In this study, the magnetic wakame biochar/Ni composites were prepared with three activating reagents of H3PO4, ZnCl2 and KOH by one-step pyrolysis activation, characterized by BET, SEM, TEM, FI-IR, XRD, Raman, and elemental analyzer, and their adsorption performance for diesel were also analyzed. The results showed that wakame biochar/Ni composites had larger specific surface area, abundant porous structure, and various reactive groups, rendering its enhancement of adsorption efficiency. The adsorption experiments indicated that the maximum adsorption capacities for diesel using WBPA 0.5, WBHZ 0.5 and WBPH 0.5 were 4.11, 8.83, and 13.47 g/g, respectively. The Langmuir model was more suitable for the adsorption isotherms process, and the Pseudo-second-order model could better describe the adsorption kinetic experimental. And the magnetic wakame biochar/Ni composites presented good stability and recyclability. This study provides a novel pattern for the high-value utilization of wakame, having huge potential in the treatment of oily wastewater.
With the rapid development of economy and industrialization, the production and consumption of petroleum products are increasing worldwide, and as a side effect of transportation, exploration and related processes, oil spill is unfortunately increasing accordingly [
Up to date, there is a variety of methods available for the treatment of oily wastewater, mainly including physical, chemical and biological method [
In recent years, with the increasingly prominent energy crisis, the preparation of biochar materials from cheap, readily available, and renewable biomass in nature has attracted increasing attention [
In this study, a series of novel magnetic biochar/Ni composites derived from wakame were synthesized with three different activators (KOH, ZnCl2 and H3PO4) by a facile and effective impregnation-pyrolysis method. The as-prepared samples were characterized by BET, SEM, TEM, FI-IR, XRD, Raman, and elemental compositions were also analyzed. Moreover, the adsorption performances for diesel wastewater using as-prepared samples were investigated by batch adsorption experiments, and the adsorption isotherms, kinetics and thermodynamics of as-prepared samples for diesel were also studied to further analyze its adsorption mechanism. Then, the magnetic recovery cycle experiments of as-prepared samples were carried out in an extra magnetic field. This study would provide a novel promising bio-absorbent prepared by macroalgae in the treatment of oily wastewater.
The wakame used in this study was purchased from Yantai City, Shandong Province, China. The wakame was washed and soaked in tap water several times to remove the salt in the wakame, and dried at 85°C. Diesel (Sinopec), Potassium hydroxide (KOH), zinc chloride (ZnCl2), phosphorus pentoxide (P2O5) and nickel chloride hexahydrate (NiCl2•6H2O) were all purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd., China All reagents used in the experiment are of analytical grade and no further purification is required.
According to our previous work [
N2 adsorption/desorption isotherm was performed on a static volumetric adsorption analyzer (Micromeritics ASAP 2010, Shanghai, China) and calculated with the method of Brunauer-Emmett-Teller (BET). The microstructure and surface morphology of as-prepared sample was observed by scanning electron microscope (SEM, S4800, Hitachi, Japan) and transmission electron microscope (TEM, Joel-2100, JEOL, Japan). The element analyzer was used to determine the contents of organic elements (C, H, N, S and N) of as-prepared sample, and the content of metallic nickel was obtained by inductively coupled plasma emission spectrometer (ICP-OES; ICAP6000, Thermo Fisher Scientific, USA). The X-ray diffraction (XRD) patterns were recorded on an X-ray Diffractometer (Ultima IV, Rigaku Corporation, Japan) in the range of 2θ from 20 to 80°. The Raman spectra were recorded by Raman spectroscopy using a 532 nm excitation wavelength (InVia-Reflex, Renishaw, UK). At the same time, the surface functional groups of as-prepared sample were analyzed by Fourier transform infrared spectroscopy (FTIR, Nicoletteis 50, Thermo Fourier, USA). And the magnetic properties were studied using a vibrating Specimen magnetometer (VSM, 7407, Lakeshore, USA).
0.1 g of magnetic biochar/Ni composites (MWB, WBPH 0.5, WBPH 1, WBHZ 0.5, WBHZ 1, WBPA 0.5 and WBPA 1) were added into 50 mL (2%, 4%, 8%, 16% and 20%) diesel solution for batch adsorption experiments, respectively, which was put into a water bath thermostatic oscillator and oscillate at 25°C for 3 h. And then under external magnetic field, the adsorbent was separated from the mixed solution at 5, 10, 20, 30, 60, 90, 120 and 180 min, respectively, and the adsorbed diesel by as-prepared sample was recovered and weighed. Moreover, the effects of diesel initial concentration and absorbent dose on the adsorption capacities of diesel were also investigated.
To evaluate the stability of the prepared adsorbent, the repeat cycle experiments were also performed. The magnetic biochar/Ni composite with adsorbed diesel was added into ethanol for desorption. The as-prepared adsorbent was recovered by magnetic separation, and dried in an oven at 85°C for 12 h. The cycles of adsorption-magnetic separation-desorption were carried out for 5 times.
The adsorption efficiency of magnetic biochar/Ni composite to diesel was calculated according to
where qt (%) is the adsorption rate of diesel by adsorbent, wt (g) is the weight of the adsorbent after adsorbing diesel, and w0 (g) is the initial weight of the adsorbent.
Statistical analysis was performed using Origin 8.5 (Origin Corp., USA); Jade 6.0 was used to process the XRD data.
The N2 adsorption-desorption isotherm was employed to determine pore characteristics of the as-prepared samples. As shown in
Sample | SBET (m2/g) | Vtot (cm3/g) | Average pore diameter (nm) |
---|---|---|---|
MWB | 9.151 | 0.011 | 4.787 |
WBPA 0.5 | 89.501 | 0.092 | 4.119 |
WBPA 1 | 25.726 | 0.033 | 4.238 |
WBHZ 0.5 | 455.626 | 0.373 | 2.141 |
WBHZ 1 | 422.897 | 0.226 | 2.275 |
WBPH 0.5 | 824.174 | 0.445 | 2.158 |
WBPH 1 | 601.501 | 0.346 | 2.302 |
Based on N2 adsorption-desorption data, the non-local density functional theory (NLDFT) model is used to calculate pore size distribution (PSD) of as-prepared sample, it can be demonstrated that the PSD of magnetic biochar/Ni composites are in the range of 3.8-4 nm, belonging to mesoporous materials. It has been demonstrated that H3PO4, ZnCl2 and KOH as activators applied in the preparation of biochar, can be beneficial to the improvement of pore properties and the increase of oxygen functional groups [
The morphology and microstructure of the magnetic biochar/Ni composites were analyzed by SEM (
TEM was employed to further analyze the pore structure of the magnetic biochar/Ni composites, it can be seen from TEM images (
According to the FTIR spectrum (
The XRD pattern (
The Raman spectrum (
Elemental analyzer and ICP-OES were employed to investigate the main components of the magnetic biochar/Ni composites, as illustrated in
Element | C | H | N | O | S | O/C | (O+N)/C | H/C | Ni |
---|---|---|---|---|---|---|---|---|---|
MWB | 51.98 | 2.47 | 2.03 | 11.28 | 0.64 | 0.163 | 0.196 | 0.570 | 28.69 |
WBPH 0.5 | 39.13 | 0.77 | 1.58 | 13.37 | 0.14 | 0.256 | 0.291 | 0.236 | 29.57 |
WBHZ 0.5 | 42.05 | 1.15 | 2.81 | 9.98 | 1.26 | 0.178 | 0.235 | 0.328 | 33.70 |
WBPA 0.5 | 43.61 | 1.01 | 0.45 | 8.79 | 0.24 | 0.151 | 0.160 | 0.278 | 37.64 |
Besides, the nickel contents of as-prepared samples have been analyzed by ICP-OES, compared with the nickel content of MWB, those of the magnetic biochar/Ni composites increase remarkably, among which, that of WBPA 0.5 is maximum (37.64%). The loading of nickel could contribute to the separation and recovery of the magnetic biochar/Ni composites after adsorption.
The adsorption performances for diesel using seven different adsorbents (MWB, WBPA 0.5, WBPA 1, WBHZ 0.5, WBHZ 1, WBPH 0.5 and WBPH 1) have been investigated, as shown in
The effects of adsorbent dosage on diesel adsorption using WBPH 0.5, WBPA 0.5 and WBHZ 0.5 have been analyzed, as shown in
In order to study adsorption equilibrium, adsorption isotherm model was chosen according to the properties and types of the system. The most commonly used models are Langmuir
where Ce (g/L) represents concentration of diesel at equilibrium, qe (g/g) and qL (g/g) are adsorption capacity of diesel at equilibrium and saturation, respectively. KL (L/mg) is the Langmuir adsorption equilibrium constant, KF [(g/g) (L/g) 1/n] is the Freundlich constant, and 1/n is the adsorption intensity factor or surface heterogeneity.
In order to study the diesel adsorption using the magnetic biochar/Ni composites, the traditional Langmuir and Freundlich adsorption isotherm models are employed to analyze the adsorption isotherms.
Adsorbent | Freundlich | Langmuir | ||||
---|---|---|---|---|---|---|
n | KF [(g/g)(L/g)1/n] | R2 | qL(g/g) | KL(L/g) | R2 | |
WBPA 0.5 | 30.05 | 3.45 | 0.965 | 4.13 | 0.481 | 0.999 |
WBHZ 0.5 | 16.47 | 6.25 | 0.892 | 8.79 | 0.231 | 0.994 |
WBPH 0.5 | 10.04 | 7.21 | 0.806 | 13.05 | 0.113 | 0.964 |
In order to further study the adsorption kinetics of the mixed system, the pseudo-first-order model
where qe (g/g) and qt (g/g) are the adsorption amount of diesel at equilibrium and at time t (min), respectively. k1 (min-1) and k2 (g/g min) are the rate constant of the PFO and PSO, respectively. kdi (g/g·min1/2) is intraparticle diffusion model constant. Ci (g/g) is boundary layer thickness constant and t (min) is time.
Adsorption kinetic as a key factor determines the uptake rate of solute, representing the adsorption efficiency of adsorbent. Pseudo-first-order (PFO), Pseudo-second-order (PSO), and Weber-Morris intraparticle diffusion model were applied for the kinetics model fitting of diesel adsorption data.
Adsorbent | qe,exp (g/g) | Pseudo-first order | Pseudo-second order | ||||
---|---|---|---|---|---|---|---|
qe,cal (g/g) | k1 (min-1) | R2 | qe,cal (g/g) | k2 (g g−1 min−1) | R2 | ||
WBPA 0.5 | 4.11 | 0.22 | 0.041 | 0.817 | 4.11 | 4.172 | 0.999 |
WBHZ 0.5 | 8.83 | 0.98 | 0.059 | 0.829 | 8.85 | 0.114 | 0.999 |
WBPH 0.5 | 13.47 | 1.55 | 0.067 | 0.856 | 13.51 | 0.074 | 0.999 |
In order to analyze the control step of diesel adsorption process, the adsorption kinetics of the magnetic biochar/Ni composites for diesel are investigated by the Weber-Morris intraparticle diffusion model, as shown in
Adsorbent | Intraparticle diffusion | ||||||
---|---|---|---|---|---|---|---|
kd1 (g/g min1/2) | C1(g/g) | R12 | kd2(g/g min1/2)× 10-3 | C2(g/g) | R22 | ||
WBPA 0.5 | 0.953 | 0.714 | 0.755 | 0.781 | 4.101 | 0.951 | |
WBHZ 0.5 | 1.997 | 1.465 | 0.768 | 0.714 | 8.652 | 0.943 | |
WBPH 0.5 | 3.172 | 1.699 | 0.891 | 0.673 | 13.467 | 0.956 |
Therefore, in addition to the intra-particle diffusion resistance, the adsorption mechanism of the system is also affected. In addition, it was observed from the FTIR spectrum that the functional groups (such as -OH, -NH and -CH) in the magnetic wakame biochar composite material have the effect of promoting the binding of diesel molecules to the active sites on the surface of the material. BET shows that high specific surface area and rich pore structure help to improve mass transfer efficiency.
The magnetic wakame bio-carbon nanocomposite material can be separated from water under the action of an external magnetic field. The key to the sustainable application of this type of magnetic biochar is its reusability and good stability. The magnetic properties of WBPA 0.5, WBHZ 0.5 and WBPH 0.5 were measured using the VSM technique under a magnetic field between −20,000 Oe and 20,000 Oe, as shown in
The magnetic wakame biochar/Ni composites were prepared with different activating reagents, which have larger specific surface area, abundant porous structure, and various reactive groups, endowing them with superior adsorption performance. The maximum adsorption capacities for diesel using WBPA 0.5, WBHZ 0.5 and WBPH 0.5 are 4.11, 8.83, and 13.47 g/g, respectively. The adsorption process is more suitable for the Langmuir model and the Pseudo-second-order model. After five adsorption-desorption cycles, the biochar/Ni composites still have higher adsorption efficiency, exhibiting good stability and reusability. Therefore, the magnetic wakame biochar/Ni composites are expected to become potential adsorbents for the treatment of oily wastewater.
This study was supported by the Fundamental Research Funds for Zhejiang Provincial Universities and Research Institutes (No. 2021J004), the National Natural Science Foundation of China (U1809214), and the Natural Science Foundation of Zhejiang Province (LY20E080014).