Engineered Nanomaterials as Emerging Contaminants in Water - Nanotechnologies for Water Environment Applications - page 579

conditions (low ionic strength, 1 to 10 mg/L of DOC, near neutral pH) present in
drinking water supplies.

Water treatment plants specifically add cationic polymer and/or metal cations
(e.g., aluminum) to neutralize negatively charged aquatic colloids like hematite (i.e., to
decrease net repulsive energy barriers between like-charged colloids). Precipitation of
aluminum hydroxide particles increases the number (N) of particles in solution (i.e., Eq.
16.8), which increases the rate of aggregation (Crittenden et al., 2005). Complex
aggregation models are available for natural water systems containing a heterogeneous
mixture of different sized colloids and particles, rather than for water containing just one
size NM, but such analysis is complex and readers are advised to look elsewhere
((Lawler, 1986)). Instead, here, a relative examination for how alum addition in water
treatment plants can remove engineered NMs during coagulation, flocculation,
sedimentation and membrane filtration will be briefly discussed based upon work of a
recent USEPA study (Zhang, 2007). Table 16.3 summarizes the effect of alum
(Al 2 (SO4) 3 • 16H 2 O) addition as a coagulant to solutions containing 5 to 10 mg/L of NMs
in the presence of 5 mg/L DOC at pH 7.8 (10 mM NaHCO 3 ) followed by flocculation in
a jar test apparatus operated under the following conditions: (a) rapid mixing
(coagulation period) for 1 minute at 100 rpm (G = 99.5 s -1 ), (b) slow mixing
(flocculation period) for 30 minutes at 30 rpm (G = 16.3 s -1 ), and (c) settling
(sedimentation period) for 1 hour. Typical alum dosages during water treatment are less
than 50 mg/L Al 2 (SO4) 3 •16H 2 O. Under these conditions poor removal of NMs were
observed by sedimentation alone (Table 16.3). After sedimentation, supernatants
containing NMs were filtered using a 0.45 μm membrane. After membrane filtration
detectable amounts of NMs remained (> 80% of the NM mass removed after 20 mg/L
alum and filtration). This research demonstrates the ability of alum to destabilize
engineered NMs and incorporate metal oxide engineered NMs into alum floc that can be
removed during water treatment. However, in all cases a detectable level of NMs was
present in all membrane filtrates, suggesting the potential exposure of NMs in drinking.
Table 16.3 Removal of NMs by alum. Initial conditions: 5 to 10 mg/L as the metal in
the metal oxide NM concentration, 5 mg/L DOC and 10 mM NaHCO 3 (pH 7.8).
Percentage of NM Removed
for Variable Alum Dosages after Sedimentation

Alum
Dosage
(mg/L) ZnO NiO TiO 2 Fe 2 O 3 SiO 2

0 < 10% < 10% < 10% 20% < 10%

40 < 10% < 10% < 10% 20% < 10%

80 < 10% < 10% 18% 20% < 10%

100 < 10% < 10% 15% 20% < 10%

150 < 10% 20% 85% 25% 50%

200 45% 60% 85% 50% 90%