Characterization and Evaluation of the Adsorption Behaviour of Salvadora Persica L. Leaves and Stems

Used/Spent Salvadora persica L. leaves and stems constitute a major portion of the waste generated by various foods and oil industries including laboratories of traditional medicines. The valorization of this waste by applying it to the remediation of industrial effluent may contribute to a cleaner and green environment. The quantitative removal of toxic metals from wastewater has been demonstrated, which may be attributed to the presence of various functional groups on the adsorbent. The probable active sites were determined by a comparative study of the FTIR spectra of the metal laden adsorbent and the free adsorbent. The adsorption behavior was found to be befitting to Langmuir model. However, Freundlich and Dubinin and Radushkevich models have also found to be almost befitting to the experimental data. The adsorption process was found to follow pseudo-first order kinetic model. However, within contact time of 50 min, the pseudo-second-order model and intraparticle diffusion model have also been followed. The evaluated thermodynamic parameters suggest the spontaneous nature of adsorption. The biomass was found to retain 98% of the sorption capacity up to 3 cycles. The proposed method offers a very simple, low cost, effective and eco-friendly alternative methodology for the determination and extraction of metal ions from wastewater samples.


INTRODUCTION
The contamination of environmental resources by heavy metals has become the prior concern because of its serious and adverse effect on the environment and consequently on human health [1].The major sources of pollution are caused by the unplanned discharge of sanitary and toxic industrial wastes, improper dumping of industrial effluent and runoff from agricultural field [2].As a consequence of the introduction of toxic metal ions into the natural ecosystem, their bioaccumulation in human bodies, and other living beings, takes place through either direct intake or food chains.Hence, their removal from natural resources has become a major concern [3,4].Excessive zinc consumption may cause different health problems, such as nausea, anemia, skin irritations, vomiting, and stomach cramps [5].
Various methods such as hydrometallutgical technologies, ion exchange, electrodialysis, reverse osmosis, precipitation and adsorption have been employed for the remediation of water resources [6,7].
Agricultural by-products is one of the rich sources of low cost adsorbents, namely peanut husk, wheat bran, rice husk etc [8][9][10][11].They possess different functional groups by virtue of their constituents namely hemicelluloses, lignin, lipids, proteins, simple sugars, hydrocarbons and starch [6].Used Salvadoraceae leaves and twigs are a waste product of various foods and oil industries including laboratories of traditional medicines [12,13].
The present work aims to investigate the potential of Salvadoraceae leaves and stems as a low-cost bio-sorbent for the removal of zinc (II) ions from aqueous medium.The study of adsorption kinetics and equilibrium isotherms can give an insight into the adsorption behavior, thereby making the adsorption mechanism more intelligible for the theoretical evaluation and interpretation of thermodynamic parameters.The adsorption capacity was determined through various kinetic models.The experimental parameters namely amount of adsorbent, concentration of metal ion, temperature and pH have been optimized for efficient removal of the metal ion.

Instruments and Equipments
The concentration of Zn (II) was determined by Flame atomic absorption spectrometry (Agilent, USA).The pH measurements were carried out with a pH meter (Bellstone, India).A mechanical shaker of 200 rpm (Bellstone, India) was used for the equilibrium studies.The FT-IR analysis (Omnic, USA) was done for characterization.Heating oven (Bellstone, India) was used for drying.A stainless steel grinder was used for reducing the adsorbent to powder.ASTM standard sieves were used for separating particles of the desired size.

Preparation of reagents and adsorbent
All reagents were of analytical grade.The Zn (II) solution was prepared (with ZnCl2) in de-ionised water.The Salvadoraceae leaves and stems were procured from the local market, in Jazan.It was extensively washed with de-ionised water and then left overnight in the oven for drying at a constant temperature of 80°C until a constant weight is observed.The dried adsorbent was crushed and then subjected to filtering through sieve for acquiring particle size of 300μm.Buffer solutions were prepared as follows: pH 1-2 (0.2 M KCl + 0.2 M HCl), pH 3-5 (0.1 M CH3COOH + 0.1 M CH3COONa), pH 6(1.0 M CH3COOH + 1.0 M NaOH), pH 8-10 (NH4Cl + NH4OH) [14,15].

Batch method for adsorption studies
The required amount of an accurately weighed adsorbent was allowed to equilibrate in contact with a suitable volume of the Zn (II) solution of appropriate concentration.The mixture was maintained at a constant pH with the appropriate buffer solution for optimum time with constant shaking.Then the mixture is subjected to filtration to separate out the filtrate, from the adsorbent, for determining the concentration of the filtrate with FAAS.

Characterization on the basis of FT-IR analysis
The interaction between the target metal ion and the adsorbent was investigated by undertaking a comparative study of the FTIR spectra of the loaded and the unloaded adsorbents (Figure 1, 2).
As illustrated in Figure 1, the loaded adsorbent underwent a relative shift of the bands at 3454-3411 cm -1 , which may correspond to ν(NH), whereby suggesting its participation in the metal binding.Again, the shift of bands, corresponding to ν(OH), from 2917 (2849) to 2360 (2338) cm -1 also indicates its role in retaining the metal ion.Moreover, the shift of band at 1622 cm -1 to 1640 cm -1 may also occur due to the participation in metal retention.As illustrated in Figure 2, the bands at 3648 cm -1 and 3328 cm -1 (representing ν(OH) and ν(NH), respectively) undergoes a shift to 3571 cm -1 and 3292 cm -1 , respectively, thereby indicating their possible involvement in the sorption process.After loading the adsorbents with zinc, the disappearance of the bands at 2926 cm -1 and 1717 cm -1 suggest the involvement of OH and CO groups, respectively, in metal binding.

Effect of contact time
The maximum saturation limit (adsorption capacity) of Salvadoraceae leaves and Salvadoraceae stems was found to be 7.94 mg g -1 and 7.75 mg g -1 , respectively.A minimum contact time of 50 min was necessary for the former to attain the maximum adsorption capacity, while the latter required a minimum of 60 min.The plot of amount of Zn (II) retained versus the equilibration time (Figure 3) illustrates the dependency of adsorption on the time

Effect of pH on removal of heavy metal
The competitive nature of the hydrogen ions (for binding with the active sites) reflects the significance of the pH of the medium.Hence, a very low retention of Zn (II) was observed at pH 1-3, as indicated in Figure 4. Thereafter, as the pH of the medium increased beyond 3, the positively charged metal ions overcome the challenge (posed by H3O + ions) for binding and hence the maximum adsorption was observed at pH 5.5 for Salvadoraceae leaves and pH 6.0 for Salvadoraceae stems.However, a decline in the adsorption of metal was observed after pH 5.5 and 6.0, which may be attributed to the deterioration of metal binding sites [16].

Effect of temperature on the uptake of metal
The favorable nature of adsorption at higher temperature may suggest the role of chemical interactive forces in the retention of the metal ions.As the temperature of the system (a suspension of the adsorbent and the metal ions in aqueous solution) rose from 20 °C to 50°C, the adsorption capacity got elevated from 7.94 mg g -1 to 10.58 mg g -1 (for Salvadoraceae leaves) and from 7.75 mg g -1 to 10.56 mg g -1 (in case of Salvadoraceae stems) within a contact time of 50 min.Before reaching the optimum contact time (50 min), the process of adsorption is kinetically controlled as reflected by the fact that with the increase in temperature the adsorption increased.Moreover, at higher temperature, more binding sites may get exposed for binding (with metal ions), whereby leading to higher adsorption of Zn (II) onto the adsorbents.

Effect of initial concentration and adsorbent mass
The resistance to the mass transfer of the metal ions (between the adsorbent and the aqueous medium) is directly related to the initial concentration of the metal ions, which provides the required driving force for transportation (of metal ions across the solution).Therefore, the retention of metal ions (by the adsorbent) is facilitated with increasing initial metal ion concentration.
On the other hand, with the increase in the amount of the adsorbent, the uptake of Zn (II) from the solution increased from 4.50 to 7.94 mg g -1 and 4.50 to 7.75 mg g -1 , for Salvadoraceae leaves and Salvadoraceae stems, respectively (Figure 5), because of the availability of more binding sites.
The Langmuir isotherm (Figure 6, 7) is commonly represented by the following mathematical expression: , where qe is the equilibrium Zn (II) concentration on the adsorbent (mol g -1 ); Ce, the equilibrium Zn (II) concentration in the solution (mol dm -3 ); qmax, the monolayer adsorption capacity of the adsorbent (mol g -1 ); and KL, the Langmuir adsorption constant (dm -3 mol -1 ) related to the free energy of adsorption.Another constant, namely, the separation factor, RL is mathematically expressed by the equation:   = 1 1+     .While Freundlich isotherm (Figure 8) is generally reproduced with the expression: ln   = ln   + 1  ln   where KF and n are Freundlich adsorption isotherm constants (dm 3 g -1 ) and Dubinin-Radushkevich isotherms (Figure 9) is represented by:ln   = ln   −  2 , where  is a constant related to the mean free energy of adsorption per mole of the adsorbate (mol 2 J -2 ); qm , the theoretical saturation capacity, and Ɛ is the Polanyi potential, which is equal to RT ln{1+ (1/Ce)}, where R (J mol -1 K -1 ) is the gas constant; and T (K), the absolute temperature.The constant  is related to the free energy E (kJ mol -1 ), expressed as:  = ln q e ln Ce S. Leaves S. Stem ln q e Ɛ 2 x 10 8 (J mol -1 ) 2 S. stems

S. leaves
The befitting nature of all the studied isotherm models, as represented in Table 1 and 2, indicates that the adsorbent contains both homogeneous as well as heterogeneous distribution of active sites (on the surface).The RL and n (Table 1 and 2, respectively) values also suggest the favorability of the adsorption process.The magnitude of E (Table 2) reflects the nature of adsorption process as of physical nature [20].

Kinetics studies
The possible reaction mechanisms, for adsorption, may be inferred from the characteristics of the befitting models of rate expressions [21].Hence, three kinetic models namely, pseudo-first-order equation (Figure 10) given as:  ; where q1 and qt are the amounts of the Zn(II) ions adsorbed at equilibrium and at time t (mg g -1 ) and k1 is the pseudo-first-order rate constant (min -1 ) of adsorption., pseudo-second order equation (Figure 11) given as: ; where q2 is the maximum adsorption capacity (mg g -1 ) for the pseudo-second-order adsorption; qt, the amount of Zn(II) ions adsorbed at equilibrium at time

S. stems
However, the pseudo-second-order model and the intraparticle diffusion model may be the other additional routes, at least within 50 min of contact time (Figure 10).Since, the linear portion (of the latter model) does not pass through the origin therefore the latter is not the rate limiting step [20,22].S. leaves

Thermodynamic parameters of adsorption
The correlation between the thermodynamic parameters, namely, ∆G 0 (free energy), ∆H • (enthalpy), ∆S°(entropy) and KL (equilibrium constant) is given as: ∆ 0 = − ln   , .The negative value of Gibbs free energies (Table 4) indicates that the adsorption process is spontaneous.An endothermic reaction is suggested by the positive value of ∆H•, while the negative value of ∆S° indicates that there is a decrease in the randomness at the interface between solid and the solution during the adsorption of Zn(II) onto the adsorbent (Figure 13).

CONCLUSION
A cheap and effective bio-sorbents, for the extraction of metal ions from aqueous solution, is offered by Salvadoeaceae leaves and stems.The thermodynamic and kinetic parameters implies the feasibility of adsorption on to Salvadoeaceae leaves and stems.Therefore, the proposed method offers a very simple, low cost, effective and eco-friendly alternative methodology for the determination and extraction of metal ions from real water samples.

Figure 1 .
Figure 1.FT-IR spectra of loaded and unloaded Salvadoraceae Leaves

Figure 2 .
Figure 2. FT-IR spectra of loaded and unloaded Salvadoraceae stems

Figure 3 .
Figure 3.The effect of contact time on adsorption of Zn (II)

Figure 4 .
Figure 4. Effect of pH on the adsorption of Zn (II)

Figure 5 .Figure 6 .
Figure 5.Effect of the adsorbent mass on the adsorption of Zn (II)

Figure 7 .
Figure 7. Langmuir plots for the adsorption of Zn (II) on to Salvadoraceae stems at different temperatures

Figure 8 .Figure 9 .
Figure 8. Freundlich plots for the adsorption of Zn (II) on to Salvadoraceae Leaves and stems

Figure 11 .
Figure 11.Pseudo-second-order plot for the adsorption of Zn (II) onto Salvadoraceae Leaves and stems

Table 1 .
Values of Langmuir isotherms constants for the adsorption of Zn (II) at various temperature

Table 2 .
Values of Freundlich and Dubinin-Radushkevichisotherm constants Pseudo-first-order plot for the adsorption of Zn (II) onto Salvadoraceae Leaves and stems

Table 3 .
Kinetic parameters for the adsorption of Zn (II)