Multicomponent ion exchange isotherms in NaX zeolite: evaluation of Cr/Ca/Mg, Cr/Ca/K and Cr/Mg/K systems

BACKGROUND: One of the most important heavy metals is Cr3+, which is commonly found together with Ca2+, Mg2+ and K+ ions in tannery wastewaters. Such metals can be removed through ion exchange process using zeolite NaX. Unfortunately, with single exchange mechanisms it is impossible to predict muti-component and competitive interactions. This work is aimed at overcoming this problem. Ion exchange isotherms of Cr3+, Mg2+, Ca2+ and K+ in ternary mixtures (Cr/Mg/Ca, Cr/Ca/K, Cr/Mg/K) using NaX zeolite at 30 degrees C, 45 degrees C and 60 degrees C are studied and reported. RESULTS: Ion exchange is dependent on temperature, and on the nature and interaction of ingoing cations. Sequential ion exchange occurs where one cation is able to displace another already located in the site. Values of the equilibrium constant of all cations investigated are close to each other, which indicates a weak preference for Cr+3. The affinity order is as follows: for Cr/Ca/Mg-NaX isotherms, Cr3+ > Ca2+ > Mg2+ at 30 degrees C, Ca2+ > Cr3+ > Mg2+ at 45 degrees C and Ca2+ > Mg2+ > Cr3+ at 60 degrees C. For Cr/Ca/K-NaX isotherms the sequence is Cr3+ > Ca2+ > K+ for the three temperatures and for Cr/Mg/K-NaX isotherms, Cr3+ > Mg2+ > K+ at 30 and 45 degrees C and Mg2+ > K+ > Cr3+ at 60 degrees C. CONCLUSION: It can be concluded that ions with a high charge such as Cr3+ are not always preferentially removed from multi-component solution. Therefore, exchanges in NaX zeolite are useful when there is no need to have high chromium removal when compared to the competitive cations investigated here. (C) 2008 Society of Chemical Industry.