Use of Mg-Al oxide for boron removal from an aqueous solution in rotation: Kinetics and equilibrium studies

Mg-Al oxide prepared through the thermal treatment of CO₃^2- intercalated Mg-Al layered double hydroxides (CO₃·Mg-Al LDH) was found to remove boron (B) from an aqueous solution. B was removed by the rehydration of Mg-Al oxide accompanied by combination with B(OH)₄^- When using twice the stoichiometric quantity of Mg-Al oxide for Mg/Al = 4, the residual concentration of B dropped from 100 to 2.8 mg/L in 480 min, and for Mg/Al = 2, it decreased from 100 to 2.5 mg/L in 240 min. In both cases, the residual concentration of B was highlighted to be lower than the current Japanese effluent standards (10 mg/L). The removal of B can be explained by way of pseudo-first-order reaction kinetics. The apparent activation energy of 63.5 kJ mol^-1, calculated from the Arrhenius plot indicating that a chemical reaction dominates the removal of B by Mg-Al oxide (Mg/Al = 2). The adsorption of B acts upon a Langmuir-type phenomena. The maximum adsorption (q_m) and equilibrium adsorption constants (K_L) were 7.4 mmol g^-1 and 1.9 × 10³, respectively, for Mg-Al oxide (Mg/Al = 2). B(OH)₄^- in B(OH)₄·Mg-Al LDH produced by the removal of B was observed to undergo anion exchange with CO₃^2- in solution. Following regeneration, the Mg-Al oxide maintained the ability to remove B from an aqueous solution. This study has clarified the possibility of recycling Mg-Al oxide for B removal.