harvest sunlight more efficiently [35]. N,S co-doped TiO 2 nanoparticles and
nanosheets have been used in photocatalysis of ibuprofen and naproxen with a
catalyst loading of 2 g/L, at pH 6. In this study, ibuprofen was removed up to an
85% with the nanoparticles, but only a 71.6% with the nanosheets, whereas for
naproxen a similar degradation (99%) was achieved on both materials. The reusability of these catalysts was of six cycles and the mechanism of degradation
proceeded through direct oxidation on the catalyst holes which leads to generation
of reactive oxygen species. Another important feature is that TiO 2 nanosheets
performance was independent from pH in the range 5–9 [36]. TiO 2 doping with
potassium ferricyanide, prepared via sol-gel, was tested toward visible light degradation of paracetamol, and it proved to be about five times faster than pure TiO 2 to
eliminate 99.1% of this pharmaceutical [37]. Another dopant that has been tested for
paracetamol removal is potassium peroxodisulfate with a 100% removal at an initial
concentration of 0.1 mM paracetamol, pH 9 and a catalyst load of 1 g/L [38].
Other nanocatalysts being developed to treat NSAIDs are NiO and NiS which
have been supported on a substrate of Fe 3 O 4 and polypyrrole to treat water polluted
with naproxen. Immobilization reduced their band gaps from 2.23 to 2.1 eV for NiS
and from 3.4 to 3.05 for NiO. In these experiments, the highest removal percentage
was achieved with the immobilized catalysts. Naproxen from real water samples
decreased its concentration in a 65% in tap water and in a 77% in pharmaceutical
wastewater with the most efficient nanocatalyst, the supported NiS [39].
ZnO and g-C 3 N 4 have also been investigated as photocatalysts that have been
tested for NSAIDs degradation. Choina et al. investigated ZnO nanoparticles of
15–30 and 100 nm to treat tetracycline and ibuprofen. They found that higher
ibuprofen removal percentages, 24% against 14%, were achieved with smaller
catalyst particles. They also observed that ibuprofen abatement was about 60%
with the lowest initial drug concentration, 5 ppm against 60 ppm [40]. In another
study, maximum ibuprofen removal (83%) was obtained with a ZnO
nanophotocatalyst (100 nm particle diameter) under 254 nm radiation with substrate
initial concentration of 1.5 mg/L and a catalyst load of 0.58 g/L within 95 min of
reaction [41]. In a comparative study between TiO 2 and ZnO nanoparticles, authors
found that, under UV light irradiation, an optimum catalyst load was 1.5 g/L for
TiO 2 and 1.0 g/L for ZnO. In this study, pH influence was stronger on TiO 2 than on
ZnO, being pH 3 more favorable for complete removal in 20 min reaction on TiO 2
and pH 7 more favorable for total abatement in 30 min reaction on ZnO. The reason
for this is attributed to the positive charge on both materials at these pH values. At
pH 3 a protonated ibuprofen ( pK a 5) will be more attracted to protonated TiO 2 , while
at higher pH values both deprotonated materials, ibuprofen and TiO 2 , will experience electrostatic repulsion that leads to a decreased degradation rate. On the other
side, ZnO point of zero charge is between 7 and 9; therefore, at pH 7 it will be
positively charged, while ibuprofen will be negatively charged which results in a
stronger electrostatic interaction between substrate and catalyst that favors degradation [42]. Moreover, ZnO supported on clay mineral fibrous sepiolite has been used
to photocatalytically remove ibuprofen, paracetamol, and antipyrine in an initial
concentration of 10 mg/L and with a catalyst load of 250 mg/L under simulated solar
Nanotechnologies for Removal of Nonsteroidal Anti-inflammatory Drug from. . .
285
nanosheets have been used in photocatalysis of ibuprofen and naproxen with a
catalyst loading of 2 g/L, at pH 6. In this study, ibuprofen was removed up to an
85% with the nanoparticles, but only a 71.6% with the nanosheets, whereas for
naproxen a similar degradation (99%) was achieved on both materials. The reusability of these catalysts was of six cycles and the mechanism of degradation
proceeded through direct oxidation on the catalyst holes which leads to generation
of reactive oxygen species. Another important feature is that TiO 2 nanosheets
performance was independent from pH in the range 5–9 [36]. TiO 2 doping with
potassium ferricyanide, prepared via sol-gel, was tested toward visible light degradation of paracetamol, and it proved to be about five times faster than pure TiO 2 to
eliminate 99.1% of this pharmaceutical [37]. Another dopant that has been tested for
paracetamol removal is potassium peroxodisulfate with a 100% removal at an initial
concentration of 0.1 mM paracetamol, pH 9 and a catalyst load of 1 g/L [38].
Other nanocatalysts being developed to treat NSAIDs are NiO and NiS which
have been supported on a substrate of Fe 3 O 4 and polypyrrole to treat water polluted
with naproxen. Immobilization reduced their band gaps from 2.23 to 2.1 eV for NiS
and from 3.4 to 3.05 for NiO. In these experiments, the highest removal percentage
was achieved with the immobilized catalysts. Naproxen from real water samples
decreased its concentration in a 65% in tap water and in a 77% in pharmaceutical
wastewater with the most efficient nanocatalyst, the supported NiS [39].
ZnO and g-C 3 N 4 have also been investigated as photocatalysts that have been
tested for NSAIDs degradation. Choina et al. investigated ZnO nanoparticles of
15–30 and 100 nm to treat tetracycline and ibuprofen. They found that higher
ibuprofen removal percentages, 24% against 14%, were achieved with smaller
catalyst particles. They also observed that ibuprofen abatement was about 60%
with the lowest initial drug concentration, 5 ppm against 60 ppm [40]. In another
study, maximum ibuprofen removal (83%) was obtained with a ZnO
nanophotocatalyst (100 nm particle diameter) under 254 nm radiation with substrate
initial concentration of 1.5 mg/L and a catalyst load of 0.58 g/L within 95 min of
reaction [41]. In a comparative study between TiO 2 and ZnO nanoparticles, authors
found that, under UV light irradiation, an optimum catalyst load was 1.5 g/L for
TiO 2 and 1.0 g/L for ZnO. In this study, pH influence was stronger on TiO 2 than on
ZnO, being pH 3 more favorable for complete removal in 20 min reaction on TiO 2
and pH 7 more favorable for total abatement in 30 min reaction on ZnO. The reason
for this is attributed to the positive charge on both materials at these pH values. At
pH 3 a protonated ibuprofen ( pK a 5) will be more attracted to protonated TiO 2 , while
at higher pH values both deprotonated materials, ibuprofen and TiO 2 , will experience electrostatic repulsion that leads to a decreased degradation rate. On the other
side, ZnO point of zero charge is between 7 and 9; therefore, at pH 7 it will be
positively charged, while ibuprofen will be negatively charged which results in a
stronger electrostatic interaction between substrate and catalyst that favors degradation [42]. Moreover, ZnO supported on clay mineral fibrous sepiolite has been used
to photocatalytically remove ibuprofen, paracetamol, and antipyrine in an initial
concentration of 10 mg/L and with a catalyst load of 250 mg/L under simulated solar
Nanotechnologies for Removal of Nonsteroidal Anti-inflammatory Drug from. . .
285