J. Cent. South Univ. (2016) 23: 1626-1632

DOI: 10.1007/s11771-016-3217-7

Removal of Pb(II) ions from aqueous solutions by litchi pericarp and its leachate

PAN Yi-min(潘轶敏)¹, JIANG Rui-xue(姜瑞雪)^{1, 2}, YANG Ji-li(杨继利)², ZHENG Hao(郑昊)², YIN Er-qin(尹儿琴)^{1, 2}

1. Environmental Science and Engineering College, Hohai University, Nanjing 210098, China;

2. Water Conservancy and Civil Engineering College, Shandong Agricultural University, Tai’an 271018, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: The adsorption capacity of Pb(II) on litchi pericarps was investigated as a function of temperature, pH, and adsorbent dose using batch experiments. The experimental data obtained were evaluated using adsorption equilibrium isotherms and a kinetic model. Additionally, the removal of Pb(II) in leachate of litchi pericarps was also evaluated. The results show that litchi pericarps exhibit a high adsorption capacity to Pb(II), with the maximum removal efficiency occurring at a temperature of 25 °C, a pH of 6.0-7.0 and an adsorbent dosage of 10 g/L. Langmuir and Freundlich isotherms and the pseudo-second-order kinetic model can all fit the equilibrium adsorption satisfactorily, with correlation coefficients (R²) of 0.9935, 0.9918 and 1.0, respectively. An average removal efficiency of 66.65% is found for Pb(II) in leachate of litchi pericarps.

Key words: litchi pericarp; Pb(II); adsorption; kinetics; isotherms; leachate

1 Introduction

Heavy metal ions are often encountered in wastewater of many industries, including wastewater of factories in the field of mineral processing, metal plating, tanneries, electronic and chemical manufacturing. Heavy metal ions can be accumulated in food chains and persist in the ecosystem because of their nonbiodegradable nature, and thus, they may pose a serious threat to aquatic and terrestrial life [1]. Therefore, wastewater containing heavy metals must be treated properly before discharging them into receiving waters.

The conventional methods for removing heavy metals from wastewater include chemical precipitation, membrane filtration, ion exchange, electro-dialysis, and reverse osmosis [2]. However, the wide application of these methods is often restricted because of their high costs, high energy consumption, production of toxic sludges, and low feasibility for small scale industries [3]. As compared to the conventional treatment methods mentioned above, the adsorption process seems to be a more promising technology for removing heavy metal ions from wastewater in terms of costs and operation [4]. Activated carbon (AC) is one of the most important adsorbents because it has a very complex structure, a variety of surface groups, impurities, and other irregularities. The removal of heavy metal ions by AC has been investigated with some success [5]. However, there are still inevitable shortcomings for the wide application of AC because of its relatively high initial costs and the need of a regeneration system [4]. Thus, at present, more effective and low-cost absorbent materials are strongly recommended for removing heavy metal ions from wastewater [6].

In recent decades, considerable research efforts for the development of low-cost adsorbents have been conducted based on inexpensive materials with local abundant availability. Among these materials, agro-wastes, which are relatively cheap and available in large quantities, contain various polymeric substances such as cellulose, hemicellulose, pectin, lignin and proteins [7]. These various polymeric compounds, in combination with the porous structure of agro-wastes, play an important role for binding heavy metal ions [8]. Thus, they are currently receiving increasing attention as adsorbent for removing heavy metals from wastewater. Various materials, based on agro-wastes, have been assessed for removing heavy metals from wastewater by adsorption, including sugarcane bagasse [1], pomegranate peel [3], banana peels [7], orange peel [9], banana leaves [10], watermelon rind [11], rice husk [12], and durian [13].

Litchi (Litchi chinensis Sonn.), a subtropical fruit, is one of the most popular fruits in Southeast Asia, especially in China. The annual litchi production is about 1.5 million tons in China [14]. Moreover, other semitropical regions began to cultivate this kind of fruit during the last 30 years [15]. Litchi pericarp accounts for more than 15% of the litchi fresh weight, thereby resulting in notable processing waste volumes [16]. Litchi pericarp contains significant amounts of chemical components such as esters, fat acids, heterocyclic compounds, cyclic ethers, cyclic olefines, and polysaccharides [17]. These chemical components bear abundant carboxyl, hydroxyl, and amide groups, which play a critical role in adsorption processes. Thus, litchi pericarp may also be used as a prospective suitable adsorbent for removing heavy metals from aqueous solution. However, to the best of our knowledge, the adsorption of heavy metals by litchi pericarp has not been reported.

The Pb(II) ion was selected as adsorbate in this work because of its widespread distribution in many industrial wastewater and its well-known strong toxic nature to aquatic life. The aim of this work is to investigate the feasibility of litchi pericarp as a bio-adsorbent for removing Pb(II) ions from aqueous solution. The effects of temperature, pH and adsorbent dose on adsorption capacity were investigated using batch experiments. The obtained experimental data were evaluated and fitted using adsorbent equilibrium isotherms and a kinetic model. Additionally, the removal of Pb(II) ions in leachate of litchi pericarps was also evaluated.

2 Methods and materials

2.1 Preparation of litchi pericarp samples

Litchis (Litchi chinensis Sonn.) were purchased from a local market of Tai’an, Shangdong, China. Pericarps were separated gently from the fruit, washed with tap water firstly, and then rinsed with deionized water three times. Subsequently, these pericarps were cut into small pieces, dried at 70 °C inside a convectional oven until reaching a constant weight. The dry litchi pericarps (LPs) were ground and sieved using a 60 mesh, and finally stocked in polythene for use.

2.2 Study of process parameters

Batch experiments were carried out using a series of polythene centrifuge tubes (100 mL) covered with Teflon sheets to prevent any foreign contamination. The effects of temperature, pH and adsorbent dose on the adsorption of Pb(II) onto LP were studied with an initial Pb(II) concentration of 50 mg/L. To study the effect of a particular parameter, this parameter has been changed progressively while keeping the other two constant. After completion of the adsorption assay, the resulting solutions were centrifuged and then filtered through a 0.45 μm cellulose acetate membrane filter. The concentrations of Pb(II) were determined in the filtrates using atomic absorption spectroscopy.

The adsorption efficiency A of Pb(II) was calculated from the following equation:

A=(C₀-C_e)/C₀×100%                          (1)

where C₀ and C_e are the initial and equilibrium Pb(II) concentrations (mg/L), respectively.

2.3 Study of adsorption isotherms

Pb(II) concentrations of 30, 40, 50, 60, and 70 mg/L were made by proper dilution of stock solutions of Pb(II). The pH was adjusted to 5.0 using 1 mol/L HCl and 1 mol/L NaOH solutions. Accurate 0.5 g of the prepared LP powder (60 mesh) was added to 50 mL of each metal solution, and agitated for 1 h. Subsequently, the suspensions were filtered through a 0.45 μm cellulose acetate membrane filter, and the filtrates were analyzed for Pb(II) using flame atomic absorption spectroscopy. Langmuir and Freundlich isotherms were plotted using standard linear equations, and the corresponding two parameters were calculated from the respective graphs.

2.4 Kinetics studies

For kinetics studies, the batch technique was used because of its simplicity. A series of polythene centrifuge tubes (100 mL) containing 50 mL Pb(II) solutions (50 mg/L) were kept in a thermostatic shaking water bath (25 °C). As described above, the pH value of the solutions was adjusted to 5.0 using 1 mol/L HCl and 1 mol/L NaOH solutions. 0.5 g of the prepared LP powder (60 mesh) was added to each tube, and the tubes were agitated mechanically at 240 r/min. At given time intervals, the solutions were centrifuged and filtered through a 0.45 μm cellulose acetate membrane filter, and the filtrates were analyzed for Pb(II) using atomic absorption spectroscopy. Pseudo-second-order model was employed to discuss the rate and kinetics of sorption of Pb(II) on the prepared adsorbent.

2.5 Removal of Pb(II) in leachate of LP

0.50 g of the prepared LP was put into a polyethylene centrifuge tube (100 mL), and 50 mL of ultrapure water (>18 MW) was added to each tube. 10 tubes were shaken simultaneously in a vapor-bath at a constant temperature of 25 °C by a vibrator working at 240 r/min. The pH values of all suspensions were approximately 5.0. The tubes were taken out after 3 h, centrifuged at 4000 r/min, and the supernatants were used as leachate of LP. The total phosphorus content (TP) in the leachate was determined by the molybdenum– antimony anti-spectrophotometric method. The further adsorption processing of Pb(II) by the LP leachate follows the steps described above, namely, agitation at constant temperature, filtering, and analysis by atomic absorption spectroscopy. The removal efficiency of Pb(II) was determined by the different concentrations of Pb(II) in the initial suspension and the filtrate.

2.6 Analysis and data processing

All experiments were carried out in triplicate, and the averages, together with the corresponding standard deviations, are used for the discussion. All chemical reagents were of analytical grade. All glassware and polyethylene tubes were soaked in 3% HCl overnight before being used in the experiments. Excel 2003 and SPSS 11.5 software packages were used to produce figures and conduct statistical analyses.

3 Results and discussion

3.1 Characterization of litchi pericarp

The properties of litchi pericarp (LP) were characterized using FTIR spectroscopic measurements. The FTIR spectra of LP (Fig. 1) exhibited a broad and intense absorption peak at 3360 cm^-1, which was assigned to the stretching vibrations of —OH group of polymeric compounds, such as cellulose, pectin, hemicellulose and lignin [18-19]. The peaks observed at 2930 cm^-1 and 1620 cm^-1 were attributed to the stretching vibrations of C—H and —COO^-, respectively [18, 20]. Finally, the peak appearing at 1060 cm^-1 was attributed to the stretching vibrations of C—OH of carboxyl and hydroxyl groups [21]. These results indicate that many functional groups (—COOH, —OH) embedded in LP would facilitate the adsorption of heavy-metal ions.

Fig. 1 FTIR spectra of litchi pericarp (LP)

3.2 Effects of operating parameters on Pb(II) adsorption

3.2.1 Effect of temperature

The effect of temperature on the adsorption efficiency of Pb(II) by litchi pericarp (LP) was investigated at 15, 25, 35 and 45 °C. The concentration of adsorbent and adsorbate was invariably kept at 10 g/L and 50 mg/L, respectively. Adsorption of Pb(II) onto LP as a function of temperature is shown in Fig. 2. The Pb(II) adsorption efficiency of LP is always more than 96% within the tested temperature range. Evidently, varying the temperature had no significant effect on the adsorption of Pb(II) onto LP, with a minimum adsorption efficiency of 96.6% at 45 °C and a maximum of 98.0% at 25 °C. These results indicate a temperature of 25 °C for the maximum removal of Pb(II) by LP from aqueous solution.

Fig. 2 Adsorption of Pb(II) ions onto LP as a function of temperature

3.2.2 Effect of pH

The pH of the solution has a significant effect on the adsorption of heavy metal ions, since it determines the surface charge of the adsorbent and the degree of ionization and speciation of the adsorbate [12]. Sorption experiments were carried out in the pH range of 3-7, keeping all other parameters constant. Figure 3 depicts the effect of pH on the Pb(II) adsorption of LP.

As evident from Fig. 3, LP had a relatively high Pb(II) adsorption capacity within the tested pH range. The minimum Pb(II) adsorption efficiency of 89.8% was observed at pH=3.0, followed by an increase upon increasing pH. The maximum adsorption of 97.6% occurred at pH=6.0-7.0. The minimum adsorption of metal ions at low pH values was also found in other fruit peels used as biosorbents, such as orange peel [2], banana peel [7], and jackfruit peel [22]. The influence of the pH may be attributed to the fact that at lower pH values, a higher concentration of H⁺ competes strongly with Pb(II) ions for active sites on the surface of LP, resulting in a reduction in Pb(II) ions binding on the adsorbent surface. An increasing pH results in more functional groups available for the binding of Pb(II) ions because of the decreased concentration of H⁺, and consequently, the Pb(II) ions adsorption is enhanced. This demonstrates that ion exchange may be considered as one of the important mechanisms responsible for the adsorption of Pb(II) ions onto LP [23-25].

Fig. 3 Adsorption of Pb(II) onto LP as a function of pH

3.2.3 Effect of adsorbent dose

Adsorbent dosage is an important parameter due to its determination of the sorption capacity of the adsorbent. Adsorption assays were carried out for a dosage of litchi pericarp (LP) varying between 2 and 40 g/L, keeping all other parameters constant. Adsorption of Pb(II) onto LP as a function of adsorbent dose is shown in Fig. 4. The minimum Pb(II) adsorption efficiency (89.8%) was observed at an LP dosage of 2 g/L, followed by a noticeable increase in the Pb(II) adsorption as the adsorbent dose increasing from 2 to 6 g/L. However, further increase in adsorbent dose beyond 10 g/L did not result in a significant increase in the Pb(II) adsorption capacity of LP. Moreover, as compared to an adsorbent dose of 20 g/L, a slight reduction in the adsorption percentage of Pb(II) could be identified at an adsorbent dose of 40 g/L. Similar results were also observed in previous reports for the adsorption behavior of metal ions by different bio-adsorbents [2]. It may be concluded from these results that at lower adsorbent dosage, the increase in the available surface area and the number of adsorption sites upon increasing adsorbents dosage results in a higher removal efficiency of Pb(II) ions. However, clustering or/and aggregation of adsorbents may occur because of a higher adsorbent dosage. Consequently, this may result in a decrease in the total surface area and the number of binding sites of the adsorbent [2, 26]. Thus, an optimum LP dose of 10 g/L is required for the removal of Pb(II) ions from a 50 mg/L solution.

Fig. 4 Adsorption of Pb(II) ions onto LP as a function of adsorbent dose (at 25 °C and pH=5.0)

3.3 Adsorption isotherms

Adsorption isotherms are established by determining the ratio between adsorbed metal ions on the adsorbents and the residual metal ions in the solution at a fixed temperature at equilibrium. Equilibrium adsorption isotherms are promising parameters for evaluating the adsorption capacity of sorbents. The most widely applied models describing adsorption equilibrium are the Langmuir and Freundlich isotherms [27]. These two important isotherms are selected for investigating the adsorption capacity of litchi pericarp (LP) in this work. The Langmuir isotherm assumes monolayer coverage of the adsorbate over a homogeneous adsorbent surface, and the biosorption of each molecule onto the surface has identical activation energy. Based on these assumptions, the Langmuir isotherms (Eq. (2)) can be expressed as follows:

                         (2)

where q_e is the amount of metal ions adsorbed (mg/g); C_e is the concentration at equilibrium (mg/L); q_m is the monolayer capacity of the adsorbent (mg/g); b is the adsorption constant (L/mg). According to this equation, a plot of 1/q_e vs 1/C_e should be a straight line with a slope of 1/q_mb and an intercept of 1/q_m.

The Freundlich isotherm supposes a heterogeneous surface with a nonuniform distribution of adsorption heat over the surface and a multilayer biosorption can be expressed. The Freundlich isotherm (Eq. (3)) can be expressed as follows:

                         (3)

where K_F and 1/n are Freundlich isotherm constants. If Eq. (3) applies, a plot of lg q_e versus lg C_e will give a straight line with a slope of 1/n and an intercept of K_F.

The adsorption of Pb(II) onto LP as a function of the initial Pb(II) concentration was studied in the range from 30 to 70 mg/L, keeping all other parameters constant with respect to optimum adsorbent dose, temperature, and pH. The Langmuir and Freundlich isotherms of Pb(II) adsorption on LP are given in Fig. 5. As evident, the plots are linear with good correlation coefficients of R²=0.9935 and 0.9918 for the Langmuir and Freundlich isotherm, respectively, indicating that both models can be used for a satisfying explanation of the equilibrium adsorption of Pb(II) ions by LP. Based on the results of Fig. 5(b), the Freundlich isotherm parameters K_F and n were determined to be 1.30 and 1.05, respectively. Additionally, the value of 1/n obtained in the present work indicates a favorable adsorption, as it lies between 0 and 1 [22]. Similar results were obtained by LASHEEN et al [2] and HUSSEIN et al [28]. According to the Langmuir adsorption isotherms shown in Fig. 5(a), the maximum adsorption capacity (q_m) for Pb(II) is 78.13 mg/g. This value is much higher than the adsorption capacity of other bioadsorbents reported in the literatures, such as 2.18 mg/g for banana peel [23], 2.50 mg/g for bagasse fly ash [29], and 42.29 mg/g for grape bagasse [30].

Fig. 5 Langmuir (a) and Freundlich (b) adsorption isotherms for sorption of Pb(II) by LP

3.4 Kinetics studies

The study of sorption kinetics in wastewater treatment is significant as it provides valuable insights into the reaction pathways and the mechanism of the sorption reactions. Additionally, the kinetics describes the solute uptake rate, which in turn controls the residence time of the sorbate at the solid/solution interface [31]. Based on the results of AHARONI et al [32], the adsorption kinetics of bivalent heavy metal ions can be described satisfactorily by the second-order kinetic model. Therefore, this model (Eq. (4)) was employed for the kinetics study of Pb(II) adsorption on LP in present work:

                             (4)

where k_s (g/(mg·min)) is the constant of the pseudo-second-order rate; q_e (mg/g) is the adsorption capacity at equilibrium; and q_t (mg/g) is the adsorption capacity at a time t (min). The equilibrium adsorption capacity q_e and the pseudo-second-order rate constant k_s can be experimentally determined from the slope and the intercept of the plot of t/q_t and t.

The graphical interpretation of the experimental data for the second-order-kinetic model is given in Fig. 6. As observed from Fig. 6, the correlation coefficient (R²) for the pseudo-second-order kinetic model was extremely high (R²=1.0). Furthermore, the theoretical value of q_e obtained from the plot is 4.86 mg/g, which agrees well with the experimental values (4.85 mg/g), suggesting that the adsorption follows the pseudo- second-order reaction mechanism, and the adsorption rate is controlled by chemical adsorption. Based on these results, LP could be considered as a promising potential bio-adsorbent for the removal of heavy metals from aqueous solutions.

Fig. 6 Pseudo-second-order adsorption kinetics of Pb(II) onto LP

3.5 Removal of Pb(II) in leachate of LP

There are many complex chemical components embedded in litchi pericarp (LP), such as polysaccharide, fat acids and phosphates [14, 33]. These chemical components would be released from LP into aqueous solution, and consequently affect the behavior of heavy metal ions in solutions. The total phosphorus (TP) concentration in LP leachate was 2.76 mg/L. The corresponding results of the removal of Pb(II) in leachate of LP are shown in Fig. 7.

As evident from Fig. 7, the concentration of Pb(II) decreased rapidly from 11.11 mg/L to 3.69 mg/L within 10 min, with an average removal efficiency of 66.65%.Pb(II) ions can precipitate with the phosphate radical (PO₄^3-) to form Pb₃(PO₄)₂ (Eq. (5)), which is insoluble in aqueous solution. Therefore, the rapid removal of Pb(II) in leachate of LP may be attributed to the co-precipitation of Pb(II) with some inorganic salts such as phosphate. This result also suggests that co-precipitation may also be an important mechanism for Pb(II) removal during the adsorption process using LP.

Fig. 7 Removal of Pb(II) in leachate of LP

↓                  (5)

4 Conclusions

Litchi pericarps (LP) exhibit a high adsorption capacity for Pb(II), with the maximum adsorption efficiency of more than 96% occurring at 25 °C, pH= 6.0-7.0 and adsorbent dosage of 10 g/L. Langmuir and Freundlich isotherms and the pseudo-second-order kinetic model can all fit the equilibrium adsorption of Pb(II) on LP satisfactorily. The maximum adsorption capacity for Pb(II) is 78.13 mg/g, indicating a great potential of LP as a bio-adsorbent for removing Pb(II). The removal of Pb(II) in leachate of LP suggests that co-precipitation might also be a potential mechanism for Pb(II) removal in the adsorption process.

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(Edited by YANG Bing)

Foundation item: Project(51208173) supported by the National Natural Science Foundation of China; Project(ZR2014EEM005) supported by the Natural Science Foundation of Shandong Province, China

Received date: 2015-04-14; Accepted date: 2015-09-10

Corresponding author: PAN Yi-min, Professor; Tel: +86-18638746311; E-mail: 75400723@qq.com