Copolymer-templated synthesis of nitrogen-doped mesoporous carbons for enhanced adsorption of hexavalent chromium and uranium

Polyacrylonitrile (PAN) homopolymer and polyacrylonitrile-block-poly(n-butyl acrylate) (PAN-b-PBA) block copolymer were synthesized via supplemental activator reducing agent atom transfer radical polymerization and used as precursors to nitrogen-doped nanocarbons. Carbonization was performed at two different temperatures (500 and 800 °C) to fabricate nanocarbons with different structural properties and nitrogen contents. Copolymer-templated nitrogen-doped carbons (CTNCs) had larger surface area and higher mesoporosity than PAN homopolymer-templated nanocarbons (PANCs), due to the presence of PBA block acting as a sacrificial porogen. Adsorption performances of PANCs and CTNCs for Cr(VI) and U(VI) species were systematically studied. Due to the well-defined structure and larger surface area, CTNCs showed stronger adsorption ability than PANCs. The nitrogen atoms incorporated into the carbon framework led to higher electrostatic attraction for Cr(VI) anions at low pH and complexation with U(VI) cations at high pH. Theoretical maximum adsorption capacities of CTNC-500 on Cr(VI) and U(VI) were 333.3 mg/g (pH = 2) and 17.2 mg/g (pH= 5), respectively. CTNCs also showed preferential adsorption for U(VI) compared to other ions, which might be explained by the hard and soft acids and bases theory. Thus, the copolymer-templated nitrogen-doped mesoporous carbons developed in this study represent a new class of nanocarbon sorbents with potential for removing heavy metal contaminants in either cationic or anionic form from aqueous media and thus mitigating environmental pollution.