Primary dechlorination

Depleted brine will be physically saturated with chlorine, and some chlorine wUl react to form hypochlorite (Section 7.5.9.1). This chlorine value represents an economic asset to be recovered and, particularly in the case of membrane cells, an intolerable contaminant in the brine treatment system. There are several approaches to this problem [208], and we cover these below. We divide them into methods aimed at recovery of the bulk of the chlorine in a useful form (primary dechlorination Section 7.5.9.2) and those whose purpose is to reduce the active chlorine to chloride and safeguard the environment or other parts of the process (secondary dechlorination Section 7.5.9.3). Some of the hypochlorite that forms in the anolyte will continue to react to form chlorate. This is a much less harmful impurity in the cells, and higher concentrations are tolerable. Many plants keep the chlorate concentration under control by natural or deliberate purges from the brine system (Section 7.5.7.2A). In others, it is necessary to reduce some of the chlorate ion to chloride in order to maintain control (Section 7.5.9.4). 

Primary Dechlorination. The dechlorination process will have some combination of the following four objectives ... 

Brine Dechlorination. Operation of the primary dechlorinator should be established next. It is not easily possible to simulate actual dechlorination. However, where vacuum dechlorination is used, it is important to establish that the vacuum and condenser systems operate properly. In the case of a conversion from mercury-cell technology, this may already be established. 

Batches of reagent should be prepared and their feed systems operated to demonstrate the ability to dose and control at correct rates. Small quantities of hypochlorite solution can be injected into the anolyte tank to simulate primary dechlorinator product ( 25 ppm as CI2) for testing of the ORP loop. 

The activating effect of a trichloromethyl group is seen in the 2-dechlorination reactions of 2-chloro-4,6-bis(trichloromethyl)-s-tria-zine (175) with arylsulfonylhydrazides (24 hr) and heterocyclic amines (3 hr) at 20° and with unbasifled primary and secondary alcohols (65°, 30 min). The 4,6-diphenyl or 4,6-bis(4-chlorophenyl) analogs do not react in this manner. ... 

Vitamin B12 catalyzed also the dechlorination of tetrachloroethene (PCE) to tri-chloroethene (TCE) and 1,2-dichloroethene (DCE) in the presence of dithiothreitol or Ti(III) citrate [137-141], but zero-valent metals have also been used as bulk electron donors [142, 143]. With vitamin B12, carbon mass recoveries were 81-84% for PCE reduction and 89% for TCE reduction cis-l,2-DCE, ethene, and ethyne were the main products [138, 139]. Using Ni(II) humic acid complexes, TCE reduction was more rapid, leading to ethane and ethene as the primary products [144, 145]. Angst, Schwarzenbach and colleagues [140, 141] have shown that the corrinoid-catalyzed dechlorinations of the DCE isomers and vinyl chloride (VC) to ethene and ethyne were pH-dependent, and showed the reactivity order 1,1-DCE>VC> trans-DCE>cis-DCE. Similar results have been obtained by Lesage and colleagues [146]. Dror and Schlautmann [147, 148] have demonstrated the importance of specific core metals and their solubility for the reactivity of a porphyrin complex. 

Mirex is a very persistent compound in the environment and is highly resistant to both chemical and biological degradation. The primary process for the degradation of mirex is photolysis in water or on soil surfaces photomirex is the major transformation product of photolysis. In soil or sediments, anaerobic biodegradation is also a major removal mechanism whereby mirex is slowly dechlorinated to the 10-monohydro derivative. Aerobic biodegradation on soil is a very slow and minor degradation process. Twelve years after the application of mirex to soil, 50% of the mirex and mirex-related compounds remained on the soil. Between 65--73% of the residues recovered were mirex and 3-6% were chlordecone, a transformation product (Carlson et al. 1976). 

Under anaerobic conditions, mirex was slowly dechlorinated to the 10-monohydro derivative by incubation with sewage sludge bacteria for two months (Andrade and Wheeler 1974 Andrade et al. 1975 Williams 1977). The primary removal mechanism for mirex was anaerobic degradation as demonstrated by the 6-month stability of the compound in nine aerobic soils and lake sediments (Jones and Hodges 1974). 

Fig. 11.7. The two pathways of dechlorination of chloramphenicol (11.39). Cytochrome P450 catalyzed oxidation to yield the oxamic acid derivative (11.40), and hydrolytic dechlorination to yield both oxamic acid (11.40) and primary alcohol (11.41) metabolites [75].

In contrast to oxidative dechlorination, the hydrolytic dechlorination of chloramphenicol replaces a Cl-atom with a OH group to yield a (monochlo-ro)hydroxyacetamido intermediate. The latter, like the dichloro analogue, also eliminates HC1, but the product is an aldehyde that is far less reactive than the oxamoyl chloride intermediate. Chloramphenicol-aldehyde undergoes the usual biotransformation of aldehydes, namely reduction to the primary alcohol 11.41 and dehydrogenation to the oxamic acid derivative 11.40 (Fig. 11.7). 

In conclusion, the oxamic acid derivative is produced by two distinct metabolic pathways, namely by oxidative and hydrolytic dechlorinations. In contrast, the primary alcohol metabolite 11.41 can be produced only by hydrolytic dechlorination and is, thus, an unambiguous marker of this pathway. The alcohol 11.41 is a known urinary metabolite of chloramphenicol in humans. 

Racemic -amino acids, namely 3,3-ethano- -alanine (193 a) and its spiro-cyclopropanated analog 193b, have been prepared by simple one-step hydrogenolytic reduction and deprotection of the primary Michael adducts 94 b and 94 d (Scheme 57) [9,21b]. As methyl esters cannot be deprotected under these conditions, only reductive dechlorination and debenzylation was observed for compounds 91t,u (Scheme 57) [7k]. This looks promising for the construction of... 

In order to determine the reaction pathway for 1,2-dichlorobenzene, Schiith fitted kinetic data and found that the primary pathway was direct reaction to benzene with a parallel reaction of sequential dechlorination through chlorobenzene to benzene. (Figure 7) These pathways were further supported by independent determination and verification of the reaction rate constant for the second hydrodechlorination step. (Schiith and Reinhard 1998)... 

It is quite plausible that some of the cis-l,2-DCE observed by us was produced by the reductive dechlorination of TCE, with the graphite at the cathode being the primary source of electrons, as in the following reaction ... 

Dichlorprop and 2,4-D are structurally related dichlorinated phenoxy-acetic acid pesticides (see Table 1). The photochemistry of 2,4-D has been studied some time ago 2,4-dichlorophenol was found as the the main product [75]. More recent photoproduct analysis studies are available for dichlorprop [76,77]. Climent et al. identified several products corresponding to dechlorination on the one hand, and to decarboxylation on the other ho-molytic bond cleavage steps were proposed to take place [76]. In a later study, Meunier et al. were able to demonstrate that 4-chloropyrocatechol is the major primary photoproduct [77]. A heterolytic cleavage of the ether bond and of the C - Cl bond was put forward as a possible mechanism (Scheme 9). 

Just as the interiors of micelles in water provide a hydrophobic environment to solubilize chlorobiphenyls and then photodegrade them by reductive dechlorination, so do octadecyl-functionalized silica gel [89], normally used as reverse phase packing in HPLC and for solid-phase extraction, and the hquid-semisohd polydimethylsiloxane [-OSi(CH3)2 -] [90,91], also used for solid-phase extraction. In both media, reductive dechlorination is the primary photochemical pathway. 

The toxic effects of chloropicrin occur very rapidly. The liver is the primary site of metabolism of this compound. Reductive dechlorination of chloropicrin serves as the basis for its multiple types of toxic action. Following an intraperitoneal or oral administration of chloropicrin, urine is the major route for excretion of its metabolites, mostly (43-47%) within the first 24 h. The urinary metabolites at 24 h are polar and nonvolatile. 

Primary, secondary, and tertiary alkyl chlorides as well as aryl chlorides undergo reductive dechlorination using Sml2, and in the presence of CO ketones are formed by photocarbonylation. ° ... 

In a recent study, Liu et al. (41) used two types of Fe p) particles in exploring the TCE degradation particles synthesized by aqueous phase reduction of ferrous or ferric iron by sodium borohydride (Fe average primary particle size = 30 to 40 nm) and commercially available reactive nanoscale iron particles (RNIP), synthesized by gas phase reduction of iron oxides in hydrogen (Fe average primary particle size = 40 to 60 nm ). Both particles had a Fe(0) core structure with similar specific surface areas (30 m /g). The shell structure for Fe was found to be oxide or oxyhydroxide of some sort while Fe contained Fe304 as the only oxide in the shell. Employed dechlorination reaction conditions included high TCE concentration (limited iron) and low TCE concentration (excess iron). The measured surface area normalized... 

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