Effects of dissolved organic matter and suspended solids on the magnetophoretic separation of microalgal cells from an aqueous environment

Abstract

Magnetic seeding of microalgal cells is required prior to the successful implementation of magnetophoretic separation to harvest microalgae from an aqueous environment. Here, poly(diallyldimethylammonium chloride) (PDDA) forms with four different molecular weights were used to surface functionalize iron oxide nanoparticles (IONPs) to promote their attachment onto the cells by electrostatic interaction. We also investigated the effect of suspended solids and natural organic matter (NOM), which are normally found in fish farm water, on the magnetic seeding process. Dynamic light scattering (DLS) and electrophoretic mobility measurement were used to confirm the successful IONP functionalization by PDDA. For particle imaging, both electron and optical microscopes were used to monitor the IONP attachment on microalgal cells. For low particle-to-cell ratios of 10,000:1 or below, the effects of NOM and suspended solids on magnetophoretic separation of microalgae were obvious, with a cell separation efficiency that rarely exceeded 20%. For a particle-to-cell ratio in the intermediate range, the formation of large clusters composed of IONPs-suspended solids/NOM-microalgal cells (which were responsive to the magnetic field) instead of the independent formation of IONPs-suspended solids and/or IONPs-NOM entities is the main reason for minimum interference of suspended solids and NOM on the magnetophoretic separation. For a particle-to-cell ratio of 20,000 or higher (equivalent to 100mg/L of IONPs), it was possible to achieve 98% separation efficiency even under the strong effects of suspended solids and NOM.