Separation/Preconcentration of Al(III) from Water Samples via Sorptive-Flotation Technique Using Silica Nanoparticles Modified with Folic Acid

In the present work, silica nanoparticles functionalized with folic acid (Nano-SiO2-FA) were synthesized and characterized thru elemental analysis, infrared spectroscopy (FT-IR), BrunauerEmmett-Teller (BET) surface area determination, and scanning electron microscope (SEM). Nano-SiO2FA was examined as a sustainable and promising sorbent for preconcentration of Al(III) from natural water samples by sorptive-flotation (SF) separation technique before its determination by flame atomic absorption spectrometry (FAAS). A several trials were made in lab, to determine the feasibility of using Nano-SiO2-FA as a sorbent and oleic acid as a surfactant. The influence of: solution pH, temperature, shaking time, surfactant, sorbent and Al(III) concentration and the presence of foreign ions that affect the sorptive-flotation process were investigated. Good results attained under optimum circumstances, at pH 3.5 and ~25°C, according to which nearly 100% of aluminum was separated. The procedure was successfully applied to recover aluminum spiked to some natural water samples. Moreover, a sorption and flotation mechanism is suggested.


Introduction
Al and Al-salts are being widely used in various industries such as drugs, beverages, packing materials, and dye industry [1]. Al high level in wastewaters prevents the growth of microorganisms which help organic materials stabilization in water. High levels of Al 3+ ions in water are unpleasant and found to be neurotoxic [2,3]. Also, it is concerned with several diseases as Parkinson and Alzheimer's [4,5], bone softening, renal failure and anaemia [6]. Al presents in soils naturally in the form of minerals as, alumino-silicates (kaolin, feldspars and micas), hydroxides and oxides [7]. Foods and water are the most important sources for Al exposure. Al levels in waters with neutral pH ranged (0.001-0.05 mg·L −1 ) [8]. According to Environmental Protection Agency (EPA), Al in drinking water has a level of (0.05-0.2 mg·L −1 ) [9]. The high detection limit (DL) of flame atomic absorption spectrometry (FAAS) and presence of matrix effects make it impossible for direct analysis of these low levels of metal ions without using a preconcentration procedure. Consequently, separation/preconcentration of analytes is required [10]. Several of separation/preconcentration techniques for metal ions in trace amount including adsorption on dissimilar adsorbents, solvent extraction [11], cloud point extraction, solid-phase extraction, co-precipitation, liquid-liquid micro-extraction have been discussed [12][13][14][15]. Recently, separation/preconcentration method namely flotation has received significant attention because it's simple and rapid method, and has good separation yield more than 95% and a wide possibility of usages for recovery purpose [16,17].
Nowadays, more interest focused on the using of chelating fibers for separation and removing metal ions from aqueous media. Chelating fibers have several advantages as large sorption capacity, ease of regeneration and high selectivity which may be attributed to high sorption kinetics, low cost, large surface areas and presence of active sites [18][19][20]. Silica one of the most commonly used sorbent in chemical analysis owing to its numerus advantages as thermal, chemical, and mechanical stability, comparing to other sorbents [21]. The silanol groups present on the silica surface is considered being weak ion exchangers and have low interaction and binding force with the target analyte [22]. Hence, certain functional groups must be immobilized on silica surface to give selectivity behavior for surface of silica. Folic acid (Fig. 1), known commercially as vitamin B9 is a water-soluble substance and exhibits versatile ligation behaviors via the carboxylate group which can act as a mono, bi or bridging ligand bind to metal ions [23][24][25]. Many studies have been reported on the binding between folic acid and metals such as Cd(II), Pb(II), Cu(II), Zn(II), Fe(III), and Hg(II) [24,26]. So, folic acid is considered being a green and ecofriendly sorbent for binding with metal ions. Owing to the above facts, this work aim to surface modification of chlorinated nanosilica with folic acid to produce Nano-SiO2-FA nanocomposite. The produced sorbent was used for the preconcentration of Al 3+ from different water samples by batch mode using sorptive-flotation (SF) technique

Apparatus 2.2.1. Flame atomic absorption spectrometry
The remaining concentration of Al 3+ ion in the filtrate was determined by Flame Atomic Absorption Spectrometry (FAAS) (GBC, SensAA Series) with air-acetylene flame under the optimal instrumental conditions shown in Table 1.

Scanning electron microscope
Scanning electron microscope (SEM, Quanta FEG 250 (Field Emission Gun), ThermoFisher Scientific, USA) at an accelerating voltage of 30 kV, magnification 14×up to 1000000, was used for the assessment of surface morphology.

Surface area
The surface area investigation of prepared sorbent was carried out with nitrogen adsorption at -196°C using surface area analyzer (QUANTACHROME-NOVA @ 2000e series).

Flotation cell
The flotation process was occurred in a cylindrical test tube with 1.5 cm inner diameter and 29 cm length.

Methodology 2.3.1. Preparation of sorbent 2.3.1.1. Synthesis of nanosized silica
Synthesis of nanosized silica was carried out using cetyltrimethylammonium bromide (CTAB) as template, sodium silicate powder (Na2SiO3.9H2O) as Si source and HCl as pH controlling agent. 12 g CTAB was placed in 460 mL of doubly distilled water, followed by stirring for 15 min then 80.377 g of Na2SiO3. 9H2O was added to the mixture followed by stirring for 30 min. Concentrated HCl added to the mixture for pH adjustment at 9. The stirring non-stop for 4. The resulting bulky white gelatinous precipitate was transferred to a vessel of Teflon and left for 24 h at 25 o C. After that, the final product was filtered and washed with doubly distilled water after that dried at 50 o C for 6 h finally, the sample was calcined at 550 o C for 6 h [27].

Immobilization of silylating reagent
2.0 g of prepared nano-SiO2 were mixed with 50 mL of dry toluene in a 250 mL flask. Then, 4.0 mL of (3-chloropropyl)trimethoxysilane was added and the product refluxed at 110°C with non-stop stirring for 6 hrs. The final product of nano-SiO2-Cl separated and washed by diethylether and ethanol many times then dried at 50°C.

Functionalization of chlorinated nanosilica with folic acid
2.0 g of prepared nano-SiO2-Cl was added to 50 mL of dry toluene in a 250 mL flask, then 2.0 g of folic acid was added and the mixture refluxed at 110°C and non-stop stirring for 6 hrs. The final product of nano-SiO2-FA separated by filtration and washed by diethylether and ethanol many times and finally dried at 50°C.

Batch method for metal ion uptake via sorptive-flotation (SF) technique
10 mL aqueous solution containing defined amounts of Al 3+ ion, sorbent and (HNO3 or NaOH for pH-controlling) was placed in a flotation tube followed by shaking for the optimum time, after that 3 mL of HOL (with optimum concentration) was added, finally the tube was inverted 20 times upside down by hand then kept 5 min standing for complete flotation.
The removal % of Al 3+ ions calculated as following:

Effect of foreign ions
To study the applicability of the proposed procedure, the effect of interfering ions which could interfere with Al 3+ ions removal using Nano-SiO2-FA was tested under the optimum conditions.

Desorption study
To test sorbent desorption, 25 mg Nano-SiO2-FA was added to 25 mL solution containing 10 ppm Al 3+ at pH 3.5 and 25 o C and shacked for 30 min. The sorbent was filtered and washed with doubly distilled water, to remove the unloaded Al 3+ . Then, 5 mL of HNO3 of different concentrations was added to Nano-SiO2-FA-Al 3+ complex followed by shaking for 15 min finally, mixture was filtrated and Al 3+ concentration was measured in the filtrate. The desorption ratio (D%) was determined by equation (2): where Cd is the concentration of Al 3+ in desorption solution (ppm); Vd is the volume of the desorption solution (L); and V is the volume of solution (L), Co (ppm) and Ce (ppm) initial and equilibrated Al 3+ ion concentrations, respectively.

Analytical application
Natural water samples were collected from different locations in Egypt (Mansoura, Gamasa, Ras El-Barr, El-Manzalah and Alexandria), at a depth of 50 cm from the upper level, then all these samples were filtered then total dissolved salts (TDS) and pH were determined and finally all samples acidified with HNO3 and preserved in a dark polyethylene bottle in a refrigerator at 5 o C for future use. The applicability of Nano-SiO2-FA for uptake of the Al 3+ ion from different natural water samples was studied for spiked concentration. The investigates were performed using 25 mL of filtered sample at pH 3.5 containing 25 mg sorbent and 1×10 -3 mol/L HOL, finally the mixture shacked for 30 min. The removal % was determined by equation (1).

Sorbent characterization 3.1.1. Elemental analysis (C, H and N)
The obtained results from elemental analysis of Nano-SiO2-FA sorbent have been shown in Table 2. It's obvious that, after the chlorination step and modification of chlorinated nanosilica, the presence of nitrogen content considered as a good indication for insertion of the folic acid moieties onto the chlorinated nanosilica.

FT-IR analysis
The FT-IR spectra of different steps for prepared sorbent are illustrated in Figure 2.

Scanning electron microscope
SEM was used to evaluate surface morphology and existence of Nano-SiO2-FA nanocomposite in the form of nanoparticles. Figures (3a, 3b) represent the SEM graphs for Nano-SiO2 and Nano-SiO2-FA, respectively. From Figures (3a, 3b) it was notice that, the Nano-SiO2 is consists of homogenous, uniform and almost spherical nanoparticles. But the immobilized FA on Nano-SiO2 surface was found to be in aggregate forms, which refers to the covering of sorbent surface with folic acid.

Surface area
The surface area found to be 455.438 m 2 /g.

Batch process using sorptive-flotation (SF) 3.2.1. Influence of pH
The removal % of Al 3+ ions over pH range (2-6) was studied to get optimum pH. Figure 4 shows the effect of the pH on the removal % of 20 ppm of Al 3+ ions using 30 mg of Nano-SiO2-FA and 1×10 -3 mol.L -1 of HOL. At pH< 3, functional groups of Nano-SiO2-FA are protonated with H3O + ions and the overall surface charge on the sorbent becomes positive. Thus, at pH below 3, the removal efficiency of Al 3+ is low which can be attributed to the competition between Al 3+ ions and protons for the active sites of sorbent surface. At pH range (3-6), there are lower competition between H3O + and Al 3+ ions for the https://doi.org /10.37358/Rev.Chim.1949 Rev. Chim., 71 (8) active sites of sorbent surface therefore more sites are easily available for metal ion binding, so the removal % of metal ions is increased and the optimal pH value found to be 3.5.

Influence of sorbent and sorbate concentrations
Two runs of trials were done to test the influence of Nano-SiO2-FA dosage ( Figure 5) and Al 3+ concentration ( Figure 6) on the removal % of Al 3+ ions from aqueous medium at pH 3.5 using 1×10 -3 mol.L -1 of HOL. Figure 5 shows that, the removal % of Al 3+ improved with raising Nano-SiO2-FA dose, while it reduced with increasing Al 3+ concentration ( Figure 6). Achieving the maximum separation of Al 3+ ions at a higher Nano-SiO2-FA dose may be owing to increasing number of active sites present on the sorbent surface and available to Al 3+ . Therefore, 30 mg of Nano-SiO2-FA can be appropriate amount for the removal of Al 3+ with a concentration ≤ 20 ppm.

Influence of surfactant concentration
A run of trials were performed to remove 20 ppm of Al 3+ ions from aqueous solutions at pH 3.5, using 30 mg of Nano-SiO2-FA and different concentrations of HOL [1×10 -3 -8×10 -3 ] mol . L -1 . The data obtained in Figure 7 confirmed that, a quantitative separation of Al 3+ ions was achieved at HOL concentration of (1×10 -3 -4×10 -3 mol . L -1 ). The removal of Al 3+ ions reduced at higher concentrations, a phenomenon that can be attributed to the formation of a stable hydrated envelop of surfactant on the surface of air bubble, or due to the formation of hydrated micellar layer on the sorbent surface. In both cases, the hydrophobicity of the sorbent surface decrease the flotation efficiency [32,33].

Influence of shaking time
The effect of shaking time was also examined utilizing 20 ppm of Al 3+ ions, 30 mg of Nano-SiO2-FA and 1×10 -3 M of HOL at pH 3.5. The range of shaking time was (1-10) min. The attained results in Figure 8 exposed that, the removing % enhanced to its maximum value after shaking time of 5 min. Subsequently, 5 min of shaking was believed to be sufficiently for the quantitative removal of Al 3+ ions.

Influence of temperature
For such study, one solution containing 20 ppm of Al 3+ ions and 30 mg of Nano-SiO2-FA and a second solution containing 1×10 -3 mol.L -1 of HOL were heated up or cooled to the same temperature utilizing a water bath. Surfactant was rapidly poured onto the Al 3+ solution, after which the mixture was floated. The found results in Figure 9 confirmed that, the removal % of Al 3+ ions decreased with temperature increasing.

Effect of foreign ions
To check the selectivity of the suggested methods and in order to investigate the relevance of Nano-SiO2-FA for separation of Al 3+ ions, the interference of several foreign ions on the removal of Al 3+ ions was examined with pre-optimized experimental procedures. All cations were used as chlorides salt while the anions were applied as sodium salts. The data showed maximum tolerable concentration of foreign ions, with relative error ≤ 5%. The results attained in Table 3 indicated that, all the studied foreign ions with relatively high concentrations (compared to that of Al 3+ ions) have no adverse effect on the analysis of Al 3+ by the mentioned techniques. Although Mg 2+ and Ca 2+ ions may form magnesium and calcium oleats, the expected harmful effect on the recovery of Al 3+ ions was not observed. Consequently, the suggested sorptive-flotation procedure could find its own applications for the recovery of Al 3+ ions from different water matrices.

Desorption study
Desorption tests contribute to regenerate the sorbent, also to recover Al 3+ ions from the used sorbent moreover protecting the environment from solid waste disposal difficulties. Tries were made to desorb Al 3+ ions from the metal laden sorbent using different concentrations of HNO3. The effect of HNO3 concentration (0.1, 0.5, 1.0 and 1.5 M) on the elution yield was assessed and the results offered in Table  4 display that quantitative extraction was achieved at concentrations of 1.5 M of nitric acid and quantitative yield (>96%) was obtained when 5 mL of 1.5 M HNO3 was taken.

Analytical applications
To examine the application of flotation procedures, a run of trials were conducted to recover spiked concentration of Al 3+ ions added to some natural water samples. The results present in Table 5  the recovery was quantitative and satisfactory (~ 100%) with a relative standard deviation (RSD) does not exceed 1.15 %. Table 5. Recovery of 20 ppm of Al (III) ions added to some water samples:

Suggested mechanism for sorptive-flotation process:
Prior to talk about the probable mechanism, the following points should be taken into account: 1. Utmost metal ions are separated by: (i) adsorption onto sorbent surface by co-precipitation as M(OH)(s); (ii) flocculation by adsorption of hydrolytic forms or (iii) complexation with surface functional groups [34,35].
3. The presence of N-H, C=O and O-H groups on sorbent surface was confirmed by the characteristic bands at 3530, 3419 and 1695 cm -1 , respectively.
4. Oleic acid begins to dissociate at pH ≥ 5.2 [37]. Consequently, the proposed mechanism for sorptive-flotation can be as follows: 1. At pH < 3, the charge on sorbent surface becomes positive due to protonation with H3O + ions. Thus, at pH below 3, the removal efficiency of Al 3+ is owing to the adsorption of hydrolytic species of aluminum on sorbent surface.
2. In the pH range 3-6, where the maximal removal of Al 3+ ions occurred, adsorption may be electrostatically in nature and taking place via co-precipitation of the colloidal positive precipitates of aluminum.
3. Then, the adsorbent-adsorbate system is made hydrophobic by combination with un-dissociated surfactant molecules through H-bonds and/or chemically with oleate anions, then the resultant aggregates are floated to solution surface with the support of air bubbles. 4. In alkaline medium, the removal of Al 3+ ions decreases which may be attributed to the inability of adsorption of the negative species, Al(OH)4 -, or negative oleate ions on the negative surface of sorbent [38].

Conclusions
Preparation and using of nanosilica functionalized with folic acid in clean technology has discussed in the present work. Nano-SiO2-FA was successfully prepared thru loading of folic acid on the surface of chlorinated nanosilica (Nano-SiO2-Cl) and its structure was proved by several assessments. Nano-SiO2-FA was used as an effective sorbent for selective separation and preconcentration of Al 3+ ions from natural water samples using sorptive-flotation separation method before its determination by flame atomic absorption spectrometry (FAAS). The experimental data showed that the Al 3+ removal by Nano-SiO2-FA was dependant on pH. Also, it was indicated that the prepared sorbent can be regenerated easily using HNO3 as eluent. Finally, the separation method was applied to the preconcentration of Al 3+ ions from real samples without matrix interference. Tap