Manipulating Bioavailability to Manage Remediation of Metal Contaminated Soils.
Nanthi Bolan, Soil and Earth Sciences, Turitea, Massey University, Palmerston North, New Zealand
Unlike organic contaminants, most metals do not undergo microbial or chemical degradation and the total concentration of these metals in soils persists for a long time after their introduction. For diffuse distribution of metals, remediation options generally include amelioration of soils to minimise the metal bioavailability. Bioavailability can be minimised through chemical and biological immobilisation of metals using a range of inorganic and organic compounds. The more localised metal contamination found in urban environments is remediated by metal mobilization processes that include bioremediation (including phytoremediation) and chemical washing. In this paper, the role of various inorganic and organic soil amendments in the (im)mobilization of metals in soils in relation to managing their remediation will be presented. Bioavailability of a chemical in the soil environment has been defined as the fraction of the total contaminant in the interstitial porewater (i.e., soil solution) and soil particles that is available to the receptor organism. Bioavailability of metals in soils can be examined using chemical extraction and bioassay tests. Chemical extraction tests include single extraction and sequential fractionation. Bioassay involves plants, animals and microorganisms. A range of chemical extractants including mineral acids, salt solutions, buffer solutions and chelating agents have been used to predict the bioavailability of metals in soils. Sequential fractionation schemes are often used to examine the redistribution or partitioning of metals in various chemical forms. The bioavailability of metals in soils has recently been examined using the in-vitro Physiologically Based Extraction Test and the In-Vitro Gastrointestinal method. Measurements of metal bioavailability and toxicity in soils using soil microorganisms are receiving increasing attention, as microorganisms are more sensitive to heavy-metal stress than plants or soil macrofauna. The rapid development of molecular techniques and their continued successful application to the study of soil microbial ecology and function provides significant future potential for the monitoring of soil pollution impacts. A number of amendments are used either to mobilise or immobilize heavy metals in soils. The basic principle involved in the mobilization technique is the release the metals into soil solution, which are subsequently removed using plants. Whereas in the case of the immobilization technique the metal concerned is removed from soil solution through adsorption, complexation and precipitation reactions, thereby rendering the metals unavailable for plant uptake and leaching to groundwater. Chelating agents which have high affinity for metal ions can be used to enhance the solubilization of metals in soils through the formation of soluble metal chelates. These compounds have been found to be very effective in the solubilization of metals such as Cu and Pb, thereby enhancing their subsequent uptake by plants. However, these chelating may induce the solubilization of other than the target metals which may be phytotoxic. Furthermore the solubilization of metals can result in their increased leaching to groundwater, especially in the absence of active plant growth. A large number of studies have provided conclusive evidence for the mitigative value of phosphate compounds to immobilize metals in soils, thereby reducing their bioavailability for plant uptake and mobility for transport. Phosphate compounds enhance the immobilization of metals in soils through various processes including: direct metal adsorption, phosphate anion-induced metal adsorption, and precipitation of metals with solution P as metal phosphates. Precipitation as metal phosphates has been proved to be one of the main mechanisms for the immobilization of metals, such as Pb and Zn in soils. These fairly stable metal-phosphate compounds have extremely low solubility over a wide pH range, which makes P application an attractive technology for managing metal-contaminated soils. Although liming is primarily aimed at ameliorating soil acidity, it is increasingly being accepted as an important management tool in reducing the toxicity of heavy metals in soils. Liming, as part of the normal cultural practices, has often been shown to reduce the concentration of Cd, Pb and other metals in edible parts of crops. The effect of liming materials in decreasing metal uptake by plants has been attributed to both decreased mobility in soils and to the competition between Ca2+ and metals ions on the root surface. The major sources of organic composts include biosolid and animal manures. Immobilization of metals by these amendments is achieved through adsorption, complexation and redox reactions. Metals form both soluble and insoluble complexes with organic constituents in soils, thereby regulating their bioavailability. Since one of the primary objectives of remediating contaminated sites is to manipulate the bioavailability of metals, in-situ (im)mobilization using some of the soil amendments that are low in heavy metal content may offer a promising option. However, a major inherent problem associated with the mobilization technique is that in the absence of active plant growth the solubilized metal may be subject to leaching. Similarly in the case of the immobilisation technique the immobilised metal may become plant available with time through natural weathering process or through breakdown of high molecular weight organic-metal complexes.