Chemical methods, dechlorination

Direct electrolytic dechlorination of 9-chloroanthracene at a mercury electrode occurs at about -1.65 V (see) in a layer of adsorbed cetyltrimethylammonium bromide on the electrode surface233. Similarly, electrochemical degradation of trichloroethylene in acetonitrile resulted in quantitative conversion to chloroacetylene, which was reduced further to acetylene at a more negative reduction potential (-2.8 V) in 96% yield234. Reductive destruction of 1,3,5-trichlorobenzene in the cathode compartment could be observed235. Electrochemical methods presumably can be used for decontamination of chemical warfare agents such as mustard derivatives as an alternative to the chemical methods such as base-catalyzed dehydrohalogenation236. 

MeS02-CBs are resistant to degradation by strong acids and bases, and are thus stable compounds. The sulfone group is not easily reduced by chemical methods without the risk of a concurrent dechlorination reaction. MeS02-PCBs... 

Dechlorination is the process of converting highly reactive chlorine from these waters into less reactive chloride ions prior to disposal into receiving streams. Various chemical and nonchemical techniques are currently used for disposal of chlorinated waters by water and wastewater agencies. For example, wastewater treatment plants use sulfur dioxide gas or sodium metabisulfate to dechlorinate treated effluent prior to release into receiving streams. Many water utilities often use passive, non-chemical methods such as discharge to sanitary sewers for disposal of chlorinated waters. Impurities such as organics, iron, and sulfide in the sanitary sewer exert a chlorine demand and neutralize chlorine in the released water. 

The effectiveness of various passive non-chemical methods as well dechlorination chemicals for disposal of chlorinated water is discussed in this section. Furthermore, water quality impacts, health and safety concerns, and dose calculations for dechlorination of both free and combined chlorine using these techniques are discussed. Utilities must verify the chlorine levels (measured as total chlorine) of discharged water prior to release to receiving streams regardless of the method chosen for dechlorination. 

Whenever it is not possible to dispose of chlorinated waters safely by non-chemical methods, chlorine must be neutralized using dechlorination chemicals. Several solid, liquid, and gaseous dechlorination chemicals are commercially available and are widely used by water and wastewater utilities. Benefits and limitations of various chemicals used for dechlorination are summarized below. 

When used in powder or crystal form, dechlorination chemicals (ascorbic acid and sodium thiosulfate) dissolved rapidly causing water-quality concerns, although physical methods (tablets) have been developed since to slow down dissolution rates. Sodium sulfite, when used in tablet form, was very effective in dose control. One tablet was sufficient to dechlorinate 2 mg/L of chloraminated water to below 0.1 mg/L for 45 min when water was released at 100 gpm. Finally, these field tests also indicated that the flow rates of chlorinated waters can significantly impact the efficiency of dechlorination operations. 

Because endrin and endrin aldehyde are listed as hazardous substances, disposal of wastes containing these compounds is controlled by a number of federal regulations (see Chapter 7). Land disposal restrictions apply to wastes containing endrin or endrin aldehyde (EPA 1986d, 1987b). Chemical treatment (reductive dechlorination) or incineration are possible disposal methods (HSDB 1995 IRPTC 1985). Past disposal... 

Microbial fouling is best dealt with before biofilm becomes mature. Biofilm protects the microorganisms from the action of shear forces and biocidal chemicals used to attack them. Microbes can be destroyed using chlorine, ozone, ultraviolet radiation, or some non-oxidizing biocides (see Chapters 8.2.1,8.2.2, 8.1.8, and 8.2.5, respectively). An effective method to control bacteria and biofilm growth usually involves a combination of these measures. Specifically, chlorination or ozonation of the pretreatment system, followed by dechlorination to protect the membranes, or UV distraction followed by periodic sanitation with a non-oxidizing biocide used directly on the membranes. 

The rate constants calculated by EF profiles (Equation (4.6)) are necessarily crude as several assumptions must hold the initial enantiomer composition is known, only a single stereoselective reaction is active, and the amount of time over which transformation takes place is known. These assumptions may not necessarily hold. For example, for reductive dechlorination of PCBs in sediments, it is possible for degradation to take place upstream followed by resuspension and redeposition elsewhere [156, 194]. The calculated k is an aggregate of all reactions, enantioselective or otherwise, involving the chemical in question. This includes degradation and formation reactions, so more than one reaction will confound results. Biotransformation may not follow first-order kinetics (e.g. no lag phase is modeled). The time period may be difficult to estimate for example, in the Lake Superior chiral PCB study, the organism s lifespan was used [198]. Likewise, in the Lake Hartwell sediment core PCB dechlorination study, it is likely that microbial activity stopped before the time periods selected [156]. However, it should be noted that currently all methods to estimate biotransformation rate constants in field studies are equally crude [156]. 

Recently UV treatment is also reported as an effective non-chemical dechlorination method for free and combined chlorine (25). 

Chlorofluoroearbons function as chemical intermediates, provided they are consumed rather than released into the atmosphere. CFC-113 serves as the starting material for the production of CTFE monomer. Despite the many years of research into a catalytic vapor-phase process for this conversion, the preferred current method still involves zinc dechlorination in methanol [reaction... 

In this approach the chloride was removed in the last chemical step from (17), eliminating the dechlorination of (12) and selective rechlorination of (13). Although this option provided significant synthesis advantages over the existing process, this alternative method produced a different impurity profile from that of the original process... 

Abiotic nonthermal methods include chemical processes such as reaction with molten sodium, sodium naphthalide, sodium salt in amine, catalytic dechlorination, wet air oxidation, ozonation, and physical methods including adsorption, microwave plasma, and photolysis. 

Abstract. Polychlorinated biphenyl (PCB) can be destroyed in a variety of ways. Currently, for PCB concentrations >500 mg/kg, incineration at an ANNEX I (or equivalent) facility is generally the only EPA approved method. There are additional options for destruction of liquids containing PCB at concentrations <500 mg/kg. Use of a high-efficiency boiler is an attractive option. Also, EPA has selectively approved chemical dechlorination for PCB concentrations <1%. 

Cobalt and nickel porphyrins have also been used for catalyzing the chemical dechlorination with a reductant, namely titanium(III) citrate or nanoscale Zero Valent Iron (nZVI), of e.g. atrazine, (2-chloro-4-(ethylamine)-6-(isopropylamine)-s-triazine), a widely used herbicide which is a persistent groundwater contaminant [38]. Nickel 5,10,15,20-tetrakis(l -methyl-4-pyridinium)porphyrintetra(p-toluene-sulfonate) (TMPyP) was activated by nZVI, while cobalt porphyrins (TMPyP,5,10,15,20-tetrakis(4-hydroxyphenyl)-21 H,23H-porphine-(TP(OH)P) and 4,4, 4",4 -(porphine-5,10,15,20-tetrayl)tetrakis (benzenesulfonic acid)-(TBSP)) were activated by titanium(lll) citrate as the electron donor. All these processes probably could be more efficient using electrochemical methods. 

The known methods lead to amino pyridazinones, which are useful intermediates for making agricultural and pharmaceutical chemicals, involving Raney-Ni cleavage of the hydrazino pyridazinone(7/, direct amination of the pyridazinones(72-74y), substitution of chloropyridazinone with ammonia at enhanced pressure( /5, and the dechlorination of chloropyridazinone performed in the presence of palladium on charcoal(7 / Therefore, we believe that a novel and convenient synthetic route is provided for the synthesis of aminopyridazinones, which are not easily accessible by traditional methods. 

Various methods have been used for the synthesis and smdies of the chemical and physical properties of the polymeric membrane reactor with embedded nanoparticles. These methods include sonochemical, ultraviolet (UV), and y irradiation, and chemical reduction by reducing agents such as borohydrides and hydrazine. Studies have revealed the successful synthesis of cellulose acetate thin film with embedded bimetallic nanopaiticles such as Ni/Fe, Pd/Co, and monometallic Pd for the catalytic dechlorination and hydrogenation processes (Liu et al., 1997 Liu et al., 2000 Meyer et al., 2004). Catalytically active... 

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