Evaluation and Prediction of Cadmium Removal from Aqueous

Subscriber access provided by READING UNIV

Article

Evaluation and Prediction of Cadmium Removal from Aqueous Solution by Phosphate-Modified Activated Bamboo Biochar Shihong Zhang, Han Zhang, Jian Cai, Xiong Zhang, Junjie Zhang, and Jingai Shao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03159 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Evaluation and Prediction of Cadmium Removal from Aqueous Solution by Phosphate-Modified Activated Bamboo Biochar Shihong Zhanga, Han Zhanga, Jian Caia, Xiong Zhanga, Junjie Zhanga,b, Jingai Shaob,c,* a

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong

University of Science and Technology, Wuhan 430074, China b

Department of New Energy Science and Engineering, School of Energy and Power Engineering,

Huazhong University of Science and Technology, Wuhan, 430074, China. c

Shenzhen Huazhong University of Science and Technology Research Institute, Shenzhen, 523000,

China.

*Corresponding author: Jingai Shao. E-mail address: [email protected]. Tel.: 086+027-87542417. Fax: 086+027-87545526

ABSTRACT The pollution of heavy metals especially cadmium in waste water is a big concern for environment and human beings. In this study, biochar was prepared by pyrolysis of bamboo particles after the impregnation of phosphate (Na2HPO4). The biochars before and after modification were characterized by environmental scanning electron microscope (ESEM), Brunet-Emmet-Teller (BET), and X-ray diffraction (XRD) analyses. The results indicate that the biochar surface properties are improved and the phosphate compounds are chemically bound to the functional groups on the biochar surface. The adsorption of Cd (II) by the modified biochar obeys

1 / 28

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

pseudo-first-order kinetic model and Langmuir adsorption isotherm model. Results of batch adsorption experiments demonstrate that the phosphate modification increases the absorption capacity of pristine biochar by almost 10 times to 202.66 mg/g and increases the removal efficiency of Cd (II) by 85.78%. The wastewater treatment process in a continuous-flow stirred tank reactor (CSTR) was modeled by using the PHREEQC-3, considering the influence of various operational parameters. Simulation results show that the modified biochar has a better performance during the wastewater treatment, moreover, the shorter residence time and the larger fluid pH value are favorable. Kew words: Biochar, Phosphate modification, Cd (II), Wastewater, Adsorption

1. Introduction The presence of heavy metals in the aqueous system has become a major concern in the present scenario, posing a potential threat to aquatic lives and human beings, due to their high toxicity and carcinogenicity.1,

2

Unlike organic pollutants, heavy metals are insusceptible to

biological degradation, but they persistently remain in the ecosystem.3 Cd (II) is one of the top-priority toxic heavy metals, the toxicity limit (LD50) of which in the rat is 225 mg/kg, and the excessive intake of cadmium can cause health problems like renal tubular dysfunction, bone damage, and other diseases.4 Cadmium is widely used in industries like zinc production, corrosion-resistant plating, glass coloring, and plastic stabilization, which may result in serious pollution to water or soil system without proper treatment. Thus, removal of Cd (II) from wastewater or soil is a prior step for better managing cadmium and avoiding cadmium pollution. Recently, adsorption and removal of heavy metals, by using various adsorbents, including cement kiln dust, clay minerals, sawdust and straw, activated carbon, and mesoporous silica, have 2 / 28


Page 2 of 28

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

been widely investigated.5,

6

Compared to other adsorbents, carbon material, such as activated

carbon, fullerene, and graphene, is considered as one of the most promising and efficient absorbents used in pollutants removal for their high stability, large surface area and pore volume, and broad pore size distribution.7-9 For example, biochar produced from fast pyrolysis of wood and bark during bio-oil production has been used to remove arsenic, cadmium, and lead from water by Mohan, et al. 10. The bamboo biochar has also been used as a low cost adsorbent for both organic pollutants and heavy metal removal from water due to the high BET surface area and various of functional groups (e.g., amino, carboxyl, and hydroxyl groups).11,

12

However, the adsorption

performance of the pristine bamboo biochar is not so good since the contents of the surface functional groups are still insufficient for heavy metal removal. To enhance the capacity and selectivity of biochar for pollutant removal, many attentions have been attracted to surface modification of carbon materials with more functional groups in recent years.13, 14 Many attempts to chemical modification, for example, citric-acid modification, amino modification, polyethylenimine modification, and some other methods, are used to improve the functional group contents of biochars and thus enhance their adsorption performance.15, 16 The phosphate compounds, as one of the typical agents used in remediation of soil, have a strong removal ability for heavy metals in wastewater. When soluble PO43- is added to wastewater, a continuum of different stabilization reaction mechanisms can occur. These reactions range from surface sorption of metals to particulate surfaces, through the formation of new surface metal precipitates, to the formation of discrete heterogeneous or homogeneous metal precipitates.17, 18 Therefore, it can enhance the sorption capability of the adsorbents and increase the removal efficiency of the heavy metals (such as Cd) when the phosphate is modified to the adsorbent surface. 3 / 28


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

However, there are very few studies focused on the phosphates modification of biochar so far. Therefore, in this work, the biochar obtained from the Na2HPO4 modification and pyrolysis of bamboo offcut is used for Cd (II) removal. The batch adsorption experiments are performed to compare the adsorption capacities and Cd (II) removal efficiencies of the biochars before and after the Na2HPO4 modification. Based on this, the optimal pyrolysis temperature and application fraction of the Na2HPO4 are determined. Several characterization methods, like environmental scanning electron microscope (ESEM), Brunet-Emmet-Teller (BET), and X-ray diffraction (XRD) analyses, are employed to investigate the microstructure and surface morphology of the pristine and modified biochars. The adsorption kinetics and adsorption isotherms of the biochars are determined to reveal the adsorption behavior of Cd (II) and investigate the interaction mechanism between Cd (II) and biochar. Moreover, to further estimate the removal performance of modified biochar, the geochemistry model PHREEQC-3 is applied to model and predict the wastewater treatment process in a continuous-flow stirred tank reactor (CSTR). The influence of adsorbent performance, residence time, and solution pH are evaluated.

2. Materials and Methods 2.1 Samples preparation The bamboo offcut generated during the processing of bamboo-based furniture was washed with deionized water to remove all dirt. The washed product was firstly air-dried, and then ground to sieve through screen mesh with the size of 0.25 mm. The bamboo particles were dried in an oven at 105 oC for 8 h and then chemically treated as follows. About 20 g of the powder was dissolved in a 250 mL 0.56 mol/L Na2HPO4·2H2O solution with a mass ratio of 1:1 (bamboo particles to 4 / 28


Page 4 of 28

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Na2HPO4). The mixture was then placed in a water bath with magnetic stirring for 6 h under the temperature of 50 oC. After the soaking, the suspension was cooled to room temperature. The solid material was separated by vacuum filtration, washed with deionized water and then dried at 105 oC. The bamboo particles were also chemically treated by Na2HPO4 with the mass ratio of 1:2 and 1:0.5 following the same procedure, respectively. The biochar was produced by the pyrolysis of both the phosphate-modified and untreated granular bamboo according to previous study.19,

20

The pyrolysis was conducted in a vertical

fixed-bed reactor under oxygen-limited conditions. In each trial, about 20 g of the bamboo particles were added and thermally treated in the pyrolysis system for 2 hours. A series of temperatures (350 o

C, 550 oC, and 750 oC) were prescribed for the pyrolysis to choose the most adequate temperature.

After the pyrolysis, the solid residual was collected and repeatedly washed with distillated water and dried at 105 °C for 48 h. The products obtained from pyrolysis of phosphate-treated and untreated bamboo particles were designated as pm-BC and BC, respectively. Both biochar samples were used as sorbents to remove Cd (II) from aqueous solutions in the subsequent experiments.

2.2 Characterization of samples The phase compositions of biochar samples characterized before and after chemical modification were analyzed by X-ray diffraction (XRD, X’Pert PRO, PANalytical B.V., Netherlands) and environmental scanning electron microscope (ESEM, Quanta 200, FEI, Netherlands). The surface area and porosity of the samples was conducted by ASAP 2020 (Micromeritics Instrument Corp, USA). The detailed parameters and methods were described in our previous work.21

2.3 Batch adsorption experiments The adsorption performance of the biochar was evaluated in the batch adsorption of Cd (II) at 25 5 / 28


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

o

C. In each trial, 1 g/L of the biochar was mixed with Cd (II) solution in a polyethylene bottle. Then

the mixture was shaken in a rotary shaker at 120 rpm for 24 h. Afterwards, the suspension was filtered with a syringe filter with the pore diameter of 0.2 µm, and the Cd (II) concentration in filtrate was determined. The concentration of Cd (II) in the filtrate was measured using an inductively coupled plasma mass spectrometry (ICP-MS) (ELAN DRC-e, PerkinElmer, USA) after leachate dilution. In order to determine adsorption isotherm and kinetics, the pm-BC with the best adsorption performance, produced with the favored mass ratio of bamboo particles to Na2HPO4 under the optimized pyrolysis temperature, was used in the experiments. The equilibrium adsorption isotherm experiments were conducted by varying the initial Cd (II) concentration in the range of 10~500 mg/L. The adsorption kinetics was determined by parallel experiments during a 12-hour observation, in which the Cd (II) concentration of subsamples was measured hourly. The adsorption isotherm and kinetics of Cd (II) with the BC adsorbent were also determined to compare with that of pm-BC adsorbent. Moreover, each trial was repeated for three times for experimental accuracy. The removal efficiency of Cd (II) was measured using the following equation Removal efficiency (%) =

∗ 100

(1)

Where C0 (mg/L) and Ce (mg/L) are the Cd (II) concentrations at the initial and at the equilibrium state, respectively.

3. Wastewater treatment simulation A continuous-flow stirred tank reactor (CSTR) shown in Figure 1 is widely applied in chemical- and bio-engineering for water treatment and chemicals production. The case models a wastewater treatment process when a pulse of water containing NO3- and cadmium is injected into the CSTR. The modeling is solved with PHREEQC-3 based on the theory of Tebes-Stevens and

6 / 28


Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

others.22 Except for the kinetics adsorption of Cd (II), the case also contains several interacting chemical processes that are common in wastewater treatment process: aqueous equilibrium reactions, denitrification by denitrifying bacteria, and growth of biomass (microbes). The CSTR contains no NO3- or cadmium initially, but has a certain concentration of biochar (3.75g/L) and biomass (0.35mol/L), or a solution with each 3 mmol/L of NO3- and cadmium. A constant boundary condition is applied at the inlet of the CSTR, and the closed for outlet. The physical processes considered in this case are mainly diffusion and homogeneous mixture of the solutes. For the treatment of wastewater, the residence time in the reactor is: τ=

(2)

/

Without reactions, an initial concentration in the tank changes as: = , ∗

/!

(3)

Where V denotes water volume (L) and c denotes solutes concentration (mmol/L). The subscripts tank and in mean the inside and inlet of the tank, respectively.

Figure 1. Schematic diagram of CSTR The NO3- is assumed to degrade in the presence of hydrogen-producing bacteria and hydrion by 7 / 28


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

the reaction:23 NO3- + H+ + 2.5 H2 → 0.5 N2 + 3 H2O

(4)

Where H2 is released by the hydrogen-producing bacteria during the degradation of substrate. Biodegradation relies on the catalyzing action of enzymes produced by microbes. Therefore, the maximal rate depends on the concentration of the enzymes or the bacteria. The variable kmax in the Monod rate equation 24 can be rewritten to account explicitly for the concentration of bacteria: "# "

'

= $%&

#

( )/*,+,- .#

∗

/

)/*,0* ./

(5)

Where µmax is the bacterial growth rate (s-1), B is concentration of biomass (mol/L) and Y is the yield factor (mol biomass/ mol substrate) which expresses how much of substrate is converted into biomass. S is the concentration of NO3- (mg/L), H is the concentration of hydrogen-producing bacteria (mg/L), t is time (s), and k½ is the half-saturation constant (mg/L). As substrate is degraded and transformed into energy and organic molecules which can be used for biosynthesis, the number of bacteria increase: "' "

= 1

"# "

2'" ∗ 3

(6)

The kBd denotes the bacterial death rate (s-1) and the equation (5) and (6) are coupled. Biodegradation augments when the microbes increase in number, and more biodegradation produces more bacteria. The decrease of substrate concentration first becomes notable when sufficient microbes are present, and the process stops when all the substrate has been consumed. Tebes-Steven and Valocchi 25 defined kinetic sorption reactions for Cd2+ by the rate equation: 4" = 2% "

56

7

(7)

Where RCd is the rate Cd2+ adsorption, si is the sorbed concentration (mol/g biochar), km is the mass transfer coefficient (h-1), and kd is the distribution coefficient (L/g). 8 / 28


Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

In this modeling, all kinetic reactants are assumed to be immobile, so that the sorbed species are not transported and the concentration difference along the vertical direction of the CSTR is ignored. The biomass reaction adds “H 0.0”, or zero moles of hydrogen, in other words, it does not add or remove anything from solution. The assimilation of carbon and nutrients that is associated with biomass growth is also ignored in this simulation. The kinetic parameters used in this modeling are listed in Table 1. Table 1. Kinetic parameters used in the wastewater treatment modeling Parameter

Description

Parameter value

Source

B

Concentration of biomass

0.35 mol bacteria / L

Appelo and Postma 23

Y

Yield factor of microbes

0.34 mol bacteria / mol NO3-


µmax

Growth rate of bacteria

1.01e-5 s-1


k1/2,NO3

Half-saturation constant for NO3-

1.21e-5 mol /L


k1/2,H2

Half-saturation constant for H2

1.14e-2 mol /L


kBd

Death rate of bacteria

1.37e-9 s-1


km,B

Mass transfer coefficient for pristine biochar

1.69 h-1

Experimental data

km,p

Mass

modified 0.47 h-1

Experimental data

transfer

coefficient

for

biochar kd,B

Distribution coefficient for pristine biochar

0.1249 L / g

Experimental data

kd,p

Distribution coefficient for modified biochar

26.20 L / g

Experimental data

4. Results and discussion 4.1 Characterization of biochars 4.1.1 Environmental scanning electron microscope (ESEM) analysis ESEM was conducted to observe the microstructure of the biochar. Figure 2 shows the ESEM images of BC0c (unmodified biochar with the pyrolysis temperature of 750 oC), pm-BC3c (with the biomass to Na2HPO4 mass ratio of 1:0.5 at the pyrolysis temperature of 750 oC), and pm-BC3c with 9 / 28


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

Cd (II) adsorbed. Figure 2a shows that the vascular bundles of pristine bamboo tends to agglomerate and biochar demonstrates a turbostratic crystallitic structure after the thermally treatment, which may be due to the enhanced graphitization effect.26 Moreover, after the chemical modification, the smooth surface of the bamboo biochar becomes coarse and it is covered with granules, as shown in Figure 2b. The covered white collective dots indicates Na2HPO4 nanoparticles are dispersed well on the surfaces of the biochar. After Cd (II) absorption, as is shown in Figure 2c, the previously observed granules loaded on the pm-BC3c decrease dramatically, which indicates that Na2HPO4 may react with Cd (II) and other compounds are produced. Further examinations were conducted through XRD analysis shown in Figure 3.

(a)

(b)

(c)

Figure 2. ESEM images of (a) BC0c (b) pm-BC3c and (c) pm-BC3c after Cd (II) adsorption 4.1.2 Porosity analysis of biochar The surface area, pore volume and average pore diameter of bamboo and biochar samples are shown in Table 2. The pm-BC3c is observed to have a BET surface area of 65.31 m2/g which is 14 times larger than that of raw bamboo while much smaller than that of BC0c. The micropore surface area of raw bamboo increases from 0.01 m2/g to 35.63 m2/g after thermally treatment and sequentially increases to 56.05 m2/g due to chemical modification. The BET results reveal that the 10 / 28


Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

modification leads to an increase in cumulative pore volume. Especially, the pore volume increased from 1.35 cm3/g for the BC0c to 3.95 cm3/g for the pm-BC3c. It is also observed that the pores of pm-BC3c are in microporous form whereas those of raw bamboo and BC0c are not, which can be more intuitively found in the evolution of the average pore diameters. The combination of micropores with the larger pores due to the chemical modification can explain the increase in pore volume and decrease in surface area. Above all, the thermally and chemically treatment can both increase the surface area and pore volume of raw bamboo. After Na2HPO4 modification, the synthesized material becomes more porous and gains the higher absorption capacity. Table 2. Surface area, pore volume and average pore diameter of bamboo and biochars Samples Parameters

Raw bamboo

BC0c

pm-BC3c

BET surface area (m /g)

4.30

80.29

65.31

Micropore surface area (m2/g)

0.01

35.63

56.05

Cumulative volume of pores (cm3/g)

0.01

1.35

3.95

Cumulative volume of micropores (cm3/g)

0.01

0.45

3.84

Average pore diameter (nm)

57.53

22.41

4.57

2

4.1.3 X-ray diffraction (XRD) analysis The high-angle XRD diffractograms (Figure 3) are used to identify the crystalline phases formed upon both the BC0c and pm-BC3c, and also the modified biochar after adsorbing cadmium. As is shown in Figure 3a, the peaks of the BC0c at 20.87o and 26.62o agree well with the inorganic components such as SiO2 (XRD standard gallery PDF card No. 05-0490). The diffraction peaks of the modified biochar at 20.92o, 26.69o, 30.12o and 31.29o shown in Figure 3b belongs to the combination of Na, P, and O, probably NaPO3 (XRD standard gallery PDF card No. 11-0648). Moreover, the peaks of 31.29o and 34.10o belong to P2O5 (XRD standard gallery PDF card No. 11 / 28


Energy & Fuels

023-1301), which indicates that Na2HPO4 is loaded on the biochar. After adsorbing cadmium, the peaks of the modified biochar at 30.02o, 34.03o and 40.62o agree well with those in CdCO3 (XRD standard gallery PDF card No. 085-0989). Therefore, the removal process of cadmium by the Na2HPO4-modified biochar, in addition to physical absorption, also includes chemical reactions.

Counts

The details of the reaction process need to be further discussed. 1400 1200 1000 800 600 400

(a) 20.87

Counts

5

10

15

1400 1200 1000 800 600 400 200

20

26.62

25

30

35

31.29 26.69 30.12 24.14 20.92 34.10

5

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

10

15

1800 1500 1200 900 600 300

20

25

30

35

40

45

50 (b)

39.31

40

45

50

26.62 20.83 24.18 30.02 31.21

10

15

20

25

30

55 (c)

34.03

5

55

35

40.61

40

45

50

55

2θ (Degree) Figure 3. XRD spectra of (a) BC0c (b) pm-BC3c and (c) pm-BC3c after Cd (II) adsorption

4.2 Adsorption performance of pm-BCs In order to select the most suitable impregnation ratio (mass ratio of Na2HPO4 to raw bamboo) and pyrolysis temperature for chemical and thermal treatment on the bamboo particles, adsorption capacity and Cd (II) removal efficiency of the pm-BCs were compared. As shown in Figure 4, the

12 / 28


Page 13 of 28

adsorption capacity of unmodified biochar (BC0) as well as the biochars with impregnation ratio of 1:0.5, 1:1, and 1:2 (namely, BC1, BC2, and BC3, respectively) under different pyrolysis temperatures were graphed. It can be seen that the pyrolysis temperature has limited influence on the adsorption capacity of both pristine biochar and modified biochars except for the pm-BC3, the capacity of which increased from 131.26 mg/g to 238.71 mg/g with the pyrolysis temperature increasing from 350 oC to 550 oC. By comparison, the pyrolysis temperature of 550 oC may be more applicable for raw bamboo thermal treatment, which is due to the increase of aromatic groups of biochar under the higher temperature.27 According to Chen, et al. 28, the aromatic groups enhanced partition effect during the adsorption process. On the other hand, the adsorption capacity is affected by the impregnation ratio largely. The best adsorption capacity is gained with the impregnation ratio of 1:2 and the higher ratios, according to the similar studies,29-31 might impede the production of micropores and result in the smaller surface area of biochars, hence the lower adsorption capacity is obtained. 250 o

Adsorption capacity (mg Cd(II)/g biochar)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

200

350 C o 550 C o 750 C

150

100

50

0 BC0

pm-BC1

pm-BC2

pm-BC3

Samples

Figure 4. Adsorption capacity of BC and various pm-BCs produced at different temperatures. 13 / 28


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

The removal efficiency of BC and various pm-BCs to 50 mg/L of Cd (II) solution were listed in Table 3 and the results agree well with that of the adsorption capacity. It can be concluded that, after modification by Na2HPO4, the removal performance of biochars to Cd (II) is increased dramatically. Especially, the removal efficiency of pm-BC3 with a pyrolysis temperature of 750 oC reaches as high as 97.49 %, which is more than 10 times of that of the unmodified biochar. Based only on comparison results of the removal efficiency of various biochars, the pm-BC3c (impregnation ratio of 1:2, pyrolysis temperature of 750 oC) was used to investigate the adsorption kinetics and adsorption isotherms of Cd (II). While, in view of the general performance as well as the energy efficiency, the pm-BC3b (impregnation ratio of 1:2, pyrolysis temperature of 550 oC) may be the most favorable adsorbent for further industrial applications. Table 3. Removal efficiency of BC and various pm-BCs produced at different temperatures to 50 mg/L of Cd (II) solution. Pyrolysis temperature (oC)

Removal efficiency (%) BC0

pm-BC1

pm-BC2

pm-BC3

350

10.51±0.51

25.99±1.17

78.81±3.70

54.70±2.61

550

5.30±0.25

21.59±1.03

86.58±4.31

97.47±4.68

750

9.11±0.43

18.10±0.83

75.54±2.95

97.49±4.33

4.3 Adsorption kinetics and adsorption isotherms 4.3.1 Adsorption kinetics Figure 5 represents the effect of contact time on cadmium uptake in the modified and unmodified biochars. The sorption of Cd (II) to both BC0c and pm-BC3c samples shows two phases: a rapid initial sorption during the first three hours followed by a much slower phase till reaching the adsorption equilibrium after about 10 hours. The rapid sorption phase could be due to 14 / 28


Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

rapid occupation of easily accessible external sorption sites like outer sphere complexation.32 The second phase could be ascribed to the formation of inner layer complexes. Moreover, the kinetic inhibition of cadmium movement through narrow pore channels may be another cause. Comparing to BC0c, the pm-BC3c shows the faster and higher sorption of cadmium, indicating that the biochar gains larger adsorption sites after modification by the Na2HPO4. As is shown in Figure 5, the pseudo-first-order (Eq. (8)), the pseudo-second-order models (Eq. (9)) and the intraparticle diffusion model (Eq. (10)) are used to describe the sorption kinetic data:

ln:; : = ln :; 2< =

>

=

*

(8) (9)

>

: = 2? = .A B

(10)

Where qt and qe are the amounts of sorption cadmium at time t and at the equilibrium respectively. k1, k2, and kip are the first-order, second-order and intraparticle diffusion apparent adsorption rate constants, respectively. C is a constant for the intraparticle diffusion model. The sorption kinetics data fit well with both the pseudo-first-order and pseudo-second-order models. Especially, the corresponding R2 of first-order model reached 0.89 for BC0c and 0.96 for pm-BC3c (Table 3), which are larger than that of the pseudo-second-order model. According to Dechow, et al.

33

, the pseudo-first-order and pseudo-second-order models describe the kinetics of

the solid-solution system based on mononuclear and binuclear adsorption, respectively, with respect to the sorbent capacity. Good fit of pseudo-first-order model implies that the adsorption of Cd (II) by both pristine and modified biochars tend to be mononuclear. In addition, the intraparticle diffusion model was also applied to further analyze the adsorption kinetics and to assess the importance of diffusion during the adsorption process. The intraparticle 15 / 28


Energy & Fuels

diffusion model is also well fitted with a R2 of 0.7 for the BC0c, indicating that cadmium was retained with other possible mechanisms. In addition to the diffusion of Cd (II) through the solution to the exterior surface of the composite biochar, the gradual sorption may also due to the intraparticle diffusion that is rate-limiting. The intraparticle diffusion model does not fit well for the pm-BC3c, this may be due to the diameter of micropore which is relatively small compared to the larger molecule-size of cadmium. 240 210

Amount of adsorpted Cd (II) (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

180 150 pm-BC3c BC0c Pseudo-first order kinetic model Pseudo-second order kinetic model Intraparticle diffusion kinetic model

120 90 60 30 0 0

1

2

3

4

5

6

7

8

9

10

11

Adsorption time (h)

Figure 5. Kinetics of cadmium adsorption onto the modified (pm-BC3c) and raw biochar (BC0c) at 25 oC (initial Cd (II) 210 mg/L; pH = 7.2) Table 4. Kinetics models parameters for the adsorption of Cd (II) onto BC0c and pm-BC3c.

Sample

Pseudo-first order kinetic model

Pseudo-second order kinetic model

Intraparticle diffusion model

qe (mg/g)

k1 (h-1)

R2

qe (mg/g)

k2 mg/(g·h)

R2

kip mg/(g·h0.5)

C (mg/g)

R2

pm-BC3c

202.28

2.11

0.96

224.34

0.01

0.90

51.82

87.13

0.37

BC0c

23.31

0.59

0.89

28.94

0.02

0.85

8.48

0.33

0.70

4.3.2 Sorption isotherms In order to further investigate the adsorption properties of the pristine and modified biochar, 16 / 28


Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

the adsorption equilibrium isotherms of Cd (II) were determined at 25 oC and pH = 7.0. The sorption data were analyzed on the basis of the Langmuir, Freundlich, and Langmuir-Freundlich models as expressed in Eqs. (11)-(13), respectively:

:; =

>C DE

(11)

Evaluation and Prediction of Cadmium Removal from Aqueous

Recommend Documents