A bio-reactive barrier sequence for petroleum hydrocarbon capture and degradation in low nutrient environments
Introduction
The migration of petroleum hydrocarbon contaminants in soil water can be effectively intercepted through the installation of permeable reactive barriers (PRB) (Mumford et al., 2013). In a PRB, soil water flows through a single material, or sequence, of reactive materials, wherein the contaminants are physically adsorbed and chemically or biologically degraded (Yeh et al., 2010). The finite operational capacity of many reactive materials, such as granular activated carbon (GAC), requires regeneration or the replacement of materials and subsequent costly disposal in appropriate landfills (Prasse et al., 2015). In the context of petroleum hydrocarbon remediation in remote and cold regions, such as the Antarctic, high transport and disposal costs challenge the feasibility of remediation technologies often employed in temperate environments (Camenzuli and Freidman, 2015).
One strategy to overcome these limitations is the use of biofilms in so called bio-reactive barriers. The interaction between petroleum hydrocarbons and reactive materials provides a carbon supply for the attachment and growth of microorganisms transported in the soil water. Where this flow is sufficient for growth, this can result in biofilm formation generating a bio-reactive barrier (Mumford et al., 2015). Growth can be promoted in the bio-reactive barrier if nutrient supply is controlled and the media composition is uniform (Careghini et al., 2013). As a result, biodegradation can occur in a more controlled manner than in soil, enhancing the performance of the reactive material and the longevity of petroleum hydrocarbon treatment within a PRB. This can facilitate a lower frequency of material replacement and disposal, leading to reduced costs (Snape et al., 2001).
Sequenced PRBs have previously been installed in the Antarctic to intercept and degrade petroleum hydrocarbon spills (Mumford et al., 2013; Statham et al., 2016). These PRBs contained materials for controlled nutrient release, as well as fuel capture and biodegradation (Mumford et al., 2013). Mumford et al. (2015) reports that the use of nutrient amended zeolites promoted large populations of hydrocarbon degrading bacteria. More recent studies have also supported the increased biodegradation of petroleum hydrocarbons in the presence of nutrient amended zeolites (Freidman et al., 2016a). Yet, despite the high retention of petroleum hydrocarbons on GAC and high cell counts observed, low degradation was recorded (Mumford et al., 2015). This suggests that microbial cells attached to the GAC may be limited in their ability to desorb and degrade petroleum hydrocarbon contaminants adsorbed to the GAC. The heterogeneous pore structure of GACs, in which micropores (<2 nm Ø), mesopores (2–50 nm Ø) and macropores (>50 nm Ø) are present, may be a contributing factor (Aktas & Cecen, 2007). GAC is primarily comprised of micropores, however the relative proportion of pore types can vary between carbonaceous materials and activation methods (Aktas & Cecen, 2007).
Two mechanisms have been reported to contribute to the biological desorption of petroleum hydrocarbons and regeneration of GAC (Aktas & Cecen, 2007). These are 1.) desorption due to a concentration gradient induced by biofilm activity, and 2.) desorption due to exo-enzymatic reactions (Aktas & Cecen, 2007). In the first mechanism the concentration of petroleum hydrocarbons in the liquid phase is reduced due to microbial activity. Desorption then occurs from the GAC and petroleum hydrocarbons become available for further biofilm formation (Ng et al., 2010). In the second mechanism, enzymes secreted by microbial cells diffuse into the pores of the GAC and react with the adsorbed petroleum hydrocarbon compounds causing hydrolytic transformation (Ng et al., 2010). Xiaojian et al. (1991) suggest that many exo-enzymes are too large to facilitate desorption across much of the GAC pore network. It is estimated that for enzyme catalysed desorption to occur, the diameter of the GAC pore must be at least three times greater than the size of the enzyme. As the average molecular diameter of exo-enzymes is greater than 3–4 nm, Xiaojian et al. concluded that the pore diameter must be larger than 10 nm. This excludes the micropores as sites available for desorption but still leaves the mesopores and macropores as possibilities for enzyme activity.
The importance of pore size has also been shown in studies where mesopores within GAC were more efficiently regenerated than micropores, due to the small pore diameter of micropores and overlapping of adsorption forces from the opposite walls within these pores (Bansal & Goyal, 2005; Putz et al., 2005). Other factors that can also influence the accessibility of petroleum hydrocarbons on GAC include the carbon activation process, which has been found to be highly irreversible on thermally activated carbons (Aktas & Cecen, 2007). Thermal activation procedures in an inert atmosphere can remove surface oxide groups from GAC, resulting in a highly reactive pore network that strongly binds organics when in contact with water (Aktas & Cecen, 2006).
A comprehensive investigation into the biological performance of GAC in a sequenced PRB is challenging at bulk scale for several reasons. Firstly, the contaminant flux and hydraulic conductivity of PRBs in environments subject to freeze-thaw cycling is variable (Mumford et al., 2014). Bulk scale mixing of PRB materials and the presence of multiple petroleum hydrocarbon products may also conceal the contribution of individual materials to biodegradation (Mumford et al., 2013). While previous works have shown that nutrient amended zeolites can enhance the biodegradation of petroleum hydrocarbons in soil water (Freidman et al., 2016a), there are further challenges to be overcome. Lag phases have been observed in biofilm formation and degradation of petroleum hydrocarbons is incomplete, suggesting that adsorption materials are required to manage risk and prevent migration.
This study builds upon the previous research of Mumford et al. (2015) and Freidman et al. (2016a) by investigating the biodegradation of special Antarctic blend (SAB) diesel within a laboratory scale sequenced PRB (Fig. 1a–f). SAB diesel is primarily comprised of n-C9-C14 alkanes and is the most commonly used fuel by Australia in Antarctica (Snape et al., 2005). The laboratory design was chosen to investigate the influence of nitrogen delivery, achieved through soil cation interaction with ammonium exchanged zeolite, for the promotion of biofilm formation and biodegradation of petroleum hydrocarbons on GAC (Fig. 1a–c). Ammonium exchanged zeolite was replaced with inert sand in control flow cells (Fig. 1b). Furthermore, the influence of low temperature (Fig. 1d), porosity (Fig. 1e) and SAB diesel concentration (Fig. 1f) were investigated alongside the initial sequence (Fig. 1a) to expand the application of this study to potential temperate water treatment applications.
Section snippets
Sequenced PRB materials
The conditioning of ammonium exchanged zeolite and the physicochemical properties of Antarctic soil collected from Australia's Casey Station are reported in previous works (Freidman et al., 2016a). The surface area of GAC and zeolites was determined by nitrogen (N2) adsorption at 77 K (−196 °C) using the Micrometrics Accelerated Surface Area and Porosimetry System (ASAP 2010, Micrometrics) and the Brunauer-Emmett-Teller method recommended by the manufacturer (Table 1). The limitations of N2
Antarctic soil and zeolite dynamics
The delivery of cations in Antarctic soil water exchanged in a controlled manner with NH4+ in the pores of the zeolite over 432 BV passing (Fig. 2). Fig. 2 and Table 2 show that the release of NH4+ was not significantly influenced by temperature or increasing the concentration of SAB diesel in the soil from 5000 mg kg−1 to 30,000 mg kg−1 across 432 BV passing. The concentration of NH4+ in the GAC bed following injection was similar to flow cells employing ammonium exchanged zeolite (Fig. 2).
The
Discussion
The exchange of NH4+ with soil water cations promoted biofilm growth and the biodegradation of SAB diesel on the surface of nutrient amended zeolites consistent with previous studies (Mumford et al., 2013; Freidman et al., 2016a). The low biodegradation rates reported for soil and ammonium exchanged zeolite at 4 °C, when compared with these materials at 23 °C, have been thoroughly documented and the data presented here is consistent with previous reports (Ferguson et al., 2003; Mumford et al.,
Conclusion
This study shows that the delivery of NH4+ is essential for the establishment of bio-reactive materials within PRBs in low nutrient environments. While adsorption of SAB diesel was observed on GC1200 GAC and PICABIOL GAC, the carbon supply was shown to be largely unavailable for biofilm formation and biodegradation within the GAC pore network. The exhaustion of the GC1200 GAC and PICABIOL GAC sorption capacity resulted in desorption of some naphthalene and dodecane from the solid phase to the
Acknowledgments
The authors acknowledge the financial support of Australian Antarctic Science Projects 4029 and 4036, as well as the Particulate Fluids Processing Centre (PFPC) at The University of Melbourne. We extend our thanks to Greg Hince and Lauren Wise at the Australian Antarctic Division for processing the TPH data and Simon Crawford at The University of Melbourne for FESEM sample preparation. Sally Gras is supported by The ARC Dairy Innovation Hub (IH120100005).
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