Sorption of Co 2+ and Sr 2+ by waste-derived 11 Å tobermorite

Newsprint recycling gives rise to significant volumes of waste sludge which can be de-watered and combusted for energy-recovery. The residual combustion ash, whose primary crystalline constituents are; gehlenite (Ca ₂Al ₂SiO ₇), åkermanite (Ca ₂MgSi ₂O ₇), β-dicalcium silicate (Ca ₂SiO ₄) and anorthite (CaAl ₂Si ₂O ₈), is currently consigned to landfill disposal. It is demonstrated herein that a mixed product of Al-substituted 11 Å tobermorite (Ca ₅Si ₆O ₁₈H ₂ · 4H ₂O) and katoite (Ca ₃Al ₂SiO ₁₂H ₈) can be synthesised from newsprint recycling combustion ash via a hydrothermal route. Batch sorption studies confirm that this mixed product is an effective sorbent for the exclusion of Co ²⁺ and Sr ²⁺ from acidic aqueous media. Kinetic sorption data are analysed in accordance with the pseudo-first- and pseudo-second-order models, and steady-state data is fitted to the Langmuir and Freundlich isotherms. The Langmuir and pseudo-second-order models are found to provide the most appropriate descriptions of the sorption processes. The maximum uptake capacities for Co ²⁺ and Sr ²⁺ at 20 °C are 10.47 and 1.52 mg g ^-1 , respectively, and the respective apparent pseudo-second-order rate constants are estimated to be 5.08 × 10 ^-3 and 6.96 × 10 ^-3 g mg ^-1 min ^-1.