Decontamination of water by combined solar advanced oxidation processes and biotreatment - Solar Energy Sciences and Engineering Applications - page 378

The ferric iron complex has different light absorption properties depending on the
ligand, so Equation 12.2.5 takes place with different quantum yields and also at dif-
ferent wavelengths. Consequently, pH plays a crucial role in the efficiency of the
photo-Fenton reaction, because it strongly influences which complexes are formed.
Thus, pH 2.8 has frequently been postulated as optimum for photo-Fenton treatment,
because there is no precipitation yet and the predominant iron species in solution is
[Fe(OH)] 2 + , the most photoactive ferric iron-water complex. In fact, as shown in its
general form in Equation 12.2.5, ferric iron can form complexes with many substances
and undergo photoreduction. Carboxylic acids are of special importance because they
are frequent oxidation intermediate products, and ferric iron-carboxylate complexes
may have much higher quantum yields than ferric iron-water complexes.
Fe 3 + complexes present in mildly acidic solutions absorb an appreciable amount of
light in the UV and into the visible region, and may complex with certain target com-
pounds or their by-products. These complexes typically have higher molar absorption
coefficients in the near-UV and visible regions than aquo complexes. Polychromatic
quantum efficiencies from 0.05 to 0.95 are common in the UV/visible range (Pignatello
et al., 2006), making the photo-Fenton process suitable for being driven by sunlight.
12.2.1 Solar photo-Fenton hardware
Much solar detoxification system component equipment (Blanco and Malato, 2003) is
identical to what is used for other types of water treatment, and construction materials
are available on the market. Most piping may be made of polyethylene or polypropy-
lene, but not metal or composite materials that could be degraded by the oxidizing
conditions of the process. Neither may reactive materials that would interfere with
the photocatalytic process be used. All materials used must be inert to degradation by
solar UV light so they last the minimum required system lifetime. Photocatalytic reac-
tors must transmit UV-Vis light efficiently because of the process requirements. The
best reflecting/concentrating material is aluminium, because, while aluminium coated
mirrors are low-cost and highly reflective in the UV-Vis band of the terrestrial solar
spectrum, the reflectivity (reflected radiation/incident radiation) from 300 to 400 nm
of traditional silver-coated mirrors is very low. Aluminium, which is the only metal sur-
face that is highly reflective throughout the ultraviolet spectrum, has a reflectivity range
from 92.3% at 280 nm to 92.5% at 385 nm. Comparable values for silver are 25.2%
and 92.8%, respectively. Aluminium also reflects perfectly in the visible range. The
photocatalytic reactor must be transparent to UV-Vis radiation. Visible range transmis-
sivity of different materials is usually high, and it is in the UV range where restrictions
appear. The choice of materials that are both transmissive to UV light and resistant to
its destructive effects is limited. Common materials that meet these requirements are
fluoropolymers, acrylic polymers and several types of glass. Quartz has excellent UV
transmission as well as good temperature and chemical resistance, but its high cost
makes it completely unfeasible for photocatalytic applications. Fluoropolymers are a
good choice of plastics for photoreactors due to their good UV transmittance, excel-
lent ultraviolet stability and chemical inertness. However, in order to achieve minimum
pressure resistance, the wall thickness of the fluoropolymer tube has to be increased,
which in turn lowers its UV transmittance. Other low cost polymeric materials are