dc.description.abstract |
Goethite (α-FeOOH), being a model adsorbent, has a very highly reactive surface. Therefore,
it is considered to be an efficient adsorbent in soil systems and plays an important role in
controlling the mobility of trace contaminants, like Pb, Ni, Cu, Cr, As, Cd, Co, Zn, and Cu
etc. Being the most abundant iron oxide mineral in almost all the soil types, many researchers
have focused, recently, on the sorption properties of metal doped goethite, not only due to its
greater resemblance with natural goethite, but also for its possible use as an adsorbent in
water purification technology.
In line with this trend of research, the present study reports the characterization of Pb, Ni and
Cu-doped goethite and their adsorption properties for chromate with stepwise comparison to
pure
goethite.
Further,
characterization
and
chromate
adsorption
properties
of
microcrystalline and amorphous phases of copper/iron mixed oxides, are also a part of this
study. All the solid samples, used in the present investigation, are prepared according to a
well known procedure reported in the literature. The amounts (%) of Pb 2+ , Ni 2+ , Cu 2+ ions
doped in goethite samples were 0.38, 0.25 and 0.32, while the microcrystalline goethite and
amorphous phases of copper/iron mixed oxides contained 6.27 and 11.31% Cu 2+ ions
respectively. All these solids are characterized for Surface Area, pH of Point of Zero Charge
(pH pzc ), XRD, TG-DTA, TEM/SEM and FTIR analyses.
XRD and TEM/SEM analyses of all the goethite samples suggest the solids to be crystalline
and doping of the metal ions have only slightly affected the unit cell parameters of the
crystals. TG-DTA analyses reveal that all the goethite samples exhibit similar thermal
behavior with a distinct peak for the degradation of doped metal hydroxides, present in the
solid matrix.. Furthermore, an increase in the concentration of Cu in the precipitate results in
the formation microcryastalline goethite and amorphous phase of copper/iron mixed oxide
with a marginal increase in the surface area and pH pzc of the resultant solid phases.
xDissolution study of goethite in KNO 3 and 303K suggest that goethite and its metal doped
counterparts are quite stable in the pH range 4-7. In all the solid samples dissolution is
maximum at the lowest pH of 3 and decreases with the increasing pH. Dissolution of pure
goethite is inhibited by increasing the concentration of chromate, while in the metal-doped
goethites, it has no effect on their dissolution. The amorphous and microcrystalline mixed
oxides of copper/iron also observed to follow the dissolution behavior of metal doped
goethites.
Adsorption studies of chromate suggest that doping of metal ions increases the chromate
adsorption capacity of goethite from 62.8 to 72.2% for Pb, from 61.4 to 82.92% for Ni and
from 64.4 to 84.5 for Cu doping in it. The adsorption studies carried out at pH 3, 5 and 7
suggest that the adsorption of chromate is maximum at low pH values and decreases with
increasing pH by all the solid samples used here. The effect of temperature is, however,
different for different samples. In case of Pb and Cu-doped goethite, an increase in
temperature decrease the adsorption capacity of the solid, while for all the other samples
adsorption capacity is observed to increase. The amorphous copper/iron mixed oxide sample
has been found to be the best adsorbent for chromate of all the solids used in the present
investigation.
The values of isosteric heat of adsorption, calculated from the Clausius Clapyron Equation,
are positive for Ni doped goethite and negative for Cu and Pb-doped goethite. In case of
amorphous phases of copper/iron mixed oxides these values are also found to be positive.
The values of isosteric heat of adsorption for chromate adsorption by all the solids are
consistent with the effect of temperature determining the endothermic/exothermic nature of
the surface reactions. Langmuir equation is applicable to the data under all the experimental
conditions. The constants of this model calculated for the adsorption of chromate by all the
xisolids coincide well with the adsorption capacity of the respective solids obtained from the
experimental data.
TEM/SEM analyses suggest that doping of foreign elements in the iron oxide structure, not
only increases the sizes of the solid particles, but also increase their surface roughness.
Theses changes in the solid particles result in an increase in the surface area, H + /OH - ions
adsorption capacity and hence increase the net surface positive charge. These changes
collectively increase the adsorption capacity of the doped solids. FTIR analyses suggest that
all the goethite samples show bands at 636, 793 and 894cm -1 which are the characteristic
bands of goethite and are due to OH bending vibrations. Similarly, the goethite samples show
bands at 1383 and 833cm -1 for NO 3 - moiety, which either disappear or present with reduced
intensity after chromate adsorption. Thus, all the experimental findings suggest that chromate
is adsorbed by all the solid samples through innersphere complexation at pH 5 and 7 by
replacing OH - , NO 3 - and CO 3 2- anions from the surfaces. However, at the lowest pH of 3,
some outersphere complexes of chromate are also formed on the surface of solids. |
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