Synthesis of nano-silica oxide for heavy metals decontamination from aqueous solutions

doi:10.21203/rs.3.rs-2750499/v1

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Synthesis of nano-silica oxide for heavy metals decontamination from aqueous solutions

https://doi.org/10.21203/rs.3.rs-2750499/v1

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In the current study, nano-silica oxide (nano-SiO₂) was fabricated via the sol-gel technique. Then, the prepared nano-SiO₂ was characterized using X-ray diffractometer (XRD), transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), and Brunauer-Emmett-Teller. Furthermore, the fabricated nano-SiO₂was applied for the adsorption of lead Pb (II) and chromium Cr (VI) from aqueous solutions. Additionally, the influence of different operating factors such as contact time, nano-SiO₂dose, initial pH, initial metal ions concentration, and the temperature was investigated. The nano-SiO₂dosages of 0.5 and 1 g L^-1 were selected as the optimal adsorbent dose for the removal of Pb (II) and Cr (VI), respectively. These doses achieved a removal efficiency of 83.29% and 78.65% for Pb (II) and Cr (VI), after 60 and 90 min, respectively, (at initial pH values of 7, using 10 mg L^-1 initial metals concentration, and at room temperature). Lastly, kinetic and equilibrium studies were competently fitted using the pseudo-second-order and Freundlich models, respectively. Adsorption thermodynamic studies designate the spontaneous and thermodynamic of the adsorption processes nature. These results reveal the efficiency of the fabricated nano-SiO₂as an adsorbent for heavy metals removal from aqueous solutions.

Silica-based nanoparticles

Adsorption

Heavy metals removal

Operating parameters

Optimization

Kinetics

Isotherm

Thermodynamics

With the expansion of the economy and industrial manufacture, various heavy metals are surely released into water through the effluent runoffs of several industries including fertilizers, steel, dyes, batteries, mining, textiles, leather, alloying, electroplating, tannery, paper, food, cosmetics, etc., causing environmental water pollution [1]. This leads to the accumulation of heavy metal ions which are non-biodegradable and can enter the human bodies via the feed chain, and then immediately threaten human health if they surpass the standard range; due to their high toxicity, carcinogenicity, and bio-recalcitrance [2], which makes it mandatory to efficiently remove toxic heavy metals from water.

Various conventional techniques are used to eliminate heavy metal ions from water including membrane separation, solvent extraction, chemical precipitation, adsorption, ion exchange, reverse osmosis, etc. [3]. Although the aforementioned approaches are effective and meet the discharge limits, most of them still produce secondary waste. Adsorption was compared to various water treatment technologies, and it was found to be more cost-effective than evaporation, aerobic digestion, anaerobic digestion, ion exchange, electrodialysis, reverse osmosis, precipitation, and oxidation [4].

Consequently, adsorption is regarded as a viable alternative for the removal of hazardous heavy metals from water; due to its accessibility, flexibility, and excellent proficiency [5]. A great perspective for the removal of heavy metals from contaminated water is revealed from the adsorption on the surface of solid adsorbents [6]. Due to its simple design, inexpensive initial investment, and minimal area requirement, it is more advantageous than other methods [7]. As a result of these qualities, the adsorption process is gaining great interest from researchers in the treatment of water contaminated with heavy metals [8].

To name a few, activated carbon, metal oxides, clay, etc. are some of the main conventional adsorbents applied for the removal of heavy metals [9]. These traditional adsorbents have limitations in terms of their reusability, recyclability, and adsorption capabilities [10]. To overcome these drawbacks, novel adsorbent materials in nano sizes are fabricated and utilized for water decontamination [11].

Presently, nanoparticles (NPs) have drawn significant attention as adsorbents for water decontamination; due to their huge specific surface area, low flocculent generation, and availability of many active groups for the binding of heavy metal ions [12]. Additionally, they can be recycled and reused repeatedly, which makes them both economical and highly desirable [13].

A variety of NP types; including chitosan, carbonaceous, metallic, bimetallic, metal oxide, polymer-based, ferrite, magnetic, and zeolite, have been investigated in previous years for heavy metals removal from water [14]. These NPs eliminate the heavy metals by adsorbing them on their surfaces [15]. Among the above-mentioned NPs that are demonstrated in scientific investigations are silica-based NPs. It is a type of metal oxide NPs that owns a significant deal of promise for adsorbing heavy metal ions; due to their distinctive qualities, including a large surface area, controllable surface characteristics, and clearly defined pore size. Moreover, nano-silica-based materials provide an eco-friendly and non-toxic adsorbent [16]. Different types of silica-based NPs have been investigated by many researchers for the removal of heavy metals removal from water and resulted in boosted adsorption capacity and selectivity. Li et al. [17] examined the nitrilotriacetic acid-silica gel (NTA-silica gel) for the effective removal of Pb (II), Cd (II), and Cu (II), and Kotsyuda et al. [18] examined amino-functionalized silica NPs for Cu (II) ions removal, and they found that silica-based NPs were very efficient for heavy metals removal with high adsorption capacity. Additionally, Najafi et al. [19] also shed light on the elimination of Pb (II), Cd (II), and Ni (II) using four different silica-based NPs; amino-functionalized nano-silica hollow spheres; nano-silica spheres; non-functionalized; and amino-functionalized nano-silica gel, and found that functionalized nano-silica was more effective than others examined silica-based NPs with maximum adsorption capacities.

The main operational conditions such as initial pH value, adsorbent dose, contact time, temperature, and initial pollutant concentration substantially affect the efficiency of the adsorption process; as reported by lots of scientific investigations [20]–[23]. Therefore, optimal conditions operational are worthy of attention. However, from the literature review, we can conclude that there are no specific favorable operating conditions for heavy metals removal using NPs; as it differs by using different NPs and pollutants. Consequently, to study the influence of each parameter; a wide range must be investigated within the same operating circumstances of the adsorption experiment, and according to the resulting removal observations, the optimum values can be highlighted.

To the best of our knowledge, the synthesis of silica-oxide NPs has been made in several studies using different techniques. However, its application in the removal of heavy metals from aqueous solution is lacking in the literature works. Moreover, the optimal operating conditions for the heavy metal’s adsorption onto the fabricated silica-oxide NPs are missing.

In the present study, therefore, silica oxide nanoparticles (nano-SiO₂) were synthesized and characterized. Then, the synthesized nano-SiO₂ was employed as an adsorbent material for heavy metals removal [i.e., Pb (II) and Cr (VI)] from aqueous solutions, using batch adsorption methods. Furthermore, the removal performance was assessed and optimized through testing different operational conditions such as contact time (0–180 min), initial pH (1–11), synthesized nano-SiO₂ dosage (0.1–8 g L^− 1), initial heavy metals concentration (5–100 mg L^− 1), and solution temperature (room temperature–85°C). Additionally, thermodynamics, kinetics, and isotherms of the adsorption process were investigated.

2.1 Materials

Tetraethylorthosilicate (99.9% Alfa Aesar, UK), acetic acid (Sigma-Aldrich), PVP (Sigma-Aldrich), and absolute ethanol (99.8%, Acros Organics) have been used for the preparation of the nano-SiO₂. In this study, all of the chemicals were utilized without any additional purification.

2.2 Fabrication of nano SiO₂

The nano-SiO₂ was synthesized via the sol-gel technique. 2.5 mL acetic acid was added to 5 mL Tetraethylorthosilicate (TEOS) solution under continuous mixing for 10 min. Then, 5 mL of 10 wt % solution of PVP in ethanol was poured dropwise into the above-mentioned mixture and the solution was stirred for 1 h to make a sol. To evaporate the water and organic components, the produced gel was dried at 90°C for three hours. Finally, the resulting gel was then ground into nanoparticles after being calcined at 600°C for 3 h.

2.3 Characterization of the synthesized nano-SiO₂

X-ray spectra of the prepared nano-SiO₂ were specified using Shimadzu XRD-6100 with Cu–K_α radiation at λ = 1.54 Å to examine the crystallinity of the synthesized material. The functional groups of the fabricated nano-SiO₂ were investigated via Fourier transform infrared spectroscopy (Shimadzu, FTIR-8400S). The morphological structure and chemical composition of the synthesized nano-SiO₂ were detected by transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) (JEOL JSM-6010LV). Furthermore, the thermal stability of the prepared nano-SiO₂ was characterized using TGA, (SII 6300, Exstar). The heating rate was 10°C/min up to 1000°C under N₂ and the weight loss of the material was recorded. The Brunauer-Emmett-Teller (Beckman Coulter SA3100, Brea, CA, USA) was used for the measurement of the surface area and the pore size of the fabricated nano-SiO₂.

2.4 Experimental design and procedures

Two parallel sets of batch experiments were carried out to evaluate the adsorption performance of synthesized nano-SiO₂ toward the removal of lead and chromium. Both series consisted of five sequential experiments to investigate the influence of several operational parameters on the adsorption efficiency including contact time (0–180 min), initial pH (1–11), nano-SiO₂ dose (0.1–8 g L^− 1), initial heavy metals concentration (5–100 mg L^− 1), and temperature (room temperature–85°C), respectively. These values have been selected according to the different standard values used in most of the former studies [24], [25]. In the beginning, single-metal stock solutions (individual metal) of Pb (II), and Cr (VI) with the chosen concentrations were prepared by dissolving lead nitrate; Pb(NO₃)₂, and potassium dichromate (K₂Cr₂O₇) in distilled water; then, stock solutions were kept in volumetric flasks in dark at 4°C, and the experimental work was designed as follows.

The first experiment was designed to assess the effect of contact time, where a certain dose of 1 g L^− 1 of synthesized nano-SiO₂ was mixed with the artificial heavy metals contaminated solutions in a one-liter glass beaker of a constant concentration of 10 mg L^− 1 of each heavy metal, at a constant pH value of 7, and room temperature, for 180 min. At specific intervals, which ranged from 5 to 180 min, samples of the suspension solutions were extracted with a 5 mL syringe and then filtered and analyzed to determine the optimal contact time for each metal.

Secondly, to examine the influence of the initial pH value, using the optimal contact time of each heavy metal, where the same operating conditions of the previous adsorption experiment were kept constant except for the initial pH value. The pH values of the metallic solutions were modified using 0.1 M of NaOH or HCl and the pH range of (1–11) was investigated. After that, the metal ions concentrations and removal efficiencies were assessed. According to the resulting removal observations, the optimum pH values were highlighted. Thirdly, the range of adsorbent dose of (0.1–8 g L^− 1) was investigated, at the optimum operational conditions, and ambient temperature.

Next, the impact of initial heavy metal ions concentration (5–100 mg L^− 1) on the removal efficiency was examined at the resulted optimal contact time, initial pH value, nano-SiO₂ dose, and ambient temperature. Lastly, to explore the effect of reaction temperature on the removal efficiency, using the optimal contact time, dose, initial pH value, and ions concentration, the temperature range of (average room temperature–85°C) was tested.

A typical lab-scale jar tester was used for all of the experiments. It mainly involves six graded cylinders with a 1 L capacity, six impellers placed on stainless steel shafts, and a variable-speed motor. Two 90° turbine blades with a 5 cm diameter and 2 cm height make up the impeller. There was a 0 to 200 rpm range for the impeller rotation speed. The optimum rotational speed was kept constant throughout all experiments at 150 rpm.

The desired initial pH value was adjusted with 0.1 M of NaOH or HCl (when needed); however, the medium pH was not controlled during the experiment. The validity of the resulting data was proved by doing all the adsorption experiments in duplicates, and the mean values were applied in data analysis. Following the completion of the water treatment experiments, the final pollutant concentration was determined and the pollutant removal percentage by synthesized nano-SiO₂ was calculated using Eq. (1):

Removal% = [(C_o − C_e)/C_o] * 100 (1)

where C_o is the initial pollutant concentration (mg L^− 1), and C_e is the final concentration (mg L^− 1).

2.5 Equilibrium, kinetics, and thermodynamics adsorption behavior of nano-SiO₂

Thermodynamic parameters of the adsorption process towards the investigated water contaminants [i.e., Pb (II) and Cr (VI)] were assessed in order to examine the nature of adsorption processes onto the fabricated nano-SiO₂. Furthermore, using the Langmiur, Frendlich, and Temkin isothermal equations, the equilibrium behavior of the adsorption processes onto nano-SiO₂ was evaluated. Pseudo-first-order, pseudo-second-order, Elovich, and intraparticle kinetic models were used to assess adsorption kinetics.

2.5.1 Thermodynamics modeling for the adsorption process

The variations in enthalpy (ΔH°), standard free energy (ΔG°), and entropy (ΔS°) are the main thermodynamic characteristics that should be considered to explore the nature of any adsorption process. Using the Van't Hoff equation, the values of the standard enthalpy and entropy may be determined as follows [26]:

$$\text{ln}Kc= \frac{\varDelta S^\circ }{R}-\frac{\varDelta H^\circ }{RT}$$

where T is the solution's temperature in Kelvin, R is the universal gas constant; equal to 8.314 J mol^− 1 K^− 1, K_c is given by F_e/(1 − F_e), and F_e=(C_o− C_e)/C_o; is the portion adsorbed at equilibrium. The intercept and the slope of the Van’t Hoff plot are used to drive the values of ΔS° and ΔH°, respectively. Moreover, the following equation allows for the determination of the activation energy (E_a) [27]:

E _a = ΔH° + RT (3)

2.5.2 Equilibrium isotherm analysis for adsorption of Pb (II) and Cr (VI)

To study the adsorption behavior of the synthesized nano-SiO₂ for Pb (II) and Cr (VI) decontamination, the Langmuir, Freundlich, and Temkin models were used. The C_e/q_e versus C_e plot of the Langmuir linearized can be made as follows:

$\frac{C\text{e}}{q\text{e}}$ = $\frac{1}{qmK}$ + $\frac{C\text{e}}{qm}$ (4)

where C_e is the equilibrium concentration of the adsorbate (mg L^− 1), q_e is the quantity of Pb (II) and Cr (VI) adsorbed at equilibrium (mg g^− 1), K_L and q_m are the Langmuir constants referred to as the adsorption energy (L mg^− 1) and maximum monolayer adsorption capacity (mg g^− 1), respectively [28].

Additionally, log q_e was plotted against log C_e to study the equilibrium data using Freundlich linear (Eq. 5); where n_F and K_F are Freundlich constants correlated to the intensity and capacity of adsorption, respectively [29]. The Temkin isotherm model, which is expressed as in (Eq. 6), was used to examine the adsorption data of Pb (II) and Cr (VI) onto the synthesized nano-SiO₂; where A is the Temkin isotherm constant (L g^− 1); and B = RT/b is constant correlates to adsorption heat (J mol^− 1) [30]:

Log q_e = log K_F + $\frac{1}{nF}$log C_e (5)

q _e = BlnA + BlnC_e (6)

2.5.3 Kinetic models of Pb (II) and Cr (VI) adsorption

Ln (q_e-q_t) = ln q_e – k₁t (7)

where the quantities of adsorbed Pb (II) and Cr (VI) ions (mg g^− 1) at time t (min) and equilibrium are denoted by [q_e and q_t], respectively, and the rate constant for the first-order reaction is k₁ (min). Moreover, the pseudo-second-order kinetic model was used to examine the decontamination kinetic data and is depicted as follows [32]:

t/q _t = (1/k₂q_e²) + t/q_e (8)

where k₂ (g mg^− 1 min^− 1) is the second-order rate’s constant. Additionally, the Elovich equation was studied for the adsorption of many water contaminants by [29]:

q _t = α + β ln t (9)

where α is the initial adsorption rate (mg g^− 1 min^− 1) and β is related to the activation energy of adsorption (g mg^− 1) and the level of surface coverage. The slope and intercept of the linear plot of q_t against ln t can be used to drive the values of α and β, respectively. Finally, Weber and Morris's equation was used to investigate the potential effect of intraparticle diffusion on the adsorption process for the studied contaminants [33]:

q _t = k _i t ^1/2 + C (10)

where k_i is the intraparticle diffusion rate constant, and C values provide an estimation of the boundary layer’s thickness. If intraparticle diffusion does place, q_t against t^1/2 will be linear, and the rate-limiting mechanism will only be due to intraparticle diffusion if the plot passes through the origin. If not, a different mechanism in addition to intraparticle diffusion is at work [34].

2.6 Instruments and Analytical Methods

A series of flocculators with 6 independent positions (Jar Tester, VELP Scientifica company, Model FC6S, USA) was used to conduct the experiments. The pH value and temperature were measured by a pH-ORP-TEMP Bench Meter (Adwa, Model AD1030, Hungary). An atomic absorption spectrophotometer (HACH, Model DR6000) was used for the analysis of heavy metals present in the solution.

3.1 Physico‑chemical characteristics of the fabricated nano-SiO₂

XRD pattern of the prepared nano-SiO₂ is provided in Fig. 1. A typical broadband cantered at 2θ = 23.08° is detected, which can be allocated to the nano-SiO₂ [35]. Moreover, there are no other diffraction peaks can be noted. Figure 1 confirms that the X-ray diffractogram illustrated the amorphous nature of the prepared nano-SiO₂ [36].

Figure 2 shows the FTIR spectrum of the fabricated nano-SiO₂. The band at 795 cm^− 1 is attributed to the bending vibration of Si-O [30] where the oxygen moves at the right angle to the Si-Si lines in the Si-O-Si plane [37]. The absorption observed at 951 cm^− 1 matches the stretching vibration of Si-OH. The above-mentioned FTIR results demonstrated outstanding nano-SiO₂ fabrication [38].

In Fig. 3, the TEM and EDX images of the synthesized nano-SiO₂ are shown. The TEM images reaffirmed a chain-like agglomeration of nanoparticles. The particle size swung from 50 to 75 nm, and the NPs had a semi-spherical shape with a low tendency to aggregate. On the contrary, the elemental mapping (Fig. 3(B, C, and D)) illustrated the presence of Si and O in the synthesized nanomaterial. Additionally, the EDX pattern of nano-SiO₂ in Fig. 3(E) demonstrated the existence of the elements.

Thermogravimetric analysis (TGA) of the prepared nano-SiO₂ is shown in

Figure 4, where two main degradation stages were detected. The first degradation step of 13.16% at 113°C may be due to the loss of water molecules and atmospheric gases that are combined with the fabricated nano-SiO₂ [39]. The second degradation stage with a total weight loss of 10.36% swung from 113 to 988°C which was assigned to the cleavage of the backbone of the synthesized nano-SiO₂ [40]. According to these data, the prepared nano-SiO₂ elucidated excellent thermal characteristics compared with other nanomaterials [26], [41], [42].

Figure 5 shows the fabricated nano-SiO₂’s adsorption-desorption curve. Both the total pore diameter and the specific surface area of the produced nano-SiO₂ were determined to be 0.104 cm³ g^− 1 and 78.38 m² g^− 1, respectively. From the abovementioned outcomes, the synthesized nano-SiO₂ achieved a significantly high surface area, which confirms its potential applicability to adsorb the pollutants from wastewater [41].

3.2 Application of synthesized nano-SiO₂ for Pb (II) and Cr (VI) adsorption

Using the batch method at room temperature; the feasibility of Pb (II) and Cr (VI) adsorption onto the fabricated nano-SiO₂ from aqueous solutions was examined [40]. In addition, the impact of contact time, initial pH value, nano-SiO₂ dose, initial pollutant concentration, and temperature were investigated according to the experimental design that was previously reported [24], [25]; to set the optimal operating parameters controlling the adsorption process.

3.2.1 Effect of contact time

The influence of contact time on the removal efficiency of Pb (II) and Cr (VI) using 1 g L^− 1 from the prepared nano-SiO₂ was investigated over 180 min, and the change in the concentration of the metals at specific time intervals was monitored, as presented in Fig. 6. The obtained results indicated that the metal ions removal rate was rapid at the initial period until the equilibrium state was reached. Then, the adsorption rate slowdown, and a slight increase was recorded irrespective of time. The large accessible surface area and functional groups of the fabricated nano-SiO₂ material may be responsible for this high adsorption rate in the first phase, which causes a faster collision velocity between the metal ions and the adsorption sites [43]. On the other hand, in the second phase of adsorption; after equilibrium, the active sites were saturated with metal ions, and the adsorbent surface became exhausted, limiting further uptake and constituting a non-significant adsorption rate [25]. According to these results, at neutral pH, the optimum equilibrium time for the adsorption of Pb and Cr onto the prepared nano-SiO₂ was considered 60 and 90 min, with the highest removal percentage of 84.15, and 78.65%, respectively; additional time could not cause any significant removal. Similar trends were previously registered in the literature (i.e., 60 and 90 min for Pb and Cr, respectively) [44]–[47].

Moreover, the prepared nano-SiO₂ represents an efficient and time-saving adsorbent for heavy metals compared with other previously prepared nanoparticles such as carbon nanotubes (CNTs) [48], copper ferrite nanoparticles (CuFe₂O₄) [49], and CuO nanoparticles [50] that recorded 360, 220, and 100 min optimal contact time, respectively for the removal of 10 mg L^− 1 initial Pb concentration, and compared with Cr removal within 240, and 180 min using alumina nanoparticles immobilized zeolite (Al₂O₃ NPs in zeolite) [51] and, tetraethylenepentamine-functionalized nano-magnetic polymer (TEPA-NMPs) [52], respectively.

Additionally, higher removal rates of Pb (II) were observed along the experiment period, as compared to those in Cr (VI) under the same operational conditions as shown in Fig. 6, as suggested by the Hard Acid Soft Base (HSAB) hypothesis, which describes metal ion interactions based on their underlying chemistry, both the type of the metals and their species in the solution has a role in adsorbent-metal binding. According to the HSAB theory, the hardness of the two metals examined in this study can be classified as Cr (VI) > Pb (II) [53], [54]. Overall, these results demonstrated the crucial effect of contact time on the Pb (II) and Cr (VI) adsorption process onto the fabricated nano-SiO₂.

3.2.2 Effect of pH

In the adsorption process, solution pH is crucial; it influences both the contaminants' ionization level and the adsorbent's surface charge [55]. Therefore, the effect of pH on the adsorption process was investigated at a range from 1 to 11, and it was noted that the neutral circumstances were favorable for the Pb (II) and Cr (VI) adsorption onto fabricated nano-SiO₂. The results indicated that increasing the initial pH from 1 to 7 significantly increased metals removal efficiency (Fig. 7), then, an inconsiderable removal efficiency was recorded with the progressed increase in pH from 7 to 11. Subsequently, the initial pH value of 7 was picked as the optimum pH for the adsorption of Pb (II) and Cr (VI) onto synthesized nano-SiO₂. This is consistent with the optimum pH for the adsorption of Pb (II) and Cr (VI) using nanoparticles reported by similar studies [51] and [56], which tested different pH values, and found that the optimum pH value was 7.

The explanation for the adsorption mechanism of Pb (II) and Cr (VI) onto nano-SiO₂ at different pHs can be due to both the point of zero charge (pzc) for SiO₂; as well as the nature of Pb and Cr as positively charged ions. The pzc for SiO₂ was reported to be around 3 [57]. Subsequently, the surface of SiO₂ is positively charged at any pH value below the pzc, and negatively charged at any pH value over the pzc. The surface of the synthesized nano-SiO₂ is positively charged at low pH values (pH < 3), so there is a repulsion force between the positively charged Pb (II) and Cr (VI) and the positively charged surface of synthesized nano-SiO₂. Otherwise, when the pH of the solution increased (pH > 3), the surface of the synthesized nano-SiO₂ becomes negatively charged, and as a result, the adsorption of positively charged Pb (II) and Cr (VI) is enhanced due to the electrostatic attraction.

3.2.3 Effect of nano-SiO₂ dosage on the adsorption process

The adsorbent dosage is one of the adsorption process' most important parameters since it has a strong effect on the adsorbent capacity at a constant initial pollutant concentration [58]. For this reason, the effect of the synthesized nano-SiO₂ dosage on both Pb (II) and Cr (VI) removal was investigated, utilizing the range of (0.1–8 g L^− 1), as shown in Fig. 8. As seen in the figure, it can be realized that the removal efficiency was considerably enhanced by increasing nano-SiO₂ dosage from 0.1 to 0.5 g L^− 1 for Pb (II), and from 0.1 to 1 g L^− 1 for Cr (VI), respectively. After that point, a minor increase in removal percentages was noted with the continued increase to 8 g L^− 1. The resultant trend was informed by various related studies [29], [32], [59]–[61]. These findings may serve as evidence that when nano-SiO₂ concentrations increase, more active sites become available for the adsorption of heavy metal ions [41], [62]. On the other hand, the degradation in the ability of nano-SiO₂ to adsorb the tested pollutants at high dosages may be due to the unsaturated adsorption residual sites on its surface [59], [60]. Consequently, based on the observed results, the optimum dosages of the synthesized nano-SiO₂ were picked as 0.5 and 1 g L^− 1 for Pb (II), and Cr (VI), respectively, which are suggested as economically reasonable dosages for the adsorption processes. These values were confirmed by the optimum dose for the adsorption of Pb (II) and Cr (VI) reported by Rahbari et al. [63] and Hosseini et al. [64], who examined several dosages, and found that the optimum was 0.5 and 1 g L^− 1 for Pb (II) and Cr (VI), respectively.

3.2.4 Effect of initial Pb (II) and Cr (VI) concentration on the removal performance

Using the optimal contact time, initial solution pH, and nano-SiO₂ dose, for each metal, separately; the impact of the initial concentration of Pb (II) and Cr (VI) was examined within the range of 5 to 100 mg L^− 1. Figure 9 demonstrates that for both metals, the removal efficiency was inversely proportional to the initial concentration. The obtained results indicated that removal efficiency was shown to drop, going from (87.2–49.16%) and from (81.5–43.3%) as the initial concentration raised from 5 to 100 mg L^− 1 for Pb (II) and Cr (VI), respectively, which is totally consistent with the previously published research [30], [29],[32],[65]. This trend may be related to the inability of the fabricated nano-SiO₂ to provide the active sites required for the higher concentrations of Pb (II) and Cr (VI). The active sites on the nano-SiO₂ are sufficient for Pb (II) and Cr (VI) reduction and adsorption in the case of low quantities of these metals. High metal ion concentrations, on the other hand, prevent the proper interaction between the adsorbents and metal ions because the reactive sites are unable to absorb [66]. Our findings demonstrated the effectiveness of the synthesized nano-SiO₂ in the Pb (II) and Cr (VI) decontamination of aqueous solutions at different initial concentrations.

3.2.5 Effect of solution temperature on the heavy metals’ removal efficiency

Figure 10 elucidated the impact of temperature on Pb (II) and Cr (VI) removal efficiency using the fabricated nano-SiO₂. The adsorption of both Pb (II) and Cr (VI) was shown to increase as the solution temperatures raised from 30 to 85°C, demonstrating that high temperatures were favorable for the adsorption process. This behavior may be due to the development of other adsorption sites onto the nano-SiO₂ at higher temperatures, which enhances the diffusion rate of the metal ions into the pores of the fabricated nano-SiO₂ [28]. These results indicated that the Pb (II) and Cr (VI) adsorption onto the nano-SiO₂ is an endothermic process [29], [40].

3.3 Equilibrium, kinetics, and thermodynamics adsorption behavior of nano-SiO₂

3.3.1 Thermodynamics modeling for the adsorption process

According to Fig. 11(a and b), for various examined Pb (II) and Cr (VI) concentrations, the Van't Hoff plot of lnK_c versus 1000/T yields a straight line with a respectable value of correlation coefficient (R²), and by using the intercept and slope of the Van't Hoff plot, the values of ΔS° and ΔH° were estimated. The estimated thermodynamic parameters values of ΔH°, ΔG°, ΔS°, and E_a at various temperatures for Pb (II) and Cr (VI) are presented in Table 1. The spontaneous and thermodynamic nature of the adsorption process of Pb (II) and Cr (VI) onto the fabricated nano-SiO₂ is indicated by the negative values of free energy changes (ΔG°) [29]. On the other hand, the positive values of enthalpy (ΔH°) demonstrate that the adsorption processes are endothermic [62]. Yet, the increased disorder at the liquid/solid interface during the adsorption processes is illustrated by the positive values of entropy (ΔS°) [30].

Table 1

Thermodynamic parameters for Pb (II) and (VI) adsorption onto nano-SiO₂
Metal	Temp.	1000/T	K_c	lnK_c	ΔG⁰	E_a	ΔH⁰	ΔS⁰
Metal	(K)	1000/T	K_c	lnK_c	(kJ·mol^− 1)	(kJ·mol^− 1)	(kJ·mol^− 1)	(J·mol^− 1·K^− 1)
Pb (II)	303	3.30	9.97	2.30	-5.79	16.95	14.43	67.02
	313	3.19	12.39	2.52	-6.55	17.03
	328	3.05	17.46	2.86	-7.80	17.16
	343	2.92	19.16	2.95	-8.42	17.28
	358	2.79	24.67	3.21	-9.54	17.41
Cr (VI)	303	3.30	3.68	1.30	-3.29	18.15	15.63	62.25
	313	3.19	4.28	1.45	-3.78	18.23
	328	3.05	5.90	1.77	-4.84	18.36
	343	2.92	7.10	1.96	-5.59	18.48
	358	2.79	9.64	2.27	-6.74	18.61

3.3.2 Equilibrium isotherm analysis for adsorption of Pb (II) and Cr (VI)

To study the adsorption behavior of the synthesized nano-SiO₂ for Pb (II) and Cr (VI) decontamination, the Langmuir, Freundlich, and Temkin models were used. The Langmuir linearized plot of C_e/q_e against C_e (Eq. 4) gives a straight line with a high correlation coefficient (R² = 0.984 for Pb (II), and 0.99 for Cr (VI), respectively). The parameters of the examined equilibrium isotherm models are reported in Table 2, and the constants for the isotherm models were calculated from the slope and intercept of the linear plots are shown in Figs. 12 and 13. By comparing the three models' linearization fitting, it was revealed that Langmuir and Temkin's R² values were lower, indicating that the Freundlich model is the best match for describing the decontamination processes of Pb (II) and Cr (VI) onto nano-SiO₂. Moreover, the values of the separation factor (R_L) are 0.523 and 0.575 for Pb (II) and Cr (VI), respectively, which fall in the range from 0 to 1. This data supports that the Langmuir model is an accurate description of the adsorption process. Besides, the Freundlich adsorption intensity (n_F) values were more than the unity, and recorded 1.771 and 1.767 for the adsorption of Pb (II) and Cr (VI), respectively, showing that the adsorption processes onto the synthesized nano-SiO₂ are favorable [67]. The Freundlich isotherm, which provides the best fit to the equilibrium data, predicts that metal ions would physically adsorb onto the synthesized nano-SiO₂ by multilayer adsorption on heterogeneous surfaces [68].

Table 2

Isotherm parameters of Langmuir, Freundlich, and Temkin models for Pb (II) and Cr (VI) removal using the fabricated nano-SiO₂
Metal	Langmuir parameters			Freundlich parameters			Temkin parameters
Metal	q_m (mg g^− 1)	K_L (L mg^− 1)	R²	K_F (mg g^− 1)	n_F	R²	A (L g^− 1)	B (J mol^− 1)	R²
Pb (II)	117.74	0.091	0.992	12.22	1.771	0.995	1.57	21.25	0.978
Cr (VI)	51.42	0.074	0.993	4.8	1.767	0.995	1.2	9.46	0.984

3.3.3 Kinetic models of Pb (II) and Cr (VI) adsorption

The pseudo-first order, pseudo-second order, Elovich, and intraparticle diffusion kinetic models were used to analyze the mechanism and kinetics for the adsorption process of Pb (II) and Cr (VI) onto the fabricated nano-SiO₂ from aqueous solution. The four examined kinetic models for the adsorption of Pb (II) and Cr (VI) ions onto the synthesized nano-SiO₂ from aqueous solutions were compared, and their correlation coefficient values for linear plotting were listed in Table 3. Yet, there was a considerable difference between experimental q_e and the attained q_e from the pseudo-first-order model, despite the high R² for pseudo-first-order. As a result, the pseudo-first-order model proved ineffective in explaining how Pb (II) and Cr (VI) were adsorbed on the nano-SiO₂ surface. Concerning the pseudo-second-order model, the model constants were estimated from the slope and the intercept of the linear relation between t/q_t and t. The significance of the pseudo-second-order model was confirmed by the high R² (0.999) and the lower difference between the experimental and estimated q_e from the model. These findings support the hypothesis that the Pb (II) and Cr (VI) adsorption processes were mostly governed by the pseudo-second kinetic model [69].

Table 3

Pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion kinetic parameters for Pb (II) and Cr (VI) removal onto nano-SiO₂
Kinetic model	Parameter	Value
Kinetic model	Parameter	Pb (II)	Cr (VI)
Pseudo-first order	q_exp. (mg g^− 1)	8.42	7.87
	q_theor. (mg g^− 1)	7.29	5.54
	K₁ (min^− 1)	0.07
	R²	0.997	0.996
Pseudo-second order	q_exp. (mg g^− 1)	8.42	7.87
	q_theor. (mg g^− 1)	9.1	8.9
	K₂ (g mg^− 1 min^− 1)	0.015	0.008
	R²	0.999	0.999
Elovich kinetic model	α (mg g^− 1 min^− 1)	1.76	0.31
	β (g mg^− 1)	1.45	1.6
	R²	0.957	0.994
Intraparticle diffusion kinetic model	C (mg g^− 1 min^− 1)	4.12	2.70
	k (g mg^− 1)	0.41	0.47
	R²	0.75	0.902

The purpose of this study was to examine the feasibility of removing Pb (II) and Cr (VI) from aqueous solutions using silica-oxide nanoparticles (nano-SiO₂). Consequently, firstly, nano-SiO₂ was successfully prepared, characterized, and used for Pb (II) and Cr (VI) removal from aqueous solutions using batch adsorption experiments. Secondly, the main operational conditions were investigated, and the findings showed that increasing contact time, solution pH, adsorbent dosage, and temperature improved the removal efficiency of heavy metals. However, the increase in initial metal concentration hurt the treatment process. The optimal nano-SiO₂ dose for the removal of Pb (II) and Cr (VI) was determined to be 0.5 and 1 g L^− 1, respectively. These dosages resulted in clearance efficiencies for Pb (II) and Cr (VI) of 83.29% and 78.65%, after 60 and 90 min, at initial pH of 7, using 10 mg L^− 1 initial metals concentration, and at room temperature. The equilibrium isotherm model’s applicability of Pb (II) and Cr (VI) adsorption process obeys the order of Freundlich > Langmuir > Temkin, assumes multilayer adsorption on heterogeneous surfaces demonstrating the anticipated physical metal ion adsorption onto the fabricated nano-SiO₂. The kinetic investigation of the Pb (II) and Cr (VI) adsorption process onto nano-SiO₂ indicated that the adsorption kinetics may be represented by the pseudo-second-order model. The synthesized nano-SiO₂ was identified as an effective adsorbent for the removal of Pb (II) and Cr (VI) from aqueous solutions.

Availability of data and material

All data generated or analyzed during this study are included in this manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

This manuscript was partially funded by Science, Technology & Innovation Funding Authority (STDF) under the grant number 46138.

Authors' contributions

All authors contributed and were involved in all stages, including the conceptualization of the idea, designing experiments, conducting measurements, and explanation of data. Synthesis and characterization of nano-silica oxide were performed by Eslam Salama. The experimental work was conducted by Soheair Naeemand Naira Meky. Naira Meky and Eslam Salama wrote the original draft text, prepared figures, analyzed and discussed the data, and gave explanations. All authors reviewed, edited, and approved the final manuscript.

Acknowledgments

The authors acknowledge Aswan University and Environment and Natural Materials Research Institute (ENMRI), City of Scientific Research and Technological Applications (SRTA-City) for providing all the needed facilities to conduct this research.

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Synthesis of nano-silica oxide for heavy metals decontamination from aqueous solutions

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Version 1

Abstract

Figures

1 Introduction

2 Materials and methods

2.1 Materials

2.2 Fabrication of nano SiO₂

2.3 Characterization of the synthesized nano-SiO₂

2.4 Experimental design and procedures

2.5 Equilibrium, kinetics, and thermodynamics adsorption behavior of nano-SiO₂

2.5.1 Thermodynamics modeling for the adsorption process

2.5.2 Equilibrium isotherm analysis for adsorption of Pb (II) and Cr (VI)

2.5.3 Kinetic models of Pb (II) and Cr (VI) adsorption

2.6 Instruments and Analytical Methods

3 Results and discussion

3.1 Physico‑chemical characteristics of the fabricated nano-SiO₂

3.2 Application of synthesized nano-SiO₂ for Pb (II) and Cr (VI) adsorption

3.2.1 Effect of contact time

3.2.2 Effect of pH

3.2.3 Effect of nano-SiO₂ dosage on the adsorption process

3.2.4 Effect of initial Pb (II) and Cr (VI) concentration on the removal performance

3.2.5 Effect of solution temperature on the heavy metals’ removal efficiency

3.3 Equilibrium, kinetics, and thermodynamics adsorption behavior of nano-SiO₂

3.3.1 Thermodynamics modeling for the adsorption process

3.3.2 Equilibrium isotherm analysis for adsorption of Pb (II) and Cr (VI)

3.3.3 Kinetic models of Pb (II) and Cr (VI) adsorption

4 Conclusions

Declarations

References

Status:

Version 1

Synthesis of nano-silica oxide for heavy metals decontamination from aqueous solutions

Status:

Version 1

Abstract

Figures

1 Introduction

2 Materials and methods

2.1 Materials

2.2 Fabrication of nano SiO2

2.3 Characterization of the synthesized nano-SiO2

2.4 Experimental design and procedures

2.5 Equilibrium, kinetics, and thermodynamics adsorption behavior of nano-SiO2

2.5.1 Thermodynamics modeling for the adsorption process

2.5.2 Equilibrium isotherm analysis for adsorption of Pb (II) and Cr (VI)

2.5.3 Kinetic models of Pb (II) and Cr (VI) adsorption

2.6 Instruments and Analytical Methods

3 Results and discussion

3.1 Physico‑chemical characteristics of the fabricated nano-SiO2

3.2 Application of synthesized nano-SiO2 for Pb (II) and Cr (VI) adsorption

3.2.1 Effect of contact time

3.2.2 Effect of pH

3.2.3 Effect of nano-SiO2 dosage on the adsorption process

3.2.4 Effect of initial Pb (II) and Cr (VI) concentration on the removal performance

3.2.5 Effect of solution temperature on the heavy metals’ removal efficiency

3.3 Equilibrium, kinetics, and thermodynamics adsorption behavior of nano-SiO2

3.3.1 Thermodynamics modeling for the adsorption process

3.3.2 Equilibrium isotherm analysis for adsorption of Pb (II) and Cr (VI)

3.3.3 Kinetic models of Pb (II) and Cr (VI) adsorption

4 Conclusions

Declarations

References

Status:

Version 1

2.2 Fabrication of nano SiO₂

2.3 Characterization of the synthesized nano-SiO₂

2.5 Equilibrium, kinetics, and thermodynamics adsorption behavior of nano-SiO₂

3.1 Physico‑chemical characteristics of the fabricated nano-SiO₂

3.2 Application of synthesized nano-SiO₂ for Pb (II) and Cr (VI) adsorption

3.2.3 Effect of nano-SiO₂ dosage on the adsorption process

3.3 Equilibrium, kinetics, and thermodynamics adsorption behavior of nano-SiO₂