Removal of polycyclic aromatic hydrocarbons from soil amended with biosolid or vermicompost in the presence of earthworms (Eisenia fetida)
Introduction
Earthworms are found in a wide range of soils and may represent 60–80% of the total soil biomass (Bouché, 1992). They are the largest and most easily quantified component of the soil biota and play a mayor role in soil fertility as soil mixers. Earthworms burrow through the soil turning it over continuously, maintaining its fertility and structure, and improving aeration and water infiltration capacity (Edwards, 1998). As such, earthworms function as soil engineers, changing the substrate through which they burrow, further developing the soil ecosystem (Eijsackers et al., 2001).
It has been reported that biostimulation with organic or inorganic fertilizers introduces additional nutrients into a contaminated ecosystem and increases the population of the indigenous microorganisms (Pankrantz, 2001). Recently, hydrocarbon removal from soil has been investigated using glucose, sawdust, manure, biosolids and compost as biostimulants (Namkoong et al., 2002). Biosolid is a source of nutrients for earthworms in vermicomposting (Contreras-Ramos et al., 2005) and contains microbes that might accelerate removal of hydrocarbons (Namkoong et al., 2002). The release of nutrients from biosolids is slow, thereby reducing possible losses and thus contamination of the environment.
Earthworms accumulate many lipophilic organic pollutants from the surrounding soil environment, not only through passive absorption through the body wall of the dissolved fraction in the interstitial water, but also by intestinal uptake during the passage of soil through the gut. This accumulation increases as the concentration of the pollutant in the soil environmental increases (Belfroid et al., 1995). The activities of earthworms thus contribute to the biodegradation of organic contaminants, such as phthalate, phenanthrene and fluoranthene (Ma, 1995). Some reports indicate that other annelids, such as aquatic Polychaetes, can metabolize benzo[a]pyrene, because they possess cytrochrome-P450 enzymes capable of degrading this compound (Driscoll and McElroy, 1997). The same enzymatic activity was found in terrestrial earthworms, such as Eisenia fetida (Achazi et al., 1998). Autochthonous microorganisms degrade hydrocarbons (Johnsen et al., 2005), but if earthworms are added to soil, they will improve aeration, and stimulate microbial activity, thus increasing biodegradation.
Few studies, however, have reported using biosolids, vermicompost and/or earthworms to remediate polycyclic aromatic hydrocarbons (PAHs)-contaminated soil. In this work, removal of three PAHs, phenanthrene (Phen), anthracene (Anth) and Benzo[a]pyrene (BaP) was monitored in sterilized or unsterilized soil with or without biosolid or vermicompost and with or without earthworms during an aerobic incubation at 22 ± 2 °C for 70 days. The objectives of this study were to investigate the effect of (i) the autochthonous soil microorganisms, i.e. unsterilized soil without earthworms, (ii) the earthworms, i.e. sterilized soil plus earthworms, (iii) a combination of the autochthonous soil microorganisms plus earthworms, i.e. unsterilized soil plus earthworms, and (iv) soil organic material amendments, i.e. biosolid or vermicompost on removal of PAHs in soil.
Section snippets
Materials used
Hydrocarbons were obtained from Sigma (USA) with purity >96% for Phen, >99% for Anth and >97% for BaP. Acetone was obtained from J.T. Baker (USA) with purity 99.7%. It would have been interesting to use 14C-labeled PAHs to study mineralization, but they cannot be imported into Mexico for security reasons.
Vermicompost and earthworms used
The earthworm E. fetida was cultivated in biosolid obtained from a wastewater treatment plant at Lerma, Edo. de México mixed with cow manure. The EC of the biosolid was 5.7 dS m−1 while that of
Results
On average, approximately 75% (C0) of the added Phen was extractable from soil at day zero (Table 1) and the concentration of Phen did not change significantly in sterilized soil over time independent of addition of organic material (Table 2, Table 3). The concentration of Phen decreased sharply in unsterilized soil and only 25% was recovered after 7 days (Table 3). The removal rate of Phen was fastest in soil with biosolid added and lowest in the unamended soil (Table 2). The removal rate of
Extractability of the PAHs
Song et al. (2002) reported recoveries of 93% for Anth, 74% for Phen, and 71% for BaP from soil with 98% sand using an ultrasonic method. Similar recoveries of 96% for Anth, 78% for Phen, and 90% for BaP were reported for the soil of Acolman using mechanical shaking and ultrasonication (Betancur-Galvis et al., 2006). Buco et al. (2004) also reported recoveries of 90% for BaP. Average recoveries (C0) in the work reported here were 99% for Anth, 75% for Phen and 100% for BaP at day zero (Table 1
Conclusion
Earthworms have a great potential to remove hydrocarbons from soil, even PAHs that are resistant to degradation, such as BaP. Earthworms are extremely resistant to toxic PAHs and tolerate concentrations normally not encountered in soil. Applying earthworms to a contaminated site might be an environmentally friendly way to remove hydrocarbons. However, a limitation might be the cost of the large amounts of earthworms required and the necessity to supply sufficient substrate while maintaining the
Acknowledgments
We thank M. Luna-Guido and J.M. Ceballos-Ramirez for technical assistance. The research was funded by the Department of Biotechnology and Bioengineering, Cinvestav and Secretaría del Medio Ambiente y Recursos Naturales (SEMARNAT) project FOSEMARNAT-2004-01-219. S.M. C.-R. and D.A.-B. received grant-aided support from Consejo Nacional de Ciencia y Tecnología (CONACyT), México.
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