The mobilization of metalloids in soils for plant uptake and leaching to groundwater can be minimized by reducing their bioavailability through chemical and biological immobilization. There has been interest in the immobilization of metalloids using a range of inorganic compounds such as lime, P fertilizers (e.g., phosphate rocks) and alkaline waste materials, and organic compounds such as bio-solids.
Immobilization of As may be achieved by (i) changing the physical properties of the soil so that As is more tightly bound and therefore becomes less bioavailable; (ii) chemically immobilizing As either by sorption onto a mineral surface or by precipitation as a discrete insoluble compound; and/or (iii) mixing the contaminated soil with uncontaminated soil, thereby increasing the number of As-binding sites.
A number of organic and inorganic amendments are known to immobilize a range of metalloids including As by chemical adsorption. These include ion-exchange resin, ferrous sulfate, silica gel, gypsum, clay minerals such as bentonite, kaolin, and zeolite, green sand, and liming materials.
These materials are naturally occurring and nontoxic with a large specific surface area and a significant amount of surface charge. The use of naturally occurring clay minerals such as zeolite as adsorbents is a novel method for the remediation of metalloid-contaminated soils.
Liming is increasingly being used as an important soil management practice in reducing the toxicity of certain metalloids in soils. In addition to the traditional agricultural lime, a large number of studies have examined the potential value of other liming materials as immobilizing agents in reducing the bioavailability of a range of metalloids in soils.
However, the effect of liming soils on As mobility has been rather inconsistent. Lime addition to As-contaminated soil induces the formation of CaH(AsO4) 2, thereby reducing the soluble As in the soil solution for plant uptake and leaching. However, the solubility product of this compound is greater than that for Fe and Al arsenates, which are readily formed in most soils.
BIOLOGICAL REMEDIATION ARSENIC
Bioremediation of soils contaminated with organic compounds such as pesticides and hydrocarbons is widely accepted in which native or introduced microorganisms and/or biological materials, such ascompost, animal manures, and plant residues, are used to detoxify or transform contaminants.
There has been increasing interest in the application ofthis technology for the remediation of metalloid-contaminated soils, especially for those metalloids that undergo biological transformation. Although it has several limitations, this technology holds continuing interest because of its cost effectiveness.
The unique aspect in bioremediation is that it relies mainly on natural processes and does not necessarily require the addition of chemical amendments other than microbial cultures and biological wastes. Because As undergoes biological transformation in soil, appropriate microorganisms may be used for the remediation of As contaminated soils. Existing and developing in situ bioremediation technologies may be grouped into the following two broad categories;
(I) Intrinsic bioremediation is where the essential materials required to sustain microbial activity exist in sufficient concentrations that naturally occurring microbial communities are able to degrade the target contaminants without the need for human intervention. This technique is better suited for remediation of soils with low levels of As over extensive areas.
(II) Engineered bioremediation relies on various approaches to accelerate in situ microbial degradation rates. This is accomplished by optimizing the environmental conditions by adding nutrients and/or an electron donor/acceptor, thus promoting the proliferation and activity of existing microbial consortia. It is favored for highly contaminated localized sites.
Three major approaches could be used in bioremediation of As-contaminated soils; (i) As could be immobilized into microbial cells through biosorption (bioaccumulation), (ii) toxic As(III) could be oxidized to less toxic As(V), and (iii) As compounds could be removed from the soil by volatilization.
- Bioaccumulation: Microorganism exhibits a strong ability to accumulation (bioaccumulation) As from a substrate containing very low concentration of this element. Bioaccumulation is activated by two processes, namely biosorption of As by microbial biomass and its by-products and physiological uptake of As by microorganisms through metabolically active and passive processes. Factors such as soils pH, moisture and aeration, temperature, concentration and speciation of As, soil amendments, and rhizosphere are known to influence the process of bioaccumulation of As in microbial cells. While a number of bacterial and fungal species have been known to bioaccumulate As, some algal species are also known to accumulate As.
- Microbial redox reactions: heterotrophic bacteria have been found to oxidize toxic As(III) in soils and sediments to less toxic As(V) and thus could play an important role in the remediation of h. Because As(V) is strongly adsorbed onto inorganic soil components, microbial oxidation could result in immobilization of As.
A dissimilatory metalloid reduction has the potential to be helpful mechanism for both intrinsic and engineered bioremediation of contaminated environments. Arsenic can be reduced to Aso, which is subsequently precipitated as a result of microbial sulfate reduction. Because arsenite is more soluble than As (V), the latter can be reduced to As(III) using bacteria in soil and subsequently leached.
(iii) Methylation of As: A variety of microbes could transform inorganic As into its metallic hydride or methylated forms. Due to their low boiling point and/or high vapour pressure, these compounds are susceptible to for volatilization and could easily be lost. Biomethylation of As in soils and aquatic environment is well documented as important in controlling the mobilization and subsequent distribution of arsenicals in the environment