(397j) Study on the Adsorption Behavior of Activated Red Mud  for Cr(VI) Ions from Aqueous Solution

Study on the adsorption behavior of activated red mud

for Cr(VI) ions from aqueous solution

Lei Wang, Pengjie Gao, Shuqin
Liang, Dan Zhang, Suyu Jiang, Huiping Li*

School
of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, Henan
450001, China

*Corresponding
author phone: +86 0371 67781807; Fax: +86 0371 67781807;

E-mail: huipingli@zzu.edu.cn  (H.Li.)

Abstract: The red mud (RM) was
activated with different methods, followed by adsorption experiments for
optimal conditions. The results revealed that the adsorption effect of modified
RM by FeCl₃was most
efficient. The removal efficiency of Cr(VI) was 99.6 %
under the optimal conditions with initial Cr(VI) concentration 100 mg/L, adsorbent
concentration 8.0 g/L, pH 5, 180 min and
25 ¡æ. The adsorption capacity
decreased with the temperature increasing from 25 to 45 ¡æ. The thermodynamic parameters showed that the sorption of Cr(VI) onto Fe-RM was
feasible, spontaneous and exothermic in nature. The Freundlich and
pseudo-second-order model provided the best correlation with the experimental
data compared to other models. It was concluded that the Fe-RM can be used as
effective sorbent for treating the Cr(VI) ions from industrial wastewaters.

Keywords: Modified red mud; Chromium; Adsorption; Thermodynamic property; Kinetic

1. Introduction

Chromium compounds are
present in the effluents from various industries that include metal plating, textile
dyeing, leather tanning, pigments and mining industries and pose severe threat
to eco-environment and human health because of its toxicity, high
carcinogenicity and oxidation [1]. A number of
treatment technologies such as adsorption [2],
biosorption, ion exchange [3], membrane filtration,
and reduction-precipitation etc, have been
conducted for the removal of chromium ions from wastewaters.

Red mud (RM) is a fine-textured waste material generated during the
combined process of alumina production from bauxite. Several studies
have reported that activated red mud has also demonstrated strong adsorption
capability to adsorb different pollutants from effluents [4].

In this study, the effect of
different parameters containing adsorbent concentration, initial pH and contact
time on the removal efficiency were investigated in a batch equilibrium
experiments. Several models of isotherms and sorption kinetics were
commissioned to fit these experimental equilibrium data.

2. Experimental sections

For the thermal
treatment, the sample was calcined at various temperatures (200-1000 ¡æ). Batches of dry RM were suspended in 100ml of 0.5-5.0 mol/L HCl
solutions. For the activation with ferric chloride, the aqueous solutions
containing different concentrations of FeCl₃ were prepared: 0.1, 0.3
and 0.5 mol/L.

The sorption
tests took place at controlled adsorbent concentration (0-15 g/L), initial
pH (1-9) and contact time (0-240 min). Batch sorption
experiments were carried out to estimate the property of sorbent in the removal
of Cr(VI) from solution.

The removal
efficiency and uptake capacity of Cr(VI) were calculated as follows:

       
                          (1)

                                                     (2)

where c_o and c_e (mg/L) are the
initial and equilibrium concentrations of Cr(VI), respectively, m (g) is
the mass of dry adsorbent used, and V (L) is the volume of solution, q_e
(mg/g) is the equilibrium Cr(VI) uptake capacity£¬h is the Cr(VI)removal efficiency.

3. Results and discussion

As the temperature
increases, there was gradual increase in Cr(VI) removal and it was maximum
at T = 600 ¡æ when the T
exceeded in 600 ¡æ, a sharp decrease in
the chromate
removal efficiency being observed (Fig. 1(a)). A faster increase in the acid concentration
up to 3.0 mol/Lcaused an increase in the efficiency of Cr(VI)
removal, thereafter a gradual decrease in the efficiency was observed (Fig. 1(b)).
It can be observed that the removal efficiency decreases with increasing
initial Cr(VI) concentration. The adsorption capacity of Cr(VI) increased with
increase in initial Cr(VI)
concentration from 20 to 120 mg/L (Fig. 1(c)).

Fig.
1 Effect of different calcination temperature (a)¡¢HCl
concentration (b) and FeCl₃ concentration (c)

The results showed that the
sorptive properties of Fe-RM were the best. The subsequent experiments were
carried out with red mud activated with a 0.3 mol/LFeCl₃
solution.

When the concentration
of Fe-RM was increased from 1 to 8 g/L, the removal efficiency of Cr(VI)
increased from 23.4 to 69.5 %. At higher concentrations of Fe-RM, the removal
efficiency did not increase significantly (Fig. 2(a)). As the pH increases,
there was little increase in the efficiency and it was maximum at pH = 5 when
the pH exceeded in 5, a sharp decrease in the efficiency being observed (Fig. 2 (b)).The efficiency of
Cr(VI) removal increased from 24.6 % in the 5 min to 68.7 % as the contact time
was increased to 180 min, then little change of removal efficiency was observed
that the indicated equilibrium condition was achieved (Fig. 2 (c)).

Fig. 2
Effect of sorbent concentration (a)¡¢pH (b) and contact time (c)
on the removal efficiency

To study the effect of
temperature on
the sorption capacity,
the temperature ranging
from 25 to 45 ¡æ were carried out at the optimum conditions in 100
mL of Cr(VI) solutions with the initial Cr(VI) concentration 100 mg/L¡¢contact time 4 h¡¢sorbent concentration
8 g/L¡¢pH 5.

Fig .
3 Effect of temperature on the sorption capacity.

It was found that the
temperature had a negative effect on the uptake capacity (Fig .2). The optimum temperature was selected as 25 ¡æ for further sorption experiments.

Thermodynamic behavior of Cr(VI)
sorption onto Fe-RM was investigated using the thermodynamic parameter [5].

The thermodynamic
parameters (DG¡¢DH¡¢DS) showed that the sorption of Cr(VI) onto Fe-RM was
feasible, spontaneous and exothermic in nature.

There were several
expressions for analyzing the experimental data, being that the most frequently
used models were the Langmuir, Freundlich [5], and Tempkin isotherm models.

Several well-known
kinetic models, Lagergren [6], Morris-Weber ,
Elovich and pseudo -second-order models have been applied to fit these
experimental data, obtained for sorption of Cr(VI) onto Fe-RM.

Fig. 4 Frendlich isotherm (a)¡¢pseudo-second
order (b) and intra-particle diffusion kinetic plot (c)

The sorption behavior of Fe-RM-Cr(VI) system
was consistent
with the Freundlich model (Fig. 4 (a)), which indicated that adsorption process
existed in multi-molecular physical adsorption. The correlation
coefficient for the pseudo-second-order model was 0.9983 (Fig. 4 (b)), which
led to believe that the kinetic model provided a best correlation factor for
sorption of Cr(VI) onto Fe-RM in contrast to other models. The Figure indicated
that the sorption process happened through surface sorption and intra-particle
diffusion (Fig. 4 (c)). As seen from Table 2 and 3, there is
chemical
sorption from Tempkin and Elovich model. Thus, the sorption process was
completed through physisorption and chemisorption.

4.
Conclusions

In the present study, the red
mud was activated via calcination, HCl soaking and FeCl₃
modification, respectively. The red mud treated by FeCl₃ hadthehigher sorption performance of Cr(VI) than that of other methods. The maximum Cr(VI)
removal efficiency was 99.6% at the optimal condition of pH 5, contact time 180 min, sorbent
concentration 8 g/L, and initial Cr(VI)
concentration 100 mg/L.
The
calculated thermodynamic parameters (¦¤H£¬¦¤G£¬¦¤S) showed the sorption
to be feasibility, exothermic and spontaneous with decreased randomness at the
solid-liquid interface. The Langmuir, Freundlich and Tempkin isotherm models were
employed for Cr(VI) sorption onto Fe-RM, and it was proved that the Freundlich isotherm model offered higher
correlation factor than other models. The kinetic process was found to fit very well to
pseudo-second-order model. The sorption process was completed through
physisorption and chemisorption. Based on all results, Fe-RM can be used as an
efficient and alternative sorbent for the treatment of wastewaters containing
Cr(VI) ions because of the advantages of being natural, low-cost adsorbent and
having high sorption capacity.

5.
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