Environmental Engineering Reference
In-Depth Information
for example, ethylenediaminetetraacetic acid (edTA), citric acid, salicylic acid, or indirect acceptors via HO
∙
reaction,
such as methanol, acetic acid, or formaldehyde (eq. 9.11).
ROHHO OH
+→
•
•
(9.11)
M
z
+
→
•
→
ROH
•
ROH
M
0
(9.12)
The reaction with h
V
+
or HO
∙
reduces the electron-hole recombination and generates very strong reducing radicals [7, 10, 11],
which allow the reduction of species that are not likely to be reduced directly by e
C
−
, as is the case for Pb(II) and As(V)
[10, 19-21]. A synergetic effect generally exists between oxidative and reductive processes [7, 10], especially in the presence
of organic compounds that can undergo oxidation.
Photocatalytic-based treatments can take advantage of these transformations in order to obtain the following:
1. A metallic solid form deposited on the photocatalyst surface; in this way, the species is removed from the solution. This
procedure can also be applied for metal recovery (copper, platinum, silver, gold).
2. A less toxic soluble species (Cr(VI) to Cr(III), As(III) to As(V)), also easier to be removed by an ulterior treatment.
9.3
ArseNic
Arsenic contamination in water can be anthropogenic (mining, use of biocides, wood preservers), but it mainly comes from
natural sources due to dissolution of minerals in surface water or groundwater, or to volcanic processes [22, 23]. Chronic inges-
tion of arsenic for prolonged periods of time results in arsenicosis, a hydric disease, which causes severe skin lesions including
pigmentation changes, palmoplantar keratosis, and other syndromes, ending generally in cancer [24]. Arsenic in drinking water
constitutes a serious problem, affecting several million people all over the world. The world Health Organization (wHO) [24]
recommends 10 µg l
−1
as the maximum allowable As concentration in drinking water, a value that most national regulatory
agencies accept as the norm.
Arsenic in water can be found in its tri- and pentavalent oxidation states; the predominant As(III) form is arsenite (H
3
AsO
3
),
while As(V) frequently occurs as arsenates (H
2
AsO
4
−
or HAsO
4
2−
). Arsenite removal from water is difficult; it is more mobile
and with a toxicity 20 times higher than that of As(V).
Conventional water treatments used to remove As are oxidation/coagulation/adsorption processes on iron or aluminum
hydroxides, ionic interchange, activated alumina, lime softening, and reverse electrodialysis and osmosis [25, 26]. Transformation
to As(V) makes the application of conventional technologies such as ion exchange and adsorption easier. However, new emerg-
ing techniques should be investigated for low-cost solutions to the arsenic problem, especially for low-income populations, as
mentioned in some references [22, 27, 28].
Photocatalysis of arsenic systems was described in our previous review [10] where examples of oxidative TiO
2
HP [29-34]
and mechanisms for As(III) oxidation [30, 35] were reported in studies that spanned concentrations from the micro- to the
millimolar range, showing in every case very fast oxidation in 10-100 min. On the other hand, reduction of As(V) to As(III)
or As(0) is thermodynamically favored even with mild reductants, as can be inferred from the Latimer diagram for As
(fig. 9.2). further reduction to arsine (As(−3)) is more difficult. All the reductive steps are less favored at higher pH values.
0.148V
0.012 V
0.560V
0.240V
-0.225V
H
3
AsO
4
As(OH)
3
AsH
3
As(0)
0.372V
figure 9.2
Latimer diagram connecting the different As species [36].
