Bioremediation of Polluted Soil

The formation of soil is influenced by parent material, climate, activity of microorganisms and macroorganisms, and the topography of the place. Soil texture is defined as the relative proportions of sand, silt, clay, and colloid fraction. Humus is defined as the total of the deeply transformed organic compounds in soil. Microbial activity in soil is limited by water activity, dissolved oxygen, and nutrient concentrations. Soil is a habitat for viruses, bacteria and archaea, fungi, algae, protozoa, and nematodes. Microbial function in soil is the biogeochemical cycling of carbon and other elements following accumulation of metabolites and evolution of a biosphere. The biogeochemical cycle of carbon includes assimilation of CO₂ from the atmosphere and the mineralization of organic matter. The biodegradation of organic matter by fermentation and by anaerobic and aerobic respiration is the most important process in wastewater and solid waste biotreatment. The anaerobic decomposition of organic matter requires the participation of four metabolic groups of bacteria: (1) hydrolytic bacteria; (2) fermentative acidogenic bacteria; (3) acetogenic bacteria (fatty acid oxidizing bacteria); and (4) methanogens. The biogeochemical cycle of nitrogen includes nitrogen fixation by free-living and symbiotic nitrogen-fixing bacteria, ammonification, nitrification by ammonium- and nitrite-oxidizing bacteria, and denitrification. The electron donors for denitrification can be such organic substances as methanol and ethanol and inorganic substances such as H₂, S, H₂S, or Fe²⁺. The biogeochemical cycle of sulfur includes the oxidation of organic sulfur, H₂S, or S, and sulfate reduction. The biogeochemical cycle of phosphorus includes mineralization of organic phosphorus, assimilation of phosphate, precipitation or solubilization of phosphate, and the accumulation of polyphosphate under aerobic conditions. The biogeochemical cycle of iron includes oxidation of Fe(II) and reduction of Fe(III). Fe(II) is stable under neutral pH and anaerobic conditions. The solubility of Fe(III) and Fe(II) depends on the formation of their chelates with organic acids. The problems of soil bioremediation could be: (1) absence of indigenous soil microorganisms capable of degrading soil pollutant; (2) suitable temperature; (3) provision of oxygen for microbial growth and oxidation; (4) provision of soluble inorganic nutrients for microbial growth; and (5) the soil pores plugging with microbial biomass. There are numerous methods for in situ (air sparging, bioventing), on-site (landfilling, composting), and off-site bioremediation of soil using slurry-phase cultures or biotreatment in a rotating drum.