Effect of FA and IS on aggregation of CeO 2 NPs

The zeta potential of CeO2 NPs as a function of electrolyte (NaCl or CaCl2)

concentration is shown in Figure 3.8. The zeta potentials of CeO2 NPs were negative for

all NaCl concentrations in the presence of FA (Fig. 3.8a). At 2 mg/L FA, the zeta potential of CeO2 NPs first decreased from -33.68 ± 12.35 mV to -56.64 ± 30.85 mV when the

concentration of NaCl increased from 10 to 100 mM. It then increased to -24.97 ± 53.14 mV when the concentration of NaCl increased to 1000 mM. At 5 mg/L of FA, the zeta potential of CeO2 NPs first decreased from -65.46 ± 9.04 mV to -73.78 ± 13.14 mV when

the concentration of NaCl increased from 10 to 50 mM. It then increased to -21.79 ± 55.41 mV at 500 mM NaCl but slightly decreased at 1000 mM NaCl (-32.58 ± 33.31 mV). At 10 mg/L of FA, the zeta potential of CeO2 NPs first decreased from -63.44 ± 10.30 mV at 10

mM to -75.93 ± 23.28 mV at 50 mM, increased and was close to 0 mV at 500 mM, and slightly decreased with the increase of NaCl concentration and was -21.42 ± 56.68 mV at 1000 mM of NaCl.

The zeta potentials of CeO2 NPs were negative for all CaCl2 concentrations in the

presence of FA (Fig. 3.8b). At 2 mg/L FA, the zeta potential increased from -43.85 ± 11.07 mV to -23.02 ± 4.09 mV when the concentration of CaCl2 increased from 1 mM to 15 mM.

However, it decreased to -25.40 ± 14.99 mV when the concentration of CaCl2 increased to

20 mM of CaCl2. At 5 mg/L FA, the zeta potential increased from -40.96 ± 10.35 mV to

-21.23 ± 7.78 mV when the concentration of CaCl2 was elevated from 1 to 15 mM. Then it

decreased to -23.68 ± 5.36 mV when the concentration of CaCl2 was 20 mM. At 10 mg/L

concentration of CaCl2 increased from 1 to 15 mM and then slightly decreased to -16.69 ±

10.94 mV at 20 mM CaCl2.

3.3.3.2 Hydrodynamic diameter measurements

Figure 3.9 shows the hydrodynamic diameter of CeO2 NPs as a function of time at

different concentrations of NaCl or CaCl2 in the presence of FA. The hydrodynamic

diameter of CeO2 NPs increased as time increased for all electrolyte concentrations in the

presence of FA. At each FA concentration, the average hydrodynamic diameter of CeO2

NPs increased during the first 100 seconds and ~ 10 min, when the concentration of NaCl increased. For example, at 2 mg/L FA and during the first 100 seconds, the average hydrodynamic diameters of CeO2 NPs increased by 0.67% (128.32 ± 6.38 nm), 13.24%

(144.35 ± 11.46 nm), 73.11% (220.66 ± 34.07 nm), and 131.20% (294.71 ± 51.76 nm) at 50, 100, 500, and 1000 mM NaCl, respectively, when compared to at 10 mM NaCl (127.47 ± 6.35 nm) (Table 3.4). This evidence indicates that in the presence of FA, the increase of monovalent Na+ _{concentration leads to the enhancement of aggregation of CeO}

2 NPs. At

each NaCl concentration, when the concentration of FA increased, the average hydrodynamic diameter of CeO2 NPs decreased during the first 100 seconds and ~ 10 min.

For example, at 1000 mM NaCl and ~ 10 min, the average hydrodynamic diameters of CeO2 NPs decreased by 39.95% (493.46 ± 23.41 nm) and 42.78% (470.20 ± 26.51 nm) at

5 and 10 mg/L FA, respectively, when compared to at 2 mg/L FA (821.76 ± 68.74 nm). This finding indicates that in the presence of monovalent Na+_{, the increase in FA}

For all three FA concentrations, the average hydrodynamic diameters of CeO2 NPs

at higher CaCl2 concentrations (> 1 mM) largely increased at ~10 minutes compared to

during the first 100 seconds. For example, at 2 mg/L FA and 5 mM CaCl2, the average

hydrodynamic diameters of CeO2 NPs at ~10 minutes increased by 148.08% (756.05 ±

47.19 nm), when compared to during the first 100 seconds (304.76 ± 50.73 nm). In addition, the average hydrodynamic diameters of CeO2 NPs under three FA concentrations at both

runs at higher CaCl2 concentrations (> 1 mM) were significantly greater than at 1 mM

CaCl2. For example, at 5 mg/L FA and during the first 100 seconds, the average

hydrodynamic diameters increased by 96.88% (170.32 ± 20.74 nm), 251.10% (303.74 ± 48.94 nm), 214.78% (272.32 ± 29.82 nm), and 166.99% (230.97 ± 36.96 nm) at 5, 10, 15, and 20 mM CaCl2, respectively, when compared to 1 mM CaCl2 (86.51 ± 4.12 nm). In

addition, at the same FA concentration and same electrolyte concentration, the hydrodynamic diameter of CeO2 NPs was larger at 10 mM Ca2+ than at 10 mM Na+, which

indicates that influence of divalent Ca2+ _{on the aggregation of CeO2 NPs was more efficient}

than monovalent Na+_{. For example, at ~10 minutes and 5 mg/L FA, the hydrodynamic}

diameter of CeO2 NPs was 165% greater at 10 mM Ca2+ (762.78 ± 53.45 nm) than at 10

mM Na+ _{(287.84 ± 13.01nm).}

3.3.3.3 Size distribution

Figure B-6 shows that CeO2 NPs particle size was distributed at different

hydrodynamic diameters in the presence of FA as a function of NaCl/CaCl2 concentration

10 min, respectively. At 2 mg/L FA, during the first 100 seconds, when the concentration of NaCl increased from 10 to 50 mM, the size distribution remained in the range of 101 ~ 150 nm (Fig. B-6a). When the concentration of NaCl increased from 50 to 1000 mM, the size distribution increased from 101 ~ 150 nm to 201 ~ 400 nm. At 2 mg/L FA, at ~ 10 min, when the concentration of NaCl increased from 10 to 1000 mM, the size distribution increased from 101 ~ 150 nm to 801 ~ 1000 nm (Fig. B-6b). At 5 mg/L FA, during the first 100 seconds, when the concentration of NaCl increased from 10 to 50 mM, the size distribution mainly remained in the range of 151 ~ 200 nm (Fig. B-6c). When the concentration of NaCl increased from 50 to 100 mM, the size distribution mainly decreased from 151 ~ 200 nm to 101 ~ 150 nm. When the concentration of NaCl increased to 1000 mM, the size distribution mainly increased to 201 ~ 400 nm. At 5 mg/L FA, at ~ 10 min, when the concentration of NaCl increased from 10 to 50 mM, the size distribution remained in the range of 201 ~ 400 nm (Fig. B-6d). When the concentration of NaCl increased to 100 mM, the size distribution decreased to 101 ~ 150 nm. When the concentration of NaCl was 500 mM, the size distribution increased to 601 ~ 800 nm. When the concentration of NaCl increased to 1000 mM, the size distribution decreased to 401 ~ 600 nm. At 10 mg/L FA, during the first 100 seconds, when the concentration of NaCl increased from 10 to 100 mM, the size distribution remained in the range of 101 ~ 150 nm (Fig. B-6e). When the concentration of NaCl increased from 100 to 1000 mM, the size distribution mainly increased from 101 ~ 150 nm to 201 ~ 400 nm. At 10 mg/L FA, at ~ 10 min, when the concentration of NaCl increased from 10 to 1000 mM, the size distribution increased from

At 2 mg/L FA, during the first 100 seconds, when the concentration of CaCl2

increased from 1 to 5 mM, the size distribution increased from 51 ~ 100 nm to 201 ~ 400 nm (Fig. B-6g). When the concentration of CaCl2 increased to 20 mM, the size was still

distributed mainly in the range of 201 ~ 400 nm. At 2 mg/L FA, at ~ 10 min, when the concentration of CaCl2 increased from 1 to 20 mM, the size distribution mainly increased

from 51 ~ 100 nm to 601 ~ 800 nm, then decreased to 401 ~ 600 nm, and finally increased to 801 ~ 1000 nm (Fig. B-6h). At 5 mg/L FA, during the first 100 seconds, when the concentration of CaCl2 increased from 1 to 10 mM, the size distribution increased from 51

~ 100 nm to 201 ~ 400 nm (Fig. B-6i). When the concentration of CaCl2 increased to 15

mM, the size was still distributed in the range of 201 ~ 400 nm. When the concentration of CaCl2 was 20 mM, the size distribution was mainly within the range of 201 ~ 400 nm, but

partially decreased to 151 ~ 200 nm. At 5 mg/L FA, at ~ 10 min, when the concentration of CaCl2 increased from 1 to 10 mM, the size distribution mainly increased from 51 ~ 100

nm to 601 ~ 800 nm (Fig. B-6j). When the concentration of CaCl2 increased to 15 mM, the

size distribution mainly decreased to 401 ~ 600 nm. However, the size distribution remained in the same range (401 ~ 600 nm (60%) and 601 ~ 800 nm (40%)) when the concentration of CaCl2 increased from 15 to 20 mM. At 10 mg/L FA, during the first 100

seconds, when the concentration of CaCl2 increased from 1 to 15 mM, the size distribution

increased from 51 ~ 100 nm to 201 ~ 400 nm (Fig. B-6k). When the concentration of CaCl2

increased from 15 to 20 mM, the size distribution remained in the range of 201 ~ 400 nm. At 10 mg/L FA, at ~ 10 min, when the concentration of CaCl2 increased from 1 to 10 mM,

However, the size distribution remained in the same range (401 ~ 600 nm (80%) and 601 ~ 800 nm (20%)) when the concentration of CaCl2 increased from 10 to 15 mM. When the

concentration of CaCl2 was 20 mM, the size distribution mainly increased to 601 ~ 800 nm.

3.3.3.4 Aggregation rate

The aggregation rate as a function of electrolyte concentration (NaCl or CaCl2) was

shown in Fig. B-3. The aggregation rate of CeO2 NPs increased (with some fluctuations at

2 and 5 mg/L FA) with increasing NaCl and CaCl2 concentrations. No attachment

efficiency values can be obtained in the presence of FA, a finding that indicates that CeO2

NPs are more stable in the presence of FA than in the absence of FA.

3.3.5 Net energy

Figure 3.10 shows net energy in the presence of FA at different NaCl or CaCl2

concentrations. In the Na+ _{- FA systems, a repulsive energy barrier at 2 mg/L FA was found}

at lower NaCl concentrations (5.48 kT at 10 mM NaCl and 13.20 kT 50 mM NaCl) (Fig. 3.10a). At 5 mg/L FA, the repulsive energy barriers were 99.90, 95.00, and 40.20 kT, at 10, 50, and 100 mM NaCl, respectively (Fig. 3.10b). At 10 mg/L FA, the repulsive energy barriers were 77.90 and 78.00 kT, at 10 and 50 mM of NaCl, respectively. This finding indicates that FA stabilizes the CeO2 NPs (Fig. 3.10c). In the Ca2+ - FA systems, repulsive

energy barriers only exist at 1 mM Ca2+_{. Moreover, when FA concentration increased from}

3.4 Discussion