Insight into the ion exchange in the adsorptive removal of fluoride by doped polypyrrole from water

In this study, the polypyrrole (PPy) samples doped with Cl^- (PPy-Cl), SO₄^2- (PPy-SO4) and SO₄^2-+Cl^- (PPy-SO4+Cl) were synthesized by chemical polymerization for the adsorptive removal of fluoride ion from water. The structure and morphology of the as-prepared PPy samples were characterized by FT-IR, BET, SEM, XPS, and zeta potential. The adsorption experiments revealed that the PPy-Cl exhibited faster kinetics and higher adsorption capacity (13.98 mg/g), more than 4 times that of PPy-SO4 (3.08 mg/g) and PPy-SO4+Cl (3.17 mg/g). The kinetics of the adsorption followed the pseudo-second-order model and the adsorption isotherm data fitted well to the Langmuir model. FT-IR, EDX, and XPS tests for PPy samples before and after fluoride adsorption demonstrated that anion exchange between F^- and Cl^- or SO4^2- was the prior mechanism for fluoride ion removal from water. Cl^- was more favorable than SO₄^2- in the ion exchange with F^-. Meanwhile, the Cl^- or SO₄^2- exchanged with F^- was mainly bound to the active nitrogen that accounts for 6% of the total nitrogen in PPy molecular matrix. Further study of zeta potential and pH influence experiment demonstrated the electrostatic interaction is auxiliary interaction for the fluoride removal by doped PPy samples.

A Review of Battery Materials as CDI Electrodes for Desalination

The world is suffering from chronic water shortage due to the increasing population, water pollution and industrialization. Desalinating saline water offers a rational choice to produce fresh water thus resolving the crisis. Among various kinds of desalination technologies, capacitive deionization (CDI) is of significant potential owing to the facile process, low energy consumption, mild working conditions, easy regeneration, low cost and the absence of secondary pollution. The electrode material is an essential component for desalination performance. The most used electrode material is carbon-based material, which suffers from low desalination capacity (under 15 mg·g−1). However, the desalination of saline water with the CDI method is usually the charging process of a battery or supercapacitor. The electrochemical capacity of battery electrode material is relatively high because of the larger scale of charge transfer due to the redox reaction, thus leading to a larger desalination capacity in the CDI system. A variety of battery materials have been developed due to the urgent demand for energy storage, which increases the choices of CDI electrode materials largely. Sodium-ion battery materials, lithium-ion battery materials, chloride-ion battery materials, conducting polymers, radical polymers, and flow battery electrode materials have appeared in the literature of CDI research, many of which enhanced the deionization performances of CDI, revealing a bright future of integrating battery materials with CDI technology.