Coagulation performance
Secondary biological effluent from
a municipal wastewater treatment plant was used as raw water (the raw
water quality is shown in Table S2) to test the coagulation performance.
The MHC settling time (5 min) was shorter than conventional coagulations
(20 min) (Figure S2, close to previous reported results8, 20). The MHC settling time was also shorter than
MSC (15 min), especially for UV254, TP, and
NO3--N (Figure 4 b, c, d). The
settling time of the two coagulants for turbidity removal is similar,
which is because particulate contaminants are easier to aggregate into
large complexes and can quickly precipitate under gravity. The rapid
settling speed of MHC can be attributed to the covalent bonds between
magnetic components and coagulants. Magnetic flocs formed when MHC
combined with contaminants, which settled rapidly by both magnetic force
and gravity. Moreover, the MHC flocs have higher density, which are
easier to settle than flocs formed by physical mixing. For MSC, however,
the lack of covalent bonds means some flocs are not combined with
magnetic seeds. These flocs have smaller density and can only settle
slowly by gravity rather than by magnetic force.
Turbidity removal was high for both MHC and MSC (>97.1%)
(Figure 4a). For UV254 and TP (Figure 4b, c), removal
efficiency was much higher with MHC (78.3% for UV254,
98.8% for TP and from 0.55 mg/L to 0.0065 mg/L) than MSC (67.5% for
UV254, 78.2% for TP and from 0.55 mg/L to 0.12 mg/L).
The TP removal efficiency by MSC was relatively low, which may be
related to the low initial TP concentration. More importantly, MSC
(Figure 4d) and conventional coagulations (Figure S2) were comparatively
ineffective for removing NO3--N,
whereas NO3--N concentration decreased
from 11.3 mg/L to 3.1 mg/L (72.2% removal) with MHC coagulation (Figure
4d). In conclusion, for contaminants that can be removed by traditional
MSC, MHC had a higher removal efficiency and settling speed. MHC was
also effective for removing contaminants that cannot be removed by
conventional coagulations and traditional MSC (e.g.,
NO3--N).
We investigated DOM removal at the molecular level when using MHC.
According to the element composition, DOM is divided into CHO, CHON,
CHOS, and CHONS. After coagulation by MHC, above four types DOM
decreased significantly, especially CHON compounds (Figure 5a). However,
the relative abundance of CHON did not change obviously after
conventional coagulants treatment (Figure S3). This shows that MHC is
significantly different from conventional coagulants. MHC can remove
DOM, especially DON that is resistant to conventional coagulants.
Van Krevelen diagrams21 (see the SI for the Van
Krevelen diagram method) were applied to further elucidate the
performance of MHC for removal of DON with different element ratios.
Based on the H/C and O/C element ratios, DON can be divided into seven
compound classes, including lipids (Lip), aliphatic/proteins (Ali/Pro),
lignins/carboxylic rich alicyclic molecules-like (CRAM), carbohydrates
(Car), unsaturated hydrocarbons (UH), aromatic (Aro), and tannin (Tan).
DON in raw water was distributed centrally in the CRAM region (Figure
5b), and the relative abundance of CRAM was the highest (Figure 5d),
indicating that these components had a high contribution to the DON in
the raw water. Other researches have had similar
results.22, 23 It may be related to CRAM’s resistance
to biodegradation and the refractory nature caused by the structural
diversity found within CRAM and its substantial content of alicyclic
rings and branching.24 However, the dots in the CRAM
region disappeared partially after MHC coagulation (Figure 5c), and the
relative abundance reduced by about 2/3 (Figure 5d), revealing that MHC
coagulation can effectively reduce the number of CRAM contaminants
species and concentration of DON. Nitrogen-containing organic compounds
are toxic, and various nitrogen compounds can be converted between each
other, such as NO3--N and
DON.25, 26 MHC can effectively remove DON in water,
reducing the possibility of DON transformation to
NO3--N.