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Adsorption of methyl violet from aqueous solutions by the biochars derived from crop residues


Bioresource Technology 102 (2011) 10293–10298

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Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech

Adsorption of methyl violet from aqueous solutions by the biochars derived from crop residues
Ren-kou Xu a,?, Shuang-cheng Xiao a, Jin-hua Yuan a,b, An-zhen Zhao a
a b

State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, P. O. Box 821, Nanjing, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

a b s t r a c t
The adsorption of methyl violet by the biochars from crop residues was investigated with batch and leaching experiments – adsorption capacity varied with their feedstock in the following order: canola straw char > peanut straw char > soybean straw char > rice hull char. This order was generally consistent with the amount of negative charge of the biochars. Zeta potentials and Fourier transform infrared photoacoustic spectroscopy, combined with adsorption isotherms and effect of ionic strength, indicated that adsorption of methyl violet on biochars involved electrostatic attraction, speci?c interaction between the dye and carboxylate and phenolic hydroxyl groups on the biochars, and surface precipitation. Leaching experiments showed that 156 g of rice hull char almost completely removed methyl violet from 18.2 L of water containing 1.0 mmol/L of methyl violet. The biochars had high removal ef?ciency for methyl violet and could be effective adsorbents for removal of methyl violet from wastewater. ? 2011 Elsevier Ltd. All rights reserved.

Article history: Received 13 June 2011 Received in revised form 17 August 2011 Accepted 21 August 2011 Available online 26 August 2011 Keywords: Adsorption Biochar Crop residues Methyl violet Waste water of dye

1. Introduction Dyes are widely used in the textile, printing, dyeing and food and paper-making industries. The wastewater from these industries contain dyes or pigments which contaminate surface water (Carneiro et al., 2010), ground water (Dubey et al., 2010) and even soils through irrigation (Topa? et al., 2009; Zhou and Wang, 2010). Methyl violet is a member of the basic dyes, a group with high brilliance and intensity of colors and that inhibit photosynthesis of aquatic plants (Hameed, 2008). Repeated or prolonged exposure to the methyl violet (2B) can produce target organ damage (Hameed, 2008). Therefore, it is necessary to remove these dyes from wastewater prior to discharge into water bodies. Removal of dyes and pigments from aqueous solutions via adsorption processes is a simple method known to be relatively low-cost, and such effective technology has been adopted widely by water treatment plants. The removal ef?ciency of dyes via adsorption mainly depends on the choice of the adsorbents employed. Activated carbon (Faria et al., 2004; Azizian et al., 2009; Soldatkina and Sagaidak, 2010; Chen et al., 2010), natural clay min? erals (Dogan and Alkan, 2003; Guiza et al., 2004), nano-particles (Tian et al., 2010; Liu et al., 2011), plant biomass (Hameed, 2008; Ofomaja and Ho, 2008; Li et al., 2010; Cengiz and Cavas, 2010) and ?y ash (Gupta et al., 2005; Mall et al., 2006) have been used to remove methyl violet and other basic dyes from aqueous solu-

? Corresponding author. Tel.: +86 25 86881183; fax: +86 25 86881000.
E-mail address: rkxu@issas.ac.cn (R.-k. Xu). 0960-8524/$ - see front matter ? 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.08.089

tions. However, recent attention has been given to some alternative low-cost materials of suf?cient suitability and selectivity for the removal of dyes from aqueous streams. In the partial or total absence of oxygen, thermal decomposition of plant-derived biomass (oxygen-limited pyrolysis) has been manipulated to yield a solid carbon-rich residue generally referred to as biochar (Sohi et al., 2010). There are ample oxygen-containing functional groups on biochars, e.g. carboxylate (–COO?), –COH and hydroxyl (–OH) (Chun et al., 2004; Yuan et al., 2011). The biochars normally carry a net negative charge on their surfaces due to the dissociation of oxygen-containing functional groups (Yuan et al., 2011; Inyang et al., 2010), and therefore can be used as low-cost adsorbents to remove organic pollutants and heavy metal cations from water (Yang and Sheng, 2003; Chun et al., 2004; Zhu et al., 2005; Wang and Xing, 2007; Mohan et al., 2007; Qiu et al., 2008; Cao et al., 2009). Several investigations have examined the adsorptive removal of organic pollutants from aqueous solutions using biochars (Yang and Sheng, 2003; Chun et al., 2004; Zhu et al., 2005; Wang and Xing, 2007; Cao et al., 2009); however, few involved the removal of dyes (Qiu et al., 2009). The mechanisms for adsorption of dyes on biochars remain to be determined. In this study, biochars from the straws of canola, soybean and peanut were prepared at 350 °C by oxygen-limited pyrolysis. Batch and column experiments were used to study the adsorption of methyl violet by these biochars. A biochar produced from rice hulls during bio-gas production was chosen for comparison. The objectives were to evaluate the abilities of biochars generated from crop residues to remove methyl violet from aqueous solutions and to probe the adsorption mechanisms for the dye on the biochars.

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2. Methods 2.1. Preparation of biochars The straws of canola, soybean and peanut were collected from cropland in a suburb of Nanjing, China. These plant materials were air dried at room temperature and ground to pass a 1-mm sieve. The ground straws were ?ll up ceramic crucibles, each with a ?tting lid, and then pyrolyzed in a muf?e furnace. The pyrolysis temperature was raised to 350 °C at a rate of approximately 20 °C min?1 and held constant for 4 h (Chun et al., 2004), and then allowed to cool to room temperature. Three replicates were done for each crop straw during the biochar-generating process. All biochars were then ground to pass a 1-mm sieve to examine their physical and chemical characteristics without further treatment. The biochar produced from rice hulls during bio-gas production was used for comparison. The biochar was obtained from a biogas station in a suburb of Nanjing, China. To obtain the water-washed biochar, 90-g biochar samples were packed in a polyethylene plastic column with an interior diameter of 5 cm. Then the biochar was leached by deionized water until the electrical conductivity of leachate <20 lS/cm. The biochar sample was taken out and air dried. 2.2. Physical and chemical properties of the biochars The pH of the biochar was measured in deionized water at a 1:5 w/w ratio. The biochar samples were each thoroughly mixed and allowed to equilibrate for 1 h. The pH was then measured using an Orion 720 pH meter with a combination electrode. The cation exchange capacity (CEC) of the biochar was measured by a modi?ed ammonium-acetate compulsory displacement method (Gaskin et al., 2008). Of the biochar, 0.2000 g was leached ?ve times with 20 mL of deionized water, and the leachates collected together. The K+, Na+, Ca2+ and Mg2+ in the leachates were determined as the soluble base cations of the biochar. After the ?fth run of leaching with deionized water, the biochar was leached with 20 mL of 1 M sodium-acetate (pH 7) ?ve times, and the leachates collected together. The K+, Ca2+ and Mg2+ in the leachates were determined as exchangeable base cations. The biochar samples were then washed ?ve times with 20 mL of ethanol to remove the excess Na+. Afterwards, the Na+ on the exchangeable sites of the biochar was displaced by 20 mL of 1 M NH4-acetate (pH 7) ?ve times, and the CEC of the biochar calculated from the Na+ displaced by NH4+. The Ca2+, Mg2+, K+ and Na+ in the leachates were determined using the same methods described above. Fourier transform infrared photoacoustic spectroscopy (FTIRPAS) spectra were recorded for the biochars from canola and peanut straws before and after adsorption of methyl violet using a Nicolet 380 spectrophotometer (Thermo Fisher Scienti?c, USA) equipped with a photoacoustic cell (Model 300, MTEC, USA) (Yuan et al., 2011). A sample was placed in the cell-holding cup (diameter 5 mm and height 3 mm) and the cell purged with dry helium at 10 mL min?1 for 10 s, then the sample was scanned from 4000 to 500 cm–1 with a resolution of 8 cm–1 and a mirror velocity of 0.48 cm s?1. The Boehm titration procedures for the analysis of oxygencontaining functional groups followed those previously established (Boehm, 2002). Each biochar sample (0.5000 g) was accurately weighed and reacted with 50.0 mL of 0.05 M NaHCO3, Na2CO3 and NaOH in 100-mL polypropylene bottles for 24 h with continuous shaking at 25 °C. The solutions were then ?ltered and the amounts of each ?ltrate (20.0 mL) determined by titration with HCl (0.20 M) solution. The numbers of acidic sites of various types were calculated on the basis of the assumption that NaHCO3 neutralized carboxyl groups only, Na2CO3 neutralized carboxyl and

lactonic groups, and NaOH neutralized all acidic (including phenolic) groups. Blank samples, containing no biochar, were also run with each reaction base. There was a duplicate for each treatment. To determine the zeta potential of the biochar, 0.0450 g of the sample (0.054-mm sieved) was placed in a 250-mL conical ?ask, and 180 mL of deionized water with or without 0.1 mM methyl violet was then added to each ?ask. The suspension pH was adjusted within a range 3.0–7.0 with NaOH or HCl. The suspensions were dispersed ultrasonically for 1 h at 25 ± 1 °C in a bath-type sonicator at a frequency of 40 kHz and a power of 300 W. After the sample was allowed to stand for 3 d, its electrophoresis mobility was measured using a JS94G + microelectrophoresis apparatus (Shanghai Zhongchen Digital Technique Equipment Ltd Co, Shanghai, China) and the zeta potential values calculated using speci?c software (Yuan et al., 2011). The suspension pH was determined and used to construct graphs of the relationship between the zeta potential of the biochar and pH.

2.3. Adsorption experiments Adsorption isotherms: biochar samples of 0.2000 g were weighed in 80-mL centrifuge bottles, 25 mL of methyl violet solution with varied concentrations (0.1, 0.3, 0.5, 0.7, 1.0, 1.5 and 2.0 mM) was added into each of the bottles. The suspensions were then shaken in a constant-temperature water bath at 25 ± 1 °C for 2 h. After standing for 22 h, the suspension pH was checked and then the suspensions were centrifuged to separate the solid phase at 9900 rpm for 10 min. Methyl violet in the bulk solutions was determined spectrophotometrically at 580 nm. The amount of methyl violet adsorbed by biochars was calculated from the difference between the total amount added and the amount remained in the equilibrium solution. The adsorption isotherms were obtained by plotting the amount of methyl violet adsorbed against bulk equilibrium concentration of methyl violet solutions. Effects of pH and ionic strength on adsorption of methyl violet: methyl violet solutions of 1 mM were prepared with deionized water and 0.01 M and 0.1 M NaCl solutions, respectively. Of these methyl violet solutions, 25 mL was added to 80-mL centrifuge bottles and 0.200 g of biochar was then added into each bottle. The pH of the suspensions was adjusted with HCl or NaOH to different values in the range 5.0–9.0. Then the suspensions were shaken in a constant-temperature water bath at 25 ± 1 °C for 2 h. After standing for 22 h, the suspension pH was checked and the suspensions centrifuged to separate the solid phase at 9900 rpm for 10 min. Methyl violet in the bulk solutions was determined spectrophotometrically at 580 nm. The amount of methyl violet adsorbed by the biochars was calculated as above.

2.4. Column leaching experiment A polyethylene plastic column (30 cm long and interior diameter of 5 cm) was used for the leaching experiment. Into the column, 156 g of rice hull char (about 20 cm high) was packed very carefully to a uniform bulk density. Inertia quartz layers were used at the top and bottom of the column to disperse ?ow throughout the entire area and to reduce the impact of the ?ow on the colloid movement. The biochar-packed column was leached with 1.0 mmol L?1 methyl violet solution from the top of the column using a peristaltic pump (?ow rate 5.56 ? 10–9 m3 s?1) and the leachate was collected continuously at the bottom of the column in 100 mL measuring ?asks. The methyl violet in the leachate was determined as previously mentioned.

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250 Adsorption of methyl violet (mmol/kg)

Table 2 Parameters of Langmuir equation for the biochars derived from straws of peanut and soybean and rice hull. Biochar Peanut straw char Soybean straw char Rice hull char Qm (mmol/kg) 256.4 178.6 123.5 K(L/mmol) 39 56 40 R2 0.996 0.992 0.999

200

150

100 Canola straw char Peanut straw char Soybean straw char Rice hull char 0 0 0.2 0.3 0.4 0.5 0.1 Equilibrium concentration of methyl violet (mmol/L) 0.6

The Langmuir equation is normally used to ?t the adsorption isotherms of ions by different adsorbents, and was also used in this study:

50

1=Q ? 1=?KQ m C? ? 1=Q m
where C is the equilibrium concentration of methyl violet in solution, Q is the amount of methyl violet adsorbed, Qm is the maximum amount of methyl violet adsorbed, and K is a constant related to binding strength. The equation was used to ?t the adsorption data for peanut and soybean straw chars and rice hull char (Fig. 1 and Table 2); with all correlation coef?cients (R2) > 0.992. Thus, the Langmuir equation ?tted the adsorption data well and could be used to describe adsorption of methyl violet by the biochars derived from these crop residues. 3.2. Effects of pH and ionic strengths on the adsorption of methyl violet by biochars The adsorption of methyl violet increased with the increase in ionic strength when pH < 8.5 (Fig. 2), suggesting that in addition to electrostatic interaction there were other mechanisms for the adsorption of methyl violet by the biochars. Biochars contain soluble salts (Yuan et al., 2011), and when peanut straw char was washed with deionized water to remove salts, the adsorption of methyl violet decreased greatly compared with the original peanut straw char – probably due to lower ionic strength for the washed biochar, especially under neutral–acid conditions (Fig. 3). Therefore, soluble salts in biochars should increase the adsorption capacity of biochars for methyl violet. Methyl violet has limited solubility in water, and readily forms precipitates on biochar surfaces when the dye is at higher ionic strength. This may be the main reason for the increase of adsorption capacity for the dye with increased ionic strength of the reaction system. At low equilibrium concentration of methyl violet, the amount of the dye adsorbed by peanut straw char and water-washed peanut straw char was similar. At higher equilibrium concentrations of methyl violet, peanut straw char adsorbed more than water-washed peanut straw char and the difference in adsorption between the two increased with further increases in equilibrium concentration of methyl violet (data not shown). Thus, soluble salts in biochar showed greater effect on adsorption of methyl violet at its higher equilibrium concentration. The adsorption of methyl violet increased slightly with increased pH under neutral–acidic conditions, but increased sharply from pH 7.7 to 8.7 (Figs. 2 and 3). The biochars used in this study carried a large amount of phenolic –OH (Table 1). Under neutral– alkaline conditions, the dissociation degree of the phenolic –OH

Fig. 1. Adsorption isotherms of methyl violet on the biochars derived from rice hulls and straws of canola, peanut and soybean.

3. Results and discussion 3.1. Adsorption isotherms of methyl violet by biochars Adsorption capacity of biochars for methyl violet varied with the four types of crop residues (Fig. 1). Canola straw char showed the greatest adsorption for methyl violet, follow by peanut and soybean straw chars, and the rice hull char had the lowest adsorption capacity. This order was generally consistent with the CEC of these biochars. Methyl violet is a basic dye and carries a positive charge, and biochars normally carry a net negative charge on their surface (Inyang et al.,2010; Yuan et al., 2011). Electrostatic interaction between negatively charged surfaces and the positively charged dye can occur (Solpan et al., 2003). CEC represents the ? negative surface charge of the biochars. Of the biochars, canola straw char had the greatest CEC (Table 1), which led to the greatest electrostatic interaction between it and methyl violet and so the highest adsorption capacity. The electrostatic attraction between cationic dyes of methyl green and methyl violet and activated carbon can greatly increase the adsorption capacity when the carbon carries a negative surface charge (Dai, 1994). Therefore, electrostatic interaction greatly affects the adsorption capacity for methyl violet by the biochars. After the adsorption experiment, the equilibrium solution pH was examined and showed that solutions were alkaline, since canola, peanut and soybean straw chars had alkaline pH values (Table 1). The equilibrium solution pH values were in the ranges 9.82–10.21, 9.19–9.41, 8.35–8.44 and 8.28–9.34 for canola, peanut and soybean straw chars and rice hull char, respectively. Alkaline pH is favorable for the adsorption of methyl violet. The negative charge of biochars originated from the dissociation of oxygen-containing groups of the biochars. Thus, the negative charge became more negative with increased system pH, which led to increased electrostatic interaction between biochars and methyl violet. This will be further discussed in the next section.

Table 1 Contents of oxygen-containing functional groups, pH, and CEC of the biochars derived from straws of canola, peanut and soybean and rice hull. Biochar Canola straw char Peanut straw char Soybean straw char Rice hull char pH 8.0 8.9 9.0 6.4 CEC (cmol/kg) 152.1 81.2 97.8 86.4 Carboxylic (cmol/kg) 9.0 6.4 6.4 13.4 Lactonic (cmol/kg) 23.7 36.9 28.2 21.1 Phenolic (cmol/kg) 106.4 160.0 139.2 87.3

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Fig. 2. Effects of pH and ionic strength on adsorption of methyl violet by canola straw char.

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Fig. 4. Effect of methyl violet adsorption on zeta potential of canola straw char (A) and peanut straw char (B).

3.3. Zeta potential and FTIR-PAS spectra before and after methyl violet adsorption The zeta potential is the potential in the sliding plane of colloidal particles, and its value and sign are related to surface charge of the particles. Protonation and deprotonation of functional groups can create a net charge on the surfaces of the solid particles, which can form an electrical double-layer in the solution phase near the surfaces. Zeta potential values were measured as a function of solution pH for peanut and canola straw chars (Fig. 4); and for both biochars were negative in the pH range 3.0–8.0, indicating that the biochar particles carried negative charges on their surfaces. The zeta potential of the biochars became more negative with increased pH, suggesting that the amount of negative charge increased with increased pH. The presence of methyl violet shifted the zeta-potential–pH curve of both biochar particles in a positive direction (Fig. 4), and the difference in zeta potential between systems of biochar and that of biochar + methyl violet increased with increased suspension pH. These results suggested that methyl violet can be speci?cally adsorbed by biochars, and the contribution of speci?c adsorption increased with increased suspension pH. Electrostatic adsorption of ions by the solid charged-surfaces will not tend to affect the surface charge and surface potential of colloidal particles,

Fig. 3. Effect of pH on adsorption of methyl violet by peanut and water-washed peanut straw chars.

increased with increased pH, making the negative charge on biochars more negative. Thus the electrostatic attraction of biochars to methyl violet increased with increasing pH. Similar change trends of methyl violet adsorption with pH were observed in systems of ? perlite, wood sawdust, activated carbon and bagasse ?y ash (Dogan and Alkan, 2003; Mall et al., 2006; Ofomaja and Ho, 2008; Chen et al., 2010). For methyl violet removal from aqueous solutions, the effective pH was 9 and 10 for bagasse ?y ash and wood sawdust (Mall et al., 2006; Ofomaja and Ho, 2008), respectively, consistent with observations in the present study. Biochars normally have alkaline pH, and after reaction of biochars with methyl violet solutions, the equilibrium solution pH was neutral–alkaline as previously mentioned – thus aiding the removal of methyl violet by the biochars. The higher equilibrium solution pH for canola and peanut straw chars, compared to the other two biochars, is one reason for their greater adsorption capacity for methyl violet (Fig. 1).

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because the adsorbed ions exist in the diffuse layer of electric double-layers on the particles. However, speci?c adsorption of ions will change the surface charge and surface potential of colloidal particles, because these ions get into the Stern layer of electric double-layers and form chemical bonds with the solid particle surfaces. Moreover, during the process of cation speci?c adsorption, some positive charges are transferred to the surface of the biochars. This process likely made the negative charge of biochars less negative or even led to net positive charge, and hence the zeta potential of biochars became less negative or changed from negative to positive accordingly. Therefore, methyl violet was adsorbed speci?cally by the biochars in addition to the electrostatic interaction between the dye and the biochars. FTIR-PAS spectra supported the speci?c adsorption of methyl violet on biochars. Before adsorption methyl violet, the peaks at 3437 and 3421 cm–1 in spectra of canola and peanut straw chars (Fig. 5) were assigned to phenolic –OH stretching (?z?imen and

Absorbance

ERsoy-Meri?boyu, 2010), and the peaks shifted to 3417 and 3406 cm–1 after the adsorption of methyl violet. The peaks at 1589 and 1593 cm–1 were assigned to –COO– antisymmetric stretching of canola straw char and peanut straw char (Qiu et al., 2008); after adsorption of methyl violet, there were only small shifts in the corresponding peaks, within the range of determining error. The absorption peaks at 1396 and 1400 cm–1 in the spectra of canola and peanut straw chars were assigned to –COO– symmetric stretching (Lammers et al., 2009); the adsorption of methyl violet shifted the corresponding peaks to 1389 and 1381 cm–1. These results suggested speci?c interactions between methyl violet and the functional groups of phenolic –OH and –COO–. The weak peak at 1180 cm–1 in the spectra of canola straw char with methyl violet adsorbed was assigned to C–N stretching in methyl violet – a shoulder was observed in a similar position in the spectra of peanut straw char with methyl violet adsorbed. These results suggested that methyl violet was adsorbed to the surface of the biochars. The absorption peaks at 1065 and 1045 cm–1 in the spectra of canola and peanut straw chars were assigned to the bands of the out-of-plane bending for carbonates (Lammers et al., 2009). The peak intensity clearly decreased after methyl violet adsorption, suggesting interactions between carbonates in biochars and methyl violet. The results of zeta potential and FTIR-PAS combined with adsorption isotherms and effect of ionic strength suggested that the adsorption of methyl violet on biochars involved electrostatic attraction, speci?c interaction between the dye and the groups of –COO– and phenolic –OH, and surface precipitation. 3.4. Column leaching experiments with rice hull char To probe the adsorption of methyl violet by biochars under leaching conditions, rice hull char was used to adsorb methyl violet in column leaching experiments (Fig. 6). Before 182 sampling times, the concentration of methyl violet in leachate from the leaching column with rice hulls was <0.02 mmol L?1. The cumulative volume of leachate was 18.2 L and cumulative amount of methyl violet removed from aqueous solution was 116.7 mmol kg?1, which was slightly lower than the maximum adsorption by the biochar predicted by the Langmuir equation (Table 2). Adsorption capacity of canola, peanut and soybean straw chars for methyl violet was greater than that of rice hull char, indicating that the three biochars had greater removal ef?ciency for

3417

1593

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Canola straw char with MV

Canola straw char

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1.2 1 0.8 0.6 0.4 0.2 0 0 50 100 Sampling times 150 200

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Peanut straw char

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Wacenumber (cm )
Fig. 5. The FTIR-PAS spectra for canola straw char and peanut straw char before and after adsorption of methyl violet (MV). Fig. 6. Dynamics of methyl violet concentration in leachate after methyl violet solution went through the biochar column during leaching experiment (the initial concentration of methyl violet was 1.0 mM).

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methyl violet than rice hull char. These biochars could be effective adsorbents for the removal of methyl violet from wastewater. The removal ef?ciency of methyl violet by rice hull char was greater than that by water-washed rice hull char (Fig. 6), consistent with the previous data (Fig. 3). Soluble salts in the biochar increased the adsorption of methyl violet and thus its removal ef?ciency as mentioned above. In addition, during water-washed process some soluble alkaline materials were removed from the biochar, which also decreased the adsorption of methyl violet. This is because biochars adsorbed greater amount of methyl violet under alkaline conditions (Figs. 2 and 3). The leachate pH provided evidence. The leachate pH from column of water-washed rice hull char is in the range from 5.8 to 6.5, while the leachate pH from column of rice hull char ranged from 7.6 to 9.5. Therefore, the biochars are more effective at removal of the dye from waters without pre-treatment. Biochars are also biofuel. The biochars with dye adsorbed can be treated through burning, and thus the secondary pollution can be avoided. 4. Conclusions The adsorption capacity for methyl violet by four crop residuederived biochars varied with the feedstock in the order: canola straw char > peanut straw char > soybean straw char > rice hull char. The results of zeta potential and FTIR-PAS combined with adsorption isotherms and effect of ionic strength indicated that the adsorption of methyl violet on biochars involved electrostatic attraction, speci?c interaction between the dye and the groups of –COO? and phenolic –OH, and surface precipitation. The biochars had high removal ef?ciency for methyl violet and could be effective adsorbents for removal of methyl violet from wastewater. The biochars are more effective at removal of the dye from waters without pre-treatment. Acknowledgements This study was supported by the Knowledge Innovation Program Foundation of the Chinese Academy of Sciences (KZCX2YW-Q10-3). We are greatly grateful to Dr. Changwen Du of Institute of Soil Science, Chinese Academy of Sciences for his help with collecting the FTIR-PAS spectra of the biochars. References
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