## 4 Process of Remediation for Oil-Contaminated Soil
### Techniques for Biological Remediation  
Bioremediation, or biodegradation, is how microorganisms digest crude-oil molecules through enzymatic processes, resulting in carbon dioxide, biomass, and water-soluble chemicals. Some bacterial types prevalent in the climate, such as mould, yeast, archaea, and fungus, may feed on crude-oil molecules [29; 30; 31]. Compared to chemical or physical reform, bioremediation is frequently employed because of its benefits of tremendous impact, quick decomposition rate, and absence of minor contamination.  
Biological cleaning uses bacteria that can eat hydrocarbons, producing CO\({}_{2}\) and H\({}_{2}\)O as waste. It was found in the water's coastline, sediments, columns, and surfaces. Microbes that consume hydrocarbons arise naturally and increase after a spill as more nutrient sources become accessible. Heavy components are too complicated for bacteria to break down immediately and may take months or years. As far as biodegradable hydrocarbons are available, biodegradation begins quickly after a spill. It peaks during the first month following the spill but may be controlled by environmental nutrients.  
Onsite bioremediation involves scraping polluted subsoil. Biological methods, such as hydrocarbon breakdown by physical procedures, chemical processes, or microorganisms such as soil venting or air sparing, are used for removing toxins from subsoil and groundwater. The hydrocarbon degradation method may need vertical and horizontal drilling, and it works better on sandy soils than clay soils. Surface pollution may be treated using spreading devices. It also increases aerobic bacteria growth by providing them with oxygen.  
Ex situ bioremediation approaches need off-site soil treatment. Incinerating, chemically extracting, or washing the soil removes hydrocarbons. Steam stripping, combustion, extraction, and biological treatments are major ex situ treatment procedures. Chemical extraction uses solvents to dissolve or suspend soil pollutants. It may be used on various crudes and soils but works best on low-clay soil offsite or onsite, although off-site is faster and more efficient. In situ or ex situ surfactant washing is possible. Ex situ procedures may be used if the quantity of polluted soil is modest or if the pollution happened in an industrial or residential location. Offsite treatment is more effective since the pH, temperature, salinity, and moisture may be controlled.  
Figure 12: Relationship between the cement content and specific gravity of contaminated soil.  
Bioremediated oil-polluted soil utilising microbial consortium and agricultural residues degraded oil ingredients [32]. After 40 days, 68% of the oil was decomposed using the water of pigs and other additives, proving that specific strains of bacteria may be employed in bioremediation. Ochroductrum, Stentrophomonas maltophilia, and Pseudomonas aeruginosa degrade ore oil (3% \(v/v\)) by more than 80% [33], whereas a separate combination of bacteria degraded diesel-contaminated soil by 89% after 365 days. Most hydrocarbons are hydrophobic and bioavailable; therefore, bioremediation may not work [34].