Impact of transportation and distribution of heavy metal on biofilm structure and function in PAH removal

Research Goals: The primary objective of this study was to investigate the interaction between heavy metals and biofilms when PAHs are the only carbon source, and the impact of transfer and distribution of heavy metals on biofilm systems. The specific aspects which will be investigated in this research include: a) the impact of heavy metals on PAH removal by a multi-species biofilm; b) the impact of substrates, PAHs and alternative substrates on PAH removal and heavy metal transportation; c) the impact of other environmental factors, such as DO and pH; d) heavy metal transport and distribution dynamics in the biofilm matrix; e) spatial distribution of EPS, microbial cells and heavy metals.

Overview: Polycyclic Aromatic Hydrocarbons (PAHs) are a class of several hundred individual compounds containing at least two condensed rings.  They are produced through incomplete combustion and are receiving increased attention because of their toxicity to human health and to natural ecology. Although they are not readily biodegradable and are persistent under natural conditions, biological processes have been applied to remediate PAH contaminated soils and ground water.

Biofilm processes have been introduced into bioremediation. For example, subsurface biofilm barriers (biowalls) are constructed by injection or establishment of selected microorganisms suitable for biodegradation of concerned contaminants into the subsurface, and development of a well functioning biofilm matrix for contaminant entrapment and degradation.  This technology may be potentially useful at locations with restricted access to the subsurface, since excavation is not necessary and there is no obvious depth limitation.

Heavy metal wastes are often produced simultaneously with PAHs in mining, ore refinement, coal combustion, and so on.  They are also present with PAHs at many hazardous waste sites due to co-disposal.  Heavy metals will not only influence human health, but also may inhibit microbial activity. However, the biofilm matrix provides diversity of sorption sites for heavy metal ions, which could reduce the chance for them to enter the microbial cells.  Although some researchers have shown higher affinity of bacterial surfaces for metal binding compared to extracellular polymeric substances (EPS), others have demonstrated that EPS may also play a crucial role in biosorption of heavy metals.

In order to facilitate the application of biofilm processes in bioremediation of contaminated soils and ground water, further research is needed to illuminate the mechanisms of heavy metal transfer and distribution in the biofilm matrix and their impact on biofilm functions. Microelectrode and confocal laser scanning microscopic technologies will be used to demonstrate the spatial transportation and distribution of heavy metals in the biofilms.

Progress to Date: The presence of heavy metals may be a crucial factor influencing the application of biofilm processes for bioremediation of contaminated soils and ground water. Environmental conditions will also have a significant impact on bioremediation. Naphthalene and cadmium are selected as target compounds. As the pH value increased, cadmium sorption in the biofilm increased and naphthalene removal decreased.  Addition of 10 mg/L of cadmium produced a significant impact on biofilm when the pH was above 7.5.  The minimum inhibition capacity for Cd was about 5 μgCd/mg VS, and the range between 5 and 10 μgCd/mgVS was defined as the cadmium effective inhibition range.  Acetate, added as an alternative substrate, competed with naphthalene and did not reduce the cadmium toxicity.  Hydrogen peroxide, added as supplement for dissolved oxygen, improved not only the cell growth, but also the cadmium uptake/efflux cycle, which made the biofilm more vulnerable to cadmium attack.  Cadmium was shown to transfer faster than naphthalene into biofilms, retard naphthalene removal and reduce the removal rate.

Future Directions: Studies on the micro-environmental profiles in the biofilm matrix need to be conducted to help explain the mechanisms of cadmium mass transfer and distribution in biofilm processes.

Application of Knowledge: Bioremediation of contaminated soil and groundwater, biofilm processes in waste treatment. PAH degradation pathway, heavy metal distribution in soil and groundwater and impact of heavy metal on microorganisms.

Techniques Incorporated: Aatomic adsorption spectrometry, gas chromatography, gas chromatography/mass spectrometry, microelectrodes (DO, pH, ORP and ISE), and confocal laser scanning microscopy (CLSM).

Keywords: PAHs biodegradation, biofilms, heavy metals, microelectrode and confocal laser scanning microscopy