Differential dissolution of interlayer, octahedral and tetrahedral cations of vermiculite in oxalic acid

Abstract Physical and/or chemical changes such as refinement, component dissolution, exchange/adsorption, structural evolution and recombination of phyllosilicate minerals occur continuously in a naturally weakly acidic water environment. To compare the differential dissolution of cations that occupy various sites in vermiculite, trioctahedral vermiculite was dissolved in various concentrations of oxalate for 24 h and in 0.2 M oxalate for various durations. The concentration of ions in the leaching solution and the phase, structure and morphology of the original samples and acid-leached samples were analysed. Structural analysis showed that the 001 reflections of vermiculite gradually shifted to a higher angle and eventually disappeared after the dissolution of interlayer cations caused by acid leaching. The amount and rate of dissolution of each cation in the vermiculite showed that the octahedral cation Mg 2+ is more soluble than Fe 2+ and Fe 3+ . The dissolution rates of Al 3+ , Mg 2+ and Ca 2+ were greatest in the first 4 h and then decreased gradually. Amorphous silicon dioxide and calcium oxalate were formed during acid leaching, and calcium oxalate was formed in the first 4 h. After leaching with oxalate for various periods, the cation-exchange capacity (CEC) of the samples first increased and then decreased. Micromorphology analysis showed that the acid erosion process started from the edges. The results of this work contribute to our understanding of many natural geochemical processes, and they will be useful for several applications such as soil improvement, ecological restoration and environmental protection.