Structural charge location dictates speciation and lability of copper in swelling layer silicates

Clay minerals with high selectivity for metals have potential for remediating contaminated water and surface soils. A highly-charged swelling mica (Na-2-mica) and montmorillonite were assessed for their capacities to sorb Cu and resist subsequent desorption. Sorption onto Na-2-mica was studied at Cu loadings of 150, 500, 1500, and 5000 mg/kg in 10 g/L Na-2-mica suspensions with 1 mM KNO ₃ as the background electrolyte at pH 5. Copper sorption onto montmorillonite was tested under the same conditions at a Cu loading of 1500 mg/kg. Desorption was performed with 10 replenishments (i.e., desorption steps) of 1 mM KNO ₃ followed by 5 desorption steps with 0.1 M KNO ₃. About 1.3%–1.4% of the initial Cu was desorbed after 10 desorption steps with 1 mM KNO ₃ in both 1500 mg/kg clay systems. Subsequently, 5 desorption steps with 0.1 M KNO ₃ removed all the Cu from the montmorillonite, whereas approximately 23% of the Cu remained sorbed to Na-2-mica. The 500 mg/kg and 5000 mg/kg Cu Na-2-mica systems followed similar desorption trends as did the 1500 mg/kg Cu system. The resistance of Na-2-mica to Cu desorption was due to strong Cu retention mechanisms, including chemisorption and retention of partially hydrated and interacting Cu ions in a crowded interlayer space, as revealed by electron paramagnetic resonance spectroscopy (EPR). Strong retention of partially hydrated Cu by ion exchange is promoted by the high structural charge in the tetrahedral sheets of Na-2-mica. In contrast, montmorillonite reversibly retained fully hydrated Cu ions by electrostatic attraction (i.e., ion exchange), probably facilitated by structural charge located a greater distance from Cu in the octahedral sheet. Overall, the Na-2-mica`s distinct electrostatic surface characteristics explain the lower degree of hydration and the greater strength of outer-sphere Cu complexes than those observed in the montmorillonite system. These results indicate that Na-2-mica shows more promise for Cu immobilization under acidic conditions.