Abstract
The dissolved state of chromium in seawater has long been studied and in particular which species, trivalent or hexavalent, is predominant. Research results are not, however, consistent, although the method used has usually been the same—using ferric hydroxide as coprecipitation carrier (Table 11–7). We have studied the coprecipitation behaviour of chromium with ferric hydroxide and other metal hydroxides in the presence of naturally existing inorganic and organic ions to attain an accurate analytical method for determining chromium content, as well as to estimate its behaviour in natural waters8,9. We report here that neither Cr(VI) nor organic species coprecipitate with ferric hydroxide in seawater; Cr(VI) can be quantitatively captured by the coprecipitation with bismuth hydroxide without any pretreatment (such as reduction); and the role of manganese oxide should be considered in addition to the amount of dissolved oxygen to understand the redox system of chromium in natural waters. The inconsistency of the past research may therefore be partly because the presence of organic species was not taken into account and was analysed as either Cr(VI) or Cr(III) or overlooked entirely.
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