Kinetics of heavy-metal removal and recovery in sepiolite

Fixed beds of Mg-enriched sepiolite were percolated through Co2+, Cu2+, Zn2+, Cd2+ and Pb2+ single- and multicomponent heavy-metal solutions to study both the dynamic interactions between mineral and heavy-metal cations and the ion-sorption kinetics. The metal concentrations in the eluates were determined by atomic adsorption and/or inductively-coupled plasma and kinetics by the classical kinetic approach, using isothermal experiments at room temperature. The experimental results suggest that: (i) the amount of heavy metal sorbed by the mineral increases for smaller cations; ii) the sepiolite sorption efficiency sequence is, for single component solution, Pb2+ < Cd2+ < Co2+ < Zn2+ < Cu2+, and; for multicomponent solution, Pb2+ = Co2+ < Cd2+ < Zn2+ < Cu2+. Therefore, the ability of sepiolite to remove Cu2+, Zn2+, Cd2+ and Pb2+ is virtually independent of the competitive cation interactions, whereas its affinity for Co2+ is lower when other metals coexist in the solution. The cationic sorption-exchange equilibrium constants (k), obtained by fitting the data with Langmuir equations are: k(Co)(2+) = 4.798 x 10(-3), k(Cu)(2+) = 3.424 x 10(-3) k(Zn)(2-) = 2.907 x 10(-3), k(Pb)(2+) = 1.009 X 10(-2) [meq min](-1), k(Cd)(2+) = 1.187 X 10(-2) [meq min](-1/2) for monocomponent solution experiment. The desorption study concerning the nature of eluting agents shows that Mg2+ is more effective than Na+ in removing heavy metals. Rapid kinetics and equilibrium of exchange of Mg2+ for heavy metals were observed; this is thought to indicate that the main process occurs at easily accessible sites at the outer and channel surface. Moreover, other processes can be assigned to the exchange of Mg2+ in the octahedral sites at the channel edges. (C) 2000 Elsevier Science B.V. All rights reserved.