Abstract

The authors present a review of chromium in the industry together with detailed reports of the remediation of chromium pollution by wetlands. Chromium is the buzzword of both tanners and environmental scientists. Because it has several advantageous properties over other tanning agents, 80-90% of leather today is tanned with chromium salts (Basic Chrome Sulphate). In pre-tanning processes hides and skins are soaked; hair and keratinous debris in the epidermis are removed and bated. The pelts are then acid pickled to prepare for addition of BCS salts. After full penetration of BCS in the pelts, pHi s raised slightly by adding weak alkaline salts. This accelerates the de-protonation of hexaquachromium (III) complexes forming more stable μ-hydroxo-μ-sulphato complexes on one hand and formation of di-, tri- or oligo-nuclear complexes by the way of polymerisation on the other. The contemporary process is exclusively based on single bath procedure and utilises Cr(III) only. In contrast, the obsolete double bath process, which involved the in situ reduction of chromate [Cr(VI)], is environmentally and toxicologically hazardous.

The East Calcutta Wetlands (ECW; lat. 22°33´ – 22°40´N; long. 88°25´ – 88°35´E), currently a special conservation site (Ramsar Site No.1208), at the eastern fringe of Kolkata city, West Bengal, India was contaminated by composite industrial and municipal wastewater continuously over the past years. A large volume (ca. 50 000 m3 day-1) of highly contaminated effluent flows through a network of canals within the ECW ecosystem and are finally discharged into the Kultigong River, nearly 40km away from Calcutta city. On the journey through the canals, this composite wastewater is used for agricultural irrigation as well as for pisciculture in wastewater-fed fisheries. Chromium, being popularly and exaggeratedly known as a possible toxic constituent of tannery effluent has attracted the attention of environmental scientists as to whether it has any detrimental effect on ecosystems and biota when the effluent is used for pisciculture and agriculture.

The present study mainly focuses on the adverse effects of chromium, if any, in the abiotic and biotic components of an ecosystem when discharged to the environment. Though it has been proved that ECW played a crucial role in ameliorating contaminated industrial effluent, thus improving water quality and minimizing the detrimental effects on biota, care still has to be taken to stop insensitive handling and wastage of this valuable mineral resource. In India, the tanning sector inflicts a heavy toll on the chromium resource, as around 30-50% of chromium that is consumed for leather processing, flows down the drain. It would be our moral duty to check this colossal wastage and think seriously about the recovery and reuse of spent chromium from tanneries.