Study of Biosorption and Desorption Process of Cu (II), Cr (VI), Pb (II) and Zn (II) Ions by Using Peels of Citrus aurantifolia

Heavy metal pollution in the aquatic water bodies via the discharge of various toxic heavy metals from industrial effluents has been a major concern in the present era. Various physical and chemical processes are available to solve this problem of heavy metal pollution. Biosorption is considered as a potential alternative for the removal of heavy metals from waste waters as compared to other conventional processes. In the present work, biosorption of Cu(II), Cr(VI), Pb(II) and Zn(II) ions from aqueous solutions was carried out by using peels of Citrus aurantifolia. The peels were found to be efficient in the biosorption of all four metal ions under study. The biosorption process was found to be influenced by factors like contact time, temperature, pH, turbidity as well as biosorbent dose. Further, the change in characteristics of Citrus aurantifolia after biosorption process was studied by using E-SEM, EDAX and FT-IR analysis. The adsorption isotherm studies revealed that Freundlich isotherm model showed better fir to experimental data as compared to Langmuir isotherm model. The results were found to be significant statistically. The regeneration of biosorbent was carried out by desorption study by using certain eluents.


Introduction
The discharge of industrial, domestic and agricultural wastes in water bodies' cause deposit of pollutants especially heavy metals in sediments which endanger health of living organisms. These heavy metals, in abundance, can be toxic through direct action of the metal itself or through their inorganic salts or via organic compounds from which the metal can become easily detached or introduced into the cell. Some heavy metals which are hazardous to humans include lead, mercury, cadmium, arsenic, copper, zinc, and chromium [1]. Since most of heavy metals are not converted into nontoxic end products and/or remain persistent in the environment, their concentrations must be reduced to acceptable levels before discharging them into environment. Over the decades, several methods have been devised for the treatment and removal of heavy metals from industrial effluents. These include reverse osmosis, electrolysis, ultrafiltration, ion exchange and chemical precipitation. The disadvantages of the conventional treatment methods have made it imperative for a standard solutions were prepared by diluting the stock solutions to appropriate volumes.

Biosorption process
100 mL of metal ions solution (50 mg/L) viz. Cu (II), Cr(VI), Pb(II) and Zn(II) ions were treated with 1 % of biosorbent in 250 mL conical flask at 30 o C and the solution was shaken at 150 rpm in an orbital shaker. After an hour, the solution was filtered using Whatmann filter paper No. 1. The aqueous solution of biosorbent under study was used as blank. The concentrations of the metal ions in the filtrate were determined by using ICP-AES (ARCOS, Simultaneous ICP Spectrometer, IIT-SAIF Bombay).

Calculation of biosorption capacity and percentage biosorption
The biosorption capacity (qe) determines the amount of metal ions adsorbed per gram of biosorbent (Kariuki et al., 2017) and it was calculated as follows: qe (mg/g) = (Co -Ce) * V (1) m Percentage of biosorption of metal ions was calculated as follows: Biosorption (%) = (Co -Ce) * 100 (2) Co Where Co is the initial metal ion concentration (mg/L) Ce is the final metal ion concentration after biosorption process (mg/L) V is the Volume of solution (L) m is the mass of biosorbent (g)

Factors affecting biosorption process
The effect of various factors viz. pH, temperature, biosorbent dosage, contact time and turbidity of aqueous solutions on biosorption process by using peels of Citrus aurantifolia was studied by using batch experiments.
To study the effect of pH on biosorption process, 1 g of biosorbent was added to 100 mL of aqueous solutions containing 50 mg/L of each ions viz. Cr(VI), Cu(II), Pb(II) and Zn(II) in 250 mL conical flasks and the biosorption process was studied at different pH viz. The effect of contact time on biosorption process was studied by adding 1 g of biosorbent to 100 mL of aqueous solutions containing 50 mg/L of Cr(VI), Cu(II), Pb(II) and Zn(II) ions at pH 8 and the biosorption process was carried out for different time period viz. 60, 120 and 180 minutes at 30 o C. 1 g of biosorbent was added into 100 mL of aqueous solutions having different turbidity and containing 50 mg/L of Cr(VI), Cu(II), Pb(II) and Zn(II) at pH 8 and biosorption process was carried out for 60 minutes at 30 o C. The turbidity of the solution was adjusted to 10, 50 and 100 NTU using hexamine and hexamethylene sulphate mixture. The aqueous solutions containing the biosorbents (blank) were kept under same conditions in all the above experiments for reference. All the solutions were subjected to shaker conditions at 150 rpm in an orbital shaker. After the biosorption process all the solutions were filtered by using Whatmann filter paper No. 1 and the filtrates were analyzed for residual metal ions by using ICP-AES (ARCOS, Simultaneous ICP Spectrometer, IIT-SAIF Bombay). All the experiments were performed in triplicates and percent biosorption of the average values were reported.

Characterization of biosorbent:
The physical and chemical properties as well as surface characteristics of untreated biosorbent were studied. The change in the surface characteristics of biosorbent after treatment with metal salt solutions were analyzed as follows: For Environmental Scanning Electron Microscope (E-SEM) analysis, the biosorbent was placed on the sample stub and dried under IR light for 2-3 minutes followed by platinum coating for 600 seconds using JEOL JFC-1600 Auto fine Coater. The biosorbents were then scanned using E-SEM (FEI QUANTA 200 E-SEM, IIT-SAIF Bombay) operated at 15 kV under different magnifications. Energy Dispersive X-ray Analysis (EDAX) in conjunction with Scanning Electron Microscope was used to perform elemental analysis of the biosorbent. The energy of the beam was used in the range of 10-20 keV. This causes X-rays to be emitted from the irradiated material. The X-rays were generated in a region about 2 microns in depth. By moving the electron beam across the biosorbent, a 2-D (two dimensional) image of each element present in the biosorbent was acquired. The EDAX analysis of biosorbent was carried out at IIT-SAIF Bombay. Fourier Transform InfraRed (FT-IR) Spectroscopy was used as a qualitative technique to assess the presence of functional groups on the surface of the biosorbent. The study was carried out by using potassium bromide (KBr) disc method. The spectrum of the biosorbent was measured within range of 400 to 4000 cm −1 wavenumbers. The biosorbent was mixed with KBr in the ratio of 1:10 and was converted into pellet. The FT-IR spectrum was recorded on a FT-IR spectrophotometer (FT/IR 4100 type A C208161016, Nanotechnology Research Centre, B. K. Birla College, Kalyan.) using a standard light source and TGS detector.

Study of adsorption isotherm
The equilibrium relationship between the metal ions concentration in the aqueous solution and on the biosorbent at 30 o C was studied by using Langmuir and Freundlich adsorption isotherm models. The amount of adsorbate (metal ions) per unit weight of adsorbent (biosorbent) against the equilibrium concentration of the adsorbate remaining in solution was plotted for each metal ions concentration. The linearized graph of log qe versus log Ce was plotted for Freundlich isotherm while Ce/qe versus Ce was plotted in the case of Langmuir isotherm. The parameters for both the isotherms were calculated by using Microsoft excel 2010. The linear form of the Langmuir equation can be expressed as follows: Based on the further analysis of Langmuir equation, the dimensionless parameter of the equilibrium known as separation factor (RL) is expressed by: where Co is the initial concentration of metal ion (mg/L) Ce is the equilibrium concentration of metal ion (mg/L) qe is the amount of the metal ions adsorbed (mg/g) per unit mass of the biosorbent qm and b are Langmuir constants evaluated from slope and the intercept respectively The RL parameter is considered as a reliable indicator of the adsorption process. RL value between 0 to 1 indicates favorable adsorption condition [17,19]. The linearized Freundlich equation can be expressed as follows: log qe = log Kf + 1 log Ce (5) n Where Kf is an indicator of adsorption capacity n is the adsorption intensity The constant 'n' gives an idea of the grade of heterogenicity in the distribution of energetic centers and is related to magnitude of adsorption driving force. The 'n' values between 2 to 10 indicate good adsorption condition [20][21][22][23].

Statistical analysis
Chi-square test was applied to the data collected from the experiments conducted for the metal ions removal from the metal salt solutions having initial metal ions concentration of 10 to 50 mg/L by using 1 % biosorbent dose at pH 8. The two main parameters used in this test were experimental value obtained for biosorption capacity (qexp) and biosorption capacity calculated from model (qcalc).
The Chi-square (χ 2 ) test can be calculated as follows: Where qe exp (mg/g) is the equilibrium capacity obtained experimentally qe cal (mg/g) is the equilibrium capacity obtained by calculation from the model.
If the data from the model are similar to the experimental data, the value of χ 2 will be small and if they differ, χ 2 value will be larger [24,25].

Desorption process
For the study of desorption process, 2 g of air dried biosorbent loaded with metals

Results and Discussion
The plant material was authenticated and was found to be the peels of Citrus aurantifolia. Figures 1a and 1b shows dried and powder form of peels of Citrus aurantifolia respectively. It has been reported that the pH of biosorbents in the range of 6-8 can be used for many applications [26][27]. The bulk density less than 1.2 g/mL represents that biosorbents are fine in nature [28]. Iodine Index for commercial adsorbent ranges from 300 to 1200 mg/g [29]. By comparing the above data with the values of characteristics shown in table 1, it can be observed that peels of Citrus aurantifolia may have good adsorption properties. The percent biosorption and biosorption capacity (mg/g) by the biosorbent are shown in table 2. The peels were found to be more efficient in the biosorption of Pb(II) and Cr(VI) ions as compared to Zn(II) and Cu(II) ions from the aqueous solution. The biosorption process was found to be affected by various factors.

Effect of pH:
As the pH of the solution increases, the concentration of H + ions in the solution reduces. Hence pH towards alkaline range indicates that the number of H + ions is less which decreases the competition between proton and metal ion. Thus, increased pH is an indication that the ligand is available for metal ion binding and so biosorption is enhanced [30]. In the present work it was observed that as the pH of the aqueous solution increased from 2 to 8, the biosorption process by peels was found to increase but further increase in pH to 10 led to reduction in the biosorption process of all four metal ions under study ( Figure 2).

3.2.
Effect of temperature: According to adsorption theory, adsorption decreases at higher temperature and molecules adsorbed earlier tend to desorb from the surface at elevated temperature [31]. This may be due to the fact that when the temperature of the solution increases, the attractive forces between biomass surface and metal ions are weakened and adsorption process declines. [32] Showed that the biosorption process of Cd (II) and Ni (II) ions by using Glycine max pod decreased with increase in temperature from 35 to 50 o C. In the present work, the biosorption process was found to increase with the rise in temperature from 10 to 30 o C but further increase in temperature to 50 o C showed marginal changes in biosorption process by the biosorbent under study (Figure 3).

Effect of biosorbent dose:
The dose of biosorbent (0.5 to 3 %) was found to be a factor influencing biosorption process. It was observed that as the dose of the biosorbent increased from 0.5 to 1 %, the biosorption process was increased. This may be due to the fact that increasing dose of biosorbent increases the number of active sites on biosorbent to bind to metal ions but further increase of biosorbent might lead to crowding of biosorbents and making them unavailable to bind to metal ions (shell effect). The decrease in biosorption with increase in biosorbent dosage to 3 % might have been due to shell effect (Figure 4).

Effect of contact time:
The biosorption capacity of the biosorbent for metal ions viz. Cu(II), Cr(VI), Pb(II) and Zn(II) were found to increase with rise in contact time from 60 to 180 minutes ( figure 5). This may be because of the increase in time duration for the metal ions to bind to the active sites on the biosorbent. Similar result was observed by [33] in which they showed that biosorption process enhanced with increase in contact time.

Effect of turbidity:
The efficiency of biosorption process was found to decrease with increase in turbidity from 10 to 100 NTU of the aqueous solution. The decrease in the biosorption process may be due to the adsorption of positively charged metal ions to the negatively charged ions present in the turbid solution, thereby rendering them unavailable for the biosorption process ( Figure 6). Geoffrey et al., [34] observed that as turbidity of aqueous solution increased from 20 to 50 NTU, the biosorption process of Ni(II) and Pb(II) ions by using Moringa oleifera seed was reduced.  The surface morphology of biosorbent showed more roughness after biosorption process (Figure 7b) as compared to its surface before biosorption (Figure 7a). The roughness may be due to the interaction and deposition of metal ions on the surface of bio-sorbent. Hence, the surface morphology of bio-sorbent are modified after treatment with metal salt solutions.

Figure 7. E-SEM images of bio-sorbent (a) before (b)after treatment
The SEM-EDAX analysis confirmed the deposition of metal ions on the surface of the biosorbent as the weight (%) of the heavy metals viz. Cu, Cr, Pb and Zn was detected after biosorption process (Figure 8b). Cu (0.32 %) which was detected on the biosorbent before biosorption process (Figure 8a) was found to increase after biosorption process. This indicates that biosorption has taken place. Further the weight (%) of lead was found to be maximum as compared to other three heavy metals under study.

Figure 8. EDAX images of biosorbet (a)before (b)after treatment
FT-IR spectroscopy was used to obtain information about functional groups present on the biosorbent. Figures 9a and 9b represent the FT-IR spectra of untreated and treated biosorbent respectively. Table 3 represents the location wavenumbers and corresponding vibration and functional groups on the biosorbent.  Considerable changes in the wavenumbers were not observed in most of the cases after the biosorption process. It may be because the electrostatic interactions between metal ions and functional groups were not strong enough to shift the absorption band frequencies or wavenumbers related to the functional groups present on the biosorbent surface. It indicated that there might be weak coordination between the metal ions and functional groups on the surface of biosorbents [35]. The analysis of equilibrium data is very important to develop an equation which precisely represents the results and it can be used for design purposes [36]. In the present work, the experimental equilibrium data (qe values) was investigated by using mathematical models viz.     figure 10, it was observed that the removal of all four metal ions was almost negligible by using deionized water as compared to other desorbing agents under study. Hence, deionized water was considered as noneffective desorbing agent (eluant). The chelating agent (EDTA) showed maximum desorption of metal ions viz. Cu(II), Cr(VI), Pb(II) and Zn(II) from biosorbent. The desorption of Pb(II) ions was found to be more as compared to other three metal ions under study. Further the desorption process was found to be better by NaOH as compared to HNO3. Desorption process studied by [37] showed similar result in which EDTA was found to be effective as a desorbent for the recovery of Pb(II) ions from green algae Cladophora fascicularis.

Conclusion
The peels of Citrus aurantifolia, an agricultural waste can be considered as a good biosorbent for removing Cu (II), Cr(VI), Zn(II) and Pb(II) ions from aqueous solution. The result was supported by adsorption isotherm studies. The desorption process revealed that the we can regenerate the biosorbent under study for further cycles of adsorption-desorption process along with possibility of recovery of metal ions. The application of present biosorbent for the removal of metal ions from