Magnetic separation of ferrihydrite from wastewater by magnetic seeding and high-gradient magnetic separation

Abstract Ferrihydrite, a member of iron oxides family, has been used as an adsorbent for the removal of heavy metal ions from industrial wastewater. The success of the operation depends mainly on the efficient removal of ferrihydrite from the aqueous phase. Hence, the emphasis of this study was given on the separation of ferrihydrite by high-gradient magnetic separator (HGMS) to overcome solid/liquid separation difficulties of ferrihydrite. This paper clarifies the seeding of ferrihydrite with magnetite mineral and its effects on the operation parameters in HGMS. Magnetic seeding technique was used to make magnetic separation available for the removal of ferrihydrite, and the magnetite mineral was chosen as a seeding material. The method clearly involves the attachment of ferrihydrite to a magnetic seed material and subsequent magnetic separation of ferrihydrite–magnetite coagulates. In seeding process, finely divided magnetite particles were entrapped in ferrihydrite precipitate evolving by increasing pH of solution. In relation with the hydrolysis properties of Fe(III) ion, there is a pH range where the seeding performance is optimal. Iron/magnetite ratio had a marked effect on the separation of seeded ferrihydrite precipitate by HGMS. This ratio determines the operation parameters such as magnetic field strength and flow rate on which the cost and performance of HGMS depend. Test results showed that when used with magnetic seeding technique, HGMS has the potential to overcome separation difficulties associated with adsorption-based treatment techniques with its advantages of high-performance, high-capacity and low-space requirements.