Adsorption of Hexavalent Chromium from Aqueous Solutionby Using Activated Red Mud Jyotsnamayee Pradhan, Surendra Nath Das, and Ravindra Singh Thakur1 Regional Research Laboratory CSIR, B
Trang 1Adsorption of Hexavalent Chromium from Aqueous Solution
by Using Activated Red Mud
Jyotsnamayee Pradhan, Surendra Nath Das, and Ravindra Singh Thakur1
Regional Research Laboratory (CSIR), Bhubaneswar 751013, India
Received January 7, 1999; accepted April 14, 1999
Adsorption by activated red mud (ARM) is investigated as a
possible alternative to the conventional methods of Cr(VI)
re-moval from aqueous synthetic solutions and industrial effluents.
Adsorption characteristics suggest the heterogenous nature of the
adsorbent surface sites with respect to the energy of adsorption.
Various factors such as pH, contact time, Cr(VI) concentration,
amount of adsorbent, and temperature are taken into account, and
promising results are obtained The applicability of the Langmuir
as well as Freundlich adsorption isotherms for the present system
is tested The loading factor (i.e., milligrams of Cr(VI) adsorbed
per gram of ARM) increased with initial Cr(VI) concentration,
whereas a negative trend was observed with increasing
tempera-ture The influence of the addition of anions on the adsorption of
Cr(VI) depends on the relative affinity of the anions for the surface
and the relative concentrations of the anions © 1999 Academic Press
Key Words: activated red mud; adsorption; hexavalent
chro-mium removal.
INTRODUCTION
Rapid industrialization and usage of heavy metals in
indus-trial processes have resulted in an unprecedented increase in
the heavy metal flux into groundwater and industrial effluents
Like many heavy metals, chromium in traces is necessary for
life processes However, with higher concentration of this
element in environment and the consequent increase in human
intake, chromium concentrations have reached toxic levels and
manifested in a variety of ailments such as dermatitis,
conges-tion of respiratory tracts, and perforaconges-tion of the nasal septum
It is also a proven mutagen which may lead to cancer (1)
Chromium can exist in several valence states of which the
trivalent and hexavalent forms are common and the hexavalent
form is highly toxic
In view of the pollution hazard caused by hexavalent
chro-mium, several methods of removal have been reported,
includ-ing chemical precipitation, reverse osmosis, ion exchange,
foam flotation, electrolysis, and adsorption Among all the
above mentioned methods, adsorption is an economically
fea-sible alternative A variety of materials are used as adsorbents
for Cr(VI), and various studies have been published
document-ing its adsorption on activated carbon (2), starch xanthate (3), alumina (4), low-grade manganese ore (5), crushed coconut shell (6), fly ash (7), sawdust (8, 9), rice husk carbon (10), wood charcoal (11), bituminous coal (12), and lignite (13) Removal of chromium by different physical and chemical methods has been reviewed (14) In the present study the material used is an industrial waste/byproduct of the aluminum industry Here, an attempt is made to prepare activated red mud and to study its feasibility as an adsorbent for removal of hexavalent chromium from aqueous solution/industrial efflu-ents The process is investigated as a function of pH, time, concentration of adsorbate, amount of adsorbent, and temper-ature The effects of other extraneous anions on the adsorption
of Cr(VI) is also investigated
EXPERIMENTAL
Activated red mud is prepared (15) by simple acid dissolu-tion followed by ammonia precipitadissolu-tion and drying at 110°C Procedural details are given in our earlier paper (16) Analyt-ical grade reagents are used to prepare KCl solution and buffer solution to maintain the ionic strength and pH of the medium The solutions of Cr(VI) are prepared from AR quality
K2Cr2O7 Adsorption experiments for Cr(VI) were carried out in 100-ml stoppered conical flasks by taking appropriate amounts
of potassium dichromate solution and activated red mud The ionic strength of the medium was maintained by adding 1 M KCl Acetic acid–sodium acetate buffer was used to maintain the pH of the solution in the range 3.0 –5.9 and KH2PO4– NaOH buffer in the range 6.0 – 8.0, and the final volume was made up to 50 ml After gentle shaking for a stipulated contact time in a mechanical shaker, the contents were filtered through
G4 crucibles Concentrations of Cr(VI) in the filtrate were determined spectrophotometrically (17) using diphenylcarba-zide solution at l 5 540 nm The percentage of Cr(VI)
ad-sorbed was determined from the ratio of chromium (VI) in the solution and particulate phases:
Cr~VI!ads~% of chromium ~VI! adsorbed!
5Cr~VI!in2 Cr~VI!eq
Cr~VI!in 3 100, [1]
1 To whom correspondence should be addressed.
Article ID jcis.1999.6288, available online at http://www.idealibrary.com on
Copyright © 1999 by Academic Press
Trang 2where Cr(VI)in and Cr(VI)eq are the initial and equilibrium
concentrations
Each run was made in duplicate All of the pH
measure-ments were made with an Elico-Digital pH meter (model
LI-120) using a combined glass electrode (Model CL-51) All
of the spectrophotometric measurements were made with a
Chemito-2500 recording UV-Vis spectrophotometer using
10-mm matched quartz cells
The pHpzcvalue of ARM was determined by potentiometric
acid-base titration following the method of Parks and Bruyn
(18) where 0.2 g of the sample was suspended in KNO3
solutions at concentrations of 0.1, 0.01, and 0.001 moles/L as
a supporting electrolyte The surface charge (s°) in coulombs/g
was calculated by taking the difference between H1and OH2
ions added to attain a particular pH, as reported earlier (19)
RESULTS AND DISCUSSION
Effect of Contact Time
Adsorption experiments were carried out for 24 h to find the
optimum contact time The kinetics of adsorption of hexavalent
chromium at pH 5.2 (Fig 1) show that the equilibrium is
attained in about 2 h There is no significant change in
equi-librium concentration after 2 h up to 24 h It is further observed
that the removal curve is smooth and continuous indicating the
possibility of the formation of monolayer coverage of Cr(VI)
ion at the interface of the ARM
Effect of pH
The initial pH is varied between 3.5 and 7.0 The adsorption
of hexavalent chromium at a fixed Cr(VI) concentration (20
mg/L) as a function of equilibrium pH is shown in Fig 2 It is evident from the figure that the adsorption is higher at lower
pH, it is maximum at pH 5.2, and it suddenly decreases becoming almost negligible at pH 7.06 A similar type of behavior is also reported for the adsorption of anionic species
on metal oxides/oxyhydroxides (19 –23), fly ash (24, 25), and coal (26) The effect of pH on the adsorption capacity of activated red mud may be attributed to the combined effect of
pH on the nature of activated red mud surfaces, adsorbed Cr(VI) species, and the presence of acid and base used to adjust the pH of the solution To explain the observed behavior of Cr(VI) adsorption with varying pH, it is necessary to examine various mechanisms such as electrostatic attraction/repulsion, chemical interaction, and ion exchange which are responsible for adsorption on sorbent surfaces
From the stability diagram (27), it is evident that the most prevalent forms of Cr(VI) in aqueous systems are acid chro-mates (HCrO4 2), chromates (CrO4 2), dichromates (Cr2O7 2), and other oxyanions From the stability diagram for the Cr(VI)–H2O system, it is evident that at low pH, acid chromate ions (HCrO4 2) are the dominant species As the pH increases, there is little increase in the percentage of adsorption, and it is maximum at pH;5.2 When the pH increases to 6, a sharp
decrease in the percentage of adsorption is observed This may
be caused by a decrease in net positive centers on the surface
of the adsorbent due to adsorbed Cr(VI) species which results
in weakening of electrostatic forces between the adsorbate and adsorbent and ultimately leads to a reduction in the sorption capacity When the pH increases beyond 6.0, a gradual de-crease in the percentage of adsorption is observed which may
be due to the competition between OH2 and chromate ions
FIG 1. Adsorption of Cr(VI) on activated red mud as a function of time.
FIG 2. Adsorption of Cr(VI) on activated red mud as a function of pH.
Trang 3(Cr2O7 2), the former being the dominant species at higher pH
values The net positive surface potential of sorbent decreases
resulting in weakening of electrostatic forces between sorbate
and sorbent which ultimately lead to the lowering of the
sorption capacity Similar results are also observed with fly
ash–wollastonite (25) and coal (26)
Determination of pH pzc
The pH at point of zero charge (pzc) was found to be approx
8.5 (Fig 3) which is comparable to the values reported earlier for
untreated red mud (28) as well as pure systems of alumina (4) and
goethite (20) This is in agreement with our experimental
obser-vation showing almost zero adsorption at pH.7.0
Effect of Temperature
The percentage of adsorption of Cr(VI) on activated red mud
was studied as a function of temperature in the range 298 –323
K The results are presented in Fig 4 There is a decrease in the
percentage of adsorption with a rise in temperature which may
be due to higher desorption caused by an increase in the
thermal energy of the adsorbate
The uniformity or heterogeneity of the surface sites of an
activated red mud sample has been deduced from the isosteric
heats of adsorption as a function of adsorption density using
the Clausius–Clapeyron equation (29) The isosteric heats of
adsorption are calculated from adsorption isotherms at two
different temperatures:
DHr5 R ln@C2/C1#/~1/T22 1/T1!, [2]
whereDHris the isosteric heat of adsorption in kJ/mole at a
given adsorption density, R is the gas constant, and C1and C2
are the equilibrium concentrations of the ion at temperatures T1
and T2 If the isosteric heat of adsorption is independent of adsorption density, then the surface is homogenous, and if it decreases with increasing adsorption density, then the surface
is heterogenous (30) A decrease in the percentage of adsorp-tion with a rise in temperature and variaadsorp-tion in the isosteric heats of adsorption support the heterogenous nature of the activated red mud sample Similar types of observations are also made in selenite adsorption on iron oxyhydroxides (19) and manganese nodules (20) This may be due to different types of adsorption sites or the interaction of adsorbing ions
Effect of Adsorbent and Adsorbate Concentration
The percentage of Cr(VI) adsorption with varying amounts
of activated red mud and Cr(VI) concentration is presented in Figs 5 and 6 An increase in percentage of adsorption with higher amounts of adsorbent and a decrease with higher con-centration of adsorbate indicate that the adsorption is depen-dent upon the availability of the binding sites To determine the adsorption capacity of the sample, the equilibrium data for the adsorption of Cr(VI) are analyzed in the light of the Langmuir adsorption isotherm model Experimental data points are fitted into the Langmuir equation:
C/X 5 1/~bXm! 1 C/Xm, [3]
where C/X is the amount of Cr(VI) adsorbed per unit weight of
FIG 3. Surface charge of ARM as a function of pH in the presence of
KNO
FIG 4. Adsorption of Cr(VI) on activated red mud as a function of temperature.
Trang 4the sample, C is the Cr(VI) concentration in equilibrium
solu-tion, b is a constant related to the energy of adsorpsolu-tion, and Xm
is the adsorption capacity of the sample Figure 7 depicts the
Langmuir plot of C/X vs C for the experimental data points.
The correlation coefficient is found to be 0.99 Xm and b are
calculated from the Langmuir equation by applying the least
squares method to the lines of Fig 7 and found to be 30.74 mmol/g and 0.2691, respectively
Ka values (31), which represent the apparent equilibrium constant corresponding to the adsorption process, can be
cal-culated as the product of Langmuir equation parameters b and
Xm The apparent equilibrium constant was found to be 8.2737 mmol/g, which can be used as a relative indicator of the red mud’s affinity for chromate ions (32) The adsorption values were fitted to the Freundlich isotherm model and were found to
be in order (Fig 8) The linearity of the Langmuir and Freund-lich isotherms indicate that the adsorption is a surface phenom-enon (4)
Effect of Competitive Ions
The effect of different competitive ions like nitrate (NO3 2), sulfate (SO4 2), and phosphate (PO4 2) on the adsorption of Cr(VI) was studied at various concentrations It was observed that the percentage of adsorption of Cr(VI) decreased with increasing concentrations of externally added ions The affinity sequence for adsorption of such anions on ARM is PO4 2
SO4 2 NO3 2 Increasing dosages of these ions from 5 to 20 mg/L had little effect Similar observations were made for fluoride removal using treated alum sludge (33)
Desorption Studies
After adsorption, the resulting Cr(VI) containing ARM is safe for disposal The stability of this sludge from the resolu-bilization point of view was studied It was found that the
FIG 5. Adsorption of Cr(VI) as a function of the amount of adsorbent.
FIG 6. Adsorption of Cr(VI) on activated red mud as a function of initial
concentration.
FIG 7. Langmuir plot of Cr(VI) adsorption on activated red mud at room temperature.
Trang 5Cr(VI) release from the sludge in aqueous medium after
con-tact time of.72 hours was negligible However, partial
de-sorption of Cr(VI) is observed in a strongly alkaline medium
(pH 8) The solution obtained after Cr(VI) adsorption was
subjected to AAS and none of the trace metals was found to be
in the detectable range Thus the release of harmful trace
metals into the environment after adsorption of Cr(VI) is ruled
out
Applicability to Industrial Effluents
Adosrption studies were extended to the effluents from the
sodium dichromate and basic chromium sulphate industries
containing very high values of Cr(VI) using ARM under
sim-ilar conditions In all cases the percentage of adsorption was
found to be more than 90% Details of the results obtained will
be communicated in a separate paper
CONCLUSION
From the previous discussions, it may be concluded that
hexavalent chromium adsorption on activated red mud is a
surface phenomenon and the Langmuir and Freundlich
iso-therm curves show linearity The best conditions for adsorption
were found to be pH 5.2 and a temperature of 303 K in the
concentration range 2–30 mg/L with a solid:liquid ratio of
1:500 The presence of other ions in solution influenced
ad-sorption Although the nitrates had very little effect, ions like
sulphate and phosphate had noticeable effects due to the higher
selectivity of the activated red mud surface for these ions Thus, the activated samples of red mud serve as an excellent alternate adsorbent for removal of hexavalent chromium from aqueous medium
ACKNOWLEDGMENT
The authors are grateful to the Director, Regional Research Laboratory at Bhubaneswar for kindly permitting the publication of this paper.
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FIG 8. Freundlich plot of Cr(VI) adsorption on activated red mud at room
temperature.