Bioremediation is a method used to clean up polluted areas by using microorganisms that are already present or added to the site. The term "in situ" means "on site" and refers to the location where an event happens. In the context of bioremediation, in situ means that the cleanup happens at the polluted location without moving the contaminated materials. Bioremediation helps remove pollutants such as hydrocarbons, chlorinated compounds, nitrates, toxic metals, and other harmful substances through chemical processes. Microorganisms used in bioremediation can be introduced to the site or grown there by adding fertilizers and nutrients. Common areas treated with bioremediation include groundwater, aquifers, and polluted soil. Aquatic environments affected by oil spills have also improved with bioremediation, such as the Deepwater Horizon oil spill in 2010 and the Exxon Valdez oil spill in 1989. There are two types of bioremediation, determined by where the process takes place. Ex situ bioremediation happens at a location away from the polluted site and involves moving the contaminated materials. In situ bioremediation occurs at the site of contamination and can be further divided based on the type of metabolism involved, such as aerobic (using oxygen) or anaerobic (without oxygen), and the level of human involvement.
History
In 1972, the Sun Oil pipeline spill in Ambler, Pennsylvania led to the first use of in situ bioremediation in a commercial setting to clean hydrocarbons from polluted areas. In 1974, Richard Raymond filed a patent titled Reclamation of Hydrocarbon Contaminated Ground Waters, which helped make in situ bioremediation a widely used method for cleaning contaminated groundwater.
Classifications of In situ Bioremediation
Accelerated in situ bioremediation is a method where specific types of microbes are encouraged to grow by adding nutrients or an electron donor to a polluted area. In aerobic bioremediation, oxygen is the nutrient added to the soil. In anaerobic bioremediation, other substances like benzoate or lactate are often used as electron donors or acceptors. In some cases, microbes are added directly to the site. This process is called bioaugmentation and is used when a specific type of microbe is needed to break down pollution but is not naturally present in enough numbers. Accelerated in situ bioremediation is used when the right microbes are not naturally available or when the site lacks enough nutrients to support their growth.
The Raymond Process is a type of accelerated in situ bioremediation developed by Richard Raymond. It involves adding nutrients and electron acceptors to a polluted area, especially groundwater. This process creates a loop system where polluted groundwater is pumped to the surface, mixed with nutrients and oxygen, and then returned to the ground upstream. This helps microbes grow and break down contaminants.
In areas where microbes need oxygen to function, oxygen can be added through injection wells or injection galleries. Oxygen is often the key factor that determines how quickly and effectively bioremediation works.
Ozone can also be used to add oxygen to a polluted site. Though ozone is a strong chemical that can harm some microbes, it dissolves easily in water and quickly breaks down into oxygen. Within 20 minutes of being injected, half of the ozone becomes oxygen. Ozone is introduced to the soil either as a gas or dissolved in water.
In anaerobic bioremediation, electron donors and acceptors are added to help anaerobic microbes grow and break down pollutants.
Monitored natural attenuation is a type of bioremediation where little or no human action is taken. It relies on naturally occurring microbes in the soil to slowly reduce pollution over time. During this process, the site is regularly checked to track progress. This method is used after other cleanup methods have removed the pollution source.
Uses of In Situ Bioremediation
Hydrocarbons are sometimes called polycyclic aromatic hydrocarbons (PAHs). These substances have structures made of aromatic rings. They are created when materials like fossil fuels are burned. Hydrocarbons are found in land and water environments, which can harm the environment, wildlife, and food safety. These substances may also cause changes in DNA and cancer, which can harm humans and animals.
To remove these pollutants, methods like aerobic and anaerobic remediation can be used. One effective anaerobic method is Biochar. Biochar is made from plant parts, microbe remains, and farming waste through a process called pyrolysis. It has a porous and absorbent structure that helps microorganisms break down pollutants in soil. Biochar is rich in carbon, which helps it trap PAHs. Biochar absorbs the most pollutants when pyrolysis is done at high temperatures because higher heat increases its surface area. To improve Biochar's ability to remove pollutants, acid and alkali are often added as activating agents. Biochar works in both aerobic and anaerobic conditions, but it is most effective under anaerobic conditions because it supports microorganism growth, which helps break down pollutants. Byproducts from Biochar can improve soil quality. Biochar is also used in creating biogas, a renewable fuel made from waste materials like animal and plant matter. Biogas is mostly methane and carbon dioxide. This process happens in a digester, where Biochar’s structure helps microorganisms break down waste. Biochar also leaves behind digestates, which are useful as nutrient-rich fertilizer and soil conditioner. However, digestates may also contain harmful inorganic heavy metals.
Hydrocarbons are divided into two groups: LMW-PAHs, which have fewer aromatic rings and lower molecular weight, and HMW-PAHs, which have more aromatic rings and higher molecular weight. The more aromatic rings a PAH has, the less soluble it is, making it harder to break down.
Soil naturally contains microbes that use hydrocarbons for energy and growth. Up to 20% of soil microbes can break down hydrocarbons. These microbes can help reduce hydrocarbon pollution through natural or accelerated processes. The main way microbes break down hydrocarbons is through aerobic metabolism. The end products of this process are carbon dioxide and water. The ease of breaking down hydrocarbons depends on their structure. Long chains of aliphatic carbon are easiest to break down, while short, branched, or quaternary aliphatic hydrocarbons are harder to break down. Alkenes are more easily broken down when their chains are fully saturated. Many microbes in soil can break down aromatic hydrocarbons, and some can do this without oxygen. The ability of microbes to break down polynuclear aromatic hydrocarbons depends on the number of aromatic rings in the compound. Compounds with two or three rings are broken down more easily, while those with four or more rings are harder to break down. Microbes in soil can break down polynuclear aromatic hydrocarbons with fewer than four rings through aerobic processes. For larger compounds, the only effective method is cometabolism, where microbes break down substances without using them as their main energy source. Under anaerobic conditions, some species of the fungus Phanerochaete can break down certain polynuclear aromatic hydrocarbons using a peroxidase enzyme.
Microbes can break down chlorinated aliphatic compounds through three main processes: anaerobic reduction, oxidation, and cometabolism under aerobic conditions. Microbes that can break down these compounds are rare in nature. Compounds with one or two carbon atoms and little chlorine are most easily broken down by soil microbes. The most common method for breaking down chlorinated aliphatic compounds is cometabolism.
Chlorinated aromatic hydrocarbons are difficult to break down, and many microbes cannot degrade them. These compounds are often broken down through reductive dechlorination under anaerobic conditions. Polychlorinated biphenyls (PCBs) are mainly broken down through cometabolism. Some fungi can also break down these compounds. Studies show that adding biphenyl to a site increases PCB breakdown because the enzymes used to break down biphenyl also help break down PCBs.
Benefits
In situ bioremediation, which takes place at the location of contamination, has a lower chance of spreading pollution to other areas compared to ex situ bioremediation, where the polluted material is moved to different locations. In situ bioremediation can also cost less money and clean up pollution more quickly than ex situ bioremediation.