Dechlorination ascorbic acid

Formate is an excellent hydride source for the hydrogenolysis of aryl halides[682]. Ammonium or triethylammonium formate[683] and sodium formate are mostly used[684,685]. Dechlorination of the chloroarene 806 is carried out with ammonium formate using Pd charcoal as a catalyst[686]. By the treatment of 2,4,6-trichloroamline with formate, the chlorine atom at the /iiara-position is preferentially removed[687]. The dehalogenation of 2,4-diha-loestrogene is achieved with formic acid, KI, and ascorbic acid[688]. 

In recent years more utilities have begun to use ascorbic acid (vitamin C) for dechlorination (23). Vitamin C has long been used in the medical field for dechlorination of tap water prior to use for kidney dialysis treatment. Vitamin C reacts with chlorine to produce chloride and dehydroascorbate. The reactions with chlorine and chloramine are shown below ... 

Ascorbic acid is reasonably stable in a dry state with a shelf life of about 1-3 yr (23). However, it rapidly oxidizes in solution. A 1% solution may remain at approx 80% potency after 10 d. A 0.02% solution will degrade to 0% within 3 d. Ascorbic acid is also currently available in tablet form for dechlorination applications. Release of ascorbic acid-containing waters under some conditions may reduce the pH of the receiving streams. Use of vitamin C is reported to have other potential benefits as it is an essential vitamin for healthy fish (23). Also, it can easily strip manganese oxide stains from reservoir surfaces and thereby promote better disinfection (once the vitamin C is exhausted). Vitamin C (ascorbic acid) is NSF certified, allowing it to be used in drinking water treatment to remove or reduce chlorine levels. 

Sodium ascorbate is the sodium salt of ascorbic acid. Most of sodium ascorbate reactions with chlorinated waters are similar to those of ascorbic acid. However, a key difference in dechlorination using sodium ascorbate is that it does not lower the water pH. Several utilities in the Pacific Northwest have evaluated the use of sodium ascorbate for neuhalizing chlorine from potable water releases. The pH of sodium ascorbate is approximately neuhal. The expected reaction of sodium ascorbate with chlorine is shown below ... 

Because sodium ascorbate is more expensive than ascorbic acid, and more sodium ascorbate than ascorbic acid is required to neutralize the same amount of chlorine, treatment using sodium ascorbate is more expensive than treatment using ascorbic acid. However, the utilities favor the use of sodium ascorbate because this chemical does not appreciably alter the pH of the discharge or receiving waters, and the increase in chemical cost is often a minor fraction of the overall dechlorination cost (e.g., labor cost, etc.). 

Tacoma Waters, Tacoma, WA Free chlorine Sodinm metabisnlfite Sodinm snlfite Sodinm thiosnlfate Calcinm thiosnlfate Ascorbic acid Sodinm ascorbate 1% solution Stoichiometric concentrations and twice the stoichiometric concentrations needed for dechlorination 1.2 8.9 Surface water 300... 

Bureau of Water Works, Portland, OR Combined chlorine Sodinm bisnlfite Sodium sulfite Sodium thiosulfate Calcium thiosulfate Ascorbic acid Sodium ascorbate 1% solution Stoichiometric concentrations needed for dechlorination 1.1 8.0 Surface water 300... 

When no dechlorination chemical was added, the dissolved oxygen concentration of the released water decreased from an initial concentration of 11 by less than 0.3 mg/L after traveling 450 ft in one test. When stoichiometric amounts of dechlorination chemicals were added, the DO decreased by 1.18,0.3,0.55, and 0.5 mg/L in the presence of sodium metabisulfite, sodium sulfite, sodium thiosulfate, and calcium thiosulfate, respectively. When twice the stoichiometric amounts of dechlorination chemicals were added, the dissolved oxygen concentration decreased hy 1, 0.9, 0.9, and 0.7 mg/L, respectively, in the presence of these chemicals. With the addition of stoichiometric concentrations of ascorbic acid and sodium ascorbate, the DO of the water increased by 0.3 mg/L, after a travel of 450 ft. When twice the stoichiometric concentrations of these chemicals were used, the DO decreased hy 0.2 mg/L. 

In summary, results indicated that sodium metabisulfite had a greater impact (1.0-1.18 mg/L depletion) on the DO concentrations of the water tested. Sodium sulfite, sodium thiosulfate, and calcium thiosulfate decreased the DO concentration hy 0.3-0.9 mg/L, depending on the amount of dechlorination chemical used. Ascorbic acid and sodium ascorbate had the least impact on the DO of the water tested. 

When no dechlorination chemical was added, the chlorine concentration decreased from 1.05 to 0.95 mg/L after 1000 feet (Fig. 2). This indicated that only a small amount (0.1 mg/L) of the chloramines dissipated through chlorine demand of paved surfaces. Sodium bisulfite, sodium sulfite, ascorbic acid, and sodium ascorbate neutralized all detectable chlorine to below 0.1 mg/L within 2 ft downstream of the mixing hose (approx 2 s). Sodium thiosulfate neutralized more than 80% of the chlorine within 2 ft. However, chlorine concentrations decreased below 0.1 mg/L (the discharge limit in most states) after about 500 ft (elapsed time 3 min, 2 s). Calcium thiosulfate neutralized 60% of the chlorine within 2 ft and neutralized 90% of the chlorine after 1000 ft (elapsed time 7 min, 10s). 

Dechlorination Using Ascorbic Acid Powder/Sodium Thiosulfate Crystals... 

The field studies indicated that all of the dechlorination chemicals were effective in neutralizing free and combined chlorine to below 0.1 mg/L. In most cases the stoichiometric amount of dechlorination chemicals removed more than 90% of the chlorine. However, the reaction rates and the water quality impacts varied with the type, amount, and form of the chemicals used. In general, the rates of dechlorination using sodium/calcium thiosulfate were slower than those using the other chemicals. However, studies by others indicated that dechlorination of wastewater samples using sodium thiosulfate was more rapid that using ascorbic acid (24). 

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. 

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