THE ROLE OF ORGANIC AMENDMENTS IN THE BlOREMEDIATION OF HEAVY METAL-POLLUTED SOILS

摘要

Pollution of land by heavy metals or metalloids (As, Cd, Cr, Cu, Mn, Mo, Ni, Pb, Se, U, Zn etc.) is a worldwide problem, which arises principally from anthropogenic sources such as mining, industry, and agriculture. Affected soils frequently have, inaddition to the contaminants, poor nutrient availability and structure, low organic matter content and microbial biomass, and restricted or non-existent vegetation cover. This can lead to further physical deterioration and erosion and hence disseminationof the contaminants. Organic amendments (compost, manures, peat, and agroindustrial by-products) can, to a certain extent, remedy such physico-chemical problems. In addition, these amendments can alter the distribution of heavy metals between soil fractions of differing "bioavailability". Their ability to do so is determined by the nature of the soil and that of the organic matter of the amendment, particularly its degree of humification, but also by the effects of the amendment upon factors such as soil pH, redox potential, and cation exchange capacity. Hence, highly-humified materials, for example peat or compost, as well as materials of high pH, able to immobilize heavy metals, are particularly effective for reducing metal bioavailability. The effects of organic amendments on heavy metal bioavailability and other soil chemical and physical properties can facilitate the re-establishment of the natural vegetation or, more commonly, active bioremediation in which the deliberate introduction of plantsand/or microorganisms is used to remove or detoxify the pollutant(s), physically stabilize the polluted soil or immobilize the pollutant(s). However, other properties of the amendments must be borne in mind, particularly their own concentrations of heavy metals and possible short- or long-term solubilization of heavy metals in the soil. This chapter includes discussion of the scale of heavy metal pollution, the particular problems associated with polluted soils, strategies for soil remediation, and therole of organic amendments in bioremediation and their mechanisms of action. In addition, a case study, reporting research into the use of organic amendments in the remediation of a site affected by a spill of acidic mine waste at Aznalcollar (Spain), is presented. It is suggested that the extent to which organic amendments will be used in future bioremediation schemes will depend on financial factors, particularly the cost of such procedures relative to other methods, and on legislation driving the recycling of organic materials.