From Removal to Immobilisation: A Closed Mass Balance with Multi-Element Internal Controls Reveals the Fate of Chromium in Vermicomposted Tannery Sludge

Vermicomposting of chromium-laden tannery sludge is widely reported to remove 30–96% of the metal, but such figures rest on concentration change alone. We tested this in 60-day mass-closed reactors, comparing composting with vermicomposting by Eisenia fetida and Eudrilus eugeniae, with and without bamboo biochar. Against a nominal baseline corrected for measured dry-matter loss, the composting control at matched sludge loading recovered 102.7% of the expected chromium and the vermicomposting treatments only 35.4–44.4%. Chromium cannot volatilise or drain from these reactors; the exchangeable plus carbonate-bound pool never exceeded 0.75%, and earthworm biomass could hold at most 0.8% of the missing chromium, so the deficit is not due to removal. Chromium shifted towards the Fe–Mn oxide-bound fraction (72.6–74.4%) in three out of four vermicomposting treatments, consistent with retention in a form that partial acid digestion does not fully liberate, pending confirmation by total decomposition. Across thirteen elements, the deficit was unrelated to biological essentiality, and inert titanium returned 46.9% apparent removal. Hexavalent chromium fell further under vermicomposting (30.6–45.1% of expected) than under composting (55.3–67.8%), most consistently with biochar. Vermicomposting detoxifies and immobilises chromium without removing it; reuse should be judged on mobility, speciation, and total recoverable chromium.