Charge-switchable zwitterionic nanomagnets for wastewater remediation†

Abstract

Enormous amounts of toxic synthetic dyes and inorganic contaminants, such as heavy metals, are regularly discharged into local water bodies unregulated and untreated through effluents from a wide range of industries. Designing industrial methodologies that limit or eliminate the unloading of harmful substances in the surrounding environment has become a requisite for sustainable growth. Although the magnetic separation-based adsorption technique seems quite promising, the functional moieties on the nanoparticle surface often restrict the choice of target pollutants, limiting their universal applicability. Here, we explore the utility of a zwitterion-coated magnetic adsorbent for the easy separation of both positively and negatively charged contaminants from water. Water-dispersible monodispersed nitrilotriacetic acid-functionalized superparamagnetic iron oxide nanoparticles (NTA@SPIONs) were prepared on a large scale following a simple one-pot route. The zwitterionic nanoparticles exhibit surface charge reversibility with a change in pH. The charge-switching property of the nanomaterial was exploited for the removal of cationic and anionic contaminants, such as dyes and heavy metal ions. By proper tuning of the medium pH, methylene blue (MB), a cationic dye, and Congo red (CR), a benzidine-based anionic azo dye, were separated from the aqueous dispersion with the help of the NTA@SPIONs. Under the same working principle, chromium, a highly toxic heavy metal both in cationic and anionic form, was successfully separated from the contaminated water. Low-gradient magnetic separation makes the process rapid, easy, and efficient, and also avoids the chances of secondary pollution.