Enhanced MOF performance in chromium (VI) removal from water using tailored MOF-polymer composites

Functionalizing the internal and external surface of MOFs with polymers allows for tailor-made improvements in their performance for chemical separations. In this work, various MOF/polymer composites are screened for the extraction of Cr(VI) from water. One material, which consists of polyserotonin (PS) inserted into Fe-BTC (Fe-BTC/PS), outperformed other screened materials in acidic media (pH = 3). The material offers a maximum removal capacity of 106 mg/g, which is approximately 10 times higher than that of the bare MOF Fe-BTC (9.8 mg/g) at pH 3. The mechanism for Cr(VI) extraction is achieved via a combined adsorption reduction mechanism, which is driven by the highly porous MOF combined with a redox active polymer, respectively. Furthermore, for the best performing material, a protective external polymeric coating was applied which allowed Cr(VI) decontamination in even more acidic medium (pH 2). In a similar fashion, the integration of polyserotonin into more acid stable Cr/Zr-MOFs (MIL-101(Cr) and UiO-66(Zr)) showed improved removal of toxic Cr(VI) from acidic aqueous solution. Finally, the Fe-BTC/PS composite was also able to reduce the Cr(VI) concentration in chromium spiked real world river water samples at neutral pH and low Cr(VI) concentration to levels below the WHO recommended guideline of 50 ppb with an adsorbent dosage of only 0.25 g/L.