Facile hydrothermal synthesis of calcium silicate nanostructures for removal of Hg(II) and Cd(II) ions from aqueous media

ABSTRACT In this work, we have synthesised a mixture consisting of sodium calcium silicate and calcium silicate nanostructures in the presence and absence of the hydrothermal method. The synthesised products were identified utilising many techniques such as XRD, EDS, FT-IR, HR-TEM, and FE-SEM. The XRD demonstrated that the mean crystallite size of the product, which was synthesised in the absence of the hydrothermal method, is 72.71 nm. The average crystallite size of the products, which were synthesised using the hydrothermal method at 160℃ for 4, 8, and 16 h, are 44.85, 34.89, and 37.38 nm, respectively. HR-TEM confirmed that all the products consist of irregular and rod shapes. The product, which was synthesised after 8 h of hydrothermal treatment, was operated as an adsorbent for the efficient removal of toxic ions (Cd(II) and Hg(II)) from aqueous media. The maximum adsorption capacity of the adsorbent towards Hg(II) and Cd(II) ions are 177.94 and 199.20 mg/g, respectively. The removal process of the studied ions was fitted well with the pseudo-second-order kinetic model and Langmuir equilibrium isotherm. Thermodynamic parameters confirmed that the removal process of the studied ions was spontaneous, chemical, and endothermic.


Introduction
Heavy metals in industrial wastewater are a worldwide issue.Heavy metals bioaccumulate in the muscle tissues of many aquatic organisms such as fish and hence they pose a threat to those organisms and the public health of their consumers [1,2].Plants and soil are intensely affected by heavy metals because these metals cause disorder in growth processes, enzyme activity, and photosynthesis [3,4].Heavy metals are involved in many industries and commercial activities and hence this makes a great chance of their spread in the water sources.Cadmium constitutes many health problems such as bone lesions, cancer, diarrhoea, nausea, and lung insufficiency.The World Health Organization (WHO) reports that the maximum permissible concentration of cadmium in drinking water is 3 µg/L [1,[5][6][7][8][9].Besides, mercury constitutes many health problems such as kidney failure, lung destruction, and nervous system weakness.The maximum permissible concentration of mercury in drinking water is 1 µg/L [1,[10][11][12][13].There are a large number of scientific papers that have used many methods for removing heavy metals from aqueous solutions such as reverse osmosis, flocculation, and adsorption [14][15][16].The adsorption method is at the top of these methods in terms of wide use because it is cost-effective, simple, and easy to operate [17][18][19][20][21][22][23][24][25].There are a large number of papers for removing Cd(II) ions using various adsorbents such as NaZ zeolite [26], surfactant modified zeolite [26], MnO 2 loaded D301 resin [27], steam activated sulfurized carbon [28], polyacrylamide-grafted iron(III) oxide [29], modified chitosan [30], cross-linked chitosan/polyvinyl alcohol [31], akaganeite [32], and ethylenediamine-functionalised multi-walled carbon nanotubes [33].The adsorption capacity of the aforementioned adsorbents is 16.37, 15.86, 77.88, 24.70, 147.20, 38.50, 142.90, 17.10, and 25.70 mg/g, respectively.Many researchers have published a lot of papers for removing Hg(II) ions using various adsorbents such as 2-mercaptobenzothiazole treated clay [34], 2-mercaptobenzimidazole-clay [35], mesoporous silica coated by magnetic nanoparticles [36], Fe 3 O 4 -SiO 2 -graphene quantum dot [37], dithiocarbamate functionalised Fe 3 O 4 [38], MnO 2 /carbon nanotubes [39], SiO 2 /carbon nanotubes [40], and activated carbon [41].The adsorption capacity of the aforementioned adsorbents is 2.71, 102.49, 14.00, 68.00, 85.00, 58.80, 142.00, and 25.88 mg/g, respectively.Nevertheless, there is an intense demand for the synthesis of very effective and low-cost adsorbents for the uptake of toxic ions such as heavy metals.Therefore, in this work, low-cost calcium silicate nanostructures were facilely synthesised as a novel mixture by a simple procedure in the presence and absence of the hydrothermal method.The synthesised products were utilised as a novel and effective adsorbents for the uptake of Hg(II) and Cd(II) ions from aqueous media.Also, the adsorption equilibrium, kinetics, and thermodynamics were studied.The negative charge resulting from the substitution of some divalent calcium atoms for some trivalent silicon atoms is neutralised by positive sodium or hydrogen ions.Positive sodium or hydrogen ions are easy to replace with other ions such as Hg(II) and Cd(II).Hence, these compounds possess a high ability for ion exchange and consequently water treatment.Therefore, this paper describes new, innovative, and novel results regarding the environmental field.

Fabrication of calcium silicate nanostructures
The silicon solution was prepared in the laboratory as the following; 10 g of silicon source (sodium metasilicate pentahydrate) was dissolved in 60 mL of double distilled water.Besides, the calcium solution was prepared in the laboratory as the following; 3 g of calcium chloride dihydrate was dissolved in 60 mL of double distilled water.After that, the freshly prepared calcium solution was added from a burette to the freshly prepared silicon solution drop by drop with constant stirring for 40 min at room temperature.Furthermore, the mixture was continuously heated at 130°C until 80 mL remains.The produced gel was transferred into 100 mL autoclave (Teflon lined stainless) then hydrothermally treated at 160°C for 4, 8, and 16 h.The precipitate was carefully filtered, washed with double distilled water, and dried at 55°C for 28 h.The products, which was hydrothermally synthesised after 4, 8, and 16 h, were abbreviated as C2, C3, and C4, respectively.The previous steps were repeated but in the absence of the hydrothermal treatment and the product was abbreviated as C1.

Characterisation of the calcium silicate nanostructures
The D8 Advance Bruker diffractometer with 0.15 nm K α Cu radiations was employed to identify the crystalline phases and estimate their sizes.Also, energy-dispersive X-ray spectroscope (EDX) was employed for elemental analysis of the calcium silicate nanostructures.Besides, Nicolet single beam spectrometer was performed to obtain the Fourier transform (FT-IR) spectra of the calcium silicate nanostructures.Furthermore, the Sigma 300VP Field Emission scanning electron microscopy (FE-SEM) and the JEOL 2100 High-resolution transmission electron microscopy (HR-TEM) were employed to identify the morphology of the calcium silicate nanostructures.Total pore volume, average pore radius, and BET surface area of the calcium silicate nanostructures were calculated from N 2 isotherms at −196℃ utilising Quantachrome (Model NOVA touch LX4).

Removal of Hg(II) and Cd(II) ions from aqueous media
The batch adsorption tests for the removal of Hg(II) or Cd(II) ions from aqueous media by the C3 product were performed as follows; 100 mg of the C3 product was mixed with 100 mL of 200 mg/L of pH-controlled Hg(II) or Cd(II) aqueous solution.Then, the mixture was stirred for several temperatures and times.After that, the suspensions were filtered then analysed to determine the concentration of Hg(II) or Cd(II) ions applying inductively coupled plasma optical emission spectrometry.
The quantity of the adsorbed toxic ions (Hg(II) or Cd(II)) per gram of the C3 product (Q, mg/g) was evaluated applying Equation (1).Also, the % removal (% R) of toxic ions (Hg(II) or Cd(II)) using the C3 product was evaluated utilising Equation (2).
where, C o (mg/L) is the initial concentration of toxic ions (Hg(II) or Cd(II)) whereas C e (mg/L) is the equilibrium concentration of toxic ions (Hg(II) or Cd(II)).V (L) is the volume of toxic ions (Hg(II) or Cd(II)) aqueous solution.m (g) is the exploited quantity of the C3 product.

Characterisation of the synthesised products
Figure 1(a-b) shows the XRD patterns of the C1 and C2 products, respectively.Also, Figure 2 (a-b) shows the XRD patterns of the C3 and C4 products, respectively.All the products consist of sodium calcium silicate and calcium silicate.Peaks at 2Ɵ (hkl) equals 25 [43].The average crystallite size of C1, C2, C3, and C4 products are 72.71,44.85, 34.89, 37.38 nm, respectively.Hence, the average crystallite size increases according to the following order; C3< C4< C2< C1.So, the optimum hydrothermal time for the synthesis of the products is 8 h.Total pore volume (cc/g), average pore radius (A o ), and BET surface area (m 2 /g) of the C1-C4 products are presented in Table S1.Hence, the BET surface area increases according to the following order; C3> C4> C2> C1.
Figure 3(a-d) shows the EDS patterns of the C1, C2, C3, and C4 products, respectively.All the products comprise of Ca, Si, Na, and O as shown in Table S1.The Si/Al of C1, C2, C3, and C4 products is 1.67, 1.61, 1.60, and 2.45, respectively.Figure 4(a-b) shows the FT-IR spectra of the C1 and C2 products, respectively.Also, Figure 5(a-b) shows the FT-IR spectra of the C3 and C4 products, respectively.The results proved the presence of all the distinguished bands for the synthesised products.The A-O-A (A = Si and/or Ca) bending vibrations of the C1, C2, C3, and C4 products were observed at 454, 460, 453, and 459 cm −1 , respectively.The A-O-A (A = Si and/or Ca) internal symmetric stretching vibrations of the C1, C2, C3, and C4 products were observed at 657, 598, 603, and 672 cm −1 , respectively.The A-O-A (A = Si and/or Ca) external symmetric stretching vibrations of the C1, C2, C3, and C4 products were observed at 796, 790, 785, and 790 cm −1 , respectively.The A-O-A (A = Si and/or Ca) internal asymmetric stretching vibrations of the C1, C2, C3, and C4 products were observed at 1041, 1047, 1041, and 1057 cm −1 , respectively.The A-O-A (A = Si and/or Ca) external asymmetric stretching vibrations of the C1, C2, C3, and C4 products were observed at 1494, 1450, 1489, and 1451 cm −1 , respectively.The bands, which were observed in C1, C2, C3, and C4 products at 1643, 1655, 1649, and 1644 cm −1 , are due to the bending vibrations of adsorbed water molecules, respectively.The bands, which were observed in C1, C2, C3, and C4 products at 3510, 3469, 3474, and 3457 cm −1 , are due to the stretching vibrations of adsorbed water molecules, respectively.Figure 6(a-d) shows the FE-SEM images of the C1, C2, C3, and C4 products, respectively.All the products consist of irregular and rod shapes.Figure 7(a-d) shows the HR-TEM images of the C1, C2, C3, and C4 products, respectively.All the products consist of irregular and rod shapes.

pH
The influence of pH value on the % removal (% R) of toxic ions (Cd(II) and Hg(II)) by the C3 sample was performed in the pH range of 2.5-8.5 and the results are presented in Figure 8(a).
The results demonstrated that the % removal of toxic ions (Cd(II) and Hg(II)) increased with increasing pH from 2.5 to 6.5 where reached 87.5 and 75% at pH 6.5, respectively.Thereafter, the % removal of Cd(II) and Hg(II) ions remained nearly constant when the pH changed from 6.5 to 8.5.Also, the influence of pH value on the adsorption capacity of C3 sample towards toxic ions (Cd(II) and Hg(II)) was performed in the pH range of 2.5-8.5 and the results are presented in Figure 8(b).The results demonstrated that the uptake capacity of the C3 sample towards toxic ions (Cd(II) and Hg(II)) increased with raising pH value from 2.5 to 6.5 where reached 175.00 and 150.00 mg/g at pH 6.5, respectively.Hence, 6.5 is considered the optimum pH for the uptake of toxic ions (Cd(II) and Hg(II)) using the C3 sample.The point of zero charge of the C3 sample is 5.89.Therefore, when the pH value is 6.5, the surface of the C3 sample is surrounded by negative ions (OH − ) that can attract positive ions (Heavy metals), and therefore the separation efficiency increases.

Time
The influence of time on the % removal (% R) of toxic ions (Cd(II) and Hg(II)) by the C3 sample was performed in the time range of 10-100 min and the results are presented in Figure 9(a).The results demonstrated that the % removal of toxic ions (Cd(II) and Hg(II)) increased with increasing time from 10 to 70 min where reached 87.5 and 73.5% at 70 min, respectively.Thereafter, the % uptake of toxic ions (Cd(II) and Hg(II)) remained nearly constant when the time changed from 70 to 100 min.Also, the influence of time on the adsorption capacity of C3 sample towards toxic ions (Cd(II) and Hg(II)) was performed in the time range of 10-100 min and the results are presented in Figure 9(b).The results demonstrated that the uptake capacity of the C3 sample towards Cd(II) and Hg(II) ions increased with increasing time from 10 to 70 min where reached 175 and 147 mg/g at 70 min, respectively.Hence, 70 min is considered the optimum time for the removal of toxic ions (Cd(II) and Hg(II)) using the C3 sample.
The kinetic experimental data were accomplished and investigated to calculate the rate constant of adsorption using pseudo-first-order (Equation ( 3)) and pseudo-secondorder (Equation ( 4)) equations [20][21][22]24,[44][45][46].Q e (mg/g) is the C3 equilibrium uptake capacity towards toxic ions (Hg(II) or Cd(II)).Q t (mg/g) is the C3 uptake capacity at the contact time t.K 1 (1/min) is the rate constant of the pseudo-first-order.K 2 (g/mg.min) is the rate constant of the pseudo-second-order.The plots of the pseudo-first-order (1 st order) and pseudo-second-order (2 nd order) results were clarified in Figure 10(a-b), respectively.The data show that the uptake process of toxic ions (Hg(II) or Cd(II)) was matched well with the pseudo-second-order kinetic model as shown in Table S2.

Temperature
The influence of temperature values on the % removal (% R) of toxic ions (Cd(II) and Hg(II)) by the C3 sample was performed in the temperature range of 298-328 kelvin and the results are presented in Figure 11(a).The results demonstrated that the % removal of toxic ions (Cd(II) and Hg(II)) decreased with increasing temperature from 298 to 328 kelvin where reached 59.5 and 30% at 328 min, respectively.Also, the influence of temperature on the uptake capacity of the C3 sample towards toxic ions (Cd(II) and Hg(II)) was accomplished in the temperature range of 298-328 kelvin and the data are presented in Figure 11(b).The results demonstrated that the uptake capacity of the C3 sample towards toxic ions (Cd(II) and Hg(II)) decreased with increasing temperature from 298 to 328 kelvin were reached 119 and 60 mg/g at 328 kelvin, respectively.Hence, 298 kelvin is considered the optimum temperature for the uptake of toxic ions (Cd(II) and Hg(II)) using the C3 sample.The thermodynamic parameters of entropy change (ΔS o ), enthalpy change (ΔH o ), and standard free energy (ΔG o ) were determined using Equations ( 5) and ( 6) to investigate the nature of Cd(II) or Hg(II) uptake on the C3 sample [20][21][22]24,[47][48][49][50][51].
Figure 12 displays the plots of ln K d versus 1/T.Besides, the values of thermodynamic parameters were listed in Table S3.The negative values of standard free energy (ΔG o ) confirm the spontaneous nature of the uptake of toxic ions (Hg(II) or Cd(II)).Also, the negative values of enthalpy change (ΔH o ) confirm the endothermic and chemical nature of the uptake of toxic ions (Hg(II) or Cd(II)).Besides, the positive values of entropy change (ΔS o ) confirm the strong affinity of the C3 sample towards toxic ions (Hg(II) or Cd(II)).

Influence of concentration
The influence of concentration on the % removal (% R) of toxic ions (Cd(II) and Hg(II)) by the C3 sample was performed in the concentration range of 200-350 mg/L and the results are presented in Figure 13(a).The data showed that the % removal (% R) of toxic ions (Cd(II) and Hg(II)) decreased with increasing concentration from 200 to 350 mg/L where      reached 55.71 and 48.57% at 350 mg/L, respectively.Also, the influence of concentration on the uptake capacity of the C3 sample towards toxic ions (Cd(II) and Hg(II)) was performed in the concentration range of 200-350 mg/L and the data are presented in Figure 13(b).The results demonstrated that the uptake capacity of the C3 sample towards toxic ions (Cd(II) and Hg(II)) increased with raising concentration from 200 to 350 mg/L where reached 195 and 170 mg/g at 350 mg/L, respectively.The equilibrium experimental data were accomplished and investigated to explain the mechanism by which the adsorbate is adsorbed by the adsorbent using Langmuir (Equation ( 8)) and Freundlich (Equation ( 9)) equations [20][21][22]24,[52][53][54][55].
The plots of the Langmuir and Freundlich results were clarified in Figure 14(a-b), respectively.The data show that the removal process of toxic ions (Hg(II) or Cd(II)) was matched well with the Langmuir isotherm as clarified in Table S4.The uptake capacity of the C3 sample towards Hg(II) and Cd(II) ions are 177.94 and 199.20 mg/g, respectively.The adsorption performance of the C3 product towards toxic ions (Hg(II) and Cd(II)) was compared with that of further adsorbents in the literature as displayed in Table S5.Noticeably, the C3 product outperformed the majority of the adsorbents because it has a high uptake capacity.

Desorption and reusability
An suitable desorbing medium is necessary for the recovery of the toxic ions (Hg(II) or Cd(II)) from the C3 product.About 0.10 g of the C3 product was stirred with 100 mL of 200 mg/L of toxic ions (Hg(II) or Cd(II)) solution (at pH 6.5 and 298 Kelvin) for 70 min.After that, the C3 solid phase was carefully filtered and washed various times with hot distilled water to exclude the non-adsorbed toxic ions (Cd(II) or Hg(II)).The loaded C3 product was stirred for 20 min with 100 mL of 0.50 M of some desorbing media (Hydrochloric acid, nitric acid, thiourea, and EDTA disodium salt) for three adsorption/desorption cycles.The desorption fraction (% D) was evaluated using Equation (11) [22].where, V H (L) is the consumed volume of desorbing medium.C H (mg/L) is the concentration of Hg(II) or Cd(II) ions that exist in the desorbing medium.Tables S6 and 7 demonstrated that the use of 0.50 M EDTA disodium salt can recover above 99.00% of the adsorbed Hg(II) or Cd(II) ions.

Conclusions
Sodium calcium silicate and calcium silicate nanostructures were facilely synthesised as a novel mixture in the absence and presence of the hydrothermal method.XRD peaks at 2Ɵ (hkl) equals 25 47.56 (600), and 48.27 (4 1 10) confirm the presence of calcium silicate.HR-TEM confirmed that all the products consist of irregular and rod shapes.The product, which was synthesised using the hydrothermal method at 160 oC for 8 h, has the lowest average crystallite size (34.89nm) and was utilised as an adsorbent for the efficient removal of Cd(II) and Hg(II) ions from aqueous media.The maximum adsorption capacity of the adsorbent towards Hg(II) and Cd(II) ions is 177.94 and 199.20 mg/g, respectively.The highest % removal of the studied ions was successfully achieved at temperature, time, and pH values equal 298, 70 min, and 6.5 Kelvin, respectively.Besides, the removal process of the studied ions is exothermic, chemical, and was matched well with the Langmuir equilibrium isotherm and pseudo-second-order kinetic model.

Figure 8 .
Figure 8.The impact of pH on the % uptake of studied ions (a) and uptake capacity of the C3 product (b).

Figure 9 .
Figure 9.The impact of time on the % uptake of studied ions (a) and uptake capacity of the C3 product (b).

Figure 10 .
Figure 10.The plots of the pseudo-first-order (a) and pseudo-second-order (b) results.

Figure 11 .
Figure 11.The impact of temperature on the % uptake of studied ions (a) and uptake capacity of the C3 product (b).

Figure 12 .
Figure 12.The plots of ln K d versus 1/T.

Figure 13 .
Figure 13.The impact of concentration on the % uptake of studied ions (a) and uptake capacity of the C3 product (B).

Figure 14 .
Figure 14.The plots of the Langmuir (a) and Freundlich (b) results.