Sorption of Lead(II), Cadmium(II), and Copper(II) Ions from Aqueous

College of Life & Environmental Sciences, Huangshan University, .... specific surface area based on the BET model by using Micromeritics ASAP2020 (U.S...
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Sorption of Lead(II), Cadmium(II), and Copper(II) Ions from Aqueous Solutions Using Tea Waste Shunli Wan,*,† Zhaozhao Ma,† Yao Xue,† Minghai Ma,† Shengyou Xu,† Liping Qian,† and Qingrui Zhang‡ †

College of Life & Environmental Sciences, Huangshan University, Huangshan, 245041 Anhui, P.R. China College of Environmental & Chemical Engineering, Yanshan University, Qinhuangdao, 066000 Hebei, P.R. China

‡

S Supporting Information *

ABSTRACT: In the present study, the sorption ability of three metal ions, lead, cadmium, copper, from aqueous solution by tea waste was investigated. Sorption of the evaluated toxic metals by tea waste was pH-dependent, and kinetic data for three metal ions not only indicated a quick sorption process but also were excellently represented by the pseudo-second-order model with all correlation coefficients R2 > 0.97. In addition, the sorption processes of three metal ions by tea waste in different temperatures could be described satisfactorily by both Langmuir and Freundlich isotherms. According to calculated results by the Langmuir equation, the maximum removal capacities of Pb(II), Cd(II), and Cu(II) were 33.49, 16.87, and 21.02 mg/g, respectively. Fourier transform infrared (FT-IR) analysis of the tea waste samples laden with different metals indicated that multiple functional groups were involved in the sorption of metal ions, and the carboxyl group (CO) and bonded−OH group were primary binding sites in lead and cadmium removal, while the −CN stretching and the carboxyl group were primary binding sites in copper removal. All the results reported strongly implied the potential of tea waste as an economic and excellent bioadsorbent for removal of metal ions from contaminated waters.

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INTRODUCTION The presence of metals like Pb(II), Cd(II), Hg(II), Cr(VI), Cu(II), etc., in the aquatic environment is of great concern because of their extreme toxicity and carcinogenic effects toward human health and aquatic organisms.1,2 Usually, these toxic metal ions have been released into the environment as industrial wastes in the smelting and mining industry, electroplating, and leather manufacturing as well as fossil fuel combustions.3,4 Currently, various physical, chemical, or biological technologies such as chemical precipitation,5,6 nanofiltration and ultrafiltration,7,8 solvent extraction,9,10 sorption11−13, and biological treatment14 have been repeatedly attempted to overcome metals contamination. Among these treatment procedures, sorption as one of the most widely used methods for metals removal has earned more and more attention for its high removal efficiency and clean process. Over the last decades, various materials have been developed as adsorbents for toxic metal ions removal from natural waters or industrial wastewaters,15−17 but high cost and complex preparation conditions usually limit their practical promotion and application. Recently, low cost adsorbents developed from natural materials or certain waste materials (or byproducts) from agricultural or industrial activities have gained greater attention due to many obvious advantages such as easy availability, comparable efficiency, resource generation and pollution abatement, and a number of low-cost biomass; for example, walnut shells,18 dry tree leaves,19,20 rice hulls,21,22 chicken feather,23 dairy-manure,24 saw dust,25,26 and corn cobs27 had shown excellent sorption capacities for toxic metal ions from solution. © 2014 American Chemical Society

Tea leaves as one of numerous biomass are porous materials with network structure, and the main dry matter of them are cellulose and hemicelluloses, lignin, condensed tannins, and structural proteins.28 FT-IR spectrum of tea leaves in earlier research confirms the existence of various functional groups such as carboxyl, phenolic hydroxyl, and amine groups on their surface,29,30 and these functional groups have always been considered to effectively form some physicochemical interactions, e.g. ion exchange or inner-sphere complex formation, with metal ions. Therefore, the surface chemical property of tea leaves determines that it might be an excellent adsorbent for metal ions removal from contaminated water. In recent years, with the improvement of People’s living standard, tea has already become an essential drink in daily lives. According to statistics, approximately 1.1 million tons per year of tea leaves were consumed in China,31 and that results in the disposal of spent tea waste that has gradually become an unavoidable challenge. In the current study, we selected tea waste as a sorbent for toxic metals removal and investigated the potential of tea waste in removal of three toxic metal ions, lead, cadmium and copper, from aqueous solutions. Sorption behaviors of lead(II), cadmium(II), and copper(II) onto tea waste were evaluated in terms of the influence of solution pH, sorption kinetics, system temperature, and solid particle size. Simultaneously, FTIR spectroscopy was also provided to preliminarily reveal its underlying binding mechanism. Received: Revised: Accepted: Published: 3629

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Figure 1. Scanning electron micrographs of the tea waste used in the present study: (a) 200 μm and (b) 50 μm.

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°C, 2300 °C. The absorbance of the samples was read in triplicate. When the content was lower than 1 mg/L, it was determined by an atom fluorescence spectrophotometer (AFS) with an online reducing unit (AF-610A, China) containing NaBH4 and HCl solution. The micrographs of tea waste were taken with a scanning electron microscope (LEO 1530VP, Germany) by fracturing the species in liquid nitrogen and then coating it with gold powder. FT-IR spectra of tea waste before and after metal sorption were obtained from a Nexus 870 FTIR spectrometer (USA) with a pellet of powered potassium bromide and sorbent in the range 400−4000 cm−1. A N2 sorption−desorption test onto tea waste was carried out at 77 K to determine pore size distribution based on the BJH model and specific surface area based on the BET model by using Micromeritics ASAP2020 (U.S.).

MATERIALS AND METHODS Reagents. All chemicals are of analytical grade or more pure and were purchased from Shanghai Reagent Station (Shanghai, China). The stock solutions containing the target metals were prepared by dissolving their corresponding nitrates in the deionized water. Adsorbent Preparation. The tea waste used for the experiments was obtained from tea plants located in the Green tea region in Huangshan, China. Soluble and colored components were removed from tea leaves by washing with boiling water. This operation was repeated until the water was completely colorless. Finally, these tea leaves were washed with deionized water and vacuum-desiccated at 333 K for 24 h until they reached a constant weight. The dried tea waste was grinded, sieved, and finally stored in sealed glass bottles. The tea waste of particle size between 250 and 880 μm was used for all the experiments except for the effect of particle size tests. Batch Sorption Experiments. 100.0 mg of tea waste was introduced into 250-mL polyethylene Erlenmeyer flasks containing 100 mL of solution of known concentration of target metals. A 1.0 M HNO3 or NaOH solution was used to adjust the solution pH throughout the experiment when desired. Detailed chemical compositions of the sorption systems are described in the corresponding figure captions. The flasks were then transferred to a Model G-25 incubator shaker with thermostat and shaken at 180 rpm for 24 h at a desired temperature. The time was deemed sufficient to ensure apparent equilibrium as determined by preliminary kinetic tests (data not shown). For the kinetic test, the initial solution volume was 500 mL, and a 0.5-mL aliquot was sampled at various time intervals. The sample was then filtered to remove any fine tea waste particles and analyzed for the metal ion. Uptakes of target metals were calculated by conducting a mass balance before and after the test. Note that all the batch runs were performed in duplicate for data analysis. Analyses. Concentration of all the metal ions in aqueous were usually determined by atomic absorption spectroscope (Beijing Puxi Co. China). For Pb(II) determining, Pb hollow cathode lamp was used at the conditions of wavelength 283.3 nm, flow gases 1500 mL/min, and burning temperature 2400 °C. For Cd(II) and Cu(II), the corresponding hollow cathode lamp was used at wavelength 228.8 nm, 324.7 nm; flow gases 1300 mL/min, 1200 mL/min; and burning temperature 2300

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RESULTS AND DISCUSSION

Characterization of Tea Waste Particles. Scanning electron micrographs (Figure 1) displayed that the surface of tea waste particles was uneven and a large number of wide pores existed in its external region. Generally, the rough surface and widely distributed pores can offer greater surface area and more binding sites for metal. BET specific surface area and average pore diameter of the tested tea waste are around 0.86 m2/g and 3.62 nm, respectively, which are in close agreement with that reported by Amarasinghe et al. 32 Pore size distribution of sorbent indicated that pores of tea waste are mainly in the microporous region (Figure 2). Effect of Solution pH. Effect of solution pH on retention of Pb(II), Cd(II), and Cu(II) ions by tea waste was examined, and the results were presented in Figure 3. As illustrated, higher solution pH (in the test pH range 1−6) is more favorable for uptake of metals onto tea waste. Specifically, at pH 2−3 range the removal efficiency is very low and rapidly increases between pH 3 and 5. The pH-dependent trend could be reasonably explained by the ion-exchange process between the surface functional groups, e.g. phenolic hydroxyl and carboxyl, of the adsorbent and the cations present in the solution. Uyanık et al.28 suggested that ion exchange mechanism between metal ions (M2+) and tea waste could be represented as eq 1: R‐(OH)2 + M2 + ⇄ −R‐(O)2 ‐M + 2H+ 3630

(1)

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Figure 4. Sorption kinetics of three metal ions onto tea waste at 298 K (0.30 g of adsorbent was added into 500 mL of solution containing 20 mg/L of each metal ion, pH 5.0 ± 0.2).

Figure 2. Pore size distribution of tea waste used in the present study.

Table 1. Kinetic Parameters for Three Metals Uptake onto Tea Waste at 298 K metals 2+

Pb Cd2+ Cu2+

qe

R2

0.68 5.22 2.37

29.47 12.39 16.48

0.996 0.993 0.971

Pb(II), Cd(II), and Cu(II) onto tea waste can be well represented by the pseudo-second-order model. The calculated qe values (29.47 mg/g for Pb(II), 12.39 mg/g for Cd(II), 16.48 mg/g for Cu(II)) were also close to the experimental data (26.33 mg/g for Pb(II), 11.58 mg/g for Cd(II), 15.62 mg/g for Cu(II)). The slower sorption speed for tea waste compared to several usual metal oxides sorbents may be due to lower BET surface areas.11,13 Sorption kinetic results for three metals onto tea waste are similar to other bioadsorbents.20,21,26 Sorption Isotherms. Ambient temperature is usually a crucial factor for sorption process. Accordingly, the effect of temperature on lead, cadmium, and copper removal by tea waste was experimentally quantified. Sorption isotherm experiments were performed at five different temperatures, i.e., 288 K, 298 K, 308 K, 318 K, and 328 K. The results are illustrated in Figure 5 and further correlated by the Langmuir and Freundlich models represented by the following equations34 1 1 1 = + qe KLqmCe qm (4)

Figure 3. Effect of solution pH on the removal of three metal ions onto tea waste at 298 K (0.1 g of sorbent was added into 100 mL of solution with 20 mg/L of each metal ion).

It is plausible that lower pH values are less favorable for metals retention onto tea waste, which is consistent with the results presented in Figure 3. Certainly, the carboxyl groups located in tea waste surface also contribute the pH-dependent trend by the possible process described in eq 2: R‐(COOH)2 + M2 + ⇄ −R‐(COO)2 ‐M + 2H+

k, 10−3 min−1

(2)

The above deduce will be indirectly elucidated in the following FT-IR spectroscopy. Sorption Kinetic. The uptake of lead, cadmium, and copper versus contact time for tea waste was conducted, and the results are presented in Figure 4. It can be seen that initial sorption of three metals onto tea waste was very quick, followed by a gradual sorption approaching equilibrium within 3 h. Kinetic data for three toxic metals were then represented by the pseudo-second-order model33 t t 1 = + qt qe k 2qe 2 (3)

qe = K f Ce1/ n

(5)

where Ce is the equilibrium concentration of metal ions in solution (mg/L); qe is the amount adsorbed at equilibrium concentration (mg/g); qm is the maximal sorption capacity (mg/g); KL (L/mg) is a binding constant, and Kf and n are the Freundlich constants to be determined. The results in Table 2 and Figure 5 indicated that the Langmuir model can represent Pb(II) and Cd(II) sorption processes more reasonably than the Freundlich model, and Cu(II) removal can be represented by the Freundlich model more appropriately. The calculated qm values (33.49 mg/g for Pb(II), 16.87 mg/g for Cd(II)) by the Langmuir model are a little higher than the experimental data (30.66 mg/g for Pb(II), 15.56 mg/g for Cd(II)). The fractional value of 1/n (0 < 1/n < 1) obtained for the Cu(II) sorption

where qe and qt are the amount of metal adsorbed per unit weight of adsorbent (mg/g) at equilibrium and at time t, respectively, and k2 is the second-order sorption rate constant. The fitting results of three metals were shown in Table 1, and the high correlation coefficients (R2) indicated that uptakes of 3631

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Pb(II), Cd(II), and Cu(II) are −1504 kJ/mol, −1708 kJ/mol, and −2030 kJ/mol, respectively,36 which indicates that affinity of three cations toward tea waste is in the sequence Pb(II) > Cd(II) > Cu(II) and that it is also consistent with the sorption capacities sequence observed from Freundlich model fitting results. The anomalous sorption behavior of copper(II) indicated the presence of other mechanisms for Cu(II) removal by tea waste besides ion exchange. This speculation can be further corroborated by the following FT-IR spectroscopy analysis. Moreover, the sorption capacities of Pb(II), Cd(II), and Cu(II) by tea waste and other lignocellulosic materials have been compared, and the related results were listed in Table 3. On the whole, the sorption capacities of three toxic metal ions onto tea waste are higher than the other listed lignocellulosic materials. Another noteworthy observation is that the sorption capacities of three metal ions by tea waste were slightly increased with elevation of temperature, which implied the endothermic nature of sorption process on tea waste. Thermodynamic parameters, such as the change of free energy of sorption (ΔG), enthalpy change of sorption (ΔH), and the standard entropy change (ΔS), were calculated according to the van’t Hoff equation37 ΔG = ΔH − T ΔS

(6)

ΔG = −nRT

(7)

d(ln Ce) ΔH = d(1/T ) R

(8)

where T is the absolute temperature in K, and R is the gas constant. The positive ΔH and the negative ΔG in Table 4 validated that the sorption processes of three toxic metals onto tea waste were all essentially endothermic and spontaneous. The reason for endothermic reaction is possible that the energy is essential for promoting ion exchange process between metal ions and H+ of hydroxyl and carboxyl groups of tea waste as above in eqs 1 and 2.19,35,38 Of course, the positive values of ΔS suggest the increased randomness at the solid/solution interface during the sorption of three metals ions onto tea waste. Effect of Sorbent Particle Size. Effect of tea waste particle size on uptake of Pb(II), Cd(II), and Cu(II) ions was examined, and the results were presented in Figure 6. As illustrated, the removal efficiency of three metals increased with the decline of tea waste particle size. The similar trend had been observed on metal ions sorption by other biomass such as tree leaves20 and rice hulls.39 The particle size dependent phenomenon might result from different surface areas of various particle sizes of tea waste. In other words, the smaller particle size of sorbent possessing the higher surface area can provide more functional groups, namely, binding sites, for metal ions removal. Fourier-Transfer Infrared Analysis. In order to gain further insight into the mechanism of metal sorption onto tea waste, FT-IR spectrum of the tea waste samples laden with different metals was conducted by comparison with the native ones. As seen in Figure 7, FT-IR analysis of tea waste laden metal and the fresh one displayed that a number of absorption peaks had shifted after toxic metal sorption. Detailed band variation data of different sorption peaks of tea waste before and after metal uptake were presented in Table 5. As shown, compared to the native tea waste, the band frequency of tea

Figure 5. Sorption isotherms of three metal ions onto tea waste at 288 K, 298 K, 308 K, 318 K, and 328 K (pH 5.0 ± 0.2): (a) Pb(II); (b) Cd(II); (c) Cu(II).

system based on the Freundlich model clearly indicated the active sites of tea waste surface are heterogeneous for Cu(II) binding. Besides, another conclusion that tea waste prefers metal ions sorption in the sequence Pb(II) > Cu(II) > Cd(II) can be drawn from Table 2 easily. This may be explained by the different hydration energy of each ion which can reflect the capability for the ion to interact with the functional groups on tea waste surface.32 In general, divalent cations with lower hydration energy are more favorable for ion exchange.35 However, the calculated Gibbs free energies of hydration of 3632

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Table 2. Isotherm Constants for Three Toxic Metals Uptake onto Tea Waste Langmuir model

Freundlich model

metals

temp (°C)

KL (L/mg)

qm (mg/g)

R2

Kf (mg1‑nLn/g)

n

R2

Pb(II)

15 25 35 45 55 15 25 35 45 55 15 25 35 45 55

0.47 0.57 0.44 0.51 0.53 0.39 0.33 0.34 0.30 0.29 0.11 0.09 0.08 0.08 0.09

25.09 26.98 29.30 31.16 33.49 13.96 15.04 15.51 16.04 16.87 16.41 18.54 19.75 20.97 21.02

0.983 0.996 0.991 0.976 0.976 0.981 0.981 0.999 0.977 0.993 0.961 0.957 0.924 0.914 0.918

12.39 14.65 13.87 15.06 14.19 5.86 6.65 6.89 6.64 6.25 4.84 4.45 4.27 4.33 4.46

6.18 5.88 5.0 4.76 4.16 5.22 5.0 4.76 4.55 4.32 3.57 3.13 2.94 2.86 2.77

0.914 0.894 0.954 0.950 0.963 0.963 0.891 0.876 0.899 0.923 0.975 0.986 0.961 0.978 0.980

Cd(II)

Cu(II)

Table 3. Reported Sorption Capacities for Several Lignocellulosic Materials

sorbent tea waste rice hulls saw dust wheat straw soybean straw corn stalk corn cob oat straw

Pb(II) sorption capacity (mg/g)

Cd(II) sorption capacity (mg/g)

Cu(II) sorption capacity (mg/g)

refs

33.49  15.90 3.11

16.87  5.76 9.96

21.02 11.83 8.06 4.48

this paper 21 25, 26 27

6.83

2.02

5.44

27

6.00 3.93 18.84

5.15 10.75 4.70

3.78 2.18 5.18

27 27 27

Figure 6. Effect of sorbent particle size on the removal of three metal ions onto tea waste at 298 K (0.1 g of tea waste was added into 100 mL of solution with 20 mg/L of each metal ion; solution pH 5.0 ± 0.2).

waste after metal sorption displayed significant changes at the sorption bands of 617, 1039, 1236, 1533, 1649, 2927, and 3359 cm−1. These results implied that the −CN stretching, CO stretching, −SO3 stretching, secondary amine group, aliphatic C−H group, and bonded−OH groups had played an important role in removal of three metal ions by tea waste.40 However, the roles of above functional groups were different in retention of various metals which can be easily verified by the different band frequency changes of various sorption peaks (Table 5) before and after each metal ion uptake. Specifically, the CO stretching (carboxyl) and bonded−OH groups were primary sorption sites for lead and cadmium binding, while the −CN stretching and CO stretching groups were primary sorption sites for copper binding. The special sorption mechanism of Cu(II) compared to Pb(II) and Cd(II) confirmed the above explanation of sorption capacities sequence of three toxic metals by tea waste. Also, the ion exchange mechanism of the CO stretching and bonded−OH groups with metals also

confirmed the deduce of above-mentioned pH-dependent. In addition, combined effects of multiple functional groups illustrated that tea waste is an excellent biosorbent toward removal of toxic metal ions. Environmental Implications. As a product coming from human daily consumption, tea waste is easily accessible and inexpensive. Using tea waste as a bioadsorbent for metal ions uptake not only purifies contaminated waters including toxic metal but also achieves the purpose of wastes resource regeneration. Moreover, the sorption capacities for three metals onto tea waste were influenced less seriously by high concentration of Ca(II) which usually coexist with target metal ions in practical contaminated waters (Figure S1). The property of tea waste increases the probability for its practical

Table 4. Thermodynamic Parameters of Pb(II), Cd(II), and Cu(II) Sorption onto Tea Waste ΔG (KJ/mol)

ΔS (J/mol·K)

metals

qe (mg/g)

ΔH (KJ/mol)

15 °C

25 °C

35 °C

45 °C

55 °C

15 °C

25 °C

35 °C

45 °C

55 °C

Pb(II) Cd(II) Cu(II)

20 12 12

25.1 10.5 11.4

−14.8 −12.5 −8.5

−14.6 −12.4 −7.8

−12.8 −12.2 −7.5

−12.6 −12.0 −7.6

−11.3 −11.8 −7.6

138.5 79.9 69.1

133.2 76.8 64.4

123.1 73.7 61.4

118.6 70.8 59.7

110.9 68.0 57.9

3633

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is a potential candidate for removing toxic metals from contaminated water.

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ASSOCIATED CONTENT

S Supporting Information *

Effect of coexisting calcium ion on three toxic metal ions retention by tea waste at 298 K was presented in Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-559-2546552. Fax: +86-559-2546552. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 7. FT-IR spectra of tea waste samples loaded with different metal ions obtained at solution equilibrium pH of 4.9−5.2 and 298 K (metal capacities preloaded on adsorbent are as follows: Pb(II), 17 mg/g; Cd(II), 9.4 mg/g; Cu(II), 4.0 mg/g).

ACKNOWLEDGMENTS We greatly acknowledge the financial support from National Science Foundation for the Excellent Youth Scholars of Universities and Colleges of Anhui Province (2013SQRL091ZD), the Natural Science research project of Huangshan University (2013xkj003), the Natural Science Foundation of Anhui Education Department (KJ2012A259), and the Characteristic Major Foundation of Anhui Province (20100986).

application. Certainly, disposal of the exhausted adsorbent laden with metal ions is crucial for its practical application. Uyanık et al.28 and Goyal and Ahluwalia30 proposed that metals sorbed can be recycled or concentrated by igniting the saturated tea waste, and Mondal41 also provided a procedure of using diluted acid as regeneration for the tea waste laden metals. It is noteworthy that both the above methods might cause secondary pollution of toxic metals. Therefore, the exhausted tea waste should be carefully disposed to prevent toxic metals adsorbed into the environment again. Related research is under way, and preliminary results indicate that curing the exhausted tea waste might be a nice technology.

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REFERENCES

(1) Bosch, X. Cadmium cuts two ways: The heavy metal mutates DNA, and then prevents cells from repairing the damage. Science 2003, 609, 1. (2) Li, M.; Liu, Q.; Guo, L. J.; Zhang, Y. P.; Lou, Z. J.; Wang, Y.; Qian, G. R. Cu(II) removal from aqueous solution by Spartina alternif lora derived biochar. Bioresour. Technol. 2013, 141, 83. (3) Mishra, A.; Malik, A. Simultaneous bioaccumulation of multiple metals from electroplating effluent using Aspergillus lentulus. Water Res. 2012, 46, 4991. (4) Stoll, A.; Duncan, J. R. Enhanced heavy metal removal from waste water by viable, glucose pretreated Saccharomyces cerevisiae cells. Biotechnol. Lett. 1996, 18, 1209. (5) Chen, Q. Y.; Luo, Z.; Hills, C.; Xue, G.; Tyrer, M. Precipitation of heavy metals from wastewater using simulated flue gas: Sequent additions of fly ash, lime and carbon dioxide. Water Res. 2009, 43, 2605. (6) Fu, F. L.; Xie, L. P.; Tang, B.; Wang, Q.; Jiang, S. X. Application of a novel strategy -Advanced Fenton-chemical precipitation to the treatment of strong stability chelated heavy metal containing wastewater. Chem. Eng. J. 2012, 189−190, 283.

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CONCLUSIONS A bioadsorbent tea waste was prepared by repeatedly washing commercially available tea leaves with boiling water until the water was completely colorless. Sorption of Pb(II), Cd(II), and Cu(II) onto tea waste were found to be pH-dependent due to the ion-exchange mechanism of the CO stretching and bonded−OH groups with toxic metal ions. Pb(II) showed higher sorption capacity because of greater hydration energy compared to Cd(II) and Cu(II). Besides, the sorption processes of three metals onto tea waste were endothermic and spontaneous, and the smaller particle size of sorbent possessing the higher surface area is more favorable for toxic metal ions binding. All the results demonstrated that tea waste

Table 5. FTIR Spectral Characteristics of Tea Waste before and after Metal Ions Sorptiona wavenumber (cm−1)

differences

native tea waste

after lead sorption

after cadmium sorption

after copper sorption

D1

D2

D3

assignment

617 1039 1236 1533 1649 2121 2927 3359

621 1057 1232 1533 1653 2121 2925 3351

611 1053 1232 1535 1651 2121 2925 3338

611 1059 1232 1529 1653 2121 2925 3356

+4 +18 −4 0 +4 0 −2 −8

−6 +14 −4 +2 +2 0 −2 −21

−6 +20 −4 −4 +4 0 −2 −3

-CN stretching CO stretching -SO3 stretching secondary amine group CO stretching amine group aliphatic C−H group bonded−OH groups

a

D1 is the wavenumber difference in corresponding absorption peaks of tea waste before and after lead binding; D2 is for cadmium, and D3 is for copper. 3634

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dx.doi.org/10.1021/ie402510s | Ind. Eng. Chem. Res. 2014, 53, 3629−3635