Enhanced adsorption of lead ions by enzymatically synthesized poly(m-phenylenediamine)-graphene oxide composites

Adsorption is considered efficient for removing metal ions dissolved in aquatic environments. For the successful performance of adsorption processes, the development of adsorbents possessing high adsorption capacity is essential. Herein, we report the enzymatic synthesis of composites consisting of m-phenylenediamine polymer (pmPDA) and graphene oxide (GO) by laccase and the adsorption properties of these composites for Pb2+, a representative toxic metal ion. Especially, the composite synthesized with initial 1:1 mass ratio of m-phenylenediamine monomer and GO was found to have the largest adsorption capacity for Pb2+. The Langmuir isotherm for the adsorption of Pb2+ by GO, pmPDA, and this composite, respectively, revealed that the maximum adsorption capacity, qmax, of this composite was the highest (2,164 µmol/g) being almost four times higher than that for pmPDA (564.7 µmol/g). The qmax for GO was 984.3 µmol/g being about two times higher than for pmPDA but less than a half of qmax for this composite. The composite was estimated to contain 56.1 wt% of pmPDA as examined by thermogravimetric analysis. This study demonstrates that the combination of the high surface area of GO and the functionality of pmPDA can significantly enhance the adsorption capacity for Pb2+.