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

Metals including lead, chromium, arsenic, zinc, cadmium, copper and mercury can cause significant damage to the environment and human health as a resu lt of their mobilities and solubilities. The selection of the most appropri ate soil and sediment remediation method depends on the site characteristic s, concentration, types of pollutants to be removed, and the end use of the contaminated medium. The approaches include isolation, immobilization, tox icity reduction, physical separation and extraction. Many of these technolo gies have been used full-scale. This paper will review both the full-scale and developing technologies that are available. Contaminants can be isolate d and contained to minimize further movement, to reduce the permeability of the waste to less than 1 x 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 /stabi lization, which contains the contaminants in an area by mixing or injecting agents. Solidification encapsulates contaminants in a solid matrix while s tabilization involves formation of chemical bonds to reduce contaminant mob ility. Another approach is size selection processes for removal of the larg er, cleaner particles from the smaller more polluted ones. To accomplish th is. 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 p rocesses involve passing a low intensity electric current between a cathode and an anode imbedded in the contaminated soil. Ions and small charged par ticles, in addition to water, are transported between the electrodes. This technology have been demonstrated in the U.S. full-scale, in a limited mann er but in Europe, it is used for copper, zinc, lead, arsenic, cadmium, chro mium and nickel. The duration of time that the electrode remains in the soi l, and spacing is site-specific. Techniques for the extraction of metals by biological means have been nor extensively applied up to this point. The m ain 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 degreesC. Pl ants such as Thlaspi, Urtica, Chenopodium, Polygonum sachalase and Alyssim have the capability to accumulate cadmium, copper, lead, nickel and zinc an d can therefore be considered as an indirect method of treating contaminate d soils. This method is limited to shallow depths of contamination. Soil wa shing and in situ Rushing involve the addition of water with or without add itives including organic and inorganic acids, sodium hydroxide which can di ssolve organic soil matter, water soluble solvents such as methanol, nontox ic cations, complexing agents such as ethylenediaminetrtraacetic acid (EDTA ), acids in combination with complexation agents or oxidizing/reducing agen ts. Our research has indicated that biosurfactants, biologically produced s urfactants, may also be promising agents for enhancing removal of metals fr om contaminated soils and sediments. In summary, the main techniques that have been used for metal removal are s olidification/stabilization. electrokinetics, and in situ extraction. Site characteristics are of paramount importance in choosing the most appropriat e remediation method. Phytoremediation and bioleaching can also be used but are not as well developed. (C) 2001 Elsevier Science B.V. AU rights reserv ed.