. On a meq/kg basis, planar nitrate sorbs more on surfactant-modified zeolite surfaces than tetrahedral chromate. In the presence of sulfate or nitrate, chromate sorption is hindered due to competition for sorption sites. Quantitative sorption of nitrate and chromate and desorption of bromide indicate that the sorption of oxyanions is primarily due to surface anion exchange.

Preferential transport of nitrate to a tile drain in an intermittent-flood-irrigated field: Model development and experimental evaluation

A comprehensive field experiment was conducted near Las Nutrias, New Mexico, to study field-scale flow and transport in the vadose zone. The field data were analyzed in terms of a two-dimensional numerical model based on the Richards equation for variably saturated water flow, convection-dispersion equations with first-order chemical decay chains for solute transport, and bimodal piecewise-continuous unsaturated hydraulic functions to account for preferential flow of water and nitrate-nitrogen (NO3-N; loosely used as NO3 ) following flood irrigation events at the experimental site. The model was tested against measured NO3 flux concentrations in a subsurface tile drain, several monitoring wells and nested piezometers, and against resident NO3 concentrations in the soil profile (obtained at 52 spatial locations and four depths along a transect). NO3 transport at the field site could be described better with the bimodal hydraulic functions than using the conventional approach with unimodal van Genuchten-Mualem type hydraulic functions. Average resident nitrate concentrations measured across the soil profile were predicted reasonably well. However, NO3 flux concentrations in the subsurface tile drain and piezometers at the field site were occasionally underestimated or overestimated depending upon the irrigation sequence in three field benches, probably reflecting unrepresented three-dimensional regional flow/transport processes. Limiting the capture zone to a region closer to the tile drain did lead to a better match with observed sharp increases and decreases in predicted NO3 flux concentrations during the irrigation events. On the basis of this result we inferred that the preferential flow intercepted by the tile drain was generated in close proximity of the drain and essentially oriented vertically. In summary, our study suggests that irrigation scheduling in adjacent field plots, drainage design (e.g., spacing between tiles, drain depth, drain diameter) and effectiveness (e.g., drain blockage), preferential flow in (horizontal) surface-opened shallow cracks and (vertical) macropores, and transient regional groundwater flow can add significant uncertainty to the predictions of (local-scale) flow and transport to a tile drain.

Using multiple experimental methods to determine fracture/matrix interactions and dispersion of nonreactive solutes in saturated volcanic tuff.

The objective of this research was to investigate the effects of matrix diffusion on solute transport in fractured volcanic tuff. Two tuff cores were studied, one with a matrix porosity of 0.27 and the other with a porosity of 0.14. The matrix permeabilities of the cores were 4.7×10−15 and 7.8×10−19 m2, 5 and 9 orders of magnitude less than the respective fracture permeabilities. This suggested that the cores could be modeled as dual-porosity systems with no flow in the matrix but significant solute storage capacity. Two types of tracer tests were conducted in each fractured core: (1) iodide was injected in separate experiments at different flow rates and (2) two tracers of different matrix diffusion coefficients (bromide and pentafluorobenzoate (PFBA)) were injected in another test. A difference in the maximum concentrations of the solutes and the extended tailing of the breakthrough curves were assumed to be indicative of diffusive mass transfer between the fracture and the porous matrix of the cores. Interpreting the results from both methods allowed the identification of matrix diffusion and dispersion effects within the fracture by simultaneously fitting the data sets (with known constraints) using a relatively simple conceptual model. Estimates of mass transfer coefficients for the fractured cores were also obtained.

REGENERATION OF SURFACTANT-MODIFIED ZEOLITE AFTER SATURATION WITH CHROMATE AND PERCHLOROETHYLENE

Surfactant-modified zeolites (SMZ) have drawn recent attention as sorbents due to their removal of multiple types of contaminants and low material cost. However, like most sorbents, SMZ has a finite sorption capacity for different contaminants. The potential applications, economics, and efficiency of SMZ as a sorbent are related to the ability to regenerate the material. This paper reports several methods to regenerate chromate- and perchloroethylene-saturated SMZ. Regeneration of chromate-saturated SMZ was achieved by flushing with a sodium carbonate/sodium hydroxide solution. However, this high-pH solution increased the counterion competition for chromate sorption sites and decreased the chromate sorption capacity of the regenerated SMZ. As an alternative regeneration method, chromate sorbed to SMZ was reduced to Cr(III) in situ using sodium dithionite solution. Although reduction with dithionite restored the chromate sorption maximum, the chromate sorption intensity was lowered, possibly due to the competition by sulfate (generated from oxidation of dithionite) for chromate sorption sites. Carbonate-regenerated SMZ showed no loss of sorption affinity for perchloroethylene (PCE) compared to virgin SMZ. Air sparging of PCE-saturated SMZ fully regenerated the SMZ. The results show that it is possible to regenerate and re-use SMZ following saturation with anionic or nonpolar organic contaminants.

Pilot Test of a Surfactant-modified Zeolite Permeable Barrier for Groundwater Remediation

Two pilot-scale tests of surfactant-modified zeolite (SMZ) permeable barriers were conducted at the Large Experimental Aquifer Facility of the Oregon Graduate Institute. The tests were performed in an 8.5-m-wide, 8.5-m-long, 3-m-deep concrete tank. The SMZ was installed in a 1-m-wide, 6-m-long, 2-m-deep barrier frame in the center of the tank. The rest of the tank was filled with sand to form a simulated aquifer. A three-dimensional sampling array consisting of 405 sampling points was installed in the tank. Controlled water flow across the tank was maintained using ten upgradient injection wells and ten downgradient withdrawal wells. A specific discharge of 0.17 m day−1 was imposed, resulting in an average linear groundwater velocity of approximately 0.5 m day−1 in the sand. The upgradient wells allowed injection of a three-dimensional contaminant plume composed of 10 mg L−1 (0.19 mmolL−1) Cr, in the form of chromate, and 1.8 mg L−1 (0.011 mmol L−1) perchloroethylene (PCE).

Electrokinetic ion transport through unsaturated soil: 1. Theory, model development, and testing.

An electromigration transport model for non-reactive ion transport in unsaturated soil was developed and tested against laboratory experiments. This model assumed the electric potential field was constant with respect to time, an assumption valid for highly buffered soil, or when the electrode electrolysis reactions are neutralized. The model also assumed constant moisture contents and temperature with respect to time, and that electroosmotic and hydraulic transport of water through the soil was negligible. A functional relationship between ionic mobility and the electrolyte concentration was estimated using the chemical activity coefficient. Tortuosity was calculated from a mathematical relationship fitted to the electrical conductivity of the bulk pore water and soil moisture data. The functional relationship between ionic mobility, pore-water concentration, and tortuosity as a function of moisture content allowed the model to predict ion transport in heterogeneous unsaturated soils. The model was tested against laboratory measurements assessing anionic electromigration as a function of moisture content. In the test cell, a strip of soil was spiked with red dye No 40 and monitored for a 24-h period while a 10-mA current was maintained between the electrodes. Electromigration velocities predicted by the electromigration transport model were in agreement with laboratory experimental results. Both laboratory-measured and model-predicted dye migration results indicated a maximum transport velocity at moisture contents less than saturation due to competing effects between current density and tortuosity as moisture content decreases.