280957 Hexavalent Chromium Remediation by Electrokinetic Transport and Zero-Valent Iron Nanoparticle Injection: Effects of Organic and Inorganic Groundwater Constituents

Monday, October 29, 2012: 9:33 AM
326 (Convention Center )
Ryan Thacher and Massoud Pirbazari, Sonny Astani Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, CA

Hexavalent chromium (CrVI) contamination is prevalent in aquifers around the world as a result of numerous anthropogenic activities. Chromium is used in many industrial processes for corrosion inhibition and also for adding pigment in dyes, paints, inks, and plastics. The highly toxic nature of CrVI makes it a target contaminant to remove from water sources, however its high solubility makes complete removal difficult. Treatment of CrVI in aquifers is generally limited to ex situ methods such as pump-and-treat or soil excavation, which are often costly and invasive. The growing field of nanotechnology however offers potential for in situ groundwater remediation. Nanoparticles can be injected directly into aquifers for remediation purposes, and can travel long distances with water flow through soil pore space treating contaminants as they move. Zero-valent iron nanoparticles (nFe0) have the capacity to react with many common pollutants such as heavy metals and radionuclides, by reducing them to insoluble precipitates through an adsorption-reduction process mechanism. Mass transfer of CrVI to nFe0 surface reactive sites is followed by the reduction to CrIII, corresponding to the oxidation of Fe0 to Fe2+. Another source of Fe2+ is from the oxidation of Fe0 by H+, which can reduce CrVI as well during the oxidation of Fe2+ to Fe3+. The production of CrIII is advantageous due to its negligible toxicity and low solubility leading to precipitation as Cr(OH)3, or Cr-Fe (oxy)hydroxides on the nano iron surface.

Once injected into groundwater, nFe0 will react with most chemical and biological species present. Groundwater contains natural organic matter and minerals, which have been found to interact with nFe0, and affect their reductive capacity. Nonetheless, it is important that nFe0 may be toxic to microbial communities present in the aquifer materials. In many cases these microbial communities are naturally assisting in the remediation process.

In this study a groundwater treatment strategy of integrating electrokinetic contaminant transport with nFe0 injection is evaluated. Through the application of an electric field across a contamination plume in an aquifer, CrVI will migrate towards the anode electrode due to the negative charge carried by the chromate ion. This technique is especially advantageous in aquifers that are difficult to access, such as below developed land, or in soils of low hydraulic conductivity. Upon concentration of CrVI at the anode, nFe0 can be injected directly into the aquifer near the anode to reduce CrVI to CrIII.

This study investigates 1) the capacity of surface stabilized nFe0 for CrVI reduction under oxic and anoxic conditions, the effects of humic acid and calcium on CrVI reduction, as well as toxic effects of nFe0 to microbial communities, and 2) optimizing the integration of electrokinetic and nFe0 technologies for CrVI remediation in a soil column. The interactions between nFe0 and organic and inorganic constituents under oxic and anoxic conditions will be discussed in terms of their effect on CrVI reduction. Adsorption isotherm studies will compare the effects of these constituents on CrVI removal.

Furthermore, laboratory-scale experiments were conducted with a continuous flow of simulated groundwater spiked with CrVI pumped through a 40-mesh silica sand soil column under an applied electric potential. The columns were oriented such that water flowed from anode to cathode, while CrVI migrated in the opposite direction towards the anode. The electric potential application was varied from 0.5-2.0 V/cm such that transport by electrokinetic phenomena counteracted the advective transport of CrVI towards the cathode by water flow to promote CrVI accumulation near the anode. Surface stabilized nFe0 were injected into the sand column at specific time intervals directly adjacent to the anode reservoir where CrVI concentrations were highest. Samples were taken from multiple locations within the soil column, as well as from the cathode reservoir effluent to determine the degree of CrVI removal by electrokinetic and nFe0 integration. The results from the aforementioned studies will be discussed during presentation.

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