Principles of bioremediation

“Remediate” means to solve a problem, and “bioremediation" means to use biological organisms to solve an environmental issue such as contaminated soil or water. The term bioremediation is used for a natural remedy approach to decrease or clean up contamination. According to the EPA 2006, bioremediation is a treatment that uses naturally occurring microorganisms to break down dangerous substances into less toxic or non-toxic materials (Gonzalez et al. 2019).

Bioremediation is used to treat sites contaminated with organic materials (USEPA 2001a). However, it can also be applied to immobilize inorganic contaminants such as heavy metals, although this is a developing area (Shanna and Reddy 2004). The most common organic pollutants typically include polycyclic aromatic hydrocarbons (PAHs), benzene, toluene, ethylbenzene and xylene (BTEX), polychlorinated biphenyls (PCBs), pesticides and herbicides (Sharma and Reddy 2004, USEPA 2001a).

Microorganisms are ideally accommodated to the task of contaminant elimination because they have enzymes that enable them to use contaminants as food or energy. Without the activity of microbes, the Earth would be buried in wastes, and the nutrients required for the survival of life would be locked up in detritus. Microbes can successfully degrade and dimmish human-made contaminants in the soil. However, it depends on three factors: the presence of types of organisms, contaminants nature, and the geological and chemical conditions at the contaminated sites (NAP 1993).

In bioremediation, microorganisms with biological activity, including algae, bacteria, fungi, and yeast, are used to digest toxic contaminants (USEPA2001b, Sharma and Reddy 2004). The process of biodegradation occurs in the presence of oxygen or without oxygen, known as aerobic and anaerobic digestion, respectively. An example of hazardous oil spill digestion and degradation and its conversion into the water and harmless gases by microbes is shown diagrammatically in Figure 1. Another example of microbial degradation of organic contaminants into water and CO, is shown by the simplified equation (1).

Organic contaminants + 02 —> H20 + C02 + cell material + energy (1)

Similarly, sulfate-reducing bacteria use ferric non or S04~ as an electron acceptor. They reduce it to ferrous iron and H,S. Denitrifying bacteria such as ThiobaciJlus denitrificans, Micrococcus denitrificcms use N03“ as an electron acceptor and emit NO,-, N,0. and N, as reduced gases. Likewise, methanogens use CO, as an electron acceptor to produce CH4.

Under the aerobic conditions, microorganisms can transform several organic contaminants to carbon dioxide, water, and microbial cell mass. Aerobic bioremediation utilizes oxygen as the electron acceptor (Parsons Corporation 2004). Aerobic metabolism is generally more utilized and can be useful for hydrocarbons, and other organic compounds, such as petroleum hydrocarbons and methyl tertiary-butyl ether. Several organisms can degrade hydr ocarbons (HC) using oxygen as the electron acceptor and HC as carbon and energy sources. Aerobic technologies may also alter the ionic form of metals (EPA 2006).

Anaerobic bioremediatiou includes microbial reactions happening in the absence of oxygen and encompasses many processes, including fermentation, methauogenesis, reductive dechlorination, and sulfate- and nitrate-reducing conditions (EPA 2000, Tomei and Daugulis 2013). Depending on the contaminant, a subset of these activities may be cultivated. In anaerobic metabolism, nitrate, sulfate, carbon dioxide, oxidized materials, or organic compounds may replace oxygen as the electron acceptor. Heavier petroleum products such as lubricating oils commonly take a longer time to biodegrade than the lighter products. However, it is also not practical to use to enhance aerobic bioremediation to address petroleum contamination in low permeability, especially in clay soil (EPA

2004). A plume moving with groundwater flow typically produces distinct redox zones—once an electron acceptor is depleted, a new redox reaction utilizing a new electron acceptor occurs. The

Diagram representing the basic concept of bioremediation (USEPA 2001b)

Figure 1. Diagram representing the basic concept of bioremediation (USEPA 2001b).

electron acceptor that would lead to the next largest production of energy during the reaction will dominate (EPA 2000). The main reason for the lack of knowledge and skepticism of bioremediation is that the technology needs experience not only of such fields as environmental engineering but also hydrology. The multidisciplinary characteristics of bioremediation present problems not only for clients and regulators but also for the vendors of environmental clean-up services (NAP 1993).

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