Effective Remediation of Lead Ions from Aqueous Solution by Chemically Carbonized Rubber Wood Sawdust : Equilibrium , Kinetics , and Thermodynamic Study

Rubber wood sawdust was carbonized into charcoal by chemical treatment which was used for removal of lead ion from aqueous solution. The work involves batch experiments to investigate the pH effect, initial concentration of adsorbate, contact time, and adsorbent dose. Experimental data confirmed that the adsorption capacities increased with increasing inlet concentration and bed height and decreased with increasing flow rate. Adsorption results showed a maximum adsorption capacity of 37mg/g at 308K. Langmuir, Freundlich, and Temkin model adsorption isotherm models were applied to analyze the process where Temkin was found as a best fitted model for present study. Simultaneously kinetics of adsorption like pseudo-first-order, pseudo-second-order, and intraparticle diffusionmodels were investigated.Thermodynamic parameters were used to analyze the adsorption experiment. Fourier transform infrared spectroscopy, scanning electron microscope, and energy dispersive X-ray spectroscopy confirmed the batch adsorption of lead ion onto chemically carbonized rubber wood sawdust.


Introduction
Water pollution due to contamination of toxic lead ions is a serious problem for human health and environment.Lead discharged from petrochemical, organic, and inorganic fertilizer, oil refineries, and automobile industries contaminates the ground water and surface water [1,2].According to the US Environmental Protection Agency (EPA), World Health Organization (WHO), and Indian Standard (IS), permissible limit of lead ion in drinking water is 0.015 mg/L, 0.01 mg/L, and 0.05 mg/L, respectively.Commercial methods such as filtration, chemical oxidation/reduction, electrochemical treatment, chemical precipitation, membrane separation, and ionexchange methods have been also utilized for the remediation of heavy metal contamination.Due to inefficiency and high operating cost these processes are not so much effective.Therefore in this study we adopted adsorption process for removal of lead ion.Many biosorbents, for example, Caulerpa lentillifera [3], sago waste [4], sawdust [5], jute [5], coir [6], algae [7], waste tea leaves [8], ground nut shell [5], rice husk ash [9], and papaya wood [10], were previously investigated by various researchers for removal of lead ions.
In this study chemically carbonized rubber wood sawdust (CCRWSD) was utilized for removal of lead ions from synthetic wastewater.The pH effect, initial concentration, contact time, and adsorbent dose in batch mode were investigated.In addition isotherm, kinetic, and thermodynamic study were carried out to compare the experimental data and to understand the adsorption behavior of lead ion onto CCRWSD.

Preparation of Biomass.
Rubber wood sawdust was collected from rubber wood processing industry, Nagechera, Tripura, India.The carbonization and activation were done in two steps, where in first step rubber wood sawdust (10 gm) was added in concentrated sulphuric acid (11   for carbonization and kept for 10 minutes.Then in second step carbonized black slurry was mixed with concentrated nitric acid (6.6 mL, 65% m/m) and kept in air oven at 150 ∘ C for 24 hours.Carbonized sawdust was centrifuged with deionized water until the pH became neutral and dried in 110 ± 2 ∘ C. Screening of the dried CCRWSD was done to get desired particle sizes ranging from 0.5 to 1 mm.

Characterization of the Adsorbent.
Atomic absorption spectrophotometer (Perkin Elmer Model AAS 700) was utilized to determine the lead concentration in effluent and influent samples.Fourier transform infrared spectroscopy (FTIR) (Bruker 3000 Hyperion, Germany) was used to study the functional group present in adsorbent.Scanning electron microscope (SEM) (JELO JSM7600F) supported by EDS (energy dispersive X-ray spectroscopy) (Oxford AZtech energy system) was utilized to characterize the surface of adsorbent.

Batch Experiment.
Batch experiment was carried out in a thermostatically controlled magnetic stirrer at 30 ∘ C with the speed of 120 rpm.A 0.5 g of CCRWSD was utilized at different concentration (5 mg/L to 30 mg/L) and at different pH (2.1 to 6.8).The experiment was done at different adsorbent dose (0.25 g/L to 5 g/L).For thermodynamic study the adsorption experiment was conducted in three different temperatures (303 K, 313 K, and 323 K).After completing every batch experiment the solution was filtered and sample was tested to know the residual lead concentration.

Results and Discussion
3.1.Characterization of the Adsorbent.Before and after adsorption functional groups were identified by using FTIR in the range of 400-4000 cm −1 as shown in Figures 1 and 2. In CCRWSD the peak at 1708.58 cm −1 and 1612.56 cm −1 shows the presence of C=O group and -COO − group.Another major peak is observed at 1162.34 cm −1 which is due to C-O group.The peak at 3419 cm −1 is due to O-H stretch from carboxyl group (O=C-OH and C-OH) and peak 2923.40 cm −1 represents the presence of O-H stretch (hydrogen bonded -COOH).Important fact is that the shifting of peaks is observed after adsorption of lead ion which is a strong evidence of adsorption in the CCRWSD.After adsorption  of lead ion, peaks are shifted because carboxyl and hydroxyl group may release proton and form new complexes with lead ion.SEM analysis (Figures 3 and 4) suggests the change in surface structure before and after adsorption of lead ion.On the other hand, EDS results also confirm the feasibility of lead adsorption process (Figures 5 and 6).

Batch Result Analysis.
Adsorption capacity is increased with the increasing of solution pH from 2.1 to 5.6 and then starts to decrease with increasing pH as shown in Figure 7.At low pH adsorption capacity is low because of higher concentration of H + ion in the solution which occupies the binding sites of adsorbent.
Effect of contact time on adsorption of lead ion onto CCRWSD is shown in Figure 8.It is observed that the rapid adsorption is taking place in first half an hour and thereafter rate of lead ion adsorption decreases.After some time period of adsorption surface sites are occupied by lead ion and adsorption process decreases.
It is found that adsorption capacity increases from 9.82 to 36.4 mg/g with the increase in initial lead ion concentration from 5 to 30 mg/L.At lower concentration maximum adsorption sites are available for adsorption of lead ion.On the other hand at higher concentration many lead ions left unabsorbed in the solution as the entire binding site get saturated by lead ion.Adsorption capacity increases from 3.96 mg/g to 37 mg/g with the decreasing of adsorbent dose from 5 g/L to 0.25 g/L.As binding sites of adsorbent increase with the increasing adsorbent amount the removal percentage increases.
Langmuir adsorption isotherm [11] equation is expressed as where   (mg/g) is the amount of lead adsorbed at equilibrium,   (mg/L) is the equilibrium concentration of lead ion in solution,  (L/mg) is the Langmuir constant, and   (mg/g) is the maximum adsorption amount of lead ion per unit weight of adsorbent to form a complete monolayer on the surface bound at   .The values of   and  are calculated from the slopes and intercepts of linear plots of   /  versus   (Figure 9) and are illustrated in Table 1.
Langmuir isotherm can be checked by the value of   (2) [12] as isotherm is irreversible (  = 0), favorable (0 <   < 1), linear (  = 1), or unfavorable (  > 1): where  0 (mg/L) and  (L/mg) are denoted by initial concentration of lead ion and Langmuir constant.In experiment   values are found to be 0.859-0.960which indicates the favorable lead adsorption.Linear form of the Freundlich isotherm is expressed as [13] ln where   and  are known as Freundlich coefficients,   (mg/g) is the amount of lead ion adsorbed, and   (mg/L) is the concentration of lead solution at equilibrium.explained in Table 1.The value of  is 2.44 for lead adsorption on CCRWSD.As the value of  lies between 1 and 10, it can be concluded that Freundlich model is favorable for lead adsorption studies.
is expressed as /, where  (J/mol) is the Temkin constant and  (K) is the absolute temperature in Kelvin. (8.314 J/mol K) is the gas constant and  (L/g) is the Temkin isotherm constant.The plot of   versus ln   (Figure 11) gives the values of , , and .The values of the Temkin constants () and correlation coefficients ( 2 ) are shown in Table 1.Value of  2 is greater than 0.99 which justifies that model is well fitted for adsorption of lead ion on the surface of CCRWSD.Linearized form of Dubinin-Radushkevich isotherm equation [15] is expressed as where   (mg/g) is the number of lead ions adsorbed per unit weight of CCRWSD,   (mg/g) is the maximum adsorption capacity, and  (mol 2 J 2 ) is the activity coefficient. (kJ/mol) is the adsorption energy and  is the Polanyi potential. is expressed as where  (J/mol K) and  (K) are the gas constant and temperature.The plot of ln   versus  2 (Figure 12) gives the value of  (mol 2 /J 2 ) and   (mg/g).The equation which is used for the calculation of adsorption energy is expressed as These parameters give the information about sorption mechanism, either chemical ion-exchange or physical sorption.D-R parameters are calculated from the plot of ln   versus  2 × 10 5 and listed in Table 1.From calculation it is found that the value of adsorption energy  is 12.70 kJ/mol.The value of adsorption energy is greater than 8 kJ/mol and smaller than 16 kJ/mol which indicates chemisorption phenomenon [16].
After calculation of all isotherms it is observed that the value of correlation coefficients ( 2 ) for Temkin model is higher than the other isotherms from which it can be concluded that Temkin model is the best fitted model for this adsorption process.Langmuir, Freundlich, and Dubinin-Radushkevich are also well fitted models for the present system; hence the adsorption process can be described by these models also.In Langmuir model maximum adsorption capacity (  ) at monolayer is measured whereas Dubinin-Radushkevich deals with the maximum adsorption capacity (  ) at microspores volume of CCRWSD (Table 1).As a result value of the adsorption capacity from Langmuir model is higher than the Dubinin-Radushkevich model.Furthermore, high value of  (L/mg) indicates that there is a strong interaction between the functional group of the adsorbent and lead ion which leads to an efficient adsorption process.

Adsorption Kinetics.
Adsorption kinetic studies were conducted to have a better and broader understanding of the dynamic behaviour of lead adsorption onto CCRWS.Pseudofirst-order, pseudo-second-order, and intraparticle diffusion models are applied to study the adsorption process.
Pseudo-first-order model [17] is expressed as where   (mg/g) and   (mg/g) are the amount of lead ions adsorbed at time  (min) and at equilibrium per unit weight of the adsorbent.  (mg/g) is the amount of lead ion adsorbed at any time and  1 (min   values of  2 are close to the unity.Similarities indicate the better agreement of pseudo-first-order model with the experimental data.Pseudo-second-order model [18] is expressed as where  2 (g/mg min) is the pseudo-second-order rate constant.  (mg/g) and   (mg/g) are the maximum capacity at equilibrium and at time  (min).Values of  2 (g/mg min) and   (mg/g) are determined by using the plot of /  versus  (Figure 13).The results are shown in Table 2 where values of theoretical adsorption capacities are not closer to the experimental data which concludes that the pseudo-secondorder model is not suitable to explain the kinetics of the present adsorption system.Adsorption process is also checked by applying intraparticle diffusion model which is expressed as [19]   =    0.5 + , (10) where   (mg/g min) is the intraparticle diffusion constant.The plot of   versus  0.5 (Figure 14) gives the values of   .
Results of the model are summarized in Table 2, where it can be seen that the values of  2 for intraparticle diffusion model are lower than the other models.So intraparticle diffusion model is not sufficient to explain the present system.

Thermodynamic Study.
Thermodynamic study of lead ion adsorption by CCRWSD was conducted in three different temperatures (303, 313, and 323 K).Gibbs free energy (Δ) (kJ/mol) of adsorption can be calculated by using (11) and (12) [20].Enthalpy change (Δ) (kJ/mol) and entropy change (Δ) (J/K mol) are also the most important thermodynamic parameters which influence the adsorption process and they can be calculated by using (13) and ( 14) [20]: ln where  denotes gas constant,   (mg/g) is the adsorption capacity at equilibrium, and   (mg/L) is the concentration at equilibrium at absolute temperature  (K).The slope and intercept of ln   versus 1/ give (Figure 15) the value of Δ and Δ.A summary of calculated thermodynamic parameter is given in Table 3.The negative values of Δ for lead ion adsorption indicate that the process is spontaneous and positive value of Δ (+10.474kJ/mol) is an indication of

Table 1 :
Parameters of Langmuir, Freundlich, and Temkin adsorption isotherm constants for adsorption of lead onto CCRWSD.
−1 ) is the pseudo-first-order rate constant.The plot of log(  −  ) versus  (not shown in figure) at different concentration gives the values of  1 and   .The values of  1 and   are shown in Table2, where values of   are quite similar with experimental data and

Table 2 :
A pseudo-first-order, pseudo-second-order, and intraparticle diffusion model parameter for adsorption of lead ion onto CCRWSD.
t (min g/mg)