Catalytic reduction of hexavalent chromium using flexible nanostructured poly(amic acids)

Conducting polymers can be tuned by manipulating the delocalized π electron system for chemical and electrocatalytic applications. We hereby describe the reduction of Cr(VI) to Cr(III) by flexible nanostructured conducting poly(amic acid) (PAA) in both solution phase and as a thin film on a gold electrode. Sodium borohydride was used as a reducing agent to prepare different sizes (3-20 nm) of palladium nanoparticles (PdNPs). The effects of experimental parameters such as particle size, temperature, and Cr(VI) concentration on the kinetics and efficiency of reduction process were investigated. Results show that in PAA solution, Cr(VI) was efficiently reduced by 85.9% within a concentration range of 1.0 × 10^-1-1.0 × 10² mM. In the presence of PdNPs and heat (40 °C), the reduction efficiency increased to 96.6% and 99.9% respectively. When employed on a solid electrode, PAA undergoes a quasi-reversible electrochemistry in acidic media with reduction efficiency for Cr(VI) at 72.84%. The method was validated using both colorimetric and Electron Paramagnetic Resonance techniques, which confirmed the formation of Cr(III) as the product of catalytic reduction. Additional characterization conducted using transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) confirmed that there was no significant change in Pd particle size and distributions after dispersion in PAA whereas its phase and oxidation state remained unchanged. Electrochemical characterization showed the reversible and recyclable features of PAA thus confirming its dual role as catalyst stabilizer and reducing agent. This approach provides a significant advantage over conventional methods such as bioremediation which typically require longer time for complete reduction.