Adsorption and reduction: combined effect of polyaniline emeraldine salt for removal of Cr(VI) from aqueous medium

In this study, we have reported the removal of Cr(VI) ions by polyaniline (PANI) particles from aqueous medium. PANI in its emeraldine salt (ES) form can interact with Cr(VI), which is present as $\text {HCrO}_{\text {4}}^{\boldsymbol {-}} $ in two ways. The adsorption of $\text {HCrO}_{\text {4}}^{\boldsymbol {-}} $ ions due to the electrostatic interaction between partially positively charged PANI backbone and Cr(VI) anions causes the major portion of Cr(VI) removal and a small portion of Cr(VI) is reduced to Cr(III) by PANI (ES). The adsorption follows Langmuir adsorption isotherm and second-order kinetic model. It is observed that the removal of Cr(VI) is negligibly effected by the presence of other anions in the aqueous medium. The adsorption capacity of PANI (ES) is found to be 123 mg g−1, which is very high compared to activated carbon-based materials. The adsorbed anions can be desorbed by converting PANI emeraldine salt (ES) to PANI emeraldine base (EB). The EB form of PANI can be converted into ES form by treating with acid, which can be reused as adsorbent. It is important to note that the PANI (ES) is oxidized by $\text {HCrO}_{\text {4}}^{\boldsymbol {-}} $ ions which decrease the hydrophilicity of the surface of PANI particles. This causes the decrease in adsorption capacity of recycled PANI.