Effect of HA and IS on aggregation of CeO 2 NPs

3.3.2.1 Zeta potential measurements

The average zeta potential of CeO2 NPs as a function of NaCl or CaCl2

concentration in the presence of HA is shown in Figure 3.5. In the Na+_{- HA systems, the}

average zeta potential of CeO2 NPs was negative (Fig. 3.5a). At 2 mg/L HA, the average

zeta potential first increased from -28.55 ± 14.95 mV to -13.16 ± 32.49 mV when the concentration of NaCl increased from 10 to 50 mM. It then decreased to -35.88 ± 42.43 mV when the concentration of NaCl was 100 mM. It reached close to 0 mV when the concentration increased from 500 to 1000 mM of NaCl. However, the average zeta potential of CeO2 NPs at 5 mg/L HA showed an opposite trend at 2 mg/L HA. It first

decreased from -16.71 ± 4.93 mV to -28.73 ± 18.80 mV when the concentration of NaCl increased from 10 mM to 50 mM, increased to -22.08 ± 15.86 mV at 100 mM of NaCl, and finally decreased to -45.88 ± 64.76 mV at 1000 mM of NaCl. The average zeta potential was much more negative for lower NaCl concentrations (< 500 mM) at 10 mg/L HA than for those at 2 and 5 mg/L HA.

In the Ca2+_{- HA systems, the zeta potential was also negative (Fig. 3.5b). The zeta}

potential ranged from -20 to -30 mV for all concentrations of CaCl2 (1 – 20 mM) and HA

At lower concentrations of Na+ _{(< 100 mM), the zeta potential was more negative}

at high concentration of HA (10 mg/L HA) than at lower concentrations of HA (2 and 5 mg/L HA). This finding indicates that the increase of HA results in the decrease of zeta potential at the same concentration of monovalent Na+_{. However, the increase of}

concentration of divalent Ca2+ _{does not contribute to a significant change of zeta potential.}

The zeta potential in the presence of HA (Fig. 3.5) was more negative than that in the absence of HA (Fig. 3.1). This finding indicates that the addition of HA leads to a change in the surface charge of CeO2 NPs, which affects the aggregation of CeO2 NPs.

3.3.2.2 Hydrodynamic diameter measurements

Figure 3.6 shows the hydrodynamic diameter of CeO2 NPs as a function of time in

the presence of HA at different NaCl concentrations (ranging from 10 to 1000 mM) or CaCl2concentrations (ranging from 1 to 20 mM). When the NaCl concentration increased,

the average hydrodynamic diameter of CeO2 NPs increased (Figs. 3.6a, b, and c). For

example, under 2 mg/L of HA at ~10 minutes, the average hydrodynamic diameter increased by 0.84% (94.19 ± 2.78 nm), 4.72% (97.82 ± 4.33 nm), 195.54% (276.06 ± 6.67 nm), and 499.80% (560.27 ± 30.77 nm) at 50, 100, 500, and 1000 mM NaCl, respectively, compared to at 10 mM NaCl (93.41 ± 2.73 nm) (Table 3.3). This finding indicates that in the presence of HA, the increase in monovalent Na+ _{concentration decreases the CeO2 NP}

stabilization and promotes CeO2 NP aggregation. However, when the HA concentration

by 31.47% (383.93 ± 17.66 nm) and 48.65% (287.72 ± 13.69 nm) at 5 mg/L, and 10 mg/L of HA, respectively, compared to at 2 mg/L HA (560.27 ± 30.77 nm). This result indicates that, in the presence of monovalent Na+_{, the increase in the HA concentration stabilizes}

CeO2 NPs and deters their aggregation.

The average hydrodynamic diameter of CeO2 NPs increased as CaCl2 concentration

increased in the presence of HA (Figs. 3.6d, e, and f). For example, at ~10 minutes, the average hydrodynamic diameter at 10 mg/L of HA increased by 14.88% (109.41 ± 3.37 nm), 568.21% (636.40 ± 47.63 nm), 762.43% (821.38 ± 47.79 nm), and 775.80% (834.11 ± 69.06 nm) at 5, 10 ,15, and 20 mM CaCl2, respectively, compared to at 1 mM CaCl2

(95.24 ± 2.23 nm). This outcome indicates that, in the presence of HA, the increase of divalent Ca2+ _{concentration destabilizes CeO}

2 NPs and increases their aggregation. The

average hydrodynamic diameter of CeO2 NPs at ~10 minutes decreased at lower CaCl2

concentrations (< 15 mM) when the HA concentration increased from 2 to 10 mg/L. However, the average hydrodynamic diameter of CeO2 NPs at ~10 minutes increased at a

high CaCl2 concentration (20 mM of CaCl2) as the concentration of HA increased from 2

to 10 mg/L. For example, under 20 mM CaCl2 and at ~10 minutes, the average

hydrodynamic diameters increased by 3.71% (828.58 ± 43.44 nm) and 4.41% (834.11 ± 69.06 nm) at 5 and 10 mg/L HA, respectively, when compared to at 2 mg/L HA (798.91 ± 49.30 nm). This finding indicates that the increase in HA concentration destabilizes CeO2

NPs in the presence of high divalent Ca2+ _{concentrations (20 mM). This result differs from}

the average hydrodynamic diameters at lower divalent Ca2+ _{concentrations (< 15 mM) and}

3.3.2.3 Size distribution

Figure B-5 shows the CeO2 NPs particle size distributed at different hydrodynamic

diameters in the presence of HA as a function of NaCl/CaCl2 concentration over an

approximately 10-minute incubation period during the first 100 seconds and at ~ 10 min. At 2 mg/L HA, during the first 100 seconds, when the concentration of NaCl increased from 10 to 100 mM, the size distribution remained constant in the range of 51 ~ 100 nm (Fig. B-5a). When the concentration of NaCl increased to 1000 mM, size distribution increased to 201 ~ 400 nm (Fig. B-5a). At 2 mg/L HA, at ~ 10 min, when the concentration of NaCl increased from 10 to 100 mM, the size distribution mainly stayed in the range of 51 ~ 100 nm (Fig. B-5b). When the concentration of NaCl increased to 1000 mM, the size distribution essentially increased to 401 ~ 600 nm. At 5 mg/L HA, during the first 100 seconds when the concentration of NaCl increased from 10 to 100 mM, the size distribution mainly remained constant and was within a range of 51 ~ 100 nm (Fig. B-5c). When the concentration of NaCl increased to 1000 mM, size distribution mainly increased to 151 ~ 200 nm. At 5 mg/L HA and at ~ 10 min, when the concentration of NaCl increased from 10 to 100 mM, the size distribution mainly remained constant and was within a range of 51 ~ 100 nm (Fig. B-5d). When the concentration of NaCl increased to 1000 mM, the size distribution largely increased to 201 ~ 400 nm. At 10 mg/L HA, during the first 100 seconds when the concentration of NaCl increased from 10 to 100 mM, the size distribution remained constant within a range of 51 ~ 100 nm (Fig. B-5e). When the concentration of NaCl increased to 1000 mM, size distribution increased to 151 ~ 200 nm. At 10 mg/L HA,

distribution mainly remained constant and was within a range of 51 ~ 100 nm (Fig. B-5f). When the concentration of NaCl increased to 1000 mM, the size distribution mainly increased to 201 ~ 400 nm. At 2 mg/L HA, 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-5g). When the concentration of CaCl2 increased to 20

mM, the size was still distributed mainly in the range of 201 ~ 400 nm, and the size of only a small portion of the nanoaggregates increased to 401 ~ 600 nm. At 2 mg/L HA, at ~ 10 min, when the concentration of CaCl2 increased from 1 to 15 mM, the size distribution

mainly increased from 51 ~ 100 nm to 801 ~ 1000 nm (Fig. B-5h). When the concentration of CaCl2 increased to 20 mM, half of the size was distributed in the range of 601 ~ 800 nm,

and half in the range of 801 ~ 1000 nm. At 5 mg/L HA, during the first 100 seconds, when the concentration of CaCl2 increased from 1 to 10 mM, the size distribution mainly

increased from 51 ~ 100 nm to 201 ~ 400 nm (Fig. B-5i). When the concentration of CaCl2

increased to 20 mM, size was still distributed mainly in the range of 201 ~ 400 nm. At 5 mg/L HA, at ~ 10 min, when the concentration of CaCl2 increased from 1 to 15 mM, the

size distribution mainly increased from 51 ~ 100 nm to 801 ~ 1000 nm (Fig. B-5j). When the concentration of CaCl2 increased to 20 mM, size was still distributed mainly in the

range of 801 ~ 1000 nm. At 10 mg/L HA, during the first 100 seconds, when the concentration of CaCl2 increased from 1 to 10 mM, the size distribution mainly increased

from 51 ~ 100 nm to 201 ~ 400 nm (Fig. B-5k). When the concentration of CaCl2 increased

from 10 to 15 mM, the size distribution remained in the range of 201 ~ 400 nm. When the concentration of CaCl2 increased to 20 mM, the size was distributed mainly in the range of

201 ~ 400 nm but only a small portion increased to 401 ~ 600 nm. At 10 mg/L HA, at ~ 10 min, when the concentration of CaCl2 increased from 1 to 20 mM, the size distribution

mainly increased from 51 ~ 100 nm to 801 ~ 1000 nm (Fig. B-5l).

3.3.2.4 Aggregation rate

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

shown in Figure B-2. As illustrated in the figure, the aggregation rate of CeO2 NPs

increases with increasing NaCl and CaCl2 concentrations. No attachment efficiency values

can be obtained in the presence of HA, a factor that indicates that CeO2 NPs are more stable

in the presence of HA than in the absence of HA.

3.3.2.5 Net energy

Figure 3.7 shows the net energy between CeO2 NPs in the presence of HA and

electrolyte (NaCl or CaCl2). In the Na+ - HA systems, the repulsive energy barriers existed

only at lower NaCl concentrations (84.00 kT at 10 mM NaCl and 61.30 kT at 50 mM NaCl) for 10 mg/L HA and at a lower NaCl concentration (1.25 kT at 10 mM NaCl) for 2 mg/L HA. In the Ca2+ _{- HA systems, the repulsive energy barrier existed solely as 2.40 kT at 1}

mM CaCl2 and 5 mg/L HA.