Envlron. Scl. Techno/. 1992, 26,1944-1950
Determination of Hexavalent Chromlum in Sludge Incinerator Emissions Using Ion Chromatography and Inductively Coupled Plasma Mass Spectrometry Elizabeth J. Arar' and Stephen E. Long Technology Applications, Inc., US. Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268
Theodore D. Martln and Seymour Gold Environmental Monitoring Systems Laboratory, US. Environmental Protection Agency, 26 West Martin Luther King Drive, Cincinnati, Ohio 45268
A unique approach is described using ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS) for the determination of hexavalent chromium [Cr(VI)] in wastewater sludge incinerator emissions. Quartz fiber filters, spiked with an isotopically enriched ("Cr or 53Cr)chromate salt, were used to collect emission particulates. The enriched Cr(V1) isotope was used to monitor the reduction of Cr(V1) during sample collection using a pseudo-first-order reaction model and to calculate the rate of deposition of native Cr(V1) on the filters. At the end of the sampling period, the Cr(V1) was extracted from the filters with 0.1 N sodium hydroxide and determined by IC using postcolumn derivatization with 1,5-diphenylcarbohydrazide.To determine the ratio of enriched Cr(V1) to the native Cr(V1) emitted from the incinerator, an additional aliquot of the sample extract was preconcentrated by IC and the isotopic composition of the Cr(V1) fraction determined by ICP-MS. ~
Introduction Due to the carcinogenicity of airborne Cr(VI), the US. Environmental Protection Agency (USEPA) has proposed regulations limiting the amount of total Cr that may be found in sewage sludge incinerator emissions (I). In setting the proposed limits on Cr emissions, the agency assumed that 1%of the total Cr emitted exists as Cr(V1). As a result of the highly reactive nature of the combustion gases, Cr(VI) is most likely reduced to Cr(III), which is not known to be carcinogenic; however, there are incinerator conditions which can favor the formation of Cr(V1) (2, 3). Studies have indicated that when l i e and ferric chloride are used as sludge conditioners, the conversion of Cr(II1) to Cr(V1) can be as high as 100% (2). Highly oxidizing combustion conditions and chlorine in the feed-waste can also promote the conversion of Cr(III) to Cr(VI) (2,3). One of the objectives of this work was to determine the Cr(V1) to total Cr ratio in the emissions from incinerators reporting significant levels of Cr in their sludge and operating under conditions favoring the formation of Cr(V1). Two types of incinerators most commonly in use today are multiple hearth and fluidized bed. Multiple-hearth incinerators consist of three zones-the drying, combustion, and cooling zones with a rabble arm moving the sludge through the different zones. A fluidized-bed incinerator uses fluidized sand to facilitate the mixing and heating of the sludge. Because a fluidized-bed incinerator requires less excess air during combustion, it has been shown that less Cr(V1) is found in the emissions (2). The data collected in this study were from three different multiple-hearth incinerators, one of which used lime and ferric chloride as sludge conditioners. 1944 Environ. Sci. Technol., Vol. 26, No. IO, 1992
Incinerator emissions present unique problems for sampling and analysis. The efficiency of modern scrubber systems in removing relatively nonvolatile metals such as chromium necessitates the use of sensitive analytical techniques for their determination. Venturi scrubbers were in place at the three incinerators sampled for this study, and one incinerator was also equipped with a wet electrostatic precipitator (wet-ESP). Chromium removal efficiency by a venturi scrubber is normally in the range 50-90% and removal using a wet-ESP can exceed 99%. Air sampling techniques used by past researchers have employed either filters or impinger solutions (4-6). Filter and impinger collection techniques for Cr speciation were compared prior to this work. Based on the results of those comparisons, the decision was made to continue subsequent work with filters utilizing an EPA method 5 filter sampling scheme (7). One problem associated with particulate collection on filter media, however, is the preservation of the oxidation state on the filter during the sampling period. In the method 5 scheme, the filters must be heated at 250 O F to prevent moisture condensation on the filter even though heating increases the rate of reduction of Cr(V1). Preservation of the oxidation state of Cr during conventional impinger sampling is also problematic. Sludge incinerator emissions contain volatiles, semivolatiles, and oxides of nitrogen, sulfur, and carbon, which are concentrated in the impinger solutions. Method 5 type impinger solutions with a beginning pH of 12 become neutral to acidic at the end of a 2-h sampling period. Cr(V1) in this medium is quickly reduced to Cr(II1). Another problem with impinger sampling is the 2-fold or more dilution of the initial solution which occurs as moisture in the gases is condensed in the impingers. Cr(V1) collected in 100-200 d of impinger solution is usually below the quantifiable limit of detection of ion chromatographic and conventional analytical techniques. However, dilution effects using filter sampling are controllable by adjusting the volume of the extraction solution. IC possesses several advantages for the determination of Cr(VI) in that the separation of Cr(II1) from Cr(VI) can be achieved at neutral to alkaline pH as opposed to older techniques in which the pH must be acidic to achieve Cr(V1)-selectivechemistry (8-14). In dilute solutions (
