Rice husk (RH) and rice stalk (RS), the abundant biomasses, have been tentatively explored as the renewable biochar which were prepared by means of hydrothermal and activation processes, and the synthetic procedure is quite simple and easy to be scaled up for industrial applications. In this work, the removal of malachite green (MG) was investigated by KMnO4-treated RH and RS as the efficient adsorbents at various experimental conditions. Various operational parameters such as initial dye concentration, adsorbent dosage, and solution temperature in batch systems were investigated on the use of RH and RS. The adsorption isotherm model (Langmuir and Freundlich isotherms), kinetic model (pseudo-first-order, pseudo-second-order, and Elovich models), and the adsorption mechanism (intraparticle diffusion and Boyd models) were studied. It showed that that the Langmuir model and Elovich model are suitable for describing the adsorption process, and the diffusion rate of surface adsorption and the particle diffusion rate jointly affect the reaction rate of adsorption. This facile, efficient, and template-free synthesis strategy holds great promise for preparing novel porous biochar from renewable biomass resources for application in adsorbents.
The disposal of textile wastewater is currently a major problem from a global viewpoint [
The commonly used adsorbents include activated carbon adsorbents and metal and nonmetal oxide adsorbents. The most representative adsorbent is activated carbon, and the adsorption performance of activated carbon is quite good. However, disadvantages of the activated carbon include relatively expensive and the lower mechanical strength. Therefore, there is a great interest in finding inexpensive and effective alternatives to the existing commercial activated carbon, such as molecular sieve, silica gel, active aluminum, polymer adsorbent and biological adsorbent, and so on. Our previous work has shown that the mesoporous molecular sieve (MCM-41, SBA-15) has a good adsorption effect on metals or dyes. However, the preparation method of the mesoporous molecular sieve is complex, and it demands high energy of prepared adsorbent. Furthermore, this technology has some drawbacks such as consuming a lot of template and water [
Rice husk (RH) and rice straw (RS) are the main agricultural biomass in China. Their main components include cellulose (about 35 wt.%, 37.4 wt.%), hemicellulose (about 25 wt.%, 44.9 wt.%), lignin (about 20 wt.%, 4.9 wt.%), and silica fume (about 20 wt.%, 13.1 wt.%) [
The process for production of biochar is one of the feasible methods. Biochar is being developed as a candidate with great potential for climate change mitigation. Hydrothermal carbonization (HTC) refers to the technology of heating biomass in aqueous suspension to 180–250°C in a closed system to produce energy-intensive, high-carbon, and hydrophobic solid hydrates [
Biochar can be generally acquired by simple carbonization and activation treatment of cheap and easily available natural biomass wastes or carbonaceous minerals that are activated by different porogens. Herein, RH and RS were employed as precursor for preparation of biochar by hydrothermal carbonization and KMnO4 oxidation. The remarkably enhanced specific surface areas, versatile pore texture with the coexistence of both micropores, and meso/macropores, apparently increased hydrophilicity which made the as-prepared biochar high-performance adsorbing materials. The adsorption mechanism of MG was investigated.
Rice husks and rice stalks were taken from the farmland around Changchun, washed, and ultrasonically removed from the surface adherends, and then dried at 60°C in an oven to constant weight; MG, formulated into a stock solution with a mass concentration of 1 g·L−1, was diluted to the required concentration according to the experimental needs; KMnO4, NaCl, NaOH, and HCl are all analytically pure; experimental water is deionized water.
Two grams of rice husk was put into a polytetrafluoroethylene lining containing 20 mL of deionized water or KMnO4 solution (0.1 mol·L−1, 0.5 mol·L−1, and 1 mol·L−1), fully infiltrated and placed in the reaction vessel, and then heated to 180°C for 4 h. After cooling down to room temperature, the resulting product was cross-washed with acetone and deionized water until the filtrate was nearly colorless. Samples were collected by vacuum filtration, washed with deionized water until pH of the washed water was around 7.0, and dried at room temperature for 24 h. The hydrothermal biochar prepared from the rice husk was recorded as RH, KRH0.1, KRH0.5, and KRH1. The rice straw was used as raw material to prepare samples RS, KRS0.1, KRS0.5, and KRS1 using the same procedure.
The morphologies of KRH
The effects of initial dye concentration, adsorbent dosage, and solution temperature for adsorption of MG onto KRH
The concentration of MG in the solution was determined using an ultraviolet-visible spectrophotometer (UV-1700), and the adsorption amount
As shown in Figure
SEM of KRH0.5 and KRS0.5.
Parameters of pore structure of the samples.
Sample | BET surface area (m2·g−1) | Pore volume (cm3·g−1) | Average pore diameter (nm) |
---|---|---|---|
RH | 15.77 | 0.05 | 3.83 |
KRH0.1 | 6.77 | 0.04 | 3.81 |
KRH0.5 | 20.61 | 0.27 | 3.42 |
RS | 7.54 | 0.11 | 4.32 |
KRS1 | 27.02 | 0.08 | 3.83 |
The FT-IR spectrum of the four groups of samples displayed a number of absorption peaks (Figure
(a) The FT-IR spectra of RH, KRH0.5, RS, and KRS0.5. (b) The FT-IR spectra of KRH0.5 and KRS0.5 before and after MG adsorption.
By comparing the spectra of KRH0.5 and KRS0.5 after MG adsorption (Figure
As shown in Figure
Effect of KRH0.5 and KRS0.5 dosage on the adsorption of MG.
The initial dye concentration has a pronounced effect on its removal from aqueous solutions. The adsorption of MG on KRH0.5 and KRS0.5 was investigated as a function of contact time at the different initial MG concentrations in the range of 0–180 min at room temperature, and the results are presented in Table
Effect of initial concentration of MG on adsorption.
|
KRH0.5 | KRS0.5 | ||
---|---|---|---|---|
|
% |
|
% | |
10 | 24.93 | 92.28 | 23.37 | 86.86 |
20 | 54.81 | 91.42 | 50.49 | 84.15 |
25 | 69.21 | 91.35 | 64.91 | 77.92 |
50 | 137.2 | 86.62 | 112.9 | 73.17 |
80 | 206.9 | 87.32 | 147.2 | 62.12 |
100 | 262.0 | 83.24 | 156.7 | 53.27 |
125 | 302.9 | 80.37 | 179.1 | 47.67 |
150 | 324.2 | 74.64 | 180.1 | 41.58 |
The adsorption isotherms express the specific relation between the concentration of adsorbate and its degree of accumulation onto adsorbent surface at a constant temperature [
Adsorption isotherm parameters.
Samples | Langmuir isotherm | Freundlich isotherm | ||||
---|---|---|---|---|---|---|
|
|
|
|
|
|
|
KRH0.5 | 1119 | 0.00286 | 0.995 | 5.069 | 1.189 | 0.991 |
KRS0.5 | 295 | 0.01165 | 0.993 | 9.821 | 1.673 | 0.972 |
The essential feature of the Langmuir model can be expressed in terms of a dimensionless constant separation factor (
The values of
Plot of the separation factor for MG onto KRH0.5 and KRS0.5 versus the initial dye concentration.
The value of
The linear determined coefficients of the fitting lines of different isothermal equations are the tools used at most to indicate whether they are suitable for describing the adsorption reaction process. Fitting degree of KRH0.5 was expressed as the Langmuir model > Freundlich model, while the fitting degree of KRS0.5 was expressed as the Langmuir model > Freundlich model. According to the isothermal nonlinear adsorption curve in Figure
(a) Langmuir isotherms of KRH0.5 on MG. (b) Freundlich isotherms of KRS0.5 on MG.
Assuming that the adsorption is controlled by the diffusion step, the single-layer adsorption completed through boundary diffusion is mainly described, which is expressed as follows [
After the boundary condition
Assume that the adsorption is affected by the chemical adsorption rate, involving electron sharing or electron transfer between the adsorbent and the adsorbate. The whole adsorption process, including the complex adsorption reactions including external liquid film diffusion, surface adsorption, and intraparticle diffusion, can comprehensively reflect the adsorption kinetics mechanism between liquid and solid. The rate equation of the reaction can be expressed by the following expression [
Integrating equation (
Assume that the active sites of the adsorbent are heterogeneous, showing different activation energies. During the adsorption process, as the surface coverage increases, the adsorption rate decreases with the increase of time, which is not suitable to describe the single reaction mechanism, which is generally expressed as follows [
Given that
The nonlinear fitting results of the kinetic model are shown in Figures
Adsorption kinetics of KRH0.5 on MG.
Adsorption kinetics of KRS0.5 on MG.
Adsorption kinetic parameters.
Pseudo-first-order kinetic model | Pseudo-second-order kinetic model | Elovich equation | |||||||
---|---|---|---|---|---|---|---|---|---|
|
|
|
|
|
|
|
|
| |
KRH0.5 | 126.3 | 0.185 | 0.944 | 136.1 | 0.002 | 0.985 | 60.72 | 16.42 | 0.998 |
KRS0.5 | 110.7 | 0.156 | 0.932 | 120.7 | 0.002 | 0.980 | 44.32 | 16.46 | 0.999 |
The intraparticle diffusion process tends to control the adsorption rate in systems with high adsorption rates, high adsorption mass concentrations, and large adsorbent particle sizes. In order to understand the mechanism and rate control steps affecting the adsorption kinetics, the kinetic experimental results were fitted to Webber’s intraparticle diffusion model and Boyd diffusion model [
In the MG attachment process, the adsorption dynamics generally consist of three consecutive steps: (i) transport of adsorbate molecules from bulk solution to the outer surface of adsorbent (film diffusion); (ii) transport of the adsorbate molecules within the pores of the adsorbent, which occurs on the external surface (intraparticle diffusion or pore diffusion); (iii) sorption of the adsorbate molecules on the interior surfaces of the pores and capillary spaces of the adsorbent. To distinguish the dye adsorption process mechanism, the Weber–Morris equation is used for fitting the experimental data [
As shown in Figure
Intraparticle diffusion plots for the MG adsorption onto KRH0.5 and KRS0.5.
Assuming that the adsorption resistance is concentrated at the adsorbent particle boundary, the Boyd model can be used to identify the rate-limiting factors in the adsorption process, and its equation is as follows [
In the figure of
Boyd plot for adsorption of MG onto KRH0.5 and KRS0.5.
By measuring the isoelectric point, the pHpzc values of RH and RS were 7.77 and 7.67, respectively, and the pHpzc of KRH and KRS were 5.26 and 5.47, respectively. Under the hydrothermal condition with KMnO4 as the solute, the surface of biochar had a large number of acidic groups, mainly from acidic oxygen-containing functional groups such as carboxyl group and phenol hydroxyl group. They were acidic by dissociating protons, making the zero-electric point of biochar pHpzc < 7. After the surface of biochar is modified by KMnO4, the surface net charge of biochar and the nature of attractions between the molecules were suggested to be one of the reasons attributed for the adsorption capacity of biochar.
Rice husk (RH) and rice stalk (RS), the agricultural by-product waste, can be used as an effective alternative low-cost adsorbent for the removal of MG from wastewater. By the adsorption experiment results of MG, isoelectric point measurement, and FT-IR characterization, it was proved that there were a large number of acidic functional groups on the surface of KRH0.5 and KRS0.5. The adsorption reaction was affected by the initial dye concentration, adsorption time, dosage, etc. Considering the adsorption effect and economic benefits, 0.05 g samples were taken to adsorb 50 mg·L−1 at room temperature for 120 min, and the adsorption rate was verified for many times to be over 90%. The Langmuir isotherm model was the best for the description of the adsorption equilibrium of both dyes onto the biochar. The kinetic studies showed that the dye adsorption process followed Elovich kinetics models and the intraparticle diffusion was the control step of the adsorption rate, but it was not the only rate controlling step.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This work was supported by the Natural Science Foundation of Jilin Province Department of Education (nos. JJKH20170234KJ and JJKH20190857KJ).