Reversible As(V) adsorption on magnetic nanoparticles and pH dependent desorption concentrates dilute solutions and realizes true moving bed reactor systems

Concentrating dilute streams is a key operation in numerous industries. The work presented here investigates the use of magnetic nanoparticles as transport reagents to mechanically move specific compounds against concentration gradients. Using a model system (arsenate), a cyclic, two-step process for concentrating solutes can be realized, based on a reversible, pH dependent adsorption equilibrium and two stirred tanks: adsorption of the model ion As(V) from a dilute aqueous stream (large tank) onto chemically functionalized magnetic nanoparticles allows water purification at the mg per liter level. Magnetic separation thereby results in an energy efficient removal of the noxious ion from a large water volume (no pumping, magnetic sedimentation/flocculation) and transfer into a much smaller, separate volume (small tank). Changing the acidity (pH) within the much smaller tank allows desorption of the collected ion As(V) into a small volume and thereby permits efficient enrichment. This process scheme, therefore, splits a highly dilute stream into a purified stream (A) and a concentrate stream (B) by using a true moving bed. Long time recycling of the functional magnetic nanoparticles mainly depends on the efficiency of magnetic filtration, under the conditions of reversible adsorption and by using stable magnetic nanoparticles (carbon-coated metal nanoparticles).

► Metal magnetic nanoparticles as true moving beds.
► Arsenate adsorption and enrichment for water purification or analysis.
► Process design estimates for the purification of 3000 m3 water.
► Solute enrichment up to 430 times per run.
► Highly stable and reusable magnetic ion exchange resins.