Bioresource Technology 133 (2013) 495–499

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Bioresource Technology

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Kinetics and mechanisms of hydrogen sulfide adsorption by biochars

0960-8524/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.biortech.2013.01.114

⇑ Corresponding author. Tel.: +86 21 34206925.E-mail address: gqsh@sjtu.edu.cn (G. Shen).

Guofeng Shang a, Guoqing Shen a,⇑, Liang Liu a, Qin Chen b, Zhiwei Xu c

a School of Agriculture and Biology, Key Laboratory of Urban Agriculture (South), Ministry of Agriculture, Shanghai Jiao Tong University, Shanghai, PR Chinab Anju Environmental Protection Technology Co., Ltd., Shanghai, PR Chinac Suzhou Faith & Hope Membrane Technology Co., Ltd., PR China

h i g h l i g h t s

" Biochars derived from agricultural/forestry wastes were a promisingadsorbent of H2S.

" H2S breakthrough capacity is relatedto local pH within the pore system ofbiochars.

" The adsorption kinetics of H2S bybiochars was modeled by Michaelis–Menten equation.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 October 2012Received in revised form 15 January 2013Accepted 17 January 2013Available online 7 February 2013

Keywords:Hydrogen sulfideBiocharsKineticsAdsorptionPyrolysis

a b s t r a c t

Three different biochars as cost-effective substitutes for activated carbon (AC) were tested for theirhydrogen sulfide (H2S) adsorption ability. The biochars were produced from camphor (SC), bamboo(SB), and rice hull (SR) at 400 �C by oxygen-limited pyrolysis. The surface area (SA), pH, and Fourier trans-form infrared spectras of the biochars and AC were compared. The maximum removal rates and the sat-uration constants were obtained using the Michaelis–Menten-type equation. The three biochars werefound to be alkaline, and the SAs of the biochars were much smaller than that of the AC. The H2S break-through capacity was related to the local pH within the pore system of the biochar. The order observed interms of both biochar and AC adsorption capacity was SR > SB > SC > AC. SR efficiently removed H2Swithin the inlet concentration range of 10–50 lL/L. Biochars derived from agricultural/forestry wastesare a promising H2S adsorbent with distinctive properties.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Hydrogen sulfide (H2S) is one of the most common compoundsthat can be found in petrochemical plants, coal gasification plants,wastewater treatment plants, man-made fiber paper, and otherproduction processes (Latos et al., 2011; Lebrero et al., 2011). Thiscompound is extremely toxic to the central nervous system even atlow doses and corrosive to concrete and steel (Burgess et al., 2001;

Lee et al., 2006). H2S is a major air pollutant and also a cause of rainacidification.

Numerous studies on H2S adsorption using activated carbon(AC) have been conducted because of the increase in deodorizationproblems (Bagreev and Bandosz, 2000; Bashkova et al., 2009).However, the manufacture of AC requires high temperature, highpressure, and an activation process (Boehm, 1994). Traditionally,only ACs impregnated with caustics were considered suitablematerials. Although caustic-impregnated and catalytic carbonshave been proven to work efficiently as H2S removers, certain dis-advantages with the application of such carbons have been ob-served, including (i) self-ignition at low temperature, (ii) low

496 G. Shang et al. / Bioresource Technology 133 (2013) 495–499

capacity for physical adsorption attributable to the filling of thepore system with the impregnate, (iii) special precautions are re-quired for use with alkalis, and (iv) difficulties in regeneration afterwashing with water. All aforementioned factors directed the atten-tion of numerous studies toward unmodified, as-received ACs.

As a precursor of AC, biochars have received considerable atten-tion in the past decades (Azargohar and Dalai, 2006; Hayes, 2006;Renner, 2007; Moussavi and Khosravi, 2012). Biochar is the car-bon-rich product of the thermal decomposition of organic materialunder a limited supply of O2 and at relatively low temperatures(<700 �C) (Hale et al., 2011). Biochar has been known to act as asuper-sorbent for organic contaminants in soil/sediment (Pandeyet al., 1997; Sattar et al., 1991; Lou et al., 2012; Inyang et al.,2012). Biochar and AC differ primarily in their preparation method,source material, and the resulting physiochemical properties of theproducts. In contrast to AC, biochar use could be a cheaper remedi-ation technology as the waste source materials would essentially befree, and the production of biochar at lower temperatures is moreenergy-efficient and less cost-intensive (Kumar et al., 2006) thanAC production. A previous study reported on the potential of bio-char derived from camphor to adsorb H2S at various temperatures(100–500 �C) and demonstrated that the different sizes of biocharsand the different pyrolysis temperatures for the camphor particlemarkedly affect H2S adsorption. The biochar with particle size rang-ing from 0.3 to 0.4 mm possesses a maximum sorption capacity at apyrolysis temperature of 400 �C (Shang et al., 2012). Further studiesmust be conducted to understand better the mechanisms of biocharH2S adsorption because biochar characteristics depend not only onthe pyrolysis temperature but also on biochar feedstock.

This study aimed to determine the efficiency of H2S adsorptionby three different biochars derived from camphor, bamboo, andrice hull at a pyrolysis temperature of 400 �C. A comparative studywith AC was also conducted. Removal kinetics were used to evalu-ate the H2S removal rate of biochars.

2. Methods

2.1. Materials

Shell-derived AC is a commercial product purchased fromChangzhou Bihai Environmental Protection Technology Co., Ltd.Three different biochars were produced from agricultural/forestrywastes such as camphor (SC), rice hull (SR), and bamboo (SB).The wastes were cut into small pieces, washed, and baked in theoven at 60 �C for 48 h. The pieces were then ground into small par-ticles by using a crusher. The sizes of the particles were determinedto be between 0.3 and 0.4 mm after screening. The waste was pyro-lyzed at 400 �C under an O2-free atmosphere in a ceramic fibermuffle furnace. The heating rate was initially set at 10 �C/minand increased to the selected pyrolysis temperature, with a holdingtime of 5 h at the final temperature. The samples were cooled byventilation to 30 �C with nitrogen gas.

2.2. Physicochemical characteristics

The pH of the carbon surface was measured in deionized waterwith a 1:5 (wt/wt) ratio. Samples were thoroughly mixed and al-lowed to equilibrate for 1 h. The pH was measured with a digitalpH meter. Carbon and nitrogen contents were determined by ele-mental analysis–stable isotope ratio mass spectrometer (Vario ELIII/Isoprime, Germany). The ash content of the biochars and ACwas measured as follows: the samples of the biochars and AC wereplaced into crucibles. The crucibles were placed into a muffle fur-nace and baked at 900 �C for 2 h and then cooled down at roomtemperature. The ash content was obtained by calculating the

difference between the mass of the biochars and AC before andafter baking.

2.3. Measurement of H2S breakthrough curves

To evaluate the removal capacity of biochars, H2S adsorptionexperiments were performed at room temperature using a labora-tory-scale apparatus (Fig. 1). The three biochars were packed intoquartz glass columns (inner diameter = 12 mm; height = 300 mm).The biochar bed was 150 mm high, with a bulk density of 0.12 g/cm3 and a porosity of 35%. The quartz sand was packed aboveand below the biochar bed. The source gas containing 50 lL/LH2S and 500 lL/L water vapor was passed through the column ofthe adsorbent at 40 mL/min. The inlet H2S concentration was chan-ged from 10 to 50 lL/L by diluted compressed air. The load of H2Sinto the columns was changed by controlling the inlet concentra-tion and/or space velocity. The outlet H2S was collected by airbagsevery 2 min and monitored using gas chromatography (GC). Thetest was stopped at the breakthrough concentration of 50 lL/L.After every test, excess H2S was absorbed using a sodium hydrox-ide solution. The adsorption capacity of each biochar was calcu-lated by integration of the area above the breakthrough curves.From the H2S concentration in the inlet gas, flow rate, break-through time, and mass of biochar were also calculated.

2.4. GC chromatography

The concentration of H2S in the exhaust gases from the biocharbed was monitored using a Shimadzu GC (Model GC-2010). Theseparation was conducted at CNW (a German company) CD-1 col-umn (bonded dimethyl siloxane = 100%, length = 30 m, internaldiameter = 0.32 mm, and df = 5.00 lm). The GC oven heating pro-cedure was conducted as follows: (1) initial heating temperatureof 80 �C for 2 min; (2) temperature increased to 150 �C at 20 �C/min; and (3) temperature held at 150 �C for 18 min. The injectortemperature was 80 �C. An FPD detector with a sulfur filter andan opening temperature of 220 �C was used.

2.5. Fourier transform infrared (FTIR) analysis

The FTIR spectra of the biochar samples were obtained usingdiffuse reflectance FTIR spectroscopy. The biochars were groundto 0.1 and 0.5 mg of each sample was placed onto the Ge windowof a Nicolet 5700 FTIR with an attenuated total reflectance attach-ment (OMNI Sampler Nexus). Spectra were obtained over 256scans with a KBr beam splitter, set at a resolution of 4 cm�1, rang-ing from 4500 to 650 cm�1 and with an aperture size of 34 cm. Thereflectance was measured and analyzed using OMNIC v7.1 withHapp–Genzel apodization and Mertz phase correction.

3. Results and discussion

3.1. Characterization of the biochars

The physicochemical characteristics of the three differentbiochars and AC used in this experiment are shown in Table 1. Bio-mass type significantly affects pH and SA. The SA of the biocharsand AC ranged from 20.35 to 850.00. All biochars were alkaline,whereas AC was neutral. The highest pH was 10.56 for SR, andthe lowest was 7.05 for AC. These values are typical for most bioch-ars generated at high temperature (Lehmann and Joseph, 2009).Higher SR pH suggests potential to adsorb acidic H2S (Dudka andAdriano, 1997). Among the biochars, the highest SA was 115.34for SR, which was 7.3 times lower than that of AC. AC is a high-sur-face-area carbon, which is most often produced by physical and

Fig. 1. Schematic of a laboratory for H2S removal. 1. Compressor; 2. Flow meter; 3. H2S gas; 4. Valve; 5. Quartz sand; 6–8. Biochars; 9. GC; 10. NaOH solution.

Table 1Physicochemical characteristics of the three different biochars and activated carbon(AC).

Properties AC SC SB SR

pH 7.05 ± 0.03 9.55 ± 0.02 10.21 ± 0.01 10.56 ± 0.02Mass loss (%) – 70.22 ± 0.05 75.26 ± 0.04 64.78 ± 0.02Ash content (%) 51.23 ± 0.03 53.32 ± 0.05 45.87 ± 0.07 48.54 ± 0.06N (%) 0.61 ± 0.06 0.66 ± 0.08 0.08 ± 0.04 0.72 ± 0.06C (%) 54.53 ± 0.04 62.94 ± 0.06 46.32 ± 0.07 53.59 ± 0.05SAa (m2/g) 850 ± 0.15 20.35 ± 0.14 58.01 ± 0.12 115.34 ± 0.18

a SA, BET-N2 specific surface area.

0 250 500 750 1000 1250 1500 1750 2000

0

10

20

30

40

50

60

Time (Min)

Con

cent

ratio

n (µ

L/L

)

Fig. 2. Breakthrough curves of H2S on different biochars.

G. Shang et al. / Bioresource Technology 133 (2013) 495–499 497

chemical activation. The AC SA was several times larger than thatof the precursor (Azargohar and Dalai, 2006). However, as a precur-sor of AC, biochars are produced without chemical activation. TheSAs of the biochars were smaller than that of AC. The biomass typeshad no significant effect on ash, N, and C content and mass loss.The average ash, N, and C content of the three biochars were49.24%, 0.49% and 53.28%, respectively. No significant differenceswere found compared with AC. The pH and SA are key factors forcarbon material to absorb H2S; however, no evidence showed thatthe change of elemental composition affected the H2S adsorptioncapacity (Hung et al., 2000).

The important peaks of FTIR spectra of the three biochars andthe AC are characterized below. A broad peak at 3420 cm�1, whichis associated with the hydroxide (OH) stretching vibration in SRwas the strongest among all samples, indicating that the OH con-tent of SR was more than that of SC, SB, and AC. A peak appearedin all samples at 1730 cm�1

, corresponding to the C@O stretchingvibration. Compared with the three biochars, the C@O stretchingvibration was weakest in the AC. For SR and SB, the sharp peakappearing at the wave number of 1433 cm�1 can be used to con-firm the COO stretching vibration. The carboxyl that lost hydrogenions became the COO�. Therefore, the COO group is alkaline. Nosuch peak appeared at the wave number of 1433 cm�1 in SC andAC. The FTIR spectra are consistent with the pH of the three bioch-ars and the AC in Table 1.

3.2. H2S breakthrough capacity

The H2S breakthrough curves for the three different biocharsand the AC are shown in Fig. 2. The breakthrough time measuredfor AC was short, and the breakthrough curve was rather steep,indicating that the material is a poor H2S adsorbent. By contrast,

the breakthrough times of the three biochars were longer, espe-cially those of SB and SR, indicating better retention of H2S on bio-char compared with AC. The breakthrough capacities derived fromthe curves are summarized in Table 3. AC exhibited a low capacity(35.6 mg/g), whereas SR seemed a good H2S adsorbent with acapacity of 382 mg/g. The order in terms of adsorption capacityfor biochars and AC is SR > SB > SC > AC. On the basis of the surfacechemistry and the performances of carbons, Adib et al. (1999a,b)suggested that the H2S breakthrough capacity is governed by localpH within the pore system. Acidic pH suppresses dissociation ofH2S; consequently, its oxidation to sulfur is limited. The proposedmechanism involves (Eq. (1)) H2S adsorption on the C surface, (Eq.(2)) H2S dissolution in a water film, (Eq. (3)) dissociation of H2S inan adsorbed state in the water film, (Eq. (4a)) surface reaction ofadsorbed O2 with the formation of elemental sulfur (Eq. (4b)) orsulfur dioxide, and (Eq. (5)) further oxidation of SO2 to H2SO4 inthe presence of water (Adib et al., 2000, 1999a,b).

H2Sgas!KH H2Sads ð1Þ

H2Sads!Ks H2Sads-liq ð2Þ

H2Sads-liq!Ka HS�ads þHþ ð3Þ

Table 2Maximum and complete removal capacity of H2S by stationary bed using three biochars as well as maximum removal rates (Vm) and saturation constants (Ks) obtained by kineticanalysis.

Biochars Maximum removal capacity (lL/L) Complete removal capacity (lL/L) Vm (g S/kg dry material h) Ks (lL/L)

SC 43.8 ± 0.04 39.2 ± 0.10 0.036 ± 0.01 9.90 ± 0.12SB 46.5 ± 0.05 40.9 ± 0.12 0.058 ± 0.02 28.72 ± 0.11SR 49.2 ± 0.03 45.4 ± 0.09 0.070 ± 0.02 173.36 ± 0.15

10 15 20 25 30 35

400600800

10001200140016001800200022002400260028003000

SB: Y=495.2+17.2X (R2=0.9938) SC: Y=275+27.8X (R2=0.9962) SR: Y=2476.6+14.3X (R2=0.9938)

C IN/R

CIN

Fig. 3. Kinetic analysis of H2S removal by three different biochars together withcorrelation coefficient, R2.

498 G. Shang et al. / Bioresource Technology 133 (2013) 495–499

H�ads þ O�ads!KR1 Sads þ OH� ð4aÞ

H�ads þ 3O�ads!KR2 SO2ads þ OH� ð4bÞ

SO2ads þ O�ads þH2Oads!KR3 H2SO4ads ð5Þ

Hþ þ OH� ! H2O ð6Þ

where H2Sgas, H2Sads-liq, and H2Sads correspond to the H2S in the gas,liquid, and adsorbed phases, respectively; KH, KS, Ka, and KR1, KR2,and KR3 denote the equilibrium constants for related processes(adsorption, gas solubility, dissociation, and surface reaction con-stants); O�ads is dissociatively adsorbed O2; and Sads, SO2ads, and H2-

SO4ads represent sulfur, SO2, and H2SO4 as the end products of thesurface oxidation reactions, respectively.

The mechanism of H2S removal by biochars probably differsfrom that of the ACs. As proposed by Hedding and Rao (1976), atthe virgin carbon surface, dissociation of hydrogen sulfide occursin the film of adsorbed water and then hydrogen sulfide ions orHS� are oxidized by oxygen radicals to elemental sulfur. On theother hand, when caustic is present, it catalyzes oxidation to ele-mental sulfur until all base is exhausted (Turk et al., 1992). There-fore, the significant decrease in the adsorption capacity of ACscorresponding to exhaustion is usually caused by the formationof sulfuric acid. Only a small decrease in pH is observed amongthe biochars, which regain their basic pH after exhaustion (Adibet al., 2000, 1999a,b).

3.3. Kinetic analysis of adsorption by biochar

The data for the kinetic analysis were obtained after 10 h byincreasing the concentration of H2S from 10 to 50 lL/L at the con-stant space velocity of 318 h�1 at three different instances for eachbiochar. The removal rate of H2S in this study (in stationary bed)was assessed in similar ways as previously reported (Hirai et al.,1990; Kim et al., 1998). By assuming the plug flow of H2S gas inthe stationary bed, the Michaelis–Menten-type equation was ap-plied as follows:

�dCdl¼ VmC

Ks þ C� Sa

F� a ð7Þ

¼ VmCKs þ C

� 1L � SV

� a; ð8Þ

where C, H2S concentration (lL/L); l, length of column (m); Vm,maximum removal rate (g S/kg dry material h); Ks, saturation

Table 3H2S breakthrough time, saturation time, and breakthrough capacity of the differentbiochars.

Sample Breakthroughtime (min)

Saturationtime(min)

Breakthroughcapacity (mg/g)

AC 120 210 35.6SC 360 600 109.3SB 580 1450 336.7SR 620 1645 382.7

constant (lL/L); Sa: cross section of stationary bed surface area(m2); F, gas flow rate (m3/h); L, height of packed peat (m); SV, spacevelocity (h�1) = F/Sa L; a, conversion coefficient (kg dry material/g S).

The conversion coefficient a defined in Eq. (9) was used to con-vert the units of concentration to lL/L.

a ¼22:4þ 273þT

273

� �� 106

32:1� 1000�W

V; ð9Þ

where: T, temperature (�C); W, dry weight of biochar (kg); V,volume of biochar (m3); 32.1, the atomic weight of sulfur.

Integrating Eq. (8) under the condition of C = C0 at l = 0 andC = Ce at l = L, we obtain

aSVðC0 � CeÞ

¼ Ks

Vm� 1ðC0�CeÞ

lnðC0=CeÞ

þ 1Vm

ð10Þ

Setting R = SV (C0 � Ce)/a and Cin = (C0�Ce)/ln(C0 � Ce), Eq. (10)is simplified to:

Cin

R¼ Ks

Vmþ Cin

Vmð11Þ

The relationship between Cin/R and Cin is shown in Fig. 3; thecorrelation equations are included in the removal rate Vm, andthe saturated constant Ks are listed in Table 2. The value of Vm ofH2S for SR is larger than those of SC and SB. As the overall removalrate was determined by both Vm and Ks, which are dependent onthe biochar, the kinetic equation using Vm and Ks in Eq. (7) for eachbiochar was compared. The removal rate of SR was superior tothose of the other biochars in the range of the H2S concentrationtested. When the concentration was below 20 lL/L, SC performedmore efficiently than did SB. However, when the concentrationrange was beyond 20 lL/L, SB performed performed more effi-ciently than did SC. Thus, among the three biochars, SR is the mostappropriate for H2S removal.

G. Shang et al. / Bioresource Technology 133 (2013) 495–499 499

4. Conclusions

Biochars derived from agricultural/forestry wastes were provento be a promising adsorbent of H2S with distinctive properties. Thebreakthrough capacities and removal rate of SR was superior tothose of SB, SC, and AC because SR possessed a higher pH value.The biochar samples were basic with higher quantities of oxy-gen-containing functional groups than commercial AC. The FTIRspectra of the biochars provide evidence of the presence of somesurface structures such as OH, COO, and C@O. Further research isneeded to apply biochars derived from other wastes in the removalof H2S.

Acknowledgements

This study was supported by the National Science and Technol-ogy Pillar Program (2012BAD15B03), the Special Fund for Scienceand Technology Innovation of Shanghai Jiao Tong University, Pro-ject 2010, and the Shanghai Agricultural Commission (Grant No.2010-2-3).

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