New treatment method for boron in aqueous solutions using Mg-Al layered double hydroxide: Kinetics and equilibrium studies

Mg-Al layered double hydroxides (LDHs) intercalated with NO₃^- (NO₃·Mg-Al LDHs) and with Cl^- (Cl·Mg-Al LDHs) were found to take up boron from aqueous solutions. Boron was removed by anion exchange of B(OH)₄^- in solution with NO₃^- and Cl^- intercalated in the interlayer of the LDH. Using three times the stoichiometric quantity of NO₃·Mg-Al LDH, the residual concentration of B decreased from 100 to 1.9mgL^-1 in 120min. Using five times the stoichiometric quantity of Cl·Mg-Al LDH, the residual concentration of B decreased from 100 to 5.6mgL^-1 in 120min. It must be emphasized that, in both cases, the residual concentration of B was less than the effluent standards in Japan (10mgL^-1). The rate-determining step of B removal by the NO₃·Mg-Al and Cl·Mg-Al LDHs was found to be chemical adsorption involving anion exchange of B(OH)₄^- with intercalated NO₃^- and Cl^-. The removal of B was well described by a pseudo second-order kinetic equation. The adsorption of B by NO₃·Mg-Al LDH and Cl·Mg-Al LDH followed a Langmuir-type adsorption. The values of the maximum adsorption and the equilibrium adsorption constant were 3.6mmolg^-1 and 1.7, respectively, for NO₃·Mg-Al LDH, and 3.8mmolg^-1 and 0.7, respectively, for Cl·Mg-Al LDH. The B(OH)₄^- in B(OH)₄·Mg-Al LDH produced by removal of B was found to undergo anion exchange with NO₃^- and Cl^- in solution. The NO₃·Mg-Al and Cl·Mg-Al LDHs obtained after this regeneration treatment were able to remove B from aqueous solutions, indicating the possibility of recycling NO₃·Mg-Al and Cl·Mg-Al LDHs for B removal.