Functionalized biochar-supported magnetic MnFe2O4 nanocomposite for the removal of Pb(ii) and Cd(ii)

In this study, a novel magnetic biochar-MnFe2O4 nanocomposite (BC/FM) was prepared using low-cost corn straw and MnFe2O4 by sol–gel/pyrolyzing route using egg white, which has abundant functional groups (–NH2 and –COOH). Following that, its composition, morphology and structure was characterized by various techniques including SEM-EDX, BET, XRD, and VSM. Batch experiment of the adsorption for Pb(ii) and Cd(ii) including influence of pH, kinetics, isotherm and thermodynamics was also studied. The results demonstrated that biochar could effectively support MnFe2O4, which displayed high dispersion on the surface of the biochar and possessed abundant functional groups and high surface area contributing to superior performance on Pb(ii) and Cd(ii) removal. Therein, MnFe2O4 with high magnetism is convenient for separating the magnetic BC/FM from an aqueous medium. Adsorption experiment results indicate that Pb(ii) and Cd(ii) removal by BC/FM was closely related to pH with the best value of pH 5.0, and the process reached equilibrium in 2 h. The adsorption process is well-described by the pseudo-second-order kinetic model and Sips (Freundlich–Langmuir) model. Thermodynamic studies suggest that the adsorption process is spontaneous and exothermic. The maximum experimental adsorption capacity of BC/FM is 154.94 and 127.83 mg g−1 for Pb(ii) and Cd(ii), respectively, in single-solute system, which is higher than that of some of the other adsorbents of biochar or biochar-based composites. In bi-solute system, the preferential adsorption order of BC/FM for the two metals is Pb(ii) prior to Cd(ii). Finally, FTIR and XPS analysis verified that the main mechanism of Pb(ii) and Cd(ii) removal by BC/FM is by forming Pb/Cd–O or complexation of carboxyl and hydroxyl and ion exchange. Therefore, the prepared magnetic BC/FM composite, as an excellent adsorbent, exhibited potential applications for the removal of Pb(ii) and Cd(ii) from wastewater.


Introduction
In the past decades, with the rapid pace of economic development, increasing amount of heavy metal ions such as lead and cadmium were released into the environment, which then entered the human body through various ways, leading to severe health issues in human beings. 1,2 As one of the toxic heavy metals, lead can cause many diseases related to digestive, circulatory, respiratory and other systems, and even death when it enters the human body. 3,4 As for another heavy metal ioncadmium, it can accumulate in the body and cause diseases of the lung, bone and kidney. 5,6 Comparing with their presence alone, the coexistence of lead and cadmium will pose a higher hazard to environmental pollution and human health. Therefore, it is essential to develop promising techniques for simultaneous removal of Pb(II) and Cd(II) from wastewater prior to their discharge into the environment.
So far, various methods including chemical precipitation, electrochemical processes, ion exchange, and adsorption have been developed to deal with heavy metals from wastewater. [7][8][9][10] Among these technologies, adsorption is a relatively advantageous method for water treatment processes because of low cost, simple operation and high efficiency. [10][11][12] A variety of adsorbents for wastewater treatment have been developed. Biochar (BC) is a pyrolytic carbonaceous organic matter that is oen produced under an oxygen-limited condition from waste biomass, which is cheap and easy to obtain. Due to its characteristic porous structure, high specic surface area, abundance of functional groups and a high ion exchange capacity, BC was chosen as a well-suited host material that can stabilize and disperse nanoparticles to further enhance the adsorption capacity of the composite materials. Moreover, BC has been deemed to be an ideal material for the removal of metal ions from an aqueous medium. 13,14 Previous studies have suggested that biochars derived from different biomass can remove heavy metals, for instance Pb and Cd, effectively. [15][16][17][18] However, unprocessed biochars usually have lower adsorption ability for metal ions than activated carbon. In addition, the separation and recycling from aqueous solutions aer adsorption would be hard to achieve. 3,13 These disadvantages have become an obstacle to their practical application. To solve these problems, the preparation of magnetic biochar composites through the introduction of magnetic particles has become a research hotspot in recent years. 13,14 Magnetic biochar composites have good separation performance from water under an external magnetic eld. Besides, the enhancement of removal efficiency for metal ions has been experimentally veried. [19][20][21][22] Various types of magnetic particles, such as iron oxide (Fe 2 O 3 , g-Fe 2 O 3 , and Fe 3 O 4 ), 13,21 ferromanganese binary oxide, 22 magnetic gelatin 14 and iron oxy-hydroxides, [23][24][25] were used to achieve magnetic properties in biochar materials. Manganese ferrospinel MnFe 2 O 4 , however, exhibits outstanding magnetic property and structural stability due to the change in valence state of iron. Moreover, the magnetic MnFe 2 O 4 material has been proved to be an outstanding adsorbent for metal ions in the aqueous medium. 26,27 Due to the active functional carboxyl and hydroxyl groups on the surface allowing chemical interactions, the MnFe 2 O 4 usually has a large adsorption capacity for metal ions. The sorption abilities of MnFe 2 O 4 for removing Pb(II) from the aqueous solutions have been reported. However, MnFe 2 O 4 exhibits several disadvantages, such as weak mechanical strength, ease of agglomeration and low removal efficiency. Accordingly, in recent years, the development of magnetic composite materials with MnFe 2 O 4 has attracted wide attention. [28][29][30] Motivated by these observations, we developed a new magnetic composite comprising of MnFe 2 O 4 and biochar (BC) for metal ions removal from aqueous medium with high efficiency. When the MnFe 2 O 4 is combined with biochar, the obtained composite is expected to show both good adsorption performance and magnetic property, and to maximally utilize the benets of both MnFe 2 O 4 and BC for heavy metal removal.
In this study, magnetic MnFe 2 O 4 particles were successfully immobilized on biochar by the sol-gel/pyrolyzing route using egg white that has abundant functional groups (-NH 2 and -COOH). It not only overcomes the problem of biochar recovery but also stops the aggregation of MnFe 2 O 4 and improves the adsorption ability of contaminants. This study was designed to elucidate the respective roles of biochar and MnFe 2 O 4 , and evaluate the performance of BC/FM for the removal of Pb(II) and Cd(II) from the coexistence system, which is different from previous studies on heavy metal removal from single-component batch solution using biochar-based composites. Their adsorption behavior was investigated through characteristic and batch adsorption experiments. Their adsorption mechanisms were further investigated. It is of great social signicance to control pollution and reduce emission of heavy metal wastewater. At the same time, it is of great signicance to seek new ways for maize straw waste resource utilization and promote agriculturally sustainable development of low carbon material loop.

Chemical reagents
All chemicals used in this experiment were of analytical grade. Pb(II) and Cd(II) stock solutions were obtained by dissolving Pb(NO 3 ) 2 and Cd(NO 3 ) 2 in deionized water. Different concentrations of Pb(II) and Cd(II) working solutions were prepared by diluting the above stock solutions before use.

Preparation of adsorbents
BC/FM was prepared through sol-gel/pyrolyzing route using corn straw as substrates and the detailed steps are as follows. Firstly, a homogeneous solution was prepared by stirring 60 mL egg white for 30 min. Then, 8.013 g Fe(NO 3 ) 3 $9H 2 O and 4.7 mL Mn(NO 3 ) 2 were slowly added into the above homogeneous egg white solution and stirred vigorously. 31 5 grams corn straw (ground to pass 1 mm sieve) was added in the mixed solution and stirred evenly, evaporated in an 80 C water bath. Subsequently, in nitrogen atmosphere, the mixed solution was pyrolyzed for 5 h in a muffle furnace at 500 C, and then cooled. Composite material preparation was completed, the material was named BC/FM. The obtained BC/FM was ground to pass 0.15 mm sieve and was washed using deionized water until reaching neutrality, and were then dried. Bare MnFe 2 O 4 nanoparticles were synthesized by a similar process in the absence of corn straw. A schematic diagram of the BC/FM fabrication is shown in Fig. 1.

Characterization of samples
The morphologies of MnFe 2 O 4 , BC and BC/FM were studied using scanning electron microscopy, energy dispersive X-ray spectroscopy (SEM-EDX, Quanta 400), and high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F30). The specic surface area was examined based on nitrogen adsorption experiments (BET, Builder SSA-4200). X-ray diffraction measurement was performed using X-ray diffractometer (XRD, Philips APD 3720). The samples were scanned in the range of 2q from 10 to 80 with a 0.02 step at a scanning speed of 4 min À1 . The magnetization of BC/FM was determined using SQUID magnetometer at 700 K. The surface functional group of the samples was evaluated using Fourier transform infrared spectroscopy at a resolution of 2 cm À1 (FTIR, Nicolet 6700). Xray photoelectron spectrometer (XPS, ESCALAB 250Xi) was used to analyze the surface chemical composition of BC/FM before and aer adsorption of Pb(II)/Cd(II).

Adsorption experiments
Adsorption experiments were performed in a 150 mL conical ask with stirring at 150 rpm to study the adsorption effects of Pb(II) or Cd(II) by BC/FM. The experiment of pH inuence on adsorption was carried out in the range of pH 2-8 using 0.1 M NaOH and HCl solutions. For the adsorption kinetics, 50 mg BC/FM was added to 40 mL Pb or Cd solutions, and aliquots were sampled at different time intervals (1-300 min). The adsorption isotherm experiments in the single solute system were carried out at three temperatures of 25 C, 35 C and 45 C, and for the bi-solute system, the temperature was 25 C. Aer adsorption, the mixtures were ltered by 0.22 mm lters and then the concentration of the metal ions were analyzed by an atomic absorption spectrophotometer (PE-AA800). All experiments were performed in triplicate. Adsorption capacity and removal rate were calculated according to the following equations: where q t is the adsorption quantity at time t (mg g À1 ); c 0 is the initial concentration (mg L À1 ) in the pre-adsorption solution; c t is the instantaneous concentration (mg L À1 ) at time t (min); V is the sample volume (L); m is the dosage of BC/FM (g); R is the removal rate.

Adsorbent characterization
The surface morphology and chemical composition of BC, MnFe 2 O 4 and BC/FM were characterized using SEM-EDX, TEM and HRTEM, and typical images are given in Fig. 2. The surface of biochar derived from corn straw was smooth and porous without impurities as seen in Fig. 2a. As shown in Fig. 2b, the prepared MnFe 2 O 4 exhibited irregular sponge-like bulk particles with signicant aggregation, which led to the decline in the adsorption effect. 28 In comparison, the spongiform MnFe 2 O 4 particles for BC/FM composite were uniformly dispersed and loaded onto the biochar surface as in Fig. 2c. It can be seen that the biochar effectively prevents the agglomeration of MnFe 2 O 4 particles, and ensures that the BC/FM composite still has a large surface area. 32 Moreover, the EDX indicated the presence of C, O, Fe and Mn in the BC/FM composite (Fig. 2d). The peaks of Fe and Mn further show that MnFe 2 O 4 was loaded onto the biochar framework. The peak of C conrms the existence of biochar, while the peak of O was ascribed to both MnFe 2 O 4 and the oxygen-containing functional group of biochar. Fig. 2e and f exhibits the TEM and HRTEM images of a typical sample of BC/ FM nanoparticles. It is displayed that the samples are homogeneous square or polygon shaped particles. The average particle size is lower than 10 nm. The HRTEM image exhibits well dened fringes as shown in Fig. 2f, and the inset expresses the enlarged pattern of the dotted box, which shows that the interplanar spacing is 0.256 nm and can be attributed to the (311) planes of MnFe 2 O 4 . The XRD pattern is shown in Fig. 3. For BC, the wide peak located at about 24 represents a typical diffraction pattern of amorphous carbon. 33 The expected spinel iron ferrite structure is present in the XRD patterns of BC/FM, which proves that MnFe 2 O 4 exists in BC/FM. The diffraction peaks are located at 30.20 , 35.6 , 43.24 , 57.26 and 62.9 (as major peaks), which agrees with the crystal planes (220), (311), (400), (422) and (440) of MnFe 2 O 4 (JCPDS: 38-0430). 34 The additional peaks located at about 32 and 45 in BC/FM are assigned to NaCl (PDF number 00-005-0628) in biochar ash.
The characteristics of magnetization curves determine the practicability of ferromagnetic materials. 35 Magnetic properties of BC/FM were determined using a vibrating sample magnetometer (Fig. 4), and its magnetization saturation (M s ) was 33.188 emu g À1 . At 700 K, coercivity (H c ) and remnant magnetization (M r ) were 86.389 Oe and 7.866 emu g À1 . According to the experimental results, BC/FM dispersed in an aqueous Paper medium was separated within 20 seconds by an external magnetic eld of 20 mT. These results suggest that BC/FM has outstanding magnetic property, which is conducive to the separation of adsorption and desorption regeneration.
The BET test results showed that specic surface area of BC/ FM was 38.05 m 2 g À1 , which is greater than those of pure MnFe 2 O 4 (2.68 m 2 g À1 ), 36 and sponge-like porous MnFe 2 O 4 (29.80 m 2 g À1 ). 31 The signicant increase in BET surface area was expected to improve the adsorption process of both the metals.

Effect of pH
In this study, the inuence of pH on the adsorption process is mainly manifested in two aspects: properties of BC/FM and speciation of two metals. The effect of pH on the adsorption by BC/FM of both the metals is shown in Fig. 5. To exclude the interference of precipitation, the experimental pH range of 2-8 was selected. 35 As shown in Fig. 5, for Pb(II) and Cd(II), the adsorption process is evidently related to the solution pH, and the removal rate increases with the pH from 2 to 8. At pH 2.0, the removal percentages of both the metal ions were low. For Pb(II), the removal percentage increased evidently and reached 90.9% as the solution pH increased from 2 to 4; it reached 99.0% when the pH increased to 5.0. The removal percentage remained stable between pH 5 to 8. In case of Cd(II), when the pH increased from 2 to 5, the removal percentage showed a gradual increase. The removal rate reached 73.5% at pH 5.0, and a small increase aer pH 5.0 was observed. On the one hand, pH is a signicant environmental factor directly related to the surface nature of BC/FM. The effect of pH on the surface characteristics of BC/FM was derived from two components, namely, BC and MnFe 2 O 4 . Heavy metals were bound to BC mainly through ion exchange and complexation with functional groups, for instance carboxyl and hydroxyl group of BC. 37-39 MnFe 2 O 4 mainly bind heavy metals by complexation.
where R 1 and R 2 denotes the surface of BC and MnFe 2 O 4 , respectively. Therefore, low pH is not conducive to bind heavy metals by both BC and MnFe 2 O 4 due to the competing H + . Besides, due to the presence of MnFe 2 O 4 , BC/FM is mainly composed of surface hydroxyl groups. At lower pH, BC/FM surface is positively charged and repels metal ions. When the pH increases, the adsorption sites become negatively charged, and more metal cations could be removed. The experimental results shown in Fig. 5 indeed conrm the above explanation.
On the other hand, speciation of metal ions is also affected by the solution pH. 35,40 According to the speciation of Pb and Cd at different pH, the removal of both the metals is explained further. At solution pH 2-5, the main species of lead is Pb(II). 40 Therefore, free lead can be adsorbed on BC/FM. When pH is above 6, formation of PbOH + and Pb(OH) 2 takes place and the concentration of free lead ions starts to decrease. At pH below and above 8, the predominant species of Cd(II) is Cd 2+ and CdOH + , Cd(OH) 2 , respectively.
For Pb(II) and Cd(II), the removal rate is relatively low at pH 2 and then it increases to 99.0% and 73.5%, respectively, at pH 5 (Fig. 5). When the solution pH reaches 8.0, due to the presence of OH À , adsorption and precipitation occur simultaneously. The nal pH aer equilibrium can also be measured, which is higher than the initial value (Fig. 5). On the basis of the analysis, pH 5.0 was selected as the best pH value for further study.

Adsorption kinetics
The effect of reaction time on Pb(II) and Cd(II) removal by BC/FM is shown in Fig. 6a and b. Within the rst 30 min, the removal   This journal is © The Royal Society of Chemistry 2019 rate of both the metal ions was fast, then dropped off gradually and eventually reached equilibrium at 120 min.
Kinetic study provides vital information about the adsorption mechanism. In order to determine the appropriate sorption kinetics model, three widely used kinetic models, pseudo-rst-order kinetics model, pseudo-second-order kinetics model and the intraparticle diffusion model were chosen to describe kinetic experimental data. Their nonlinear equation is as follows: The kinetic model equation of intraparticle diffusion is described as follows: where q e is the equilibrium adsorption capacity (mg g À1 ); q t is the adsorption capacity at time t (mg g À1 ); k 1 , k 2 , k ip are the equilibrium rate constants of the above three models, respectively (min À1 , mg g À1 min, mg g À1 min À0.5 ); t is the adsorption time (min); C is a constant.
The tting results of the three adsorption kinetic models are shown in Fig. 6, and the parameters calculated are listed in Tables 1 and 2. Evidently, the correlation coefficient (R 2 ) of the pseudo-second-order kinetics model for both the metal ions was over 0.99 and was greater than that of the pseudo-rst-order kinetics model. In addition, for both Pb(II) and Cd(II), the theoretical q e values calculated from the pseudo-second-order kinetics model is closer to the experimental values(q e,exp ). Thus, these results indicate that the kinetic model of pseudosecond-order is better suited to describe the removal process   Fig. 6c and d and Table 2 depict the results of the intraparticle diffusion model tting for Pb(II) and Cd(II) on BC/FM. It indicated that both the metal ions migrated rapidly in the initial stage, then diffused relatively slowly, and nally reached the binding sites of BC/FM. Similar diffusion mode was also studied for Pb(II) and Cd(II) removal by HMO-BC 37 and in other researches. 41,42 The calculated constant C (Table 2) is a non-zero value, implying that the adsorption mechanism of both the metals onto BC/FM is complex and intraparticle diffusion is not the only rate limiting step.

Adsorption isotherm
The study of adsorption isotherm provides vital information, for instance, sorption mechanism. The adsorption isotherm models are fundamental in describing the distribution of adsorbate on adsorbents in the equilibrium state. Freundlich, Langmuir and Sips (Freundlich-Langmuir) adsorption isotherms were employed to explain adsorption equilibrium data in this study. The equations for the three models are provided as follows: Freundlich model: Sips ðFreundlich À LangmuirÞ model : (14) where c e is the concentration of Pb(II) or Cd(II) at the equilibrium state (mg g À1 ); q e is the balanced adsorption capacity of Pb(II) or Cd(II) (mg g À1 ); Q m indicates the maximum adsorption capacity (mg g À1 ); K 1 , K 2 and K LF , n, n LF and a LF are constants of Langmuir, Freundlich and Sips (Freundlich-Langmuir) model, respectively. 3.4.1 Single solute (Pb or Cd) adsorption isotherms. The sorption isotherms of both the metals onto BC/FM at three temperatures (25 C, 35 C and 45 C) are displayed in Fig. 7. The adsorption process of both the metal ions onto BC/FM is closely related to temperature, and the adsorption capacity increases with the increase in the solution temperature from 25 C to 45 C. This result suggests that the adsorption of both the metal ions onto BC/FM is an endothermic process. The ts to three isotherm models at three different temperatures are depicted in Fig. 7, and the isotherm parameters are given in Table 3.
According to the comparison of the correlation coefficients (R 2 ), Sips adsorption isotherm model is more suitable for tting experimental data than the other two models. This result suggests that the removal process of both the metals onto BC/ FM is controlled by diffusion and saturated monomolecular adsorption at low and high concentration of Pb(II) and Cd(II), respectively. 35 The experimental maximum adsorption capacity for Pb(II) and Cd(II) is 154.94 and 127.83 mg g À1 , respectively. Compared with some other adsorbents of biochar and biocharbased composites reported in other studies (Table 4), adsorption ability of BC/FM for Pb(II) and Cd(II) is greater than most of them. To a certain extent, the high adsorption capacity of both the metal ions on BC/FM can be attributed to the loaded MnFe 2 O 4 . The high affinity of MnFe 2 O 4 with Pb(II) and Cd(II)    26 The separation factor R L values calculated by K 1 (R L ¼ 1/(1 + K 1 C 0 )) were all within 0-1, suggesting that the adsorption of both the metals onto BC/FM composite was favorable. 22 Moreover, the Freundlich constant (n) values indicated that Pb(II) and Cd(II) is easily adsorbed by BC/FM. 43 3.4.2 Bi-solute (Pb + Cd) adsorption isotherms. Competitive adsorption occurs when two metals coexist in a solution. 44 In order to understand the mutual effects of the two metal ions, the studies of adsorption by BC/FM in a bi-solute solution were conducted at 25 C. The sorption isotherms of the two metals in a bi-solute system are shown in Fig. 8 and the corresponding isotherm parameters are given in Table 5.
For Pb(II), the adsorption capacity was hardly affected by the addition of Cd(II) at 20 and 100 mg L À1 (Fig. 8a). In comparison, the adsorption of Cd(II) decreased markedly by the addition of Pb(II) and the effect was greater at 100 mg Pb L À1 than 20 mg Pb L À1 . The Langmuir adsorption capacity of Cd(II) decreased to Table 3 Adsorption isotherm parameters for the adsorption of Pb(II) and Cd(II) onto BC/FM in a single system  42.628 and 30.266 mg g À1 , respectively (Fig. 8b), demonstrating the competition for the binding sites between them. To summarize, the preferential adsorption order of BC/FM for the two metals is Pb(II) > Cd(II). The relative adsorption for metal ions is mainly related to their ionic radius, ionic potential, hydrolysis state and chemical properties. 35 The larger Pb 2+ ion size (1.32Å) versus Cd 2+ ion (1.03Å) may be connected to the higher Pb(II) adsorption. 35 The ionic potential (ionic charge/radius ratio) of Pb 2+ ion and Cd 2+ ion is 3.3 and 1.9 respectively, which leads to stronger Pb 2+ ionic interaction force versus Cd 2+ , thus Pb 2+ will be adsorbed preferentially. With respect to hydration energy, Pb(II) (À1504 kJ mol À1 ) has lower hydration energy than Cd(II) (À1708 kJ mol À1 ), which indicates that Pb(II) is more favorable for ion exchange with BC/FM. 37 As for other aspects, the complex nature of BC/FM composite and the abundant functional groups also play a vital role in the relative adsorption.

Adsorption thermodynamics
To reveal the adsorption thermodynamic characteristic of the two metals by the prepared BC/FM, the Gibbs free energy (DG ), enthalpy change (DH ), and entropy change (DS ) of sorption were calculated using the van't Hoff thermodynamic equations: where T is the absolute temperature (K) and R is the universal gas constant (8.314 J mol À1 K À1 ). The thermodynamic parameters obtained are given in Table 6. DG < 0 implies that the adsorption process of both the metal ions on BC/FM is spontaneous, while DH > 0 indicates that the process is endothermic and is favorable at higher temperatures. DS > 0 implies the increase in randomness owing to the adsorption of Pb(II) and Cd(II) from the solution. Therefore, the above results show that the adsorption process in this study was mainly chemisorption.

Sorption mechanism
3.6.1 FTIR analysis. The FTIR spectra of BC, BC/FM, BC/ FM-Pb, and BC/FM-Cd samples are shown in Fig. 9. The broad band around 3400 cm À1 can be attributed to -OH group stretching vibration. The peaks at 2922 and 2852 cm À1 are ascribed to -CH 2 groups. Compared with pristine BC, a new peak appears at 602 cm À1 on BC/FM, which could be ascribed to Fe-O, 26 typical of spinel ferrite, but it moves to 601 cm À1 and  22 3.6.2 XPS analysis. In order to further investigate the removal mechanism of Pb(II) and Cd(II) by BC/FM, XPS analysis on BC/FM and Pb/Cd-BC/FM were performed (Fig. 10).  shows that elements K and Na existed in BC/FM, but disappeared aer Pb(II) or Cd(II) adsorption. Since Na and K have strong ion exchange ability, it proves the existence of the ion exchange reaction in Pb(II) and Cd(II) removal process. The Mn 2p 3 and Fe 2p 3 peaks are found on BC/FM surface before and aer Pb/Cd adsorption. The signicant peaks corresponding to Pb 4f and Cd 3d were observed on BC/FM surface aer Pb and Cd were adsorbed, respectively.
The species of Pb and Cd adsorbed on BC/FM surface was also analyzed (Fig. 10b and c). XPS of Pb 4f in Fig. 10b showed a signicant shi to a lower binding energy for 4f 7/2 at 138.23 eV in comparison with Pb(NO 3 ) 2 that was centered at 139.5 eV for 4f 7/2 . The results indicate that not a mere electrostatic interaction but a specic sorption could explain the mechanism of Pb adsorption onto BC/FM, and the interaction was attributed to Pb-O binding or hydroxyl and carboxyl binding. Binding energy of Cd 3d 3/2 and Cd 3d 5 These results suggest that there is a strong complexation between Pb(II), Cd(II) and MnFe 2 O 4 as well as oxygen-containing functional groups on BC/FM, and the complexation played a vital role in the removal process of Pb(II) and Cd(II) by BC/FM. In addition, the role of the ion exchange reaction can not be ignored in this process.

Conclusion
A functionalized biochar-supported magnetic MnFe 2 O 4 nanocomposite (BC/FM) was developed through sol-gel process using egg white and pyrolysis at 500 C. The SEM-EDX, BET, XRD, FTIR and XPS analyses show that the magnetic MnFe 2 O 4 adhere onto the surface of the biochar, and Pb(II) and Cd(II) were absorbed by BC/FM composite successfully. The results of adsorption experiments indicate that the removal process for Pb(II) and Cd(II) in a single-solution is pH-dependent. The adsorption preference of the two metals by BC/FM in bi-solute system was Pb(II) prior to Cd(II) and the competition effects existed. The BC/FM composite exhibited an outstanding adsorption ability towards both the metals in the system of single or double solute. XPS and FTIR analysis veried that the main removal mechanism of Pb(II) and Cd(II) by BC/FM was the formation of Pb/Cd-O or complexation of carboxyl and hydroxyl with both the metals ions and ion exchange. Thus, BC/FM composite has been proved to be an outstanding adsorbent for Pb(II) and Cd(II) removal from wastewater.

Conflicts of interest
There are no conicts to declare.