Effect of acid treatment on red mud properties with implications on Ni(II) sorption and stability

• Sorption properties of acid treated red mud from aluminum industry were considered.
• Increase of acid concentration lead to sorbents decreased conductivity and pHPZC.
• Ni(II) sorption increased with pH in the range 2–4 and remained constant at pH 4–7.
• Capacities and stabilities abruptly dropped using HCl concentrations >0.25 mol/L.
• Loss of major buffering minerals, sodalite and calcite, adversely affected sorption.

Red mud, a heterogeneous mineral waste from aluminum industry, can be beneficially utilized as a sorbent. High alkalinity and the amount of water soluble ions can limit some of its applications. This study’s aim was to explore and correlate physicochemical properties and sorption efficiencies of acid treated red mud. Sorbents (BRM0.05–BRM1), obtained by varying HCl concentration (0.05–1 mol/L), were characterized in terms of mineralogical composition, surface functional groups, electrical conductivity (EC) and the point of zero charge (pHPZC). Removal of Ni(II) ions was tested in batch conditions. Sorbents EC and pHPZC values decreased with the increase of acid concentration. Structural analysis revealed that sodalite and calcite phases were completely lost starting from the samples BRM0.25 and BRM0.5, respectively. Along with the sodalite dissolution, gibbsite precipitation was observed. Ni(II) sorption increased with the increase of initial pH from 2 to 4, while at pH > 4 it depended only on sorbent material. A radical drop of maximum sorption capacities was associated with the increase of acid concentration to 0.25 mol/L, whereas additional rise to 1 mol/L caused minor fluctuations. Fully loaded sorbents BRM0.25–BRM0.1 also exhibited considerable lower stability over a range of investigated pH conditions. Dissolution of sodalite and calcite, and the consequent reduction of buffering capacities can be identified as the main reasons for decreasing sorption efficiency. Sample BRM0.05 was efficient in terms of both sorption and stability, yet environmentally more benign than the original red mud due to the lower alkalinity and content of water soluble ions.