1. Biological Remediation of Soil: An Overview of Global Market and Available Technologies

Due to a wide range of industrial and agricultural activities, a high number of chemical contaminants is released into the environment, causing a significant concern regarding potential toxicity, carcinogenicity, and potential for bioaccumulation in living systems of various chemicals in soil. Although microbial activity in soil accounts for most of the degradation of organic contaminants, chemical and physical mechanisms can also provide significant transformation pathways for these compounds. The specific remediation processes that have been applied to clean up contaminated sites include natural attenuation, landfarming, biopiling or composting, contained slurry bioreactor, bioventing, soil vapor extraction, thermal desorption, incineration, soil washing and land filling (USEPA 2004).

Biological remediation using microorganisms and plants is generally considered a safe and less expensive method for the removal of hazardous contaminants. The microorganisms have the primary catalytic role in degrading or mineralizing various contaminants and converting non-toxic by-products during soil bioremediation processes (Seshadri and Heidelberg 2005; Head et al. 2006; Gomez et al. 2007). Plants have an inherent ability to detoxify some xenobiotics in soil by direct uptake of the contaminants, followed by subsequent transformation using enzymes similar to detoxification enzymes in mammals, transport and product accumulation (Macek et al. 2008). Phytoremediation, with the associated role of rhizospheric microorganisms, is therefore an important tool in bioremediation processes. Various bioremediation configurations as options for treatment of different classes of chemicals have been evaluated (Hughes et al. 2000). Natural attenuation and electron donor delivery were considered as options for remediation of chlorinated solvents, biostimulation for treatment of chlorinated solvents and phenols, bioventing for polycyclic aromatic carbons (PAHs); landfarming or composting were options for nitroaromatics, phenols, monoaromatic hydrocarbon and PAHs (Prince 1998; Mishra et al. 2001). Slurry bioreactor processes were considered suitable for treatment of all of the above mentioned chemicals. Optimizing the environmental conditions in bioremediation processes ensures that the physiological and biochemical activities are directed towards biodegradation of the target contaminants. Environmental factors influencing biological activity include moisture, temperature, pH, oxygen, soil type and chemical nature of contaminant for aerobic degradation and redox potential for anaerobic degradation (Van Hamme et al. 2003).