Reusable ZIF-8@chitosan sponge for the efficient and selective removal of congo red

One of the challenges to sustainability is to efficiently and conveniently remove organic dyes from wastewater. Herein, a light ZIF-8@chitosan (CS) composite sponge with bulk density of 0.18 g cm−3 was prepared by an in situ method. The composite sponge possesses a three-dimensional porous structure with abundant chelating sites, which can efficiently and selectively remove congo red (CR). The maximum adsorption capacity (987.01 mg g−1) is higher than most chitosan-based adsorbents. High selective adsorption towards CR over other dyes is also demonstrated. Over 90% adsorption capacity was retained after five adsorption–desorption cycles. In addition, a column-mounted equipment for fixed-bed cycle wastewater treatment was assembled with a freestanding ZIF-8@CS sponge, which exhibited high CR removal efficiency of 99%. Based on the systematic spectroscopy characterization, the dominant role in the superior adsorption performance is attributed to the electrostatic interactions, hydrogen bonds, π–π stacking and metal coordinating interaction between the ZIF-8@CS sponge and CR.