In this study, infused tea leaves as a low-cost adsorbent have been used in the removal of the Pb 2+, Fe 2+ and Cd 2+ ions from aqueous solution. The adsorption study was carried out in a batch process and the effects of parameters such as initial pH, adsorbent dose, contact time and initial metal ion concentration were investigated. Experimental results showed that the maximum adsorption of metal ions occurred at pH 5 for Pb 2+ and Fe 2+ and at pH 6 for Cd 2+. Adsorption of metal ions increased with increasing adsorbent concentration and contact time. The isothermal data for the adsorption of metal ions by infused tea leaves were found to fit well with the Langmuir equations. Based on the experimental data of the Langmuir isotherm model, the maximum adsorption capacities of the metal ions onto the infused tea leaves were found in the order of Pb 2+ > Cd 2+ > Fe 2+ with the adsorption capacities of 26.32 mg ·g ǃ, 14.29 mg ·g ǃ and 12.38 mg ·g ǃ respectively. The adsorption process followed the pseudo-second order reaction and the corresponding rate constant were found to be 4.30 × 10 Dž g ·mg ǃ ·min ǃ, 1.75 × 10 ǃ g ·mg ǃ ·min ǃ and 1.45 × 10 DŽ g ·mg ǃ ·min ǃ for Pb 2+, Fe 2+ and Cd 2+ ions respectively.
The rapid growth of the manufacturing industries such as chemicals, foods, motor vehicles, paper, electronics, etc., have caused a lot of pollutions in the world especially in developing countries such as China, India, Brazil as well as Malaysia. The environmental pollution by heavy metals such as Cd, Zn, Fe, Ni, and Pb has become one of the many serious pollution issues. These heavy metals are often discharged into the environment by a number of industrial and domestic wastewater processes [
Any natural products that are produced in bulk will be a potential biosorbent for the removal of heavy metals from the aqueous environment. This is because the main components of natural products are lignin and cellulose which contain high amount of electronegative functional groups such as amino, hydroxyl, carboxylic acid and ester. These electronegative functional groups can thus provide good chemical interactions between the biosorbent and the metal ions [
Tea is a tropical plant and it belongs to the Theaceae family. The main tea producers in the world are the two most populous countries, that is, China and India, followed by Kenya and Sri Lanka [
All the chemicals used were of analytical grade. The stock solutions (1000 mg∙L−1) of Pb2+, Fe2+ and Cd2+ were prepared in distilled water from their respective nitrate salts. All the standard solutions were prepared by diluting the stock solution with distilled water. Sample of tea leaves were obtained from packaged products of the BOH plantation Malaysia, the biggest tea producer in Malaysia, contributing to 70% of the total production in Malaysia.
Consumed infused tea leaves were used in this study. They were first washed with hot distilled water (90˚C) to remove the remaining soluble and colored components. This process was done repeatedly until the discharging distilled water became colorless. The washed infused tea leaves were dried in an oven at 70 degree for 24 hours. It was then ground and sieved to obtain a powdered material with particle size ranging from 0.5 - 1.0 mm. This material was used for the metal adsorption studies without any physical or chemical treatment and will be called infused tea leaves.
The batch adsorption experiments for individual metal ions were carried out at a constant temperature (28˚C) on a rotary shaker (300 rpm) using 125 mL capped conical flasks. For each set of experiment, 0.2 g of infused tea waste was added to each 100 mL metal ion solutions of various concentrations (20 - 100 mg∙L−1) for isotherm study and 50 mg∙L−1 for kinetic study. The mixtures which contain the infused tea leaves and the respective metal ion were agitated on a rotary shaker at 300 rpm for 105 min for both the isotherm and kinetic studies. The mixtures were then filtered through Whatman no. 40 filter paper to remove particulates and the filtrate was analyzed with atomic absorption spectrometry (Shimadzu AA6200) to determine the concentration of metal ions. The same procedure was applied to the control samples with the same process and concentration but without infused tea leaves. The amounts of metal ion adsorbed on the biosorbent were determined from the difference of metal ion concentrations in the initial and final suspensions. The batch experiments were performed in triplicate and the mean value was used for each set of data. The amount of metal ions adsorbed at equilibrium, qe (mg∙g−1) and the percentage removal (%) of metal ions were calculated as follows [
where Co and Ce are the initial and equilibrium concentrations of metal ions (mg∙L−1), V is the volume of solution (mL) and W is the weight of infused tea leaves used (g). The effect of pH on metal ions removal was examined by varying the pH from 2 to 7, with initial metal ion concentrations of 50 mg∙L−1, infused tea leaves of 0.20 g/100mL and agitation at 300 rpm for 105 min at 28˚C. The initial pH of the metal ions was adjusted by addition of 0.10 M HCl or NaOH.
The functional groups present in the infused tea leaves were characterized by a Perkin Elmer Spectrum 100 Fourier transform infrared (FT-IR) spectrometer fitted with Attenuated Total Reflection (ATR). In this analysis, the infused tea leaves with or without metal salts were analyzed directly without blending with KBr. The Spectra of the samples were recorded from 4000 to 650 cm−1.
ions. On higher pH values, a slight decrease in adsorption for Pb2+, Fe2+ and Cd2+ ions were observed. This is because at higher pH, metal precipitations such as Pb(OH)2, Fe(OH)2 and Cd(OH)2 were observed due to the existence of OH− ions in the adsorption medium. At lower pH such as pH 2, very little adsorption took place (0% Cd2+, 12.76% Pb2+, and 3.55% Fe2+). This could be due to the competition between the H+, produced from the protonation of the active sites of the adsorbent, and the M+ from the metal ions [
The removal of metal ions from aqueous solution was significantly increased with the increase in the amount of adsorbent dosage (
due to the fact that at lower adsorbent dosage, the metal ions were competing for limited adsorption sites. There were excess of metal ions in the solution and gave a lower percentage removal of metal ions. At higher adsorbent dosage, there were more adsorption sites available for similar number of metal ions in the solution and caused the drop of the amount of metal ions adsorbed per unit mass of adsorbent. This explains why the percentage removal of metal ions increased at higher adsorbent dosage but the adsorption capacity (q) drops to a lower value. The similar trend has also been reported for other biomass materials such as cotton ball [
The adsorption of the three metal ions by the infused tea leaves versus contact time is illustrated in
studies conducted by researches such as Gok and Aytas [
The amount of time required to reach equilibrium for the adsorption of the three metal ions is practically the same and is within 75 - 90 minutes. From
Usually the effects of electronegativity and ionic radius have been used by different researchers in explaining the trend of adsorption capacity separately. However, we strongly believe that a combination of both the electronegativity and ionic radius will be more appropriate in explaining the adsorption capacity for a particular metal ion onto a biosorbent. Hence, the covalent index of metal ions will be used in explaining the trend of the adsorption capacity in this study and it can be calculated using the following equation [
where Xm and r are the electronegativity and ionic radius of the metal ions respectively. The value of 0.85 is a constant assumed to reflect the radius of O and N donor atoms [
Nevertheless, what we can conclude in this study is that both the ionic radius and electronegativity do play important roles in determining the adsorption capacity of the infused tea leaves towards the three metal ions. Furthermore, it is also obvious that Pb2+ has a much higher adsorption capacity compared to the other two metal ions. The much higher value of covalent index of the Pb2+ indicates that Pb2+ has a higher degree of binding capacity towards the biosorbent. Our results show that Pb2+ is the most easily bonded component to the binding sites of the infused tea leaves, followed by Cd2+ and Fe2+. Similar results were found by other researches using different biosorbents. For examples, the use of Neurospora crassa for the removal of Pb(II) and Cu(II) from aqueous solutions [
The adsorption isotherm represents the relationship between the amounts of adsorbate adsorbed on the surface of a unit weight of solid adsorbent at a fixed temperature at equilibrium. It is usually described by certain constants whose values express the surface properties and affinity of the adsorbent. In the present work, Langmuir and Freundlich isotherms are used to describe the adsorption isotherm of the three metal ions. The Langmuir isotherm model assumes that there is a finite number of identical adsorption sites on the surface of the adsorbent and it is only valid for monolayer adsorption. Once a metal ion occupies a site, then there is no further adsorption taking place at that site [
where Ce is the equilibrium concentration (mg∙dm−3) of metal ion, qe is the amount of adsorption at equilibrium (mg∙g−1), qm is qe for a complete monolayer (mg∙g−1), b is an equilibrium constant of Langmuir (dm3∙mg−1). A plot of Ce/qe versus Ce should indicate a straight line of slope 1/qm and an intercept of 1/bqm.
The essential features of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter RL, which is defined by the following equation [
where Co is initial concentration (mg∙dm−3) and b is the Langmuir constant (dm3∙mg−1).
The value of RL indicates the pattern of the isotherm. If the RL > 1, the adsorption is unfavorable, and if RL = 1, there will be a linear adsorption. If 0 < RL < 1, the adsorption is favorable, and if RL = 0, the adsorption is irreversible.
The Freundlich isotherm is an empirical equation based on adsorption on a heterogeneous surface of an adsorbent [
where KF and 1/n are the Freundlich constants of the systems, indicating the adsorption capacity and adsorption intensity, respectively. The adsorption constants of Freundlich isotherm Kf and 1/n can be determined from the intercept and slope of logqe versus logCe. The 1/n value is usually dependent on the nature and strength of the adsorbent as well as the distribution of active sites [
The Langmuir and Freundlich constants and their correlation coefficient values (R2) for the three metal ions calculated based on these isotherm models are summarized in
Metal ions | Langmuir isotherm model | Freundlich isotherm model | ||||
---|---|---|---|---|---|---|
qm (mg/g) | b (dm3/mg) | R2 | Kf | 1/n | R2 | |
Cd2+ | 14.29 | 0.1077 | 0.996 | 3.25 | 0.334 | 0.952 |
Pb2+ | 26.32 | 0.5071 | 0.995 | 10.56 | 0.258 | 0.833 |
Fe2+ | 12.38 | 0.0622 | 0.993 | 1.58 | 0.456 | 0.947 |
isotherm, indicating that there is a good agreement between the experimental data and the Langmuir isotherm parameters. Whereas, for the Freundlich isotherm, the values of correlation coefficients for the three metal ions were found to be lower (0.952 for Cd2+, 0.830 for Pb2+ and 0.947 for Fe2+) than those of the Langmuir isotherm, indicating that the set of equilibrium data is not in as good agreement as in the case of the Langmuir isotherm. This confirms that the metal ions/infused tea leaves adsorption data follows the Langmuir isotherm.
A plot of Ceq/qe versus the various concentrations of the three metal ions at equilibrium (Ceq) is shown in
In this study (
The removal of heavy metal ions using tea waste as an adsorbent has been reported by several other researchers. The treatment conditions of the tea waste, the types of metal ions being removed, and the maximum adsorption capacities for each study are summarized in
The adsorption kinetics of the removal of metal ions by the infused tea leaves was described using the pseudo-first-order and pseudo-second-order models. The pseudo-first-order in the linear form is generally expressed as follows [
where qe and qt are the amount of metal ions adsorbed per unit weight on the adsorbent (mg/g) at equilibrium, and at time t, respectively, and k1 is the rate constant of the adsorption (min−1). Plot of
Treatment given | Cd | Pb | Fe | Cu | Ni | Cr6+ | Reference |
---|---|---|---|---|---|---|---|
None | 65 | 48 | [ | ||||
None | 43a | [ | |||||
Na2S | 81 | [ | |||||
None | 10a | [ | |||||
None | 55b | [ | |||||
None | 18b | [ | |||||
None | 11 | 9 | [ | ||||
Acid washed | 79 | 27 | [ | ||||
None | 14 | 26 | 12 | This work |
aaverage value of different flow rates. bmaximum adsorption obtained at 60˚C.
where k2 is the rate constant of adsorption (g∙mg−1∙min−1), and qe and qt are the amounts of metal ions adsorbed at equilibrium and at time t (mg∙g−1), respectively. The constant k2 can be determined experimentally by plotting t/qt against t. The initial adsorption rate, h (mg∙g−1∙min−1) can be calculated from the pseudo-second-order by the following equation [
The corresponding results of the two kinetics models are summarized in
First-order kinetic model | Second-order kinetic model | Intraparticle diffusion | |||||||
---|---|---|---|---|---|---|---|---|---|
metal ion | qexp (mg∙g−1) | k1 (min−1) | q1 (mg∙g−1) | R2 | k2 (g∙mg−1∙min−1) | q2 (mg∙g−1) | R2 | kp (mg∙g−1∙min−1/2) | R2 |
Cd(II) | 8.11 | 4.80 × 10−2 | 9.25 | 0.754 | 1.45 × 10−2 | 8.53 | 0.994 | 3.8885 | 0.966 |
Pb(II) | 21.21 | 6.03 × 10−2 | 15.85 | 0.967 | 4.30 × 10−3 | 24.10 | 0.998 | 0.8385 | 0.965 |
Fe(II) | 6.71 | 5.44 × 10−2 | 2.51 | 0.975 | 1.75 × 10−1 | 3.53 | 0.996 | 4.0569 | 0.976 |
(24.10 mg∙g−1), followed by Cd2+ (8.53 mg∙g−1) and Fe2+ (3.53 mg∙g−1). The reasoning is that Pb2+ which has a larger ion radius and electronegativity as compared to the other two metal ions is better accommodated by the infused tea leaves as discussed in the previous section. In addition, the calculated q2 values for the three metal ions were found relatively closer to the experimental qexp values compared to the q1 values for the three metal ions (
In order to understand the adsorption mechanisms and the rate limiting steps that may affect the kinetics of adsorption, the kinetic experimental results were fitted to the Weber’s intraparticle diffusion model. The rate parameters for intraparticle diffusion (kp) at different initial concentrations can be determined using the following equation [
where C is the intercept and kp is the intraparticle diffusion rate constant (mg∙g−1∙min−1/2), which can be evaluated from the slope of a linear plot of qt versus t1/2. According to this model, the plot of qt versus t1/2 should be linear if intraparticle diffusion is involved in the adsorption process and if the plot passes through the origin then the intraparticle diffusion is the only rate-limiting step [
Like all other biomass, infused tea leaves are composed of cellulose, hemi-cellulose and lignin [
Before the adsorption of Pb2+ onto the infused tea leaves (
C=O stretching vibration of non-ionic carboxyl groups (-COOH) or their esters (-COOCH3) and the symmetric and asymmetric stretching vibrations of ionic carboxyl groups (-COO−) are observed at 1622.72 cm−1 [
The present study shows that infused tea leaves like the most other natural absorbents are able to remove Cd2+, Pb2+ and Fe2+ ions from aqueous solutions. The equilibrium isotherms between metal ions and infused tea leaves have been developed and analyzed according to two isotherm equations, namely the Langmuir and Freundlich adsorption isotherm models. The results from this study are found to fit well to Langmuir adsorption isotherm model with the maximum adsorption capacity of 14.29 mg∙g−1, 26.32 mg∙g−1, and 12.38 mg∙g−1 respectively for Pb2+, Cd2+ and Fe2+ ions. A comparison of the kinetic models on the overall adsorption rate showed that the adsorption system is best described by the pseudo-second order kinetic model, and the rate constants are found to be 4.30 × 10−3 g∙mg−1∙min−1, 1.75 × 10−1 g∙mg−1∙min−1 and 1.45 × 10−2 g∙mg−1∙min−1 for Pb2+, Fe2+ and Cd2+ ions respectively. Kinetic results of the adsorption also indicate that chemical sorption is the basic mechanism involved in this system. The FTIR results show that OH, C=O, C-O, and C-H are the main functional groups of infused tea leaves that are involved in the metal ions adsorption. As a conclusion, the results show that infused tea leaves which are a natural waste can be used as an alternative low-cost and environment-friendly biosorbent for the removal of heavy metals from contaminated water.
We thank the Faculty of Applied Sciences and Computing of Tunku Abdul Rahman University College for financial support. We also wish to thank Dr. Teh Geok Bee, Dr. Lo Fook Loong and Ms. Selvi for their valuable comments and help in proof reading of the manuscript.
Chen Son Yue,Kok How Chong,Cheah Cheng Eng,Ling Siang Loh, (2016) Utilization of Infused Tea Leaves (Camellia sinensis) for the Removal of Pb2+, Fe2+ and Cd2+ Ions from Aqueous Solution: Equilibrium and Kinetic Studies. Journal of Water Resource and Protection,08,568-582. doi: 10.4236/jwarp.2016.85047