Permeable Reactive Barriers for the Removal of Heavy Metals: Lab-Scale Experiments with Low-Grade Magnesium Oxide by Andre´s Navarro, Josep M Chimenos, David Muntaner, and A. Ine´s Ferna´ndez Abstract The attenuation of heavy metals in ground water depends on factors such as solid-solution equilibrium or the solubility product (K sp ) of the solid phase. Metals are pH dependent, and the solubility of heavy metal hydroxides such as lead, cad- mium, zinc, and nickel is minimal within pH range 9 to 11. MgO as material in a passive remediation system for removing heavy metals from contaminated ground water is very useful because of its minimal environmental impact and low solubility reaches a maximum pH of 10. Low-grade MgO (LG-MgO) was found to be suitable and economically feasible for use as permeable reactive barrier filler for remediation of ground water contaminated by heavy metals. The results of this study suggest that the coprecipitation of heavy metal hydroxides and magnesium hydroxide on the LG-MgO particle surface and sorption in the neoformed precipitates are the main mechanisms of heavy metal release from contaminated ground water. Remediation is not affected by the presence of other metals, which ensures that the range of application is very broad in both subterranean and surface water. LG-MgO used as a reactive medium for the treatment of contaminated water significantly di- minishes the amount of heavy metals in ground water. Introduction The difficulty and high cost of conventional techniques for the remediation of contaminated ground water, such as impermeable barriers and systems to extract and treat ground water, have in recent years caused in situ passive techniques to be favored. These techniques are grouped under the name PIR (passive in situ remediation). They include permeable reactive barriers (PRBs), constructions that are permanent and semipermeable or replaceable. Such barriers intercept the pollutants that move in ‘‘feather’’ or ‘‘plume’’ flows through the saturated zone (Vidic and Pohland 1996; Mc- Govern et al. 2002; Wilkin and McNeil 2003; Ferna ´ndez- Sa ´nchez et al. 2004). The main advantage of PRBs compared to conventional techniques is a reduction in the costs of oper- ation and maintenance. However, the biggest drawback of PRBs is the difficulty of quantifying their average life span. Precipitation, adsorption, and other phenomena within the PRBs lead to a reduction in pore space and hydraulic con- ductivity, and also to a decrease in the reactive capacity of the medium. Therefore, the barrier needs to be renewed after a few years (EPA 1998) or has to be built in such a way that worn-out material can be extracted easily (Day et al. 1999). The materials used in reactive barriers should not cause counterproductive reactions, toxic intermediary products, or products that react with contaminants to detrimental effect. Likewise, the material should not interrupt the flow of ground water, should be low cost, and should have a long service life. Thus, the characterization of both the reactive materials and those in the aquifer is vital in analyzing the effect of variables on barrier efficiency. The mobility of heavy metals in ground water is con- trolled by reactions that cause metals to be absorbed or to precipitate, as well as by chemical reactions that keep met- als associated with the solid phase. A range of technology is available for the remediation of contaminated ground water by immobilization or removal of heavy metals (Evanko and Dzombak 1997). The use of PRBs for the remediation of pollutants such as Cr and halogenated sol- vents has been relatively common (Day et al. 1999). Sev- eral types of treatment barriers that arrest the transport of metals are being tested (Blowes et al. 2000; Naftt et al. 2002). Trench reactive materials that are being investigated include zeolite, hydroxyapatite, sulfate-reducing bacteria, elemental iron, and limestone, mainly for the remediation of Pb and hexavalent Cr (Vidic and Pohland 1996; Evanko and Dzombak 1997; Ludwig et al. 2002). Previous experi- ences in the laboratory and in fieldwork have shown that it is possible to remove metals such as Hg, U, and Cu by Copyright ª 2006 The Author(s) Journal compilation ª 2006 National Ground Water Association. 142 Ground Water Monitoring & Remediation 26, no. 4/ Fall 2006/pages 142–152