Mesoporous activated biochar for As(III) adsorption: A new

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Materials and Interfaces

Mesoporous activated biochar for As(III) adsorption: A new utilization approach for biogas residue Dong Xia, Heng Li, Zheng Chen, Peng Huang, Dun Fu, Qingbiao Li, and Yuanpeng Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b04468 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Industrial & Engineering Chemistry Research

Mesoporous activated biochar for As(III) adsorption: A new utilization approach for biogas residue Dong Xiaa,c, Heng Lib*, Zheng Chenb, Peng Huangd, Dun Fua, Qingbiao Lia and Yuanpeng Wanga* aDepartment of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, and The Key Laboratory for

Synthetic Biotechnology of Xiamen City, Xiamen University, Xiamen 361005, P.R. China bKey laboratory of Estuarine Ecological Security and Environmental Health, Tan Kah Kee College, Xiamen University, Zhangzhou, P.R. China. cSchool of Chemistry, University of Leeds, Leeds, United Kingdom LS2 9 JT dSchool of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH 14 4AS, UK

ABSTRACT:

To develop a new utilization approach of biogas residues, mesoporous activated biochar (MB) was prepared from biogas residues and used for adsorption of As(III) from aqueous solution. MB-ZnCl2-600, which was activated with 2.5M zinc chloride solution at 600 oC, showed superior thermostability and good mesoporous distribution. The total surface area and the mesoporous surface area of the prepared biochar reached up to 892.3 m2/g and 701.7 m2/g respectively, and its pore diameters mainly ranged from 4 nm to 40 nm. Batch adsorption of As(III) revealed a high theoretic adsorption capacity of 5861 µg/g. The kinetic data revealed good fitting with the Elovich model, whilst the adsorption data demonstrated excellent correspondence with the Langmuir model. Experimental results confirmed that As(III) adsorption capacity is positively correlated with the mesoporous surface area (R2 = 0.813) of the adsorbent, confirming that mesopores in the MB-ZnCl2-600 adsorbent dominated As(III) adsorption. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) demonstrated that SiO2 provides crucial adsorption sites for the removal of As(III) and that organic functional groups on the biochar surface bonded with As species during adsorption. This study is the first time to prepare MB with good mesoporous distribution from low-cost and wide-source starting material of biogas residues, can be used as a supporting framework for catalysts and other adsorbent applications.

1. Introduction The use of renewable and clean energy instead of fossil fuels has become a crucial issue worldwide in the 21st century. As a result, methane projects to meet the requirements of sustainable energy and utilize low-grade biomass (i.e. animal manure and crop stalks) have become popular.1 According to a 2013 estimation by the Ministry of Agriculture of China, there were more than 15,000 large-scale (>500 m3) and medium-scale (300–500 m3) agricultural biogas plants since 2013 in China. However, the massive amounts of digested by-products generated by these projects, such as biogas residue, can easily cause environmental and ecological issues. These biogas residue after methane production will be largely possible discharged into rivers and lakes in the rural areas, causing seriously drinking water contaminants and damaging cultivating fisheries. According to recent investigations, in the United Kingdom alone, anaerobic digestion of plants produces approximately 277,000 tonnes of biogas residue each year.2 Therefore, development of ways to dispose of biogas residue properly has become an important challenge.3 Biogas residues mainly consist of hemicellulose, lignin and cellulose, which are easy to obtain from rural areas. These natural organics are readily available and suitable for the production of biochar materials.4,5 Importantly, resourcilization of biogas residues can solve environmental-related concerns and produce value-added products. For example, Yao et al. discovered that biochar made from digested sugar beet tailings is effective for the removal of pollutants from wastewater.6

Biogas residues have been successfully used to prepare biochar with a pivotal role in disposing pollutants, such as organic pigments,7 ammonia nitrogen and heavy metals.8,9 However, the disordered morphologies of the prepared biochars often lead to great variations in their adsorption performances. Thus, methods to produce structurally ordered and uniform biochar have become a research priority, as the resultant products may provide higher adsorption capacity, excellent thermal and electrical conductivity and superb energy storage ability.10 Dehkhoda et al. verified that mesoporous structures could significantly decrease the electrode resistance and enhance the capacitive behaviour of KOH-activated biochar.10 Owing to their excellent chemical stability, extensive mesostructure and pore accessibility,11 mesoporous carbon has been applied to different fields, such as energy storage,12 catalysis,13 gas separation and pollutant adsorption.14,15 Furthermore, biochars with highly porous structures possess promising potentials to support reported ultrafine nanocrystals.16-24 Studies on the preparation of mesoporous carbon materials from waste biomass, such as coconut shells,25 industrial laundry sewage sludge,26 banana peel and mangosteen peel,27,28 have increased in recent years. The mature technology of biochar preparation from biogas residues and the similarity between the physical and chemical properties of mesoporous carbon materials and biochar indicate the possibility of preparing mesoporous carbon from biogas residue. To the best of our knowledge, no report on the preparation of mesoporous activated biochar (MB) from biogas residues has yet been published. We believe that utilisation of this low-cost and wide-source material for advanced applications will not only solve waste disposal

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ARTICLE problems but also expand the resource utilisation of biogas residues. The metalloid As, which can cause different types of cancer in the skin, bladder and lung, is the most prevalent contaminant in aqueous solutions.29 As(III) is 60 times more toxic than As(V) and more difficult to remove from the environment, because it exists in a neutral form (H3AsO3) in natural water.3032 Our previous investigation revealed that biochar derived from the biogas residues of pig manure is effective for aqueous As(III) removal, which mainly proceeds via ligand exchange to form Zn–O–As(III).9 However, to better understand the adsorption process of As(III) on biochar materials, the mechanisms of As(III) adsorption onto the surface of biochars in aqueous solution should be further studied. In this study, a well-structured MB produced from biogas residue with a large mesoporous surface area and desirable mesopore distribution is investigated and used to adsorb As(III) from aqueous solutions. The chemical, physical and functional properties of the prepared MB are studied, and the mechanisms of adsorption of As(III) onto the surface of the prepared MB are discussed.

2. Method and materials 2.1 Materials Biogas residue from pig manure was obtained from Yongan Town, Shuangliu District, Sichuan. The biogas residue was airdried in an oven at 105 °C and crushed into smaller pieces prior to activation treatment. Specifically, the air-dried biogas residue was put into a small grinder with blades (Model: 31000 r/min, 850 W, XL-600B), followed by full power grinding 10 minutes. The sieved biogas residue was stored in a vial for subsequent utilisation. Other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used without further purification. All of the As(III) aqueous solutions were diluted to the desired concentration using deionized water from a concentrated stock solution of 1,000 mg/L. 2.2 Preparation of mesoporous activated biochar The detailed procedures of preparing mesoporous activated biochars are presented in Figure 1. In detail, aqueous solutions of ZnCl2 (1, 2, 2.5 and 3 M) were prepared. Each solution and the biogas residue were mixed at a proportion of 5 mL:1 g followed by stirring for 0.5 h in a beaker (i.e. put 1 gram biogas residue into 5 mL 2.5 M ZnCl2 solution forming well mixture under vigorous stirring). Subsequently, the obtained mixtures were thermally pyrolysed to form activated carbon in tube furnace in N2 inert atmosphere. The prepared mixture was dried in an oven at 105 °C for approximately 24 h, after which the dried samples were pyrolysed at different temperatures of 550, 600, 650,700 °C, 750 °C and 800 °C. All samples were maintained at each temperature for 2 h in a tubular furnace (SK2-1-12, Tianye, China) and allowed to cool down to ambient temperature under a N2 gas flow rate of 100 mL/min. Thereafter, the pyrolysed samples were rinsed with 0.1 M HCl solution for about 12 h to remove residual ZnCl2 and added

with 0.1 M NaOH solution to adjust the pH of the aqueous layer to 7.0 ± 0.2. Finally, the mixtures were rinsed with deionized water thrice and dried in an oven at 105 °C for approximately 5 h. The fabricated activated biochar within lager number of mesopores in its internal structures, therefore, the prepared materials in this study were named mesoporous activated biochars (MB). The obtained samples were kept in a desiccator for further analysis and labelled MB-ZnCl2-X, where MB represents the mesoporous activated biochar and X represents the pyrolysis temperature applied to the sample. Amongst the prepared MB samples, MB-ZnCl2-600, which was activated with 2.5 M ZnCl2 solution at 600 °C, exhibited the best structure-function properties (Table S1). Thus, all subsequent experiments were conducted on using MB-ZnCl2600. 2.3 Analytical methods The C, H, O, S and N contents of MB-ZnCl2-600 were analysed using an elemental analyser (VarioEL III, Elementar, Germany). X-ray diffraction (XRD) analysis was carried out on an Ultima IV X-ray diffractometer (Rigaku, Japan) operating at 35 kV and 10 mA with CuKα radiation ( = 0.15418 nm) at 40 kV and 30 mA. Scanning was performed at a rate of 10°/min. The thermal stability of MB-ZnCl2-600 was assessed by thermogravimetric analysis (TGA) under an air atmosphere over temperatures ranging from 30 °C to 1000 °C (STD Q600). The specific and mesoporous surface areas of samples were measured using the Brunauer–Emmett–Teller (BET) method, and the pore distributions of the samples were determined using an ASAP 2020 instrument with N2 as the adsorbent at -195.74 °C (Micromeritics Tristar 3020). All samples were degassed at 300 °C for 2 h before being assayed. The samples of the prepared MB-ZnCl2-600 before and after arsenite adsorption were characterized by Fourier transform infrared spectrophotometric analysis (FT-IR, Nicolet 6700, Thermo Electron, USA). The surface morphology of the MB-ZnCl2-600 was observed using scanning electron microscopy analysis (SEM, ZEISS SIGMA, Germany). X-ray photoelectron spectroscopy (XPS) was used to determine the elemental properties before and after As(III) adsorption (Quantum-2000, USA). For XPS assays, samples were compressed into a thin pellet. To avoid As(III) oxidation at high temperature, the powder of MB-ZnCl2-600 after As(III) adsorption was dried in a vacuum oven at 30 °C. 2.4 Adsorption of As(III) from aqueous solutions MB-ZnCl2-600 was exposed to an As(III) aqueous solution to evaluate its adsorption performance. The adsorbent dosage was 2 g/L, the pH of the aqueous solution was adjusted to 7.0 ± 0.2 before adsorption, and the initial As(III) concentration was 5,000 μg/L. Adsorption kinetics were studied by pouring 50 mL of the As(III) aqueous solution (5,000 μg/L) and 100 mg of the adsorbent (2 g/L) into a 100 mL conical flask and placing the flask in a thermostatic shaker at ambient temperature (180 rpm). Aliquots of the As(III) aqueous solution were taken from


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Industrial & Engineering Chemistry Research ARTICLE

the flask at predetermined intervals of 0.5, 1, 1.5, 2, 3, 4, 6, 8,

where C0 and Ce (μg/L) are the initial and the equilibrium

Figure 1. Corresponding schemes of production of biogas residue from pig manure and preparation of mesoporous activated biochar.

10, and 12 h. The adsorbent and As(III) wastewater was then separated using a 0.2 μm membrane filter (Millipore). The concentration of residual As(III) in the separated solution was detected by inductively coupled plasma-optical emission spectrometry (ICP-OES, Jitian AF-610B, China). Three parallel experiments were conducted, and the mean value was recorded as the final measurement. Adsorption isotherm studies were carried out at concentrations ranging from 500 μg/L to 5,000 μg/L. Sixty milligrams of the biochar was added to a conical flask containing 30 mL of an As(III) aqueous solution (2 g/L). The mixture was then stirred in a thermostatic shaker at ambient temperature (180 rpm) for approximately 12 h to achieve equilibrium. ICP-OES was used to determine the residual As(III) concentration in the As(III) wastewater. The As(III) removal rate and adsorption quantity of adsorbent per unit mass were calculated using equations (1) and (2):

(

𝐶𝑒

)

As(III) removal % = 1 ― 𝐶0

𝑞𝑒 =

𝑉(𝐶0 ― 𝐶𝑒) 𝑚

(1) (2)

concentration of the solution, respectively; qe (μg/g) is the equilibrium adsorption capacity; V is the volume (L) of As(III) solution and m is the mass (g) of adsorbent used. The Elovich, Lagergren’s pseudo-first-order and Ho’s pseudosecond-order models were used to analyse the adsorption kinetics of MB-ZnCl2-600 by non-linear regression. The mathematical equations of the kinetic models used are listed below.

𝑞𝑡 = 𝑚 + 𝑛ln 𝑡

(Elovich)

ln 𝑞𝑒 ― 𝑞𝑡 = ln 𝑞𝑤 ― 𝑘1𝑡 (Pseudo-first-order) 𝑡 𝑞𝑡

𝑡

1

― 𝑞𝑒 = 𝑘 𝑞2

(Pseudo-second-order)

(3) (4) (5)

2 𝑒

Where in the Elovich model, n is a desorption constant (g/µg), m is the initial rate of adsorption (µg/g). qe and qt are the amount of As(III) absorbed (µg/g) at equilibrium and at contact time t (h), respectively. k1 (h-1) and k2 (g/µg·h) are the rate constants for the pseudo-first-order and pseudo-second-order models, respectively. To determine the adsorption capacity of MB-ZnCl2-600 for As(III) wastewater, the Langmuir and Freundlich adsorption models were employed. The two equations are listed below:



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ARTICLE (Langmuir) (Freundlich)

𝑞𝑚𝐶𝑒𝐾𝐿

𝑞𝑒 = 1 + 𝐶𝑒𝐾𝐿 𝑞𝑒 =

𝐾𝐹𝐶1𝑒 ― 𝑛

(6)

3. Results and discussion 3.1 Preparation of mesoporous activated biochar

(7)

Where qe is the As(III) concentration at equilibrium, qm is the maximum As(III) uptake capacity (µg/g), and KL is related to the energy of adsorption (L/µg). KF is the Freundlich affinity coefficient (μg1-nLn/g), and n is the Freundlich linearity constant. OriginPro 2015 software was used to fit and analyse the adsorption kinetics and adsorption isotherms in this work.

The detailed procedures of producing mesoporous activated biochar (MB) using biogas residue from pig manure after anaerobic digestion are exhibited in Figure 1. Clearly, the biogas residue from anaerobic digestion reactor was grinded into fine powders followed by mixing with various concentrations of ZnCl2 solutions. Subsequently, the obtained mixtures were thermally pyrolysed to form activated carbon in tube furnace in N2 inert atmosphere. After washing and pH

ACS Paragon Plus Environment Figure 2. Nitrogen adsorption-desorption isotherms of 1M-ZnCl2 (a), 2M-ZnCl2 (b), 2.5M-ZnCl2 (c), and 3M-ZnCl2 (d) activated biochars prepared at different temperatures, and corresponded pore diameter distributions (a), (b), (c), and (d).

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neutralizing, the fabricated activated biochar within lager number of mesopores in its internal structures, therefore, the prepared materials in this study were named mesoporous activated biochars (MB). The pronounced sample in this study was MB-ZnCl2-600, which was prepared using 2.5M ZnCl2 solution at 600 oC. Figure 2 and Table S1 clearly imply that MB-ZnCl2-600 exhibited the best adsorption-desorption curves

Brunauer-Emmett-Teller (BET) and scanning electron microscopy (SEM) techniques were respectively used to understand the mesoporous distribution and morphological features of the asprepared MB-ZnCl2-600 adsorbent (Figure 3). BET analysis showed typical type-IV hysteresis loops at a relatively high pressure region in the nitrogen adsorption/desorption isotherms of the samples (Figure 3a), which explicitly proves that MB-ZnCl2-600 contains a

Figure 3. (a) Nitrogen adsorption/desorption isotherms and, (b) pore diameter distributions of MB-ZnCl2-600 adsorbent. SEM images of MB-ZnCl2600 with different various magnifications (c - f). (Red arrows and red circles mark the mesopore sites)

and pore size distributions among all produced MBs, therefore, the MB-ZnCl2-600 adsorbent will be specifically characterised in this study. 3.2 Characterization of mesoporous activated biochar

large number of mesopores in its structure. Interestingly, the pore diameter distribution of MB-ZnCl2-600 was between 4 and 40 nm (Figure 3b), thus confirming that the sample has an outstanding mesoporous structure. The BET data in Table 1 reveal that the specific surface area of MB-ZnCl2-600 is 892.3 m2/g and its



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ARTICLE Table 1. Parameters of various mesoporous biochars prepared from different starting materials and MB-ZnCl2-600.

aS

Starting materials

Activators

Biochar name

SBET a (m2/g)

Smeso b (m2/g)

Ratio c (%)

Ref.

Rice husk

La(NO3)3

RHBC

543.9

305.7

56.2

33

Coconut shell

NaOH

COSHTC

876.1

319.2

36.4

34

Phragmites australis

C2H7O2N

PAB

1074

585

54.5

35

Prosopis juliflora

H2O2

HC-24-300

277.5

127.2

45.8

36

Pine

O2/H2O

1A

906

< 331

< 36.5

37

Biogas residue

---

dBC-600

70.9

25.5

38.2

This work

Biogas residue

NaOH

BC-NaOH-600

115.8

51.1

44.1

This work

Biogas residue

ZnCl2

MB-ZnCl2-600

892.3

701.7

78.6

This work

BET

= total surface area; bSmeso = mesoporous surface area; cRatio = Smeso/ SBET×100%; dBC = biochar

mesoporous surface area is 701.7 m2/g (approximately 78.8% of the total surface area); these values are significantly larger than that of most reported mesoporous carbon materials.33-37 These characteristics directly indicate that the prepared MBZnCl2-600 is an interesting mesoporous biochar material. Importantly, with and without using NaOH as an activator, for example BC-600 and BCNaOH-600, exhibited substantially smaller both the total surface areas and the mesoporous surface areas compared with MB-ZnCl2600 (Table 1, Table S2, Figure S1 and Figure S2). A large number of mesopores are clearly distributed on surface of MB-ZnCl2-600 (Figure 3c-f), and the majority of the pore

diameters found were in the mesoporous ranges. The red arrows and circles clearly marked the mesopore positions in the adsorbent, with diameters obviously smaller than 50 nm (Figure 3e-3f). The analytical results of the SEM images are consistent with above BET analysis. Compared with previously reported non-mesoporous biochar,38 MB-ZnCl2-600 exhibited a larger mesoporous surface area and uniform mesopore distribution. Moreover, the mesopores of MB-ZnCl2-600 are of the ideal diameter range, which is a significant difference between the reported mesoporous and non-mesoporous biochars. Biogas residues from pig manure, which mainly include hemicellulose, lignin, and cellulose, are mainly composed of C (24.47 %) and O (69.61 %).39 After pyrolysis, the content of C increased to 56.95 % whilst that of O decreased to 28.89 % (Table S3). These changes indicate that the hemicellulose, lignin and cellulose in the raw biogas residue were successfully pyrolysed. The ash content of MB-ZnCl2-600 also dropped significantly to 25.76 % after thermal treatment, which was close to the TGA analysis (Figure 4a). MB-ZnCl2-600 exhibited great thermostability that can maintain high thermal stability up to 500 oC in air atomsphere (Figure 4a), and the major mass loss at 500-580 °C was attributed to the combustion of some components of the biochar structure to produce CO2 and H2O at high temperatures.40 The DSC thermogram of the combustion process of MB-ZnCl2-600 varied with increasing temperature. The greatest heat flow rate and mass loss were observed at 550 °C. XRD (Figure 4b) demonstrated that both the prepared MB-ZnCl2-600 and ashes present amorphous structural characteristics. The main component was SiO2, and the two primary diffraction peaks appeared at 20.90° and 26.68° in both samples.41,42 More diffraction peaks of SiO2 were exposed after thermal treatment, which suggests that SiO2 was covered by carbon. Based on the XRD analysis, the prepared MB-ZnCl2-600 clearly possesses a large amount of SiO2 in its internal and external structure. 3.3 Adsorption kinetic study

Figure 4. (a) TGA and DSC results of MB-ZnCl2-600 adsorbent. (b) XRD patterns of the MB-ZnCl2-600 and the corresponded ashes.

The optimal adsorption conditions (i.e. the best adsorbent dosage and the most favourable pH) were tested before carrying out adsorption kinetics and isotherms studies. Figure 5a and Figure 5b manifest that the best adsorbent dosage was 2g/L and the optimal pH for As(III) adsorption was 7, respectively. Followed on the selected conditions, adsorption kinetics assays of the prepared MB-ZnCl2-


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600 were carried out to examine the adsorption efficiency of the adsorbent toward As(III).

second-order model can best describe the As(III) adsorption behaviour, thus suggesting that the mechanism of As(III) adsorption

Figure 5. (a) Effect of adsorbent dosage on As(III) adsorption capacity and removal rate.(Adsorbent: MB-ZnCl2-600, Initial arsenic concentration was 5 mg/L. pH = 7.) (b) Effect of initial pH on MB-ZnCl2-600 adsorption of As(III) (Initial conc. was 5 mg/L and adsorbent was 2g/L). Kinetics (c) and isotherms (d) of As(III) adsorption onto MB-ZnCl2-600 adsorbent.

The experimental data of As(III) adsorption are shown in Figure 5c, and the fitting results of the mathematical models are presented in Table 2. The correlation coefficients (R2) of the adopted models indicated that both the Elovich model (R2 > 0.985) and the pseudo-

includes multiple processes.43 Aa a consequence, this result proposed that the adsorption mechanisms should contain several aspects, which need to be discussed separately in detail. According to both the pseudo-first-order and pseudo-second-order kinetic models,

Table 2. Adsorption kinetics and isotherm parameters for As(III) adsorption onto the MB-ZnCl2-600 adsorbent

Kinetics Models Pseudo-first-order

Pseudo-second-order

Elovich

Parameters

Isotherms Models

qe (ug/g)

909.2

k1 (h-1)

0.971

R2

Parameters KF (ug1-nLn/g)

1.157

n

1.124

0.951

R2

0.973

qe (ug/g)

1008

qm (ug/g)

5861

k2 (ug/mg·h)

1.400

KL (L/ug)

0.104

R2

0.985

R2

0.977

qe (ug/g)

901.9

k1 (h-1)

161.1

R2

0.985

Freundlich

Langmuir



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ARTICLE sorption of As(III) onto the adsorbent showed two stages: very rapid adsorption during the first 4 h of reaction followed by a gradual decrease in adsorption until equilibrium is reached. This rapid adsorption process could be ascribed to the large mesoporous surface area of the adsorbent (701.7 m2/g), which provides readily accessible external surface adsorption sites. The excellent pore distribution in the MB-ZnCl2-600 adsorbent also facilitates metal binding in aqueous medium.44 The slow stage could be attributed to both the decrease in number of available adsorption sites and the lower concentration of the adsorbate, as high concentrations of adsorbate offer strong driving forces to overcome the mass transfer resistance between the liquid phase and the solid phases.45 Eventually, adsorption equilibrium was established within 10 h with a theoretic equilibrium adsorption capacity of 1008 µg/g of pseudo-secondorder kinetic model, and 909 µg/g of pseudo-first-order kinetic model, respectively (Table 2). While the experimental value of equilibrium adsorption capacity was 1,159 µg/g, which was very close to pseudo-second-order kinetic model analysis. In terms of correlation coefficient, the pseudo-second-order kinetic model also exhibited significantly high correlation with As(III) adsorption behaviour (Table 2), further indicating that pseudo-second-order kinetic model can well describe As(III) adsorption behaviours onto the MB-ZnCl2-600 adsorbent. Apart from excellent fitting results, the well-described pseudo-second-order kinetic model also indicated that chemisorption occurred during As(III) sorption process. 3.4 Adsorption isotherm study The adsorption isotherms of As(III) adsorption onto MB-ZnCl2600 adsorbent are presented in Figure 5d, and the fitting parameters of the applied models are listed in Table 2. Both the Freundlich model and Langmuir model can describe As(III) adsorption behaviours well, with correlation coefficient R2 > 0.973 and R2 > 0.977, respectively. However, in the Freundlich isotherm, the empirical parameter n is used to determine the degree of favorability in the adsorption process. The n (1.124) in this work indicates a strong favorability in the MB-ZnCl2-600 system. The high correction coefficient of Freundlich model also explained that As(III) is adsorbed in the form of heterogeneity on the surface of MB-ZnCl2600 adsorbent, which is in line with the mesoporous activated biochar complex microstructures. However, the higher fitting correlation coefficient of Langmuir model also confirms that complicated As(III) adsorption behaviours onto MB-ZnCl2-600 adsorbent, as a consequence, adsorption mechanisms are essentially required to understand adsorption behaviours in this work. The theoretic maximum As(III) adsorption capacity of MB-ZnCl2-600 was 5861 µg/g (Table 2), which indicated that the produced adsorbent has high As(III) adsorption potentials. The data listed in Table 3 are compared with the maximum adsorption capacities of biochar-based adsorbents produced from different starting materials, such as Kans grass biochar,30 sludge-derived biochar,46 activated carbon47 and buckwheat husk biochar.48 Compared with coal-based mesoporous activated carbon,49 the prepared MB-ZnCl2-600 showed better performance for As(III) removal. The MB-ZnCl2-600 adsorbent exhibited an As(III) adsorption capacity lower than those of other biochar materials, such as Al-enriched biochar derived from abandoned Tetra Paks.50 However, this is an indication of using the

excellent properties of mesoporous biochar produced in this work to dope with metal oxides for enhancing As(III) adsorption capacities. In the future, the applicability of MB-ZnCl2-600 for working as supports for different adsorbents, catalysts and supercapacitors should be investigated. Table 3. Comparison of As (III) adsorption capacities of MB-ZnCl2-600 with other adsorbents from the literature

Adsorbents

T (oC)

qm (μg/g)

Ref.

Kans grass biochar

500

2004

30

Sludge-derived biochar

500

5650

46

Wheat straw biochar

500

7.128

47

Buckwheat husk biochar

450

510

48

Coal-based activated carbon

950

1491

49

MB-ZnCl2-600

600

5861

This work

3.5 Determination of the As(III) adsorption mechanism Ligand exchange, surface complexation, electrostatic interactions and interactions between different surface functional groups and the aqueous As species are the major established mechanisms for As removal.9,51,52 As(III) in neutral wastewater mainly exists as uncharged H3AsO3 and, therefore, is difficult to remove. This work demonstrates that the porous structure and surface functional groups of MB-ZnCl2-600 are beneficial for adsorption of As(III) from aqueous solutions. 3.5.1 Influence of mesopores Mesopores significantly contribute to adsorption and act as primary channels for transporting adsorbates; they also accelerate the mass transfer of pollutants.53,54 Large amounts of mesopores on adsorbents have been proven to speed up the rate of diffusivity of molecular compounds.55 More importantly, the generation of mesopores provides more adsorption sites and exposes more minerals from the biochar to enhance the adsorption capacity of an adsorbent. Li et al. verified that slightly increasing the mesopore distribution of biochar leads to significantly enhanced adsorption performance.56 Mesopores introduce more organic functional groups and volume, further increasing the adsorption capacity of the adsorbent and enhancing the removal rate of contaminants.56 Li et al. demonstrated that the adsorption performance of biochar is dominated by mesoporous areas instead of the total surface area of the adsorbent.57 In this work, different mesoporous activated biochars were used to adsorb As(III) from aqueous solutions to demonstrate that mesopores facilitate the adsorption of As(III). Figure 6a and 6b show that, amongst the adsorbents studied, the mesoporous surface area of MB-ZnCl2-600 was the largest and exhibited the best As(III) adsorption performance. To explore possible relationship between mesopores and adsorption efficiency, linear fitting analyses of pore size distribution and adsorption efficiency were conducted. As shown in Figure 6c, the mesoporous surface area was positively


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Figure 6. Relationship between the mesoporous surface areas and the adsorption of As(III) on mesoporous materials prepared at different temperatures with 2.5 M zinc chloride (a), relationship between the mesoporous surface areas and the adsorption of As(III) on mesoporous materials prepared at 600 oC with different zinc chloride concentrations (b). Correlation analysis between the mesoporous surface areas and the As(III) removal rates (c) and correction analysis between the total surface areas and the As(III) removal rates (d). Initial arsenic concentration was 200 µg/L, adsorbent content was 2.0 g/L and initial pH was 7.0.

correlated with the As(III) removal rate (with R2 = 0.813), while the correlation coefficient between the total surface area and the As(III) removal rate only exhibited with R2 = 0.672 (Figure 6d), which confirmed that mesoporous structures played dominated role in As(III) adsorption. Moreover, the generation of mesopores resulted in the exposure of more SiO2 adsorption sites and organic functional groups. These results are consistent with previously reported studies demonstrating that mesoporous activated biochar prepared from biogas residues in this study have properties similar to that of mesoporous carbon made from valuable pure substances.12 3.5.2 Influence of SiO2 Figure 7a shows the FT-IR results of MB-ZnCl2-600 before and after As(III) adsorption. A new peak at 791 cm-1 appeared after As(III) sorption, which implies that As was adsorbed successfully onto the surface of the adsorbent. This finding is consistent with a previous investigation showing that the stretching vibrations of As– O are mainly centred between 650 and 950 cm-1.58 The signals of the functional group Si-O-Si shifted from 1070 cm-1 to 1080 cm-1 after the reaction, which indicates that SiO2 functions as As(III) adsorption sites (Figure 7a). To investigate the shifting of the binding energy of Si and demonstrate that SiO2 plays an important role in As(III) sorption, XPS was employed. Figure 7b shows that

the surface of MB-ZnCl2-600 is covered with C, O and Si. Besides, there was nearly no changes in terms of element variations (i.e. C, O and Si) before and after adsorption, this explained that the composition of as-prepared MB is high stability, confirming no variations of compositions before and after adsorption. New peaks corresponding to As(3d) and As(LMM) appeared on the surface of the adsorbent after As(III) adsorption (Figure 7b). The As(3d) electron binding energy is shown in Figure S3. The binding energy of the prominent peak corresponding to As(3d) is 43.91 eV, thus suggesting that As(III) is not oxidised. This finding is in agreement with the FT-IR results and proves that As species are adsorbed by MB-ZnCl2-600. The Si(2p) electron binding energy showed an obvious shift after As(III) adsorption (Figure 7c), which demonstrates that adsorption of As(III) is related to the surface of the SiO2 sites. Taken together, the FT-IR and XPS results illustrate that SiO2 acts as crucial adsorption sites for As(III) removal. 3.5.3 Influence of O-containing functional groups



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ARTICLE

Figure 7. FT-IR spectra (a), XPS analysis (b) and spectra analysis of Si (2p) (c), C (1s) (d) and O (1s) (e) of the MB-ZnCl2-600 before and after As(III) adsorption.

Figure 7d shows that the C(1s) electron binding energy shifted after adsorption. The peak at 284.8 eV was assigned to graphite–C (C–C) whilst that at 287.4 eV was attributed to C=O. These results indicate that amorphous and graphite C are the major species adsorbing As(III).40 Only one peak of the O(1s) electron binding energy appeared after adsorption of As(III) onto MB-ZnCl2-600 (Figure 7e). The binding energy at 532.7 eV can be ascribed to C=O or C–OH.59 Although the O(1s) electron binding energy only shifted slightly, all of the results thus so far demonstrate that bonding effects exist between organic O-containing functional groups and As species during the adsorption process. Therefore, O-containing functional groups on the surface of the prepared MB-ZnCl2-600 promote As(III) removal. The adsorption mechanism of As(III) onto MBZnCl2-600 (Figure 8) mainly consists of three steps: (1) mesopores

facilitate As(III) sorption and provide adsorption sites; (2) exposed SiO2 groups function as important adsorption sites; and (3) bonding effects occur between O-containing functional groups and As species in the course of adsorption. The complicated adsorption mechanisms of As(III) adsorbed onto MB-ZnCl2-600 adsorbents were well in line with the previous Elovich model assumptions.

4. Conclusions Biogas residue obtained from pig manure after anaerobic digestion was successfully converted into the mesoporous activated biochar (MB-ZnCl2-600) with excellent structure-function properties and outstanding capacity for As(III) adsorption. The as-prepared MBZnCl2-600 adsorbent exhibited large mesoporous surface area up to 701.7 m2/g, and the pronounced mesoporous diameters mainly


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Figure 8. The graphical abstract of the As(III) adsorption mechanisms onto the MB-ZnCl2-600.

ranged from 4 nm to 40 nm. The kinetic data of As(III) adsorption onto MB-ZnCl2-600 followed well with the Elovich model (with R2 > 0.985), indicating multiple As(III) adsorption mechanisms. Besides, the adsorption process appeared to follow both the Freundlich model and the Langmuir model well (with R2 > 0.973 and R2 > 0.977, respectively). The theoretic maximum As(III) adsorption capacity of MB-ZnCl2-600 can reach up to 5861 μg/g according to Langmuir model. As(III) adsorption experimental results demonstrated that adsorption of As(III) onto the prepared biochar is directly related to the mesopores, confirmed by the well linear relationship between mesoporous surface area and As(III) removal rate (with R2 > 0.813). FT-IR and XPS spectra verified that SiO2 adsorption sites and O-containing functional groups contributed to As(III) adsorption onto MB-ZnCl2-600 adsorbent. This study expands the resource utilisation approach for biogas residues, and suggests the potential applications of mesoporous activated biochars prepared from biogas residues as multifunctional mesoporous supports for catalysts and other adsorbents.

Author information Corresponding Authors *Heng Li, Key laboratory of Estuarine Ecological Security and Environmental Health, Tan Kah Kee College, Xiamen University, Zhangzhou 363105, P.R. China. Email: [email protected] *Yuanpeng Wang, Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, No. 422, Southern Siming Road, Xiamen 361005, P.R. China. Email: [email protected]

Supporting Information statement The Supporting Information includes 3 tables and 3 figures. The structure-function properties of different activated biochars are shown in Table S1, Table S2, Figure S1 and Figure S2. Elemental compositions analysis of biogas residue and MB-ZnCl2-600 are

shown in Table S3. XPS spectra analysis of As(3d) in MB-ZnCl2600 after As(III) adsorption is shown in Figure S3.

Table of Contents graphic

Acknowledgements The National Natural Science Foundation of China (21206143, 41271260), the Program for New Century Excellent Talents in University (NCET-12-0326), the Natural Science Foundation of Fujian Province of China (2018J05073), the Program for Young Excellent Talents in University of Fujian Province (201847), and the Project of Educational Scientific Research of Junior Teacher of Fujian Province of China (JT180792).

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Mesoporous activated biochar for As(III) adsorption: A new

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