D10: Impact of Contaminants - Hydromod

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Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas EUROLAKES D10: Impact of Contaminants FP5_Contract No.: EVK1-CT1999-00004 Version: 4.0 Date: 25/07/01 File: D10-vers.4.0.doc Page 54 of 136 8.2 REDUCTION OF FOREIGN SUBSTANCES BY MICRO-ORGANISMS [FRITSCHE 1998] mentions, that the biological reduction (biodegradation) is the reduction of the complexity of an organic compound. Normally, a total mineralisation does not happen in the nature because a part of the organic compounds are used for the cell growth. Natural materials are metabolised by micro-organisms. During the evolution micro-organisms had developed a multiplicity of enzymes for the metabolism of natural materials. Because of the industrial development during the last 50 years microorganisms have an added contact with new organic substances. Several of this xenobiotics are metabolised if they are similar to natural materials. If the organisms have any respective enzymes, the reduction stays away. Because of mutation and genetic recombination the evolution is proceeding. In this manner micro-organisms regenerate themselves with new, changed or advanced metabolic efficiencies for the metabolism of foreign matter. But it must be allowed that the evolution takes place slowly and need a pressure of selection. A pressure of selection is also only infrequent at contaminated locations because beside the persistent environmental chemicals other organic compounds exists [FRITSCHE 1998]. Microbial Elimination of Heavy Metals [FRITSCHE 1998] refers that heavy metals can not be metabolised but oxidised or reduced and converted into other compounds. For the elimination of heavy metals from the environment three mechanisms are important: the biotransformation, the biosorption and the sulphide precipitation. At the methylation by fungi and Methanobacterium for the micro-organisms detoxification functions happen but for the environment they mean the relocation from one medium into the gaseous phase connected with the formation of more toxic compounds for human and animals. Toxic chromates and dichromates are reduced by bacteria into lower toxical trivalent chrome. The fixation at metallothioneines (Metallthioneine) at bacteria is evidenced for copper, zinc and cadmium. Metallothioneines are lower molecular rich in cystein polypeptides where heavy metals are bounded at SH-groups [FRITSCHE 1998]. He points out that, in contrast to the bioaccumulation the biosorption is not bounded to living cells. With bacillus-cells preparations were made adsorbing cadmium, chrome, copper, mercury and zinc not selective. The sorption results from highly dilutions. Through stripping the separation of the heavy metals results and through alkali-treatment the biomass is reactivated. In soils a fixation of heavy metals in melanin results through fungi. The fixation of metals were proved with the high in melanin "black yeast" Aureobasium pullans. Due to H2S-fixation through sulphide reducing bacteria in anaerobic sediments heavy metal sulphide precipitation takes place. Reduction of Chlorinated Aromatic Substances [FRITSCHE 1998] refers that there are only a few specialists which are able to use single and twice chlorinated aromatic substances as C and energy sources under selective conditions. The cometabolic turnover to hydroxylated metabolites is more common. They are able to receive more or less bound residues with the soil matrix. If a dehalogenation of the chlorine aromatic substances take place, a mineralisation is possible. For determinably single and twice halogenated aromatic substances four mechanisms are evidenced. The de-chlorination after ring splitting is the most important one. The
Integrated Water Resource Management for Important Deep European Lakes and their Catchment Areas EUROLAKES D10: Impact of Contaminants FP5_Contract No.: EVK1-CT1999-00004 Version: 4.0 Date: 25/07/01 File: D10-vers.4.0.doc Page 55 of 136 other mechanisms are the oxidative de-halogenation, the hydrolytic de-halogenation and the reductive de-halogenation [FRITSCHE 1998]. He describes, the prerequisite at the de-chlorination after ring splitting is, that the enzymes preparing and realising the ring splitting are such non-specific that they can also transform chlorinated analogous substances. This low specificity is infrequent at bacteria and only with some efforts such strains can be concentrated. Obviously they are infrequent in the nature. On one hand strains with non-specific enzymes must be found on the other hand it must be minded that no enrichment of toxic metabolite take place. The reduction reaction and the regulation of the respective enzymes result very specific. For this reason it is difficult to get a strain which is able to use productive a wide spectrum of chlorinated aromatic substances for the growth. Failure to pressure of selection makes sure, that e. g. the metabolism of the phenoxy acid herbicide 2,4-D (2,4-dichlorophenoxy acetic acid) is unrealised. Moreover substances can arise, which are further metabolised only with difficulty and by their reactivity they are bound to the soil matrix. As an example dichlorinebrenzcatechin is mentioned which arises through oxidation from 3,4 dichlorophenol. Soil fungi like penicillium have a relative non-specific working phenol-hydroxylase. It transform a wide spectrum of single and twice chlorinated phenols to chlorobrenzcatechins. At the reduction of the above-mentioned 2,4-D, the brenzcatechin-dioxygenase is catalysing the twiceoxidation. It reacts more substrate specifically, so that the chlorobrenzcatechins temporary be accumulated [FRITSCHE 1998]. He says, it applies to the monochlorophenols, that chloro-substituted phenols are reduced in 4-position well, in 2-position moderate and in 3-position bad. There are also some specialists which partly dechlorinate pentachlorophenol (PCP). Plural the process finished at tetra-, tri- and dichlorophenols. Aerobic bacteria like rhodococcus chlorophenolicus realise a para-hydroxylation to tetrachlorohydrochinon (para-Hydroxylierung zu Tetrachlorohydrochinon). Via hydrolytic and reductive reactions this substance is dechlorinated to 1,2,4-trihydroxybenzene. Organisms like phanerochaete chrysosporium build several metabolites such as the methoxylated compound pentachloroanisole. At the aerobic reductive dehalogenation a wide spectrum of tetra, tri and dichlorophenoles emerge. Thus, possibilities to abolish these environmental chemicals microbial arise because twice chlorinated phenoles are mineralised aerobically. This way the basics for the development of specific bioremidiation methods exist [FRITSCHE 1998]. Limits of Degradability - Polychlorinated Biphenyls and Dioxins Multi-chlorinated aromatic compounds build up of two aromatic rings, are hardly reduced aerobically because the chlorine substitutes impede the enzymatic attack. Thereby the number and the position of the chlorine substitutes and the structure of the molecule play an important part. If this differentiation is not be made seeming contradictory propositions about the persistence level of such compounds take place [FRITSCHE 1998].
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D10: Impact of Contaminants - Hydromod

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