Utilisation of wastes for low-cost synthesis of chitosan composites with nanosized sodium aluminium silicate hydrate and geopolymer/zeolite A for the removal of Hg(II) and Pb(II) ions from aqueous media

ABSTRACT In this work, nanosized sodium aluminium silicate hydrate and geopolymer/zeolite A samples were obtained using hydrothermal and precipitation methods, respectively. In these methods, aqueous solutions of sodium hydroxide were utilised for the extraction of aluminium and silicon from waste aluminium cans and rice husk ash, respectively. Also, chitosan composites with the aforementioned samples were fabricated. The properties of the obtained nanosized samples and their chitosan composites were identifiedutilizingFT-IR, XRD, HR-TEM, and FE-SEM tools. The average crystallite size of the sodium aluminium silicate hydrate and geopolymer/zeolite A samples is 39.72 and 22.56 nm, respectively. Moreover, the obtained nanosized samples and their chitosan composites were utilised as adsorbents for the effective uptake of Pb(II) and Hg(II) ions from aqueous media. The maximum capacities of sodium aluminium silicate hydrate, geopolymer/zeolite A, sodium aluminium silicate hydrate/chitosan, and geopolymer/zeolite A/chitosan adsorbents towards Hg(II) ions are 88.65, 125.94, 168.35, and 211.86 mg/g, respectively. The maximum capacities of the previous adsorbents towards Pb(II) ions are 123.61, 160.51, 211.86, and 269.54 mg/g, respectively. Furthermore, the uptake processes of Pb(II) and Hg(II) ions using the obtainednanosized samples and their chitosan composites are exothermic, chemical, spontaneous, and well-matched with Langmuir equilibrium isotherm and pseudo-second-order kinetic model.


Introduction
In recent years and until now, water pollution is considered one of the mainconcernsforscientists and environmentalists. Amid the several classifications of water pollutants, metal ions have attracted lots of interest because it causes many diseases such as kidney failure and nerve atrophy. Anthropogenic sources particularly industrial factories can discharge a lot of metal ions into the water with or without caution. Release of metal ions in water sources such as lakes and rivers can prompt numerous damages and dangerous problems [1][2][3][4]. Aquatic living organisms such as fishes adsorb the metal ions then these ions accumulate CONTACT Ehab A. Abdelrahman dr.ehabsaleh@yahoo.com, ehab.abdelrahman@fsc.bu.edu.eg Supplemental data for this article can be accessed here.
(HgCl 2 ), sodium hydroxide (NaOH), ethylenediaminetetraacetic acid tetrasodium salt dihydrate (Na 4 -EDTA), and lead nitrate (Pb(NO 3 ) 2 ). Waste aluminium cans were gathered from the local Egyptian market. Moreover, rice husk was gathered from a rice mill near Com Hamada City, Beheira Governorate, Egypt. All chemicals were obtained from Sigma-Aldrich Company and used as provided.

Synthesis of sodium aluminium silicate hydrate and geopolymer/zeolite A
Firstly, the silicon solution was prepared as follows: the rice husk was put in an ignition furnace at 750°C for8 h for generation of rice husk ash. After that, 2500 mg of the rice husk ash was moved into a 0.25 L stoppered flask containing 0.1 L of 1.5 mol/LNaOH solution for refluxing the mixture at 155°Cfor 2 h. Secondly, the aluminium solution was prepared as follows: 1500 mg of the outer part (i.e. not top or bottom) of aluminium waste was put in a 0.25 L beaker containing 0.05 L of 2 mol/L NaOH solution for dissolving it under the action of heat. The aluminium solution was added to the silicon solution under the action of stirring. The stirring of the produced mixture was continued for 120 min. Moreover, the mixture was heated at 120°C until 80 mL remained. The remaining volume was transferred into a 0.1 L Teflon lined autoclave for treating it hydrothermally at 160°C for 13 h. The precipitate was separated via a centrifuge, washed six times with 0.13 L of hot distilled water, and dried at 65°C for 13 h for obtaining sodium aluminium silicate hydrate which was abbreviated as R1. The prior steps were repeated without performing hydrothermal treatment for obtaining geopolymer/zeolite A which was abbreviated as R2.

Synthesis of sodium aluminium silicate hydrate/chitosan and geopolymer/ zeolite A/chitosan composites
The synthesis of composites was accomplished as suggested by the prior work [13]. Briefly, 2 g of sodium aluminium silicate hydrate or geopolymer/zeolite A was mixed with2 g of chitosan. Then, the mixture was added to 0.126 L of 1.04 mol/L acetic acid solution under the action of stirring. The stirring of the produced mixture was continued for 8 h. After that, the mixture was added to 0.250 L of 0.65 mol/L sodium hydroxide solution under the action of stirring. The stirring of the produced mixture was continued for 8 h. Furthermore, the precipitate was separated via a filter paper, washed six times with 0.13 L of hot distilled water, dried at 65°C for 12 h, and ground using agate mortar. The sodium aluminium silicate hydrate/chitosan and geopolymer/zeolite A/chitosan samples were abbreviated as R1/Ch and R2/Ch, respectively.

Effect of pH
To investigate the effect of pH (2-7) on adsorption, 50 mg of adsorbent (R2, R1, R2/Ch, or R1/Ch) was added to 0.05 L of 500 mg/L aqueous solution of Hg(II) or Pb(II). Also, adjustment of pHwas accomplished using 0.1 mol/LHCl or NaOH. Then, the mixture was stirred at 510 rpm at the desired pH for 180 min.

Effect of time
To investigate the effect of time (5-180 min) on adsorption, 50 mg of adsorbent (R2, R1, R2/Ch, or R1/Ch) was added to 0.05 L of 500 mg/L aqueous solution of Hg(II) or Pb(II) which was adjusted to pH = 7. Then, the mixture was stirred at 510 rpm for the desired time.

Effect of temperature
To investigate the effect of temperature (298-318 K) on adsorption, 50 mg of adsorbent (R2, R1, R2/Ch, or R1/Ch) was added to 0.05 L of 500 mg/L aqueous solution of Hg(II) or Pb(II) which was adjusted to pH = 7. Then, the mixture was stirred at 510 rpm for 60 min at the desired temperature.

Effect of concentration
To investigate the effect of concentration (125-600 mg/L) on adsorption, 50 mg of adsorbent (R2, R1, R2/Ch, or R1/Ch) was added to 0.05 L aqueous solution of Hg(II) or Pb(II) which was adjusted to pH = 7. Then, the mixture was stirred at 510 rpm for 60 min at the desired concentration.

Effect of ionic strength
To investigate the effect of ionic strength using different concentrations of sodium chloride (0.1-1 mol/L) on adsorption, 50 mg of adsorbent (R2, R1, R2/Ch, or R1/Ch) was added to 0.05 L of 500 mg/L aqueous solution of Hg(II) or Pb(II) which was adjusted to pH = 7. Then, the mixture was stirred at 510 rpm for 60 min at the desired concentration.

Effect of regeneration and reusability
For the regeneration of the adsorbent, the adsorbent was recovered by adding 0.05 L of 0.5 M Na 4 -EDTA solution to the loaded adsorbent then the solution was stirred for 120 min at ambient temperature. After that, the recovered adsorbent was separated, washed six times with 0.13 L of hot distilled water, and dried at 65°C. For reusability experiments, the recovered adsorbent was tested 4 times for the uptake of Hg(II) or Pb(II) ions as previously described.
After studying the previous effects, the adsorbent was removed via centrifugation then the concentration of Hg(II) or Pb(II) ions were examined by inductively coupled plasmaoptical emission spectrometry (ICP-OES).
The adsorption efficiency (% R) of Pb(II) or Hg(II) was estimated using Equation (1): The adsorption capacity (Q, mg/g) of adsorbents was estimated using Equation (2): where C o (mg/L) and C e (mg/L) are the initial and equilibrium Hg(II) or Pb(II) concentration, respectively. m A (g) is the mass of adsorbent and V S (L) is the volume of the Hg(II) or Pb(II) solution.

Characterisation techniques
X-ray diffraction (XRD) patterns of the obtained products were performed on a PAN analytical x, PERT PRO utilizing CuK α X-ray radiation (λ = 1.540 Å). Fourier-transform infrared spectroscopy (FT-IR) spectra of the obtained products were recordedutilizingKBr disk on a Perkin Elmer FT-IR spectrophotometer in the 4000-400 cm −1 region. The morphologies of the obtained products were performed via field emission scanning electron microscopy (FE-SEM) (JSM5410JEOL). The high-resolution scanning electron microscopy (HR-TEM) images of the obtained products were performed via a transmission electron microscope (JEOL 2100) at a speeding voltage of 200 kV. The concentration of Hg(II) or Pb(II) ions was estimated via inductively coupled plasma-optical emission spectrometry (ICP-OES) (Thermo Scientific, 6500 Duo).       [11][12][13][14][15][16][17][18]66,[70][71][72][73]. The proposed structure of the chitosan composites with nanosized zeolites or geopolymers was clarified in a prior work [15]. Figure 3(a-d) represents the FE-SEMimages of the geopolymer/zeolite A, sodium aluminium silicate hydrate, geopolymer/zeolite A/chitosan, and sodium aluminium silicate hydrate/chitosan products, respectively. The geopolymer/zeolite A and sodium aluminium silicate hydrate products composed of a sphere and irregular shapes with an average size of ca. 0.21and 0.34 µm, respectively. Also, the chitosan composites are composed of a rough and flaky surface. Figure 4(a,b) represents the HR-TEM images of the geopolymer/zeolite A and sodium aluminium silicate hydrate, respectively. The geopolymer/zeolite A and sodium aluminium silicate hydrate products composed of a sphere and irregular shapes with an average diameter of ca. 27.62 and 46.28 nm, respectively.

Effect of pH
From figure 5 it is noticeable that due to the change in values of pH, the removal efficacy (% R) or adsorption capacity (Q) of R1, R2, R1/Ch, and R2/Ch adsorbents toward Hg(II) or Pb(II) ions varyextensivelywhere the optimum pH, which corresponds to the highest value, is 7. The % R of Hg(II) ions utilising the aforementioned adsorbents at the optimum pH is 16, 21.4, 30, and 40.6%, respectively, as shown in Figure 5(a). Also, the adsorption capacity of the abovementioned adsorbents towards Hg(II) ions at the optimum pH is 80, 107, 150, and203 mg/g, respectively, as shown in Figure 5(c). Besides, the % R of Pb(II) ions utilising the aforementioned adsorbents at the optimum pH is 22, 30, 39, and 52%, respectively, as shown in Figure 5(b). Moreover, the adsorption capacity of the abovementioned adsorbents towards Pb(II) ions at the optimum pH is 110, 150, 195, and 260 mg/g, respectively, as shown in Figure 5(d). Thus, the efficiency of the adsorbent materials towards the removal of Hg(II) or Pb(II) ions is according to the following order: R1< R2< R1/Ch<R2/Ch. R2 outperformed R1 because the crystallite size of the R2 sample is less than that of the R1 sample. Hence, the surface area of R2 is more than that of the R1 sample [16]. Also, R1/Ch or R2/Ch outperformed R1 or R2 because chitosan composites remove metal ions by ion exchange and chelation via NH 2 and OH groups whereas R1 or R2 remove metal ions via ion exchange only through the exit of sodium ions and the entry of metal ions instead as shown in Scheme 1. Besides, R2/Ch outperformed R1/Ch because the chitosan functional groups on R2/Ch is more than that of R1/Ch. Moreover, the values of % R or Q decrease at low pH values because these values make the adsorbent materials surrounded by a positive charge that expels the positive metal ions [12][13][14][15].

Effect of time
From Figure 6 it is noticeable that due to the change in values of time, the removal efficacy (% R) or adsorption capacity (Q) of R1, R2, R1/Ch, and R2/Ch adsorbents towards Hg(II) or Pb(II) ions vary extensively and the optimum time is 60 min. The % R of Hg(II) ions utilising the aforementioned adsorbents at the optimum time is 16.2, 22, 30, and 40%, respectively, as shown in Figure 6(a). Also, the adsorption capacity of the above-mentioned adsorbents towards Hg(II) ions at the optimum time is 81, 110, 150, and 200 mg/g, respectively, as shown in Figure 6(c). Besides, the % R of Pb(II) ions utilising the aforementioned adsorbents at the optimum time is 21.6, 29.6, 39.2, and 51.8%, respectively, as shown in Figure 6(b). Moreover, the adsorption capacity of the above-mentioned adsorbents towards Pb(II) ions at the optimum time is 108, 148, 196, and 259 mg/g, respectively, as shown in Figure 6(d). Pseudo- first-order (Equation (3)) as well as pseudo-second-order (Equation (4)) kinetic equations are utilised for expressing the uptake of Pb(II) or Hg(II) ions by the produced adsorbents and their chitosan composites [11][12][13][14][15][16].  logðQ e À Q t Þ ¼ logQ e À K 1 t=2:303 where Q t (mg/g) is the removal capacity of the constructed products at a certain time t while Q e (mg/g) is the removal capacity of the constructed products at equilibrium. Besides, K 2 (g/mg.min) is the pseudo-second-order rate constant whileK 1 (1/min) is the pseudo-first-order rate constant. Figure 7(a,b) represents the pseudo-first-order model of the uptake of Hg(II) and Pb(II) ions using the constructed products, respectively. Also, Figure 7(c,d) represents the pseudo-second-order model of the uptake of Hg(II) and Pb(II) ions using the constructed products, respectively. The results of the previously mentioned kinetic models confirm that the removal process of Hg(II) and Pb(II) ions by R1, R2, R1/Ch, and R2/Ch adsorbents is well fitted with pseudo-second-order model as shown in tableS1and S2, respectively.
%R= 100 À %R ð Þ ½ �V=m (7) Figure S1 represents the plot of ln K d versus 1/T for the uptake of Hg(II) and Pb(II) ions utilising the constructed products, respectively. Tables S3 and S4 contain the enthalpy, Gibbs-free energy, and entropy parameters for the uptake of Hg(II) and Pb(II) ions, respectively. The results of the previously mentioned thermodynamic parameters confirm that the removal process of Hg(II) and Pb(II) ions by R1, R2, R1/ Ch, and R2/Ch adsorbents is spontaneous (Sign of Gibbs-free energy is negative), exothermic (Sign of enthalpy is negative), and chemical (value of enthalpy is more than 40 kJ/mol).

Effect of concentration
Calculation of adsorption capacity of the adsorbents (R1, R2, R1/Ch, and R2/Ch) clarifies that usually with increasing the initial concentration of Hg(II) and Pb(II) ions, the adsorption capacity is increased as shown in figure S2C-D, respectively. Besides, % removal of Hg(II) and Pb(II) ions is decreased with increasing the initial concentration of the prior ions as displayed in figure S2A-B, respectively. Adsorption isotherms gotten through linear fitting of experimental results with Langmuir and Freundlich models are introduced in Equations (8) and (9), respectively [11][12][13][14][15][16].
where Q m (mg/g) is considered the maximum adsorption capacity of the constructed products while K L (L/mg) is considered the Langmuir constant. Moreover, K F (mg/g)(L/ mg) 1/n is considered the Freundlich constant. Furthermore, 1/n is considered the heterogeneity factor. Besides, the value of Q m from Freundlich isotherm was calculated using Equation (10) [11][12][13][14][15][16]. Figure S3A-B represents the Langmuir isotherms for the uptake of Hg(II) and Pb(II) ions, respectively. Also, Figure S3C Table S7 shows the effect of ionic strength on the removal of Hg(II) and Pb(II) ions using the synthesised adsorbents in the presence of different concentrations of sodium chloride in the range from 0.1 to 1 mol/L. The decrease in % R or Q may be due to competition between sodium ions and metal ions for the active sites of the utilised adsorbents.

Reusability study
Any efficient adsorbent must exhibit enough reusability. For both Hg(II) and Pb(II) ions, there are not any considerable decreases in the values of capacity after four cycles as shown in figure S4A-D and figure S5A-D, respectively.

Conclusion
Aqueous solutions of sodium hydroxide were utilised for the extraction of aluminium and silicon from waste aluminium cans and rice husk ash, respectively. After that, the extracted solutions were mixed then treated using precipitation and hydrothermal methods to synthesise geopolymer/zeolite A and sodium aluminium silicate hydrate nanosized samples, respectively. Also, chitosan composites with the obtained nanosized samples were fabricated. Moreover, the obtained samples and their chitosan composites were utilised for the efficient removal of Pb(II) and Hg(II) ions from aqueous media. tively. Additionally, the uptake processes of Pb(II) or Hg(II) ions using the obtained nanosized samples and their chitosan composites are exothermic, chemical, spontaneous, and well matched with Langmuir equilibrium isotherm and pseudo-secondorder kinetic model.

Disclosure statement
No potential conflict of interest was reported by the author(s).