hexavalent chromium (Cr6+)

Hexavalent chromium (Cr6+, also written Cr(VI)) is the +6 oxidation state of chromium, in compounds such as chromic acid (CrO3), chromates (CrO4²⁻) and dichromates (Cr2O7²⁻). It is used as a corrosion-inhibiting plating on steel hardware (the bright yellow-iridescent finish called "yellow chromate" or "trivalent equivalent yellow" in modern RoHS-compliant alternatives), as a passivation treatment on cadmium and zinc platings, and historically as a pigment (chrome yellow, chrome orange) in plastics, paints and inks. In contrast, trivalent chromium (Cr3+) is essential to human nutrition, non-toxic at dietary levels, and is the form used in modern RoHS-compliant plating.

The toxicity contrast between Cr6+ and Cr3+ is extreme. Cr6+ is a Group 1 human carcinogen (IARC), causally linked to lung cancer in workers inhaling chromate dust or chromic acid mist; it is also a potent skin sensitiser (chromate eczema affects 10-20% of exposed metalworkers) and damages renal tubules. Cr6+ readily crosses cell membranes via sulphate transporters; once inside the cell it is reduced to Cr3+ generating reactive oxygen species and DNA damage. Cr3+ does not cross membranes efficiently and lacks the carcinogenic mechanism.

Regulatory restrictions are EU-led. The RoHS Directive (2002/95/EC, recast 2011/65/EU) bans hexavalent chromium above 0.1% by weight in homogeneous material in EEE placed on EU market. India's E-Waste (Management) Rules 2022 mirror the limit at 1,000 ppm in homogeneous material. The EU REACH SVHC list includes chromium trioxide and several chromate salts, requiring authorisation for any continued use; the global transition has been to alternative chromium-free passivations (trivalent chromium, zinc-nickel, zinc-aluminium) since 2007.

For e-waste recyclers, hexavalent chromium hazard appears in three contexts. Legacy yellow-chromate hardware on steel from pre-2007 equipment — the bright iridescent finish on screws, brackets, and fasteners may contain Cr6+ at 50-500 mg/m² of plated surface; shredding generates Cr6+-bearing dust that captures in baghouse fines and requires hazardous-waste disposal. Chrome-pigmented plastics — older yellow and orange plastics may contain lead chromate pigments combining the hazards of both lead and Cr6+. Leather and textile components in old computer cases and bag components — chrome-tanned leather under wet conditions can release Cr6+ at 1-50 mg/kg, posing groundwater contamination risk in landfill scenarios. The pragmatic management approach is wet-process shredding for pre-2010 hardware (prevents Cr6+ dust generation), segregated baghouse for the dust stream (with hazardous-waste disposal of the captured fines at Rs 12-25 per kg), and avoidance of mixed-feed shredding that contaminates downstream recovered plastic and metal streams with Cr6+ above thresholds for sale into food-contact or toy applications.