Field Dechlorination Studies

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]. 

On a weight-to-weight basis, approx 1.775 parts of sodium sulfite are required to remove one part of chlorine (17). Sodium sulfite produces sodium sulfate and hydrochloric acid with free and combined chlorine. Ammonia may be present as NH3 or NH +, depending on pH. Although it produces HCl, field studies (18,19) indicated that the reaction does not appreciably decrease solution pH during dechlorination. 

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. 

The field study indicated that, when no chemical was added, free chlorine concentration in the water did not decrease significantly. Chlorine concentrations decreased from 1.2 to approx 1.0 mg/L after a travel of 450 ft (4 min, 10 s) on the semi-paved, asphalt road. When stoichiometric concentrations of dechlorination chemicals were added, most of the chemicals neutralized chlorine instantaneously. Samples analyzed 2 ft (reaction time approx 1 s) downstream of the diffuser contained less than 0.1 mg/L of chlorine. An exception to this trend was calcium thiosulfate. When calcium thiosulfate was added, chlorine concentrations decreased to 0.2 mg/L within 2 ft and residual chlorine was reduced to less than 0.1 mg/L after a travel of 200 ft (reaction time 1 min 40 s). 

EBMUD uses combined chlorine for disinfection. The purpose of the field study at EBMUD was to evaluate dechlorination when chemicals were placed either as tablets or as powder within the path of the chlorinated water. The following dechlorination chemicals were evaluated ... 

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). 

Chlorinated ethenes are subject to a variety of microbial degradation processes that include reductive dechlorination (Vogel et al., 1987 Maymo-Gatell et al., 1997), aerobic oxidation, anaerobic oxidation (Bradley and Chapelle, 1996), and anaerobic cometabolism (McCarty and Semprini, 1994). Both, laboratory studies (Bradley and Chapelle, 1998), and field studies (Chapelle and Bradley, 2000) show that the efficiency of chlorinated ethene biodegradation depends on ambient redox conditions. Therefore, reliable tools to measure the redox conditions are crucial to imderstand and even predict chlorinated ethene degradation. 

The intention of this paper is to evaluate as to whether redox parameters derived from the concepts outlined above are predictive with respect to the degradation rate of chlorinated ethenes observed in the field. We used data from 13 field studies where in-situ degradation rates had been determined together with a (partly incomplete) analytical protocol of redox sensitive species. A novel approach will be derived to predict redox conditions based on the assumption that reductive dechlorination of chlorinated ethenes is at steady-state. 

Semkiw ES, Barcelona MJ (2011) Field study of enhanced TCE reductive dechlorination by a full-scale whey PRB. Ground Water Monit Remed 31(l) 68-78... 

The anaerobic dechlorination of PCBs has been extensively studied both in microcosms and in field samples from heavily contaminated sites in the United States. Three main patterns have been fonnd—N that removed flanked meto-chlorines, P that removed para-chlorines, and LP that removed nnflanked para-chlorines (Bedard et al. 1998). By contrast, ort/to-chlorines were more recalcitrant. These experiments, which have been discussed in Chapter 9, Part 2, laid the fonndation for analysis of a field situation and an appreciation of the effect of long-term exposnre of contaminated lake sediment (Magar et al. 2005a,b). Substantial dechlorination took place in buried sediment cores (35-40 cm) compared with the snrface sediment cores (0-5 cm). Although there were some variations among the cores, the di- and trichlorinated biphenyls were produced at the expense of the... 

All pesticides that can come into contact with the environment are subject to a risk assessment. The basis for this risk assessment is provided by data from environmental fate and environmental toxicity studies, which are carried out in the laboratory or under field conditions. The fate (adsorption, degradation, and mobility) of the active substance must be studied in soil, air, water, and sediments. The laboratory studies are frequently performed with C-labeled substances to make the mass balance easier. It is important to know how a substance degrades in the environment, because sometimes the degradation products are more persistent than the parent substance. DDT, for instance, is converted to metabolites by stepwise dechlorination (Eq. 11.9). The metabolites (e.g., DDD or DDA) can be found in soil for many years after the DDT itself is degraded. 

Dechlorination of Water Releases Field Dechlorination Studies Acknowledgments References... 

Field Dechlorination Studies at Bureau of Water Works, Portland... 

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