Lexikon

decomposition or breakdown of a substance through the action of microorganisms, such as bacteria or fungi.

Decomposers of organic matter are found in the soils. These groups of living organismsm perform different functions:

• Microflora: certain types of bacteria and fungi are the major or primary decomposers; they are capable of digesting complex organic matter and transforming it into simpler substances that can be utilised by other organisms;

• Microfauna: certain types of protozoa and nematodes feed on or assimilate microbial tissues and excrete mineral nutrients;

• Mesofauna: includes a large number of organisms, ranging from small arthropods like mites (Acari) and springtails (Collembola) to potworms (Enchytraeidae). They break up plant detritus, ingest soil and organic matter or feed on primary decomposers thereby having a large influence on regulating the composition and activity of soil communities;

• Macrofauna: including ants, termites, millipedes and earthworms, contribute to organic matter decomposition by breaking up plant detritus and moving it down into the soil system thereby improving the availability of resources to microflora (through their nest building and foraging activities).

enhanced biodegradation or biostimulation means the addition of nutrients to encourage the growth of indigenous contaminant-degrading microorganisms, is one of the most mature methods of bioremediation. It is applicable to both chlorinated and unchlorinated dissolved hydrocarbons.

Biostimulation is dependent on indigenous organisms and thus requires that they are present and that their environment can be altered in a way that will have the desired bioremediation effect. In addition to an explanation of the concept of biostimulation, this chapter discusses critical aspects of site biogeochemistry, characterization and monitoring, combined biological technologies, and research needs.

Source: US-EPA, Clu-In: http://www.clu-in.org/techfocus/default.focus/sec/Bioremediation_of_Chlorinated_Solvents/cat/Overview/

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 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 NO₃^2-, SO₄^2-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 SeO₄^2- reduction to selenite: SeO₃^2- and Se⁰, arsenate AsO₄^3- reduction to arsenite: AsO₃^3- 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.