Bioremediation of Contaminated Soil
Bioremediation is defined as the use of biological processes to degrade, break down, transform, and/or essentially remove contaminants or impairments of quality from soil and water. Bioremediation is a natural process which relies on bacteria, fungi, and plants to alter contaminants as these organisms carry out their normal life functions. Metabolic processes of these organisms are capable of using chemical contaminants as an energy source, rendering the contaminants harmless or at least making them less toxic. This paper summarizes the general processes of bioremediation within the soil environment, focusing on biodegradation of petroleum hydrocarbons. The effect of soil conditions on rate of biodegradation of hydrocarbons is addressed. Further, limitations and potential of both ex situ and in situ bioremediation as viable alternatives to conventional remediation are explained and addressed.
Many substances known to have toxic properties have been introduced into the environment through human activity. These substances range in degree of toxicity and danger to human health. Many of these substances either immediately or ultimately come in contact with and are sequestered by soil. The emerging science and technology of bioremediation offers a method to detoxify contaminants. Bioremediation has been demonstrated and is being used as an effective means of mitigating:
- halogenated organic solvents
- halogenated organic compounds
- non-chlorinated pesticides and herbicides
- nitrogen compounds
- metals (lead, mercury, chromium)
Bioremediation technology exploits various naturally occurring mitigation processes: natural attenuation, biostimulation, and bioaugmentation.
- Bioremediation which occurs without human intervention other than monitoring is often called natural attenuation. This natural attenuation relies on natural conditions and behavior of soil microorganisms that are indigenous to soil.
- Biostimulationalso utilizes indigenous microbial populations to remediate contaminated soils.Biostimulation consists of adding nutrients and other substances to soil to catalyze natural attenuation processes.
- Bioaugmentationinvolves introduction of exogenic microorganisms (sourced from outside the soil environment) capable of detoxifying a particular contaminant, sometimes employing genetically altered microorganisms.
During bioremediation, microbes utilize chemical contaminants in the soil as an energy source and, through oxidation-reduction reactions, metabolize the target contaminant into useable energy for microbes. By-products (metabolites) released back into the environment are typically in a less toxic form than the parent contaminants. For example, petroleum hydrocarbons can be degraded by microorganisms in the presence of oxygen through aerobic respiration. The hydrocarbon loses electrons and is oxidized while oxygen gains electrons and is reduced. The result is formation of carbon dioxide and water. When oxygen is limited in supply or absent, as in saturated or anaerobic soils or lake sediment, anaerobic (without oxygen) respiration prevails. Generally, inorganic compounds such as nitrate, sulfate, ferric iron, manganese, or carbon dioxide serve as terminal electron acceptors to facilitate biodegradation.
Three primary ingredients for bioremediation are:
1) presence of a contaminant,
2) an electron acceptor, and
3) presence of microorganisms that are capable of degrading the specific contaminant.
Generally, a contaminant is more easily and quickly degraded if it is a naturally occurring compound in the environment, or chemically similar to a naturally occurring compound, because microorganisms capable of its biodegradation are more likely to have evolved. Petroleum hydrocarbons are naturally occurring chemicals; therefore microorganisms which are capable of attenuating or degrading hydrocarbons exist in the environment. Development of biodegradation technologies of synthetic chemicals such as DDT is dependent on outcomes of research that searches for natural or genetically improved strains of microorganisms to degrade such contaminants into less toxic forms.
Microorganisms have limits of tolerance for particular environmental conditions, as well as optimal conditions for pinnacle performance. Factors that affect success and rate of microbial biodegradation are nutrient availability, moisture content, pH, and temperature of the soil matrix. Inorganic nutrients including, but not limited to, nitrogen, and phosphorus are necessary for microbial activity and cell growth. It has been shown that “treating petroleum-contaminated soil with nitrogen can increase cell growth rate, decrease the microbial lag phase, help to maintain microbial populations at high activity levels, and increase the rate of hydrocarbon degradation”. However, it has also been shown that excessive amounts of nitrogen in soil cause microbial inhibition. Walworth et al. (2005) suggest maintaining nitrogen levels below 1800 mg nitrogen/kg H2O for optimal biodegradation of petroleum hydrocarbons. Addition of phosphorus has benefits similar to that of nitrogen, but also results in similar limitations when applied in excess.
All soil microorganisms require moisture for cell growth and function. Availability of water affects diffusion of water and soluble nutrients into and out of microorganism cells. However, excess moisture, such as in saturated soil, is undesirable because it reduces the amount of available oxygen for aerobic respiration. Anaerobic respiration, which produces less energy for microorganisms than aerobic respiration and slows the rate of biodegradation, becomes the predominant process. Soil moisture content “between 45 and 85 percent of the water-holding capacity (field capacity) of the soil or about 12 percent to 30 percent by weight” is optimal for petroleum hydrocarbon degradation.
Soil pH is important because most microbial species can survive only within a certain pH range. Furthermore, soil pH can affect availability of nutrients. Biodegradation of petroleum hydrocarbons is optimal at a pH of 7 (neutral); the acceptable range is pH 6 – 8.
Temperature influences rate of biodegradation by controlling rate of enzymatic reactions within microorganisms. Generally, speed of enzymatic reactions in the cell approximately doubles for each 10˚ C rise in temperature. There is an upper limit to the temperature that microorganisms can withstand. Most bacteria found in soil, including many bacteria that degrade petroleum hydrocarbons, are mesophiles which have an optimum temperature ranging from 25˚ C to 45˚ C. Thermophilic bacteria (those which survive and thrive at relatively high temperatures) which are normally found in hot springs and compost heaps exist indigenously in cool soil environments and can be activated to degrade hydrocarbons with an increase in temperature to 60˚ C. This finding “suggested an intrinsic potential for natural attenuation in cool soils through thermally enhanced bioremediation techniques”.
Contaminants can adsorb to soil particles, rendering some contaminants unavailable to microorganisms for biodegradation. Thus, in some circumstances, bioavailability of contaminants depends not only on the nature of the contaminant but also on soil type. Hydrophobic contaminants, like petroleum hydrocarbons, have low solubility in water and tend to adsorb strongly in soil with high organic matter content. In such cases, surfactants are utilized as part of the bioremediation process to increase solubility and mobility of these contaminants. Additional research findings of the existence of thermophilic bacteria in cool soil also suggest that high temperatures enhance the rate of biodegradation by increasing the bioavailability of contaminants. It is suggested that contaminants adsorbed to soil particles are mobilized and their solubility increased by high temperatures.
Soil type is an important consideration when determining the best suited bioremediation approach to a particular situation. In situ bioremediation refers to treatment of soil in place. In situ biostimulation treatments usually involve bioventing, in which oxygen and/or nutrients are pumped through injection wells into the soil. It is imperative that oxygen and nutrients are distributed evenly throughout the contaminated soil. Soil texture directly affects the utility of bioventing, in as much as permeability of soil to air and water is a function of soil texture. Fine-textured soils like clays have low permeability, which prevents biovented oxygen and nutrients from dispersing throughout the soil. It is also difficult to control moisture content in fine textured soils because their smaller pores and high surface area allow them to retain water. Fine textured soils are slow to drain from water-saturated soil conditions, thus preventing oxygen from reaching soil microbes throughout the contaminated area. Bioventing is well-suited for well-drained, medium, and coarse-textured soils.
In situ bioremediation causes minimal disturbance to the environment at the contamination site. In addition, it incurs less cost than conventional soil remediation or removal and replacement treatments because there is no transport of contaminated materials for off-site treatment. However, in situ bioremediation has some limitations:
1) it is not suitable for all soils,
2) complete degradation is difficult to achieve, and
3) natural conditions (i.e. temperature) are hard to control for optimal biodegradation.
Ex situ bioremediation, in which contaminated soil is excavated and treated elsewhere, is an alternative.
Ex situ bioremediation approaches include use of bioreactors, landfarming, and biopiles.
- In the use of a bioreactor, contaminated soil is mixed with water and nutrients and the mixture is agitated by a mechanical bioreactor to stimulate action of microorganisms. This method is better suited to clay soils than other methods and is generally a quick process.
- Landfarming involves spreading contaminated soil over a collection system and stimulating microbial activity by allowing good aeration and by monitoring nutrient availability.
- Biopiles are mounds of contaminated soils that are kept aerated by pumping air into piles of soil through an injection system.
In each of these methods, conditions need to be monitored and adjusted regularly for optimal biodegradation. Use of landfarming and biopiles also present the issue of monitoring and containing volatilization of contaminants. Like in situ methods, ex situ bioremediation techniques generally cost less than conventional techniques and apply natural methods. However, they can require a large amount of land and, similar to in situ bioremediation, complete degradation is difficult to achieve, and evaporation of volatile components is a concern.
If the challenges of bioremediation, particularly of in situ techniques, can be overcome, bioremediation has potential to provide a low cost, non-intrusive, natural method to render toxic substances in soil less harmful or harmless over time. Currently, research is being conducted to improve and overcome limitations that hinder bioremediation of petroleum hydrocarbons. On a broader scope, much research has been and continues to be developed to enhance understanding of the essence of microbial behavior as microbes interact with various toxic contaminants.