Polypyrrole-Modified Polyaniline Cryogels with Enhanced Cr(VI) Removal Performance

Polypyrrole/polyaniline-poly(vinyl alcohol) (PPy/PANI–PVAl) aerogels were prepared by PPy deposition on PANI–PVAl cryogel substrates by oxidative chemical vapor deposition (oCVD; o-PPy/PANI–PVAl) and chemical deposition in solution (c-PPy/PANI–PVAl). Scanning electron microscopy showed that the macroporous structure (pore size ≈1–20 μm) of the PANI–PVAl precursors was preserved after the modification. Elemental analysis showed that the chemical polymerization approach resulted in the material with a much higher PPy fraction, compared to oCVD (≈35 and ≈3 wt %, respectively), leading also to significantly higher total conducting polymer fraction relative to PVAl (1.45 and 0.47, respectively). Raman mapping supported the results of the elemental analysis, revealing that as a result of the oCVD approach, PPy is mainly deposited on the surface of the material with its presence beyond 75 μm from the surface of the material being significantly limited. On the other hand, in the case of c-PPy/PANI–PVAl aerogel, the PPy distribution throughout the aerogel was uniform, although its protonation/oxidation degree was lower, compared to oCVD. The composition of the composite aerogels was found to affect their performance in Cr(VI) removal. c-PPy/PANI–PVAl had the highest equilibrium fraction of removed Cr(VI), compared to o-PPy/PANI–PVAl and PANI–PVAl (95, 87 and 73%, respectively). The maximum adsorption capacity for c-PPy/PANI–PVAl aerogel, calculated from the Langmuir model (217 mg g–1) was comparable to the corresponding values of the other PANI-, PPy- and PPy/PANI-based materials. The potential for reusability of the modified aerogel for the removal of Cr(VI) from the aqueous medium was assessed for 4 consecutive adsorption–desorption cycles. Therefore, PPy-modified PANI aerogels were shown to be attractive materials for Cr(VI) removal.