Acidulous Elements

Various catalysts have been used, including activated carbon, bauxite, and bimetallic oxides. The sulfur dioxide is then absorbed in a solution of ammonium sulfite and bisulfite acidulation of this yields ammonium sulfate and elemental sulfur. 

The DPHSE technique has also been used for the determination of organic pollutants and metals in fly ash and coal, respectively. The extraction of dioxins [48,179] and PAHs [180] from fly ash was accomplished with toluene [48,180] or a toluene-methanol mixture [179], with results as good as or even better than those provided by Soxhiet extraction for 24 h. On the other hand, the extraction of major ash-forming elements (Fe, AI, Ca, Mg, Na and K) [148] and minor inorganic pollutants (As, Se and Hg) [46] from coal was done with acidified water. In the latter case, a combination of static and dynamic extraction was found to provide quantitative recoveries within a shorter time and with less dilution of the extracts than dynamic extraction alone. Acidified water is more corrosive than pure water, so the high temperatures required for extraction (150-200°C) call for the use of an extractor made of a material more corrosion-resistant than steel hastel-loid. However, in proportions above 4%, nitric acid — the acidulant most frequently added to the water — has been found to result in clogging of the system and the restrictor, so the recommended acid concentration is much lower than that. 

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