Evaluation of synthetic Birnessite utilization as a sorbent for cobalt and strontium removal from aqueous solution

• Prepared Birnessite has good sorptive behavior and thermal and chemical stability.
• Contaminants are removed as CoCl+ through chemical reaction and Sr2+ via H-bond.
• Empirical model was deduced to correlate the removal with time and temperature.
• Transient removal mechanism obey mechanistic double-diffusive-barrier model.

Remediating aqueous solution contaminated by cobalt and strontium using synthetic Birnessite was investigated. Birnessite structure and chemical properties were evaluated; the material belongs to K-Birnessite family with alternate triclinic and hexagonal layers, where water molecules and potassium ions are interlaced between MnO layers. It is thermally stable and chemically very stable at slightly acidic conditions (pH = 5), and its stability under slightly acidic to alkaline conditions (5 = pH < 10) is higher than that of acidic media pH < 5. Equilibrium sorptive behavior was investigated by conducting batch experiments and modeling species distribution for these experiments to identify preliminary the geochemical suitability of Birnessite. The material remediates efficiently aqueous solutions (% uptake >90%) under acidic and slightly alkali conditions. The removal process is achieved by exchanging hydrogen ions in the interlayer hydroxyl group and CoCl+ or Sr2+ species. Transient removal investigations were carried out and cobalt removal was found faster than strontium. The reaction is spontaneous, endothermic and of increased disorder. The value of enthalpy change indicates that CoCl+ is bonded to Birnessite via chemical reaction, where Sr2+ is bonded via hydrogen bond in a double step reaction. Transient data analysis using rate models were used to deduce empirical equations for sorbed contaminants amounts as a function of contact time and ambient temperature. Transient removal mechanism was further investigated using double-diffusive-barrier model to identify the transport parameters through solid–liquid interface and bulk Birnessite particles. Reaction equilibrium isotherm follows Generalized Langmuir model and the sorbent has higher affinity to stabilize strontium.