Chromium (chromium)

Chromium (Cr) is a heavy metal that exists in effluent in two main forms with very different toxicities: trivalent chromium (Cr³⁺), which is far less toxic and even a trace nutrient, and hexavalent chromium (Cr⁶⁺), which is highly toxic, carcinogenic and mobile in water. The effluent standards reflect this: the inland-surface-water limit is 2.0 mg/L for total chromium but only 0.1 mg/L for hexavalent chromium, a twenty-fold stricter cap on the dangerous form.

The hexavalent form is the serious hazard — it is a confirmed carcinogen (the existing glossary covers hexavalent chromium specifically), causes skin ulceration and respiratory harm, and contaminates groundwater readily. Chromium pollution from tanneries and electroplating has caused some of India's worst documented groundwater-contamination cases.

For recyclers, chromium is relevant chiefly to e-waste and metal recycling (chromium plating on components and connectors, chromium in stainless steel and alloys) and to any stream carrying electroplating or metal-finishing residues. Stainless-steel scrap and chrome-plated parts are common in e-waste and metal recycling, and processes that strip or dissolve chromium coatings generate chromium-bearing effluent, often including the toxic hexavalent form.

The practical treatment is a two-step chemistry: because hexavalent chromium is both the toxic form and hard to remove directly, it is first reduced from Cr⁶⁺ to Cr³⁺ (using a reducing agent such as sodium metabisulphite under acidic conditions), and then the trivalent chromium is precipitated as hydroxide by raising the pH with lime or caustic, and removed as sludge. Meeting the 0.1 mg/L hexavalent limit specifically requires the reduction step — simple precipitation alone does not remove Cr⁶⁺. For e-waste and metal recyclers handling chrome-plated or stainless material, recognising and treating hexavalent chromium correctly is essential, and the chromium-bearing sludge must then be managed as hazardous waste.