Environmental Engineering Reference
In-Depth Information
phenol was decomposed completely using three irradiation techniques in the presence of NPs. However, in the case of ionizing
radiation, phenol decayed more efficiently than in the case of uV photocatalysis. Also, the absorbed energy of the ionizing
radiation (γ-ray and electron beam) needed for phenol decomposition was much lower than that for uV photocatalysis.
The addition of TiO
2
in the case of ionizing radiation had a significant effect on phenol decomposition since phenol was
also efficiently decomposed in the absence of TiO
2
NPs [6]. However, the presence of TiO
2
NPs drastically increased the
removal of total organic carbon by ionizing radiation. ionizing radiation processes induced the formation of radical products
from water molecules such
e
a
−
,
H atoms, and OH radicals. These water decomposition products were responsible for phenol
decomposition by ionizing radiation.
5.5.2
Nanocomposites for phenol degradation
zinc oxide (znO) NPs are gaining significant attention as a photocatalyst for the degradation of organic pollutants because of
the generation of a more negative potential in the electrons derived from them as compared to those derived from TiO
2
[42,
43]. Meshram et al. [24] investigated the removal of phenol using znO-bentonite nanocomposite as a photocatalyst under uV
irradiation in a batch as well as a continuously stirred tank reactor (CSTR). Clay is widely available and inexpensive and is an
attractive substrate for the immobilization of various photocatalysts. ion exchange intercalation process was utilized to inter-
calate znO NPs (20-30 nm) in the lattice structure of the bentonite matrix. The advantages of intercalation are (i) proper
dispersion of nanosized photocatalysts in a solid support and thus the generation of distinct reaction sites; (ii) participation of
solid support in the adsorption of the contaminants, which can increase the rate of photocatalytic degradation; and (iii) less
amount of photocatalyst required for the degradation of the organic pollutant [24].
The results from the batch experiments showed that the removal of phenol followed first-order reaction kinetics with
langmuir type of adsorption characteristics [24]. The CSTR experiment showed that phenol can be continuously removed from
aqueous stream using znO-bentonite nanocomposite as a photocatalyst under uV irradiation. The utilized znO-bentonite
nanocomposite achieved ~70% removal of phenol from the effluent at a low flow rate and under basic pH conditions (12.0),
with the generation of phenoxide ions. The photocatalytic degradation of phenol using znO-bentonite nanocomposite resulted
in the formation of adipic acid and 2,4,6-triphenoxy phenol through a radical mechanism.
5.6
the impact of Nms oN voc removal By other processes
Salih et al. [44] investigated the impact of the presence of three commercially available NPs in aqueous stream on TCE
adsorption by gAC. The presence of background materials in natural water can highly impact the adsorption efficiency in
removing VOCs by activated carbon. Since NMs are emerging as new contaminants in water, how their presence impacts the
efficiency of VOC removal by gAC needs investigation. using gAC as adsorbent, Salih et al. [44] conducted TCE adsorption
isotherms and column breakthrough experiments in the presence and absence of silicon dioxide (SiO
2
NPs), titanium dioxide
(TiO
2
NPs), and iron oxide (Fe
2
O
3
NPs) NPs. The results of the TCE adsorption isotherms showed that the presence of neither
of the three NPs had an impact on TCE adsorption by gAC. The authors attributed this lack of effect to the fast adsorption
kinetics of TCE on gAC. On the other hand, during the column breakthrough studies, the presence of any of the three NPs
decreased the amount of TCE adsorbed on gAC. This is a result of the preloading pore blockage phenomenon. The presence
of Fe
2
O
3
NPs, which had the highest particle size distribution (PSD) among the investigated NPs, resulted in the fastest TCE
breakthrough followed by TiO
2
NPs, while SiO
2
with the smallest PSD showed the least impact.
5.7
challeNges iN the use of Nms for voc remediatioN
Although NPs proved efficient for the removal of VOCs from aqueous solution, these investigations were conducted only in a labo-
ratory scale setup and thus the scale-up of these processes to pilot and full scale may or may not be practical or economically
feasible. There are many challenges facing the use of NMs for environmental remediation such as (i) the availability of suppliers that
can provide large quantities of NMs at economically viable prices; (ii) the integration of NMs into existing water treatment plants;
and (iii) the fate and toxicity of NMs, which are critical issues that have to be taken into account when selecting NMs for water
remediation [5]. However, as research progresses, the properties of NMs can be enhanced, for example, by synthesizing more stable
NMs, finding environmentally friendly techniques to introduce functional groups on NMs, and finding energy-efficient techniques
to separate NMs from aqueous solution at the end of the remediation process. Therefore, more research is still needed before a
conclusion can be made with regard to the use of NMs for the remediation of VOCs in aqueous solutions.
