Adsorptive behavior, isothermal studies and kinetic modeling involved in removal of divalent lead from aqueous solutions, using Carissa carandas and Syzygium aromaticum

Abstract This study is focused on the biosorption of lead(II) ion onto surface of Carissa carandas and Syzygium aromaticum biomass from aqueous solution. The operating parameters, pH of solution, biomass dosage, contact time, initial metal ion concentration, and temperature considerably affect the biosorption efficiency of Pb(II). Biosorbent C. carandas leaf powder showed higher sorption efficiency than that of biosorbent S. aromaticum powder under identical experimental conditions. It was observed that the lead(II) removal percentage was found highest of 95.11% for C. carandas and 91.04% for S. aromaticum at contact period of 180 min. Also, it was observed that the regression coefficient (R2 = 0.99) for the pseudo-second-order kinetic model is higher in comparison with the pseudo-first-order kinetic model and the calculated value of qe for the pseudo-second-order kinetic model is very close to the experimental value, which indicates that it fits well with the equilibrium data for Pb(II) sorption from aqueous solutions on biosorbents. Also, the adsorption of Pb(II) onto C. carandas was best described by the Freundlich isotherm model.


PUBLIC INTEREST STATEMENT
Potable water is a great concern worldwide and the researchers are trying hard for its purification and sustainability. Heavy metals are important pollutant present in water, which may cause serious adverse effects on the health of human beings, animals, and environment. Various modern and promising techniques are nowadays popular. Biosorption is one of them. The present research is a modest effort to remove lead from aqueous solutions by using Carissa carandas and Syzygium aromaticum as adsorbents. Where, we are reporting adsorptive behavior, isothermal studies, and kinetic modeling involved in removal of divalent lead from aqueous solutions. This research opens a new door to the researchers to try naturally available adsorbents or agriculture wastes to remove heavy metals from water and wastewater.

Introduction
Industrialization is the biggest source of heavy metals pollution in environment. Contrasting organic pollutants, the mainstream of which are vulnerable to biological degradation, heavy metal ions is not degradable into undamaging end products (Gautam, Sharma, Mahiya, & Chattopadhyaya, 2015). Heavy metals have been exceptionally released into the environment due to speedy industrialization and have become a major global concern. Heavy metals like cadmium (Cd), zinc (Zn), copper (Cu), nickel (Ni), lead (Pb), mercury (Hg), and chromium (Cr) are major habitually detected in industrial wastewaters, which instigate from metal plating, mining activities, smelting, battery manufacture, tanneries, petroleum refining, paint manufacture, pesticides, pigment manufacture, printing and photographic industries, etc. (Forgacs, Cserháti, & Oros, 2004;Mahiya, Lofrano, & Sharma, 2014a).
Lead exposure is toxic or poisonous. It causes many dangerous diseases like encephalopathy, nephropathy, anemia, mental retardation, seizures and it forms complexes with oxo-groups in enzymes and affect the hemoglobin synthesis (Ademorati, 1996;Schümann, 1990).
The purpose of the present study is to evaluate the efficiency of C. carandas and S. aromaticum as biosorbent for removal of divalent lead from aqueous solution. Maximum adsorption capacity of biosorbent, adsorption intensity of the adsorbate on biosorbent surface, and biosorption potentials of biosorbent were estimated by Langmuir and Freundlich isotherms, respectively. In the present study C. carandas and S. aromaticum leaves were used for biosorption of lead(II) from aqueous solutions.
Batch adsorption experiments were carried out at ambient temperature (300 K) as a function of solution pH (2-12), biosorbent dosage (20-100 g/L), contact time (60 min interval and up to 300 min), and initial metal ion concentration. Then, equilibrium isotherms and kinetic data parameters were evaluated. The prepared adsorbent was characterized by SEM analysis and FTIR analysis.

Chemicals
Analytical-grad chemicals were used in this work without further purification. To avoid any interference of other ions, all solutions were prepared using double distilled water. To evaluate the significance of the adsorbent, stock solutions (1,000 mg/L) of Pb(II) were prepared by dissolving 1.831 g of (CH 3 COO) 2 Pb•3H 2 O in 1,000 mL volumetric flask and make up to the mark with double distilled water. All the required working solutions were prepared by diluting the stock solution with double distilled water. Dilute solutions of 0.1 M HNO 3 and 0.1 M NaOH were used to adjust pH of metal ion solutions using a pH meter.

Adsorbents
The leaves of C. carandas and buds of S. aromaticum were collected from local field of Pushkar (India) and from local market of Jaipur (India), respectively. These were washed with distilled water, dried in sunlight, then 60°C for 24 h in hot air oven. Finally, the dried leaves of C. carandas and S. aromaticum were grinded in clean electric mixer and stored in a dry and clean plastic bag.

Experimental
The affinity of C. carandas and S. aromaticum to adsorb lead(II) were studied in batch experiments. All experiments, performed at room temperature and fixed volume of lead(II) ion solution in 100 mL was stirred with desired biosorbent dose (2-10 gm) for the period of three hours.
After a defined time interval, samples were withdrawn from the shaker, filtered by Whatman filter paper No. 1, and the supernatant solutions were analyzed for Pb(II) ion concentration using an atomic absorption spectrometer (thermo scientific solar S-series AA spectrometer). The removal of lead was calculated according to following expression: where C 0 and C i are the initial and equilibrium concentrations (mg/L).

Effect of pH on biosorption
To study the effect of pH on adsorption of lead(II) ions, the batch equilibrium studies at pH values in the range of 2-10 were carried out. The results are furnished in Table 1 and the variation is presented in Figure 1. It was observed that after pH 10 solid precipitation of Pb(OH) 2 , PbOH + , Pb(OH) 2 , and Pb(OH) − 3 was occurred. Therefore, a pH range of 2-10 was used during the analysis (Issabayeva, Aroua, & Sulaiman, 2006;Rajkumar, Sefra, & Praveen, 2015).
It is observed that biosorption of heavy metal is critically linked with pH of the solution. The pH of solution is a very important contributing and controlling factor in the adsorption process, for this the role of hydrogen ion concentration was examined at different pH. The effect of pH was studied at the Pb(II) concentration of 100 mg/L, biosorbent dosage of 2 g/100 mL at the pH range 2-10. It was observed that with the increase in the pH of the solution the percentage removal of Pb(II) increased up to the pH 4 for C. carandas and pH 2 for S. aromaticum. Further increase in pH value decreased the percentage removal of Pb(II) up to pH 10 (Table 1).

Effect of contact time on biosorption
As shown in Figure 2 the effect of contact time on biosorption of lead(II) was performed. It was observed that the lead(II) removal percentage was the lowest of 87.98% for C. carandas and 82.22% for S. aromaticum at 60 min of contact time and the highest of 95.11% for C. carandas and 91.04% for S. aromaticum at contact period of 180 min. After equilibrium reaction, increase in contact time did not affect the biosorption process of lead(II) ion. Hence the contact time of 180 min was selected for further experiments. The observations are given in Table 2. Similar results were observed for removal of lead(II) by Shaik, Yadamari, Yakkala, and Gurijala (2015). Figure 3 illustrates the variation of adsorption efficiency with varying adsorbent dosage using C. carandas and S. aromaticum, which shows that the adsorption efficiency increases with an increase in adsorbent dosage. A graph was plotted between the different dosage of biosorbent C. carandas and S. aromaticum and the resultant percentage removal of Pb(II). The effect of different biosorbent   dosage (2-10 g/100 mL) with percentage removal of lead is shown in Figure 3. The figure shows a marginal increase in lead removal with increasing biomass concentration (Table 3).

Effect of initial metal concentration
The biosorption capacity of C. carandas and S. aromaticumas a function of the initial concentrations of lead have been studied at different concentrations of Pb(II) in batch experiments. Increasing the initial concentration of Pb(II) in a batch study resulted in decreasing percentage of Pb(II) removal because evidently the biosorbent was approaching its saturation uptake capacity. In batch study using C. carandas and S. aromaticum biomass percentage removal of Pb(II) decreased from 95.11 to 71.12% and 90.04 to 68.98% when the initial concentration of Pb was increased from 100 to 1,000 mg/L (Table 4 and Figure 4). Figure 5 shows the biosorption of Pb(II) for varied temperatures at 180 min of contact time. As shown in Figure 5

Isothermal studies
An adsorption is a quantitative relationship describing the equilibrium between the concentrations of absorbate in solution and its concentration of adsorbent. To better understand the Pb(II) ion adsorption characteristics to the C. carandas and S. aromaticum, we applied two well-known two-parameter isotherms: Langmuir and Freundlich isotherm models. Langmuir adsorption (Langmuir, 1918) isotherm assumes monolayer adsorption of solutes onto a surface of adsorbent with a finite number of identical sites and can be expressed as where C e (mg/L) and q e (mg/g) are the liquid phase concentration and solid phase concentration of adsorbate at equilibrium, respectively, and Q 0 (mg/g) and b (L/mg) are the Langmuir isotherm constants.
Figure 6(a) and (b) show the linear plots of C e /q e vs. C e for both adsorbents, which are used to determine the value of q max and b. The values obtained are given in Table 6. The R 2 values indicate that Langmuir isotherm fairly well predicts the adsorption process of Pb(II) ions by the C. carandas and S. aromaticum. The highest q max was observed for the C. carandas. The Freundlich isotherm is known to well describe heterogeneous system for non-ideal adsorption. The experimental data on Pb(II) adsorption were fitted to the Freundlich adsorption isotherm (Freundlich, 1907), which can be expressed as: where K F [mg/g (L/g) 1/n ] is the Freundlich constant related to the bonding energy, n is the heterogeneity factor which depicts the extent of deviation from linearity of the adsorption. It indicates the degree of non-linearity between solution concentration and adsorption. Table 6 lists the calculated Freundlich and Langmuir isotherm constants. Based on regression coefficient (R 2 ) values, the experimental data for adsorption of Pb(II) onto C. carandas and S. aromaticum powder fit better to Langmuir isotherm model than the Freundlich isotherm model. The adsorption of Pb(II) onto C. carandas was best described by the Langmuir isotherm model. The maximum adsorption capacities (K L ) estimated from the Langmuir isotherm model for Pb(II) were 2,018 mg/g and 5,414.18 mg/g for C. carandas leaves and S. aromaticum, respectively (Figure 7).

Adsorption kinetics
The study of adsorption kinetics of biosorption of lead(II) ion by C. carandas and S. aromaticum is important as it provides efficacy of adsorption mechanism and valuable insight into the reaction pathways and it also describes the solute uptake rate. For analysis of sorption kinetics of Pb(II), kinetic models such as pseudo-first order and pseudo-second order have been used.
The linearized pseudo-first-order kinetic model takes the following form: where q is the amount of lead(II) (mg/g) at time t (min), q e is the amount of lead adsorbed at equilibrium (mg/g), and K 1 is the equilibrium rate constant of pseudo-first-order adsorption (min −1 ). The plot of log (q e − q t ) vs. t gave a straight line for the first-order adsorption kinetics.
The pseudo-second-order kinetic model considered in this study is given as: where K 2 (gmg −1 min −1 ) is the second-order reaction rate constant.
The experimental data and the parameters of both models are tabulated in Table 7. It was observed that the regression coefficient (R 2 : 0.99) for the pseudo-second-order kinetic model is higher in comparison with the pseudo-first-order kinetic model and the calculated value of q e for the pseudo-second-order kinetic model is very close to the experimental value. Similar experimental results indicate that the pseudo-second-order kinetic model fits the equilibrium data for heavy metal ion (4) q t = q e − q e,exp = (−k 1 t)    sorption on biomasses from aqueous solutions quite well. Figure 8 shows the pseudo-first and pseudo-second order for both biosorbents and 100 ppm Pb(II) ion concentration.

SEM images
The SEM images of adsorbents before and after adsorption are given in Figures 9 and 10, respectively, for C. carandas and S. aromaticum. From these images, it is clear that there is significant difference in the appearance of the adsorbent surfaces. Images clearly highlight the action of adsorption on the surfaces of both the adsorbents and strengthen the view point of the researchers.

Conclusions
This study is focused on the biosorption of lead(II) ion onto of C. carandas and S. aromaticum biomass from aqueous solution. The operating parameters, pH of solution, biomass dosage, contact time, initial metal ion concentration, and temperature are effective on the biosorption efficiency of Pb(II). Biosorbent C. carandas leaf powder showed higher sorption efficiency than that of biosorbent S. aromaticum powder under identical experimental conditions. Also, the adsorption of Pb(II) onto C. carandas was best described by the Freundlich isotherm model.
However, we suggest that it also necessary to investigate the efficacy of C. carandas leaves powder to treat real industrial effluents. There is a ready supply of agricultural wastes worldwide. The  use of such materials will not only convert into low-cost effective adsorbents, but also provide a green solution to their disposal.
Since, several parameters including migration of metal ions from bulk solution to the surface of the adsorbent through bulk diffusion and the adsorption of metal ions at an active site on the surface of the adsorbent by chemical reactions play very important role in deciding the adsorption kinetics and adsorption mechanism; the actual adsorption mechanism is yet to be discussed and explained.