|
(W/EH009) Chromium Toxicology and Speciation: The effects of pH on Cr(III) and Cr(VI) toxicity. Mlilo, Thabani1, Jackson, Graham1, Day, Jenny2, 1 2 ABSTRACT- Chromium is a particularly useful metal whose chemical and physical properties have made it an integral part of global industrial development. The application of chromium to activities which include electroplating, production of steels, leather tanning and wood treatment, have lead to the production of chromium containing wastes, which unless disposed of in the proper manner may result in the pollution of the aquatic environment. In order to predict the impact of chromium pollution and thus create sound environmental management strategies, it is important to understand the behaviour of this metal in natural aquatic systems. Chromium is a transition metal and therefore is able to exist in a variety of oxidation states ranging from -2 to +6 with the most stable being +3 and +6. In water Cr(III) and Cr(VI) form a number of ionic species in a process known as chemical speciation. Aquatic organisms are able to take up some of these ionic species which are referred to as the bioavailable fraction of the total metal concentration. Some of the bioavailable ionic species have toxic effects on aquatic organisms, disrupting the biochemical pathways within the organisms, and may adversely affect biota in the aquatic ecosystem. The speciation of Cr(VI) and Cr(III) and thus their toxicity is dependant on a variety of environmental properties the most important being pH and redox potential of the aquatic medium. The aim of this study was to investigate the relationship between the effects of pH on the speciation of chromium and its subsequent toxicity to two aquatic invertebrates;Daphnia pulex, a standard laboratory organism and Paramelita nigroculus, an indigenous freshwater amphipod. The computer speciation model JESS (Joint Expert Speciation System) version 6 was used to model and predict the speciation of chromium as well as identify the bioavailable species over a range of pH's and the results compared to the toxicity results obtained. Key words: Chromium, Toxicity, speciation, freshwater |
