Determination of Ni(CO)₄, Fe(CO)₅, Mo(CO)₆, and W(CO)₆ in sewage gas by using cryotrapping gas chromatography inductively coupled plasma mass spectrometry

Abstract

Evidence for the occurrence of Ni(CO)₄ in addition to Mo(CO)₆ and W(CO)₆ in fermentation gases from a municipal sewage treatment plant is presented for the first time. The gases were sampled at the top of the sewage sludge digester using Tedlar bags, and were analysed using cryotrapping followed by gas chromatography coupled with inductively coupled mass spectrometry (GC-ICP-MS). The use of an ICP-MS as an element-specific detector gives sufficiently low detection limits for metals and was coupled to a packed column gas chromatograph. This method provides information about the speciation of volatile transition metals in contrast to previously used methods for the determination of Ni(CO)₄ in gas samples. The element-specific detection of three different isotopes (m/z 58, 60, 62) and the correspondence of the samples' retention times with those of the standard provided convincing evidence that Ni(CO)₄ is present in the fermentation gas. The concentrations found were in the sub-ppb level, which is at least one order of magnitude lower than the threshold level of 1 ppb (v/v). In addition, Mo(CO)₆ and W(CO)₆ were also measured in the sub-ppb range in contrast to the absence of Fe(CO)₅. The stabilities of Ni(CO)₄, Fe(CO)₅, and Mo(CO)₆ were tested in a carbon monoxide atmosphere. In the presence of distilled water, the following order of stability was found after 11 weeks: Fe(CO)₅<Ni(CO)₄<Mo(CO)₆. In the presence of an aqueous solution containing nickel, molybdenum, tungsten and iron, however, only Fe(CO)₅ was significantly decomposed (<0.3% recovery); Ni(CO)₄ and Mo(CO)₆ were stable after 11 weeks. No W(CO)₆ was formed. The low stability of Fe(CO)₅ in the presence of water could be the reason why no volatile iron compound was found in sewage gas. This study showed that GC-ICP-MS can be employed to identify species-specific traces of metal carbonyls in process gases such as sewage gas.

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