Enhancing polychlorinated biphenyl dechlorination in fresh water sediment with biostimulation and bioaugmentation

Abstract

Polychlorinated biphenyls (PCBs) are toxic compounds ubiquitously distributed in ecosystems. Microbial attenuation of these contaminants is a potential means of remediation. Two promising microbial PCB remediation technologies, biostimulation and bioaugmentation, were investigated in different sediments. Biostimulation experiments in which electron donor was supplied (H₂ via elemental iron, Fe⁰) resulted in only a marginal improvement in the dechlorination of amended 2,3,4,5-tetrachlorobiphenyl (2,3,4,5-CB), likely because of an inadequate population of indigenous H₂-utilizing dechlorinators. Extensive dechlorination was observed, however, after bioaugmenting microcosms with a PCB-dechlorinating enrichment culture. Dechlorination of 2,3,4,5-CB began prior to the 20th day of incubation and proceeded to 2-chlorobiphenyl. This extensive dechlorination activity was maintained in both sediments over 70 d at 10 and 25 °C. This research demonstrates that although past studies of biostimulation were promising, a great deal must be known about the PCB-dechlorinating organisms present before successful biostimulation is expected. Bioaugmentation, however, appears to be a promising PCB remediation strategy and should be further pursued.

Introduction

Polychlorinated biphenyls (PCBs) are a family of anthropogenic compounds with physical and chemical properties that are beneficial for certain industrial applications (e.g., dielectric fluids, plasticizers, flame retardants, and carbonless paper). The same properties that made PCBs attractive for industrial use also rendered them recalcitrant, allowing them to accumulate in the environment. This raises concerns because exposure to PCBs has been linked to cancer and other adverse health effects, such as reproductive deficiencies, developmental complications, immuno-suppression, and dermatological disorders in humans and other animals (World Health Organization, 2003).

Established PCB treatment technologies such as incineration, landfilling, thermal desorption, and chemical dehalogenation can be costly (USD $50–$1000 per ton) and usually involve dredging or excavation followed by disposal (Dàvila et al., 1993, Caruana, 1997). In situ anaerobic bioremediation, however, has the potential to be both cost effective and cause minimal environmental disturbance. Bench-scale studies of anaerobic dechlorination of PCBs have encountered long lag periods, however, ranging from weeks to months. Researchers have attributed these long lag times to inadequate biomass, slow growth of dechlorinating bacteria, and hydrophobicity of PCBs (Tiedje et al., 1993). Two strategies proposed to accelerate microbial PCB dechlorination are stimulation of existing PCB dechlorinators (biostimulation) or inoculation of contaminated sites with exogenous PCB-dechlorinating cultures (bioaugmentation) (Bedard, 2003).

Bioaugmentation involves the addition of an active contaminant-degrading culture to enhance a desired degradation process (Witt et al., 1999). Bioaugmentation has proven to be a successful technology in both laboratory and field applications for contaminants like methyl tert-butyl ether (Salanitro et al., 2000), mixtures of benzene, toluene, ethylbenzene, and xylene (Da Silva and Alvarez, 2004), carbon tetrachloride (Mayotte et al., 1996, Dybas et al., 1998, Witt et al., 1999, Dybas et al., 2002), chlorinated ethenes (Ellis et al., 2000, Major et al., 2002), and chlorophenols (Quan et al., 2004). With respect to PCB contamination, indigenous microorganisms may not have the capability to dechlorinate PCBs, or more likely, may not be present in sufficient numbers to substantially desorb and dechlorinate the total mass of PCBs present in the sediment. The addition of exogenous PCB-dechlorinating microorganisms should create a PCB sink, leading to desorption of previously non-bioavailable PCBs. Several studies in which exogenous alternative chlorinated or brominated compounds were used to prime, or enrich, sediment for dechlorinators (Bedard et al., 1996, Bedard et al., 1997, Wu et al., 1997, Wu et al., 1999) suggest that PCB sinks can be established via increased PCB-dechlorinator populations. In these primed environments dechlorinating biomass is allowed to grow rapidly in the presence of high concentrations of available chlorinated or brominated electron acceptors. This ultimately leads to the extended PCB dechlorination of weathered contaminants. Bioaugmentation with a robust PCB-dechlorinating culture should yield similar results.

Studies in which anaerobic sediments were bioaugmented with enriched cultures of PCB-dechlorinators have met with some success (Bedard et al., 1997, Wu and Wiegel, 1997). Although the addition of bioaugmented cultures has not been able to effectively dechlorinate weathered PCBs when added alone, significant dechlorination of historical contamination has been documented when a PCB primer is amended to sediment at the same time (Bedard et al., 1997, Wu and Wiegel, 1997). These studies suggest that bioaugmentation can successfully enhance PCB dechlorination, but that it may require the concomitant addition of a biostimulant. Our laboratory has observed enhanced PCB dechlorination when an electron donor (Fe⁰) was added as a biostimulant (Rysavy et al., 2005). Concomitant bioaugmentation and Fe⁰ addition therefore may result in increased PCB-dechlorination compared to Fe⁰ addition or bioaugmentation alone.

One goal of this research was to further explore the utility of Fe⁰ addition for the biostimulation of PCB dechlorinators (Rysavy et al., 2005). Previous work has shown the level of H₂ produced cathodically from Fe⁰ to be critical to the extent, rate, and pathway of dechlorination (Sokol et al., 1994, Rysavy et al., 2005). Thus, an optimal Fe⁰ loading for the biostimulation of PCB dechlorination should exist. A second goal was to investigate bioaugmentation in conjunction with biostimulation.