Chromium speciation in tannery effluent .2. Speciation in the effluent and in a receiving estuary

The forms and transformations of chromium were characterised in the effluent of an Irish tannery before and after discharge into an estuary. The three primary effluent streams in the tannery were studied, i.e. the chrome and sulphide streams and the final composite effluent. Chromium was fully speciated in the final effluent, the discharge plume, and in the estuary outside discharge periods. The partitioning Of chromium was also studied in the sediments, and transformations due to the remobilisation and oxidation of Cr(III) were followed. These data were compared to a reference site. Mixing of the chrome and sulphide streams resulted in the binding of Cr(III) to organic ligands, notably proteins. The speciation of chromium in the final effluent was dominated by Cr(III) particulates with a mean concentration of 72.4 mg Cr l(-1). Some 80-90% of these particulates were inorganic while 10-20% were bound to organic ligands. Dissolved concentrations averaged 3.6 mg l(-1) at pH 10. However, a 45% fall in dissolved Cr(III) was recorded after lowering the pH of the final effluent to that of the receiving waterway, i.e. pH 7.6-8.1. Approximately 50% of the remainder was determined to be bound to proteinaceous ligands with 50% bound to stable organic ligands. However, only the latter species was detectable in the water column outside discharge periods. Low levels of dissolved organic Cr(III) were also found to leach from both control and contaminated sediments, although concentrations were higher in the contaminated sediments. Cr(VI) was undetectable in the estuary (<3 mu g l(-1)) and local sediments were found not to oxidise Cr(III) in the effluent. In comparison, a number of sediments obtained from other sites readily oxidised experimentally added Cr(III) in the tannery effluent matrix. Elevated levels of organic and inorganic chromium in suspended solids were detectable at all sites studied in the contaminated estuary. Sediment Cr concentrations corrected for grain size were also elevated at all sites with highest concentrations being associated with fine grain sediments. However, total Cr concentrations in the sediments were relatively low compared to other tannery contaminated sites due to good dispersion of the effluent in the estuary. Copyright (C) 1996 Elsevier Science Ltd