MHC bioregeneration
Donnan dialysis combined with bio-denitrification was applied to regenerate MHC (Figure 9a). Donnan dialysis (the principles are in SI) is a process that promotes migration of target ions across a membrane.36 The transport of target ions is promoted by the electrochemical potential gradient caused by the ion composition on both sides of the membrane, while the transfer of co-ions is prevented by the ion exchange membrane. As shown in Figure 9b and c, during the regeneration process, the NO3--N concentration in the two chambers first increased, then decreased after ~ 5 hours, and reached equilibrium after 10 hours. It shows that NO3- was firstly desorbed from the MHC into the desorption solution, then transported across the membrane to the stripping solution driven by the electrochemical potential produced by NaCl, and finally denitrified by microorganisms. After three cycles of coagulation and regeneration, the NO3--N removal using MHC decreased from 72.2% to 61.8% (Figure 9d). This shows that the Donnan dialysis combined with bio-denitrification method can be used for MHC multiple regeneration cycles without significant reduction in contaminants removal efficiency. The method minimizes chemical reagent use and does not generate secondary high-concentration wastewater. The ion exchange membrane separates microorganisms from flocs, thereby preventing the microorganisms from attaching to the floc surface and thus avoid biofilm fouling.37 Magnetic components and coagulant components are covalently bound in MHC, which allows effective recovery of the integral structure.
Conclusions
In order to achieve rapid and advanced wastewater treatment in areas with limited space, we synthesized a novel magnetic hybrid coagulant. MHC settling time was 2/3 shorter than traditional magnetic seeding coagulation. MHC improved the removal efficiency of contaminents that can be removed by traditional MSC and also efficiently removed dissovled contaminents that are difficult to remove using most conventional coagulants, especially NO3--N and DON. MHC peformed well over wide ranges of initial NO3--N concentration, pH, and coexisting ions, reducing the need for costly chemical pre-treatments. Although sulfate slightly reduced NO3--N removal, it has little effect on the practical water treatment porformance. The efficient NO3--N removal by MHC stems from the electrostatic adsorption of quaternary ammonium N atoms, which can promote nitrate dehydration and stable adsorption to quaternary ammonium N atoms. The applied magnetic field can also increase the interaction potential and collision efficiency between particles to promote the combination, precipitation, and removal of dissolved water contaminents such as NO3--N. In addition, components of used MHC can be recovered through magnetic separation and regenerated multiple times through Donnan dialysis combined with bio-denitrification. MHC can improve the effluent water quality with a short hydraulic retention time, threreby reducing treatment footprint compared with other technologies. As such, MHC has broad application potential in the land-scarce areas for treating black odorous water bodies, secondary biological effluent, and other similar water quality challenges.