Removal of lead from aqueous solutions by low cost and waste biosorbents (lemon, bean and artichoke shells)

Water contamination that is caused by heavy metal leads to serious environmental problems and it has lethal effects on living organisms. Even if lead (II) in particular is found in low amounts in the environment, it has negative effects on living organisms and, if it is above a certain degree, it shows a toxic effect. Heavy metal ions such as Pb(II) are toxic and carcinogenic at relatively low concentrations (Tirtom et al. 2012). Chronic exposure to high amounts of lead can result in various and considerable damages to systems of the body, damage to the kidneys, liver and reproductive system, basic cellular processes and brain functions. Moreover, high blood pressure, anemia, insomnia, headache, dizziness, irritability, weakness of muscles, hallucination and renal damage, lead poisoning, coma and death can be considered among the most substantial consequences (Malakootian et al. 2006; Fenglian & Wang 2011). Before being released as waste to the environment, the removal of the metal ions is significant. Among the many methods available to remove heavy metals from wastewater, the most common ones are chemical precipitation, ion-exchange, filtration, electrochemical treatment, adsorption and reverse osmosis. Adsorption is recognized as an effective and economic method for heavy metal wastewater treatment. In addition, the adsorption is sometimes reversible, the adsorbents can be regenerated by proper desorption, and they do not result in the formation of harmful substances (Dabrowski 2001; Crini 2006; Doyurum and Çelik, 2006; Gülbaş et al. 2012). According to recent studies, the usage of low cost and easily available biosorbents with high removal capacity for heavy metal ions is seen as more convenient in terms of providing a contribution to the environment (Crini 2005; Malakootian et al. 2009). In light of the above, waste biosorbents have emerged as an economic and eco-friendly option. Agricultural wastes, natural substances, industrial byproducts and wastes have been studied as adsorbents for heavy metal wastewater treatment (Ansari et al. 1999; Mohan & Singh 2002; Yoshihiro et al. 2005). Various different adsorbents and biosorbents can be employed for the treatment of wastewater laden with heavy metal (Bhattacharyya & Gupta 2008; Fenglian & Wang 2011). In the literature the adsorption efficiency of rice husk, maize cobs and sawdust for the removal of Pb(II) from aqueous systems is examined (Abdel-Ghani et al. 2007). Different forms of inexpensive, non-living plant material such as potato peels (Aman et al. 2008; Kaczala et al. 2009), eggshell (Jai et al. 2007), coffee husks (Oliveira et al. 2008), seed shells (Amudaa et al. 2009), black gram husk (Saeed et al. 2005) and sugar-beet pectin gels (Mata et al. 2009), sugarcane bagasse (Kausar et al. 2016), fungal dead biomass composite with bentonite (Rashid et al. 2016), vigna radiata biomass (Naeema et al. 2017), peanut husk (Kausar et al. 2018), and so on, have been widely investigated as potential biosorbents for heavy metals. In accordance with many natural adsorbents made up to now, the adsorption capacity of our adsorbents is quite high. In addition, our adsorbents form too much waste. It is very economical to use the materials found in nature so that they can remove heavy metals from water.

In this study, bean, lemon and artichoke shells were used as natural biosorbents in order to removal Pb(II) from aqueous solution. In the first process of the studies, the effect of optimum contact time on the capacity of metal adsorption was determined for the natural biosorbents.

Next, the effects of the concentration of Pb ions, pH, and temperature on the capacity of metal adsorption of natural biosorbents were determined. Characterization of the surface morphology of the biosorbents was performed on both adsorbents before and after lead adsorption.

Three different biosorbents, lemon, bean and artichoke shells, were used in order to examine the removal of lead (II) from aqueous solutions. All reagents used throughout this study were analytical-reagent grade. De-ionized water was used for all reagent solutions. Pb(NO₃)₂ (99.5–100.2%), HCl, Tris, CH₃COOH, NaCH₃COO·3H₂O, NH₃, and NH₄Cl were purchased from Merck.

Lead concentration measurements were carried out using a Varian 220 FS atomic absorption spectrophotometer (AAS). An atomic absorption spectrophotometer equipped with a lead hollow cathode lamp at the wavelength of 193.7 nm was used for the determination of lead in sample solutions. The pH of the solution was measured with a Hanna P211 microprocessor pH-meter using a combined glass electrode. The shaking experiments were carried out in a Labart SH-5 thermostated electronic shaker and drying was carried out in a Nüve EN 400 drying oven. Surface morphology of samples was determined by Philips (FEI) XL30-SFEG Scanning Electron Microscope.

Stock solution (1000 mg L⁻¹) of Pb(II) was prepared by using ultrapure grade (99.5–100.2)%) Pb(NO₃)₂ 4H₂O. The stock solution was then diluted to give standard solutions of appropriate concentration.

Lemon, bean and artichoke shells were sieved with a grinding mill to obtain 63 μm particle size and used in the adsorption studies. The biosorbents were used without any chemical pre-treatment. The samples were washed and air-dried and then kept in desiccators.

Adsorption experiments were carried out by batch technique. 0.50 ± 0.0004 g of each biosorbent was put in a beaker containing 25 mL of 20 mg L⁻¹ lead (II) solutions. The shaking was carried out in a thermostated electronic shaker under constant stirring at 160 rpm. After decantation, the concentration of lead (II) was analyzed by AAS at 193.7 nm by acetylene/air flame method. The effect of contact time was studied between 5 and 240 minutes and the initial metal concentration was fixed at 20 mgL⁻¹.

For desorption studies, 0.1 g of biosorbent was loaded with 25 mL of 100 mg L⁻¹ lead (II) solution. Lead (II) loaded biosorbents were collected, and gently washed with distilled water to remove any unadsorbed lead (II) ions. The sorbents were contaminated with 25 mL of 0.01 M EDTA, 0.1 M HCl and 0.1 M HNO₃ solutions at 25 °C. The amount of desorbed lead (II) was determined as mentioned before.

The time-dependent behavior of lead (II) adsorption was measured by varying the contact time between the adsorbate and adsorbent in the range of 5–240 min. The percentage removal of lead (II) from aqueous solution as a function of contact time indicates that higher removal of lead (II) was observed in the case of lemon and artichoke shells. After 10 min, 90% lead (II) removal was obtained by artichoke shells and after 60 min, 90% lead (II) removal was obtained by lemon shells. After 10 min, 81% lead (II) removal was obtained by bean shells. The results for the effect of contact time on lead removal by lemon, artichoke and bean shells are presented in Figure 1. Figure 1 shows that uptake of lead (II) by bean and artichoke shells was rapid within the first 10 min of contact time. Equilibrium was reached at around 60 min, and beyond this time no change in adsorption occurred with lemon shells. Thus, the equilibrium time was maintained at 60 min in subsequent adsorption studies with lemon shells. In addition, optimum contact time was chosen as 10 min for bean and artichoke shells. Results of this study are entirely in accordance with the results of Inglezakis et al., which showed that 120 min is a sufficient time for Pb adsorption on bentonite and clinoptilolite (Inglezakis et al. 2007).

The pH of the aqueous solution is an important parameter in adsorption processes. The effect of pH is presented in Figure 2. The effect of pH on the adsorption of lead (II) ions was studied at different pH values (3.0 − 9.0) using lemon, artichoke and bean shells (500 mg) at a constant lead (II) ion concentration (20 mg L⁻¹). The effect of pH on adsorption of lead was studied at room temperature by using buffer solutions. As can be seen, lead (II) adsorption was observed due to the increasing positive ions on the adsorbent surface at low pH. At higher pH values, there should be competition between H⁺ ions and lead (II) ions to occupy the adsorption sites. Also, as the solution pH value decreases, the adsorbent surface is more positively charged. Therefore, there would be a repulsion force between the surface and adsorbate species. At pH <6, the free ion Pb²⁺ is the predominating species. At pH < 6, positively charged metal ions are attracted to the surface of the biosorbent. The main species at pH 7–10 are Pb(OH)⁺ and Pb(OH)₂ and thus the removal of Pb²⁺ is possibly accomplished by simultaneous precipitation of Pb(OH)₂ and sorption of Pb(OH)⁺. By increasing the solution pH, there was an increase in ligands carrying negative charges, which resulted in higher binding of cations. Although high adsorption was observed at low and high pH, we carried out all studies at the natural water pH without using any buffer solution. Considering these results and the limitations concerning the discharge of acidic and alkaline effluents to the environment, neutral pH is logical and acceptable in the removal process.

In this study, the adsorption process was assessed at different temperatures between 20 and 40 °C at an increment of 5 °C units. The contact time and pH were kept constant during the temperature experiments. Figure 3 shows that the temperature had no important effect on the adsorption of lead (II). Maximum adsorption was observed at 20 °C. It can be seen in Figure 3 that over 20 °C adsorption decreased a little. As seen in Figure 3, adsorption capacity decreased a little with an increase in temperature. With increasing temperature, the attractive forces between the lemon-metal ion, artichoke-metal ion and bean shells-metal ion are weakened, and then sorption decreases. The adsorption temperature was chosen as 20 °C.

Thermodynamic values are shown in Table 1. The negative values of ΔS (−0.00545, −0.05017, −0.02185 kJ/molK) suggest decreased randomness during adsorption and reflect the affinity of the lemon, artichoke and bean shells for lead (II) ions. The negative value of ΔG (−5.6786, −5.5758, −3.1488 kJmol⁻¹) shows the adsorption process proceeds spontaneously. The negative value of ΔH indicated that the adsorption process was exothermic in nature.

Adsorption isotherms are used to express the surface properties and affinity of the adsorbent and can also be used to compare the adsorption capacities of the sorbents for pollutants in aqueous solutions. The adsorption isotherm is a functional expression that correlates the amount of solute adsorbed per unit weight of the adsorbent and the concentration of an adsorbate in the bulk solution at a given temperature under equilibrium conditions. The adsorption isotherms are one of the most useful data to understand the mechanism of the adsorption, and the characteristics of isotherms are needed before the interpretation of the kinetics of the adsorption process. Langmuir and Freundlich isotherms are most used to describe the equilibrium sorption of metal ions. The adsorption of lead ions onto lemon shell, artichoke shell and bean shell were studied at 20 °C by varying the lead concentration from 5 to 6000 mg L⁻¹ (Figure 4). All other parameters were kept constant. The results in Figure 4 show the adsorption capacity of lemon shell, artichoke shell and bean shell used. The maximum adsorption capacities were found to be 61.3 and 88.5 and 62.81 mg g⁻¹of lemon, bean and artichoke shell, respectively.

When C_e/q_e is plotted against C_e, a straight line with a slope of 1/Q and intercept was obtained, which shows that the adsorption of lead (II) followed the Langmuir isotherm model.

According to the results, the Freundlich model was found to describe adsorption more successfully than the Langmuir isotherm in respect to linearity coefficients obtained for both models (lemon, artichoke, bean shell) (R²_Freundlich = 0.9810, 0.9094, 0.8876, R²_Langmuir = 0.8797, 0.0835 and 0.7097) (Figures 5–7).

The Freundlich parameters K_F (mg g⁻¹) and n were calculated from the slope and intercept and are given in Table 1.

As can be seen in Table 2, lemon, bean and artichoke shells even without any chemical pretreatments showed better adsorption capacities for lead compared with conventional adsorbents.

The scanning electron microscopy of lemon shell, artichoke shell, bean shell and Pb(II)-loaded lemon shell, artichoke shell, and bean shell are shown in Figures 8–10. The surface change in the scanning electron microscope (SEM) micrographs indicated the structural changes in the sorbent before and after Pb(II) treatment. The general morphology of the modified chestnut shell before adsorption could be characterized as rough and folded. It was found that the surface of the Pb(II) loaded chestnut shell was a shiny material. It was seen that the surface is more uneven after adsorption for Pb(II).

Energy-dispersive X-ray (EDX) results are shown in Figures 11 and 12 of artichoke shell and bean for Pb(II). Since the biosorbents are in the cellulosic structure, C and O are visible, and the adsorbed Pb ions are also found in EDX (Tables 3 and 4). The Ca, K and Mg ions seen in EDX are due to the mineral content of the adsorbents.

Fourier transform infrared (FTIR) spectra are shown in Figure 13 of bean, lemon and artichoke shell and bean, lemon and artichoke shell with Pb(II). The bands at 3539, cm⁻¹ are assigned to the stretching vibration of the surface hydroxyl. The major bands for the bean, lemon and artichoke shell can be assigned as follows: 2921 cm⁻¹ (CH stretching vibration in CH and CH₂), 1643 cm⁻¹ (NH₂ bending vibration), and 1021 cm⁻¹ (CO stretching vibration in CONH).

The desorption process is very important as it allows the metals to be recovered and the adsorbent to be used for large-scale industrial application. (Rashid et al. 2016; Godiya et al. 2019). Studies have shown that the protons of mineral acids such as HCl, and HNO₃ can displace metals from the sorbents' binding sites (Bux et al., 1995) and that the strong chelating agent EDTA is also an efficient desorbing agent (Njikam & Schiewer, 2012) causing metal desorption by reducing the concentration of metal ions in solution. Percentage desorptions (Equation (6)) of lead (II) ions from the biosorbent are shown in Table 5. Table 5 shows the percentage of lead (II) released after treatment with desorbents such as EDTA, HCl and HNO₃. It was observed that EDTA was a more efficient desorbing agent than HCl and HNO₃ for all three biosorbents. EDTA has a high level of complexing capacity with respect to heavy metals (Zhao et al. 2014). EDTA has been widely demonstrated as a chelating agent in washing treatment (Martinez et al. 2006). Desorption rates of 74.02% 85.3% and 93.8% obtained from desorption studies of lemon, artichoke and bean biosorbents with EDTA show us that lead ions from these biosorbents can be recovered very successfully and that the biosorbents can be used again.

Lead (II) removal using lemon, artichoke and bean shells would be effective methods for the economic treatment of wastewater. Bean shell was the most effective, for which the removal reached 88.5 mg g⁻¹ of Pb at room temperature. 61.30 mg g⁻¹ and 62.81 mg g⁻¹ lead (II) ion capacities were found for lemon and artichoke shells, respectively. The optimum pH for lead removal was found to be 4.5. The adsorption process was exothermic (ΔH°ads <0) and proceeds spontaneously (ΔG° ads <0) for both adsorbents. The Freundlich models achieve good fittings for all biosorbents. This indicates that these materials have a greater capacity, and are most effective, low cost and environmentally friendly adsorbents for the removal of lead (II) from wastewater. The results prove that lead ions can easily be removed with these biosorbents in a very short time.