Removal of Heavy Metals from Contaminated Soils

Introduction

Soil contamination is a worldwide environmental problem. It is caused by either solid or liquid hazardous substances, such as toxic trace metal, mixing with the naturally occurring soil. Contaminated areas are often those with mining activities or agricultural land affected by the use of metal-containing fertilizers. Mining and mineral processing in particular generate large quantity of by-product mine wastes. These are collected in mine waste piles and tailings, and may cause severe soil contamination through spillage. The recent incident at the Talvivaara mining site in Sotkamo, Finland, is an example where a major tailings dam leakage has caused severe soil contamination by releasing waste water with high nickel and zinc concentration into the nearby area.

The main focus of this report is the removal of heavy metal contaminants from soil. These include essential heavy metals such as Fe, Cu, Mn and Ni, those which are needed by living organisms, and non-essential heavy metals such as Cd, Pb, As and Hg, which are not needed for any biochemical fundctions. Heavy metals are non-biodegradable and will accumulate in the environment. High concentrations of heavy metals beyond threshold limits pose high risks to the environmental and human health. Apart from mining, other anthropogenic sources of heavy metals include smelting, electroplating, the use of pesticide and fertilizer in agriculture, industrial discharge etc. Should heavy metals enter the food chain through soil contamination they can cause adverse health problems such as lead poisoning, kidney and brain damage.

Over the past few decades there have been increasing interests in development of technologies for the remediation of contaminated soils. The mar...

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...s such as chelators that provide key nutrients and protection against plant pathogen. However the main effect of these microbes is that their soil biological activities affect the speciation and mobility of metal ions, and therefore influences plant metal tolerance and uptake [Whiting et al.2001]. Studies have shown that some microbes can increase the metal uptake of plants [Robinsons et al.2009]. For example Chopra et al. (2007) showed that microorganism in soil from an Arsenic contamination site increased the As uptake of Agrostis tenuis by 45%.

The effectiveness of rhizoremediation highly depends on the ability of plant to promote the growth of microbes and colonize the root region. It can occur naturally plant roots release complex aromatic compounds such as flavonoids in the rhizosphere, which help stimulate the growth and activity of degrading bacteria.