Polydopamine-assisted deposition of polypyrrole on electrospun poly(vinylidene fluoride) nanofibers for bidirectional removal of cation and anion dyes

In this work, novel composite nanofibers which exhibited ‘core–shell’-like structure were fabricated through the deposition of polypyrrole (PPy) particles on the electrospun poly(vinylidene fluoride) (PVDF) nanofibers with the aid of polydopamine (PDA). First, the PVDF/DA electrospun nanofibers were prepared. Then, the self-polymerization of PDA was triggered in a Tris–HCl buffer solution. After that, the pyrrole monomer was added into the distilled water containing the PVDF/PDA fibrous membrane and the chemical oxidation polymerization of PPy was triggered by adding FeCl3. Morphological characterization confirmed that the electrospun PVDF/PDA nanofibers were homogenously coated by PPy particles. Largely enhanced hydrophilicity of the fibrous membrane surface was achieved due to the presence of many nitrogen-containing groups and the increased surface roughness of nanofibers. The adsorption abilities of the composite nanofibers were investigated using Methylene Blue (MB) and Congo Red (CR) as the probe dyes. The results showed that the composite nanofibers exhibited excellent adsorption abilities toward cation and anion dyes simultaneously, and the maximum adsorption capacities were 370.4 and 384.6 mg/g for MB and CR adsorption, respectively. The adsorption behaviors toward the two dyes could be well described by the pseudo second-order adsorption model and the Langmuir adsorption model. Further results showed that the adsorption abilities of the composite nanofibers were greatly dependent upon the pH values of the solution. Furthermore, the composite nanofibers exhibited good regeneration ability. The removal ability toward Cr(VI) was also measured and the experimental adsorption capacity was 126.7 mg/g. The excellent adsorption abilities toward cation/anion dyes and heavy metal ions endow the composite nanofibers with great potential application in wastewater treatment.