Abstract:

Objective Waste fabrics (WF) cause serious environmental pollution and waste of resources, while conventional recycling methods are known to be cumbersome, difficult for mass processing, and of low value, which is against the carbon neutrality goals and limits the sustainability of the textile industry. On the other hand, the capacitive deionization (CDI) desalination capacity is too low due to the simultaneous existence of co-ions adsorption and counterions adsorption on the surface of the electrode materials. Therefore, this paper proposes a simple and easy large-scale processing method for high-value recycling of waste fabrics aiming for efficient desalination.

Method Waste fabrics are soft and porous, with a high specific surface area and excellent mechanical properties, providing ideal mechanical support for the CDI electrode material. Under the oxidation of ferric chloride hexahydrate, pyrrole easily polymerized on the surface of waste fabrics (plain cotton fabrics) to form a polypyrrole coating (PPy/WF). Through different electrode assembly modes, PPy/WF was selected as the cathode electrode of CDI to selectively adsorb Cl^-, and MnO₂ as the anode electrode of CDI to selectively adsorb Na⁺, reducing the influence of co-ions adsorption to improve the desalination capacity and stability.

Results The results of SEM, FT-IR, and EDS proved that polypyrrole was successfully in-situ polymerized on the surface of waste fabrics. When the polypyrrole coating was polymerized on the surface of the waste fabric, the polypyrrole coating had low surface energy and showed hydrophobic properties, but PPy/WF could still be infiltrated in a very short time (150 ms) due to the large pore structure between the waste fabric fibers and the yarns. The CV curves of the MnO₂ and PPy/WF electrodes were approximately rectangular and leaf-shaped, respectively, indicating that no redox reaction occurred. This proved that the ions were adsorbed on the surface of the electrode to form an electric double-layer (EDL). The galvanostatic charge/discharge curves of MnO₂ and PPy/WF electrodes were approximately symmetric, further confirming their EDL behavior. The specific capacitance of the MnO₂ and PPy/WF electrodes decreased gradually as the current density increased from 0.1 A/g to 1.0 A/g and reached a maximum of 6.41 F/g and 80.81 F/g (at 0.1 A/g), respectively. EIS results showed that PPy/WF and MnO₂ electrodes approximated straight lines at low frequencies and semicircle curves at high frequencies, which were beneficial to the diffusion and transfer of ions. The concentration of NaCl solution declined the most when PPy/WF as the anode was coupled with MnO₂, which was possibly attributable to the asymmetric configuration of the electrode to avoid the adsorption of co-ions. The desalination capacity of the PPy/WF-MnO₂ assembled form (39.89 mg/g) was much greater than that of the symmetrically arranged CDI electrode form. The CDI Ragone plotted by the desalination rate versus desalination capacity showed that the desalination rate was higher at higher operating voltages and up to 8.42 mg/(g·min). The initial and maximum desalination capacity during the repetitive cycles reached 40.52 mg/g and 44.97 mg/g, respectively, and the desalination capacity still reached 36.18 mg/g after 30 desalination cycles with only a 10.71% reduction, showing excellent cycle stability in desalination.

Conclusion The influences of different CDI electrode assemblies on the desalination performance were investigated, and the results showed that when the PPy/WF was assembled into CDI with the cathode material MnO₂ as an anode material, excellent desalination capacity (44.46 mg/g) and desalination rate (9.81 mg/(g·min)) were guaranteed. The stable pore structure formed between the fibers and yarns in the fabric ensures excellent desalination stability (only 10.71% reduction after 30 cycles). This simple and cost-effective recycling strategy for used fabrics helps reuse resources and alleviates water scarcity. PPy/WF exhibited excellent desalination capacity, and it has attractive economic benefits by virtue of simple synthesis, large batch preparation, and low cost. It is worth noting that the overall structure of the PPy/WF has not been significantly damaged after 30 cycles, and can be repolymerized with PPy to prolong the service life of waste fabrics.

Key words: recycling of waste fabric, polypyrrole, capacitive deionization, desalination, electrode material