Interior multi-cavity/surface engineering of alginate hydrogels with polyethylenimine for highly efficient chromium removal in batch and continuous aqueous systems

Chromium(VI) is a well-known highly toxic metal ion that has serious effects on the environment. Herein, novel functional alginate beads were developed cooperatively using polydopamine (PDA)polyethylenimine (PEI)-modified CaCO3 composites (solid porogens with amine groups), with PEI acting as a granule interior and surface modification reagent. The hollow cavity structures with high-density reactive sites immobilized favorably into the interior of the alginate capsules and the PEI layers grafted onto the alginate bead surface via cross-linking led to bead-like composites with interior/surface accessible PEI, with the aim of exceptional Cr(VI) removal performance from aqueous conditions. SEM-EDS, TEM, XRD, BET, FT-IR, and XPS analysis were used to characterize these new sorbents. The adsorption properties were investigated using batch adsorption and continuous adsorption processes. The optimally designed adsorbent, namely 0.7-HS-PDA@PEI-SA@PEI, possessed excellent Cr(VI) adsorption capacity, exhibiting a superior experimental uptake capacity of 524.7 mg g(-1) for Cr(VI), which was much higher than those of most reported adsorbents. The experimental data fitted well to the Freundlich isotherm and pseudo-second-order kinetic model in a batch experiment. The experimental breakthrough curves obtained under continuous processes were correlated with the Thomas and Adams-Bohart models. Significantly, the reusability experiments showed that 0.7-HS-PDA@PEI-SA@PEI beads exhibited excellent regeneration ability, with no obvious decline in adsorption capacity after five cycles. These results indicated that the newly designed 0.7-HS-PDA@PEI-SA@PEI beads were promising for Cr(VI) removal because of their ultra-high chromium adsorption capacity, rapid adsorption rate, low cost, and facile recovery process.