Smart water-based ferrofluid with stable state transition property: Preparation and its application in anionic dye removal

This article reports a smart water-based ferrofluid that can transform from stable state to unstable state for separating the contained magnetic nanoparticles after use. The ferrofluid is synthesized by preparing polyethylenimine modified Fe 3 O 4 nanoparticles via a one-pot method, and then improved by acidification treatment and ultracentrifugation-based washing. The resultant ferrofluid and intermediate products have been systematically characterized, verifying that the ferrofluid possesses superparamagnetism, high saturation magnetization as well as strong colloidal stability, and the magnetic nanoparticles contained in the ferrofluid have small aggregation size, strong electropositivity and high saturation magnetization. It has been confirmed that the acidification treatment and ultracentrifugation-based washing greatly activate the positive charge and reduce the aggregation size of polyethylenimine modified Fe 3 O 4 nanoparticles, which is the key to our successful synthesis of this new type of ferrofluid. Furthermore, the adsorption behavior of the ferrofluid on the anionic Ponceau S dye has been systematically investigated, demonstrating that the ferrofluid can adsorb anionic Ponceau S dye in a short time (<5 min) with a maximum adsorption capacity of 140.26 mg/g. The experimental data show that the adsorption kinetics follows the pseudo-second-order mode and the Langmuir isotherm model is applicable to describe the adsorption processes . Importantly, the magnetic nanoparticles in the ferrofluid can be easy to separate from solution after adsorbing anionic Ponceau S dye, preventing secondary pollution and showing great potential in wastewater treatment .