The dried activated tannery industry sludge was used as complex biosorbent for removal of Ni(II), Co(II), Zn(II) and Cd(II) in single and multi-component system. The mechanism for toxic metals biosorption was analyzed along with equilibrium isotherm and kinetic study. The efficiency of the biosorbent was studied for real effluent treatment. Further, safe disposal of the spent biosorbent in glass form was established which is significant for commercial implementation of the biosorption technology. Zn(II) and Cd(II) showed 99% removal within 10 min while Ni(II) and Co(II) attained 98% removal at 20-24 h. The biosorbent showed >96% removal efficiency for these metals in effluents from battery manufacturing industry with simultaneous removal of Pb, Cu and Fe ions. Chemical modification of hydroxyl, carboxyl, amino, phosphate, sulfonyl and carbonyl functional groups were undertaken and surface characterization of the biosorbent was done using zeta-potential, FTIR, FESEM-EDX and XPS technique to elucidate the biosorption mechanism. The biosorption efficiency was found to decrease significantly indicating involvement of functional groups in metal binding which was confirmed by FT-IR. Rapid removal of Zn (II) and Cd(II) was due to binding with functional groups. However, the gradual removal of Ni(II) and Co(II) was governed by ionic exchange mechanism, confirmed by ICP-AES. Deconvolution of N1s and Cis XPS spectra produced two and one additional peaks respectively, suggesting formation of amino-metal complexes. Increase in atomic concentration of total oxygen explained further the formation of different metal complexes on biosorbent surface. Upto 30% of metal laden biosorbent could be inertized in phosphate glass matrix as confirmed by the XRD-analysis. No leaching of heavy metals was observed on the glass with thermal cycle at 75 degrees C for 8 h/day up to 35 days. (C) 2016 Elsevier B.V. All rights reserved.