Removal of Pb ( II ) using Hydroxyapatite from Golden Snail Shell ( Pomacea canaliculata L . ) Modified with Silica

The composites of hydroxyapatite and SiO2 were successfully synthesized. The hydroxyapatite was prepared from golden snail shells (Pomacea canaliculata L). The hydroxyapatite and hydroxyapatite-SiO2 composites were characterized using XRD, FTIR, SEM-EDS. Furthermore, hydroxyapatite and hydroxyapatite-SiO2 composites were used to remove Pb(II) from aqueous solution. Various adsorption parameters such as pH of the solution, contact time, and initial Pb(II) concentration were used to study the adsorption process. The optimum pH of the solution for removal of Pb(II) by hydroxyapatite and hydroxyapatite-SiO2 composite at pH 6 and contact time at 60 minutes. Both adsorbents follow the Langmuir isotherm. The maximum adsorption capacity of the hydroxyapatite-SiO2 composite is greater compare to hydroxyapatite, respectively 135.14 and 123.46 mg/g. The pseudo-second order kinetic model had a correlation coefficient (R 2 ) greater than the pseudo-first order so pseudo-second order kinetic is better to describe adsorption kinetics.


INTRODUCTION
Heavy metals such as lead, nickel, lead, mercury, chromium, copper are commonly found on the surface water and groundwater. Industrial wastewater has a role in the presence of heavy metals in the environment. Heavy metals are toxic compounds for humans, other living organisms, and the environment (Samuel, Shah, Bhattacharya, Subramaniam, & Singh, 2018). In addition, heavy metals have properties that are difficult to degrade and accumulate in the body of organisms. One of the heavy metals found in industrial wastewater is lead. The presence of lead in small concentrations in water is harmful to human and aquatic animals due to accumulation in the food chain (Dong, Zhu, Qiu, & Zhao, 2010). Lead is difficult to break down naturally. Lead can have a serious impact on living organisms that trigger kidney disorders, anemia, hemolysis, liver dysfunction, and neuron cell injury (Jarup, 2003;He et al., 2019). The World Health Organization (WHO) has set permitted limits for lead in drinking water 0.01 mg/L while the US Environmental Protection Agency (US EPA) sets a maximum of 0.015 mg/L. Wastewater treatment is very important before being released into the environment.
Technologies that can be used to eliminate Pb include extraction (Tawinteung, Parkpian, Dealune, & Jugsujinda, 2005), precipitation (Kavak, 2013), membrane (Hajdu et al., 2012), electrochemical (Li et al., 2017), ion exchange (Meyers, Gottlieb, & Desilva, 2013) and adsorption (He et al., 2019). Adsorption is a method based on chemical or physical interactions between pollutants and adsorbents. This method is often used in wastewater treatment because it is effective for removing heavy metal ions, simple and inexpensive technology (Gu, Wang, Mao, Yang, & Wang, 2018;He et al., 2019). The adsorption methods are developed to obtain adsorbents that are environmentally friendly, inexpensive, abundant in availability, and large adsorption capacity.
Hydroxyapatite can be synthesized using chemical reagents or extracted from materials containing calcium. Hydroxyapatite from biomaterials has the advantage of being biodegradable and low cost. Various biomaterials can be used as a source of hydroxyapatite such as fish bones (Sunil, & Jagannatham, 2016), eggshells (Wu, Hsu, Hsu, Chang, & Ho, 2016), animal bones (Miecznik, Haberko, Sitarz, Bucko, & Macherzynska, 2014). Biomaterials that are also potentially used as a source of hydroxyapatite are golden snail shells. Golden snail (Pomacea canaliculata L) is one of the animals in Indonesia which is abundant in freshwater waters and has not been used optimally. The golden snails belong to the species of snails in Mollusca phylum, animals with soft-bodied features that are protected by hard golden shells containing calcium carbonate (Campbell, Reece, & Mitchell, 2005). Golden snails are considered as one of the pests in rice plants.
In this study, hydroxyapatite was modified with silica and used to remove lead. Hydroxyapatite is prepared from golden snail shells. Silica is reported to be used to adsorb organic compounds and metal ions (Sayari, Hamoudi, & Yang, 2005;Parida, Dash, Patel, & Mishra, 2006;Dhmees, Khaleel, & Mahmoud, 2018). The influence of several parameters (pH of the solution, contact time, and initial concentration), isotherms, and kinetics were studied in this study.

Synthesis of hydroxyapatite from golden snail shell
The golden snail shell was cleaned of impurities using water and then boiled for 1 hour. The shell was separated from the meat and then dried in the sun for 1 day. The shell was crushed to the size of 100 mesh. 50 g of shell were calcined at 900 o C for 4 hours to convert the CaCO3 into the CaO phase. The calcination reaction is as follows.
CaCO3 → CaO + CO2 CaO (11.216 g) was put into a 500 mL beaker glass with 100 mL of distilled water. After that, 200 mL of 2 M nitric acid solution was added, and then the solution stirred at 600 rpm for 30 minutes at 90 o C. Subsequently, 15.84 g of (NH4)2HPO4 was added to the solution. The pH of the solution being maintained at 9-10 by dropping NH4OH at a rate of 1 mL/min using a burette. The solution was left to stand for 24 hours so that a precipitate formed. The precipitate obtained was filtered, then washed using distilled water. Finally, precipitate dried for 1 hour at 100 o C.

Synthesis of hydroxyapatite-SiO2 composite
The hydroxyapatite-SiO2 composite synthesis uses the sol-gel method (Latifi, Fathi, & Golozar, 2012). The ratio of TEOS and MTES is 1:1, the ratio of H2O and (TEOS + MTES) is 2:1, and the ratio of water and acetic acid is 7:1. A total of 50 mL of the mixture is stirred to form a sol. Subsequently, hydroxyapatite was added as much as 5 g and stirred for 2 hours. The mixture was allowed to stand for 24 hours to form a gel. The gel was dried in an oven at 80 o C for 10 hours and continued with calcination at 600 o C for 1 hour. The product obtained was crushed with a mortal to obtain a size of 100 mesh.
The crystal structure of powder was analyzed using XRD in the range of 2θ at 10-80 o . The functional groups were observed with FTIR at wavenumbers of 400-4000 cm -1 . The morphology and element content were observed using SEM-EDS.

Determination of point zero charge (pHpzc)
The measurement of pHpzc was performed based on the method reported by Jain, et al., (2018) with modification. The pHpzc of hydroxyapatite and hydroxyapatite-SiO2 composite was conducted using 50 mL NaCl 0.01 M, the initial pH (pHi) was adjusted from 2-10 using 0.1 M HCl and NaOH solution. The solution was added into a conical flask containing 0.2 g hydroxyapatite or hydroxyapatite-SiO2 composite, agitated by shaker at 120 rpm for 24 h. The final pH (pHf) of solution was measured using pH meter. The pHpzc was determined from the graph of the ΔpH(pHf-pHi) versus pHi.

Batch Adsorption Process
The effect of the pH solution is carried out in the range 2-10 was determined by adding 20 mg of hydroxyapatite-SiO2 composite to 25 mL Pb(II) solution with a concentration of 50 mg/L. HCl and NaOH 0.1 N solutions were used to adjust the pH of the solution. The contact time effect was carried out using a shaker for 0-90 minutes, while the effect of the initial concentration of Pb(II) varies from 25 to 200 mg/L. The concentration of Pb(II) after the adsorption process was determined by Atomic Absorption Spectroscopy.

RESULTS AND DISCUSSION Characterization of Hydroxyapatite-SiO2 composite
The XRD patterns of hydroxyapatite, SiO2, and hydroxyapatite-SiO2 composite are shown in Figure 1 (JCPDS No. 49-1425). In this study, the peak appears in 2θ=22.25 o . Some researchers obtained silica peaks at 22.5 o (Dhmees, Khaleel, & Mahmoud, 2018), and 23.0 o (Music, Vincekovic, & Sekovanic, 2011). The XRD pattern of the hydroxyapatite-SiO2 composite is a combination of hydroxyapatite and silica peak, and there is no shift in the 2θ. In the ZnO-SiO2 composite, the peak does not have a shift, but the high intensity peak depends on the SiO2 concentration (Widiyastuti, Machmudah, Nurtono, Winardi, & Okuyama, 2017).
The FTIR spectra of hydroxyapatite, SiO2, and hydroxyapatite-SiO2 composite are presented in Figure 2. The phosphate ion (PO 4 3− ) is a specific functional group on hydroxyapatite that can be observed at wavenumbers 900-1200 cm -1 for ν1 and ν2 ions. In hydroxyapatite, the group appears at wavenumbers 1087 and 1093 cm -1 . In hydroxyapatite-SiO2 composite, it appears at 1098 and 966 cm -1 . The presence of phosphate ion is also expressed at peak characteristics (ν3 and ν4) at 563, 626, and 910 cm -1 for hydroxyapatite and wavenumbers of 567, 602 and 966 cm -1 for hydroxyapatite-SiO2 composite (Esmaelkhanian et al., 2019). The broadband wavenumber at 2800-3600 cm -1 indicates the presence of a hydroxyl group that can be found in all spectra (Balamurugan, Kannan, & Rajeswari, 2013). FTIR spectra of SiO2 shows Si-O (δ) bonds at wavenumber 464 cm -1 for Si-O-Si (νsim and νass) and at wavenumber 806 and 1064 cm -1 . The same bonds in hydroxyapatite-SiO2 composite appear in 464, 806, and 1066 cm -1 (Ferreira, Santos, Bonacin, Passos, & Pocrifka, 2015).    The morphology of hydroxyapatite appears to have pores indicated by holes on the surface, while hydroxyapatite-SiO2 composite has a denser surface with addition of other particles on the surface (Figure 3). The hydroxyapatite has a Ca/P molar ratio of 1.64, similar to a theoretical molar ratio of 1.67. The success of the synthesis is shown by the addition of Si elements in the hydroxyapatite-SiO2 composite, as shown in Table 1. pHpzc was measured to determine the charge for an adsorbent surface or isoelectric point. The surface of the adsorbent is positively charged if the pH of the solution < pHpzc and negatively charged if the pH of the solution > pHpzc (Kausar et al., 2019). The pHpzc of hydroxyapatite and hydroxyapatite-SiO2 composite were obtained shown in Figure 4. pHpzc of hydroxyapatite and hydroxyapatite-SiO2 composite were 5.5 and 5.8, respectively.

Effect of the pH solution
The pH of the solution affects the adsorption capacity of hydroxyapatite-SiO2 to Pb ions. The surface charge of the adsorbent is affected by the pH of the solution (Kalaivani, 2014). The pH of the solution in the adsorption process was studied at range 2-10 using 25 mL of 50 mg/L Pb(II) solution. Figure 5 shows that % removal of Pb(II) increases with increasing the solution pH and optimum % removal at pH 6. The competition between H + and Pb 2+ at lower pH decreased the percentage removal. At pH of the solution > pHpzc, the negatively charged of the adsorbent. The adsorption process is effective because the adsorbent attracts Pb 2+ , which have a positive charge. Meanwhile, at pH 8, the presence of OHcaused Pb in solution to form Pb(OH)2, Pb(OH)3 and Pb(OH) + which reduced Pb 2+ absorbed by hydroxyapatite and hydroxyapatite-SiO2 composite (Xu, Tan, Chen, & Wang, 2008). Another study investigated that the precipitate of Pb(OH)2 is formed at pH 7.5 (Le et al., 2019). The % removal of Pb(II) using hydroxyapatite-SiO2 composite is greater than hydroxyapatite.  Effect of the contact time The contact time in the adsorption process was studied using 25 mL of 50 mg/L Pb(II) solution at pH 6 with the amount of adsorbent 20 mg. At the contact time of 0-60 minutes, an increase in the removal of Pb(II) was observed. The % removal of Pb(II) optimum at the contact time of 60 minutes for hydroxyapatite and hydroxyapatite-SiO2 composite ( Figure 6). The removal of Pb(II) in using hydroxyapatite is 70.4 %, and hydroxyapatite-SiO2 composite is 97.1 %. Thus, the presence of SiO2 in the hydroxyapatite increases the removal of Pb(II).

Effect of initial concentration
The initial concentration of Pb (II) was investigated in the range 25-200 mg L with 25 mL, pH of the solution 6 and amount of adsorbent 20 mg. Figure 7 shows that the initial concentration effects on the removal of hydroxyapatite and hydroxyapatite-SiO2 composite on Pb(II). The removal of Pb(II) decreases with increasing Pb(II) concentration. The optimum removal of Pb(II) at a concentration of 25 mg/L. The amount of adsorbent remains constant, while the amount of Pb(II) concentration increases. As a result, the removal of Pb(II) decreases. The same adsorption pattern was obtained by Farghali, Bahgat, Allah, and Khedr (2013) using copper oxide nanostructure on the adsorption of Pb(II).

Adsorption Isotherm
Adsorption isotherm was determined using Pb(II) concentrations in the range of 50-200 mg/L at a constant temperature and pH. In general, adsorption isotherms explains the relationship between the amount of substance absorbed per unit mass of adsorbent (Farghali et al., 2013). Two adsorption isotherm models are used namely Langmuir and Freundlich isotherms. The equation of the Langmuir and Freundlich isotherms expressed by the equation: Where (mg/L) and (mg/g) are a concentration of Pb and amount of adsorbed at equilibrium, qm (mg/g) and b represent the maximum adsorption capacity and coefficient of Langmuir is obtained from slope and intercept of the linear plot / vs . The constant (L/mg) is an empirical constant of the Freundlich isotherm, while indicates a value that is related to adsorption intensity (Rao, Ikram, & Ahmad, 2011).
The adsorption isotherm of Pb(II) on hydroxyapatite and hydroxyapatite-SiO2 composites is shown in Figures 8 and 9. The correlation coefficient value (R 2 ) of the Langmuir isotherm is higher than Freundlich isotherm, showing that the Langmuir isotherm model is more appropriate for explaining the adsorption model of Pb(II) on hydroxyapatite and hydroxyapatite-SiO2 composite. This model indicates the monolayer adsorption mechanism. Table 2 shows the Langmuir and Freundlich parameters. The maximum adsorption capacity of the hydroxyapatite-SiO2 composite is greater than hydroxyapatite. The addition of SiO2 to hydroxyapatite increases the adsorption capacity. The silanol groups have a role in reducing Pb(II) from aqueous solution. The silanol group releases H + and forms SiO-which can adsorb Pb(II) (Hu, Wang, & Pan, 2010). The value of n between 1 and 10 represents favorable adsorption (Kalaivani et al., 2014). This study obtained n > 1 for both hydroxyapatite and hydroxyapatite-SiO2 composite. Besides that, values of n between 1 and 10 describe beneficial adsorption (Rao et al., 2011).
The difference of maximum adsorption capacity using variations of adsorbent to removal Pb(II) is shown in Table 3. The hydroxyapatite and hydroxyapatite-SiO2 composite have maximum adsorption capacity for Pb(II) are 123.46 and 135.14 mg/g, respectively. The maximum adsorption capacity is higher than the other adsorbents. The result shows that hydroxyapatite-SiO2 composite can effectively use as an adsorbent of Pb(II).   (2020) In this study In this study Adsorption Kinetic Adsorption kinetic models were studied to determine the mechanism of adsorption, control of adsorption steps such as chemical reactions, transport of mass, and kinetics adsorption (Rao et al., 2011). The pseudo-first-order and pseudo-second-order are used to describe kinetic models in this study. The equations are as follows: log ( and (mg/g) are the adsorption capacity at equilibrium and every time (t). 1 and 2 are the pseudo-first-order and pseudo-second-order adsorption rate constant. The value of k is determined from the slope and intercept of the plot of the equation at different times. As shown at Table 4, the pseudosecond-order is more suitable to describe the model of adsorption Pb(II) on hydroxyapatite and hydroxyapatite-SiO2 composite. The correlation coefficient (R 2 ) of pseudo-second-order is greater than the pseudo-first-order.

CONCLUSIONS
In this study, hydroxyapatite was successfully synthesized from golden snail shells (Pomacea canaliculata L). The composite of hydroxyapatite and SiO2 was made to remove Pb(II) from aqueous solution. The optimum adsorption process was obtained at pH solution 6 and 60 minutes of contact time using hydroxyapatite and hydroxyapatite-SiO2 composites. Hydroxyapatite and hydroxyapatite-SiO2 composite are potential to be used as adsorbents. The maximum adsorption capacity of both adsorbents is greater than several adsorbents. However, the hydroxyapatite-SiO2 composite has a maximum adsorption capacity greater than hydroxyapatite. The adsorption process of the two adsorbents follows Langmuir isotherm at room temperature and also follows the pseudo-second-order kinetics.