Adsorption of Lead with Silica Gel Modified with Polyamidoamine Dendrimer and Thiomalic Acid

New adsorbents PAMAM-n.0TMSG (n=1,2,3,4) with thiomalic acid as functional group has been prepared based on polyamidoamine dendrimer modified silica gel and characterized with FTIR, SEM and TG. Microcolumn enrichment and measurement of Pb with graphite furnace atomic absorption spectroscopy (GFAAS) was investigated with PAMAM-n.0TMSG (n=1,2,3,4) as adsorbent. The adsorption conditions were optimized. The increase of grafted generation of PAMAM-n.0TMSG could effectively improved the adsorption performance of adsorbents. The adsorption capacity of adsorbents was 14.42, 16.19, 20.79 and 25.32 mg g respectively, With PAMAM-4.0TMSG as adsorbent, microcolumn enrichment and measurement of Pb with GFAAS was proposed. The relative standard deviation (R.S.D.) was 1.4% (n=11) for 0.2 μg mL of Pb. The limit of detection (LOD) of 2.9ng mL 1 was achieved. The proposed column enrichment method was applied for detection of Pb in tap water and sea water samples successfully.


Introduction
Because of the low concentration of lead and interference from co-existing substances in environmental sample, Solid-phase extraction (SPE) was often used to preconcentrate and separate trace metals from matrices [1][2][3] with macroporous adsorption materials such as such as silica-gel [4], chelating resin [5] and organic-inorganic hybrid materials [6,7]. Dendrimer-like polyamidoamine (PAMAM) has attracted considerable attention in metal ions adsorption due to its end amine and ester groups [8][9][10][11]. Considering that if silica-gel could be modified with dendrimer-like PAMAM and functional group for adsorption could be linked to the generous peripheral amine group of PAMAM, it would be possible to improve the adsorption capacity of silica-gel and selectivity.
In present work, silica-gel adsorbents PAMAM-n.0TMSG (n=1,2,3,4) with dendrimer-like PAMAM and thiomalic acid as functional groups have been prepared and characterized with FTIR, SEM, and TG, then micro-column adsorption of Pb 2+ was investigated. The experiments results showed that modification of PAMAM and thiomalic acid could effectively improve the adsorption/desorption properties of adsorbent. With PAMAM-4.0 TMSG as adsorbent, GFAAS method for analysis of Pb 2+ combined with micro-column enrichment was proposed and applied to analysis of Pb 2+ of water samples.
Silica gel ( 60-100 meshes, surface area 300-600 m 2 g í1 ) was bought from Qingdao Shuoyuan Chemical Co. Ltd., China. Ȗ-Aminopropyltriethoxysilane (Beijing Shenda Fine Chemical Co. Ltd., China) was purified by distillation at 123 under 10 mm Hg. All of the other reagents were analytical reagent grade. 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride(EDC), thiomalic acid and N-Hydroxysuccinimide(NHS) were all bought from Aladdin Chemistry Co.Ltd. Stock solution of Pb 2+ (1000 mg L í1 ) was prepared by dissolving appropriate amounts of analytical pure Pb(NO 3 ) 2 in pH=2 HNO 3 solution. Standard solutions of Pb 2+ were prepared by dilution of the stock solution just before use. Distilled water was used throughout.

2.3Enrichment Procedure
One end of the micro-column was packed with polypropylene fiber, PAMAM-n.0TMSG (n=1,2,3,4) was packed in micro-column, then the other end of the micro-column was packed with polypropylene fiber, two ends were connected with silicone tubing. In Step 1, the blank solution (its pH is the same as Pb 2+ solution) was pumped through micro-column. In Step 2, the Pb 2+ solution was pumped through micro-column and was collected for determination of Pb 2+ . In Step 3, the blank solution was pumped through micro-column. In Step 4, the eluent solution was pumped through micro-column in the reverse direction and was collected for determination of Pb 2+ . Pb 2+ concentration was determined with GFAAS. Pb 2+ adsorption percentage (A) and desorption percentage ( R) were calculated according to (1) and (2). % 100 0 0 While C 0 and C 1 (ȝg mL -1 ) are concentration of Pb 2+ in the sample solution before and after adsorption, C 2 (ȝg mL -1 ) is concentration of Pb 2+ in elutant through micro-column in step 4; V 0 and V 1 (mL) are the volume of sample solution before and after adsorption, V 2 (mL) is elutant volume.  The Fourier transmission infra-red (FTIR) spectra of PAMAM-n.0TMSG (n=1,2,3,4) are shown in Fig.1. The absorption at 3346 cm í1 is characteristic of -NH 2 and -OH stretching vibration and the absorption at 2946 cm í1 is characteristic of-CH 2 -stretching vibration. The absorption at 1644 cm í1 and 1575 cm í1 is characteristic of C=O stretching vibration in amide groups and N-H scissorvibration respectively. The absorption at 2614 cm í1 and 2558 cm í1 is characteristic of -SH vibration. The appearance of these characteristic absorptions suggested that PAMAM and thiomalic acid were successfully introduced onto the surface of silica-gel. Fig.2 presents SEM images (magnified fifty thousand times) of PAMAM-n.0TMSG (n=1,2,3,4). On the rough surface of APSG there are many pores with kinds of diameter. As the increase of PAMAM grafting generation, more surface was covered by PAMAM, indicating that dendrimer-like PAMAM had been successfully grafted onto the surface of silica gel.

3.1Characterization of Adsorbent
TG analysis was applied to evaluate the grafting effectiveness of PAMAM onto the surface of silica gel. Under N 2 atmosphere, temperature range was from 30 to 600 with heating rate of 5 min -1 during TG analysis. When the temperature reached 100 , the weight loss were about 2.09-8.23%, which represented the evaporation of water adsorbed on silica-gel. When the temperature reached 600 , the total weight loss of APSG, PAMAM-n.0TMSG(n=1,2,3,4) were 11.69, 22.42, 26.46, 27.43 and 33.08% respectively, suggesting that grafting percentage of PAMAM increased with the increase of grafting generation number of PAMAM.

3.4Effect of Flow Rate
In the sample solution flow rate range of 0.6-6.0 ml min í1 , adsorption rate of Pb 2+ reached more than 95%. So 4.2 mL min í1 was used as the sample solution flow rate.
In the eluent solution flow rate range of 0.6-6.0 ml min í1 , recovery of Pb 2+ reached more than 95% with a flow rate of 3.0 mL min -1 and with PAMAM-1.0TMSG or PAMAM-2.0TMSG as assorbent. Recovery of Pb 2+ reached more than 95% with a flow rate of 3.6-4.8 mL min -1 and with PAMAM-3.0TMSG or PAMAM-4.0TMSG as assorbent.In order to guarantee the elution of Pb 2+ , 3.0,3.0, 4.2 and 4.2 mL min -1 was used as eluent solution flow rate respectively in further experiments.

3.5Effect of Sample and Eluent Solution Volume
Under the above selected conditions, different volume (5.0-40.0 ml) sample solution of 0.2 ȝg mL -1 Pb 2+ was used respectively for sample loading. When sample solution volume was not more than 10.0 ml, adsorption rate of Pb 2+ were more than 95%. Different volume (5.0-40.0 ml) HCl solution was used as eluent, and elution recovery of Pb 2+ could reach more than 95%. So 10.0 ml sample solution and10.0 ml eluent solution were selected in the further experiments.

3.7Saturated Adsorption Capacity
Under the above selected conditions, 40.0 ȝg mL -1 Pb 2+ solution (pH=5.0) passed through micro-column with flow rate of 1.8 mL min í1 . The saturated adsorption capacity of PAMAM-n.0TMSG (n=1,2,3,4) for Pb 2+ were 14.42, 16.19, 20.79 and 25.32 mg g -1 , which indicated that the increase of grafting generation of PAMAM was helpful for increase of functional group amount and adsorption capacity of adsorbent.

3.9Analytical Performances
The characteristic data for the analytical performance of microcolumn enrichment system with PAMAM-4.0TMSG as adsorbent and GFAAS detction of Pb 2+ are investigated. With the proposed method, the relative standard deviation (R.S.D.) was 1.4% for 0.2 ȝg mL í1 of Pb 2+ . The limit of detection (LOD) was 2.9 ng mL í1 , calculated as the concentration of Pb 2+ required to yield a net peak-height absorbance that was equal to three times the standard deviation of the background signal (3ı) of the blank solution. Regression equation (five standards, n=5, C in ȝg mL í1 ) was A=3.0952*C+0.1090 in Pb 2+ concentration range of 0.05-0.3 μg mL -1 with a correlation coefficient R= 0.9990.  The proposed column enrichment method was applied for the preconcentration and determination of Pb 2+ in tap water and sea water samples (100 mL). The standard addition method was applied for evaluation of the method in analysis of water samples. The analytical results are given in Table 1. As it can be seen, the recoveries of Pb 2+ are in the range of 98.0-105.0% with R.S.D not more than 5.0%. The results indicate that the proposed method is reliable.

Summary
PAMAM-n.0TMSG (n=1,2,3,4) adsorbents withPAMAM and thiomalic acid as functional groups have been synthesized and characterized with FTIR,SEM andTG, then enrichment conditions for Pb 2+ was investigated. PAMAM-n.0TMSG showed good adsorption capability. The proposed column enrichment-GFAAS method was successfully applied for detection of Pb 2+ in tap water and sea water samples.