Bioremediation and Rhizoremediation of Polychlorinated Biphenyls (PCBs) Contaminated Soils  

Project leaders

Angelo Massacci, Asha Iuwarkar

Agreement

INDIA - CSIR-expired - Council of Scientific and Industrial Research

Call

CNR/CSIR 2012-2014

Department

Earth and Environment

Thematic area

Earth system science and environmental technologies

Status of the project

New

Research proposal

This proposal aims to investigate in the laboratory the potentiality of rhizo-assisted degradation of polychlorinated biphenyls (PCBs) following a preliminary anaerobic degradation without plants. The proposed activities will be carried out propaedeutically to the remediation of the PCB contaminated site (SIN, DM Ambiente 8/7/02) of Papigno (Terni) as foreseen in a phytoremediation plan submitted to the Environment Ministry and approved with few prescriptions. The industrial site of Papigno was built around 1900 and closed around 1970, it was built on a surface of 105.450 m2 near the Nera River and  Papigno village. The chemical plant produced calcium carbide (CaC2)  and Cianamide (CH2N2 / H2NCN). In front of the plant site there was an area used as a landfill plant ("uncontrolled") for the industrial waste of the process.  After the "industrial age", the local municipality  built two football fields and a little park on a portion of this area; the rest of the area was abandoned. After the adoption of the national legislation for soil investigation  the site was characterized; the public park and the sport area were closed till the cleaning up of the area. The main contaminanants of the soil and top soil are heavy hydrocarbons and PCB . The presence of PCBs with the maximum concentration of 3 mg per Kg has been localized in two small spots of few square metres and until 1 m depth. The approved plan is to remove all the PCB contaminated soil and to place it in a confined area where it will be cleaned by applying plant assisted bioremediation.
PCBs are very difficult contaminants to remediate. They are a class of 209 synthetic congeners, in which one to ten atoms are attached to a biphenyl ring (Ceccarini and Giannarelli, 2005). All congeners are lipophilic, those with the highest number of chlorines are most recalcitrant because chemically very stable, very toxic and. long range transportable. Their major source of environmental contamination is the illegal or improper dumping of PCBs hazardous wastes. Italian law imposes a concentration threshold limit of 1 microgram per Kg for industrial soils. Currently used techniques for reducing PCB concentration, for example hydride reduction, hydrodechlorination, dechlorination using metals, photolysis and gamma-radiation, oxidation, electrolysis, mechano-chemical degradation are both of difficult application in situ and thus expensive. Further, some of these methodologies may not degrade completely some PCB congeners. For these reasons, and given the scarcity of economic resources in these years, the cost-effective and ecofriendly technologies based on natural processes and proved to be efficient in decontaminating soils from PCBs have gained more and more interest. Bioremediation is one of these natural methods that is worth of consideration for degrading PCBs in soils (Ohtsubo et al., 2004). Its limitation is that some PCBs, those with many chlorines, can be only partially degraded by some bacteria under anaerobic conditions (Wegel and Wu, 2000). On the contrary, lower molecular weight PCBs, included those originated from the anaerobic dechlorination, seem to be completely degraded by other bacteria under aerobic conditions (Focht, 1995). Both processes, aerobic and anaerobic, are essential to a rapid PCBs soil remediation and difficult that combine under natural conditions. An opportunity is given in planted soil by rhizosphere activities where apparently at microsite level the aerobic and anaerobic conditions would alternate every twenty minutes due to spikes of oxygen release by plants to the rhizosphere soil (Smith, 1976). There are no studies that apply these finding to the remediation of PCBs. This proposal within the bilateral collaboration between CSIR and CNR, if funded, could be an opportunity to gain insights on these rhizosphera phenomena and open to a further innovation of remediation technologies based on natural methods. It has to be underlined, however, that not all plants can be applied for this rhizoremediation studies. Among the few that could be very interesting are certainly salicaceae, a family of trees that are very fast growing and can grow at very high density, up to 10,000 plants per hectare (Marmiroli et al., 2011). These two characteristics seem to be favorable to the enhancement of bacterial rhizoremediation under both aerobic and anaerobic conditions. It is, in fact, known that fast and indeterminately growing plants need to fix high amount of carbon and nitrogen to sustain more and more photosynthesizing and efficient organs. Nitrogen can be the limiting factor for this investment and thus these plants need to send roots a lot of the photosynthetically fixed carbon (between 20 and 30 percent) for increasing the root surface and then uptake more nitrogen from the soil. The carbon sent to roots reaches apical cells where it should be used for root elongation (Lynch, 1987). The high carbon concentration building up in these cells make root leaks of these substances to the rhizosphere very likely. In this case all kinds of bacteria starving close to roots will compete for this eventually leaked carbon. The simultaneous presence of some amount of oxygen, also leaking from same cells in between the few mm distance from the root surface, stimulated therein the proliferation and the activities of the aerobic microflora. The aerobic condition lasts until oxygen concentration is depleted around creating de facto the condition for a transient anaerobic condition. In synthesis, the rich and biodiverse microflora of the rhizosphere could alternate their aerobic and anaerobic activity co-metabolizing even recalcitrant PCBs by removing chlorines and preparing the biphenyl ring to the further or the complete aerobic breaking down. The integration of these activities from a micro-rhizosphera level to a large rhizosphera such that of a high density salicaceae plantation could be expected to effectively reduce the PCB soil concentration of large contaminated sites. Such expectation can be will be tested by the two collaborating groups that submit this proposal with the aim to study natural based methods to rapidly destroy the recalcitrant forms of PCBs. At the same time the proposal will include research activities focusing on the comprehension of various aspects of PCB biodegradation. For example, the joint proposal will study 1) the role and the effectiveness of secondary plant metabolites in the induction of specific metabolic degradation by specialized bacteria, 2) the effect of amendments with biosurfactants on PCBs bioavailability.
Management of PCBs contaminated soils is thus going to be a big challenge in near future due their persistence and toxicity to the environment. Innovative, cost-effective technologies based on natural methods assisted by rhizosphere activities could be a formidable opportunity to cope with environmental criticalities whose remediation is limited by the high economical costs.

Aon MA, Cabello M.N, Sarena D.E, Colaneri A.C, Franco M.G, Burgos J.L, Cortassa S 2000. Spatio-temporal patterns of soil microbial and enzymatic activities in an agricultural soil. Applied Soil Ecology 18(3 ) 239-254
Ceccarini, A and Giannarelli S 2005. Polychlorobiphenyls In Chromatographic Analysis of the Environment, Third Edition. Nov 2005 , 667 -709
Focht, 1995. Strategies for improvement of aerobic metabolism of polychlorinated biphenyls. Curr Opin Biotech 6:341-346
Lynch, J.M. 1987. The rhizosphere. Chichester: Wiley Interscience.
Marmiroli M, Pietrini F. Maestri E, Zacchini M, Marmiroli N and Massacci A (2011) Growth, physiological and molecular traits in Salicaceae trees investigated for phytoremediation of heavy metals and organics. Tree Physiol doi:10.1093/treephys/tpr088 (in press)
Ohtsubo Y, Kudo T., Tsuda M, Nagata Y. 2004. Strategies for bioremediation of polychlorinated biphenyls. Appl. Microbiol Biot 65:250-258.
Smith A. M. 1976.Ethylene in soil biology. Ann Review Phytopatol 14:53-73.
Wegel J.and Wu Q. 2000. Microbial reductive dehalogenation of polychlorinated biphenyls. FEMS Microbiology Ecology 32: 1-15.

Research goals

The objectives of this study are 1) to screen candidate plants for assisting microbial PCB degradation among fast growth trees that experimentally proved to release in the rhizosphere high amount of carbon and oxygen; 2) characterization of rhizosphere and bulk soil microbial populations for the ability to degrade PCB under aerobic and anaerobic conditions; 3) analysis of soil amendments (micorrhizae to ehance plant growth, bio-surfactants to increase PCB mobility, co-metabolites and metabolic inducers to activate the PCB degradation) to obtain higher rates of PCB degradation under aerobic and anaerobic conditions; 5) bioaugmentation of microbial PCB degraders under aerobic and anaerobic conditions and analysis of their persistence in the contaminated soil; 6) microcosm tests with soil sampled from contaminated sites with PCB under aerobic, anaerobic, xenic, and with or without the selected plants for rhizoremediation assisted PCB degradation.

Last update: 01/05/2024