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.