Heavy metal removal from aqueous solutions by tobermorites and zeolites

The specific removal of Pb²⁺, Cd2²⁺, Co²⁺, Ni²⁺ and Cr⁶⁺ from 0.02N NaCl solution by tobermorite and zeolite cation exchangers was investigated using a batch method. Tobermorites, both synthetic and natural were extremely efficient in the removal of all the heavy metals except Cr⁶⁺ and are superior to zeolites. For example, a natural tobermorite from Crestmore, CA removed 99.9% of Pb2²⁺, 90% of Cd²⁺, 95.8% Co²⁺ and 96.6% of Ni²⁺ from 0.02NNaC1 solution containing 0.002N of the heavy metal. Based on the percentage of metal ions removed by tobermorites from solutions, the order of removal for the different ions is as follows: Pb²⁺ > Cd²⁺ > Co²⁺ > Ni²⁺. The removal of heavy metals by tobermorites appears to be related to the exchange of structural Ca²⁺ by Pb²⁺, Cd²⁺, etc., analogous to the exchange of these metals in hydroxyapatites. These results suggest that tobermorites may serve as useful scavengers for trace heavy metals in waste solutions.