Protein-derived nitrogen and sulfur co-doped carbon for efficient adsorptive removal of heavy metals☆

Yawei Shi,Wei Zheng,Hao Liu,Liang Wang, *,Hongwei Zhang

1 State Key Laboratory of Separation Membranes and Membrane Processes,Tianjin Polytechnic University,Tianjin 300387,China

2 School of Environmental and Chemical Engineering,Tianjin Polytechnic University,Tianjin 300387,China

Keywords:Protein Porous carbon Heteroatoms Adsorption Heavy metal Waste water

ABSTRACT A nitrogen and sulfur co-doped carbon has been synthesized employing egg white as a sustainable protein-rich precursor.According to CHNS elemental analysis,N,S and O heteroatoms accounted for mass fractions of 3.66%,2.28%and 19.29%respectively,and the types of surface functionalities were further characterized by FT-IR and XPS measurements.Although the carbon possessed a smaller surface area(815 m2·g-1)compared to a commercial activated carbon(1100 m2·g-1),its adsorption capacity towards Co2+reached 320.3 mg·g-1,which was over 8 times higher compared to the limited 34.0 mg·g-1 over the activate carbon.Furthermore,the carbon was found to be an efficient adsorbent towards a series of metal ions including VO2+,Cr3+,Ni2+,Cu2+and Cd2+.Combined with its environmental merits,the protein derived carbon may be a promising candidate for heavy metal pollution control.

1.Introduction

Heavy metal ions,which are not biodegradable and can accumulate through the food chain,are widely present in water resources[1,2].Regarding their detrimental effects on aquatic ecosystems and human health,numerous treatment technologies,such as chemical precipitation,ion exchange,electrochemical techniques,membrane processes and adsorption,have been used to remove these harmful ions from the water environment[3-9].Among these methods,adsorption is considered as a promising approach due to its easy operation and extensive applicability for various situations[10,11].

Adsorbents tested for heavy metal removal are of great diversity,and carbon materials with large surface areas and pore volumes are probably the most widely employed ones.Not only commercial activated carbons but also emerging carbon materials,such as carbon nanotubes[11]and graphene oxides[12],have been utilized in this field.Generally,commercial activated carbons can adsorb heavy metal ions to some extent,but their adsorption capacities are not that satisfactory mainly due to the lack of surface functional groups.It is believed that the unpaired electrons provided by these functional groups can coordinate with metal ions and then promote their adsorption[13,14].Accordingly,multiform prompting agents have been applied for posttreatment surface modification of activated carbons,boosting their surface functional groups and thus improving their adsorption performances successfully[15,16].

Another way to introduce functionalities to carbon materials is through the so called in-situ doping approach,which usually leads to more uniform distributions of heteroatoms[17].By carbonization of heteroatom-rich precursors,for instance,metal-organic frameworks[18],organic salts[19]and various biomass resources[20-22],carbon materials with abundant heteroatom functional groups can be facilely derived.This in-situ approach usually leads to more uniform distributions of heteroatoms Compared to other precursors,biomass resources stand out thanks to their low cost,abundant availability and environmental friendliness,and biomass-derived carbon adsorbents have been attracting increasing attention[23,24].To date,most of the biomass-based carbon adsorbents were derived from carbohydrates and lignin,while research works concerning adsorption properties of protein-derived carbons were relatively limited.As another main component of biomass,proteins are readily available in our daily life,and large amounts of protein-rich waste,such as press cakes and fish silage,are generated in the production of foods and beverages[25].Considering the abundance of nitrogen and sulfur heteroatoms in them,proteins are ideal precursors for manufacturing of in-situ doped carbon materials[26-28],and the resulting doped carbons may be efficient adsorbents for metal ion removal.

In the present work,employing egg white as a protein-rich biomass precursor,a sustainable nitrogen and sulfur co-doped porous carbon has been synthesized and applied for adsorptive removal of a variety of heavy metal ions from water.Compared to a commercial activated carbon,the co-doped carbon possessed a smaller surface area,but its adsorption capacity was over 8 times higher compared to the activated carbon,demonstrating the promising potential applications of proteinderived carbons in heavy metal pollution control.

2.Experimental

2.1.Materials

Eggs used for carbon preparation were purchased from local markets in Tianjin.CoCl2·6H2O(98%),VOSO4·xH2O(97%),CrCl3·6H2O(98%),NiCl2·6H2O(99%),CuCl2·2H2O(99%),CdSO4·H2O(98%)were obtained from J&K Chemical Ltd.(Beijing,China).FeCl3·6H2O(99%)and nano magnesium oxide(50 nm,99%)were purchased from Aladdin Reagents(Shanghai,China).HNO3(65%-68%),ethanol(AR)and a commercial activated carbon(AC)were obtained from Tianjin Jiangtian Chemical Industry(Tianjin,China).Milli-Q ultrapure water was used in all experiments.

2.2.Synthesis of egg white derived carbon

In a typical synthesis,egg white was first diluted with water in a 1:1 mass ratio.10 g protein solution was then mixed with 1 g FeCl3·6H2O and 2 g nano magnesium oxide under stirring at 60°C overnight.The mixture was then dried by rotary evaporation,carbonized at 700 °C for 4 h under nitrogen atmosphere(100 ml·min-1)in a horizontal quartz furnace tube(SK-G08125K,Tianjin Zhonghuan Corp.,China),washed with 1 mol·L-1HNO3,water and ethanol respectively and dried at 60°C to form the egg white derived carbon(denoted as EWC).

2.3.Characterizations

The morphology of EWC was characterized by scanning electron microscopy(SEM,Hitachi S4800).The X-ray diffraction(XRD)pattern was recorded on a Bruker D8 Advance powder X-ray diffractometer using the Cu Kαradiation(λ=0.15418 nm).The Fourier transform infra-red(FT-IR)spectrum of the sample was collected using a Bruker Vertex 70 IR spectrometer(4 cm-1)in KBr media.X-ray photoelectron spectroscopy(XPS)was performed with a ThermoFisher K-Alpha spectrometer with Al Kαradiation,and the binding energies were referenced to the C 1s peak at 284.6 eV.CHNS elemental analysis was conducted on a Vario El cube instrument.The surface area was calculated from the Brunauer-Emmett-Teller(BET)method based on the nitrogen adsorption-desorption isotherm measured at-196°C on a Quantachrome Autosorb-iQ-C.

2.4.Adsorption experiments

10 mg of EWC or AC was added into a glass flask with a magnetic stirrer.Then 50 ml of heavy metal solution was added and the mixture was stirred for an appropriate time at 25°C.The initial metal concentration was～100 mg·L-1based on Co for kinetic study.For isotherm measurements,initial metal concentrations of 5-150 mg·L-1were employed.After that,the solution was separated from the carbon adsorbent by centrifugation,and the metal concentration was measured with an inductively coupled plasma(ICP)spectroscopy(Thermo Jarrell-Ash Corp.ICP-9000(N+M)).

The amount of metal ion adsorbed(qt)was calculated using Eq.(1):

Fig.1.SEM images(a,b),XRD pattern(c)and FT-IR spectrum(d)of EWC.

where C0and Ct(mg·L-1)are the metal concentrations in the solution at time=0 and t,respectively.V(ml)and W(mg)are the volume of the heavy metal solution and the weight of EWC or AC,respectively.

3.Results and Discussion

3.1.Adsorbent characterizations

As shown in Fig.1a-b,the egg white derived carbon(EWC)exhibited a packed sheet-like structure,forming a wood-ear-like morphology.XRD pattern of EWC(Fig.1c)possessed two broad peaks centered at around 27°and 43°corresponding to(002)and(10)diffractions peaks of graphitic structure[29],revealing its amorphous feature.Besides,no peaks of iron or magnesium species could be observed,indicating the effectiveness of the acid-washing procedure.

Protein is the major component in egg white,accounts for～95%on a dry-mass basis[30].When using egg white as the precursor for carbon manufacturing,the heteroatoms present in it are expected to be in-situ doped to the resulting carbon.To address this issue,CHNS elemental analysis was conducted for EWC(Table 1).As expected,abundant N,Sand O heteroatoms were detected in EWC,accounting for a total mass fraction of 25.23%.In contrast,only limited amount of O(3.48%)was detected in the commercial AC,while neither N nor S element was found.The FT-IR spectrum of EWC(Fig.1d)also indicated the existence of heteroatoms in EWC,showing bands centered at around 3410,1580,1385 and 1200 cm-1which could be assigned tovibrations,respectively[13,31,32].

Table 1 Elemental compositions and specific surface areas of the adsorbents

The XPS spectrum of EWC distinctly proved the presence of C1s,O1s,N1 s and S2p peaks(Fig.2a),in accordance with CHNS and FT-IR measurements.No peaks of iron or magnesium species could be found,again indicating the effectiveness of the acid-washing procedure,supporting the XRD result.Furthermore,deconvolution of the spectrum led to the distributions of heteroatom-containing species(Fig.2b-d).Deconvolution of the O1s spectrum gave rise to two peaks around 532 and 533 eV,assigning to CO quinone typephenol/ether type groups(O--II)[33,34].Deconvolution of the N1s spectrum gave rise to four peaks indicating the existence of pyridinic nitrogen(N-6)at 398 eV,pyrrolic/pyridone nitrogen(N-5)at 400 eV,quaternary nitrogen(N-Q)at 402 eV,and pyridine-N-oxide(N-X)at 405 eV,respectively[35,36].For sulfur,binding energies around 164 and 168 eV were attributed tofunctionalities[37].Generally,XPS shows the surface compositions while the CHNS measurement tells the bulk compositions.The elemental compositions determined by the two methods were close to each other(Table 1 and Table S1),indicating the relatively uniform distribution of heteroatoms in EWC.

3.2.Adsorption of Co2+:kinetics and isotherms

The high content of heteroatoms in EWC may be beneficial for heavy metal adsorption.Considering this,both EWC and AC were tested for adsorption of Co2+,an ion related to lung irritations,bone defects and other diseases[38].

Fig.2.The full-scale(a),O1s(b),N1s(c)and S2p(d)XPS spectra of EWC.

The effect of contact time on Co2+adsorption was investigated in the time range of 15-720 min(Fig.3a).The experimental data were better fitted to the pseudo-second-order model(see details in Supporting Information),indicating the involvement of chemical interactions in the adsorption process[39].The calculated equilibrant adsorption amount(qe)of Co2+over EWC was 290.2 mg·g-1,far exceeding the limited 30.6 mg·g-1over AC(Table 2).

Generally,a larger surface area promotes the interactions between the metal ions and the adsorbent,which is beneficial for metal ion adsorption.However,although the surface area of an adsorbent is important,the divergence in adsorption capacities between EWC and AC could not be simply explained by their different surface areas because the surface area of EWC(815 m2·g-1)was smaller than that of AC(1100 m2·g-1).Thus,the high adsorption capacity on EWC was attributed to the abundant heteroatomcontaining groups in it.These groups may act as complexing sites in the adsorption process[40,41],providing lone pair electrons to the unoccupied orbitals in heavy metal ions and thus promoting adsorption of the latter.

The adsorption of Co2+was further performed under a series of initial concentrations.As illustrated in Fig.3b,the adsorption amounts of Co2+increased with incremental initial concentrations until saturation was achieved.The data were then plotted to follow the Langmuir and Freundlich models(see details in Supporting Information),and the fitting parameters were summarized in Table 3.

As shown in Table 3,the correlation coefficients(R2)were larger for the Langmuir model compared to the Freundlich model,suggesting the better suitability of the Langmuir model for the adsorbents.The maximum adsorption capacities of Co2+(qmax)calculated by the Langmuir model were 320.3 and 34.0 mg·g-1for EWC and AC,respectively.The order of EWC＞AC was in accordance with the results in kinetic study,probably due to the stronger interaction strength between EWC and the metal ion.The order in KLvalues,which described the interaction intensity in the adsorption process[42],also reflected this point(Table 3).For the Freundlich model,the n values were larger than 1 for both adsorbents,showing the favorable nature of the adsorption[43,44].In addition,the values of KFrepresenting the adsorption amount and n representing the adsorption intensity followed the same order of EWC＞AC,further proving the credence of the results.Obviously,the higher content of complexing groups in EWC promoted the adsorption of the metal ion,enhancing the interaction intensity and boosting the adsorption capacity at the same time.

3.3.Adsorption of other metal ions and comparison with various adsorbents

Encouraged by the high adsorption capacity of Co2+by EWC,we further performed the adsorption of a series of heavy metal ions including VO2+,Cr3+,Ni2+,Cu2+and Cd2+.Interestingly,efficient adsorption was observed for all the tested metal ions(Fig.4),indicating the wide applicability of EWC.The adsorption capacity of Co2+over EWC was further compared with those in previous reports.As shown in Table 4,under similar conditions,the capacity over EWC was higher or comparable with various adsorbents.Combined with its environmental merits,EWC may be a potential candidate for promising applications in heavy metal pollution control.

Fig.3.Adsorption kinetics(a)and isotherms(b)for Co2+on EWC and AC(dosage=0.2 g·L-1).

Table 2 Kinetic models for Co2+adsorption on the adsorbents

Table 3 Isotherm models for Co2+adsorption on the adsorbents

Fig.4.Adsorption of a series of metal ions by EWC(C0=～100 mg·L-1,dosage=0.2 g·L-1).

4.Conclusions

Employing egg white as a protein-rich biomass precursor,a sustainable nitrogen and sulfur co-doped carbon has been synthesized.According to CHNS elemental analysis,N,S and O heteroatoms accounted for a total mass fraction of 25.23%,and the types of surface functionalities were further characterized by FT-IR and XPS measurements in detail.The carbon together with a commercial activated carbon(AC)was tested for adsorption of Co2+from water.The kinetic data were better fitted to the pseudo-second-order model,indicating the involvement of chemical interactions in the adsorption process.Although the carbon possessed a smaller surface area compared to AC,its adsorption capacity towards Co2+reached as high as 320.3 mg·g-1based on the Langmuir model,which was over 8 times higher than the limited 30.6 mg·g-1over AC.The heteroatom functionalities acted as complexing sites in the adsorption process,enhancing the interaction intensity between the carbon and metal ions and thus promoting adsorption of the latter.Furthermore,the carbon was found to be an efficient adsorbent towards a series of metal ions including VO2+,Cr3+,Ni2+,Cu2+and Cd2+,indicating its wide applicability.Considering its environmental merits,the protein derived carbon may be a promising candidate for heavy metal removal.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2018.11.017.

Table 4 Comparison of Co2+adsorption capacity with various adsorbents