Fast and efficient removal of dyes and Cr(VI) ion from wastewater using halloysite nanotubes-covalently coated polyurethane sponges

Inspired by the mussel adhesion protein, dopamine was used to prepare polydopamine (PDA)-coated poly-urethane sponge (PDA-PUS) via self-polymerization reaction. Halloysite nanotubes (HNTs) were grafted with gamma-aminopropyltriethoxysilane (APTES). The new organic-inorganic hybrid adsorption material (HNTs@PUS) was fabricated by covalent link of APTES-grafted HNTs and PDA-PUS through Schiff base reaction/Michael addition. The as-prepared HNTs@PUS showed much higher mechanical stability and compressive property than poly-urethane sponge (PUS). HNTs@PUS had 7.7, 7.1, 1.7 and 8.5 times adsorption capacity of PUS for Congo red (CR), Methyl orange (MO), Rhodamine 6 G (R6G) and heavy metal ion Cr(VI), respectively. HNTs@PUS had a fast adsorption rate and most of CR, MO, R6G and Cr(VI) were adsorbed by HNTs@PUS within 20 min. In addition, HNTs@PUS maintained 70.3%, 66.5%, 67.3% and 77.4% of its adsorption capacity for CR, MO, R6G and Cr(VI), respectively, after 5 cycles of regeneration. HNTs@PUS could be easily recovered to avoid secondary contamination. Besides, using HNTs@PUS as adsorbents, a continuous flow adsorption device was developed, through which anionic and cationic dyes and heavy metal ion could be continuously and effectively removed from the flowing solution. These results demonstrated that HNTs@PUS had potentially ideal attributes for use as adsorbents in the ultrafast, high efficiency and large-scale removal of anionic and cationic dyes and heavy metal ions from wastewaters. The fabrication strategy not only solved the problem of the poor recyclability and low permeability of HNTs, but also greatly improved the adsorption capacity of PUS, which might accelerate the application of natural "green " HNTs in wastewater treatment.