Remediation technologies for metal-contaminated soils and groundwater: an evaluation

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Abstract

Metals including lead, chromium, arsenic, zinc, cadmium, copper and mercury can cause significant damage to the environment and human health as a result of their mobilities and solubilities. The selection of the most appropriate soil and sediment remediation method depends on the site characteristics, concentration, types of pollutants to be removed, and the end use of the contaminated medium. The approaches include isolation, immobilization, toxicity reduction, physical separation and extraction. Many of these technologies have been used full-scale. This paper will review both the full-scale and developing technologies that are available. Contaminants can be isolated and contained to minimize further movement, to reduce the permeability of the waste to less than 1×10−7 m/s (according to U.S. guidelines) and to increase the strength or bearing capacity of the waste. Physical barriers made of steel, cement, bentonite and grout walls can be used for isolation and minimization of metal mobility. Another method is solidification /stabilization, which contains the contaminants in an area by mixing or injecting agents. Solidification encapsulates contaminants in a solid matrix while stabilization involves formation of chemical bonds to reduce contaminant mobility. Another approach is size selection processes for removal of the larger, cleaner particles from the smaller more polluted ones. To accomplish this, several processes are used. They include: hydrocyclones, fluidized bed separation and flotation. Addition of special chemicals and aeration in the latter case causes these contaminated particles to float. Electrokinetic processes involve passing a low intensity electric current between a cathode and an anode imbedded in the contaminated soil. Ions and small charged particles, in addition to water, are transported between the electrodes. This technology have been demonstrated in the U.S. full-scale, in a limited manner but in Europe, it is used for copper, zinc, lead, arsenic, cadmium, chromium and nickel. The duration of time that the electrode remains in the soil, and spacing is site-specific. Techniques for the extraction of metals by biological means have been not extensively applied up to this point. The main methods include bioleaching and phytoremediation. Bioleaching involves Thiobacillus sp. bacteria which can reduce sulphur compounds under aerobic and acidic conditions (pH 4) at temperatures between 15 and 55°C. Plants such as Thlaspi, Urtica, Chenopodium, Polygonum sachalase and Alyssim have the capability to accumulate cadmium, copper, lead, nickel and zinc and can therefore be considered as an indirect method of treating contaminated soils. This method is limited to shallow depths of contamination. Soil washing and in situ flushing involve the addition of water with or without additives including organic and inorganic acids, sodium hydroxide which can dissolve organic soil matter, water soluble solvents such as methanol, nontoxic cations, complexing agents such as ethylenediaminetetraacetic acid (EDTA), acids in combination with complexation agents or oxidizing/reducing agents. Our research has indicated that biosurfactants, biologically produced surfactants, may also be promising agents for enhancing removal of metals from contaminated soils and sediments.

In summary, the main techniques that have been used for metal removal are solidification/stabilization, electrokinetics, and in situ extraction. Site characteristics are of paramount importance in choosing the most appropriate remediation method. Phytoremediation and bioleaching can also be used but are not as well developed.

Introduction

In the United States, 1200 sites are on the National Priority List (NPL) for the treatment of contaminated soils, indicating the extensiveness of this problem. Approximately 63% of the sites on the NPL include contamination from toxic heavy metals (Hazardous Waste Consultant, 1996). For example, lead was found at 15% of the sites, followed by chromium, cadmium and copper at 11, 8 and 7% of the sites, respectively. Therefore, metal contamination is a major problem.

Cadmium, copper, lead, mercury, nickel and zinc are considered the most hazardous and are included on the US Environmental Protection Agency's (EPA) list of priority pollutants (Cameron, 1992). Sources of metals include domestic and industrial effluents, the atmosphere, runoff and lithosphere. Once metals are allowed to pass through the municipal waste treatment facility, the heavy ones return to the environment where they are persistent, cannot be biodegraded and can thus follow a number of different pathways. The metals can adsorb onto the soil, runoff into rivers or lakes or leach in the groundwater, an important source of drinking water. Exposure to the heavy metals through ingestion or uptake of drinking water (particularly where water is reused) and foods can lead to accumulation in animals, plants and humans. This phenomenon can lead to extinction or alteration of plants and animals. Metals can accumulate in the following order, river sediments, bacteria, tubicids and then fish and man if one consumes these fish.

Over the past years, use of metals such as copper, cadmium and zinc have increased substantially (Table 1). Copper is produced more than any other metal, whereas more zinc reaches the soil than any other metal. Lead use has decreased due to toxicity concerns. In Canada, according to the National Pollutant Release Inventory, approximately 13,300 ton of copper, 9500 ton of zinc, 1300 ton of lead and 33 ton of cadmium were released to the air, water and soil (NPRI, 1995).

In view of the extensiveness of metals in the environment, this paper describes the fate and transport of selected metals and technologies for remediation that are full-scale and developing. This information will assist in the selection of the appropriate technology for treatment of metal-contaminated soils and sediments.

Section snippets

Mobility of metals

In its natural form, cadmium is relatively rare and concentrated in argillaceous and shale deposits as greenockite (CdS) or otavite (CdCO3) and is usually associated with zinc, lead or copper in sulfide form (Cameron, 1992). It is a bluish-white soft metal or grayish powder. It is more mobile, though, than zinc at low pH, particularly at pH values between 4.5 and 5.5. Above pH 7.5, cadmium is not very mobile. Its divalent form is soluble but it can also complex with organics and oxides. A

Metal speciation

The term speciation is related to the distribution of an element among chemical forms or species. Heavy metals can occur in several forms in water and soils. Interest has increased in sequential extraction techniques to relate the degree of mobility with risk assessment, (i.e. the more mobile the metal is, the more risk associated with it (Bourg, 1995)) and as a method of designing remediation techniques (Mulligan et al., 1999b). Not only is total metal concentration of interest, but is also

Isolation and containment

Contaminants can be isolated and contained, to prevent further movement, to reduce the permeability of the waste to less than 1×10−7 m/s (as required by the U.S. EPA) and to increase the strength or bearing capacity of the waste (USEPA, 1994). Physical barriers made of steel, cement, bentonite and grout walls can be used for capping, vertical and horizontal containment. Capping is a site-specific proven technology to reduce water infiltration. Synthetic membranes can be used for this purpose.

Conclusions

A summary of the various remediation techniques is shown in Table 3. Physical containment is the least expensive approach but this leaves the contaminants in place without treatment. Since metals are considered relatively immobile, methods for metal decontamination have focused on solid-phase processes such as solidification/stabilization and vitrification. These processes can be performed in situ which reduces handling costs. Costs depend on presence of debris, excess moisture, contaminant

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