Uptake of cadmium, copper, and lead by microporous synthetic Na-birnessite

Removal of cadmium, copper and lead with microporous synthetic Na-birnessite (sodium-birnessite) was investigated by carrying out batch-type sorption experiments with 2 days of equilibration at room temperature. The sorption isotherms indicated that synthetic Na-birnessite showed high affinity for all three heavy metal cations. The Na-birnessite was able to take up Cd, Cu and Pb up to approximately 140, 106 and 60%, respectively of its theoretical cation exchange capacity. The above higher uptakes of Cd and Cu than the theoretical cation exchange capacity of birnessite were probably caused by exchange of not only Cd²⁺ but also CdCl⁺ species with Na⁺ and by exchange of not only Cu²⁺ but also CuCl ⁺ species with Na⁺. Some exchange of CdCl⁺ and CuCl⁺ species as well as some pH-dependent specific adsorption of the Cd and Cu cations resulted in higher than theoretical uptakes. The XRD patterns after sorption of Cd with Na-birnessite showed an increase in the d(001)-spacing from 7.144 to 7.244 Å with high Cd²⁺ concentration, which indicated that interlayer Na⁺ ions were replaced by Cd²⁺ ions. After the sorption reactions with high Cu concentrations, the XRD patterns showed that the main d(001)-spacing of the birnessite slightly increased from 7.144 to ~7.179 Å. In the case of Pb sorption, the d(001)-spacing slightly decreased to 7.133 Å from 7.144 Å of the as synthesized Na-birnessite. These results suggest that removal of heavy metal cations by Na-birnessite is likely due to both ion exchange and chemisorption, the latter due to surface complexation at the edges and outer planar surfaces of Na-birnessite. Based on these results, Na-birnessite is proposed as a potential candidate material to remove heavy metal cations from groundwater as well as industrial wastewater.