aerobic conditions or processes are those, which require oxigen. In environmental and biological systems biologically available oxigen can be derived from the atmospher in the form of atmospheric air, from the water in the form of water-dissolved oxigen for aquatic-ecosystem, or in the form of soil-air for the soil living organisms. If the oxigen is not enough or not available, the conditions are anoxic. If no oxigen is present at all, we speak about anaerobic conditions.
The biological activity in the presence of oxigen is called: aerobiosis, under anaerobic conditions: anaerobiosis.
anaerobic is a technical word which means without air. Air is generally used to mean atmospheric oxygen. Anaerobic is the opposite aerobic.
In the environmnetal technologies the absence of atmospheric oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen.
for the in situ or ex situ remediation of hydrocarbons, pesticides, chlorinated substances contaminated soils the altering oxic-anoxic or aerobic-anaerobic treatment is an efficient bioremediation alternative. The steps of the technology application are:
1. Addition of organic soil amendment, zero valent iron, and water to produce anoxic conditions.
2. Periodic tilling of the soil to promote oxic conditions.
3. Repetition of the anoxic-oxic cycle until the desired cleanup goals are achieved.
The addition of DARAMEND® organic amendment, zero valent iron, and water stimulates the biological depletion of oxygen, generating strong reducing anoxic conditions within the soil matrix. The diffusion of replacement oxygen into the soil matrix is prevented by near saturation of the soil pores with water. The depletion of oxygen creates a low redox potential, which promotes dechlorination of organochlorine compounds. A cover may be used to control the moisture content, increase the temperature of the soil matrix and eliminate runon/run off.
The soil matrix consisting of contaminated soil and the amendments is left undisturbed for the duration of the anoxic phase of treatment cycle typically 1-2 weeks. In the oxic phase of each cycle, periodic tilling of the soil increases diffusion of oxygen to microsites and distribution of irrigation water in the soil. The dechlorination products formed during the anoxic degradation process are subsequently removed trough aerobic oxic biodegradation processes, initiated by the passive air drying and tilling of the soil to promote aerobic conditions.
soil bioremediation based on aerobic oxidation means that the soil remediation is based on aerobic biodegradation. The microbiological biodegradation occurs in this case on a high redoxpotential of +0,8-+0,6 Volt. The degrading microorganisms utilise the pollutant as enbergy sources. The source of oxigen is the atmospheric air, soil air, or dissolved oxigen in soil moisture or ground water. If the oxigen-concentration is low, the technologist can increase it by aeration of the soil or the groundwater as well as by adding peroxide substances or other oxigene release compounds ORC to serve as oxigene source for the activation of the aerobic soil microbes.
soil remediation based on aerobic biodegradation is an oxidative process catalysed by microbes. Microbes, mainly bacteria utilise the contaminant as substrate for producing energy. Aerobic bacteria use athmospheric oxigen for the oxidation of the polluting organic compounds and produce inorganic products, such as CO2, NO3 and H2O. This process is also called mineralisation.
When athmospheric oxigen is limited, the biodegradation is catalysed by facultative anaerobic microbes, which use NO3 for their alternative respiration. In this case the oxidation/mineralisation products from the substrate the contaminant are alcohols or aldehydes.
anaerobic biodegradation of soil contaminants is based on the aternative respiration of soil microorganisms, using oxigen from NO32-, SO42-or CO2, as hydrogen-acceptor instead of atmospheric oxigen. Paralel to the oxidation of the contaminant energy source in this case, nitrate, sulfate and carbonate are reduced into N2 via nitrite NO2−, nitric oxide NO, nitrous oxide N2O, H2S and CH4 respectively.
There are some metals which can also be reduced and function as electronacceptor, such as ferric ion Fe3+reduction to Fe2+ or Fe0, manganic ion Mn4+ reduction to Mn2+, selenate SeO42- reduction to selenite: SeO32- and Se0, arsenate AsO43- reduction to arsenite: AsO33- or uranyl ion UO22+ reduction to uranium dioxide UO2 for the electron transport chain.
The anaerobic biodegradation of xenobiotics needs a microorganism- and metabolism-specific redoxpotential. The soil remedial biotechnology is responsible for ensuring the proper redoxpotential in the soil to control the process and run biodegradation on the optimum.
To control the redoxpotential the technologist should ensure sufficient quantity of nitrate, sulfate or any other electronacceptors in the soil.