Biologically-assisted treatment of soils both contaminated with Heavy Metals and Polycyclic Aromatic Hydrocarbons

AGNELLO, A. C.; HUGUENOT, D.; VAN HULLEBUSCH, E.D.; ESPOSITO, G.

Champs-sur-Marne

Otro; Summer School on Contaminated Soils: from characterization to remediation; 2012

Université Paris-Est

This research plan proposes the study and development of enhancedphytoremediation technologies as an environmental remediation method to treatcontaminated soils with heavy metals (HMs) and polycyclic aromatic hydrocarbons(PAHs). Biodegradable compounds are proposed as amendments to improve thephytoremediation efficiency in an environmentally friendly approach.Phytoremediation is an emerging remediation technology that can be definedas the use of plants to remove pollutants from the environment or to make themharmless (Interstate Technology and Regulatory Cooperation (ITRC) Work Group,2001). Phytoextraction and rhizodegradation are two types of phytoremediationtechnologies that can be both used together to clean-up contaminated soils (Pivetz,2001). In phytoextraction, plants have a central role as HMs are taken up by plantroots and translocated to the above ground tissues. In contrast, in rhizodegradationplants have a secondary role in the dissipation of organic contaminants. Plantsrelease root exudates that promote microbial growth and activity in the rhizosphere,which leads to the biodegradation of organic compounds such as PAHs. A key factorlimiting the efficiency of phytoremediation is the bioavailability of contaminants i.e.the ability of a pollutant to be transferred from a soil compartment to a livingorganism. For this reason, several compounds have been studied as amendmentsthat increase the bioavailability of contaminants (Evangelou et al., 2007). However,many of them are synthetic and not biodegradable substances which tend to persistlong in the environment increasing the leaching risk and can even be toxic for plantsand microorganisms. In alternative, biodegradable compounds such as low molecularweight organic acids (LMWOAs) and surfactants have also been proposed. LMWOAsare strong ligands that can hold trace elements in solution by forming solublecomplexes with HMs, which may even be taken up by plant roots. LMWOAs arenaturally present in the soil rhizosphere as they are produced by plants andmicroorganisms and, unlike synthetic chelates, they are easily biodegradable andless phytotoxic (Jones, 1998). Furthermore, LMWOAs may also have a role in theremediation of organic contaminants such as PAHs. Surfactants are surface-activesubstances with amphiphilic chemical structure, which can increase the watersolubility, and consequently the bioavailability of hydrophobic compounds (Gao et al.,2007). Besides, surfactants also have a part in removing HMs from soil surfaces,probably through the formation of complexes, micelles and ion exchange processes.Biosurfactants, i.e. surfactants produced by microorganisms are more biodegradableand less toxic, making these compounds a better choice for surfactant-enhancedbioremediation (Mulligan, 2005).The main general objective of this project is to study the effect of LMWOAs andsurfactants on the phytoremediation of soils co-contaminated with HMs and PAHs.Specific objectives include: (1) studying the effects of LMWOAs and surfactants onthe HM and PAH bioavailability, (2) analyzing and comparing the phytoremediationefficiency of LMWOAs and surfactants, and (3) proposing a combined treatment with LMWOAs and surfactants suitable for the phytoremediation of co-contaminated soils.Zinc and pyrene are chosen as representative HM and PAH contaminants that areknown to be present in multipolluted soils (Ouvrard et al., 2011). Alfalfa (Medicagosativa) will be the candidate plant as it is fast growing and has an extensive rootsystem, both characteristics desired for phytoremediation species. Besides, alfalfahas been shown to be tolerant of HMs and PAHs and has been used to removethese contaminants from the soils (Peralta-Videa et al., 2002; Fan et al., 2008). Citricacid and Tween-80® will respectively be the main LMWOA and surfactant understudy.Research activities will involve laboratory, growth chamber and greenhousestudies using the following methodologies. The bioavailability of contaminants will bestudied by in situ soil solution extraction studies. The rate of HM phytoextraction willbe assessed by the determination of plant biomass, quantification of HMs in plantparts and soils and calculation of phytoextraction parameters. The study of PAHrhizodegradation will be achieved by the quantification of PAHs in soil, calculation ofPAHs removal rate and measurement of soil microbial biomass and activity.Through this research work it is expected to achieve new insights in theemerging environmental phytotechnologies contributing to the development of moreeffective applications of phytoremediation technologies to treat soils contaminatedwith HMs and PAHs.