As dissolution of raw biomass is serious when used as an adsorbent, the cheap biochar pyrolyzed from biomass might be a good matrix. Raw cornstalk biochar was intentionally modified by cetyltrimethylammonium bromide (CTAB) to prepare the composite adsorbent designed for the removal of negatively charged pollutants. After modification, the removal efficiency for anionic dye Orange II (ORII) increased from 46.9% of the virgin cornstalk biochar to 99.7% of the CTAB-modified cornstalk biochar. The uptake of ORII proved to be favorable under acidic conditions but unfavorable under alkaline conditions. By nonlinear simulation, the Elovich model was the best to describe the adsorption kinetics. For linear simulation of the kinetic data, the pseudo-second-order kinetic model fitted the experimental points better than the pseudo-first-order model. Kinetic analysis indicated that the ORII adsorption process on the CTAB-modified cornstalk biochar might be chemical adsorption accompanied by ion exchange. At 298 K, the maximal adsorption capacity of the modified biochar is 26.9 mg/g by the Langmuir model. The adsorption of ORII increased with a rise in the reaction temperature. The enthalpy and entropy of the adsorption process are calculated to be 38.45 KJ mol−1 and 185.0 J mol−1 K−1, respectively. The negative values of
An increasing number of organic pollutants, such as dyes, endocrine-disruptors, and pharmaceutical and personal care products, have been detected in the natural environment and wastewaters [
As we know, activated carbon is well accepted as the most widely used adsorbent for water purification around the world due to its high surface area, porous structure, and special surface reactivity [
As a kind of low-cost adsorbents, these biomass related adsorbents have been widely explored for the removal of a number of pollutants including dyes and heavy metals [
Regarding the adsorptive removal of water soluble dyes, Zhang and coworkers treated cornstalk with cetyltrimethylammonium bromide (CTAB), and the prepared sorbent was used for effectively removing an anionic dye Congo red [
Orange II (ORII, mass fraction > 95%), methylene blue (MB, mass fraction > 98.5%, chemically pure), and cetyltrimethylammonium bromide (CTAB) were purchased from Beijing Chemical Reagents Company and used without further purification. Other chemicals used were of analytical grade. Deionized (DI) water was used throughout the study.
Cornstalk was collected from a farmland in Zhengzhou of Henan Province. The collected biomass was washed, dried, crushed, and sieved using a 40 mesh sieve. Then the cornstalk was pyrolyzed at 600°C for 3 h in a furnace under an oxygen-limited condition. The resultant biochar was demineralized in a 4 mol/L HCl solution for 12 h and separated by filtration. Then the residues were rinsed with deionized (DI) water to neutral solution pH and dried in an oven at 80°C overnight. One gram of the demineralized cornstalk biochar was added into 100 mL of CTAB solution (1.0%). The mixture was shaken by an orbital shaker at 120 rpm for 24 h. Then the modified biochar was separated by filtration and dried at 60°C for 4 h. Finally, the prepared CTAB-modified cornstalk biochar was stored in a desiccator for further use.
The morphologies of raw cornstalk and cornstalk biochar were recorded on a Philips Quanta-2000 scanning microscope coupled with an energy dispersive X-ray (EDX) spectrometer. FTIR spectra (KBr pellets) were recorded on a Nicolet NEXUS 470 FTIR spectrophotometer from 400 to 4000 cm−1.
The adsorption of ORII and MB on the CTAB-modified cornstalk biochar was conducted in a series of 100 mL conical flasks. For the tests of adsorption isotherm and pH effect, 20 mg of CTAB-modified cornstalk biochar was added into 50 mL of ORII solution with an initial concentration of 10 mg/L. These flasks were shaken on a horizontal shaker for 24 h at a speed of 140 rpm. For the kinetics study, 400 mg of CTAB-modified cornstalk biochar was added into 1000 mL of ORII solution with an initial concentration of 10 mg/L. Constant stirring was maintained by a magnetic stirrer. Samples were collected at a desired time interval.
The temperature was controlled at a constant value of 298 K except for the study on the adsorption isotherm at different temperatures. All of the solution pH was maintained at neutral pH except for the pH effect study. The solution pH adjustment was conducted by adding diluted HNO3 or NaOH (2 mol/L) solution.
Samples were collected and filtered through a 0.45
The removal efficiency of ORII was calculated as
The quantity of ORII adsorbed on the CTAB-modified cornstalk biochar was calculated by the following equation:
As illustrated in Figure
SEM image of raw cornstalk (a) and cornstalk biochar pyrolyzed at 600°C (b).
The FTIR spectra of the raw biochar, the raw CTAB-modified biochar, and the exhausted CTAB-modified biochar are recorded in Figure
FTIR spectra of raw biochar, CTAB-modified biochar, and exhausted CTAB-modified biochar.
The immobilization of cationic surfactant CTAB onto cornstalk biochar is intended to modify the surface charge properties of the raw biochar, which is expected to facilitate the adsorptive removal of anionic pollutants. As a comparison, the adsorption of anionic dye ORII and cationic dye MB was investigated, as illustrated in Figure
Effect of charge property of dye on the adsorption by CTAB-modified biochar. The concentration for both ORII and MB was 8 mg/L, sorbent dosage 20 mg.
As the CTAB-modified cornstalk biochar proved to be especially powerful for the adsorptive removal of ORII, the effect of the sorbent dosage was explored with an initial ORII concentration at 10 and 15 mg/L, respectively. As presented in Figure
Effect of dosage of the CTAB-modified cornstalk biochar on ORII adsorption. The concentrations for ORII were 10 and 15 mg/L, respectively.
Adsorption kinetics for ORII uptake on the CTAB-modified cornstalk biochar was investigated at pH 3.0, 5.0, 7.0, 9.0, and 11.0, respectively. Three kinetic models including pseudo-first-order, pseudo-second-order, and Elovich models were used to fit the experimental data.
The mathematical representations of the linear and nonlinear models of pseudo-first-order and pseudo-second-order kinetics are given in [
Concurrently, the Elovich model was also used for the nonlinear simulation. The Elovich model can be written as [
The adsorption process typically consists of an especially rapid initial uptake and a subsequent smooth increase to equilibrium within 24 h. Using the nonlinear regressive method, the experimental kinetic data for ORII adsorption at pH 7.0 were first simulated by pseudo-first-order, pseudo-second-order, and Elovich kinetic models (Figure
Parameters for the nonlinear kinetic models including pseudo-first-order, pseudo-second-order, and Elovich models.
Kinetic model | pH = 3 | pH = 5 | pH = 7 | pH = 9 | pH = 11 |
---|---|---|---|---|---|
Pseudo-first-order | |||||
|
0.117 | 0.159 | 0.191 | 0.139 | 0.235 |
|
20.9 | 17.2 | 14.6 | 11.3 | 7.00 |
|
0.903 | 0.901 | 0.922 | 0.956 | 0.901 |
Pseudo-second-order | |||||
|
0.00866 | 0.0150 | 0.0221 | 0.0204 | 0.0569 |
|
21.9 | 17.9 | 15.2 | 11.7 | 7.3 |
|
0.971 | 0.967 | 0.975 | 0.989 | 0.954 |
Elovich | |||||
|
10.34 | 9.997 | 9.325 | 6.360 | 4.679 |
|
2.099 | 1.234 | 0.992 | 0.878 | 0.450 |
|
0.981 | 0.992 | 0.996 | 0.968 | 0.998 |
Nonlinear adsorption kinetics at pH 7.0 and fitted curves for ORII adsorption onto the CTAB-modified cornstalk biochar.
Concurrently, the experimental data for the adsorption kinetics were also fitted by the linear pseudo-first-order and pseudo-second-order kinetic models (Figure
Linear adsorption kinetics for pseudo-first-order and pseudo-second-order simulation of ORII adsorption onto the CTAB-modified cornstalk biochar.
Additionally, as suggested by the aforementioned results, CTAB has overwhelmingly modified the surface charge property of the cornstalk biochar. However, the solution pH could influence the uptake of ORII on the CTAB-modified cornstalk biochar by altering the surface functional groups of sorbent and the dissociation of ORII molecules. The values of p
In order to evaluate the adsorption capability of the modified biochar, the adsorption isotherm was investigated at 288, 298, and 308 K, respectively. For simplicity, only the adsorption isotherm at 298 K is illustrated and simulated in Figure
Parameters of the Langmuir and Freundlich isotherm models for the adsorption of ORII onto the CTAB-modified cornstalk biochar.
Adsorption model | Temperature | ||
---|---|---|---|
288 K | 298 K | 308 K | |
Langmuir |
|
|
|
|
25.4 | 26.9 | 29.1 |
|
0.586 | 0.509 | 0.519 |
|
0.911 | 0.932 | 0.923 |
Freundlich |
|
|
|
|
15.38 | 15.43 | 16.44 |
|
8.23 | 7.43 | 7.18 |
|
0.985 | 0.982 | 0.976 |
Adsorption isotherm at 298 K and fitted curves of ORII adsorption onto the CTAB-modified cornstalk biochar.
The saturated monolayer Langmuir isotherm can be represented as [
The Freundlich isotherm is an empirical equation describing adsorption on a heterogeneous surface. It is commonly described as [
The adsorption isotherm can provide information about the surface properties and adsorption behavior of adsorbent. Judging from the experimental data and fitted isotherm curves in Figure
The thermodynamic parameters associated with the adsorption mechanism including standard Gibbs free energy change (
The composite adsorbent, CTAB-modified cornstalk biochar, was successfully prepared and used for the removal of negatively charged pollutants such as Orange II. Compared to the virgin cornstalk biochar, the modified biochar demonstrated its excellent adsorption capability for Orange II removal. The uptake of Orange II increased with a decrease of solution pH. The electrostatic interaction proved to be dominant for the uptake of the dye. Kinetic experiments indicated that the ORII adsorption process on the CTAB-modified cornstalk biochar might be chemical adsorption accompanied by ion exchange. Thermodynamic analysis indicated that the adsorption process is spontaneous and endothermic. A large amount of CTAB proved to be still combined with the stable substrate biochar after the adsorption process. These results suggest that the CTAB-modified cornstalk biochar is a promising candidate for the removal of negatively charged pollutants.
The authors declare that they have no competing interests.
The authors appreciate the finical support from the National Science Foundation of China (Grant no. 51378205) and the Foundation for University Key Youth Teacher of Henan Province of China (2013GGJS-088).