(749e) Importance of Mesopore of Activated Carbon in Removing Chromium(VI) From Aqueous Solution

Importance of mesopore of activated
carbon in removing chromium(VI) from aqueous solution

Mingxia Fan^a,b, Shitang Tong^a,c*

a. Department of Chemical Engineering,
Wuhan University of Science and Technology, 947 Heping
Avenue, Wuhan, Hubei, P. R. China, 430081

b. College of
Chemical and Environmental Engineering, Hubei University of Technology, Wuhan, Hubei, P.R. China, 430068

c. Research
Center of Green Manufacturing and Energy-saving & Emission Reduction
Technology, Wuhan
University of Science and Technology, 947 Heping
Avenue, Wuhan, Hubei, P. R. China, 430081

* Corresponding author, Tel: +86-27-86568662; Fax:
+86-27-86568662; Email address: tongshitang@wust.edu.cn

Chromium hexavalent ion (Cr(VI)) in wastewater
is highly hazardous to the ecosystem. Activated carbon adsorption is considered to be the simplest
technique due to low cost and perfect removal. Compared
to normal activated carbon (AC), mesoporous activated
carbon (MAC) is regarded to be more favorable for removal of the large sized
species from wastewater by adsorption due to easy access of the large sized
species to mesopores. The objectives
of the present work are to study the
adsorption performance of Cr(VI) on MAC from aqueous solution to verify importance of mesopores on adsorption of Cr(VI).

The
MAC samples tested were prepared from coal tar pitch
by using the nano-sized imprinting method. The
pore structural properties of the six MAC samples were examined
using N₂ adsorption at 77K as seen in Table 1. The honeycomb-like
porous structure was seen in the SEM image of the MAC (Figure 1) and the
scale provided evidence of existing large pores in MAC.

Table 1 Pore structural properties of MACs.

Figure 1. SEM image of MAC(AC04)

The working solution was prepared by dissolving potassium
dichromate in double-distilled water. pH of the
solution was adjusted by using either 0.1N NaOH or
0.1N H₂SO₄. Adsorption experiments
were carried out in a thermostatic bath oscillator. 50 mL
Cr(VI) solution of known concentration (C₀)
and initial pH was taken in a 150mL erlenmeyer flask
with a fixed dosage (2g/L) of
MAC. The mixture was agitated at a speed of 200 rpm in the thermostatic shaker
bath for a certain period of time. Except for the
runs for investigating the influence of mesopore, AC02
was used in all experiments. Equilibrium adsorption isotherms were measured in
48 hours to ensure attainment of equilibrium. The concentrations of Cr(VI) in the solution were determined by using a UV-visible spectrometer according to the standard
procedure.

Percent
removal of Cr(VI) on MAC was examined for different pH
values of the solution as shown in Figure 2. It is seen that the adsorption displayed a strong pH
dependence. A
large percent removal of Cr(VI) at low
pH can be explained by the strongly electrostatic interaction near the interfacial region between the charged
metal ions in solution and the charged surface of MAC. In acidic
solution, the charged Cr(VI) ion predominantly in
forms are Cr₂O₇ ^2-, HCrO₄^-,
CrO₄^2-, Cr₃O₁₀^2- and Cr₄O₁₃^2-
which are all negatively charged species. According to pH of the zero point
of charge (pHzpc) measured, all MAC samples have the
value of pHzpc around 7.35, indicating MACs display a positive charged surface. As a result, the
electrostatic interaction played a major role in adsorption of Cr(VI) ions from acidic solution on MAC. With an increase of
pH, the percent removal of Cr(VI) is rapidly decreased, ascribed to the competent
adsorption of the negatively charged chromate ions and OH^- on the
surface of MAC.

Figure 2. Effect of pH value on adsorption of Cr(VI) on MAC (initial
concentration of Cr(VI), 50 mg/L; temperature, 303 K; contact time, 48 hrs;
dosage, 2g/L)

On the other hand, chromium existing in solution
is mainly in two states which are Cr(VI) and
Cr(III). The
stability of these forms in
solution highly depends on pH of the solution. In knowledge of chromium chemistry, the
negative charged Cr(VI) ions are easily reduced to Cr(III) at low pH in the presence of MAC
as shown in following equations (1) and (2).

2 Cr₂O₇
^2- + 3C + 16H⁺
=4Cr^{3 +} + 3CO₂ + 8H₂O             
(1)

4 HCrO₄^-+
3C + 16H⁺ =4Cr^3
+ + 3CO₂ + 10H₂O             
(2)

It is noted
from Figure 2. that total removal of chromium is
always lower than that of Cr(VI). Particularly the difference increases with a
decrease in pH. In fact, the removal of Cr(VI) at low pH is partially due to
reduction of Cr(VI) into Cr(III) which are known to exist in various hydrolyzed
forms of [Cr(H₂O)₆]³⁺, [Cr(H₂O)₅OH]²⁺and
[Cr(H₂O)₄(OH)₂]⁺depending on pH of
the solution. Clearly, the positive charged surface of MAC does not favor to
adsorption of the positive charged Cr(III) species due
to electrostatic repulsion.

Adsorption
equilibrium isotherms of Cr(VI) on MAC were measured
as shown in Figure 3. The positive temperature dependence suggests the
chemical nature of Cr(VI) adsorption on the MAC. Langmuir,
Freundlich and Dubinin¨CRadushkevich(D¨CR) isotherms were used to interpret
equilibrium adsorption data. The values of model parameters and correlation
coefficients R² were presented in Table 2. The correlation coefficient
for three models are all >0.95, likely Langmuir model is the best
interpretation of the experimental data. The predicted maximum adsorption of Cr(VI) on MAC ranges from 42.52 to 53.79 mg/g when raising
temperature from 303 to 323K.

Figure 3. Effect of temperature on Cr(VI) equilibrium adsorption amount (initial concentration of Cr(VI), 10-200 mg/L; pH, 3;
contact time, 48 hrs; dosage, 2g/L)

Table 2 Model parameters for adsorption Cr(VI) onto MAC at different temperatures

d. q_e=K_Lq_mCe/(1+K_LCe) ; q_m: the maximum
adsorption capacity (mg/g); K_L: Langmuir constant (L/mg);

e.  q_e=K_FCe^1/n;
K_F:
a constant indicative of the relative adsorption capacity of the adsorbent
(mg/g); 1/n: indicates the intensity of the adsorption;

f.  q_e=q_De^-Be2   e=RTexp(1+1/Ce)   E= (-2B)^-1/2 ; q_D: the adsorption
capacity( mg/g); e: the Polanyi
potential; ¦¢: the mean free energy E (kJ/mol) of adsorption per
molecule of the adsorbate, (mol²/kJ²)
.

Figure 4 shows
the time dependence of removal of Cr(VI) on MAC. It is
seen that the MAC displays a rapid initial rate at three temperatures. The fast
initial rate is ascribed to the large mesopores of
MAC and the gradually lowering rate to approaching equilibrium indicates the
formation of steric hindrance in pores with
progressing of adsorption of Cr(VI).

Figure 4. Effect of time on adsorption of Cr(VI) on AC (initial concentration of Cr(VI),
50mg/L; pH, 3; dosage, 2g/L)

The measured
kinetic data were fitted into three kinetic models, the pseudo-first-order, the
pseudo-second-order and the intra-particle diffusion. The kinetic model
parameters for three models were shown in Table 3. The pseudo-second-order
kinetics is the best suitable for representing the measured kinetic data of
adsorption of Cr(VI) on MAC.

Table 3 Kinetic parameters for the adsorption of Cr(VI)
onto MAC

g.  log(q_e-q_t)=logq_e-k₁t/2.303;
q_e: the amount of solute adsorbed at equilibrium per unit mass
of adsorbent (mg/g); k₁: the rate constant (1/min).

h.  t/q_t=1/k₂q_e²+t/q_e; k₂:
the equilibrium rate constant (g/mg min).

i.  q_t=k_pt^1/2+c;
kp: the diffusion reaction rate constant (mg/g min^1/2).

Figure
5 shows importance of mesopores for removal of Cr(VI).
The percent removal increased with an increase of mesopore
porosity for different length of contact time. With short contact time, the
removal was more significantly influenced by the mesopore
porosity than with long contact time. It means that the intra-particle
diffusive resistance of the adsorbed Cr(VI) visually
occurs due to formation of steric hindrance in pores.
The larger mesopore prosity,
the higher removal approached, but the difference of removal with different
contact times was decreased. With an increase of mesopore
porosity up to near 92%, the difference of removal completely disappeared
regardless of the difference of the contact time, indicating that mesopores dominate the adsorption kinetics during removal
of Cr(VI) due to accessibility of mesopores
to the large sized adsorbate. In addition, surface
oxygen groups in MAC may be one of important factors affecting the adsorption performance
of Cr(VI), however, the experimental result will be
discussed elsewhere.

Figure 5.
Importance of mesopore in adsorptive removal of Cr(VI)
with MACs  (initial concentration of Cr(VI),
50 mg/L; temperature, 303 K; pH,3; dosage, 2g/L)