permit. In the United States, CN concentrations in treated waste- water discharged into a receiving water by a publicly owned treatment works (POTW), as well as ...
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Reliable Determination of Cyanide in Treated Water MICHAEL F. DELANEY1 AND CHARLES BLODGET1 1Department
of Laboratory Services, Massachusetts Water Resources Authority, Winthrop, Mass.
Reliable determination of cyanide in water samples is important for both wastewater and drinking water treatment plant operators and regulatory agencies. However, as a result of cyanide’s diverse chemistry, obtaining reliable results has been challenging because several chemical mechanisms can form or destroy cyanide—and some of these can occur within the sample container or during laboratory pretreatment and analysis, leading to biased results. Also, sample matrix constituents or preservation chemicals can interfere with the analytical determination. The US Environmental Protection Agency acknowledged this difficulty for wastewater samples in the 2012 Methods Update Rule by revising the
footnote for cyanide preservation to indicate that some interferences may not be mitigated and any technique for removal or suppression of interferences can be used as long as quality control measures are used to demonstrate that the technique worked. Many of the same concerns inherent in testing wastewater apply to testing drinking water. In this study, the effects of holding time, preservation, and on-line digestion and distillation on cyanide results for wastewater and drinking water were examined, including the use of field dilution as a treatment for interferences and field spikes as a means to gauge whether sample integrity was maintained.
Keywords: analytical methods, complex cyanide, cyanide, cyanide monitoring, field dilutions, field spikes, free cyanide, interferences, matrix effects, preservation, simple cyanide, total cyanide Accurate determination of low concentrations of cyanide (CN) in treated drinking water and wastewater samples is important to government regulators and to those they regulate, but it is complicated because of CN’s diverse chemistry. Wastewater utilities, such as the Massachusetts Water Resources Authority (MWRA), regulate discharges of the highly toxic chemical CN from industrial wastewater effluent sources into the sewer collection system as part of an industrial pretreatment program, as required by the National Pollution Discharge Elimination System permit. In the United States, CN concentrations in treated wastewater discharged into a receiving water by a publicly owned treatment works (POTW), as well as in drinking water produced by the water treatment facilities, are regulated by the US Environmental Protection Agency (USEPA) or a state’s environmental agency under the Clean Water Act (CWA) for wastewater or the Safe Drinking Water Act for drinking water.
REGULATIONS Because of CN’s high toxicity (USEPA 2010), USEPA has set low aquatic-life water quality criteria for free CN (22 µg/L acute and 5.2 µg/L chronic in freshwater; 1 µg/L acute and 1 µg/L chronic in saltwater) (USEPA 1985) and somewhat higher human health criteria for water and organisms of 140 µg/L as total CN (TCN; USEPA 2003). For drinking water, the maximum contaminant level (MCL) and the maximum contaminant level goal (MCLG) are both 200 µg/L (USEPA 1992). Water utilities need to be exceedingly careful to preserve the integrity of CN samples and to test them using approved methods, without resulting in JOURNAL AWWA
false positives or false negatives, which would result in the utility reporting a false detection for CN on its Discharge Monitoring Report or Consumer Confidence Report. This can be problematic because the public is highly aware that CN is a poisonous compound. CN has been reported with some regularity in regulated drinking water samples (1.6% of 120,368 samples with a median detection of 8.9 µg/L and a maximum of 1,930 µg/L; USEPA 2009), but it is unclear what fraction of these are false detections. Difficulties with TCN determinations in wastewater samples have been reported (Delaney et al. 1997), including apparent false CN formation in dechlorinated samples and negative amenable CN concentrations. Recently it was shown that field spikes could be used to demonstrate that sample integrity had been maintained on industrial wastewater samples (Delaney & Blodget 2015). Even for drinking water, it was demonstrated that apparent CN can be formed in the sample container after prescribed sample preservation (Delaney et al. 2007), causing a false positive. This formation of false CN during preservation led the investigators to distill their quarterly drinking water TCN samples on site at the time of sampling at the drinking water treatment plant (WTP) to avoid forming false positives caused by the sodium hydroxide (NaOH) preservation treatment. TCN determination using strongly acidic digestion/distillation conditions has been reported to be problematic (Solujic et al. 1999), including “production of cyanide (real or as an artifact) during distillation and associated colorimetric procedures,” leading to poor precision and accuracy. To avoid these harsh analysis conditions, TCN analyses were switched to a flow injection
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analysis (FIA) approach that uses on-line ultraviolet (UV) digestion; isolation of hydrogen CN gas through a permeable membrane (gas diffusion); and sensitive, selective amperometric detection using a silver electrode (Solujic et al. 1999). The same instrument is approved for free CN testing in drinking water by omitting the on-line UV digestion (40 CFR 141.23, 2009). There are primarily three places where CN’s chemistry can cause analysis difficulties—both false positives and false negatives: •• Sample collection and preservation in the field •• Transporting the sample container from the field to the laboratory •• During sample analysis Although in some cases it may be possible to prepare and test the sample at the treatment plant, generally that is not an option. This study was conducted to see how to increase the reliability of CN determinations on wastewater and drinking water samples within the boundaries of the approved test methods and any allowed flexibility. This included careful dechlorination, avoiding preservation with NaOH, field dilutions as a treatment for matrix interferences, and the use of field spikes to demonstrate the maintenance of sample integrity. Note that although field dilutions reduce matrix effects, they also raise the reporting limit by the dilution factor. This must be kept in mind to ensure results with elevated reporting limits still meet regulatory reporting requirements. CN sample preservation and maximum holding time. In the United States, the nominal maximum CN holding time from collection to analysis is 14 days for drinking water and wastewater samples. This maximum holding time was set by regulation, accompanied by prescribed preservation requirements, but without any supporting data to substantiate the holding time. For CWA testing (e.g., wastewater and surface water), the TCN holding time was proposed by USEPA in 1979 and set in 1984. The dechlorinating agent was proposed in 1979 as thiosulfate but was changed to ascorbic acid in the 1984 final rule. Required preservation for TCN or CN “amenable to chlorination” in Table II of 40 CFR 136 (2013) was “cool 4°C, NaOH to pH >12, 0.6 g ascorbic acid (only in the presence of residual chlorine).” The 14-day holding time had a footnote indicating that the maximum holding time is 24 hours if sulfide is present. Optionally all samples may be tested with lead acetate paper before pH adjustment in order to determine if sulfide is present. If sulfide is present, it can be removed by the addition of cadmium nitrate powder until a negative spot test is obtained. The sample is filtered and then NaOH is added to pH 12.
Data to support the TCN holding time and preservation requirements were not cited in either the 1979 proposed or 1984 final rules for 40 CFR 136. In USEPA’s 2007 CWA Method Update Rule (MUR), a lengthy footnote on CN preservation was added but was further revised and drastically shortened in USEPA’s 2012 MUR, adding American Society for Testing Materials (ASTM) D7365-09a (ASTM 2009a) on CN preservation as a reference. The 2012 MUR footnote gave laboratories some leeway: JOURNAL AWWA
There may be interferences that are not mitigated . . . any technique for removal or suppression of interference may be employed, provided the laboratory demonstrates that it more accurately measures cyanide through quality control measures described in the analytical test method.
Available CN was added to the list of CWA parameters in 1999; the approved method for this was OI Analytical (OIA)1677 (USEPA 1999). Free CN was added to the list of CWA parameters in the 2012 MUR, and the approved methods for this were listed as ASTM D7237-10 and OIA-1677-09 (USEPA 2012). The preservation and holding time requirements are the same for total, available, and free CN, but the required preservation was lowered from pH >12 to pH >10 in the 2012 MUR, without discussion; presumably this was to lessen the chance of adverse effects from high NaOH concentrations. For drinking water regulations, the Safe Drinking Water Act was originally enacted in 1974. Free CN was added as a regulated parameter in 1992 (USEPA 1992), setting both the MCL and the MCLG at 200 µg/L. The 1992 rule allowed the use of an ionselective electrode (ISE) to measure free CN and added several screening methods for TCN. It also defined the required CN preservation to be “cool 4ºC, NaOH to pH >12” and specified that “ascorbic acid should only be used in the presence of residual chlorine.” It also defined the maximum holding time as 14 days. Drinking water testing for CN under 40 CFR 141 still requires that CN samples have a holding time of 14 days and are to be preserved to pH 12 with NaOH, but a footnote to the preservation/holding time table indicates the following: “In all cases samples should be analyzed as soon after collection as possible. Follow additional (if any) information on preservation, containers, or holding times that is specified in method.” Therefore, the intention of both the drinking water and wastewater regulations is to hold samples for short enough times so that the CN concentration does not change significantly before analysis, but there is some flexibility regarding the choice of preservation. CN holding time studies. The few published experimental studies of CN holding times are summarized in Table 1. These studies show that CN spiked into unpreserved ambient or wastewater samples had good integrity for 48 h to 14 days. Also, three studies indicated that CN tended to be formed in NaOH-preserved samples. The current study explores CN preservation and holding times in greater detail.
STUDY OBJECTIVES In this study, analytical methods, sample preservation, and holding times were examined with the goal of obtaining reliable, regulatory-defensible results. In particular, the utility of field dilutions as a general treatment for interferences and field spikes were examined as a general approach for demonstrating accurate results.
MATERIALS AND METHODS Apparatus and analytical methods. Conventional TCN analyses using manual distillation and segmented flow colorimetric autoanalyzer (AAN) determinations1 were performed by USEPA
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Method 335.4 (USEPA 1993), as described previously (Delaney & Blodget 2015). Laboratory TCN distillations were performed with a “midi” scale setup that consisted of a source of chilled water and vacuum, reflux/absorber tubes, reflux impingers, absorber impingers, and cold finger condensers.2 On-site distillations were performed with a “micro” scale passive miniature distillation device.3 The analysis of distilled samples was performed by AAN, with a filter photometer at 570 nm and a 5-cm flow cell. TCN analyses were also performed by FIA, on-line UV digestion, and gas diffusion through a membrane to isolate the hydrogen CN followed by amperometric detection with a silver electrode4 per Method ASTM D7511-09 (ASTM 2009b). Free CN analyses were performed following Method OIA-167709 (OI Analytical 2009),4 which is similar to the FIA method described previously but without UV digestion. Free CN analyses were also performed by ISE5 following Standard Methods 4500CN F (Standard Methods 2012). The ISE analysis includes pH and matrix matching by adding an “ionic strength and pH adjustment” reagent that contains NaOH before ISE measurements. All CN analyses were accompanied by batch quality control tests, including a laboratory reagent blank (method blank) below the reporting limit (lowest calibration standard), a free CN or TCN laboratory-fortified blank within control limits, and a free CN or TCN laboratory-fortified matrix (matrix spike) and laboratoryfortified matrix duplicate (matrix spike duplicate) within control limits. Field spikes were prepared in the field at the time of sample collection by adding a known amount of simple or complex CN to a known volume of field sample. Matrix spikes and matrix spike duplicates were prepared in the laboratory at the time of sample analysis by adding a known amount of simple or complex CN to a known volume of field sample. Method comparison: manual distillation/colorimetry (AAN) versus FIA/UV/amperometry (FIA). To compare the conventional manual distillation TCN method (i.e., AAN) to the newer
TABLE 1
FIA/UV/amperometry TCN method, routine industrial pretreatment and POTW wastewater effluent samples submitted to the authors’ laboratory were tested by both methods. The sampling and preservation procedures are described in greater detail elsewhere (Delaney & Blodget 2015). When necessary, these samples were dechlorinated with ascorbic acid and preserved with NaOH to pH 11 or higher. None of the samples in this study was positive for sulfide using lead acetate paper, and no sulfide pretreatment was performed. An aliquot of some of the samples was spiked at the time of sample collection with 100 µg/L complex CN using potassium ferrocyanide trihydrate (K4[Fe(CN)6] ∙ 3H2O). Some of the samples were also randomly chosen as matrix spikes at the time of analysis and were spiked with 100 µg/L of free CN using potassium CN (KCN). Each sample and spiked sample were tested for TCN by both methods within the 14-day holding time. The pairs of TCN concentration results were examined using a regression analysis and a paired t-test. CN holding time study for drinking water without dechlorination. An experiment was conducted to show how quickly free CN is degraded in the presence of residual chlorine as chloramines at a typical treated drinking water concentration. A grab of MWRA tap water was collected at the MWRA central laboratory, located at the Deer Island Treatment Plant in Boston, Mass. This plant has a drinking water transit time of several days from where the drinking water is treated with ozone as the primary disinfectant, UV irradiation, residual disinfection with hypochlorite, fluoridation, corrosion control by raising the pH with carbonate, and finally residual chloramine formation with aqueous ammonia followed by hypochlorite at the MWRA John J. Carroll WTP in Marlborough, Mass. The sample had an initial total chlorine residual (TCR) of 2.1 mg/L. This sample was split and portions were spiked with 50 and 500 µg/L of free CN using KCN. Both samples were tested for free CN by FIA for the ensuing several hours to monitor the decrease of CN.
Literature survey of unpreserved holding time studies Source
Spiking
Preservation
Conclusions
Chakrabarti et al. 1978
River water samples spiked with free CN
Preserved to pH 11 with NaOH or unpreserved
• NaOH preserved—no CN loss up to 30 days • Unpreserved samples—no change after five days; CN dropped to 48% after 30 days
San Jose 2004: City of San Jose “Cyanide Attenuation Study”
POTW final effluent, marine receiving water, and deionized water spiked with complex CN
Unpreserved or preserved to pH 12 with NaOH
• No degradation over 13–14 days with the exception of the preserved effluent sample • After 14 days, preserved effluent sample had 87% of initial TCN • Some indication that cyanide was formed in NaOH preserved samples
City of Vacaville 2007
POTW final effluent dechlorinated with bisulfite and spiked with 20 µg/L complex CN
Unpreserved or preserved to pH 12 with NaOH
• Spike recovery after seven days was 86.5 and 109.5% in wastewater and 93.5% in deionized water • Some indication that CN was formed in NaOH preserved samples
ASTM 2009a: D7365-09a
Synthetic challenge matrix spiked with 200 µg/L free CN
Unpreserved
• Available CN recovery was 93% after seven days, 91% after 14 days
ASTM 2009b: D7365-09a
POTW final effluent spiked with 10 µg/L free CN
Unpreserved
• The recovery was 90% after 6 days
Stanley & Antonio 2012
POTW final effluent from four treatment plants spiked with 5 µg/L free CN
Unpreserved or preserved to pH 12 with NaOH
• No significant loss from 48 hours to nine days for unpreserved samples • Increased free CN from NaOH in 89% of samples from both chlorinated and unchlorinated effluent
ASTM—American Society for Testing Materials, CN—cyanide, NaOH—sodium hydrochloride, POTW—publicly owned treatment works, TCN—total cyanide
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CN holding time study on samples without NaOH preservation when dechlorination is not needed. An experiment was conducted to show how stable free CN is in the absence of residual chlorine. Wastewater samples consisted of treated secondary effluent from the Deer Island Treatment Plant before chlorination/dechlorination and ambient raw drinking water (Wachusett Reservoir raw surface water influent at the John J. Carroll WTP). “Matrix-free” control samples were also prepared using 0.1 M NaOH prepared with laboratory reagent water. All samples were spiked with free CN at 20 µg/L using KCN and were then immediately aliquoted unpreserved into amber 40-mL septum-capped volatile organic analysis vials with no headspace. They were kept refrigerated in the dark at
