Page 167 - Adsorbents fundamentals and applications

152 SILICA GEL, MCM, AND ACTIVATED ALUMINA

charge (ZPC) is an important, measurable property of activated alumina (or of any
sorbent). It is the pH value that is required to give zero net surface charge. The
ZPC can be measured by acid–base titration (Berube and DeBruyn, 1968; Tewari
and Campbell, 1976; James and Parks, 1982; Noh and Schwarz, 1990). At pH
below the ZPC, the alumina surface is positively charged; whereas at pH above
the ZPC, the surface is negatively charged. Therefore, at pH below ZPC, activated
aluminas will adsorb anions; whereas at pH above ZPC, cations will be adsorbed.
The typical values of ZPC for the commercial activated aluminas are in the range
of pH = 8–10, depending on the grades. The selectivity sequence for a number of
competing ions for activated alumina is (Clifford, 1999; Vagliasindi et al., 1996):

− − − − − 2–
OH > H 2 AsO 4 > Si(OH) 3 O > F > HSeO 3 > TOC > SO 4 > H 3 AsO 3
As 5+ exists in water as H 2 AsO 4 .As 3+ is hardly adsorbed because H 3 AsO 3 is
not charged. A number of plant treatment results are available (e.g., Wang et al.,
2000), all indicating that with raw water containing arsenic in the 50–70 µg/L
range, the use of fixed beds of activated alumina could produce treated water with
less than 5 µg/L As. Similarly, numerous studies have been performed for the
defluoridation of water (e.g., Singh and Clifford, 1981; Karthikeyan et al., 1994).
In 12 test runs of defluoridation of drinking water, fluoride contents <1mg/L
were achieved from a raw water with 3 mg/L by using adsorption with activated
alumina (Karthikeyan et al., 1994).
The adsorption isotherm of arsenic (as As(V)) on activated alumina is given
in Figure 6.17 (Wang et al., 2000; Rosenblum and Clifford, 1984). Beside the
oxidation state of As, pH and competing ions significantly affect the adsorption
of As. As mentioned, the best pH is below 8.2. Below this pH, the surface of
alumina has a net positive charge that can be balanced by adsorbing anions, such
− 5+
as H 2 AsO 4 (for As ), and all other anions listed above. Several studies have
shown the optimum range as 5.5–6.0 (Singh and Clifford, 1981; Rosenblum and
Clifford, 1984; Clifford, 1999). The isotherm shown in Figure 4.16 can be fitted
by the Freundlich model:
q = 0.1679C 0.452
e

where C e is the concentration in micrograms per ligand and q is the amount
adsorbed in mg/g, both for As(V). This isotherm is in agreement with other
studies (Clifford, 1999).
Alkalized Alumina. Activated alumina can be alkalized by impregnation with
alkali oxides, such as K 2 O, Na 2 O, or NH 4 OH. Alkalized alumina is a commer-
cial sorbent for flue gas desulfurization, developed since the early 1970s (e.g.,
Emerson et al., 1971). Alkalized alumina also has other interesting applications;
two will be discussed here: CO 2 removal and NO x removal.
The alkalized aluminas are prepared by impregnation with solutions of alkali
salts, for example, alkali bicarbonates. The salts are typically decomposed by
◦
heat-treatment to 500 C. The alkali oxide contents are typically 5% (wt),
but a range of 1–10% has been reported. The surface areas are reduced