Polyaniline-based acid resistant membranes for controllable ion rejection performance

Nanofiltration (NF) membrane with extraordinary acid stability and controllable separation performance is crucial for acidic wastewater treatment. In this study, we reported polyaniline (PANI)-based acid resistant membranes which can realize controllable ion transport under applied potential. To achieve this, electrically conductive membranes (ECMs) were fabricated by incorporating carboxylic multi-walled carbon nanotubes (MWNTs-COOH) and acids doped PANI via in-situ chemical polymerization. The external voltage was applied to manipulate the electrostatic interactions of ECMs with charged ions in order to regulate the ion transport. The results showed that the type of acid dopants (from small acid to polymer acid) affected the physicochemical and electrochemical properties of ECMs, and therefore influenced the ion rejection performance. ECM-PANIPSSA possessed the optimal acid stability performance among the three kinds of ECMs. It was interesting to find that all the ECMs showed elevated ion rejection rate in response to electrical voltage even immersed in the acid solution for different periods. The ion transport behavior in response to applied potential attributed to the interaction of steric hindrance, Donnan effect and dielectric effect. The proposed ion transport mechanism implied that the interaction energy barrier between ions and ECMs showed a significant change as a result of applied potential. For negatively charged membrane, the Donnan potential difference became more obvious under negative voltage, which made ion transfer more difficult. By contrast, negative voltage neutralized part of the positively charged surface, and therefore weakened the electrostatic interaction for positively charged membrane. The ECMs exhibit good ion rejection and membrane permeance by applying electrical voltage, whose performance was comparable to the current acid resistant NF membrane. This work offers a feasible guideline for the development of acid resistant membranes and opens opportunities in controllable ion transport for acidic wastewater treatment.