Dechlorinating agent

The use of subcritical water extends beyond destruction. A method for remediation of PCB-contaminated soil and sediments has been described that uses zerovalent iron as the dechlorination agent and subcritical water extraction (SWE) as the transporting medium. The initial findings indicate that this technique may be a viable method for remediation of PCB-contaminated soil and sediments (Yak et al., 1999). 

Effluents from sewage treatment plants are not allowed to contain residual chlorine in excess of tolerable values as determined by water quality standards. For example, in discharges to trout streams, the residual chlorine should not exceed 0.02 mg/L. Thus, chlorinated effluents should be dechlorinated. Sulfur dioxide, sodium sulfite, sodium metabisullite, and activated carbon have been used for dechlorination. Because sulfur dioxide, sodium sulfite, and sodium metabisulfite contain sulfur, we will call them sulfur dechlorinating agents. Dechlorination is an oxidation-reduction reaction. The chemical reactions involved in dechlorination are discussed next. 

Chemical Reactions Using Suleur Dechlorinating Agents... 

Irrespective of the type of residual chlorine present, all the species can be represented by a single species by means of the equivalence of chemical reactions. It is convenient to use HOCl to represent all these species. Thns, the chemical reactions using snlfnr dioxide as the dechlorinating agent with HOCl representing the residual chlorine species are as follows ... 

Sodium sulfite is Na2S03. Thus, the chemical reactions using sodium sulfite as the dechlorination agent are as follows ... 

Although precise control may be achieved in dosing the sulfm dechlorinating agents, some residual will exist. This residual should be minimized not only because of waste of chemicals but also because of its effect on the dissolved oxygen of receiving streams. The reactions of these residuals on oxygen are as follows for sulfm dioxide ... 

Tables 1 and 2 summarize information currently available on dechlorination efficiency, water quality impacts and regulatory issues. Table 1 shows the amount of each dechlorination agent required to neutralize one part of chlorine in distilled water at different pHs. Table 2 shows selected regulatory and toxicity information of the dechlorination chemicals.

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. 

M. Bedner, W. A. MacCrehan, and G. R. Helz, Making chlorine cleaner An investigation of alternate dechlorination agents for chloramine removal in wastewater. Proceedings of the Oxidation and Reduction Technologies for Water Treatment, Division of the Environmental Chemistry, American Chemical Society, August, 2004. 

Sodium sulfite is an odorless, solid white powder with a salty sulfurous taste that is soluble in water. It is a reducing agent that is used as a food preservative and antioxidant. Its use is prohibited in meats and other sources of vitamin Bj. Sodium sulfite is also used in the treatment of semichemical pulp in the paper industry, in the treatment of water, as a photographic developer, and in textile bleaching (antic-hlor). It has also found historical use in the water treatment field as a dechlorinating agent. 

Dechlorination. This mild dechlorinating agent is used as triethyl phosphite is used for the preparation of perchlorofulvalene. ... 

Chlorine bleaching can be done at 1 -2°Tw bleaching powder solution for 2-3 h, which is followed by dechlorinating treatment. Sodium thiosulphate, acidic sodium sulphate, ammonia, etc. can be used as dechlorinating agents. 

Sulfurous acid, calcium salt (1 1). Used as a dechlorinating agent for water. Used to preserve fruit juices, in paper processing and sugar manufacture, [dihydratej slightly soluble in H2O, EtOH. 

Following the chlorine chemical contact for disinfection, described in the previous section, residual chlorine must be removed using a dechlorination agent such as sulfur dioxide (Fig. 8). This quenching of the chlorine (or chloramine) by a strong reductant provides a potential for the chemical reduction of transformation products in the wastewater. Minimal research has addressed these reductive reactions for transformation products of synthetic organic chemicals. 

In any of these cases, prior removal of dissolved chlorine from the spent acid is desirable, and this is the subject of Section 9.1.4.4E. Method (4) appears on the list above as well as method (1) because the amount of acid generated may exceed the local demand for neutralization of alkaline waste. This situation depends on a plantwide balance and is not constrained to the battery limits of the chlor-alkali unit. Use of the acid as a dechlorinating agent, as in number (2), is limited to situations in which the treated condensate is not returned to the brine process (e.g., diaphragm-cell plants). The presence of sulfates in the stripped product makes it unsuitable for recycling. Many producers favor option number (3), when it is available. The supplier s ability to handle the material may dictate the concentration of the spent acid. 

F. Fors, L. Julia, J. Riera, J. M. Tura, and J. Saulo, Syntheses and properties of polymers from perchloro-p-xylene, perchlorobi-p-tolyl, and perchloro-m-xylene with Fe(CO)s as dechlorinating agent, J. Polym. Sci. Part A Polym. Chem. 30 2489 (1992). 

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