Journal Pre-proofs Magnetic Activated Biochar Nanocomposites Derived from Wakame and its Application in Methylene Blue Adsorption Xinxin Yao, Lili Ji, Jian Guo, Shaoliang Ge, Wencheng Lu, Lu Cai, Yaning Wang, Wendong Song, Hailong Zhang PII: DOI: Reference:
S0960-8524(20)30111-5 https://doi.org/10.1016/j.biortech.2020.122842 BITE 122842
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16 November 2019 14 January 2020 16 January 2020
Please cite this article as: Yao, X., Ji, L., Guo, J., Ge, S., Lu, W., Cai, L., Wang, Y., Song, W., Zhang, H., Magnetic Activated Biochar Nanocomposites Derived from Wakame and its Application in Methylene Blue Adsorption, Bioresource Technology (2020), doi: https://doi.org/10.1016/j.biortech.2020.122842
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Magnetic Activated Biochar Nanocomposites Derived from Wakame and its Application in Methylene Blue Adsorption Xinxin Yao a, Lili Ji b, Jian Guo c, Shaoliang Ge d, Wencheng Lu a, Lu Cai e, Yaning Wang b, Wendong Song f, Hailong Zhang b* a College
of Naval Architecture and Mechanical-Electrical Engineering, Zhejiang Ocean University, Zhoushan,
Zhejiang, 316022, China b
Institute of Innovation & Application, Zhejiang Ocean University, Zhoushan, Zhejiang, 316022, China,
College of Food and Medical, Zhejiang Ocean University, Zhoushan, Zhejiang, 316022, China
College of Port and Transportation Engineering, Zhejiang Ocean University, Zhoushan, Zhejiang, 316022, China
Donghai Science and Technology College, Zhejiang Ocean University, Zhoushan, 316000, China
of Petrochemical and Energy Engineering College, Zhejiang Ocean University, Zhoushan, Zhejiang,
316022, China. * Correspondence: [email protected]
(H.Z.); Tel.: +86-580-226-2589 (H.Z.)
Abstract In this work, magnetic wakame biochar nanocomposites for the first time had been synthesized to investigate their adsorption to methylene blue dye. As-prepared magnetic biochar samples were obtained by the impregnation method to load nickel on wakame biochar via one-step carbonization with activation agent KOH at 800 °C. The prepared samples were characterized by BET, XRD, FTIR, Raman, SEM, TEM and so on. The results exhibited that the maximum adsorption capacity of BW(Ni)0.5 to methylene blue could reach 479.49 mg/g at 20 °C. The adsorption behavior was more suitable for Langmuir isotherm equation and the kinetic data were most consistent with the pseudo second-order model. And also, the adsorption reaction was a spontaneous and endothermic process. After five cycles, it was found that BW(Ni)0.5 had a high adsorption capacity for methylene blue (117.58 mg/g). This study demonstrated that wakame biochar could have great potential in dye wastewater treatment. Keywords Wakame; Magnetic; Thermodynamics; Methylene Blue; Activated Biochar Nanocomposites
1. Introduction Methylene blue (MB) has been massively used in dye industry and textile printing and dyeing industry. With the rapid development of economy, more dye wastewater has been produced in the industry, and dye wastewater containing a large number of phenyl, amino, azo and other organic compounds continues to seriously challenge the environmental safety. Improper treatment of dye wastewater has potential to damage the environment and harm human health (Liu et al. 2013). If the concentration of MB in water is too high, it will affect the chromaticity and turbidity of water. It can irritate the skin and nervous system, seriously damaging the liver and digestive system of the human body by long-term uptake of water contaminated with dyes (Ma et al. 2015). Therefore, the treatment of dye wastewater has become a big concern in recent years. Biomass carbon can be prepared via using crop straw, pine tree residues, rice husk (Park et al., 2019; Shukla et al., 2019) and other waste resources on land. It can not only reduce the environmental pollution caused by waste incineration and decomposition, but also realize the recovery and reuse of waste resources. Biomass carbon has larger pore structure, higher surface activity and stronger adsorption capacity. The previous study has proved that it is very effective to remove harmful cationic dyes from waste water (Sewu et al., 2017). Porous biomass carbon has attracted the attention of scholars in many environmental fields due to its surface hydrophobicity, high specific surface area, large pore volume and thermal stability (Tian et al., 2016). In particular, macroalgae, because of their loose internal structure, high content of organic components, fast growth rate and low price, have aroused great interest in the study of their adsorption properties. (Zhou et al., 2018). In the marine environment, there are many kinds of algae, 3
with more than 1000 species. Seaweed is a very attractive renewable biological resource (Hou et al., 2017). Besides, seaweed grows fast and does not need any artificial cultivation (Tabassum et al., 2017). In the natural aquatic environment, it can synthesize organic matter through its own pigment and photosynthesis, which can avoid competing with crops for cultivated land and fresh water resources. The utilizations of algae carbon as adsorbents and supercapacitors have been well realized. Other researches on the preparation of seaweed carbon as supercapacitor materials and biological borax have been reported (Cui et al., 2016). The adsorption of phenol biochar prepared from brown algae activated by zinc chloride, and the removal rate was 98.31% under the optimum conditions was studied by Rathinam coworkers (Rathinam et al., 2011). Two kinds of algae biochar were prepared by activation of two kinds of algae by KOH to capture CO 2. The results showed that the pseudo-first-order model was more consistent with the experimental data, and the external mass transfer played an important role in the adsorption process (Ding et al., 2020). It was reported that algae were utilized for the removal of elemental mercury (Hg0) from flue gas, which were first prepared by pyrolysis and then modified with halide reagents when using a simple impregnation method. It was reported that biochar derived from halide modified seaweed was used to capture Hg0 in flue gas. And according to the comparative analysis of experimental data and kinetic simulation, it was found that chemical adsorption was the main control factor (Liu et al., 2018). Biochar derived from algae residue effectively adsorbed dyes in aqueous solution. Experiments showed that the maximum removal rate of Congo red dye could reach 82.6% (Nautiyal et al., 2016). Biochar derived from Chlorella BC-Cha01 can effectively remove p-nitrophenol, and the maximum adsorption capacity can reach 204.8 mg/g (Zheng et al., 2017). The preparation of high performance porous 4
carbon by pyrolysis of natural renewable algae and its application in MB adsorption has been well reported (Ahmed et al., 2019). However, to our best of knowledge, so far there is no other report on the preparation of magnetic wakame biochar as a recyclable adsorbent. In the literature, almost all the adsorption modified materials based on biomass carbon were treated by carbonization and then activated by potassium hydroxide (Wang et al., 2017). This method not only consumed a lot of energy, but also had complex, tedious and cumbersome process, and it was difficult to recover biomass carbon after adsorption as well (Chen et al., 2018). In contrast, in this study, nickel was loaded onto the surface of wakame biochar by impregnation method, and then carbonized and activated with potassium hydroxide to prepare adsorptive biochar and magnetic nanocomposites. Since these nanocomposites were associated with nickel, they were easy to recycle. The optimal ratio of adsorption materials was then obtained through batch adsorption experiments, and thermodynamics, equilibrium isotherm and kinetics for our adsorption experiments were studied by changing the concentration and temperature of the system. The main purpose of this work was to prepare magnetic wakame biochar nanocomposites for MB adsorption, recover biochar after adsorption via magnetic force, and then analyze the thermodynamic mechanism of adsorption. 2. Materials and methods 2.1 Materials Wakame (undaria) was purchased from Shandong Province, China. Wakame was washed by using distilled water several times, and dried at 80 °C for 48 hours. The dried wakame was crushed into powder and used in subsequent pyrolysis experiments. 5
Methylene blue (MB, C16H18N3SCl), potassium hydroxide (KOH) and nickel chloride hexahydrate (NiCl2·6H2O) were obtained from Shanghai Sinopharm Chemical Reagent Co. Ltd., China. The chemical reagents used in this study were analytical grade and do not need to be further purified. 2.2 Preparation of magnetic activated biochar nanocomposites derived from wakame Under room temperature, 5.0 g of wakame powder was immersed in 50 mL nickel chloride hexahydrate solution with a concentration of 0.5 mol/L for 24 h. After ultrasonically sonicated for 4 h, nickel chloride hexahydrate was fully loaded onto the surface of wakame powder. Filtration was carried out, and the obtained solid was dried in an oven at 80 °C for 24 hours. A wakame powder loaded with nickel chloride hexahydrate was obtained. The wakame powder with nickel chloride hexahydrate was then grinded and mixed with different proportions of KOH, and the ratio of wakame to KOH was 1:0, 1:0.5, 1:1, 1:1.5, labeled as: BW(Ni)0, 0.5 ,1 , 1.5, respectively, and then transferred the mixture to a tube furnace and heated at 10 °C/min-1 under nitrogen flow at 200 mL/min. The pyrolysis was conducted at 800 °C for 2 h. After cooling, the obtained magnetic activated biochar nanocomposites was rinsed to pH = 7 by using a large amount of distilled water, and dried at the condition of 80 °C for 24 h in the oven. 2.3 Characterization of magnetic activated biochar nanocomposites The surface morphology of prepared samples was observed by scanning electron microscope (SEM, Hitachi S4800). In order to detect the organic elements in samples, the contents of C, H, O, S and N were measured by element analyzer (Elementar Vario EL III). The content of nickel was obtained by inductively coupled plasma optical 6
emission spectroscopy (Thermo Fisher Scientific ICAP6000). The images of transmission electron microscope (TEM) are received on the Joel-2100 of 200 kV and dispersed on the Cu grid using porous carbon. The absorbance of residual dyes in water was determined by ultraviolet-visible spectrophotometer (UV 2600, Shimadzu). In this paper, the inorganic crystal forms of samples in the range of 20° to 80° were recorded by X-ray diffractometer (θ, Ultima IV). The Raman spectra were recorded under the excitation of Ar+ laser (532 nm) by Renishaw InVia-Reflex confocal spectrometer system (UK). At the same time, the surface functional groups of prepared samples were analyzed by Fourier transform infrared spectroscopy (FTIR, Nicoletteis 50). Specific BET surface area measurement was carried out on Micromeritics ASAP 2010 instrument (ASAP 2010, Micromeritics). 2.4. Experimental method for adsorption properties of magnetic activated biochar nanocomposites Add 0.05 g magnetic activated biochar nanocomposites to different concentrations (400 mg/L - 650 mg/L) of MB solution (Jung et al., 2016a), and the water was shaken for 5 h to reach the adsorption equilibrium. Add 0.05 g of magnetic biochar to 350 mg/L MB solution; shake the water bath for 2 h to reach the equilibrium for the best matching experiment. The mixed solution was centrifuged at 4000 rpm/min for 5 min, and the upper clear liquid was taken to measure its adsorption value. According to the absorption value, the concentration of MB in the adsorbed solution was calculated, and the adsorption amount of MB by magnetic activated biochar nanocomposites was further calculated, and at equilibrium the adsorption amount of MB was qe (mg/g). According to formula (Eq. 1), it is calculated that: 7
(C0 Ce )V m
where C0 (mg/L) is the initial concentration of MB, and Ce (mg/L) indicates the equilibrium concentration of MB. m (g) and v (L) represent the weight of magnetic activated biochar nanocomposites and the MB solution volume, respectively. 2.5. Study on thermodynamics and adsorption kinetics In this study, the kinetic analysis of MB-BW(Ni)0.5 system was carried out by increasing the solution concentration, and the kinetic data were acquired by the concentration of MB at the interval of 0 to 5 h. By changing the temperature, the concentration of MB by the time interval of 0 to 5 h was obtained at the temperature of 293, 303, 313 and 323 K, respectively. The thermodynamic data were obtained, and the dynamic thermodynamic analysis of MB-BW(Ni)0.5 system was carried out. 2.6. Experimental study on the cyclic utilization of magnetic activated biochar nanocomposites 0.05 g of magnetic activated biochar nanocomposites was added to 50 mL MB solution with a concentration of 500 mg/L, and oscillated in water bath at 20 °C for 5h. The absorbance value was determined. The magnetic activated biochar nanocomposites with MB adsorbed on the surface were separated by an external magnetic force, then added into anhydrous ethanol water bath for 10 min; and the desorption with anhydrous ethanol was carried out for 4 times until the solution was clarified. The cycles of adsorption-magnetic separation-desorption were 5 times. 3. Results and discussion 8
3.1. Experimental optimal proportion The adsorption performance of magnetic activated biochar nanocomposites with various ratios to KOH adding has been measured. As shown in Fig. 1, it can be seen that the adsorption performance of BW(Ni)0.5 is the best. Because KOH has the effect of punching (Zhang et al., 2019a), the adsorption performance decreases with the increase of the ratio to KOH. This is because under the high temperature reaction, a large amount of gas destroys the pore structure of wakame, resulting in the decrease of adsorption performance. The reaction mechanism (Zhao et al., 2019) is as following equations (Eq. 2-9), it is worth noting that the two reactions do not particularly proceed in order. For KOH activation, first, KOH is converted to K2CO3 by interacting with carbon components (Eq. 5), and then K2CO3 is decomposed into K2O and CO2 (Eq. 6), intermediates to further react with carbon to form CO (Eq. 7-9). These gases (H2O, CO2, etc.) produced by these reactions will contribute to the formation of a rich porous structure. With regard to the reduction process of metallic nickel, nickel chloride (NiCl2·6H2O) is first converted to NiO (Eq. 2), and then the metal Ni2+ is reduced to metal Ni (Eq. 3-4) by C in the carbon matrix and CO produced by the reaction. In return, reducing metallic nickel improves the degree of graphitization of the carbon matrix. Next, the adsorption properties of BW(Ni)0 and BW(Ni)0.5 were studied and discussed. 5nNiCl2+(C6H10O5)n
< Insert Fig. 1 here > 3.2. Characterization of magnetic activated biochar nanocomposites 3.2.1. Morphology analysis: SEM and TEM It has been reported that the adsorption performance of magnetic activated biochar nanocomposites, to a certain extent, depends on its surface structure (Fan et al., 2011). There are the SEM images of wakame biochars treated with KOH (the ratio to KOH, 1:0.5) and without. According to the SEM image, it can be seen that the surface of the magnetic activated biochar nanocomposites of wakame without KOH treatment is disorderly and bulky, and no obvious pore structure is observed. However, after activated by KOH, there are abundant pore structures on the surface of magnetic activated biochar nanocomposites. This indicates that KOH plays an active role in the pore formation of biochar and its pore size is mainly mesoporous and macroporous, these features are conducive and also very important for the adsorption of macromolecular dye MB (Islam et al., 2017). Also as indicated as red arrows point in the SEM image of biochar with KOH treatment, some dot features on the surface of magnetic activated biochar nanocomposites appear to be the particles of nickel, during high temperature pyrolysis, nickel ions are reduced to metal nickel, which is embedded in biochar pores (Xiao et al., 2019), indicating the successful synthesis of magnetic activated wakame biochar nanocomposites. However, these dots may occupy or even 10
block the macropores and mesopores. These will be disadvantage to the adsorption performance of wakame biochar, but by doing this the magnetic nanocomposites can be easily and conveniently recycled. In order to further illustrate some details of microporous structure of as-prepared samples, TEM micrographs have been obtained. It can be seen that there are obvious morphological changes occurred during the activation process. The morphology of BW(Ni)0.5 indicates that there are irregular pores with sponge-like figures, indicating the presence of a large number of uniform micropores (Yang et al., 2019). A large number of dense concave-convex pores suggest that after activated magnetic biochar pores have been enriched. 3.2.2. Porous determination: BET The porous structure of the magnetic activated biochar nanocomposites of wakame was identified by N2 adsorption/desorption isotherm. As illustrated in Fig. 2, asprepared sample, WB(Ni)0.5, belonged to the typical IV isotherm and the branch of adsorption-desorption isotherm was irreversible. There was a magnetic hysteresis loop between their adsorption and desorption branches, which indicated the presence of mesopores. The specific surface area of WB(Ni)0.5 was nearly 70 times larger than that of WB(Ni)0, i.e., from 11.098 m2/g to 744.15 m2/g. The total pore volume also augmented from 0.0174 cm3/g to 0.4511 cm3/g, suggesting that KOH activation could dramatically improve the specific surface area and pore volume of carbon materials (Jung et al., 2016b). This illustrated that the magnetic activated biochar nanocomposites adsorption of wakame had been greatly advanced. Based on the N2 adsorption bifurcation data, the pore size the non-local density functional theory (NLDFT) model 11
was used to analyze the distribution (PSD). As illustrated in Fig. 2, the formation of mesoporous pores in the range of 4 nm can be discovered in all biochar materials with a relatively sharp PSD, centered on 2.5 - 4.7 nm. In addition, under the condition of high temperature treatment, the level of mesoporous increased. Through the combination of N2 adsorption isotherm results and PSD analysis, it demonstrated that high temperature treatment and KOH activation could not only affect the surface area and pore volume, but also changed the pore size of the magnetic activated biochar nanocomposites materials, which could be attributed to the carbonation process. KOH and NiCl2·6H2O reacted with wakame to produce gas via the process of the thermal decomposition of organic matter in wakame, thus improving its texture features and adsorption properties (Chen et al., 2018). < Insert Fig. 2 here > < Insert Table 1 here > 3.2.3. Analysis of phase structure and chemical composition FTIR spectra of WB(Ni)0 and WB(Ni)0.5 are illustrated in the Figure. It can be seen that there are changes before and after biochar activation. The spectra show that the -OH stretching band of alcohol or phenol group is at 3300 - 3500 cm-1 (Wang et al., 2017). Due to the vibration of the primary amine (NH) group, characteristic bands appear at 1634 and 1586 cm-1 (Shi et al., 2014). The peak at 1404 cm-1 is attributed to the spectrum of nitro compounds (N-O) group. The stretching vibrations of alcohols, carboxylic acids and ethers show a wide band in the range of 1050-1200 cm-1. This is because during pyrolysis, different forms of oxygen in macroalgae are converted into 12
carbon-oxygen bonds in the form of bond chains (Lin et al., 2018). The vibration peaks below 600 cm-1 can be considered as stretching vibrations caused by the compound (MX) formed by metal ions (M) and halogen (X) (Jin et al., 2016). As can be seen from above peaks discussed that the active groups on the surface of BW(Ni)0.5, like -OH, NH, N-O, and so on, become more abundant after KOH activation treatment, which renders this nanocomposite surface more advantages to its adsorption performance to MB (Fan et al., 2017). It can be seen that the XRD diffraction peak intensity of BW(Ni)0.5 is higher than that of BW(Ni)0, which is due to the consumption of carbon in the activation process of KOH and the increase of the content of metal Ni. and the diffraction peaks are mainly centralized between 40° and 80°. The diffraction peaks of 2θ = 44.480°, 51.830°, and 76.350° correspond to elemental nickel (PDF#70-1849) for (1,1,1), (2,0,0), (2,2,0), which suggests that metal nickel is successfully loaded on the surface of BW(Ni)0.5. Due to its strong magnetic properties, it has been utilized for recycle during adsorption experiments, the adsorbent material can be magnetically recovered easily after adsorption experiments (Quah et al., 2019). The Raman spectra of WB(Ni)0 and WB(Ni)0.5 have been acquired to show the details of graphitization status. The D band near 1344 cm-1 corresponds to the defects and disorder of carbon atoms and the G band near 1588 cm-1 is related to the surface tensile motion of the sp2 carbon atom pair (Mohanty et al., 2013). The vibration process of these two phonons can be used as graphitization (Gong et al., 2017), and the secondorder peaks of 2713 and 2864cm-1 are related to the carbon atom pair. The intensity ratios (ID / IG) of the D and G bands of WB(Ni)0 and WB(Ni)0.5 are 1.03 and 1.05, 13
respectively. This indicates that the crystallinity of the biochar increases after activation, which is due to the increase in pores and structural disorder (Zhang et al., 2019b). The wider half-peak width of the band means that the structure in the disordered carbon is rich. These characteristics indicate that the biochar after activation is more conducive to the adsorption of MB. In addition, the intensities of the second-order peaks at 2713 and 2864 cm-1 were weak, indicating that the disordered carbon was dominant in the WB(Ni)0.5 samples (Chen et al., 2020). 3.2.4. Elemental analysis and ICP measurement The main components of magnetic activated biochar nanocomposites of wakame are detected by elemental analysis in order to further explore the adsorption of MB by magnetic activated biochar nanocomposites. The results of the element analyzer are shown in Table 2. It can be seen that the organic element of BW(Ni)0.5 is higher than that of BW(Ni)0. This is because BW(Ni)0.5 reacts with KOH at high temperature, consuming some organic elements, resulting in porous structures. The content of high carbon demonstrates the high specific surface areas of magnetic activated biochar nanocomposites, and the low ratio of O/C (0.283) indicates that the surface of magnetic activated biochar nanocomposites is highly hydrophobic. (O+N)/C (0.290) reflects the existence of polar groups in the structure of magnetic activated biochar nanocomposites (Ding et al., 2016), which can be beneficial to the adsorption performance to lipophilic organic compound, like MB. In order to measure the loaded nickel, ICP detection has also been carried out, and its result shows that the content of loaded metal nickel is 3.28×105 mg/kg. The load of nickel is conducive to recycling for wakame biochar. The
high nickel content facilitates the efficient separation of magnetic activated biochar nanocomposites from aqueous phase after adsorption. < Insert Table 2 here > 3.3. Study on adsorption performance 3.3.1 Effects of different initial concentrations of MB Fig. 3a shows the adsorption performance of magnetic activated biochar nanocomposites to different initial concentrations of MB at 20 °C. It has been observed that the residual concentration of MB decreases gradually in the process of contacting with magnetic activated biochar nanocomposites until the adsorption equilibrium is reached. At the initial stage of contact, because magnetic activated biochar nanocomposites have a large number of active sites, and the adsorption rate is faster. With the prolongation of adsorption time, the adsorption rate decreases gradually, MB molecules occupy a large number of active sites, and adsorption tends to reach its equilibrium. When the concentration of MB solution is low, the adsorption reaches its equilibrium rapidly, because there are a large number of active sites practicable for adsorption (Mahmoud et al., 2012). And with the increase of the concentration of MB solution, the equilibrium time becomes longer, which is due to the fact that more active sites are occupied. Competitive adsorption leads to the decrease of the attractiveness of magnetic activated biochar nanocomposites to MB (Gupta et al., 2010). 3.3.2. Effect of temperature Temperature plays an important role in the adsorption of MB by porous magnetic activated biochar nanocomposites. Fig. 3b shows that at 20, 30, 40 and 50 °C, the 15
maximum adsorption capacity of BW(Ni)0.5 to MB is 479.49, 521.25, 533.71, and 540.23 mg/g, respectively. In the range of 20 °C to 50 °C, the adsorption capacity increases with the increase of temperature. The increase of adsorption capacity may be caused by the expansion of pore size and / or activation of adsorbent surface at higher temperature (Gerçel et al., 2007). < Insert Fig. 3 here > 3.3.3. Adsorption isotherm of MB-BW(Ni)0.5 system 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 (Eq. 10) and Freundlich (Eq. 11) (Langmuir., 1916).
Ce Ce 1 qe qL K L qL
qe K F Ce1/n
where qe (mg/g) is the adsorption capacity of MB at adsorption equilibrium, qL (mg/g) represents the adsorption capacity of MB at adsorption saturation, while n, KL (L/mg) and KF ((mg/g)(L/mg)1/n), indicate constants taken via the fitting of the Langmuir and Freundlich models. As shown in Table 3, when R2 is close to 1, it illustrates a favorable correlation between the experimental data and theory models. The R2 range of Langmuir isotherm is 0.995-0.997, and that of Freundlich isotherm is 0.981-0.944. The Langmuir and Freundlich isotherm models are shown in Fig. 4. It can be seen that the Langmuir equation is more appropriate to describing the adsorption of magnetic activated biochar nanocomposites at different temperatures, indicating that the adsorption process is 16
single-layer adsorption (Sun et al., 2015), and the n value is within 1-10, indicating the favorable adsorption of MB to BW(Ni)0.5. < Insert Fig. 4 here > < Insert Table 3 here > 3.3.4. Adsorption kinetics of MB-BW(Ni)0.5 system In order to study the kinetic adsorption mechanism process of MB-BW(Ni)0.5 system, data analysis is performed by using pseudo first-order (PFO) and pseudo second-order (PSO) models (Eq: 12), (Eq: 13).
qt qe (1 e K1t )
t 1 t 2 qt k2 qe qe
where qt (mg/g) is the MB adsorption amount at time (t), k1 (min-1) and k2 (g/mg min) are the rate parameters determined from the PFO and PSO models, respectively. As shown in Fig. 4, the data is subjected to nonlinear regression analysis using a dynamic equation. It can be seen from Table 4 that the PSO model shows a higher R2 (0.9980.999), while the PFO model has a R2 of 0.889 - 0.983. The PSO fit shows that the adsorption amount at equilibrium is 398.41 mg/g. 469.48 mg/g, and 487.81 mg/g; and the experimental values are 387.45 mg/g, 464.85 mg/g, and 479.49 mg/g, respectively; therefore, the PSO model can be more suitable to fully describe the MB-BW(Ni)0.5 adsorption system. The PSO equation model is based on the chemical adsorption. These results indicate that electrons are shared or exchanged between the adsorbent and the positively charged MB ions occurred during chemisorption (Hosseinzadeh et al., 2015). 17
In fact, the functional groups on the surface of biochar, such as -OH,-NH, N-O, can promote the chemical combination of positively charged MB dye molecules to the surface of magnetic activated biochar nanocomposites. This adsorption may be caused by π-π stacking, hydrogen bonds and van der Waals forces (Liu et al., 2019). 3.3.5. Adsorption mechanism of MB-BW(Ni)0.5 system In order to understand the types of steps to control the adsorption rate, the intraparticle diffusion model (equation (14) was used to study the adsorption mechanism of BW(Ni)0.5 on MB molecules.
qt kdit1/2 Ci
where Ci (mg/g) represents the degree of adsorption and is related to the thickness of the boundary layer. The slope, kdi (mg/g min1/2), is the constant of adsorption rate. The values of kdi (mg/g min1/2) and Ci (mg/g) can be acquired by fitting the curve of qt (t1/2), as indicated in Fig. 4f. With the increase of initial concentration (400 to 600 mg/L), it can be seen that the observed kd1 and kd2 also increase due to the high initial concentration of MB molecules diffusing. kd2 is lower than kd1 because the surface of the magnetically activated biochar nanocomposite has more active sites for adsorbing MB dye molecules at the initial contact, so the adsorption rate is high, and over time, the surface of the material is large. The active site is occupied by MB molecules, and the MB molecules diffuse into the pores through the surface, thus slowing down. It has been observed from FTIR spectra that the magnetic biochar nanocomposites of wakame has various surface functional groups, such as -OH,-NH, N-O, which can promote the chemical combination of positively charged MB dye molecules to active sites on the surface of magnetic activated biochar nanocomposites. Moreover, the high BET surface 18
area of magnetic activated biochar nanocomposites and their high micropore density on the surface contribute to improved mass transfer efficiency. Fig. 4f shows two parts of linear graphs with different slopes and intercepts obtained by curve fitting. The slopes and intercepts of different stages prove different mechanisms, indicating that there are two stages of continuous mass transfer in MB-BW(Ni)0.5 system, that is, the sharp increase of the first stage. It is the gradual adsorption from the bulk of MB solution to the external mass transfer and the second part of the adsorbent surface, and diffuses in the particles in the microporous and mesoporous structure of BW(Ni)0.5. The intercept increases with the increase of concentration, which indicates that the adsorption mechanism of MB-BW(Ni)0.5 system is also affected by external interaction, not just the diffusion resistance in the particles (Değermenci et al., 2019). < Insert Fig. 4 here > < Insert Table 4 here > 3.3.6 Adsorption thermodynamics of MB-BW(Ni)0.5 system The thermodynamic parameters (entropy ΔSo, enthalpy ΔHo and Gibbs free energy ΔGo) of the biochar adsorption system of MB-BW(Ni)0.5 were calculated by using the equations, (Eq. 15 and Eq. 16) T S 0 H 0 In( K L ) RT
G 0 RTIn( K L )
where KL is the corresponding Langmuir adsorption constant, R (J/molK) is the gas parameter (8.314), and T(K) is the temperature at which the adsorption system is carried 19
out. As illustrated in Fig. 4, a chart of lnKL versus 1/T is plotted, from which thermodynamic parameters are calculated and given in Table 5. From these data, it is clear that with the increase of temperature from 20 °C to 50 °C, the Gibbs free energy (ΔGo) becomes more negative, indicating that this adsorption process takes place spontaneously. The positive value of ΔHo suggests that the adsorption capacity increases with the increase of temperature (within the temperature range of our experiments), and the adsorption is an endothermic adsorption reaction. The positive value of ΔSo indicates an increase in the number of species at the solution—solid interfaces (Khan et al., 2012). < Insert Fig. 4 here > < Insert Table 5 here > 3.3.7. Adsorption and desorption: recycling of BW(Ni)0.5 The surface of wakame biochar loaded with nickel is easy to separate from aqueous phase by an external magnetic force and it has a higher recovery rate, which is conducive and convenient for reuse. As shown in Figure, when the adsorption of MB is completed, the system turns blue color into a black turbid solution. Under the action of external magnetic field, as-prepared BW(Ni)0.5 nanocomposite is separated from the solution and attaches on the side wall of the beaker. For the five times recycling of adsorption and desorption, the adsorption capacity of BW(Ni)0.5 to MB is 450.92, 264.55, 223.89, 131.37 and 117.58 mg/g, respectively, even after the fifth cycle, BW(Ni)0.5 can remove a considerable amount of MB. It shows that the adsorption capacity of BW(Ni)0.5 to MB decreases with the increase of the number of cycles, 20
which can be due to the unavailability of some active sites after the reaction of functional groups with nanocomposites BW(Ni)0.5. Another reason may be that some mesopores and/or micropores are occupied and are not washed out during the parsing process. 3.3.8. Practical implications of this study There are many studies on the adsorption and removal of dyes in wastewater treatment. Biochar is widely used in dye adsorption because of its high specific surface area, rich pore structures and chemical functional groups on its surface. Generally, the adsorbent is in powder form, if dye is to be removed from wastewater, the adsorbent cannot be separated easily from aqueous phase. In this study, magnetic nickel was loaded on biochar by impregnation method, and metallic nickel was uniformly distributed on the surface of biochar. After the adsorption, it could be conveniently separated from water phase under the action of external magnetic field. In literature, some other magnetic biochars shows their good reusability in regeneration studies (Ma et al. 2015; Miranda et al 2014). Comparison of the adsorption capacity of MB between this work and some previously reported magnetic adsorbents (Liu et al. 2013; Ai et al 2011; Li et al 2013; Fan et al 2012), it can be seen that the magnetic wakame biochars have better performance and a good effect on the removal of MB.
4. Conclusions In this study, magnetic activated biochar nanocomposites derived from wakame have been first time successfully prepared at 800 °C by dipping method and controlling 21
the amount of activating agent KOH. At 298 K, BW(Ni)0.5 can reach its highest adsorption capacity of 479.49 mg/g MB under adsorption equilibrium. The recycling experiments using magnetic separation exhibit that after five cycles, the magnetic activated biochar nanocomposite still has a high adsorption capacity (117.58 mg/g) for MB. This study demonstrates that the high adsorption performance of BW(Ni)0.5 to MB can be expected to be utilized in large-scale industrial wastewater treatment in the future.
Acknowledgments: We thanks for the fund supported by the Fundamental Research Funds for Zhejiang Provincial Universities and Research Institutes ( No. 2019J00045) to make this work possible, also thanks to Key Research and Development Projects of Zhejiang Province of China (No. 2018C02043), Demonstration Project of Marine Economic Innovation and Development of Zhoushan City of China, and Demonstration Project of Marine Economic Innovation and Development of Yantai City of China (No. YHCX-SW-L-201705).
Supplementary information E-supplementary data of this work can be found in the online version of the paper.
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Fig. 1. Optimal proportion experiment.
Fig. 2. Nitrogen physisorption-desorption isotherms of BW(Ni)0 and BW(Ni)0.5.
Fig. 3. Effect of different factors on MB adsorbed by BW(Ni)0.5: (a) Effect of initial MB concentration; (b) Effect of temperature. BW(Ni)0.5 = 1 g/L.
Fig. 4. The kinetics and thermodynamics of the adsorption system of BW(Ni)0.5 to MB. (a) Langmuir isotherms and (b) Freundlich isotherms. (c) Plot of ln KL v/s 1/T for adsorption of MB on BW(Ni)0.5 at various temperatures between 293 and 323 K. Adsorption kinetics of MB adsorbed by BW(Ni)0.5. (d) pseudo-first-order model; (e) pseudo-second-order model; and (f) intraparticle diffusion model.
Table 1. Texture characteristics of BW(Ni)0 and BW(Ni)0.5
S BET (m2/g)
V tot (cm3/g)
Average pore diameter (nm)
Table 2. Some non-metallic elements of BW(Ni)0 and BW(Ni)0.5 obtained by elemental analyzer (mass, %).
Table 3. Parameters of the Freundlich and Langmuir adsorption isotherm models of MB adsorbed by BW(Ni)0.5 at different temperatures.
523.56 546.44 537.63 552.48
0.565 0.772 1.902 3.549
0.998 0.999 0.997 0.998
6.223 5.305 7.204 6.766
287.76 300.96 365.03 407.07
0.981 0.958 0.983 0.993
（K） 293 303 313 323
Table 4. Kinetic parameters for the adsorption of MB adsorbed by BW(Ni)0.5.
Kinetic model pseudo-first-order
Parameters 400mg/L 143.12 0.017 0.983 398.41 5.404 0.998 85.706 0.3712 0.717 2.013 358.26 0.527
qe(mg/g) k1(min-1) R2 qe(mg/g) k2(g/mg min) × 10 -3 R2 ki,1 C1 R 12 ki,2 C2 R 22
Values 500mg/L 174.81 0.013 0.889 469.48 0.694 0.999 78.409 88.801 0.694 2.982 419.04 0.788
600mg/L 203.95 0.013 0.982 487.81 0.512 0.999 82.018 92.969 0.697 3.859 421.47 0.767
Table 5. Thermodynamic parameters of MB adsorption onto BW(Ni)0.5 at different temperatures.
293 303 313 323
-29.86 -31.67 -35.06 -37.86
Credit Author Statement 39
ΔS（J/mol K） 269.87
Author Contributions: Lili Ji: Conceptualization, Methodology, Software; Xinxin Yao.: Data curation, Writing- Original draft preparation. Jian Guo, Lu Cai: Visualization, Investigation. Wendong Song: Supervision.; Shaoliang Ge, Yaning Wang: Software, Validation.; WenCheng Lu: Project administration; Hailong Zhang: Writing- Reviewing and Editing,
1) The novel magnetic activated biochar nanocomposites were first time prepared using wakame. 2) Loading nickel on the surface of wakame biochar is conducive to its recycling. 3) The adsorption process is a spontaneous endothermic process. 4) At 293 K, the maximum adsorption capacity of MB is 479.49 mg/g.