Speciation of chromium in the waste water from a tannery

Due to the different toxicity of chromium (VI) and chromium (III) and the different toxicity of free hydrated metal ions versus stable complexes of the ions, the determination of the physicochemical form of the chromium is necessary in addition to the total metal content. In this investigation a strategy of analysis has been developed which provides information about the physicochemical form, toxicity and ecochemical behaviour of the chromium in the waste water. The strategy was used for the analysis of two waste water samples. The investigations were not able to give exact quantitative results for the different species initially present because each method shifted the equilibrium state. It was shown that the more toxic chromium (VI) species was unstable in both the waste water samples investigated. Although the two waste waters came from very different sources, most of the chromium (III) in both waste waters after centrifugation were not free ionic species but were associated with macromolecular particles (complexed or colloidal). 36% and 46% of the total chromium of the two samples was bound in a very stable complex; 25% and 15% was bound as colloids. Only a small part of the chromium was ionic (and hence particularly toxic). Despite the stable binding, most of the chromium could be coprecipitated with Fe(OH)₃ or Al(OH)₃ in laboratory experiments and would probably be mineralised in natural water.