Sorption and Preconcentration of Lead on Silica Nanoparticles Modified with Resacetophenone

The silica-resacetophenone (SiO2-RATP) nanoparticles were used as a new sorbent for extraction of trace amounts of Pb(II) by batch technique. Conditions of the analysis such as preconcentration factor, effect of pH, sample volume, shaking time, elution conditions and effects of interfering ions for the recovery of analyte were investigated. The adsorption capacity of nanometer SiO2-RATP was found to be 167.24 μmol/g at optimum pH and the detection limit (3σ) was 0.58 μg/L. The adsorption equilibrium of Pb(II) on nanometer SiO2-RATP was achieved in 20 min. Adsorbed Pb(II) was easily eluted with 5 mL of 0.5 M hydrochloric acid. The maximum preconcentration factor was 60. The method was applied for the determination of trace amounts of Pb(II) in various natural water rivers.


Introduction
Heavy metal ions contamination is recognized as a priority problem in environmental protection because these represent an environmental concern when present in uncontrolled and high concentration.Lead is considered as non-essential and highly toxic element with a series of cumulative effect.Its toxicity is comparable of any other metallic element.It is toxic to the human biosystem, and is among the common global pollutants arising from increasing industrialization.So, there is need to determine lead up to micro levels [1][2][3][4][5] .Despite the selectivity and sensitivity of analytical techniques, there is crucial need for the preconcentration of trace elements before their analysis due to their frequent low concentrations in numerous samples [6][7][8][9][10][11][12] .Nowadays, nanometer materials have become more important owing to its special physical and chemical propertities.Nanoparticles exhibit intrinsic surface activity due to high surface areas and can strongly chemisorb many chemical substances.The size, surface structure and interparticle interaction of nanomaterials determine their unique properties and the improved performances and make their potential application in many areas [13][14] .Nanoparticles such as TiO 2 15-16 , Al 2 O 3 17 and ZrO 2 [18][19] have been used for the preconcentration of many metal ions and give promising results when used for trace element analysis of different samples.In present work, chemically grafted SiO 2 -RATP nanoparticles have been used for the sorption and preconcentration of lead prior to their determination by spectrophotometric method.These chemically modified SiO 2 -RATP nanoparticles are characterized by Fourier Transform Infrared Spectroscopy (FT-IR).

Experimental
Absorbance of Pb(II) was measured with UV-Vis Shimadzu-1700 spectrophotometer.The pH values were controlled by century Cp901 digital pH meter.

Reagents and standard solutions
Unless otherwise stated, all reagents used were of analytical reagent grade and all solutions were prepared with double distilled water.The 3-aminopropyltriethoxysilane of GR grade was supplied by Acros Organics (USA).Resacetophenone (RATP) was obtained from Merck (Mumbai).Nanometer SiO 2 and modified SiO 2 -RATP was synthesized according to the method reported [20][21] .

Sample preparation
Different water samples (River water) were analyzed without pretreatment.The pH was adjusted to 7.0 by using 0.2 M disodium tetraborate/boric acid buffer prior to use.

General procedure
Aliquots of sample solutions containing the analytes of interest were prepared and pH value was adjusted to the selected value with disodium tetraborate/boric acid buffer (0.2 M).Then, 20 mg of SiO 2 -RATP nanoparticles were added, and the mixture was shaken vigorously for 20 min to facilitate sorption of metal ion onto the adsorbent.Pb(II) sorbed on the sorbent was eluted with 5 mL of 0.5 M hydrochloric acid to get free metal ions, the solution was neutralized with 2 M sodium hydroxide and were determined by standard spectrophotometric method.

FT-IR spectrum analysis
The chemical grafting of resacetophenone the surface of nanometer SiO 2 was confirmed by FT-IR spectrum.FT-IR spectra of resacetophenone, nanometer-SiO 2, nanometer -SiO 2 -RATP, respectively (Figure 1).It reveals that main absorption peaks of nanometer SiO 2 (3448.0,1642.5, 1404, 1070.2, 964.2, 798.8 cm -1 ) are in agreement with standard spectrum 17 of SiO 2. The FT-IR spectrum of nanometer -SiO 2 -RATP, exhibits a band at 1613.9 cm -1 which appears to be contributed to by C=N stretching.The phenyl vibrations appears at 1493.9 cm -1 and 1373 cm -1 .The above experimental results suggest that nanometer SiO 2 has been successfully modified by resacetophenone.

Effect of pH on enrichment recovery
The adsorption of Pb(II) on nanometer SiO 2 -RATP was studied at different pH value (3.4 -10.0) following the recommended procedure.The results of effect of pH on the recoveries of the metal ions are given in (Figure 2).A quantitative recovery (>95%) was found for Pb(II) in the pH range of 7.0-10.0.

Effect of eluent concentration and volume
Elution of Pb(II) from nanometer SiO 2 -RATP sorbent was investigated by using different concentrations of hydrochloric acid and found quantitative recoveries (>95%) of Pb(II) could be obtained using 5 mL of 0.5 M hydrochloric acid as eluent.The results of effect of eluent concentration and volume are given in Table 1 and Table 2

Effect of nanometer SiO 2 -RATP amount
To optimize the amount of sorbent on quantitative retention of analyte, different amounts (10-60 mg) of SiO 2 -RATP nanoparticles were added into the solution following and the experiment was performed.Quantitative extraction of the Pb(II) >95% was obtained in the range of 20-35 mg of nanometer SiO 2 -RATP.20 mg of sorbent was found to be sufficient for further studies.

Effect of shaking time
The sorption of Pb(II) on 20 mg of RATP modified silica nanoparticles were studied for different shaking time (5-30 min).The results indicated that within 20 min the quantitative extraction of Pb(II) was >95% .

Adsorption capacity (Q S )
The adsorption capacity 22 is an important factor as it determines how much sorbent is quantitatively required to concentrate the analytes from a given solution.A breakthrough curve was obtained by plotting the concentration (mg/L) vs. the µmol of Pb(II) sorbed per gram.From the breakthrough curve the amount of SiO 2 -RATP nanoparticles for Pb(II) was found to be 167.24µ mol/g at pH 7.0 respectively.The results are given in Figure 3.

Effect of sample volume
In order to explore the possibility of concentrating low concentration of analytes from large volumes, the effect of sample volume on the retention of Pb(II) was also investigated.

Analytical precision and detection limit
Under the selected conditions, three portions of Pb(II) standard solutions was enriched and analyzed simultaneously following the experimental method.The relative standard deviation Concentration, mg/L Adsorption capacity, µ mol/g % Recovery Volume of sample solution, mg/L (RSD) of the method was 3.8% for the determination of 5.0 µg Pb(II) in 100 mL river water samples.The detection limit of this method for Pb(II) was 0.58 µg/L.

Applications
The developed method has been applied for the determination of trace Pb(II) in river water.For analysis, 200 mL of sample was extracted following the method described above.The results are given in the Table 3.

Conclusion
Resacetophenone anchored silica nanoparticles were prepared and used as solid sorbent for preconcentration and separation of trace Pb(II) prior to their determination.These analytes can be easily screened at µg/mL level with preconcentration times of 20 min and preconcentration factor of 60 for Pb(II).The method can be used as screening to estimate the total concentration of Pb(II) present in a large number of samples and thus avoiding the continuous use of expensive instrumentation in routine analysis.The results of determination have been found to be accurate and reproducible.

Figure 4 .
Figure 4. Effect of sample volume on analyte recovery.

Table 1 .
. Effect of concentration of HCl solution on elution of Pb(II) (n=3).

Table 2 .
Effect of volume of HCl solution on elution of Pb(II) (n=3).
. Analytical recovery of Pb(II) added to natural water samples.