Abstract
Anthropogenic activities, such as mining of natural resources, manufac-turing industries, modern agricultural practices and energy production have resulted in the release of heavy metals with resultant harmful im-pacts in some natural environments. Toxic heavy metals are harmful to living organisms even at low concentrations. Therefore, heavy metal contaminated sites should be remediated as heavy metals do not decompose into less harmful substances and are retained in the soil. Conventional methods are used for remediation of heavy metal contaminated soils such as heavy metal extraction, immobilization and removal of soils to landfill produce large quantities of toxic products including insoluble hydroxides and are rarely cost effective. The advent of bioremediation technologies like biosparging, bioventing and bioaugmentation has provided an alternative to conventional methods for remediating heavy metal contaminated soils. A subset of bacteria found in the rhizosphere has been found to increase the tolerance of plants to heavy metals in soil. These bacteria commonly known as plant growth promoting rhizobacteria or Plant Growth Promoting Rhizobacteria (PGPR) are showing promise as a bioremediation technique for the stabilisation and remediation of heavy metal contami-nated sites. PGPR can improve plant growth via a variety of mechanism including fixing atmospheric N to improve N status and making plants more tolerant of heavy metals. Scattered literature is harnessed to review the principles, advantages and disadvantages of the available technologies for remediating heavy metal contaminated soils and is presented.
Highlights
One of the environmental problems caused by industry is an increase in the concentration of heavy metals in the air, land and water
Plant growth promoting rhizobacteria or Plant Growth Promoting Rhizobacteria (PGPR) are a number of species of soil bacteria that grow in the rhizosphere of plants and stimulate plant growth by a variety of mechanisms [57]
Plant Growth promoting Rhizobacteria resource acquisition including assimilation of N from atmosphere, protection of host plant from contaminants concentrations because concentrations that are too high can be toxic to plants; pathogenic microorganisms and heavy metals atmospheric N) [74] [75], producing particular compounds used by plants [76] [77], solubilising nutrients (e.g. P, Fe), facilitating uptake of nutrients from soil (e.g. P, Fe) [78] [79] [80], protecting plants from possible microbial attack [81] [82], and decreasing the toxicity of heavy metals [25]
Summary
One of the environmental problems caused by industry is an increase in the concentration of heavy metals in the air, land and water. Heavy metals that are necessary for living organisms at low concentrations can become toxic at higher concentrations [2]. Heavy metals can enter organisms via direct soil ingestion, inhalation, dermal contact and intake through food and water [5]. Accumulation of heavy metals in soil is of concern to the agricultural production sector because of the potential threat to food quality and quantity as a result of increased absorption of heavy metals by plants [6]. Agricultural exports are internationally marketed on the basis of environmental safety and sustainability and so regulating heavy metal contamination is an important issue [7]-[13]
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