Phytoremediation of polyaromatic hydrocarbons, anilines and phenols

Abstract

Phytoremediation technologies based on the combined action of plants and the microbial communities that they support within the rhizosphere hold promise in the remediation of land and waterways contaminated with hydrocarbons but they have not yet been adopted in large-scale remediation strategies. In this review plant and microbial degradative capacities, viewed as a continuum, have been dissected in order to identify where bottle-necks and limitations exist. Phenols, anilines and polyaromatic hydrocarbons (PAHs) were selected as the target classes of molecule for consideration, in part because of their common patterns of distribution, but also because of the urgent need to develop techniques to overcome their toxicity to human health. Depending on the chemical and physical properties of the pollutant, the emerging picture suggests that plants will draw pollutants including PAHs into the plant rhizosphere to varying extents via the transpiration stream. Mycorrhizosphere-bacteria and -fungi may play a crucial role in establishing plants in degraded ecosystems. Within the rhizosphere, microbial degradative activities prevail in order to extract energy and carbon skeletons from the pollutants for microbial cell growth. There has been little systematic analysis of the changing dynamics of pollutant degradation within the rhizosphere; however, the importance of plants in supplying oxygen and nutrients to the rhizosphere via fine roots, and of the beneficial effect of microorganisms on plant root growth is stressed. In addition to their role in supporting rhizospheric degradative activities, plants may possess a limited capacity to transport some of the more mobile pollutants into roots and shoots via fine roots. In those situations where uptake does occur (i.e. only limited microbial activity in the rhizosphere) there is good evidence that the pollutant may be metabolised. However, plant uptake is frequently associated with the inhibition of plant growth and an increasing tendency to oxidant stress. Pollutant tolerance seems to correlate with the ability to deposit large quantities of pollutant metabolites in the 'bound' residue fraction of plant cell walls compared to the vacuole. In this regard, particular attention is paid to the activities of peroxidases, laccases, cytochromes P450, glucosyltransferases and ABC transporters. However, despite the seemingly large diversity of these proteins, direct proof of their participation in the metabolism of industrial aromatic pollutants is surprisingly scarce and little is known about their control in the overall metabolic scheme. Little is known about the bioavailability of bound metabolites; however, there may be a need to prevent their movement into wildlife food chains. In this regard, the application to harvested plants of composting techniques based on the degradative capacity of white-rot fungi merits attention.

Contamination and remediation of phthalic acid esters in agricultural soils in China: a review

“…Phytoremediation is relatively inexpensive and is an environmentally friendly approach. However, the plant growth may be inhibited under high concentrations of contaminants (Harvey et al 2002), which will limit the remediation effects. Plants used for phytoremediation are generally species with a small biomass and little economic value.…”

Phytoremediation of PAHs-contaminated Sludge in a Constructed Wetland

“…The literature indicates that the number of total rhizosphere microflora and PAHdegrading microorganisms are both higher in rhizosphere soil than in non-rhizosphere soil, and that the latter selectively increases in rhizosphere soil [19]. it has been reported that Pah dissipation corresponded to rhizosphere microflora density [20]. Additionally, root exudates are capable of providing nutrients to rhizosphere microorganisms in stabilized sludge and eliminating contaminant toxicity; as such, they increase soil heterogeneity, promote microbial growth and finally enhance the PAH degradative potential of rhizosphere microflora.…”

“…Muratova et al used reed and alfalfa to phytoremediate Pah-contaminated soil in pot experiments [22]. They achieved PAH removals of 68.7% and 74.5%, respectively, and the total number of rhizosphere microorganisms and Pah-degraders in alfalfa-vegetated soil were 1.3 and 0.7 times greater than those in non-rhizosphere soil, respectively [20]. additionally, Cui constructed reed-planted pilot-scale wetlands for sludge stabilization in which reed roots extended through the granular media layer to the sludge layer after two years of sludge loading and one year of natural resting [23].…”

Tracing the behaviour of hexachlorobenzene in a paddy soil-rice system over a growth season

“…Leaf waxes act as a barrier to the penetration of organic pollutants and with the aging of leaves, the quantity of epicuticular waxes accumulated on the leaf surface increases (Nakajima et al, 1996). This probably leads to the more intensive uptake of hydrocarbons by young leaves than by old ones (Harvey et al, 2002). Besides, the same dilution effect for HCB in roots because of plant growth applies to the HCB concentration in shoots.…”