Rare-Earth Elements as Natural Tracers for In Situ Remediation of Groundwater.

The utility of rare-earth elements (REEs) as natural geochemical tracers for the analysis of groundwater remediation was examined in several example permeable reactive barriers (PRBs). The PRBs utilize zero-valent iron and organic carbon plus limestone mixtures for contaminant treatment. Zero-valent iron removed REEs from groundwater to below detection levels (2-4 ng/L) and subsequent rebound of REE concentrations in regions down-gradient of the treatment zones was not observed. In addition, REE concentrations within and down-gradient of an organic carbon/limestone PRB were significantly reduced to <1% of influent levels. Thus, REEs are sensitive tracers for evaluating the interaction of groundwater with materials placed in the subsurface for contaminant remediation. Analysis of geochemical tracers for understanding in situ remediation becomes important in situations where down-gradient contaminant concentrations fail to decrease within expected timeframes. The field data indicated that increased solid-phase partitioning of REEs occurred with increasing pH and heavy REEs were preferentially removed compared to light REEs in ZVI systems. In the organic carbon PRB, unexpected negative europium anomalies were observed, revealing new information about redox conditions within the treatment zone. REE concentrations and shale-normalized profiles can be used as natural tracers to better understand in situ technologies for groundwater remediation.

Treatment of arsenic, heavy metals, and acidity using a mixed ZVI-compost PRB.

A 30-month performance evaluation of a pilot permeable reactive barrier (PRB) consisting of a mixture of leaf compost, zerovalent iron (ZVI), limestone, and pea gravel was conducted at a former phosphate fertilizer manufacturing facility in Charleston, SC. The PRB is designed to remove heavy metals and arsenic from groundwater by promoting microbially mediated sulfate reduction and sulfide-mineral precipitation and arsenic and heavy metal sorption. Performance monitoring showed effective treatment of As, Pb, Cd, Zn, and Ni from concentrations as high as 206 mg L(-1), 2.02 mg L(-1), 0.324 mg L(-1), 1060 mg L(-1), and 2.12 mg L(-1), respectively, entering the PRB, to average concentrations of <0.03 mg L(-1), < 0.003 mg L(-1), < 0.001 mg L(-1), < 0.23 mg L(-1), and <0.003 mg L(-1), respectively, within the PRB. Both As(III) and As(V) were effectively removed from solution with X-ray absorption near edge structure (XANES) analysis of core samples indicating the presence of As(V) in oxygen-bound form and As(III) in both oxygen- and sulfur-bound forms. XANES solid phase sulfur analysis indicated decreases in the peak amplitude of intermediate oxidized sulfur species and sulfate components with increasing distance and depth within the PRB.

In situ chemical reduction of Cr(VI) in groundwater using a combination of ferrous sulfate and sodium dithionite: a field investigation.

A field study was conducted to evaluate the performance of a ferrous iron based in situ redox zone for the treatment of a dissolved phase Cr(VI) plume at a former industrial site. The ferrous iron based in situ redox zone was created by injecting a blend of 0.2 M ferrous sulfate and 0.2 M sodium dithionite into the path of a dissolved Cr(VI) plume within a shallow medium to fine sand unconfined aquifer formation. Monitoring data collected over a period of 1020 days after more than 100 m of linear groundwater flow through the treatment zone indicated sustained treatment of dissolved phase Cr(VI) from initial concentrations between 4 and 8 mg/L to less than 0.015 mg/L. Sustained treatment is assumed to be primarily due to the reduction of Cr(VI) to Cr(III) by ferrous iron adsorbed to, precipitated on, and/or incorporated into aquifer iron (hydr)oxide solid surfaces within the treatment zone. Precipitated phases likely include FeCO3 and FeS based on saturation index considerations and SEM/EDS analysis. The detection of solid phase sulfites and thiosulfates in aquifer sediments collected from the treatment zone more than 2 years following injection suggests dithionite decomposition products may also play a significant role in the long-term treatment of the dissolved phase Cr(VI).

Treatment of hexavalent chromium in chromite ore processing solid waste using a mixed reductant solution of ferrous sulfate and sodium dithionite.

We investigated a method for delivering ferrous iron into the subsurface to enhance chemical reduction of Cr(VI) in chromite ore processing solid waste (COPSW) derived from the production of ferrochrome alloy. The COPSW is characterized by high pH (8.5-11.5) and high Cr(VI) concentrations in the solid phase (up to 550 mg kg(-1)) and dissolved phase (3-57 mg L(-1)). The dominant solid-phase minerals are forsterite (Mg2SiO4), brucite (Mg-(OH)2), and hydrocalumite [Ca4(Al, Fe)2(OH)12X x 6H2O), X = (OH)2(2-), SO4(2-), CrO4(2-)]. The method utilizes FeSO4 in combination with Na2S2O4 to inhibit oxidation and precipitation of the ferrous iron, thereby preventing well and formation clogging. Laboratory batch tests using a 0.05 M FeSO4 + 0.05 M Na2S2O4 solution indicated effective treatment of both dissolved and solid-phase Cr(VI). Contrary to treatments with FeSO4 and FeCl2 alone, the combination resulted in both complete removal of Cr(VI) from solution and sustained Fe(ll) concentrations in solution after a 24 h period. A field test involving injection of 5700 L of a 0.07 M FeSO4 + 0.07 M Na2S2O4 solution into a COPSW saturated zone (pH 11.5) indicated no well and formation clogging during injection. Examination of a core collected 0.46 m from the injection well following injection indicated effective treatment of the solid phase Cr(VI) based on analysis of water, phosphate solution, and high temperature alkaline extracts. The combined reductant solution also imparted a residual treatment capacity to the COPSW allowing for subsequent treatment of dissolved phase Cr(VI); however, dissemination of the iron in the highly alkaline environment appeared to be impeded by the inability to sufficiently lower the pH with distance from the injection well to avoid precipitation of Fe(OH)2 and likely also FeCO3. Injection of a 0.2 M FeSO4 + 0.2 M Na2S2O4 solution into another COPSW saturated zone (pH 9) indicated much more effective dissemination of the injected iron.

A permeable reactive barrier for treatment of heavy metals.

Historical storage of ore concentrate containing sulfide minerals at an industrial site in British Columbia, Canada, has resulted in widespread contamination of the underlying soil and ground water. The oxidation of sulfide minerals has released significant quantities of heavy metals, including Cu, Cd, Co, Ni, and Zn, into the ground water. A pilot-scale, compost-based, sulfate-reducing permeable reactive barrier was installed in the path of the dissolved heavy-metal plume. The permeable reactive barrier uses sulfate-reducing bacteria to promote precipitation of heavy metals as insoluble metal sulfides. Monitoring over a 21-month period indicated significant removal of heavy metals within the barrier. Copper concentrations declined from a mean concentration of 3,630 pg/L in the influent to a mean concentration within the barrier of 10.5 microg/L, Cd from 15.3 microg/L to 0.2 microg/L, Co from 5.3 microg/L to 1.1 microg/L, Ni from 131 pg/L to 33.0 microg/L, and Zn from 2,410 microg/L to 136 pg/L. Within the lower half of the barrier where tidal influences were more limited and sulfate-reducing conditions were better maintained, mean treatment levels of 2.9 microg/L (Cu), 0.1 microg/L (Cd), 0.4 microg/L (Co), 2.7 microg/L (Ni), and 6.3 microg/L (Zn) were observed.