Risk management of contaminated sites includes source reduction, site remediation, and environmental protection. Selection of optimal risk management strategies requires consideration of core objectives such as technical practicability, feasibility, and cost effectiveness of the strategy and wider environmental, social, and economic impacts.
Arriving at an optimal risk management solution for a specific contaminated site involves three main phases of the decision-making process. These include problem identification, development of problem solving alternatives (i.e., remediation technologies), and management of the site. The next section discusses the various remediation technologies considered suitable for managing As-contaminated soil and aquatic environments.
REMEDIATION OF ARSENIC CONTAMINATED SOIL
Remediation of As-contaminated soil involves physical, chemical, and biological approaches that may achieve either the partial/complete removal of As from soil or the reduction of its bioavailability in order to minimize toxicity. A large variety of methods have been developed to remediate metalloids-contaminated sites. These methods can also be applicable for the remediation of As contaminated soils. The selection and adoption of these technologies depend on the extent and nature of As contamination, type of soil, characteristics of the contaminated site, cost of operation, availability of materials, and relevant regulations.
Major physical in situ treatment technologies to remediate metalloid contaminated sites include capping, soil mixing, soil washing, and solidification. The simplest technique for reducing the toxic concentration of As in soils is mixing the contaminated soil with uncontaminated soil. This results in the dilution of As to acceptable levels. This can be achieved by importing clean soil and mixing it with As-contaminated soil or redistributing clean materials already available in the contaminated site.
Another dilution technique, especially in cultivated soils, relies on deep ploughing, during which the vertical mixing of the contaminated surface soil with less contaminated subsoil reduces the surface contamination, thereby minimizing the potential for As uptake by plants and ingestion of As by grazing animals.
However, in this method the total concentration of As in soil will remain the same. Soil washing or extraction has also been used widely for the remediation of metalloid-contaminated soils in Europe and this method may be applicable for As-contaminated soils to some extent.
The contaminated soil maybe washed with different concentrations of hydrogen fluoride, phosphoric acid, sulfuric acid, hydrogen chloride, nitric acid, per chloric acid, hydrogen bromide, acetic acid hydrogen peroxide, 3:1 hydrogen chloride–nitric acid, or 2:1 nitric acid– perchloric acid. Phosphoric acid proved to be most promising as an extractant, attaining 99.9% As extraction at 9.4% acid concentration. Sulfuric acid also attained a high percentage extraction. The acid-washed soil was further stabilized by the addition of lanthanum (La), cerium (Ce), and iron (FeIII) salts or their oxides/hydroxides, which form an insoluble complex with dissolved As. Both salts and oxides of La and Ce were effective in immobilizing As in the soil attaining less than 0.01 mg liter-1 As in the leachate.
The success of soil washing largely depends on speciation of As present in the contaminated soils, as it is based on the desorption or dissolution of As from the soil inorganic and organic matrix during washing with acids and chelating agents. Although soil washing is suitable for off-site treatment of soil, it can also be used for on-site remediation using mobile equipment.
However, the high cost of chelating agents and choice of extractant may restrict their usage to only small-scale operations. Arsenic-contaminated soil may be bound into a solid mass by using materials such as cement, gypsum, or asphalt. However, there are issues associated with the long-term stability of the solidified material.
Capping the contaminated sites with clean soil is used to isolate contaminated sites as it is less expensive than other remedial options. Such covers should obviously prevent upward migration of contaminants through the capillary movement of soil water.
The depth of such cover or “cap” required for contaminated sites should be assessed carefully. Where the water table is shallow enough to supply water to the surface (i.e., 1.5 to 2 m in most soils), dissolved As could take less than10 years to reach the surface.
Remediation, based on chemical reactions, is becoming increasingly popular largely because of a high rate of success. A number of methods have been developed mainly involving adsorption, immobilization, precipitation, and complexation reactions. However, such methods are often expensive for the remediation of large areas.
Two approaches are often used in the chemical remediation of metalloid-contaminated soils: (i) immobilization of metalloids using inorganic and organic soil amendments in order to reduce their bioavailability and (ii) mobilization of metalloids and their subsequent removal through plant uptake (phytoremediation) or soil washing.
This section discusses the immobilization techniques used for the remediation of As-contaminated soil. Chemical immobilization is achieved by mainly through adsorption/precipitation of As in contaminated sites through the addition of soil amendments.