Degradation of haloaromatic compounds

An ever increasing number of halogenated organic compounds has been produced by industry in the last few decades. These compounds are employed as biocides, for synthetic polymers, as solvents, and as synthetic intermediates. Production figures are often incomplete, and total production has frequently to be extrapolated from estimates for individual countries. Compounds of this type as a rule are highly persistent against biodegradation and belong, as "recalcitrant" chemicals, to the class of so-called xenobiotics. This term is used to characterise chemical substances which have no or limited structural analogy to natural compounds for which degradation pathways have evolved over billions of years. Xenobiotics frequently have some common features. e.g. high octanol/water partitioning coefficients and low water solubility which makes for a high accumulation ratio in the biosphere (bioaccumulation potential). Recalcitrant compounds therefore are found accumulated in mammals, especially in fat tissue, animal milk supplies and also in human milk. Highly sophisticated analytical techniques have been developed for the detection of organochlorines at the trace and ultratrace level

Effect of pH on the anaerobic dechlorination of chlorophenols in a defined medium

[[abstract]]Anaerobic dehalogenation of aromatic compounds is a well-documented phenomenon. However, the effects of operating parameters such as pH have received little attention despite their potential impact on treatment processes using dehalogenating organisms. In this work the effect of pH on the dehalogenation of 2,4,6-trichlorophenol (2,4,6-TCP) was studied using defined media containing one of several non-fermentable buffering agents (MOPS, TRICINE, BICINE, CHES), and no chloride ions. The dechlorination process was followed by monitoring the disappearance of 2,4,6-TCP, as well as the appearance of its dehalogenation products, i.e., 2,4-dichlorophenol (2,4-DCP), 4-chlorophenol (4-CP), and chloride ions. The results indicate that dechlorination occurs only if the pH is within the range 8.0–8.8. The newly formed 2,4-DCP was also dehalogenated in the process. However, even within this pH range dechlorination ceased when all 2,4,6-TCP and 2,4-DCP was converted to 4-CP. Stoichiometric amounts of all dehalogenation products (including chloride) could be recovered at any stage during the process. In addition, the biomass concentration was measured. After an initial lag phase, it appeared that the rate of dechlorination per unit biomass (proportional to the Cl− concentration divided by the biomass concentration) went through a rapid increase and then remained constant throughout the process. This indicates that the dechlorinating organism(s) either make up the entire population or constitute a stable fraction of it