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CORRESPONDENCE Comment on “Speciation of Airborne Nickel in Occupational Exposures” SIR. We agree with Wong and Wu that airborne nickel must be speciated to assess any nickel-related occupational health hazards ( I ) . We were also pleased to see data on occupational exposures to nickel appear in the literature. However, we do not think their proposed procedure for determining soluble, metallic, and oxidic nickel is generally applicable to workplaces where nickel is present. We also wish they had provided more information on the operations being carried out and on the nickel species being handled near each sampling site. Our concern for their speciation method is based on more than 10 years of work to develop, refine, and test a protocol for separating airborne nickel in the workplace into four different groups: soluble, sulfidic, metallic, and oxidic. The protocol, based on sequential selective leaches, was successfully tested on three industrial dusts in an international interlaboratory program (2). First, there can be no argument with using an aqueous leach to determine soluble nickel. However, we believe Wong and Wu should not have used glass fiber filters in connection with this assay. They found decreased concentrations of soluble nickel and increased concentrations of oxidic nickel when using a glass fiber filter compared with using a cellulose ester membrane. They attempted to explain these differences in terms of sampling periods of different length, different pore sizes of the filters, and different size distributions (which were not determined) of the three nickel species. We believe the explanation for their observations is much simpler. Every glass fiber filter that we have examined has an alkaline surface. We have found as much as 240 pequiv of alkali/g of filter for some makes. This alkalinity can convert some or all of the soluble normal nickel salts collected on the filter into insoluble basic nickel salts. Less nickel dissolves from the basic salts during the subsequent water leach, resulting in an underestimate of soluble nickel. The remaining basic salt dissolves during the hydrochloric acid leach, leading to an overestimate of the oxidic nickel. To provide experimental support for our belief, we found that Gelman type A/E filters available in our laboratory contained a natural surface alkalinity of 32 pequiv/g. We spiked two such filters with 10 pg of Ni as NiC12, dried them at ambient temperature, and then leached them in water. We recovered only 65 and 6870 of the total nickel. The remainder would report as oxidic nickel. This outcome agrees with results we obtained on other glass fiber filters and report elsewhere (2). Glass fiber filters should not be used when attempting to determine t h e soluble nickel content of airborne dust. Wong and Wu may find this hard to believe in view of the 100% recovery of soluble nickel they reported for the same glass fiber filters in their method validation study ( 3 ) . We can think of three possible explanations for the high recovery: (1)the filter was water leached before the added soluble nickel could react with the filter’s surface alkalinity; (2) the soluble nickel was added on top of and 2096
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absorbed by the additions of nickel-containing silica already on the filter and hence did not contact the filter; (3) commercial standard metal salt solutions such as the one used by Wong and Wu are likely to contain between 2 and 570 free acid as stabilizers. This amount of acid could easily have eliminated the surface alkalinity of the glass fiber filter. Second, Wong and Wu’s method for determining metallic nickel relies on a physical separation based on magnetic properties. This approach may have been suitable for the workplace they studied, but in many workplaces, metallic nickel will not exist as discrete, homogeneous particles. For example, oxide fume from high-temperature furnaces often contains a metallic core while metallic dust from reducing operations could well have cores of unreduced oxide. If the oxidic nickel took the form of a magnetic nickel-bearing spinel, it would all report as metallic nickel. Thus, Wong and Wu’s method is likely to overestimate the amount of metallic nickel and underestimate the amount of oxidic nickel. Our speciation protocol employs a selective leach for the metallic nickel phase. If that phase is completely encapsulated by an oxidic phase, our method would underestimate the amount of metallic nickel and overestimate the amount of oxidic nickel. However, we do not think this happens very often with dusts fine enough to remain airborne. Third, we think Wong and Wu have underestimated the refractory nature of oxidic nickel found in many workplaces. Its dissolution in dilute single acids like the 18.570 HC1 they used is often incomplete. Sodium peroxide fusion or digestion with fuming perchloric acid are the only ways we know to ensure the complete dissolution of any type of oxidic nickel, including refractory oxides produced at high temperatures. Since we believe speciation of airborne nickel is essential for meaningful assessment of workplace hazards, we must caution readers of ES&T that the speciation method of Wong and Wu is of limited applicability. One would have to know in advance the microscopic nature of the dust of interest to know whether their method could be relied upon. We believe we have developed a method for speciation that, while not foolproof, is better suited for general application. Registry No. Ni, 7440-02-0.
Literature Cited (1) . , Wona, J. L.;. Wu,. T.-G. Enuiron. Sci. Technol. 1991, 25, 306-309. (2) Zatka, V. J.; Warner, J. S.; Maskery, D. Enuiron. Sci. Technol., in press. (3) Wu,T.-G.; Wong, J. L.Anul. Chim.Acta 1990,235,457-460.
Vladimir J. Zatka, J. Stuart Warner*
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0 1991 American Chemical Society
