Adsorption of iodate on nanosized tubular halloysite

Naturally occurring porous minerals as adsorbents for the remediation of environmental pollution has been one of the hot topics in environmental science and material science. In this study, the adsorption of iodate (IO3−) on two nanosized tubular halloysite samples (HBHal and SCHal, from Hubei and Sichuan Province, China, respectively) were investigated for the first time. To evaluate the impact of nanopore of halloysite on the adsorption of IO3−, chemically similar platy kaolinite was used for comparative studies in the batch experiments. The adsorption of IO3− on HBHal reached equilibrium within 36 h, and the adsorption kinetics and isotherm fitted the pseudo-second-order and Langmuir equation well, respectively. Both HBHal and SCHal exhibited high adsorption capability to IO3−, with distribution coefficients (Kd's) of 131.6 ± 14.6 and 126.6 ± 3.1 mL/g, respectively, which were >30 times higher than that for kaolinite (4.2 ± 2.3 mL/g). The specific surface area normalized Kd values on halloysite samples were >13.5-fold higher than that on kaolinite. In addition, halloysite had high selectivity to IO3−, and the adsorption capacities on the two halloysite samples were slightly affected by the concurrent anions, such as SO42−, NO3−, or ClO3−. It was proposed that the curved octahedral sheet caused geometry match between three oxygen atoms in the iodate and three OH above the vacant site of the octahedral sheet. This work also gave insight into the potential application of halloysite on the adsorption of radioactive iodate in natural and engineering environments.