Mechanically reinforced quaternary amine cryogels: efficient column adsorption of chromium–methyl orange mixtures at neutral pH

Industrial wastewater containing hexavalent chromium (Cr(VI)) and methyl orange (MO) poses severe threats to aquatic ecosystems due to their high toxicity and difficulty in degradation. In this study, a microfibrillated cellulose (MFC)-reinforced polyethyleneimine (PEI) interpenetrating network cryogel was fabricated and further modified through a two-step synergistic quaternization process. The mechanical properties of the modified cryogel were validated using a universal testing machine, which showed a five-fold enhancement compared to its non-reinforced counterparts. The dynamic column adsorption behavior of the quaternized PEI/MFC cryogel at neutral pH was systematically investigated. For individual contaminant systems, the operational parameters were investigated, including flow rate, initial concentration, and bed height. In the mixed solution, a competitive adsorption relationship was observed between Cr(VI) and MO, wherein Cr(VI) preferentially occupied binding sites owing to the strong affinity of quaternary ammonium groups, resulting in a distinctive “peak–plateau” phenomenon in the breakthrough curves for MO. Breakthrough curves were analyzed using Thomas, bed depth service time (BDST), and Yoon–Nelson kinetic models, all of which exhibited strong agreement with the experimental results ( R 2  > 0.96), confirming that these models were well-suited for the design of this column. The regeneration performance of the adsorbent was evaluated using chemical regeneration methods. Based on FTIR and XPS analyses, the adsorption mechanisms can be summarized as follows: MO is captured by the cryogel mainly via ion exchange and hydrogen bonding, whereas Cr(VI) undergoes synergistic ion exchange and redox reactions. The study confirms the applicability of the synthesized cryogel as an effective adsorbent in mitigating pollutants.