Environmental Engineering Reference
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to the theoretical value (72.36%). The pH pzc of MNPs was found to be 7.1. The results of adsorption studies of MB and cR dyes
exhibited that the kinetics of the process was rapid, showing that adsorption occurred only on the surface of MNPs. The
behavior of MNPs toward varying pH was explained on the basis of pHpzc (pH pzc : ~7.1), and the adsorption of cationic MB was
found to be above pH pzc and anionic cR adsorption was higher in the lower pH range. The optimized pH for MB and cR dye
adsorption was approximately 9.2 and 6.2, respectively. The aggregation of MNPs with increase in the MNPs dose led to a non-
linear adsorption of dyes. An increase in dye concentration led to an increase in dye uptake by MNPs, but the percentage of
adsorption decreased. The monolayer adsorption capacities of MNPs (70.4 and 172.4 mg/g for MB and cR, respectively) were
obtained. Desorption studies of MB and cR dyes were also performed, and 95% cR was found to desorb at pH greater than 8.0.
However, MB desorption was more effective (85% MB desorption) when methanolic solution of acetic acid (4% (v/v) acetic
acid in methanol) was used as an eluent.
In an attempt to modify the iron oxide NPs for rendering them effective for dye removal, Inbaraj and chen [38] synthe-
sized poly(γ-glutamic acid) (PGA)-coated iron oxide NPs. PGA coating was found to improve the stability of the MNPs,
which was confirmed by the leaching experiments in different aqueous matrices (deionized water, tap water, river water,
acidic solutions, basic solutions). The MB dye experimental isotherm data fitted well with the Redlich-Peterson and
langmuir models. The maximum adsorption capacity of MB dye onto PGA MNPs obtained from the langmuir model was
78.67 mg/g. The adsorption kinetics of MB onto synthesized nanomaterials was fast (equilibrium was achieved in 5 min).
In the presence of coexisting ions, MB adsorption was lowered. A declining trend in the presence of cacl 2 was more
significant than in the presence of Nacl due to the valence effect. Acidic pH (1.0-3.0) was not favorable for the adsorption
of MB dye onto PGA MNPs due to the competition between dye cations and hydrogen ions in the solution for the
α-carboxylate anions present on the surface of the adsorbent. The ion exchange mechanism that dominated at higher pH
favored the MB adsorption onto PGA MNPs. On the other hand, desorption was favorable in acidic media (ionized water).
Salih et al. [39] investigated the fate and transport of fe 2 O 3 NPs in a GAc adsorber bed and its impact on trichloroethy-
lene (TcE) adsorption. The TEM image revealed that fe 2 O 3 NPs formed chains and branches due to intermolecular
attraction. The presence of fe 2 O 3 NPs enhanced TcE removal (70%) from the aqueous phase as compared to kinetics onto
GAc alone. But, desorption was similarly fast due to the lack of a suitable pore size distribution for permanent adsorp-
tion of TcE. characterization results revealed that only 3% of fe 2 O 3 NP total surface area was microporous resulting in
their easy desorption into bulk solution. It was concluded from the values obtained from the kinetic models that batch sys-
tems might not be optimal to study TcE adsorption. column breakthrough studies were also undertaken in which it was
observed that TcE breakthrough occurred in a shorter time in the presence of fe 2 O 3 NPs than in their absence. This was
attributed to the attachment of fe 2 O 3 onto GAc. Also, the impact of fe 2 O 3 NPs on TcE breakthrough was found to be
concentration-dependent.
Jegadeesan et al. [40] reported the sorption of As(III) and As(V) species on nanosized amorphous and crystalline TiO 2 . The
isotherms indicated TiO 2 sorption capacities as dependent on the site density, surface area, and crystalline structure. The
adsorbent surface remained almost constant for particles between 5 and 20 nm. But As(V) surface coverage increased with
the degree of crystallinity, which was confirmed by X-ray absorption spectroscopic analysis. The data indicated binuclear
bidentate inner sphere complexation of As(III) and As(V) on amorphous TiO 2 at neutral pH. Belessi et al. [41] synthesized
TiO 2 NPs and used them for the removal of reactive red 195 azo dye. The effects of pH, concentration of dye, and adsorbent
dose have been studied on the removal of dye. The equilibrium data fitted well with langmuir and pseudo-second-order
kinetic models. At pH 3.0 and 30°c, the maximum adsorption capacity was 87.0 mg/g. The kinetic studies indicated a rapid
sorption of dye in the first 30 min with equilibrium at 1 h.
few other efforts have been made to use other nano metal oxides for the removal of various aquatic pollutants. Arsenic
(As(III) and As(V)) removal using nano copper oxide has been tested [42, 43]. The kinetics of arsenic onto cuO NPs was fast,
but As(III) took a comparatively longer time than As(V) to adsorb [42]. As(III) adsorption onto cuO NPs was maximum at
pH 9.3; on the contrary, As(V) adsorption was relatively independent of pH. Sulfate and silicate are the main competing ions
in case of As(III), whereas only phosphate was found to affect As(V) adsorption. Hydrous cerium oxide (HcO) NPs have also
been reported for the removal of arsenic [44]. The kinetics of As(III) and As(V) was observed to be rapid onto HcO NPs. The
increase in pH from 3.0 to 7.0 resulted in the decrease of percent removal of As(V) from 99.7 to 54.7%. On the contrary, only
a slight increase in As (III) removal percent was noticed when the pH was increased from 3.0 to 7.0. Unlike the case of other
metal oxyhydroxides, the increase in solution pH affected As(III) removal only slightly, suggesting that a strong affinity exists
between the As(III) and HcO NPs. The repulsion effect from the increase in solution pH was easily overcome by the adsorp-
tion energy. Mainly H 2 PO 4 had an adverse effect on As(III) and As(V) removal efficiency, which was similar to the results
obtained in previous studies on arsenic adsorption. The fTIR results of HcO NPs after adsorption process exhibited the
formation of ce-O-As and As-Oce bonds in case of As(III)-HcO and As(V)-HcO NPs, respectively, indicating the formation
of inner-sphere complexes.
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