Magnetic adsorption separation (MAS) process: An alternative method of extracting Cr(VI) from aqueous solution using polypyrrole coated Fe3O4 nanocomposites

• Fe3O4 nanoparticles are coated by PPy and used as an adsorbent for Cr(VI).
• The performance is explored in a continuously operated MAS device.
• Rapid sorption kinetics and complete Cr(VI) removal are achieved.
• Enhanced magnetic separation efficiency is achieved at prevailing conditions.

A simple pilot-scale magnetic adsorption separation (MAS) device was designed, fabricated and tested in treating aqueous solution stream laden with highly toxic and mobile hexavalent chromium [Cr(VI)] ions using super-paramagnetic crystalline polypyrrole–magnetite nanocomposite (PPy–Fe3O4 NC) as a mobile scavenger. In the first experiment, the kinetics of Cr(VI) adsorption on PPy–Fe3O4 NC containing three different quantities of Fe3O4 nanoparticles were explored in a continuously stirred tank reactor (CSTR) operated in a batch mode. Thereafter, combined adsorption and magnetic separation process studies were conducted in semi-continuous and continuous operations modes whereby Cr(VI) loaded PPy–Fe3O4 NC particles were continuously isolated from the treated stream using a rare-earth permanent magnet. In the adsorption and magnetic separation systems, Cr(VI) removal and magnetic separation efficiency were used as key performance indicators. Results on batch adsorption kinetic studies showed that the quantity of Fe3O4 nanoparticles in the PPy–Fe3O4 NC influenced Cr(VI) removal whereby equilibrium uptakes of 119 mg/g, 109 mg/g and 103 mg/g were achieved using 50.4%, 58.5% and 40.3% Fe3O4 loaded NCs, respectively. Meanwhile, for semi-continuous operations, a decrease in initial Cr(VI) concentration in the reactor and an increase in the quantity of Fe3O4 nanoparticles in the NCs resulted in improved magnetic separation efficiency. Moreover, the introduction of stainless steel (SS) wool in the separation chamber enhanced the entire separation process. Finally, results obtained from a continuously operated MAS system revealed that it is possible to achieve 100% Cr(VI) removal when the residence time was set at 30 min, at a flow rate of 0.2 L/min. At those prevailing conditions, the magnetic separation efficiency was 92%.

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