Flame Atomic Absorption Determination of Gold Ion in Aqueous Samples after Preconcentration Using 9-Acridinylamine Functionalized γγ-Alumina Nanoparticles

A simple and sensitive solid phase extraction utilizing 9-acridinylamine functionalized alumina nanoparticles was developed, and their potential use for preconcentration and subsequent determination of gold by �ame atomic absorption spectrometry (�AAS) was investigated. A number of parameters, namely, type, concentration, and volume of eluent, pH of the sample solution, �ow rate of extraction, and volume of the sample, were evaluated. e effect of a variety of ions on preconcentration and recovery was also investigated. Gold ions were found to be recovered quantitatively at pH 3.0, with 0.1 mol L thiourea in 2 mol L H2SO4 as eluent. e limit of detection (LOD), de�ned as �ve times the standard deviation of the blank, was determined to be lower than 13.0 ppb. Under optimum conditions, the accuracy and precision (RSD%) of the method were >98.0 and <1.5%, respectively. To gauge its ability in terms of application to real samples, the proposed method was successfully applied for determination of gold concentration in waste water samples and one soil standard material, and satisfactory results were obtained.


Introduction
e interest on gold is not only the reason of the use of gold in jewelry and its shining color, but also it has been found that gold can be used in catalytic converters, metallurgy, energy, electronics, health and environment, and many more applications [1,2].Nowadays, as human knowledge increases, more and more usage of gold has been investigated and it becomes more valuable.Waste water from mining, electroplating industries, electronics, and jewelry making manufacturing are the examples of natural samples which contain trace amount of gold [3,4].
In this paper, for the �rst time, nanoparticles of alumina are functionalized with 9-acridinylamine group and used for extraction of trace amount of gold ions from some real samples of waste water.e optimum conditions including �ow rates of the sample and eluent solution, pH of the solution, type, and least amount of eluent for elution were studied.Also, amount of break through volume, maximum adsorption capacity, and in�uence of various cationic interferences were investigated.is method can be applied as a reliable method for gold enrichment and determination in complex environmental samples.

Experimental
2.1.Reagents and Solutions.Analytical reagent grade chemicals-from Merck Company (Darmstadt, Germany) or Fluka Company (Buchs SG, Switzerland)-were employed for the preparation of all solutions.Gold standard solution of 1000 g mL −1 was purchased from Merck (Darmstadt, Germany).Deionized water was used during the experiments.e required pH adjustments were made by use of the buffer solutions.For the pHs 1 and 2, KCl/HCl buffer solutions were used.CH 3 COOH/CH 3 COONH 4 buffers were used to adjust pH in the range of 4-6, while NH 3 /NH 4 Cl buffers were used for pHs 8-10.9-Acridinylamine, 3-(chloropropyl)-trimethoxysilane, CH 3 COOH, Na 3 C 3 H 5 O(CO 2 ) 3 , Na 2 HPO 4 , NaH 2 PO 4 , HCl, and HNO 3 were purchased from the Merck, and thiourea and thioacetamide were purchased from Fluka.

Preparation of 9-Acridinylamine Functionalized Gamma
Alumina Nanoparticles.Gamma alumina nanoparticles were prepared by the sol-gel method.Boehmite sol was prepared by the controlled hydrolysis of aluminum-tri-sec-butoxide. Details of the sol-gel synthesis are reported elsewhere [36].e sol was dried at 50 ∘ C to obtain gel pieces and heat treated at 600 ∘ C to make gamma alumina.BET analysis shows 256 m 2 gr −1 surface area for these nanoparticles.e average size of the individual -alumina nanoparticles was calculated as 93 nm using histogram program.
In order to synthesize 9-acridinylamine functionalized -alumina (py--alumina), 1 g -alumina was activated in NaOH 2 M for 4 h then suspended in 50 mL toluene, and the mixture was stirred for 1 hour.Aer this step, 2.0 g of 3-chloropropyl trimethoxy silane was added and re�uxed for 12 hours under nitrogen atmosphere.e white solid was removed from the solvent by �ltration.e solid was suspended in 50 mL of triethylamine and toluene, and then 1 gr of 9-acridinylamine was added and re�uxed for 6 hours.e white-brownish solid was removed from the solvent by �ltration.Aer this step, it was washed by toluene and chloroform then dried at room temperature.Functionalization by pyridine was con�rmed by IR spectroscopy and elemental analysis.IR spectroscopy is given as follows: IR (KBr, cm −1 )-3445 (NH), 3023 (CH, aromatic) 2913 (CH, aliphatic), 1545 (C=C), 600-800 (alumina).Elemental analysis of py--alumina sample gave 9-acridinylamine concentration of 0.49 mmol g −1 .e SEM photograph of py--alumina nanoparticles is shown in Figure 1.A schematic diagram of synthesis route is shown in Figure 2.

Instrument.
A Shimadzu AA-680 atomic absorption spectrometer (AAS) equipped with single element hollow cathode lamp (6.0 mA for gold) and 10 cm of burner head and air acetylene burner was used for the determination of gold.e wave length at 242.8 nm (resonance line), the spectral band width at 0.5 nm, and the ratio of air-acetylene at 4.7 were set.
A digital WTW Metrohm 827 ion analyzer (Switzerland) equipped with a combined glass-calomel electrode at 25 ± 1 ∘ C was used for the pH measurements.
A vacuum pump from Leybold (Germany) was used during the experiments, and an adjustable vacuum gauge and controller from Analytichem International (Harber City, CA) was used for adjusting �ow rate during experiments.

Column Procedure.
A glass column with 120 mm in length and 2 cm in diameter was used for the experiments.It was �lled with 200 mg of the alumina.Before using the column, it was washed with 5 mL dilute hydrochloric acid 1 M, 5 mL absolute ethanol, 5 mL toluene, and 20 mL distilled water to remove all organic and inorganic impurities.

Preconcentration Procedure.
A solution containing 1 g mL −1 of gold was made.Solution's pH was adjusted to to 3 using Na 3 C 3 H 5 O(CO 2 ) 3 /HCl buffer solutions.Firstly, buffer solution was passed through the column to precondition it, then 100 mL of gold solution was passed at �ow rate of 6 mL min −1 .e elution process was performed by passing 8 mL of thiourea 0.1 mol L −1 in H 2 SO 4 solution 2 mol L −1 .e eluted solutions were analyzed by FAAS for �ve times.e results were averaged and reported.

Sample Preparation.
Real samples were obtained from tap water in Tehran, Caspian Sea, and jewelry waste water.e solutions were stored in cleaned polyethylene bottles and were �ltered before usage.In order to validate the present method, a standard material sample (NCS DC 73323) with a certi�ed gold content was obtained from China National Analysis Center for Iron and Steel.Standard material sample was digested with 6 mL HCl (37%) and 2 mL of HNO 3 (65%) in a microwave digestion system.e microwave program was as follows: 2 min at 250 W, 2 min without radiation, 6 min at 250 W, 5 min at 400 W, 8 min at 550 W, and then venting for 8 min.Aer digestion, it was diluted to 50.0 mL with deionized water.e pH of solutions was adjusted by adding Na 3 C 3 H 5 O(CO 2 ) 3 /HCl buffer solutions to 3. e proposed procedure was performed on the samples, and results were reported.

Result and Discussion
e effect of different parameters on gold extraction was studied.It was tried to �nd best conditions for extraction.e in�uence of pH, effect of type, concentration, and volume of eluent, and sample and eluent �ow rates were studied, and the optimum values were obtained.

3.�. �n�uence of p�.
Considering the important role of pH on solid phase extraction, the optimum condition for the pH was obtained by passing 100 mL of different sample solutions containing 1 mg L −1 gold ion with the pH range 2-9.en, the column was washed with 8 mL of thiourea 0.1 mol L −1 in 2 mol L −1 H 2 SO 4 , and the eluent was analyzed with FAAS.e optimum condition for extraction was obtained at pH 3 (Figure 3).

Effect of Type, Concentration, and Volume of Eluent.
HCl, HNO 3 , H 2 SO 4 , thiourea, and thioacetamide were mixed in different concentrations to make eluents.ese eluents were testi�ed to get the highest recovery in elution.It was observed that 0.1 mol L −1 thiourea in 2 mol L −1 H 2 SO 4 solution provided effectiveness of the elution of Au (III) from sorbent.e optimum volume for elution was 8 mL of the eluent (Figure 4).

Sample and Eluent Flow Rates.
In order to get the optimum conditions for sample and eluent �ow rates, the pH of 100 mL of 1 g mL −1 gold solution was adjusted to 3, and the solution was passed through the column in the �ow rate range of 1-15 mL min −1 .e column was washed with 8 mL of the optimum eluent.Adsorption of gold ions in the �ow T 1: e tolerance limit of the diverse ions on the determination of gold.rates higher than 9 mL min −1 was not complete (Figure 5).So, the optimum �ow rate should be around 6 mL/min so that any small variation in the �ow rate will not affect the adsorption.e �ow rate for elution was also studied and observed so that �ow rates 3 mL min −1 and less show good recoveries.

Interfering ions
3.4.�n�uence of �nterference �ons.e efficiency of the method in the presence of different cations was studied.e cations of Na + , K + , Cs + , Mg 2+ , Ca 2+ , Cd 2+ , Fe 2+ , Cu 2+ , Pb 2+ , and Cr 3+ as their chloride salts with various concentrations were added to 100 mL of single solution containing 1 mg of gold, and the extraction procedure was followed.e results showed that presence of these cations has no effect on recovery of gold ions (Table 1).

Maximum Adsorption Capacity.
Maximum adsorption capacity of the sorbent was studied by passing 500 mL portions of aqueous single solutions containing 100 mg gold through the column, followed by determination of the effluent and retained metal ions using FAAS.e maximum capacity was 38  2 mg g −1 (0.2  0.01 mmol g −1 ).
T 2: Data of real sample analysis for Au on 9-acridinylamine alumina.
Real sample (ng mL −1 ) Added (ng mL −1 ) Found (ng mL  3.6.Analytical Performance.e enrichment factor was determined by the recommended column procedure using increasing volumes of 1 g mL −1 Au solution.e maximum sample volumes were found to be 600 mL for nano alumina with recovery greater than 98.5%.e loaded gold ions were easily desorbed from the solid phases with their respective eluent volume.Subsequently, they were subject to boiling until the volume reaches 3 mL.As a result, enrichment factors as high as 200 on nano alumina were obtained. To determine the detection limit of the present method, 500 mL blank solutions (  1) were passed through the column under the optimum experimental conditions.e values of LOD for gold on modi�ed nano alumina are 13.00 ppb.e results were obtained from CLOD =     /m, where    3.
e precision of the method under the optimum conditions was determined by performing ten replicates.e recoveries were found to be 98.5%± 1.2 on modi�ed alumina.In order to investigate the accuracy and applicability of this method, real samples and standard material were analyzed.With these analyses, the effect of different matrices on the method was studied.For sample preparation, certain amounts of gold were spiked into the samples (Table 2).As shown, in all cases, the recovery is almost quantitative.

Conclusions
Solid phase extraction procedure based on modi�ed gamma Alumina is very simple, fast, reproducible, and selective.Compared with other solid phases, 9-acridinylamine functional gamma alumina has some bene�ts, like high capacity factor, low detection limit, and high enrichment factor.Due to relative high preconcentration factor, trace metal ions at ppb level in high volume economical sample can be determined and separated by them.

F 3 :
Effect of pH of sample solution on percent recovery of Au(III) by 9-acridinylamine alumina.

F 4 :
Effect of type and concentration of eluent for desorption of Au(III) by 9-acridinylamine alumina.

F 5 :
Effect of �ow rates of sample solutions on the percent recovery of Au(III) by 9-acridinylamine alumina.