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

Abstract

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 flame atomic absorption spectrometry (FAAS) was investigated. A number of parameters, namely, type, concentration, and volume of eluent, pH of the sample solution, flow rate of extraction, and volume of the sample, were evaluated. The 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⁻¹ thiourea in 2 mol L⁻¹ H₂SO₄ as eluent. The limit of detection (LOD), defined as five 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.

1. Introduction

The 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].

Several methods have been proposed for recovery of gold from waste water such as solvent extraction [5, 6], membrane disk [7], ion exchange [8, 9], coprecipitation [10, 11], cloud point extraction [12, 13], electrodeposition [14], leaching [15], chlorination [16], cyanidation [17], and solid phase extraction (SPE). Among all these methods, solid phase extraction has some advantages. It has low cost and a high preconcentration factor, and also it is simple, rapid, and efficient [18].

The choice of adsorbent is an important stage in SPE due to control of the analytical parameters such as selectivity, affinity, and capacity [19]. So, different cartridges have been proposed including activated carbon [20], Dowex M 4195 [21], chitin [22], micro beads [23], thiol cotton fiber [24], chelating fiber [25], ion-imprinted polymers [26], modified resin [27, 28], polyurethane foam [29], activated alumina [30], and modified silica [31–34]. Among many types of adsorbents used in SPE, functionalized alumina has received great attention for their good mechanical and thermal stability and also less susceptibility to swelling or shrinking [35].

In this paper, for the first 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. The optimum conditions including flow 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 influence of various cationic interferences were investigated. This method can be applied as a reliable method for gold enrichment and determination in complex environmental samples.

2. 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 𝜇g mL⁻¹ was purchased from Merck (Darmstadt, Germany). Deionized water was used during the experiments. The required pH adjustments were made by use of the buffer solutions. For the pHs 1 and 2, KCl/HCl buffer solutions were used. CH₃COOH/CH₃COONH₄ buffers were used to adjust pH in the range of 4–6, while NH₃/NH₄Cl buffers were used for pHs 8–10.

9-Acridinylamine, 3-(chloropropyl)-trimethoxysilane, CH₃COOH, Na₃C₃H₅O(CO₂)₃, Na₂HPO₄, NaH₂PO₄, HCl, and HNO₃ were purchased from the Merck, and thiourea and thioacetamide were purchased from Fluka.

2.2. 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]. The sol was dried at 50°C to obtain gel pieces and heat treated at 600°C to make gamma alumina. BET analysis shows 256 m² gr⁻¹ surface area for these nanoparticles. The average size of the individual γ-alumina nanoparticles was calculated as 93 nm using histogram program.

In order to synthesize 9-acridinylamine functionalized γ-alumina (py-γ-alumina), 1 g γ-alumina was activated in NaOH 2 M for 4 h then suspended in 50 mL toluene, and the mixture was stirred for 1 hour. After this step, 2.0 g of 3-chloropropyl trimethoxy silane was added and refluxed for 12 hours under nitrogen atmosphere. The white solid was removed from the solvent by filtration. The solid was suspended in 50 mL of triethylamine and toluene, and then 1 gr of 9-acridinylamine was added and refluxed for 6 hours. The white-brownish solid was removed from the solvent by filtration. After this step, it was washed by toluene and chloroform then dried at room temperature. Functionalization by pyridine was confirmed 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-γ-alumina sample gave 9-acridinylamine concentration of 0.49 mmol g⁻¹. The SEM photograph of py-γ-alumina nanoparticles is shown in Figure 1. A schematic diagram of synthesis route is shown in Figure 2.

Figure 1 

SEM photograph of 9-acridinylamine modified nano alumina.

Figure 2 

A schematic model for 9-acridinylamine modified nano alumina.

2.3. 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. The 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 flow rate during experiments.

2.4. Procedure

2.4.1. Column Procedure

A glass column with 120 mm in length and 2 cm in diameter was used for the experiments. It was filled 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.

2.4.2. Preconcentration Procedure

A solution containing 1 μg mL⁻¹ of gold was made. Solution’s pH was adjusted to to 3 using Na₃C₃H₅O(CO₂)₃/HCl buffer solutions. Firstly, buffer solution was passed through the column to precondition it, then 100 mL of gold solution was passed at flow rate of 6 mL min⁻¹. The elution process was performed by passing 8 mL of thiourea 0.1 mol L⁻¹ in H₂SO₄ solution 2 mol L⁻¹. The eluted solutions were analyzed by FAAS for five times. The results were averaged and reported.

2.4.3. Sample Preparation

Real samples were obtained from tap water in Tehran, Caspian Sea, and jewelry waste water. The solutions were stored in cleaned polyethylene bottles and were filtered before usage. In order to validate the present method, a standard material sample (NCS DC 73323) with a certified 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 HNO₃ (65%) in a microwave digestion system. The 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. After digestion, it was diluted to 50.0 mL with deionized water. The pH of solutions was adjusted by adding Na₃C₃H₅O(CO₂)₃/HCl buffer solutions to 3. The proposed procedure was performed on the samples, and results were reported.

3. Result and Discussion

The effect of different parameters on gold extraction was studied. It was tried to find best conditions for extraction. The influence of pH, effect of type, concentration, and volume of eluent, and sample and eluent flow rates were studied, and the optimum values were obtained.

3.1. Influence of pH

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. Then, the column was washed with 8 mL of thiourea 0.1 mol L⁻¹ in 2 mol L⁻¹ H₂SO₄, and the eluent was analyzed with FAAS. The optimum condition for extraction was obtained at pH 3 (Figure 3).

Figure 3 

Effect of pH of sample solution on percent recovery of Au(III) by 9-acridinylamine alumina.

3.2. Effect of Type, Concentration, and Volume of Eluent

HCl, HNO₃, H₂SO₄, thiourea, and thioacetamide were mixed in different concentrations to make eluents. These eluents were testified to get the highest recovery in elution. It was observed that 0.1 mol L⁻¹ thiourea in 2 mol L⁻¹ H₂SO₄ solution provided effectiveness of the elution of Au (III) from sorbent. The optimum volume for elution was 8 mL of the eluent (Figure 4).

Figure 4 

Effect of type and concentration of eluent for desorption of Au(III) by 9-acridinylamine alumina.

3.3. Sample and Eluent Flow Rates

In order to get the optimum conditions for sample and eluent flow rates, the pH of 100 mL of 1 μg mL⁻¹ gold solution was adjusted to 3, and the solution was passed through the column in the flow rate range of 1–15 mL min⁻¹. The column was washed with 8 mL of the optimum eluent. Adsorption of gold ions in the flow rates higher than 9 mL min⁻¹ was not complete (Figure 5). So, the optimum flow rate should be around 6 mL/min so that any small variation in the flow rate will not affect the adsorption. The flow rate for elution was also studied and observed so that flow rates 3 mL min⁻¹ and less show good recoveries.

Figure 5 

Effect of flow rates of sample solutions on the percent recovery of Au(III) by 9-acridinylamine alumina.

3.4. Influence of Interference Ions

The efficiency of the method in the presence of different cations was studied. The cations of Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Cd²⁺, Fe²⁺, Cu²⁺, Pb²⁺, and Cr³⁺ 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. The results showed that presence of these cations has no effect on recovery of gold ions (Table 1).

Table 1 

The tolerance limit of the diverse ions on the determination of gold.

3.5. 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. The maximum capacity was 38±2 mg g⁻¹ (0.2±0.01 mmol g⁻¹).

3.6. Analytical Performance

The enrichment factor was determined by the recommended column procedure using increasing volumes of 1 𝜇g mL⁻¹ Au solution. The maximum sample volumes were found to be 600 mL for nano alumina with recovery greater than 98.5%. The 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 (𝑛=10) were passed through the column under the optimum experimental conditions. The values of LOD for gold on modified nano alumina are 13.00 ppb. The results were obtained from CLOD = 𝐾_𝑏𝑆_𝑏/𝑚, where 𝑘_𝑏=3.

The precision of the method under the optimum conditions was determined by performing ten replicates. The recoveries were found to be 98.5% ± 1.2 on modified 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.

Table 2 

Data of real sample analysis for Au on 9-acridinylamine alumina.

4. Conclusions

Solid phase extraction procedure based on modified gamma Alumina is very simple, fast, reproducible, and selective. Compared with other solid phases, 9-acridinylamine functional gamma alumina has some benefits, 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.