Arsenic Remediation Technologies

Arsenic contamination of soil and groundwater is a widespread problem in certain areas and originates from many different sources. In many areas, arsenic is a diffuse pollutant that cannot be traced back to a single entity. Often, the contamination is naturally occurring or resulting from widespread activities such as irrigation. The costs of this type of cleanup usually cannot be charged to individuals; therefore the government often pays the cleanup costs. In other cases, especially in the United States, much of the arsenic contamination originates from industrial processes. Arsenic is often released by such enterprises through industrial wastes, materials, and products. These types of sources often cause a site-specific arsenic contamination problem. This article will focus on common remediation techniques used in the U.S. for arsenic point source remediation in soil and groundwater.

The new EPA arsenic MCL regulations for drinking water will require better remediation and cleanup technologies to achieve the required standards. Remediation is often performed at the contamination source. Many factors determine if a specific remediation technique is suitable for the particular site; three important factors are:

· Cleanup method must be effective for the pollutant source and the specific end use.

· Method must be tested and established.

· Cost must be appropriate.

The main categories of remediation techniques for soil and groundwater arsenic contamination are: containment, ex situ (contaminant removed and treated), and in situ (contaminant treated in place). Seven common remediation techniques will be discussed: stabilization, vitrification, soil washing, pyrometallurgical, soil flushing, electrokinetic, and phytoremediation.

Containment Technologies

Containment techniques aim to contain and isolate pollutants by preventing further movement and reducing the permeability of the waste (EPA, 1994). These techniques do not actually remove the arsenic; they only serve to prevent contamination from spreading further. Two specific techniques are stabilization and vitrification.

Stabilization

Stabilization, which reduces the mobility and hazard potential of arsenic in soil and groundwater through addition of binders, is established and very common in the U.S. This remediation technique physically binds arsenic within a stabilized area. This technique also chemically reduces the hazard potential of arsenic by converting it to less mobile or toxic forms. In this process, soil is mixed with binders by injection, while still in situ, to form a slurry or paste, which then cures to a solid form. This stabilized mass remains in place. The most common binders used are Portland cement and fly ash. Other possible binders are lime, cement kiln dust, phosphate, sulphur, and polymers. Stabilization is applicable for shallow contamination, since injection and construction equipment are typically used.

Vitrification

Vitrification is a high-temperature remediation process which aims to reduce the mobility of arsenic by incorporating it into a leach resistant and chemically durable mass. Vitrification may also cause arsenic to volatilize, therefore reducing the concentration in the soil. This technology has only been applied in a limited number of cases in the U.S. This remediation process requires a large amount of energy to achieve the proper temperatures; therefore vitrification is quite costly. During the vitrification process, arsenic is chemically bonded inside a glass matrix. This is achieved by passing an electric current through the soil by means of a group of electrodes. This produces temperatures of 2000 to 3600 degrees F, which allows vitrification as well as volatilization to occur. Sometimes the volatilized arsenic must be treated. Vitrification is applicable to shallow contamination and large volumes; however, high costs often limit feasibility.

Ex Citu Treatment Technologies

Ex citu treatment technologies involve removing the contaminant or contaminated soil from the original location; arsenic remediation is then performed either on-site or at an off-cite location. Ex citu treatment actually removes the arsenic pollution for the environment. Two types of arsenic ex citu treatment are soil washing and pyrometallurgical.

Soil Washing

Soil washing is an ex citu remediation technique in which the soil is removed and treated to lower the arsenic concentration; this technique is only used in a limited number of cases. This technique uses particle size separation to reduce the arsenic concentration in the soil. Accordingly, this technique is only applicable to soil in which the arsenic prefers to attach to the fine particles. Soil washing is performed by first screening the soil to remove coarse particles, then mixing the soil with a wash solution to remove the arsenic. The wash solution consists of water often mixed with acids, surfactants, leaching agents, or chelating agents, which assist in arsenic removal. The soil particles are then separated by size, thus concentrating the arsenic with the fines, which must then be further treated. The clean, coarser particles may then be returned to their original location.

Pyrometallurgical Recovery

Pyrometallurgical recovery remediation uses high temperature furnaces to convert arsenic in soil into a product with a high arsenic concentration, which may then be sold and reused. This technique is only used on a limited basis because the recovered arsenic product often costs more to produce than importing new arsenic (EPA, 1994). In order to perform pyrometallurgical recovery, arsenic must be concentrated in the soil, through processes such as soil washing, which concentrate the arsenic in the fines fraction. This product is then heated in furnaces to 200 700 degrees C in order to volatilize the arsenic, which is then recovered for reuse (Mulligan, 2000). This process is often performed off-site because of a lack of mobile equipment. Pyrometallurgical recovery is only applicable to certain highly contaminated soils.