Implementing Phytoremediation of Petroleum ... - Springer Link

8 Implementing Phytoremediation of Petroleum Hydrocarbons Chris D. Collins Summary An evaluation of the current “state of the art” for the phytoremediation of total petroleum hydrocarbons (TPH) is given, which will allow for well-informed decisions to be made when the technology is being applied to this contamination problem. Information is provided on phytotoxicity, plant selection, and management as well as useful supplementary practical data sources. A management decision tree is presented to aid in the successful application of phytoremediation to TPH-contaminated sites. Finally, deficiencies in the current knowledge are identified, which need to be addressed to improve the effectiveness of phytoremediation to this problem. Key Words: Total petroleum hydrocarbons; plant selection; field application; decision tree.

1. Introduction 1.1. Total Petroleum Hydrocarbons in the Environment The petroleum hydrocarbons are some of the most universally detected organic pollutants in the environment because of the high industrial use of petroleum products world wide. The world petroleum consumption in 2001 was 77 million barrels per day (1), this scale of use results in a high potential for contamination from both accidental and fugitive releases. For example, in the United Kingdom alone there are estimated to be 120,000 contaminated petrol station sites with an associated remediation cost of £2.5 billion. The remediation of contaminated oil terminals and refinery sites will increase this figure. The US petroleum industry alone spent $0.8 billion dollars in 2001 on remediation (2). The large scale and economic importance of this contamination

From: Methods in Biotechnology, vol. 23: Phytoremediation: Methods and Reviews Edited by: N. Willey © Humana Press Inc., Totowa, NJ

99

100

Collins Table 1 Carbon Number Ranges and Associated Boiling Point Ranges of Different Fuels and Oils Fuel or oil

Carbon number range

b.p. range (°C)

Petroleum Jet fuel Kerosene Diesel Motor oil

C4–C12 C5–C14 C6–C16 C8–C21 C18–C34

40–200 150–275 150–300 200–325 325–600

has resulted in a significant effort in developing remediation technologies for its clean-up. Phytoremediation is one of the remediation technologies that has been developed to address this problem. Phytoremediation is advantageous when a low-cost solution is needed that can be easily applied to diffuse sources of contamination. This is typical of the large sites used by the petrochemical industry. Such sites are usually in places that do not have significant value for redevelopment (where economic pressures require a rapid clean up) so it is usually a matter of reducing liability. Phytoremediation can also be used to reduce the leaching of contaminants through soils, and hence, protect the groundwater where guidelines for contaminant levels are more stringent (3). This technology has also been used to prevent off-site migration of contaminants in groundwaters (4). Finally, within the petrochemical industry there is recognition of bioremediation strategies such as land farming, so gaining acceptance for phytoremediation should be easier than in other commercial enterprises. Before detailing the potential clean-up options currently available it is necessary to understand something of the nature of petroleum hydrocarbons or total petroleum hydrocarbons (TPHs) as they are frequently referred to analytically. TPHs are a whole collection of organic compounds that include aromatics, PAH, alkanes, and others, all with different physicochemical properties. Table 1 illustrates the carbon chain lengths of the alkanes associated with the different fuel and oil formulations and how these affect boiling point. Increasing carbon chain length will also reduce contaminant solubility and increase its partition to soil and hence reduce the availability of the contaminant to plants. In the early years of the clean-up operations of polluted soil sites the focus was often on the total remediation of the contamination. Owing to the magnitude of most soil contamination problems this often led to a stagnation of the overall remediation process because of the high costs involved. Legislators have subsequently tried to avoid this by achieving a balance between environmental effectiveness and cost using the “suitable-for-use” cleanup criterion. Phytoremediation has a role in this process because of its low cost and positive environmental impact.

Implementing Phytoremediation of Petroleum Hydrocarbons

101

At present, phytoremediation of TPH is still not a proven technology. Rigorous science is now required to determine when it is an appropriate option so the technology is not rejected because of failures from inappropriate implementation. Further information is required on those species that tolerate/ degrade TPH; how the system acts upon “aged” and recent contamination; and the influence of soil organic matter in the toxicity of TPH (5). The chemical component that causes phytotoxicity still needs to be elucidated. If these questions can be addressed then phytoremediation may be applicable to higher concentrations than those currently being considered. The role of phytoremediation in ecological restoration also requires further examination—it may well be a major application. The best combination of phytoremediation with other technologies when addressing TPH contamination also needs to be investigated. With this knowledge the quality and repeatability of the clean up of TPH by phytoremediation will be improved and its adoption by industry for routine use may then be achieved. The methodology outlined here will be useful for progressing the science underpinning phytoremediation of TPHs and other organic contaminants. 1.2. Traditional Clean-up Options for Soil TPHs 1.2.1. Excavation This involves the removal of the contaminated material from the site to landfill. It has become an increasingly difficult option to adopt because of the conflicts it creates with the sustainable goals of most brownfield-site remediation projects. Additionally, the increasing costs of disposal to landfill and the potential problems this creates with liability will see further declines in this strategy. 1.2.2. Air Sparging During air sparging volatile organic compounds are removed from soil by mechanically drawing or venting air through the soil matrix. Volatiles contain much of the toxicity and this approach is most efficient for the lighter fractions of TPH, e.g., benzene, toluene, ethylbenzene, xylene, and petroleum. This process has the benefit that it is relatively cheap, however, it often leaves a residual fraction and is only really useful for removing the volatile organic compounds. The main advantage of air sparging is the speed of this process, which can take a few weeks to 6 mo. 1.2.3. Bioremediation Bioremediation involves the use and/or enhancement of micro-organisms, which degrade the pollutants of concern. For biodegradation to be successful four conditions must be met. First, the contaminants have to act as the primary

102

Collins

carbon source for the microbial population; this will be the electron donor. Second, an electron acceptor must be available so that energy can be extracted at environmentally significant rates; this can be oxygen, nitrate, sulfate, carbon dioxide, or organic carbon. Oxygen is usually selected for its high efficiency and can be provided by aerating formed piles (6). Nitrate is also often added as a fertilizer to bioremediation systems. Third, macro- and micronutrients need to be available for the production of cellular material, usually in the ratio 100:10:1 for carbon, nitrogen, and phosphorus. Finally, conditions must not be inhibitory to the indigenous microflora, these should be soil moisture at 50–80% field capacity, pH 5.5–8.5, temp 15–45°C, and an absence of microbial toxicants. Bioremediation is attractive because it can be effective on some of the more recalcitrant components of oils, however, it can also increase costs because the soil area being treated will often need to be manipulated to accommodate the treatment (e.g., formed piles), chemicals may need to be applied, and extra degrading microbes may also be added (bioaugmentation). The time-span required can be long and hence monitoring and management costs will increase. However, bioremediation can potentially be a relatively fast treatment and can take as little as 8 wk (7). 1.2.4. Land Farming In this system contaminated soils are spread and tilled regularly and left to degrade by biological processes, in many ways it is a low-input form of bioremediation. Plants are not a component of this system. Repeated applications of contaminated material to the area being farmed will maintain the pollutantdegrading microbial population. Fertilizers may also be added to improve bacterial populations. Tilling the land improves contact between the soil and the contaminated material and is used to increase effectiveness. Land farming is a low-cost option that is considered to be a slow process, for example the treatment of drill cuttings might require 2–4 yr (8). The previously described processes can be used in combination and, for example, soil vapor extraction followed by bioremediation is considered to be a very effective strategy. They can also be used in combination with phytoremediation. Table 2 provides an overview of the abilities and costs of the different strategies. 2. Materials 2.1. Choice of Plant Grasses have been proposed for phytoremediation by many researchers because they have fibrous roots that penetrate a large soil volume and can quickly provide a good surface cover to suppress dust. Nedunuri et al. (9) found

Implementing Phytoremediation of Petroleum Hydrocarbons

103

Table 2 Summary of Remediation Technologies Available for Clean Up of the Petroleum Hydrocarbonsa Strategy

Application

Air sparging

Gasoline

aAdapted

from ref. 26.

Advantages Can remove some compounds resistant to biodegradation Effective on some nonvolatile compounds

Limitations

Costs

VOCs only

Low: low capital cost, short treatment time Bioremediation Gasoline, Possible Moderate: fuel oils lengthy because of clean up time capital outlay and management Land farming Gasoline, Uses natural Some residuals Moderate: fuel oils, degradation remain because of coal tar processes initial capital residues outlay. Phytoremediation Gasoline (?), Uses natural Efficacy not Low: low capital fuel oils (?) degradation process, really known cost ecologically benign, and soil stabalization

ryegrasses and St. Augustine’s grass to enhance TPH degradation. However, other researchers have found little treatment effect when using grass species (10). Adam and Duncan (11) found cultivated crops the most tolerant of dieselrange organics with many grasses sensitive. Plant choice is further complicated in that there may be differences in tolerance within individual species (12). Many of the recommended species in the Phytopet” database (13), which contains details of species known to be tolerant or to degrade TPH, are also grasses. Overall, therefore, there is no consensus on those species to be recommended for phytoremediation of TPH. In the light of these findings, perhaps the best strategy is to follow the Research Technologies Development Forum (RTDF) approach (see Note 1), in which a combination of cool and warm season grasses and a legume, which are known to be adapted to the local environment, are chosen. Grasses are chosen for the reasons previously outlined and a legume is included to provide nitrogen, which can be beneficial for microbial action. If contaminated groundwater needs to be treated, then a high water-use plant for example cottonwood, willow, or poplar needs to be used, which will draw the contaminated plume upward or prevent its migration off site.

104

Collins

3. Methods 3.1. Assessing Phytotoxicity Some plants are clearly tolerant of high levels of TPH, for example Lindau and Delaune (14) applied 2 L/m of South Carolina crude oil to Sagittaria lancifolia and found only short-term toxicity effects on plant growth and function. This increase in tolerance has also been reported for other wetland macrophytes (15) and short-term toxicity has been seen by a number of other researchers; Donnetti (16) reported it was completely gone after 16 wk, whereas others have found it significantly reduced after 2 wk (11). Most studies have found that TPH effects on germination occur around 5000 ppm on sensitive species, and nearly all species are affected at 25,000 ppm, but some species can tolerate concentrations of up to 50,000 ppm (16–19). Planting tolerant species at 30,000 ppm (3%) has been proposed (20) and this would seem an appropriate level. If the contamination has been subject to “aging” the tolerable contamination levels may be higher because of the loss of the phytotoxic fractions. 3.2. Implementing a Phytoremediation Strategy The first decision that must be made is whether phytoremediation is appropriate for the site. For example, are the conditions conducive to plant growth? This is defined by the Interstate Technology Resource Center (ITRC) (see Note 2) as greater than 200 d with temperatures higher than freezing and annual rainfall of greater than 10 in. If the TPH levels are such that phytotoxicity is a problem then the removal of the volatile fraction using air sparging can be considered. This might reduce toxicity to a level where healthy plant growth can be achieved. To assess whether this is appropriate for a given soil contamination the benzene, toluene, ethylbenzene, and xylene compounds could be monitored— these are a good indicator of this volatile toxic fraction and are often routinely analyzed in the initial site investigation if TPH are known to be present. Active remediation measures may also be required to deal with nonaqueous phase liquids as these cannot be treated by phytoremediation alone (21). If the TPH level is equal to or less than 3% of soil by weight then planting tolerant species can proceed either using an RTDF mix or high water use plants for polluted groundwaters. Even with nitrogen fixation from the legume in the recommended RTDF seed mix, application of fertilizer is considered good practice to encourage the phytoremediation proecess. It has been found that the best reduction in TPH in vegetated treatments occurred when the fertilizer ratio was 100:2:0.2 (C:N:P) (10). This is more than the amount required for plant growth and probably a ratio of 100:10:1 should be the target to enhance microbial action. An annual application of fertilizer may be required. In addition aeration is also advised,

Implementing Phytoremediation of Petroleum Hydrocarbons

105

this is similar to the principle for bioremediation systems (21). Initially the aeration could be provided by agricultural operations such as ploughing once the plants are established; the action of the roots should aerate the soils. If, after the site has been put under a phytoremediation system, no decline in TPH is evident, then some additional active measures may be required to start the degradation process. Once the TPH levels have been reduced to less than 1% , then species can be planted to ecologically restore the habitat. At this point a risk assessment should be carried out to see if the levels of TPH are such that the site is no longer classed as contaminated and no longer poses a risk. A schematic of these decisions based on previous research (20,22) is provided in Fig. 1. (For helpful data sources and previous field trials see Notes 3 and 4, respectively.) 4. Notes 1. RTDF is a partnership of the US Environmental Protection Agency (EPA) to develop and improve remediation technologies. 2. The ITRC is a web-based open access tool. 3. There are a number of useful data sources for those wishing to implement phytoremediation of TPH. The best database for plant selection is operated by the University of Saskatchewan (13). The PhytoPet database contains detailed information on a plant’s phytoremediation potential or tolerance of hydrocarbons and the literature source this came from. At present the data are predominantly for grass species and are biased toward Canadian conditions, but the database is being updated continuously and can be accessed over the Internet. The ITRC provides a web-based open access tool for the application of phytoremediation to a range of contaminants (22). This tool is in the form of a decision tree, which runs through a series of questions and provides recommendations as to whether phytoremediation is appropriate for the site, as well as guidance on which species and process management decisions to use. Some of these decisions have been incorporated into Fig. 1. Finally, there are a number of US EPA documents, which can be used for additional guidance and background to the phytoremediation of TPH (23–25). 4. Although a lot of research has been carried out testing phytoremediation on TPHcontaminated soils, there are very few examples of field scale trials of phytoremediation of TPH. If it is to be adopted as a technology, more “proof of concept” at this scale is required. The largest range of field trails to date has been a state-wide survey in the United States and Canada by the RTDF. Thirteen sites were chosen with a range of source terms such as a closed refinery and a manufactured gas plant; and a range of climates from subarctic to warm temperate. The research is ongoing, but to date, the results have been mixed. In only two instances have there been clear phytoremediation effects. There are a number of potential reasons for this, but possibly the greatest is that the contamination at a number of the sites is extremely variable and may have obscured some findings, despite a good plot size (min 400 sq ft) and four replicates (P. Kulakow personal communication). Overall this seems to be a disappointing finding for phytoremediation protagonists.

106

Collins

Fig. 1. Decision tree for the phytoremediation of total petroleum hydrocarboncontaminated sites. (Adapted from refs. 20 and 21.)

Implementing Phytoremediation of Petroleum Hydrocarbons

107

References 1. World Petroleum Consumption. Energy: http://www.eia.doe.gov/emeu/aer/pdf/ pages/sec11_21.pdf: Table 11.10. Last Accessed 05/01/04. 2. API (2003) U.S. Oil and Natural Gas Industry’s Environmental Expenditures 1992-2001. American Petroleum Institute, Washington DC, pp. 1–17. 3. Susarla, S., Medina, V. F., and McCutcheon, S. C. (2002) Phytoremediation: An ecological solution to organic chemical contamination. Ecol. Eng. 18, 647–658. 4. Hong, M. S., Farmayan, W. F., Dortch, I. J., Chiang, C. Y., McMillan S. K., and Schnoor, J. L. (2001) Phytoremediation of MTBE from a groundwater plume Environ. Sci. Tech. 35, 1231–1239. 5. Robinson, S. L., Novak, J. T., Widdowson, M. A., Crosswell, S. B., and Fetterolf, G. J. (2003) Field and laboratory evaluation of the impact of tall fescue on polyaromatic hydrocarbon degradation in an aged creosote- contaminated surface soil. J. Environ. Eng.-A 129, 232–240. 6. Dupont, R. R. (1993) Fundamentals of bioventing applied to fuel contaminated sites. Environ. Prog. 12, 45–53. 7. Hildebrandt, W. W. and Wilson, S. B. (1991) On-site bioremediation systems reduce crude-oil contamination. J. Petr. Technol 43, 18–22. 8. Zimmerman, P. K. and Robert, J. D. (1991) Oil-based drill cuttings treated by landfarming. Oil Gas J. 89, 81–84. 9. Nedunuri, K. V., Govindaraju, R. S., Banks, M. K., Schwab, A. P., and Chens, Z. (2000) Evaluation of phytoremediation for field-scale degradation of total petroleum hydrocarbons. J. Environ. Eng.-A 126, 483–490. 10. Hutchinson, S. L., Banks, M. K., and Schwab, A. P. (2001) Phytoremediation of aged petroleum sludge: Effect of inorganic fertilizer. J. Environ. Qual. 30, 395–403. 11. Adam, G., and Duncan, H. J. (1999) Effect of diesel fuel on growth of selected plant species. Environ. Geochem. Health 21, 353–357. 12. Wiltse, C. C., Rooney, W. L., Chen, Z., Schwab, A. P., and Banks, M. K. (1998) Greenhouse evaluation of agronomic and crude oil phytoremediation potential among alfalfa genotypes. J. Environ. Qual. 27, 169–173. 13. PhytoPet” A Database of Plants that Play a Role in the Phytoremediation of Petroleum Hydrocarbons. http://www.phytopet.usask.ca/mainpg.php. Last accessed 05/01/04. 14. Lindau, C. W. and Delaune, R. D. (2000) Vegetative response of Sagittaria lancifolia to burning of applied crude oil. Water, Air, Soil Pollut. 121, 161–172. 15. Pezeshki, S. R., Jugsujinda, A., and Delaune, R. D. (1998) Responses of selected US Gulf coast marsh macrophyte species to oiling and commercial cleaners. Water, Air, Soil Pollut. 107, 185–195. 16. Donnetti, C. (2003) Plant microbe interactions for the clean up of contaminated land. MSc Thesis, Imperial College, London, UK. 17. Adam, G. and Duncan, H. (2002) Influence of diesel fuel on seed germination. Environ. Pollut. 120, 363–370.

108

Collins

18. Harvey, B. S. (2002) Screening plant species for phytoremediation potential. MSc Thesis, Imperial College, London, UK. 19. Lin, Q. X., Mendelssohn, I. A., Carney, K., Bryner, N. P., and Walton, W. D. (2002) The dose-response relationship between No. 2 fuel oil and the growth of the salt marsh grass, Spartina alterniflora. Mar. Pollut. Bull. 44, 897–902. 20. Brown, J. L. and Nadeau, R. J. (2002) Restoration of petroleum contaminated sites using phased bioremediation. Biorem. J. 6, 315–319. 21. Dowty, R. A., Shaffer, G. P., Hester, M. W., Childers, G. W., Campo, F. M., and Greene, M. C. (2001) Phytoremediation of small-scale oil spills in fresh marsh environments: a mesocosm simulation. Mar. Environ. Res. 52, 195–211. 22. ITRC. Phytoremediation Online Decision Tree Document. http://www.itrcweb.org/ user/webphyto/envdept/phyto/wwwphyto/index.htm. Last accessed 05/01/06. 23. Environmental Protection Agency (1999) Phytoremediation Resource Guide, EPA 542-B-99-003. 24. Environmental Protection Agency (2000) Introduction to Phytoremediation, EPA/600/R-99/107. 25. Environmental Protection Agency (2001) Brownfields Technology Primer: Selecting and Using Phytoremediation: EPA 542-R-01-006. 26. Kujat, J. D. (1999) A comparison of popular remedial technologies for petroleum containated soils from leaking underground storage tanks. E. Green J. 11, 1–13.