An enclosure experiment for testing the consequences of using magnetic microparticles on water quality and sediment phosphorus pools

Abstract

By using microcosm enclosures containing brackish water (6000 µS cm-1) and natural surface sediment from an hypertrophic aquatic ecosystem, this study assessed the effects of adding iron (Fe) magnetic microparticles (MPs) on water quality, with special focus on P concentrations in water column and in the sediment. Two different treatments were considered for a constant 85:1 MPs:PMobile molar ratio: T-W, where MPs were applied above the surface layer of water, and T-S, where MPs were added directly above the sediment. Our study demonstrated the convenience of adding MPs for reducing both P concentrations in lake water and lake sediment. Both, T-W and T-S exhibited a reduction of 68 ± 6% in dissolved inorganic P concentration (DIP) on average for a 70-days oxic period and a reduction of 80 ± 8% (T-W) and 80 ± 4% (T-S) for a 5-days anoxic period. MPs also decreased reactive silicate (Si) concentration approx. by 50% on average for both periods whereas dissolved organic carbon (DOC) was only reduced by 15% after 24 h of MPs addition. Despite the drastic reduction in DIP concentration due to MPs addition, chlorophyll a (Chl a) was not reduced because total P (TP) concentration after treatments (> 200 µg L-1; before treatments: > 700 µg L-1) was still higher than that required (0.05–0.1 mg L-1) to see changes in biological community. P bound to Al oxides, clay minerals and humic substances (P→NaOH) and easily degradable organic P (Org-PLabile) showed a reduction of 15% and 12%, respectively in T-S compared to control. P bound to humic substances (P→NaOH, Humic) was reduced by 11-22% in both treatments. Finally, considering the efficiency in P and MPs removal in both treatments (T-W and T-S), we recommend T-W treatment for a future whole-lake application to ensure the effectiveness of P export (in water and sediment) and the recovery of MPs.