In many regions of the world, groundwater is contaminated with a high level of arsenic that must be treated before it can be safely used as drinking water. In situ immobilization of arsenic from groundwater within subsurface environment could have major advantages over the conventional above-ground chemical coagulation-precipitation treatment process. In this study, we develop a novel technique that can in situ emplace iron oxides onto the sand grain surface of porous media under mild chemical and temperature conditions. The technique involves sequential injections of a preconditioned ferrous iron solution and an oxidant solution and then orchestrate the advective-diffusive transport of the two reagents in porous media to create an overlapped reaction zone where ferrous iron is oxidized and precipitated on the sand grain surfaces. We demonstrate through bench-scale column tests the feasibility of using this technique to create a large-scale iron oxide-enriched reactive barrier in subsurface environment for in situ removal of arsenic. A sand filter with a fresh iron oxide coating can treat thousands of pore volumes of water contaminated with dozens of ppb arsenic before the coating needs to be regenerated. Arsenic breakthrough curves through the sand filter suggest that both reversible adsorption and irreversible precipitation are responsible for removing arsenic from the water. Unlike conventional excavate-and-fill permeable reactive barriers, the treatment capacity of our in situ created barrier can be in situ regenerated and replenished with a fresh coating.

Iron oxide-bearing minerals have long been recognized as an effective reactive media for arsenic-contaminated groundwater remediation. This research aimed to develop a technique that could facilitate in situ oxidative precipitation of Fe3+ in a soil (sand) media for generating a subsurface iron oxide-based reactive barrier that could immobilize arsenic (As) and other dissolved metals in groundwater. A simple in situ arsenic treatment process was successfully developed for treating contaminated rural groundwater using iron oxide-coated sand (IOCS). Using imbibition flow, the system facilitated the dispersive transport of ferrous iron (Fe2+) and oxidant solutions in porous sand to generate an overlaying blanket where the Fe2+ was oxidized and precipitated onto the surface as ferric oxide. The iron oxide (FeOx) emplacement process was significantly affected by (1) the initial surface area and surface-bound iron content of the sand, (2) the pH and solubility of the coating reagents, (3) the stability of the oxidant solution, and (4) the chemical injection schedule. In contrast to conventional excavate-and-fill treatment technologies, this technique could be used to in situ replace a fresh iron oxide blanket on the sand and rejuvenate its treatment capacity for additional arsenic removal. Several bench-scale experiments revealed that the resultant IOCS could treat arsenic-laden groundwater for extended periods of time before approaching its effective life cycle. The adsorption capacity for As(III) and As(V) was influenced by (1) the amount of iron oxide accumulated on the sand surface, (2) the system pH, and (3) competition for adsorption sites from other groundwater constituents such as silicon (Si) and total dissolved solids (TDS). Although the IOCS could be replenished several times before exhaustion, the life cycle of the FeOx reactive barrier may be limited by the gradual loss of hydraulic conductivity induced by the imminent reduction of pore space over time.

The overall objective of the present study was to evaluate the effectiveness of using a variety of hydrophobic biopolymers and hydrophobic biopolymer-coated sands as technically and economically feasible in-situ sediment amendments or alternative capping materials on a laboratory scale. Cutin from tomato peels, cellulolytic enzyme lignin from sitka spruce chips, and keratin azure from commercially dyed sheeps wool were isolated, prepared, tested, and evaluated as feasible hydrophobic biopolymers for the removal of selected Polycyclic Aromatic Hydrocarbons (PAHs). Testing included chemical and physical characterization, as well as the measurement of kinetics and equilibrium sorption parameters for the sorbates naphthalene, phenanthrene, and pyrene as model hydrophobic organic contaminants. Tomato peel cutin exhibited the largest overall affinity for PAHs, however, keratin azure was selected for further evaluation as the most feasible material due to its low preparation cost. Amendment of industrial sand with a stable, uniform, cross-linked keratin azure derivative was achieved to produce hydrophobic biopolymer-coated sand products containing zero, moderate, and high mass fractions of sand. Chemical and physical material parameters, as well as kinetics and equilibrium sorption parameters for the sorbates naphthalene, phenanthrene, and pyrene, were then obtained for the coated sand products. This allowed simple finite difference modeling of PAH fate and transport through a thin cap comprised of the same, insight into the specific sorption mechanisms involved, and information which could prove useful in predicting potential of keratin products to provide a suitable capping material. Conclusions and recommendations for future research focus on the technical and economical feasibility of the prepared hydrophobic biopolymers and hydrophobic biopolymer-coated sand products as capping or in-situ sediment amendments. / text