Removal of methylene blue from aqueous solution by sewage sludge-derived biochar: Adsorption kinetics, equilibrium, thermodynamics and mechanism

Accepted Manuscript Title: Removal of methylene blue from aqueous solution by sewage sludge-derived biochar: Adsorption kinetics, equilibrium, thermodynamics and mechanism Author: Shisuo Fan Yi Wang Zhen Wang Jie Tang Jun Tang Xuede Li PII: DOI: Reference:

S2213-3437(16)30460-2 http://dx.doi.org/doi:10.1016/j.jece.2016.12.019 JECE 1378

To appear in: Received date: Revised date: Accepted date:

30-8-2016 21-11-2016 17-12-2016

Please cite this article as: Shisuo Fan, Yi Wang, Zhen Wang, Jie Tang, Jun Tang, Xuede Li, Removal of methylene blue from aqueous solution by sewage sludge-derived biochar: Adsorption kinetics, equilibrium, thermodynamics and mechanism, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.12.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Removal of methylene blue from aqueous solution by sewage sludge-derived biochar: Adsorption kinetics, equilibrium, thermodynamics and mechanism

Shisuo Fan1,2*, Yi Wang1, Zhen Wang1, Jie Tang1, Jun Tang1, Xuede Li1,2

(1. School of Resources and Environment, Anhui Agricultural University, Hefei, 230036,

China;

2. Hefei Scientific Observing and Experimental Station of Agro-Environment, Ministry of Agriculture, People’s Republic of China, Hefei, 230036 ,China)



Corresponding authors. Tel/Fax: +86 551 65786311

Email addresses: [email protected] (Shisuo Fan)

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Graphical abstract

Highlights 

Municipal sludge was prepared to biochar.



Adsorption kinetic was described by the pseudo-second order model.



Equilibrium data were well fitted to the Langmuir isotherm equation. 2



The adsorption mechanism includes electrostatic interaction, ion exchange, hydrogen bond interaction, n-  interaction.

Abstract: In this study, biochar was produced from municipal sludge and was characterized by Surface area and porosity analysis, Scanning Electron Microscope–Energy Dispersive Spectrometer (SEM-EDS) and Fourier Transform Infrared Spectroscopy (FTIR). The effect factors including adsorbent dosage, contact time, pH, temperature on the adsorption properties of methylene blue (MB) from aqueous solution onto sludge-derived biochar were investigated in batch experiments. The adsorption kinetics, isotherm, thermodynamic and mechanism were also studied. The results showed that the adsorption kinetics of MB on biochar was accurately described by a pseudo-second-order model, indicating that liquid film diffusion, intra-particle diffusion and surface adsorption coexisted during MB adsorption on the biochar. The equilibrium adsorption data were well represented by the Langmuir isotherm 2

equation (R >0.99). As the initial MB concentration and temperature increased, the adsorption amount also increased and the adsorption was favorable. The thermodynamic analysis showed that MB adsorption onto sludge-derived biochar was spontaneous and endothermic. The desorption and reusability experiment indicated that sludge-derived biochar had the potential to be a reusable adsorbent for MB removal.The adsorption mechanism appeared to be related to electrostatic interaction, ion exchange, hydrogen bond interaction, n-

 interaction, etc. Thus, sludge-derived biochar can be used as an effective absorbent to remove dyes from wastewater. As a beneficial by-product of sludge, sludge-derived biochar also can mitigate the environmental burden of sewage sludge.

Keywords: Sludge-derived biochar; Methylene blue; Adsorption kinetics; Thermodynamic; Mechanism

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1. Introduction

At present, wastewater from textile, leather, paper, plastics and other industries were largely produced, especially methylene blue wastewater which may pose potential risks to human and animals [1]. In addition to large wastewater yield, the dyeing industry produces effluent that is characterized by high concentrations of aromatic pollutants, dark color, alkaline properties, weakly biodegradable components and multiplicate complex substance [2]; thus, unexpected or accidental dyeing industries discharge effluent into the rivers or lakes and could cause serious pollution. So, the treatment of dyeing wastewater is the focus of considerable water treatment research and one of the difficult challenges.

The conventional used treatment methods of dyeing wastewater involve advance chemical oxidation technology, photo-degradation, membrane treatment, biological treatment and adsorption treatment, among others [3]. Due to higher efficiency, lower cost, ease and simplicity of operation and lower sensitivity to toxic pollutants, adsorption method (typically using commercial activated carbon) has been considered superior to other treatment techniques. Nevertheless, cost considerations and complicated technology have limited the widespread application of adsorption treatment. Recently, value-added material or modified waste material has become more commonly used in the treatment of dyeing wastewater [4-6].

Nowdays, the large amount of sludge that is produced during biological wastewater treatment represents a serious burden on ecological health and society. In general, the typical techniques for sludge treatment include incineration, composting, aerobic digestion, disposal 4

in landfills and land application, among others. Resource utilization has become an important means by which to derive value from sludge and preparations to transform sludge into activated carbon is an important pathway of sludge resource utilization. Through physical, chemical or physicochemical activation, highly active and highly adsorbent sludge-derived adsorbent can be obtained [7-9].

Recently, due to the functions in soil improvement, greenhouse gases reduction, wastewater pollutant control, solid waste resource utilization, much research on biochar has been performed [10-19]. The main raw materials of biochar preparation have been forestry waste, agricultural residue and the organic fraction of municipal solid waste, among others. The typical raw materials for biochar include agricultural waste [20], manure [21], municipal sludge [22]. Compared with sludge-derived activated carbon, sludge-derived biochar lacks an activation process and was prepared at relative lower temperatures when compared with activated carbon. Therefore, the preparation process for sludge-derived biochar is relatively simple and inexpensive. Sludge-derived biochar has a large potential for transforming sludge as a value-added waste material into a resource to be utilized in pollutant control field.

Currently, the application of sludge-derived biochar to treat dyeing wastewater has been rarely reported, especially the interaction mechanism between sludge-derived biochar and MB solution. Therefore, municipal sludge was used to prepare biochar at 550°C in this research. The sludge-derived biochar was used to adsorb methylene blue (MB), a typical component of dyeing wastewater. The objectives of this study were to: 1) observe the effect of biochar mass, adsorption time and pH on MB adsorption; 2) investigate the kinetics and isotherm of MB adsorption on sludge-derived biochar; 3) characterize the mechanism of MB adsorption 5

on sludge-derived biochar. This research should provide a reference for sludge resource utilization and application of sludge-derived biochar in dyeing wastewater treatment.

2. Materials and Methods

2.1. Preparation of sludge-derived biochar

Air-dried sludge (moisture content 8.06±0.29%) was obtained from the dewatering stage of a domestic wastewater treatment plant. The wastewater treatment process consisted of an oxidation ditch treating 180,000 m3·d–1. To prepare biochar, a specified amount of sludge was put into a crucible with a lid. The crucible was put into a stainless steel cylinder. The air in the cylinder was removed by nitrogen gas to ensure an oxygen-free atmosphere. Then, the cylinder was put into the furnace and the furnace temperature was programmed to increase at a rate of 10 ºC·min-1 until it reached at 550 ºC and kept for 2h. After natural cooling, the yield rate of the sludge-derived biochar was determined gravimetrically, and pH of biochar was determined using the 24 hours-equilibrated solution of biochar and deionized water with a solid/liquid ratio of 1:10(w/w). The yield rate was 43.61% and the pH was 7.50 . Sludge-derived biochar was ground and passed through a 100-mesh sieve. The sample was saved for further adsorption experiment.

All chemicals used in this study were of analytical grade. The batch experiments were duplicated and only the average values were reported. 2.2. Characterization The textural property of the sludge-derived biochar was analyzed with a surface area and porosity analyzer (Micromeritics, Tristar II 3020) at 77K. The surface area, pore volume and 6

average pore size were calculated using the nitrogen adsorption isotherms. The multipoint Brunauer–Emmett–Teller method was employed to calculate surface area.The pore volume and average pore siz were obtained from desorption isotherms using the Barrett–Joyner– Halenda method.

Scanning Electron Microscopy (SEM, S-4800, Hitachi, Japan)- Energy Dispersive Spectrometer (EDS, X-Max, Oxford Instruments, Britain) analysis was carried out on the sludge-derived biochar to investigate the surface morphology and element distribution before and after MB adsorption.

Fourier Transform Infrared (FTIR, Nicolette is50, Thermo Fourier, USA) analysis was applied on the sludge-derived biochar to characterize the surface functional groups, and the spectrum was recorded in the wave number range of 400-4000cm-1. The production of sample pellet was mixed with KBr at a ratio 1:100 w/w. The concentration of released metals (Ca2+, Mg2+, Na+, K+) from the sludge-derived biochars in the supernatant of the equilibrium solution were analyzed by Inductively Coupled Plasma (iCAP 6300 Series, Thermo Fourier, USA). Meanwhile, the corresponding release of Ca2+, Mg2+, Na+, K+ from the biochars with deionized water was served as control. The net metal release was calculated by the difference of the values obtained from the equilibrium solution and those of the control experiment (background control for normalization). 2.3. Effect of operational parameters on MB adsorption Adsorption experiments were conducted using 50 mL centrifuges tubes with same volume of 20 mL, and MB concentration of 100 mg·L-1.Different mass of biochar was added to adjust the

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varying solid-to-liquid ratios. Different dosages of biochar were added to achieve liquid-solid ratios between 2 and 50 g·L–1 (biochar mass effect). Sub-samples were collected at pre-determined times during the 24 h of oscillation (contact time effect).1mol∙L–1 HCl or NaOH was added to the MB solution to adjust the pH in the range of 2-11 (pH effect ).Meanwhile, the raw MB solution without pH adjustment was used as a control treatment. The samples were placed in an oscillating shaker operated at 180 rpm and 25°C for 24 hours.Then, the samples were filtered through a 0.45μm membrane and the absorbance of the filtrate was measured by spectrophotometer ( the wavelength was set at 665nm, 722S, Shanghai Precision, Shanghai, China). The absorbance of the filtrate was measured by a spectrophotometer (1 cm pathlength cuvettes was used). The ultrapure water was used to zero calibration.The MB removal efficiency and amount adsorbed were calculated using Equations 1 and 2, respectively.

Removal efficiency (%)=(C0-Ce)/C0×100% (1) Adsorption amount (mg·g-1) =(C0-Ce)× (V/m) (2) In Equations 1 and 2, C0 is the initial MB concentration (mg·L–1); Ce is the equilibrium MB concentration (mg·L–1); V is the volume of MB solution (L); and “m” is the weight of adsorbent (sludge-derived biochar, g). 2.4. Adsorption kinetics Based on the above experimental parameters, a certain amount of biochar was added to a 500-mL Erlenmeyer flask containing 250 mL MB solution having an initial concentration of 50 mg·L–1, 100mg·L–1 or 150mg·L–1. The experiment was conducted at 25°C, 35°C and 45°C. Sub-samples were collected at pre-determined times and were determined. The adsorption amount of MB was calculated by: 8

qt = (C0-Ct)/C0×V/m (3) Ct (mg·L–1) is the liquid-phase concentration of MB at time t.

Then, different kinetic models were evaluated for their ability to accurately represent the adsorption process. 2.5. Adsorption isotherm According to the above experimental parameters, a certain amount of biochar was added to 50-mL centrifuge tubes containing different concentrations of MB solution (50-500 mg·L1

). The samples were placed in an oscillating shaker operated at180 rpm and either 25°C,

35°C or 45°C for a certain time. Then, the samples were filtered, measured and the adsorption amount of MB was calculated. Then, different isotherm models were evaluated for their ability to accurately represent the adsorption process.

3. Results and Discussion

3.1 Characterization of sludge-derived biochar The surface area of sludge-derived biochar was 25 m2·g-1. The total pore volume and average pore size were 0.047 cm3·g-1 and 3.828 nm, respectively. The biochar was classified as mesoporous material with average pore size in fall in the range of 2-50 nm [23]. These results suggested that biochar with higher surface areas and more pore volume may be used as potential adsorbent.

SEM-EDS of Pure MB, sludge-derived biochar and biochar absorbed MB is shown in Fig.1. As the shown in Fig.1 (a), the crystal structure of MB was obvious and the main composition elements of MB include C, Cl, S. The structure of sludge-derived biochar was coarse and more porous texture. The elements composition of sludge-derived biochar 9

involved C, O, Si, Al, Mg, Ca, P, K, S, etc (as shown in Fig.1 (b)). The surface structure of biochar absorbed MB is shown in Fig.1(c). The abundant pore structure of biochar determines its potential to adsorption of MB. Sulfur could be detected on the surface of biochar according to the EDS analysis. However, chloride could not be found in the EDS spectrum (Fig.1(c)) due to its dissolved in supernatant and the concentration of chloride in the supernatant is 1.62 mg·L-1 (the approach is titration method).

Additionally, EDS mapping analyses (Fig. 2) on the MB absorbed sludge-derived biochar samples also show the appearance of Si, O, Na, K, Mg, Ca upon adsorption treatment, which indicated that these elements may participate the adsorption process of MB onto biochar.

The FTIR spectrum of pure MB, sludge-derived biochar and biochar adsorbed MB is shown in Fig.3. Functional groups of methylene blue, sludge-derived biochar, biochar with MB adsorbed were presented in table 1. As shown in the Fig.3 and table 1, the pure MB have many functional groups, including –OH, C=C, C=N, C=S, C-S, C-H, etc [24]. As the shown in Fig.3 and table 1, the main functional groups of sludge-derived biochar contains 3424cm1

(-OH), 1618cm-1(C-N or secondary protein II), 1038cm-1(C-O and Si-O bonds), 463cm-1(Si-

O-Si) [25, 26]. Some peaks shifted or appeared after MB adsorption on the sludge-derived biochar. The peaks of 1618cm-1, 463cm-1 shifted to 1602cm-1, 476cm-1 which indicated that nitrogen containing functional and Si-O-Si groups may participate in the adsorption process. The appeared peaks of 1388cm-1 and 1331cm-1 were attributed to the presence of MB

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molecular on the biochar surface and means –CH3 and aromatic nitro groups from MB involved the interaction between MB and biochar [27, 28].

3.2. Effect operational parameters on MB adsorption

3.2.1. Effect of biochar dosage and contact time

The dosage of biochar addition was an important factor influencing MB adsorption. Fig. 4(a) shows the effect of biochar dosage on the MB removal efficiency and adsorption capacity. When the dosage of biochar was 6-8 g·L–1, the MB removal efficiency reached 98%. However, as the biochar mass increased, the adsorption amounts decreased and adsorption amount per unit mass of adsorbent decreased even when the initial concentration of MB solution was constant. Removal rate increases with an increase in adsorbent mass is due to the availability of more binding sites for adsorption. However, further increase in adsorbent mass did not affect the removal rate may be related with the unavailability of the adsorbate sites which were saturated by the MB dye molecules. Hence, the suitable sludgederived biochar dosage was 6 g·L–1 in consideration of removal rate, adsorption amount, cost of adsorbent, etc.

The contact time was also an important influencing factor for MB adsorption. The influence of contact time on MB removal efficiency and adsorption capacity is shown in Fig. 4(b). When the contact time was approximately 8 h, the adsorption efficiency reached 98%. With an increase of contact time beyond 8 h, the removal efficiency eventually reached 100% (at approximately 25 h). As contact time increased (to approximately 10 h), adsorption

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capacity first increased and then tended to stabilize, which indicated that the MB adsorption reached an equilibrium state. Thus, the proper contact time was in the range of 8-10 h. 3.2.2 Effect of solution pH The pH was a key factor affecting MB adsorption onto sludge-derived biochar. Fig.4(c) shows the MB removal efficiency and adsorption capacity of biochar under different pH. With the increase of pH, the removal efficiency and adsorption capacity also increased. When the pH was lower than 2, the MB removal efficiency was less than 85%. When the pH increased from 2 to 10, the MB removal efficiency increased from 92% to 96%. When the pH of MB solution reached 11, the removal efficiency was nearly 100%. These results showed that alkaline conditions favor the adsorption of MB on sludge-derived biochar, and indicated that high pH exited electrostatic interaction between MB and adsorbent. The effect of pH on MB adsorption can be explained as follows. When the pH was low, H+ ions occupied the limited number of possible binding sites on sludge-derived biochar and thus hindered the adsorption of MB, which is a cationic dye and has a positive electrical charge on the surface. Therefore, at low pH, MB experienced are pulsive interaction with H+ ions and this interaction went against the adsorption of MB on biochar. When the pH increased, the H+ ion concentration decreased in the solution and the competitive adsorption [29-31]. Then, the repulsive interaction between MB and H+ ions also decreased and increasing the adsorption amount. At pH 11, the adsorption capacity tended to stability. The effect of pH on adsorption process can be attributed to the electrostatic interaction [32].

A control experiment also was conducted using MB solution for which the pH was not adjusted. The MB removal efficiency was more than 95% without pH adjustment and the 12

adsorption capacity was 16.21 mg·g–1. So, the pH of the MB solution was not adjusted in the subsequent kinetic and isotherm experiments. These results showed that when sludge-derived biochar is used to treat MB wastewater in a practical application, no pH adjustment of the wastewater is required to achieve acceptable MB treatment performance. 3.3. Adsorption Kinetics To investigate the adsorption kinetics and mechanism of MB adsorption on sludgederived biochar, pseudo-first, pseudo-second, intra-particle diffusion and Elovich models (Equations 4-7, respectively) were used to describe the kinetic process and replicate results obtained from the kinetic experiments [33-38]. log( q e - q t )  log q e t 1 t   q t k 2 q e2 q e

qt  k d t 1 / 2  C

k1 t (4) 2.303

(5)

(6)

q t  ( 1 /  ) ln(  )  1 /  ln( t ) (7)

In Equations 4-7, qe is the adsorption capacity at equilibrium time (mg·g–1); qt is the adsorption capacity at t time (mg·g–1); k1 is the pseudo-first order rate constant (min–1); k2 is pseudo-second order rate constant (g·mg–1·min–1); kd is the intra-particle diffusion rate constant (g·mg-1·min-1/2) ; C is a constant; α is the initial adsorption coefficient (mg·g-1·min– ); and β is the desorption coefficient (g·mg-1).

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Fig.5 shows the adsorption kinetics curve of MB solution onto sludge-derived biochar at 25°C.

The model fitting parameters of the pseudo-first and pseudo-second order kinetic models describing MB adsorption onto sludge-derived biochar at different initial MB concentrations 13

and temperatures are presented in Table 2. The correlation coefficients indicated that the pseudo-first order (R2> 0.95) and pseudo-second order (R2> 0.99) models described the kinetic adsorption process well. Since the pseudo-second order model explains the external liquid film diffusion, surface adsorption and intra-particle diffusion processes [35], this model provided a more comprehensive and accurate reflection of the adsorption mechanism of MB onto the sludge-derived biochar than did the pseudo-first order model. The pseudo-second order model confirmed that the rate limiting step is chemisorptions [36].

As adsorption temperature increased, the maximum adsorption capacity also increased. Thus, a rise in operating temperature favored MB adsorption on biochar that means adsorption of MB on biochar was an endothermic process. At any given initial MB concentration, the adsorption rate constant increased with an increase in temperature. However, at any given temperature, the rate constant decreased with an increase in the initial MB concentration because the mass transfer did not occur well at lower MB concentration.

The fitting parameters for the intra-particle and Elovich kinetic models of MB adsorption onto sludge-derived biochar at different initial MB concentrations and temperatures are given in Table 3. When the initial MB concentration was 50 mg·L–1, neither model described the observed results well. However, when the initial MB concentration was higher than 50 mg·L– 1

, the models fitted the observed results well, which indicated that intra-particle and

chemisorption processes were the main control steps during adsorption of MB on sludgederived biochar.

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Increased temperature and higher initial concentration of MB favored the intra-particle process of MB onto sludge-derived biochar. When the initial concentration of the MB solution was low, the model fitting results were not good, indicating that intra-particle processes were not the main controlling step during adsorption, and that liquid film diffusion may also played important role during the adsorption process. With the increase of the initial MB concentration, the MB molecular gradually diffused into the inside of sludge-derived biochar particles. However, the plotted intra-particle diffusion model did not pass through the origin, indicating that although the intra-particle process of MB solution onto biochar was not the only rate-controlling step [36]. The adsorption rate also was influenced and controlled by the external diffusion step (surface adsorption and liquid film diffusion).

The Elovich model describes the heterogeneous diffusion process, which is comprehensively regulated by the reaction rate and diffusion factor. Elovich plot was found to fit well with the kinetic data as evidenced by the high values of correlation coefficients indicates that MB adsorption on biochar is a heterogeneous diffusion process and not a simple first-order reaction [38]. The adsorption of MB onto biochar was an integrative process that was controlled by reaction rate and diffusion, a conclusion that is consistent with the results of pseudo-second order kinetics model. 3.4. Adsorption Isotherm To describe the adsorption isotherm process of MB solution onto biochar, Langmuir, Freundlich and Temkin isotherm models (Equations 8-10, respectively) were examined for their abilities to accurately describe the adsorption process [39-40]. The Dubinin-

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Radushkevich (D-R) isotherm model (Equations 11-13) was also used to fit the adsorption process [41]. The Langmuir isotherm equation is: Ce 1 1   Ce q e Qmax b Qmax

(8)

The Freundlich isotherm equation is: ln q e  ln K f 

1 ln C e n

(9)

The Temkin isotherm equation is: qe 

RT R ln KT  T ln Ce bT bT

(10)

The D-R adsorption isotherm equations are: ln( qe )  ln( q m ) -  2

  RTln( 1 

Ea 

1

1 ) Ce

(11)

(12)

(13)

2

In Equations 8-13, Ce is the equilibrium concentration (mg·L–1); qe is the adsorption capacity at equilibrium time (mg·g–1); Qmax is the maximum adsorption capacity (mg·g–1); b is a Langmuir constant related to adsorption capacity (mg·g1); Kf is the Freundlich constant(L·mg–1); n is the adsorption “intensity”; KT is the equilibrium binding constant (L·mg–1); bT is the Temkin isotherm constant; R is the universal gas constant (8.314J∙mol–1∙K– ); T is absolute temperature (K); the quotient “RT/bT” is related to adsorption heat (J·mol–1);

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qe is the amount of adsorbate in the adsorbent at equilibrium (mol·g–1); qm is the theoretical isotherm saturation capacity (mol·g–1);  is the Dubinin–Radushkevich isotherm constant

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(mol2·kJ-2); Ea is the free energy per molecule of adsorbate (kJ·mol-1); and ε is the Dubinin– Radushkevich isotherm constant. Fig.6 shows the adsorption isotherm curve of MB solution onto sludge-derived biochar at 25 °C.

The fitting parameters of the four adsorption isotherm models and the correlation coefficients of model predictions versus experimental observations are presented in Table 4. Correlation coefficients (Table 4) indicated that both the Langmuir (R2> 0.99) and Freundlich (R2> 0.96) models described the adsorption process well. Hence, the adsorption of MB solution onto sludge-derived biochar occurred mainly by single molecular adsorption. With an increase of temperature, the maximum adsorption capacity also increased, which indicated that raising the operational temperature enhances adsorption behavior. The factor “1/n” in the Freundlich isotherm model can reflects the heterogeneity factor, the heterogeneity of site energies, and the adsorption intensity. Values of 1/n smaller than 0.5 indicate that the adsorbate is easily adsorbed; values of 1/n larger than 2 indicates the adsorbate is hardly adsorbed [39]. In the present study, the value for 1/n of MB adsorption onto sludge-derived biochar at all three temperatures was less than 0.5 and indicated that increasing temperature enhanced the adsorption process.

A separation or equilibrium factor (RL) has been defined based on the Langmuir isotherm such that RL=1/(1+C0b), where C0 is the initial MB concentration. When 01 means unfavorable adsorption; RL=0 indicates irreversible adsorption, and RL=1 means linear adsorption [39]. In the present study, as shown in Fig. 7, RL was less than 1 and indicated that the adsorption of MB on sludge-derived

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biochar was favorable. Furthermore, increasing the initial MB concentration enhances the adsorption process. With the increase of temperature, the decreased RL implied the favorable adsorption of MB onto the sludge-derived biochar.

The Temkin isotherm model assumes that heat of adsorption (function of temperature) of all molecules in the layer would decrease linearly rather than logarithmic with coverage. The Temkin isotherm model mainly describes the chemical adsorption process as electrostatic interaction [40]. In the present study, the Temkin isotherm fitted the observed results well (R2>0.95, Table 4). Hence, electrostatic interaction is an important mechanism affecting the interaction between sludge-derived biochar and MB. The correlation coefficients (R2>0.90, Table 4) indicated that the D-R isotherm also described the adsorption of MB onto sludge-derived biochar reasonably well. When Ea is in the range 1-8kJ mol–1, the adsorption process is a physical function [41]. When Ea is in the range 8-16 kJ mol–1, the adsorption process occurs via ion exchange mechanism [42]. When Ea is in the range16-40 kJ mol–1, the process is chemical adsorption [43]. In the present study, the Ea ranged between 17 and 19 kJ mol–1, which indicated that the MB adsorption onto sludge-derived biochar occurred through chemical adsorption. 3.5. Adsorption Thermodynamics To investigate the adsorption thermodynamics of MB adsorption onto sludge-derived biochar, Ea, ∆G, ∆H and ∆S were calculated using equations 14-16 [44].

ln k 2 

Ea  C (14) RT

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Where k2 is the pseudo-second order rate constant (g∙mg–1∙min–1); Ea is the activation energy (kJ∙mol–1,). R is the ideal gas constant (8.314J∙mol–1∙K–1); T is the absolute temperature (K).C is the constant.

G  - RTln( K ) (15) Where K is the thermodynamic equilibrium constant equal to Qmax ×b of Langmuir isotherm [45], R is the ideal gas constant, 8.314J∙mol-1∙K-1; T is the absolute temperature, K. lnK 

S H 1 (16) R

R T

Where ∆H is adsorption enthalpy (kJ mol–1); ∆S is adsorption entropy (J mol–1·K–1). Plotting the ln(K) and 1/T would obtain the ∆H and ∆S based on formula (16) [46].

The variation of activation energy (E a) for different initial MB concentrations adsorbed onto sludge-derived biochar is shown in Fig. 8. The Ea was an important index with which to analyze the adsorption type. The physisorption processes usually involves activation energies in the range of 0-50 kJ· mol-1 while higher activation energies (50-800 kJ ·mol-1) indicates chemisorptions [44].In this study, the activation energy for the adsorption of 50 mg∙L– ,100mg∙L–1, 150 mg∙L–1 MB onto sludge-derived biochar were 72.87 kJ∙mol–1, 54.45 kJ∙mol–1

1

and 47.35 kJ∙mol–1, respectively. Thus, the adsorption of MB by sludge-derived biochar occurred via chemical adsorption when the concentration of MB solution was less than 100 mg·L-1. However, the physisorption also played important role when the MB solution was 150 mg·L-1.

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The adsorption thermodynamic parameters of MB adsorbed onto biochar are presented in Table 5. ∆G values were obtained to be -3.3299, -4.3997 and -5.7809 kJ·mol-1 for 298K, 308K and 318 K, respectively. The values of ∆G were found to be negative under different temperatures which indicated the spontaneous and favorable nature of MB adsorption onto sludge-derived biochar. Meanwhile, with the increase in temperature, the more decrease of ∆G values implies that higher temperature favors MB adsorption [46]. The positive value of ∆H suggests that the adsorption of MB onto biochar was endothermic, and the positive value of ∆S indicates an increase in the degree of randomness (or disorder) at the solid/solution interface during the sorption of MB onto biochar. Generally, when the ΔH is less than 25 kJ∙mol–1, the acting force is Van der Waals' force and can be attributed to physical adsorption. When ΔH is in the range 40–200 kJ∙mol–1 , the acting force is chemical bonding and can be attributed to chemical adsorption[47]. In the present research, ΔH was 33.1396 kJ∙mol–1. Physical adsorption was predominant and chemical adsorption also existed between biochar and MB interaction [48]. Therefore, MB adsorption on biochar surfaces was a complex process. 3.6. Desorption study of sludge-derived biochar The desorption and reusability property is an important part for a particular adsorbent.The reusability of sludge-derived biochar towards adsorption of methyl blue dye molecules was studied for three cycles.The adsorption of MB (initial concentration: 50mg·L-1) was stirred with the sludgederived biochar (solid-to-liquid ratios: 6 g·L-1) for 10h at 25ºC.The adsorbent was separated from the suspension by mixed with ethanol/acetic acid eluent (v/v:9/1) for 10h and ultrasonicated for 10 min.After ultrasonication the adsorbent was washed with water for several times to remove the dye 20

molecules and dried in an oven at 80 ºC.

The removal rate of MB by sludge-derived biochar was shown in Fig.9. The removal rate was more than 60% after three cycles. The removal rate of desorbed-biochar decreased due to the effect of the prior desorption process,which solubilized some parts of biochar, changed superficial structures of biochar and subsequently led to loss or blockage of adsorption sites [49].

Meanwhile, the removal of MB by sludge-derived biochar included irreversible adsorption.

The desorption experiment indicated that sludge-derived biochar had the potential to be a reusable adsorbent for MB removal. However, the recycle properties of biochar could be improved through other approaches. Magnetization of the adsorbent is a promising method to solve the problem of recycle. The utilization of iron oxide nanomaterials has received much attention in wastewater treatment [50]. Further investigation is needed to focus on the better recycle of biochar for more effective pracitical application. 3.7. Possible mechanism of MB adsorption onto sludge-derived adsorbent The adsorption of MB on sludge-derived biochar was a complicated process and various interactions co-existed during the process.

According to the effect of pH, the adsorption mechanism referred to electrostatic interaction.

Based on the results of adsorption kinetics, the adsorption process involved physical acting and chemical bonding. The adsorption process includes the external liquid film diffusion, surface adsorption and intra-particle diffusion processes. 21

According to the results of FTIR and SEM-EDS, nitrogen functional and Si-O-Si groups may participate in the adsorption process of MB onto sludge-derived biochar. The hydrogen bonding interaction was formed between the nitrogen from MB and hydrogen from sludgederived biochar. Hydrogen bonds are usually present in most adsorption systems and are known as non-electrostatic [51].-Si-O-Si may act as an active site for dye adsorption through n-  interaction [52]. The n-  interaction was also confirmed by the shift of FTIR (464 cm-1 shifted to 476cm-1). Therefore, the n-  interaction may occur between aromatic structure of MB and Si-O-Si of sludge-derived biochar.

Ion exchange process was important mechanism involved in MB adsorption on sludgederived biochar. The concentration of released metals (Ca 2+, Mg2+, Na+, K+) from the biochars in the supernatant of the equilibrium solution were determined and deionized water was served as control experiment (background control for normalization). Then, the net amount of released cations (mequiv·g−1) were calculated and is shown in Table 6. As seen in Table 6, the net amount of released cations increased after MB adsorption and indicated that more cations were released to supernatant, especially the Ca 2+ and Na+ ions which more participates in the ion exchange process. More ions were released to the supernatant under higher MB concentration during the adsorption. The release of Na+ and K+ cations from the biomass surface in the solution after MB sorption confirmed the contribution of cation exchange mechanism [53].

Until recently, investigations of the mechanism of MB adsorption onto sludge-derived activated carbon or biochar have been rarely reported. Biochar from sludge has alkaline properties and contains oxygen-containing functional groups on the surface as well as 22

inorganic mineral components [54].The mechanism of interaction between other sludgederived absorbent and MB solution had been reported, involving oxygen-containing functional groups on the surface of the sludge [40], formation of electrostatic bonds [55-57], Shi et al [58] investigated the use of biochar produced from anaerobic granular sludge to absorb the MB solution and found that the adsorption mechanism included electrostatic interaction, ion exchange and surface complexation.

On the whole, the mechanisms that may happen in the MB adsorption process are shown in Fig.10. The possible mechanism of MB adsorption onto sludge-derived biochar includes electrostatic interaction, ion exchange, hydrogen bond interaction and n-  interaction, etc. 4. Conclusions The adsorption of MB onto sludge-derived biochar can be accurately represented using a pseudo-second order kinetics model. The Langmuir isotherm model describes the adsorption process well. Increasing temperature or increasing initial MB concentration favored the adsorption process. The results of adsorption thermodynamic analysis confirmed that MB adsorption onto sludge-derived biochar was spontaneous and endothermic. With the increase of temperature, the decrease of ∆G values implied that higher temperature favors MB adsorption. The desorption and reusability experiment indicated that sludge-derived biochar had a potential to be a reusable adsorbent for MB removal.The mechanism of MB adsorption onto sludge-derived biochar referred to electrostatic interaction, ion exchange, hydrogen bond interaction and n-  interaction, etc. Therefore, sludge-derived biochar is a promising and valuable absorbent to remove MB from wastewater. Acknowledgements 23

This study was financially supported by National Major Water Project (No.2015ZX07103007) and Anhui Agricultural University Youth Fund Project (No.2014zr004, jz2015-23).

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32

Fig.1. SEM-EDS of (a) Pure MB, (b) sludge-derived biochar, (c) biochar absorbed MB

33

Fig.2. Sludge-derived biochar adsorbed MB (EDS mapping)

34

Fig. 3. FTIR spectrum of pure MB, sludge-derived biochar and biochar adsorbed MB

35

Fig. 4. Effect of operational parameters on adsorption of methylene blue (MB) on sludge-derived biochar: (a) biochar dosages; (b) contact time; (c) pH.

36

Fig.5. Kinetic fits for MB adsorption on sludge-derived biochar (25°C) using different kinetic models: (a) pseudo-first order; (b) pseudo-second order; (c) intra-particle diffusion; (d) Elovich model.

37

Fig.6. Linear fits of the isotherm models for methylene blue adsorption on sludgederived biochar predicted by various isotherm models: (a)Langmuir; (b)Freundlich; (c) Temkin;(d) Dubinin–Radushkevich.

38

Fig.7. Effect of initial methyleneblue concentration and temperature on the Langmuir equilibrium factor (RL) for adsorption onto sludge-derived biochar

39

Fig.8. Effect of initial concentration on activation energy for methylene blue adsorption on sludge-derived biochar

40

Fig.9. Desorption and reusability study for adsorption of methyl blue onto sludge-derived biochar

41

Fig.10. Interaction mechanisms in the Biochar-MB system: a)

hydrogen bond interaction; b) n-  conjugate action; c)electrostatic adsorption; d) ion exchange process; e) other interactions.

42

Table 1. Functional groups of biochar before and after methylene blue sorption

43

Sludge-derived Biochar Methylene

Before

After

Blue

adsorption

adsorption

cm-1

cm-1

cm-1

assignment

assignment Difference

3424

bonded−OH groups

3424

3424

0

bonded−OH groups

1599

Stretch vibration of C=N and (C=C)

1618

1602

+16

assigned to C=O and C=C aromatic vibrations

1491

CH2 deformation vibration,

1388

-CH3

1396

C-N bonds in the heterocycle

1334

Aromatic nitro groups

1357

C-N bonds connected with benzene ring

1038

1038

0

1340

N-CH3 bond

463

476

-13

1252

Ar-N deformation vibration,

1224

Ar-N deformation vibration,

1182

stretching vibrations of C=S

1142

C-S bonds

887

wagging vibration of C-H in aromatic ring

44

C-O stretching and Si-O bonds Si-O-Si

Table 2. Adsorption kinetics parameters of MB on sludge-derived biochar

Temperature (°C) 25

35

Pseudo-first model concentration (mg∙L-1) 50 100 150 50 100 150 50 100

qe ( mg∙g-1) 1.083 3.148 6.901 1.083 3.110 7.313 1.019 2.114

k1 (min-1) 0.05757 0.005760 0.003450 0.05750 0.01030 0.006680 0.09760 0.01470

150

6.845

0.01220

Pseudo-second model

0.9860 0.9470 0.9730 0.9860 0.9720 0.9840 0.9970 0.9920

qe ( mg∙g-1) 8.380 16.69 22.37 8.347 16.61 21.65 8.340 16.67

k2 ( g∙mg-1∙min-1) 0.1455 0.006652 0.001843 0.3329 0.01187 0.003192 0.9275 0.02658

1.000 0.999 0.999 1.000 1.000 0.998 1.000 1.000

0.9650

22.52

0.006141

0.999

R2

(pseudo-first and pseudo-second order models)

45

R2

Table 3. Adsorption kinetics parameters of MB on sludge-derived biochar (intra-particle diffusion and Elovich models)

Temperature (°C)

25

35 45

concentration

Intra-particle diffusion model

(mg∙L-1)

Kd

C

R2



β

R2

50 100 150 50 100 150 50 100 150

0.03610 0.2167 0.3341 0.0361 0.1968 0.4052 0.0175 0.1162 0.3568

7.827 12.53 13.86 7.827 12.93 13.31 8.090 14.64 16.05

0.5676 0.8360 0.9573 0.5676 0.7676 0.9437 0.4912 0.9132 0.9788

0.1771 1.141 1.668 0.1771 1.059 2.037 0.08780 0.5457 1.625

7.427 9.586 10.16 7.427 10.39 8.754 7.889 13.47 12.71

0.7504 0.9663 0.9945 0.7504 0.9250 0.9925 0.6770 0.9783 0.9855

46

Elovich model

Table 4. Adsorption isotherm parameters of methylene blue on sludge-derived biochar for various isotherm models

Temperature (ºC) 25 35 45

b 0.1591 0.1934 0.2983

25 35 45

bT 1208 1035 936

Langmuir Qmax 24.10 28.82 29.85 Temkin KT 530.3 519.2 289.1

R2 0.9950 0.9948 0.9973 R2 0.9575 0.9608 0.9517

47

qm 6.241e-05 7.283e-05 7.852e-05

Kf 1.110 1.119 1.132 D-R β 3.300e-03 2.600e-03 2.800e-03

Freundlich 1/n 0.1042 0.1128 0.1243

R2 0.9668 0.9848 0.9689

E 17.41 19.61 18.90

R2 0.9297 0.9036 0.9219

Table 5. Thermodynamic parameters of MB adsorption on sludge-derived biochar

Temperature (K) 298

∆G (kJ∙mol-1) -3.330

308

-4.400

318

-5.781

∆H (kJ∙mol-1) 33.14

48

∆S (J (mol∙K)-1) 122.2

Table 6. The net release of Ca2+, Mg2+, Na+, K+ during adsorption by biochar at 25 ºC Samples 50 mg·L-1-25ºC 100 mg·L-1 -25ºC 150 mg·L-1 -25ºC

Ca2+ 0.005800 0.008700 0.008800

The net amount of released cations (mequiv·g-1) Mg2+ Na+ 0.001100 0.00950 0.001500 0.02010 0.001400 0.03130

K+ -0.003600 -0.003700 -0.003700

Sum 0.01280 0.02660 0.03770

Note: 50 mg·L−1−25 °C means the concentration of MB was 50 mg·L−1 and the operating temperature was 25 °C during the adsorption kinetics experiment.

49