Dechlorination effect

This section summarizes the field studies performed to compare the efficiencies of dechlorination chemicals under identical conditions to evaluate the chemical of choice for various dechlorination applications (18,19). The field tests were conducted at Tacoma Waters, WA, Portland Bureau of Water Works, OR, and East Bay Municipal Utility District (EBMUD), CA. Six dechlorination chemicals were used in solution, tablet, or powder form in these tests (Table 3). In the Tacoma and Portland studies, a 1 % solution of the dechlorination chemicals were introduced into water released from a hydrant. The field studies evaluated the rate of dechlorination, effect of overdosing, and concurrent water quality impacts when stoichiometric or twice the stoichiometric amounts of dechlorination agents were added. In the EBMUD dechlorination studies, bags, or dispensers containing tablets or powders of dechlorination chemicals were placed in the flow path of hydrant water. At all three sites, the water used for the test originated from surface water sources rather than from groundwater sources. Table 3 summarizes the chemicals, forms and dosing rates used in these studies. 

Removed)Required for Complete Dechlorination. Effect of Water ... 

R is seen from Fig. 6.35 that the catalyst reduced at 200°C has the highest activity. R cannot be reduced completely when the temperature is lower than 200° C. The change of temperature is not the significant effect on the catalyst activity when the temperature is in the range of 200°C-300°C. The activity is obviously decreased when the temperature is over 300°C. d4i,is6 illuminated the dechlorination effect, the best when the temperature is about 200°C. 

Zeng et al. have studied the effect of the temperature and time of dechlorination on the catalytic activity (Fig. 6.36). R was found that the dechlorination effect is not complete (only 96% chloride ions were removed) when the catalysts were reduced by H2 at low temperatures such as 350° C and 400° C. But there is... 

Chlorine is desirable as a bulk pretreatment biocide for inlet water, but its subsequent removal upstream of the membrane is absolutely necessary ana difficult. NaHSO,3 is a common additive to dechlorinate before membranes. It is customarily added at 3-5 mg/1, an excess over the stoichiometric requirement. NH3 is sometimes added to convert the chlorine to chloramine, a much less damaging biocide. Heavy metals present in seawater seem to amplify the damaging effects of chlorine and other oxidants. 

Some Comparisons of Substituent Effects in the Nucleophilo-DECHLORINATION OF AzA-BeNZENE AND AzA-NapHTHALENE DERIVATIVES... 

As to the electron-withdrawing substituents, the activating effect of a nitro group in the piperidino-dechlorination of 2-chloropyridine involves factors of 7.3 x 10 and 4.6 x 10 from the para and ortho positions, respectively. An ortho-cyano group was found to be... 

Substituent Effects for the Methoxy-Dechlorination of Some Fused-Ring Systems ... 

The ortho indirect deactivating effect of the two methyl groups in 2,6-dimethyl-4-nitropyridine 1-oxide (163) necessitates a much higher temperature (about 195°, 24 hr) for nucleophilic displacement of the nitro group by chloride (12iV HCl) or bromide ions N HBr) than is required for the same reaction with 4-nitropyridine 1-oxide (110°). With 5-, 6-, or 8-methyl-4-chloroquinolines, Badey observed 2-7-fold decreases in the rate of piperidino-dechlorination relative to that of the des-methyl parent (cf. Tables VII and XI, pp. 276 and 338, respectively). 

The effect of a carboxy group is illustrated by the reactivity of 2-bromopyridine-3- and 6-carboxylic acids (resonance and inductive activation, respectively) (cf. 166) to aqueous acid under conditions which do not give hydroxy-debromination of 2-bromopyridine and also by the hydroxy-dechlorination of 3-chloropyridine-4-car-boxylic acid. The intervention of intermolecular bifunctional autocatalysis by the carboxy group (cf. 237) is quite possible. In the amino-dechlorination (80°, 4 hr, petroleum ether) of 5-carbethoxy-4-chloropyrimidine there is opportunity for built-in solvation (167) in addition to electronic activation. This effect of the carboxylate ion, ester, and acid and its variation with charge on the nucleophile are discussed in Sections I,D,2,a, I,D,2,b, and II,B, 1. A 5-amidino group activates 2-methylsulfonylpyridine toward methanolic am-... 

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. ... 

A bifunctional autocatalytic effect of azinones in general is possible in certain nucleophilic reactions such as amination. Zollinger has found that 2-pyridone is the best catalyst for anilino-dechlorination of various chloroazines. It seems likely that examples of autocatalysis will be found when the substrate contains an azinone moiety. The azinone hy-products of displacement reactions may also function in this way as catalysts for the main reaction. 

The catalytic effect of protons, of bifunctional catalysts, and of base is demonstrated in the amination of chloro derivatives of pyridazine, pyrimidine, and s-triazine (Tables V and VI). Anilino-s-triazines containing NH groups act as catalysts in their own formation. The catalytic action of protons on anhino-dechlorination of 2-chloro-4,6-diamino-s-triazine and of 2-amino-4-chloropyrimidine was reported in the classic paper by Banks. ... 

The.effect of the entropy of activation was noted above for the quaternary pyridine salts (280 and 281). In future work, it may also be found to reflect the electrostatic or hydrogen-bonding interactions in transition states of amination reactions and the effect of reversible cationization of an azine-nitrogen. Brower et observed a substantial rate difference between piperidino-dechlorinations of 2-chloropyrimidine in petroleum ether and in alcohol due partly to the higher entropy of activation in the latter solvent (Table III, lines 3 and 4). 

Three comparisons of para vs. ortho activation by an additional ring-nitrogen are possible from the available data and all show the former to be more effective, as found for the nitronaphthalenes above and for monocyclic azines (Section III). In piperidino-dechlorination of chloroquinazolines (Table XV, lines 3 and 4), the 4-isomer (405) reacts 6,500-fold faster than the 2-chloro compound (400) due entirely to a 4.1 kcal decrease in the dS being the same for both. However,... 

Burns SE, JP Hassett, MV Rossi (1996) Binding effects on humic-mediated photoreaction intrahumic dechlorination of mirex in water. Environ Sci Technol 30 2934-2941. 

Wu Q, DL Bedard, J Wiegel (1997a) Effect of incubation temperature on the route of microbial reductive dechlorination of 2,3,4,6-tetrachlorobiphenyl in polychlorinated biphenyl (PCB)-contaminated and PCB-free freshwater sediments. Appl Environ Microbiol 63 2836-2843. 

Attention has been directed to the dechlorination of polychlorinated benzenes by strains that use them as an energy source by dehalorespiration. Investigations using Dahalococcoides sp. strain CBDBl have shown its ability to dechlorinate congeners with three or more chlorine substituents (Holscher et al. 2003). Although there are minor pathways, the major one for hexachlorobenzene was successive reductive dechlorination to pentachlorobenzene, 1,2,4,5-tetrachlorobenzene, 1,2,4-trichlorobenzene, and 1,4-dichlorobenzene (Jayachandran et al. 2003). The electron transport system has been examined by the use of specific inhibitors. lonophores had no effect on dechlorination, whereas the ATP-synthase inhibitor A,A -dicyclohexylcarbodiimide (DCCD) was strongly inhibitory (Jayachandran et al. 2004). 

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