Adsorption of Cd by peanut husks and peanut husk biochar from aqueous solutions

Ecological Engineering 87 (2016) 240–245

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Adsorption of Cd by peanut husks and peanut husk biochar from aqueous solutions Qiming Cheng a , Qing Huang a,∗ , Sardar Khan a,b,∗∗ , Yingjie Liu a , Zhenni Liao a , Gang Li a , Yong Sik Ok c a

Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China Department of Environmental Science, University of Peshawar, Peshawar 25120, Pakistan c Korea Biochar Research Center, Kangwon National University, Chuncheon 200-701, South Korea b

a r t i c l e

i n f o

Article history: Received 5 March 2015 Received in revised form 20 October 2015 Accepted 22 November 2015 Keywords: Adsorption Cadmium Peanut husk biochar Isothermal model Aqueous media

a b s t r a c t This study was conducted to investigate the best absorbent for cadmium (Cd) present in aqueous solutions. Among the selected absorbents (n = 22), peanut husk biochar (PHB) was the best absorbent for Cd and its adsorption reached to equilibrium within 12 h. In this study, the optimum conditions observed for Cd adsorption were pH 5.0, an initial Cd concentration of 200 mg L−1 , a PHB dosage of 40 g L−1 , and room temperature. The Cd removal efficiency of the tested biochar reached to 99.9% at these optimum conditions. EDX analysis confirmed that Cd was adsorbed onto PHB more than onto PHs. It is concluded that PHB can be used as a good remediating material for removal of Cd from contaminated environmental matrixes. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Cadmium (Cd) is a non-essential heavy metal and highly toxic to all living organisms including plants, animals, and human beings. The enrichment of Cd in the environmental matrixes occurs through natural processes and anthropogenic activities (Khan et al., 2010; Liu et al., 2013; Luo et al., 2011). Cd enters into humans and other living organisms through contaminated water and the food chain and can cause irreversible damage. Traditional methods used for removal of Cd from contaminated water have several disadvantages such as incomplete metal removal, high reagent and energy requirements and generation of toxic sludge or other waste products that require disposal and further treatment (Sud et al., 2008; Bilal et al., 2013; Won et al., 2014; Moubarik and Grimi, 2015). Efforts are being made to develop efficient and innovative methods of wastewater treatment. While developing new methods, economic feasibility and environmentally friendly concepts are of great importance.

∗ Corresponding author. Tel.: +86 92 6190559; fax: +86 592 6190977. ∗∗ Corresponding author at: Department of Environmental Science, University of Peshawar, Peshawar 25120, Pakistan. Tel.: +92 91 9216742; fax: +92 91 9218401. E-mail addresses: [email protected], [email protected] (Q. Huang), [email protected] (S. Khan). http://dx.doi.org/10.1016/j.ecoleng.2015.11.045 0925-8574/© 2015 Elsevier B.V. All rights reserved.

Biomass-based high-performance adsorbent techniques are being developed for the treatment of wastewater containing Cd and have become a hot research topic due to their easy availability and presence in wide economic range of low cost (Sud et al., 2008; Coelho et al., 2014; Sun et al., 2014; Weng et al., 2014; Ammari, ´ 2014; Sˇ ciban et al., 2011; Vafakhah et al., 2014). These techniques are quick, easy to use, and have high adsorption capacity for toxic heavy metals, especially when these toxins are present at low concentrations in wastewater (Bilal et al., 2013). However new, economical, locally available, environmentally friendly and highly effective Cd sorbents are still needed. Comparatively, peanut husks (PHs) are a very cheap material produced in agriculture and oil extracting industries. China produces peanut husks at the rate of 3.14 million tons per year with a constant annual increase (Zhang et al., 2008; Wang et al., 2010a). PHs and their derived biochar (PHB) can be used as biomass-based adsorption materials for the treatment of wastewater containing heavy metals. In recent years, different researchers have applied different biomass and biochar materials for removal of toxic metals present in environmental matrixes (Sud et al., 2008; Purkayastha et al., 2014; Khan et al., 2015; Waqas et al., 2015). However, long term application of these adsorbents for treatment of wastewater and their mechanisms needs further investigations. In this study a total of 22 kinds of materials were tested to find the best candidate for Cd removal from aqueous media. The

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80

Efficiency (%)

performance of PH and PHB was compared with other materials. The effects of various operating parameters such as initial pH, Cd concentrations, different sorbent dosages, and contact time were monitored and the optimal experimental conditions were determined. Characterization of PHs and PHB were studied using different techniques such as BET, FT-IR, SEM, and CNS and CHNS O Cl; the kinetic data were used to understand the adsorption mechanism.

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2. Methods and materials 2.1. Biomasses and preparation of biochars Pyrolyzed biochars were prepared from several feedstocks (n = 22) including mushroom waste, Wolffia, lemna, rice materials (straw, stubble, husk, and root), water hyacinth materials (stems, leaves, and roots), legume straw, oil-tea camellia seed cake, and peanut husks were collected from Xiamen, China. Further detail is given in Supporting Information (SI).

2.2. Experimental designs Initially, screening experiments were conducted to test different materials for Cd removal. The detailed information is given in SI. Adsorption experiments for PHs and PHB with different pH levels (from 1 to 6 units) were conducted for the removal of Cd from the aqueous solution. Similarly, experiments for different adsorption times (from 10 to 5760 min) at room temperature were also conducted at room temperature. For more information, see SI. In order to find the adsorption equilibrium, PHs and PHB (0.2 g each) were added into 50 mL of Cd(NO3 )2 solution (pH 5.0) with different initial Cd concentrations (from 10 to 800 mg L−1 each in triplicates). The flasks were shaken at room temperature at 200 rpm for 24 h. In order to determine the optimum biochar dosage, different dosages (from 0.1 to 5.0 g each in triplicates) were added into 50 mL of Cd(NO3 )2 solution (pH 5.0) with an initial Cd concentration of 200 mg L−1 . The flasks were shaken at 200 rpm for 24 h at room temperature.

2.3. Cd analyses and characterization PHs and PHB The concentrations of Cd were measured using inductively coupled plasma-mass spectrometer (ICP-MS: Agilent Technologies, 7500 CX, USA). The initial pH and EC of PHs and PHB were determined at a ratio of 1:10 w/v in DDW. The specific surface areas and porous textures of PHs and PHB were measured by nitrogen adsorption at 77 K using a surface area and porosimetry system (ASAP 2020M+C, USA). The surface characteristics of PHs and PHB were analyzed using field emission-scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan). Elemental analysis was performed using a CNS and CHNS-OCl analyzer (Vario MAX, Germany). The elemental analyses of PHs and PHB before and after adsorption of Cd were carried out by an energy-dispersive X-ray spectrometry (EDX, Genesis XM2, USA) detector with an SEM (Hitachi S-4800). Similarly, samples were screened for functional groups using Fourier Transform Infrared Spectroscopy (FTIR) (Nicolet iS 10 model, Thermo Scientific, USA) following the KBr tablet method, which was equipped with a TGS/PE detector and silicon beam splitter with 1 cm−1 resolution. Infrared spectra were obtained in the range of 400–4000 cm−1 . For data analyses such as the efficiency (E) of PH and PHB materials for Cd removal and two sorption isotherms (Langmuir and Freundlich models) were used, see the SI.

20

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Different materials Fig. 1. Effects (%) of different adsorbents on removal of Cd. The number 1–22 represent materials used i.e, (1) powder of mushroom waste, (2) powder of rice straw, (3) powder of oil-tea camellia seed cake waster, (4) Imported activated carbon (AC1), (5) powder of Lavenda oil extraction residue, (6) powder of rice stubble, (7) powder of rice husk, (8) powder of rice root, (9) powder of PHs, (10) powder of water hyacinth stem, (11) powder of Wolffia, (12) powder of Lemna, (13) powder of water hyacinth leave, (14) powder of water hyacinth root, (15) powder of soybean straw, (16) mushroom waste biochar, (17) water hyacinth root biochar, (18) soybean straw biochar, (19) rice straw biochar, (20) oil-tea camellia seed cake biochar, (21) Domestic activated carbon (AC-2), (22) PHB.

3. Results and discussion 3.1. Characterization and best sorbent selection The major characteristics of PHs and PHB materials are given in Table S1, while Fig. 1 summarizes the efficiencies of different materials for sorption of Cd from aqueous solution. Among the biomasses, the lowest efficiency (38.7%) was observed for mushroom waste, while the highest (86.6%) was observed for soybean straw. PHB showed significantly (P = 0.001) higher efficiency (99.2%) for the sorption of Cd as compared to the other kinds of biochars tested in this research. Furthermore, results indicated that the Cd sorption capacity of the PHB was higher than the PHs (Fig. 1), indicating that there is need for biochar preparation from PHs to use as an adsorbent for Cd from aqueous solution. The results obtained in this study are consistent with those reported in earlier papers (Table S2), but different research groups had been tested the biochar at different pH levels. 3.2. Effect of pH on Cd adsorption In this study, the pH ranged from 1-2 units was shown no significant effect on the sorption of Cd by PHB, but the higher pH 3-4 units has shown rapid sorption of Cd (Fig. 2A). However, further increases in pH (5–8) did not affect the sorption of Cd. This equilibrium pH value changed according to the biochar material tested and ranged from 4 to 5, as reported in previous studies (Sun et al., 2014). It means that Cd sorption reaction is adaptable to a wide range of pH and is dependent on the biochar characteristics determined by the temperature range of pyrolysis and feedstock materials. PH contains many compounds such as lignin, cellulose, pentosan, organic acids, and tannins that can bind metals (Ding et al., 2012). PHB contains different active functional groups ( COOH and OH) on its surface (Ahmad et al., 2014; Khan et al., 2015) (Table S3 and Fig. S1). According to earlier studies, at low pH high concentration of H+ is present in the reaction system which protonates the functional groups on the biochar surface (Elaigwu et al., 2014)

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25

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B

A 20

Cd+2 qe/mg.g-1

Cd+2 qe/mg.g-1

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PH adsorption PHB adsorption

5

PH adsorption PHB adsorpion

5

10 5 0 0 2 4 6 8 10 12 14

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PH adsorption PHB adsorption

adsorption rate of PHs

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Cd+2 qe/mg.g-1

Cd

+2

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capacity of PHs 10

40

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5 0

0 0

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Adsorbent g-1

C0/mg.L-1

25

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adsorption rate of PHB 15

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capacity of PHB

Cd

+2

qe/mg.g

-1

80

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5

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0

0 0

1

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Adsorbent g

4

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-1

Fig. 2. Adsorption of Cd present in aqueous solution: (A) the effect of contact time duration on Cd adsorption, (B) effect of different pH values on Cd adsorption, (C) adsorption isotherm of Cd by PH and PHB materials used in this research, (D) effects of different PH dosages on Cd adsorption and (E) effects of different PHB dosages on Cd adsorption.

and leads to electrostatic repulsion between the protonated functional groups and the positively charged Cd2+ (Liu and Zhang, 2011). Additionally, the high concentration of H+ can compete with Cd2+ for the sorption sites present on the PHB surfaces (Wang et al., 2010b) and reduce the Cd sorption capacity of the biochar. Furthermore, at pH 8 or higher, OH functional groups lead to formation of several low-soluble hydroxyl complexes such as Cd(OH)2 and

Cd(OH)3 which precipitate (Mohan et al., 2007). FT-IR spectra have shown different functional groups on PHs/PBH surfaces such as OH (3419–3446 wave number/cm), which may help to reduce Cd in aqueous solution (Table S3 and Fig. S1). SEM and FTIR data before and after adsorption of Cd are discussed in SI (Fig S2). pH higher than the isoelectric point of PHB and lower than 8 could produce negatively charged functional groups, which help to promote Cd

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35

A

B

30

Ce/Qe=1.2561+0.0345*Ce

Ce/Qe=3.7275+0.0372*Ce

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1.4

logQ =0.4069+0.3189*logC e e 2 R =0.9212

logQe

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logQe=0.7957+0.2201*logCe

2 R =0.9800

1.0 0.8 0.6 0.4 0.2 -2

-1

0

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logCe

4 -2

-1

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Fig. 3. Different models used for Cd adsorption experiments (A) PH Langmuir isotherm plot of Cd adsorption, (B) PHB Langmuir isotherm plot of Cd adsorption, (C) PH Freundlich isotherm plot of Cd adsorption and (D) PHB Freundlich isotherm plot of Cd adsorption.

sorption through high attraction between the positively charged Cd2+ and negative functional groups, which simultaneously stimulate the removal of Cd from environmental matrices (Elaigwu et al., 2014). It is clear that the possible mechanism for metal adsorption by PHB material involves coordination between the metal ions and the biochar functional groups, which greatly depend on the pH of solution. Another mechanism might be the physical adsorption of metal ions depending on the biochar characteristics such as porosity and surface area (Liu and Zhang, 2011; Khan et al., 2013). 3.3. Effect of time on Cd adsorption by PHB Fig. 2B shows the influence of time on Cd adsorption by PHB. The adsorption of Cd ion reached to maximum (21.8 mg g−1 ) level in 48 h (Fig. 2). After 10 min, the adsorption amount reached 14.0 mg g−1 , which was 64% of the maximum adsorption amount of Cd ions. This adsorption amount reached 89% of the maximum adsorption capacity of PHB within 12 h, while further increases were also observed (reached to 95% of the maximum adsorption capacity within 24 h) until the desorption and adsorption rates became equal in a dynamic equilibrium; PHB increased with prolonged absorption, and finally reached a stable condition. These trends in metal adsorption are consistent with those reported in

Table 1 Simulation of isotherm models and corresponding parameters. Materials

Langmuir models −1

Qmax (mg g PHs PHB

26.88 28.99

)

Freundlich models −1

KL (L g

0.0100 0.7961

)

R

Kf (L g−1 )

n

R2

0.9010 0.9722

2.5521 6.2474

3.14 4.54

0.9212 0.9800

2

previous literature (Venkata Ramana et al., 2012). Adsorption by PHB increased gradually over time and started to stabilize at 12 h. We hypothesized that at the beginning, the adsorption mechanism was related to physical and chemical adsorptions, while in the latter stage the gradual increase in the adsorption of Cd was from the surface into the micropores of the biochar material; Cd slowly moved to in pores and the adsorption capacity further increased with time, and finally reached saturation. 3.4. Optimization of the adsorbent dosage Fig. 2D and E summarize the effects of different amounts of PHs and PHB on the sorption of Cd. The Cd sorption rate was positively correlated (R2 = 0.9778/0.7875) with the amount of PHs/PHB. These findings are consistent with those previously reported by

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other researchers (Elaigwu et al., 2014). The Cd adsorption rate increased to 90% with the increase of PHB dosage to 1.0 g, while the application of 2–5 g of PHB showed stability and equilibrium of Cd absorption. A direct proportional relationship exists between the number of negatively charged sites available for metal adsorption and the quantity of biochar used for batch experiments (Fig. 2). Thus, the number of sites increased with increasing application rates of biochar; linear curve can be obtained by plotting adsorbed metal ion levels with the amount of adsorbent used (Anwar et al., 2010). However, the Cd sorption rate declined as the amount of PHB increased beyond 1 g, because these high amounts of biochars had more sorption sites which could not be saturated when the total concentration of Cd ions in the solution remained fixed (Elaigwu et al., 2014). No significant changes in the adsorption of metal ions with the addition of biochar doses beyond 1.6 g L−1 suggest that equilibrium is reached between the adsorption sites of biochar and the metal ions; once the threshold limit is crossed, no significant changes are seen in metal adsorption by the tested adsorbents (Rahmani et al., 2010). The present study demonstrated that when 0.4% (0.2 g: 50 mL) PHB was exposed to a 200 mg L−1 Cd(NO3 )2 solution with shaking at 200 rpm for 3 h, about 95% of aquatic Cd was absorbed and the Cd content in the absorbed PHB reached 25 mg g−1 (Fig. 3). 3.5. Effect of initial Cd concentrations and sorption isotherm The adsorbed Cd contents (3.0–24.3 mg g−1 ) in PHs were increased with increasing initial concentrations of Cd in the aqueous solution (Fig. 2). The quantity of Cd (3.3–28.3 mg g−1 ) adsorbed by PHB also increased with increasing concentration of Cd in the solution and finally reached to an upper adsorption limit asymptote because of the limited sorption sites on the surfaces of the adsorbent (Elaigwu et al., 2014). At low concentrations of Cd ions in solution, the available sites for adsorption sites was high, therefore all the Cd ions attached to the available sites on the adsorbent. But as the Cd ions increased in solution the extent of Cd adsorption decreased because of the saturation of the sites available for adsorption on the adsorbent. A similar trend was also reported in a previous studies conducted by Elaigwu et al. (2014). Characteristically, the hydrated ionic radius of Cd is 0.275 nm, which may lead to a swift saturation of the available adsorption sites on the adsorbent (Benhima et al., 2008). The linear forms of the Langumuir and Freundlich equations were plotted and the corresponding correlation coefficients and constants were obtained. Fig. 3 summarizes the sorption isotherm models (Langmuir and Freundlich) which describe the relationship between sorption capacity of PHs/PHB and equilibrium concentrations. For PHs, the values of R2 for the Langumuir isotherm model was 0.9010, while for the Freundlich isotherm model was 0.9212. Similarly, the R2 values for PHB materials using the Langumuir and Freundlich isotherm models were 0.9722 and 0.9800, respectively (Fig. 3). These findings indicated that R2 values for Cd ion adsorption onto the PHs/PHB were higher in the Freundlich model, showing that the adsorption Cd data fitted the Freundlich model better than the Lanumuir model. The two isotherm models are fitted within the sorption of Cd by PHs and PHB, as given in Table 1. The separation factor RL = 1/(1+bco ) is the basic characteristic index of the Langmuir isotherm model (where, co was the initial concentration of heavy metals in mg L−1 ). This is often used to judge whether a sorption process is a thermodynamically favorable process or not: when RL > 1 it is an unfavorable adsorption; when RL = 1, indicates a linear adsorption; when 0 < RL < 1, adsorption is favorable; when RL = 0, it is an irreversible adsorption (Gorgievski et al., 2013). In this experiment, the RL value was calculated and ranged from 0 to 1 and RL = 1/(1 + 0.066 × ce ), implying that Cd sorption by PHB is a

favorable process. The values of RL are negatively correlated with the initial concentration of Cd (Table 1), indicating that the sorption of Cd by PHB can be further improved with changes in initial Cd concentrations. As shown in Table 1, the maximal Cd sorption capacity by PHB determined in our experiment was more similar to that calculated using the Langmuir model (qmax = 28.99 mg g−1 ; given in Table 1). The Freundlich model is a semi-empirical equation, which describes surface sorption and multi-layer sorption under various non-ideal conditions (Ding et al., 2012). As shown in Table S3 and Figs. 3A–D, the two isotherm models were fitted in the biosorption of Cd by PHs and PHB. The correlation coefficients (r) derived from the Freundlich equations were above 0.92, indicating that the data fit this model well (Fig. 3 C–D). All values of 1/n were less than 1, indicating sorption by heterogeneous media where high energy sites were occupied first, followed by sorption at lower energy sites (Peruchi et al., 2015). 4. Conclusions PHB has good adsorption capacity for Cd under selected adsorption conditions including pH from 4.0 to 8.0, optimum adsorbent dosage 2.0 g and the optimum adsorption time 12 h. PHB adsorption efficiency for Cd was more than 90% at a concentration of 200 mg L−1 of Cd in the solution. Isotherm models indicated that the theoretical maximum adsorption capacity was 28.99 mg g−1 . SEM and FTIR spectra analyses showed that the adsorption of Cd was partly linked with surface adsorption by PHB, while complexation is another complex reaction mechanism, involving different functional groups which govern the adsorption of metal ions from aqueous solutions. PHB can be used for removal of metals like Cd from aqueous media, including wastewater, but long term field experiments are still needed. Acknowledgments We gratefully acknowledge financial support from the National Natural Science Foundation of China (41571288), Chinese Academy of Sciences Key Development Project (KZZD-EW-1602), National Science and Technology Support Program of China (2014BAD14B04), the National High-Tech R&D Program of China (2012AA06A204), and Fujian Provincial Department of Science and Technology Project (2015Y0084). The authors also thank Mr. Yijun Yan for assistance on testing of the samples. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ecoleng.2015.11. 045. References Ahmad, M., Rajapaksha, A.U., Lim, J.E., Zhang, M., Bolan, N., Mohan, D., Vithanage, M., Lee, S.S., O.K., dY.S., 2014. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99, 19–33. Ammari, T.G., 2014. Utilization of a natural ecosystem bio-waste; leaves of Arundo donax reed, as a raw material of low-cost eco-biosorbent for cadmium removal from aqueous phase. Ecol. Eng. 71, 466–473. Anwar, J., Shafique, U., Waheed uz, Z., Salman, M., Dar, A., Anwar, S., 2010. Removal of Pb(II) and Cd(II) from water by adsorption on peels of banana. Bioresour. Technol. 101 (6), 1752–1755. Benhima, H., Chiban, M., Sinan, F., Seta, P., Persin, M., 2008. Removal of lead and cadmium ions from aqueous solution by adsorption onto micro-particles of dry plants. Colloid Surf. B-Biointerfaces 61, 10–16. Bilal, M., Shah, J.A., Ashfaq, T., Gardazi, S.M.H., Tahir, A.A., Pervez, A., Haroon, H., Mahmood, Q., 2013. Waste biomass adsorbents for copper removal from industrial wastewater—a review. J. Hazard. Mater. 263, 322–333.

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