Robert W. Puls

Treatment of inorganic contaminants using permeable reactive barriers

Abstract Permeable reactive barriers are an emerging alternative to traditional pump and treat systems for groundwater remediation. This technique has progressed rapidly over the past decade from laboratory bench-scale studies to full-scale implementation. Laboratory studies indicate the potential for treatment of a large number of inorganic contaminants, including As, Cd, Cr, Cu, Hg, Fe, Mn, Mo, Ni, Pb, Se, Tc, U, V, NO 3 , PO 4 and SO 4 . Small-scale field studies have demonstrated treatment of Cd, Cr, Cu, Fe, Ni, Pb, NO 3 , PO 4 and SO 4 . Permeable reactive barriers composed of zero-valent iron have been used in full-scale installations for the treatment of Cr, U, and Tc. Solid-phase organic carbon in the form of municipal compost has been used to remove dissolved constituents associated with acid-mine drainage, including SO 4 , Fe, Ni, Co and Zn. Dissolved nutrients, including NO 3 and PO 4 , have been removed from domestic septic-system effluent and agricultural drainage.

Coupled Iron Corrosion and Chromate Reduction: Mechanisms for Subsurface Remediation

The reduction of chromium from the Cr(VI) to the Cr(III) state by the presence of elemental, or zero-oxidation-state, iron metal was studied to evaluate the feasibility of such a process for subsurface chromate remediation. Reactions were studied in systems of natural aquifer materials with varying geochemistry. Different forms of iron metal had significantly different abilities to reduce chromate, ranging from extremely rapid to essentially no effect. Impure, partially oxidized iron was most effective, with iron quantity being the most important rate factor, followed by aquifer material type and solid :solution ratio. Evidence for chromium-iron hydroxide solid solution (Cr x, Fe 1-x )(OH) 3 (ss) formation was obtained by electron probe microanalysis. A cyclic, multiple reaction electrochemical corrosion mechanism, enhanced by the development of an electrical double-layer analogue, is proposed to explain the differing iron reactivities and aquifer material effects.

The application of in situ permeable reactive (zero-valent iron) barrier technology for the remediation of chromate-contaminated groundwater : a field test

A small-scale field test was initiated in September 1994 to evaluate the in situ remediation of groundwater contaminated with chromate using a permeable reactive barrier composed of a mixture of zero-valent Fe, sand and aquifer sediment. The site used was an old chrome-plating facility located on a U.S. Coast Guard air base near Elizabeth City, North Carolina. Dissolved chromate concentrations were reduced to less than 0.01 mg/L via reduction from Cr(VI) to Cr(III) as a result of the corrosion of the Fe. As the Fe corrodes, pH increases, oxidation-reduction potential declines, dissolved oxygen is consumed, and Fe(II) is generated. Mineral phases formed as a result of the Fe corrosion include ferrous sulfides and various Fe oxides, hydroxides, and oxyhydroxides.

Effects of pH, solid/solution ratio, ionic strength, and organic acids on Pb and Cd sorption on kaolinite

Potentiometric and ion-selective electrode titrations together with batch sorption/desorption experiments, were performed to explain the aqueous and surface complexation reactions between kaolinite, Pb, Cd and three organic acids. Variables included pH, ionic strength, metal concentration, kaolinite concentration and time. The organic acids used were p-hydroxybenzoic acid, o-toluic acid, and 2,4-dinitrophenol. Titrations were used to derive previously unavailable aqueous conditional stability constants for the organometallic complexes. Batch results showed that aqueous lead-organic complexation reduced sorption of Pb by kaolinite. Cadmium behavior was similar, except for 2,4-dinitrophenol, where Cd sorption was increased. Metal sorption increased with increasing pH and decreasing ionic strength. Distribution ratios (Kds) decreased with increasing solid/solution ratio. The subsurface transport of lead and cadmium may be enhanced via complex interactions with organic wastes or their degradation products and sorbent mineral surfaces.

A two and half-year-performance evaluation of a field test on treatment of source zone tetrachloroethene and its chlorinated daughter products using emulsified zero valent iron nanoparticles

A field test of emulsified zero valent iron (EZVI) nanoparticles was conducted at Parris Island, SC, USA and was monitored for two and half years to assess the treatment of subsurface-source zone chlorinated volatile organic compounds (CVOCs) dominated by tetrachloroethene (PCE) and its chlorinated daughter products. Two EZVI delivery methods were used: pneumatic injection and direct injection. In the pneumatic injection plot, 2180 L of EZVI containing 225 kg of iron (Toda RNIP-10DS), 856 kg of corn oil, and 22.5 kg of surfactant were injected to remedy an estimated 38 kg of CVOCs. In the direct injection plot, 572 L of EZVI were injected to treat an estimated 0.155 kg of CVOCs. After injection of the EZVI, significant reductions in PCE and trichloroethene (TCE) concentrations were observed in downgradient wells with corresponding increases in degradation products including significant increases in ethene. In the pneumatic injection plot, there were significant reductions in the downgradient groundwater mass flux values for PCE (>85%) and TCE (>85%) and a significant increase in the mass flux of ethene. There were significant reductions in total CVOC mass (86%); an estimated reduction of 63% in the sorbed and dissolved phases and 93% reduction in the PCE DNAPL mass. There are uncertainties in these estimates because DNAPL may have been mobilized during and after injection. Following injection, significant increases in dissolved sulfide, volatile fatty acids (VFA), and total organic carbon (TOC) were observed. In contrast, dissolved sulfate and pH decreased in many wells. The apparent effective remediation seems to have been accomplished by direct abiotic dechlorination by nanoiron followed by biological reductive dechlorination stimulated by the corn oil in the emulsion.

Potential indicators for the assessment of arsenic natural attenuation in the subsurface

Abstract Arsenic is a priority pollutant found in soil and groundwater contaminated by arsenic pesticides and industrial wastes. Assessing the natural attenuation capacity of the subsurface for arsenic is a key step leading to successful site remediation. Chemical reactions between arsenic and several iron hydroxides (goethite, lepidocrocite and green rust) were studied. By comparing the arsenic attenuation capacity of iron hydroxides, clay minerals and feldspars, an order was established: iron hydroxides>clays>feldspars. Results showed that arsenic was more strongly bound by iron hydroxides than clays, and that the iron minerals studied can extensively oxidize As(III) to As(V). Under reducing conditions, the concentration of As(III) significantly decreases due to the presence of green rust and parasymplesite. The studies provide evidence that some minerals could act as mineral indicators for site characterization. Integration of mineral indicators with geochemical parameters should aid in determining the potential of natural attenuation of the subsurface with arsenic contamination.

Passive sampling of groundwater monitoring wells without purging: multilevel well chemistry and tracer disappearance

Abstract It is essential that the sampling techniques utilized in groundwater monitoring provide data that accurately depicts the water quality of the sampled aquifer in the vicinity of the well. Due to the large amount of monitoring activity currently underway in the U.S.A. it is also important that the techniques be efficient. It would be desirable to minimize the requirements of sampling time, equipment and quantity of contaminated waters pumped to the surface, without loss of data integrity. If representative samples could be acquired without purging the wells, increased sampling efficiency could potentially be achieved. Purging of multiple borehole volumes is largely routine, based on studies that show changes in the water chemistry as it stands in the casing and is subjected to atmospheric exposure at the top of the column. However, little data are available depicting water chemistry in the screened intervals of wells at equilibrium flow conditions, i.e. with little or no disturbance to the natural flow regime or disruption of the overlying casing waters. This study examines the differences in water chemistry between the casing and screened interval volumes of four wells at a field site, then compares the results to purged values for the same wells. Tracer experiments, utilizing both colloidal particles and dissolved species as tracers, are presented to illustrate differences in natural flushing between the screened and cased intervals. The data from the tracer removal were then utilized to estimate groundwater flow velocities in the vicinities of the boreholes. The results indicate that dedicated low-flow rate samplers set in the screened intervals are appropriate for the 6.1- and 7.6-m wells at this site and could be used without purging of the casing volume. The possibility exists that such passive sampling techniques would be useful at many of the sites currently undergoing routine monitoring.

Transport and transformation of hexavalent chromium through soils and into ground water

Abstract A detailed characterization of the underlying and adjacent soils of a chrome‐plating shop was performed to provide information on the extent of soil and aquifer contamination at the site and on the potential for off‐site migration and environmental impact. Intact, moist cores were obtained from more than 40 different locations, resulting in more than 200 discrete samples for total metal analysis, selective extraction tests, and adsorption‐reduction experiments, to assess the chemical speciation and distribution of chromium on the contaminated soils and its leaching potential. Surface analytical techniques were also used to determine chemical speciation and to further elucidate mineral fractions responsible for retention of the chromium on the soils and sediments. Adsorption and reduction capacities of the saturated aquifer sediments were variable and low, while the unsaturated soils’ reduction capacities were much greater and were correlated with depth (decreasing capacity with increasing depth)....

Multi-layer sampling in conventional monitoring wells for improved estimation of vertical contaminant distributions and mass

Abstract “Traditional” approaches to sampling groundwater and interpreting monitoring well data often provide misleading pictures of plume shape and location in the subsurface and the true extent of contamination. Groundwater samples acquired using pumps and bailers in conventional monitoring wells yield data which are largely dependent upon the length of the screened interval, the purging and sampling method employed, and the purge volume extracted prior to sample collection. Accurate delineation of plume boundaries and vertical concentration gradients is desirable, to accurately characterize waste sites and optimize remedial strategies. The objective of this study was to compare sampling results using four different sampling approaches and devices. Conventional monitoring wells were sampled with an electric submersible pump using low-flow sampling techniques and with a bailer using “traditional” sampling methods. The same wells were also sampled with a passive multi-layer sampling system (DMLS®, Margan Ltd.). Finally, aqueous concentrations were also determined in the formation adjacent to the monitoring wells studied using a Geoprobe® and short (30 cm) screens. Results indicated that “traditional” sampling methods can provide misleading information regarding contaminant distribution and mass and indeed can miss the presence of contamination altogether.

Forces dictating colloidal interactions between viruses and soil.

The fate and transport of viruses in soil and aquatic environments were studied with respect to the different forces involved in the process of sorption of these viruses on soil particles. In accordance with the classical DLVO theory, we have calculated the repulsive electrostatic forces and the attractive van der Waals forces. Bacteriophages have been used as model sorbates, while different clays have been used as model sorbents. The equations used for the determination of the change in free energy for the process (deltaG) takes into consideration the roughness of the sorbent surfaces. Results indicate that attractive van der Waals forces predominate the process of sorption of the selected bacteriophages on clays.