Bioremediation of Soils Contaminated with Both Petroleum Hydrocarbons and Heavy Metals

Soil is the loose surface of the earth crust that supports plant growth. It is an essential resource for human survival because it functions as a "universal sink" by supporting agricultural production and enhancing different economic ties. Introduction of organic and inorganic contaminants such as petroleum hydrocarbons and heavy metals including barium, cadmium and chromium into the soil due to several anthropogenic sources impacts the soil detrimentally. Interference of the soil’s optimal functions is the end result. The presence of these co-contaminants in the soil also poses serious health risks as they have been reported to be mutagenic and carcinogenic. Studies have reported past methods of remediating contaminated soils; however, such methods have several shortcomings. For example, thermal treatment, a type of physical remediation, although fast is not cost-effective. In the light of these, the use of biological agents to restore contaminated sites termed ‘bioremediation’ is considered to be a more eco-friendly and cost-effective means of remediation. Hence, these review aims to highlight some past methods of remediation which were considered effective, identify their shortcomings and discuss extensively the concept of bioremediation of the co-contaminants. The study also identifies novel, ongoing studies, and more effective ways of bioremediating the contaminants.