6 Source and Fate of Perchlorate in the Environment: A Grave …
149
ClO
−
4 (aq) + 4Fe(0) (s) + 8H
+
(aq) + 8e
−
→ Cl
−
(aq) + 4Fe
2
(aq) + 4H 2 O
(6.1)
ClO 4
− can be removed by nanoscale zero-valent iron (10 mg L
−1 ) at different
rates at different temperatures. For example, ClO 4
− can successfully remove at a
rate of 0.013 mg g
_1 hr
_1 and 1.52 mg g
_1 hr
_1 at temperatures such as 25 °C and
75 °C, respectively (Cao et al. 2005). But this process has a drawback as residual iron
will remain present in treated medium, and therefore, further treatment is required
to remove iron from the medium (Kumarathilaka et al. 2016).
6.7.2.2 Adsorption
In adsorption process, some adsorbent media such as granular activated carbon
(GAC) or activated alumina are used to remove ClO 4
− . Generally, the three influencing factors of adsorption capability of a particular adsorbent are structural characteristics of adsorbent, solution pH and surface properties of an adsorbent. In USA,
granular activated carbon is widely used to adsorb ClO 4
− during water treatment process. (Ye et al. 2012). Recently, a group of researchers identified some other modified
adsorbents such as granular ferric hydroxide to remove ClO 4
− from the water within
60 min (Kumarathilaka et al. 2016). The maximum adsorption capacity of granular
ferric hydroxide was 20 mg/g at pH 6.0–6.5 at 25 °C. Optimum removal occurred in
the pH range of 3–7 (Baidas et al. 2011).
6.7.2.3 Membrane Filtration
Pressure-driven membrane filtration by using reverse osmosis, nanofiltration, ultrafiltration and electrodialysis (ED) are considered as promising technologies for removing ClO 4
− . Both size exclusion and electrostatic exclusion are prominent pathways
for removing inorganic contaminants using such a membrane. Moreover, solution
pH and conductivity determine the degree of rejection by controlling the membrane
charge. To remove ClO 4
− from groundwater, mainly three types of semi-permeable
membranes are used: (i) high-pressure RO membrane, (ii) nanofiltration membrane
and (iii) low-pressure RO membrane. High-pressure RO membranes (>150 psi) have
been observed to remove about 99.9% of ClO 4
− (Morss 2003). Study of Yoon et al.
(2009) reveals that ClO 4
− removal is directly proportional to the pH and indirectly
proportional to the conductivity. In addition, when the ratio of solute radius to the
effective membrane pore radius is greater than 0.4, ClO 4
− removal was higher than
70% in the reverse osmosis membrane. According to study of Roach and Tush (2008)
95% of ClO 4
− can be separated by polyelectrolyte-enhanced ultrafiltration even in
the presence of tenfold excesses of competing ions such as chloride, sulphate and
carbonate. Electrostatic attraction properties between ClO 4
− and positively charged
NH 3
+ is an important mechanism which can be used for removing ClO 4
− during
hybrid/ultrafiltration process using chitosan as an adsorbent, and its equilibrium
149
ClO
−
4 (aq) + 4Fe(0) (s) + 8H
+
(aq) + 8e
−
→ Cl
−
(aq) + 4Fe
2
(aq) + 4H 2 O
(6.1)
ClO 4
− can be removed by nanoscale zero-valent iron (10 mg L
−1 ) at different
rates at different temperatures. For example, ClO 4
− can successfully remove at a
rate of 0.013 mg g
_1 hr
_1 and 1.52 mg g
_1 hr
_1 at temperatures such as 25 °C and
75 °C, respectively (Cao et al. 2005). But this process has a drawback as residual iron
will remain present in treated medium, and therefore, further treatment is required
to remove iron from the medium (Kumarathilaka et al. 2016).
6.7.2.2 Adsorption
In adsorption process, some adsorbent media such as granular activated carbon
(GAC) or activated alumina are used to remove ClO 4
− . Generally, the three influencing factors of adsorption capability of a particular adsorbent are structural characteristics of adsorbent, solution pH and surface properties of an adsorbent. In USA,
granular activated carbon is widely used to adsorb ClO 4
− during water treatment process. (Ye et al. 2012). Recently, a group of researchers identified some other modified
adsorbents such as granular ferric hydroxide to remove ClO 4
− from the water within
60 min (Kumarathilaka et al. 2016). The maximum adsorption capacity of granular
ferric hydroxide was 20 mg/g at pH 6.0–6.5 at 25 °C. Optimum removal occurred in
the pH range of 3–7 (Baidas et al. 2011).
6.7.2.3 Membrane Filtration
Pressure-driven membrane filtration by using reverse osmosis, nanofiltration, ultrafiltration and electrodialysis (ED) are considered as promising technologies for removing ClO 4
− . Both size exclusion and electrostatic exclusion are prominent pathways
for removing inorganic contaminants using such a membrane. Moreover, solution
pH and conductivity determine the degree of rejection by controlling the membrane
charge. To remove ClO 4
− from groundwater, mainly three types of semi-permeable
membranes are used: (i) high-pressure RO membrane, (ii) nanofiltration membrane
and (iii) low-pressure RO membrane. High-pressure RO membranes (>150 psi) have
been observed to remove about 99.9% of ClO 4
− (Morss 2003). Study of Yoon et al.
(2009) reveals that ClO 4
− removal is directly proportional to the pH and indirectly
proportional to the conductivity. In addition, when the ratio of solute radius to the
effective membrane pore radius is greater than 0.4, ClO 4
− removal was higher than
70% in the reverse osmosis membrane. According to study of Roach and Tush (2008)
95% of ClO 4
− can be separated by polyelectrolyte-enhanced ultrafiltration even in
the presence of tenfold excesses of competing ions such as chloride, sulphate and
carbonate. Electrostatic attraction properties between ClO 4
− and positively charged
NH 3
+ is an important mechanism which can be used for removing ClO 4
− during
hybrid/ultrafiltration process using chitosan as an adsorbent, and its equilibrium