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

Hexavalent chromium (Cr^VI) 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 Cr^VI makes it a target contaminant to remove from water sources, however its high solubility makes complete removal difficult. Treatment of Cr^VI 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 Cr^VI to nFe⁰ surface reactive sites is followed by the reduction to Cr^III, corresponding to the oxidation of Fe⁰ to Fe²⁺. Another source of Fe²⁺ is from the oxidation of Fe⁰ by H⁺, which can reduce Cr^VI as well during the oxidation of Fe²⁺ to Fe³⁺. The production of Cr^III 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, Cr^VI 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 Cr^VI at the anode, nFe⁰ can be injected directly into the aquifer near the anode to reduce Cr^VI to Cr^III.

This study investigates 1) the capacity of surface stabilized nFe⁰ for Cr^VI reduction under oxic and anoxic conditions, the effects of humic acid and calcium on Cr^VI reduction, as well as toxic effects of nFe⁰ to microbial communities, and 2) optimizing the integration of electrokinetic and nFe⁰ technologies for Cr^VI 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 Cr^VI reduction. Adsorption isotherm studies will compare the effects of these constituents on Cr^VI removal.

Furthermore, laboratory-scale experiments were conducted with a continuous flow of simulated groundwater spiked with Cr^VI 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 Cr^VI 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 Cr^VI towards the cathode by water flow to promote Cr^VI accumulation near the anode. Surface stabilized nFe⁰ were injected into the sand column at specific time intervals directly adjacent to the anode reservoir where Cr^VI 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 Cr^VI removal by electrokinetic and nFe⁰ integration. The results from the aforementioned studies will be discussed during presentation.