Bioinspired size separation of halloysite nanotubes by polydopamine-modified porous silica particles in matrix solid-phase dispersion fixed-bed column

Gradation of particles with a specific size range is crucial as it significantly impacts their properties and potential applications. In the present study, a mesoporous silica microsphere (MSMs) was synthesized and the surface chemistry was tuned by polydopamine (PDA) coating as a bioinspired polymer on the pore surfaces. The matrix solid-phase dispersion (MSPD) approach was exploited for the size classification of halloysite nanotubes (HNTs) of varying lengths. For this, the nanotubes were mixed with the as-prepared MSMs or with the PDA-modified silica microspheres (MSM/PDA) and packed in a column. The column was eluted with an aqueous solution of various pHs. The surface chemistry of MSM/PDA packings in synergy with their size exclusion nature led to the retention of the nanotubes of a specific size in the column. The results showed that the separation of HNTs by the MSM/PDA was carried out in far greater amounts (approximately 10-fold) compared with that of MSMs. The highest separation percentage of 32.87 % was achieved at pH 9 of the influent when MSM/PDA was used as packing. In addition, the size distribution analysis of HNTs in the first 10 mL of the effluent revealed that HNTs of average size of 442 nm with a lower polydispersity index value of 0.23 are achievable. Also, the various mathematical models were fitted to the experimental data for both MSMs-HNT and MSM/PDA-HNT packings. It was found that the second-order kinetic model well-suited the size separation of the nanotubes.