Dechlorination catalytic carbon

The chloramine removal efficiency of catalytic carbon is reported to be an order of magnitude greater than that of conventional activated carbons used for dechlorination. Various factors such as empty bed contact time (EBCT), influent chloramine concentration, particle size, and temperature influence treatment efficiency using catalytic carbon (11,12). In a study using water containing 2 mg/L influent chloramine concentration, an increase in EBCT from 10 to 30 s increased the volume of water treated to below 0.1 mg/L chloramine from 250 bed volumes to 11,000 bed volumes. In a different study, reducing the mesh size from 20 x 50 to 30 x 70 increased the bed volumes treated from 11,000 to 28,000 at a 30 s EBCT and 2 mg/L influent chloramine... 

The limitations to using catalytic carbon for potable water dechlorination include (i) inability to dechlorinate free chlorine (only chloramines are removed by catalytic carbon) and (ii) potential loss of carbon life due to fouling by organic compounds or oxidation by various compounds. Furthermore, catalytic carbons are generally more expensive than activated carbon as well as other dechlorination methods currently available. 

Catalytic hydrogenation of the chloro substituted starting material (103) over palladium on carbon gives the dechlorinated and hydrogenated product (109) (68JHC53). 3,4-Dihy-dropyrazino[2,3-d]pyridazines can be dehydrogenated either with 2,3-dichloro-5,6-dicyanoquinone or potassium ferricyanide (75CPB1505). 

Weckhuysen et al. found the destruction of carbon tetrachloride (CCI4) on Ce02 or La203 in the presence of oxidants. In the process, the rare earth oxides are chlorinated (Weckhuysen et al., 1999). Later they reported the catalytic destruction of CCI4 over a series of lanthanide and alkaline-earth metal oxides based catalysts in the presence of water steam (Van der Avert et al., 2004). The catalysts combined the destruction on basic oxide surface and concurrent dechlorination of the partially chlorinated solid by steam. The two noncatalytic reactions combined into a catalytic cycle in this way. 

The ion-exchange catalyst dechlorinates the plastics (e.g. PVC) and therefore avoids issnes with HCl generation and chlorine contamination of the diesel. The company claims to have processed cable waste consisting of almost 100% PVC and the catalytic depolymerization process produced diesel fuel with a chlorine content below the detection limit. The catalyst needs to be activated before use via ion exchange using soda (sodium carbonate) and lime (calcium hydroxide) in order to insert the sodium and calcium ions... 

Hagenmaier [158] studied the catalytic effect of metals, oxides and carbonates on the decomposition of chloroaromatics such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofuranes. Copper catalyses dechlorination/hydrogenation of these compounds at temperatures of about 553 K. After heating for 15 minutes the amount of most polychlorinated compoimds was below 0.1 ng. 

The dechlorinating power of carbon can be influenced by the presence of other constituents in the water supply. Ammonia will form chloramines that are less readily removed by carbon. The catalytic removal of chlorine is accelerated at a low pH and retarded at a high pH. 

One of the uses of o-chloronitrobenzene is in the production of o-chloroaniline, which is obtained by catalytic reduction with sulfided palladium/active carbon catalysts. These catalysts enable dechlorination to be suppressed. 

Activated Carbon. As a special category of catalytic decomposition of species of free chlorine, we include chemisorption on activated carbon. Beds of granular activated carbon have long been used to dechlorinate water after its puriflcation [231-233]. Because of its tendency to form surface oxides, activated carbon has the ability to abstract the oxygen generated by hydrolysis of chlorine. As an example, the reaction of HOCl can be written as ... 

Due to chlorines deleterious effects on polyamide membranes, it [and more specifically, free chlorine (i.e., hypcochlorite, + hypochlorous acid + chlorine gas + trichloride ion)] must be removed to prevent contact with the membranes. Dechlorination is relatively simple, typically using either sodium bisulfite to chemically remove free chlorine or carbon filtration to catalytically remove chlorine (see chapter 8.2.3. and 8.1.4, respectively). 

The use of chitosan-Pd nanocomposites for heterogeneous catalytic reactions is the most extensively studied system and reported in the literature. For example, aUylic substitution reactions of ( )-cinnamyl ethyl carbonate by morpholine was reported by Quignard et al. (2000) using a chitosan-supported palladium complex. Dechlorination reaction of chlorophenol to phenol, degradation of nitrophenol, and reduction of chromate were studied by Vincent et al. (2002, 2003, 2004). Hydrogenation of nitrobenzene to aniline by chitosan-impregnated palladium composites was also reported by Jin et al. (1994). Excellent review of chitosan-based materials used in the heterogeneous catalytic reactions... 

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