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.

For the highly effective removal of hexavalent chromium from aqueous solutions, a new polyethylenimine (PEI) grafted porous adsorbent, a cellulose@PEI aerogel (CPA-2) composite, was synthesized through a glutaraldehyde crosslinking reaction between the amine groups of PEI and the hydroxyl groups of cellulose. The physicochemical properties of this new adsorbent were characterized by FT-IR, SEM, EDX, XPS, etc., and the modification of grafting with PEI was demonstrated by FT-IR, EDX and XPS analyses. The effects of pH, contact time, initial concentration and PEI content on Cr(VI) sorption were systematically investigated. Experimental data were well described by the Freundlich isotherm and the pseudo-second-order model in a batch system, demonstrating that chemisorption was the rate-controlling factor for Cr(VI) removal with CPA-2. Furthermore, the experimental maximum adsorption capacity of CPA-2 was 229.1 mg g⁻¹, which was around 12 times higher than that of cellulose aerogel (CA) (18.7 mg g⁻¹). The Thomas model was well fitted to the breakthrough curves of adsorption processes under different fixed-bed conditions. Above all, the exhausted bead-like adsorbent could be easily separated and regenerated without significant loss of adsorption capacity. Accordingly, this new composite material should be a promising sorbent for sewage disposal, with advantages of high performance, low-cost, biodegradability and excellent reusability.