ABSTRACT

In this work, a flow injection analysis (FIA) method for the trace determination of lead, cadmium, nickel and cobalt in natural waters by formation of neutral chelates with ammonium pyrrolidine dithiocarbamate (APDC) was developed. The neutral chelates formed was retained in a mini-column packed with Linde type A zeolite (LTA) and type Y Faujasite zeolite (FAU) and then eluted with methyl isobutyl ketone (MIBK) to flame atomic absorption spectrometry (EAA) for its detection. Physicochemical characterization of this zeolite was carried out by Fourier Transform infrared spectroscopy and attenuated total reflectance (FTIR and IR-ATR), scanning electron microscopy and energy dispersive X-ray microanalysis (SEM-EDX) and X-ray power diffraction (XRD). Then, a FIA configuration was used with a column preconcentration system coupled to the detection system at room temperature (22˚C). The detection limit and the relative standard deviation for 5 determinations of different solutions of Pb²⁺, Co²⁺, Ni²⁺ and Cd²⁺ for FAU and LTA zeolite were calculated. The sampling frequency ranged from 18 - 35 h⁻¹ and preconcentration factors from 21 - 250 were achieved, for a sample volume of 6 mL using 20 mg of sorbents, indicating a high retention of the analytes on the zeolites material. The recoveries obtained in natural waters samples were close to 100% for all ions metal using synthetic zeolites, confirming the applicability of the method. The isotherm models of Langmuir, Scatchard, Freundlich and Dubinin-Radushkevich were used to study the equilibrium data, indicating that successfully followed the Freundlich and Dubinin-Radushkevich (D-R) isotherms at low metal ion concentration. The Freundlich parameter n varied between 0.35 - 1.01, whereas D-R isotherm yields the sorption free energy E < 8 kJ∙mol⁻¹ indicating psysisorption.

1. Introduction

The selective extraction and determination of heavy metals, especially some toxic ones, are still an intensively active research areas due to their biological mechanisms. The traditional methods for noble metals purification were precipitation and complexation procedure which needed large numbers of toxic acids and complexation agents, such as potassium or sodium cyanides that have negative impact on environment [1]. Analytical techniques such as inductively coupled plasma optical emission spectrometry (ICP-OES) [2], and inductively coupled plasma mass spectrometry (ICP-MS) [3] are available for the determination of trace metals with sufficient sensitivity for most of applications, but the high cost of maintenance and correction of inherent interferences is usually a limitation [4]. On the other hand, flame atomic absorption spectrometry (FAAS) is the most widely used analytical method in this field due to its low cost, friendly operation, high sample throughput and good selectivity. However, there are some drawbacks that lessen sensitivity of the technique, including low sample introduction efficiency and low residence time of the atoms in flame [5].

Therefore, analyte preconcentration is required. In this manner a large number of techniques for the preconcentration of lead, cadmium, cobalt and nickel, including liquid-liquid extraction [6], coprecipitation [7], microextraction [8], solid phase extraction [9], etc., have been developed [10]. The combination of flow injection analysis with atomic spectrometry (FIA-FAAS) has considerably extended capabilities of conventional atomic spectrometric methods in terms of efficiency, sensitivity, economy and freedom from interferences [11,12]. Also FIA configuration may contain mini-columns with sorbent material for the analytes preconcentration and improve selectivity in the analysis. For that reason new packing materials are still being proposed and examined [11]. Amberlites (XAD) [13,14], silica gel [15,16], bio-adsorbents [17,18], carbon nanotubes [19,20] and zeolites [21,22] have been used for preconcentration of trace metals [23], after complex formation of metal to facilitate retention on the adsorbent material., such as 8-hidroquinoline [24], APDC [21], 1-(2-Pyridylazo)- 2-naphthol (PAN) [25], ammonium diethyldithiophosphate (DDTP) [26], for ion exchange and/or complexation of metal ions has been applied to extraction-complexation of metal ions. Petit et al. [27] developed an on-line FI system for the preconcentration of Cu²⁺ onto a mini-column packed with synthetic zeolites; APDC was used to form neutral chelate of the metal, which is then eluted with MIBK and determined by FAAS. Preconcentration factors of copper ranging from 35 - 125 for Na-LTA and 30 - 65 for Na-FAU were readily achieved, related to the direct introduction of aqueous solutions into an atomic absorption spectrometer.

Zeolite [28] is an aluminum silicate that occurs both as natural and as produced synthetic. Zeolite has a threedimensional structure with pores. It consists of silicon, aluminum and oxygen ions. The silicon ions are neutrally charged in the crystal structure. Aluminum ions create negative places. To keep the charge in balance, a counterion (Na+, K+) or a proton (H+) is present in the pores. Some types of zeolite have just a large pore through the entire crystal structure, which is stipulated by the ring size. Changing the ratio of aluminum on silicon can also influence the pore size as well as the type of a counterion. All natural zeolites contain aluminum and are hydrophilic in nature [29,30]. The zeolites have great capacity for ionic exchange due to the charge equilibrium that attracts the closest cation, maintaining neutrality. The cationic exchange property is a function of the ratio of Si to Al. This capacity is expressed as the number of cations per mass or volume unit available for exchange [31]. Zeolites are mainly used as ionic exchangers (water softeners), molecular sieves, absorbents and catalysts. Many studies and investigations are carried out with different zeolites, due to its properties and characteristics [31]. The purpose of this work is to investigate the feasibility of adsorption of the neutral complex with the use of adsorption isotherms, as a modeling tool to describe the phenomenon that regulates the preconcentration (with separation from the matrix) of metal ions on sodium and calcium Y Faujasites (Na-FAU, Ca-FAU) and sodium and calcium A zeolite (Na-LTA and Ca-LTA) for designing a new sorbent material. Strong emphasis is devoted to study and evaluate the utility of the new sorbent to act as a selective solid phase extractor for separation and preconcentration of ultratrace amount of Pb²⁺, Cd²⁺, Ni²⁺ and Co²⁺, in spiked natural water samples prior to the determination by FIA-FAAS.

2. Experimental

2.1. Apparatus

A flame atomic absorption spectrometer Varian model SpectrAA 110 (Sao Paolo, Brazil), equipped with a bead impact system in the burner chamber and deuterium as background correction was used throughout. The flame composition was acetylene (flow rate: 2.0 L∙min⁻¹) and air (flow rate: 10.0 L∙min⁻¹). Nebulizer flow rate was 4.0 mL∙min⁻¹. Signals were obtained using SpectrAA-110 software, as peak heights. Absorbance measurements and currents were carried out at 217.0 nm and 4 mA for Pb, 232.0 nm and 4 mA for Ni, 228.8 nm and 4 mA for Cd and 240.7 nm and 7 mA for Co, using hollow cathode lamps Varian. The instrumental parameters were used according to the manufacturers recommendations. A pH meter Metrohm 701A (Ohio, USA) was used for pH adjustment.

A FIA system (Figure 1) equipped with: 1) one Gilson Minipuls-3 peristaltic pump (PP, Ohio, USA) four channels as liquid propulsion devices, equipped with tubes of polyvinyl chloride; two channels for sample and APDC reagent, one for water and one channel for the propulsion of MIBK, using the displacement bottle; 2) two reactions coils, assembled with politetrafluoroethylene (PTFE) tubing; 3) the sample injection was achieved using two six-way rotatory valve Rheodyne (Berlin, Germany) provided with fixed volume loops substituted by a reaction coil (MR) followed by a minicolum in valve V₂ and by another coil filled with MIBK in valve V₁. PTFE tubing (0.5 mm i.d.) was used for all connections. A GraLab900 (Ohio, USA) timer was used to select preconcentration/ elution steps.

2.2. Reagents and Samples

All chemicals reagent used were of analytical grade and ultrapure water from a Milli-Q (Barnstead NANO pureInfinity) to prepare all solutions. A 1000 mg∙L⁻¹ lead, nickel, cadmium and cobalt stock solutions were prepared by dissolving 1.000 g of the metal (Merck, Germany, 99.9% w/w), in a small volume of concentrated

(A)

(B)

Figure 1. Flow injection manifold for preconcentration and determination of trace metal ions. R: APDC reagent.

nitric acid (Fluka, Germany, 65.0% v/v, d = 1.42 g∙L⁻¹) and diluting to 1 L with 1% v/v HNO₃. Working solutions were prepared by dilution of the stock standard solution and were adjusted to optimum pH value with concentrated HNO₃. The aqueous solution of APDC (Sigma Chemical, Germany 99.0% w/w), using solid purchased was prepared daily in water. MIBK (Riedel de Haën, Germany, 99% v/v) was also used as eluent organic solvent. All bottles used for storing samples and standard solutions, as well as the glassware were washed in 10% v/v nitric acid for 24 h and finally rinsed with ultrapure water. The natural water samples were collected from Mérida City (Venezuelan) namely: tap water, ozonated water, Mucuy River and Albarregas River. All samples were filtered through 0.45 μm membrane filters, acidified to 0.03 mol∙L⁻¹ HNO₃ and stored at 4˚C in acid-cleaned polyethylene bottles in order to determine the “dissolved metal” fraction.

2.3. Mini-Columns: Zeolite Synthesis, Construction and Packing

Zeolites synthetic (Na-LTA, Ca-LTA, Na-FAU, Ca-FAU) with 60 - 100 µm particle size (Millipore, Madrid, Spain) were employed as sorbent materials. The Na-FAU zeolite used in this work was synthesized in the Kinetic and Catalysis Laboratory of the Chemistry Department, Los Andes University. The raw material for the synthesis of Na-LTA zeolite (4A, [31]) used in this study was Venezuelan Kaolin from deposits located in Bolivar state (particle size less than 200 - 270 mesh) [32].

Kaolin was subject to metakaolinitation to produce a metakaolin. The metakaolin was then zeolitized, producing Na-LTA zeolite, following a procedure reported else where [31,32]. The Ca-LTA and Ca-FAU were obtained by ion exchange using 1.0 mol L⁻¹ calcium chloride (Merck, Germany, 99.9% w/w) in a reflux system under temperature control. Amounts of 20 mg of the zeolites were used to pack the mini-columns. These were manufactured in the laboratory using PTFE tubing (4 mm id. and 0.3 cm length), sealed with a small piece of glass wool at the ends to avoid material losses and accommodated onto the FIA system. This configuration facilitated rapid replacement of the column, whenever required, thereby overcoming the deterioration of the analytical performance of the method due to the progressive deactivation of the sorbent material. The zeolites mini-column was proven to operate reliably for at least 200 sorption/elution cycles after washing with ultrapure water.

2.4. Characterization

The following tests were performed to characterize the synthesis and exchanges of Ca-FAU zeolite and Ca-LTA zeolite: FTIR, IR-ATR, SEM-EDX and XRD. FTIR and IR-ATR were performed using a FTIR Perkin Elmer (Germany) Spectrom RX1 and IR-ATR Perkin Elmer (Germany) model Spectrum 400, software-controlled Spectrum v.6.3.4.

The zeolites samples for testing were prepared using KBr pellet technique for FTIR and by the IR-ATR technique; the samples were placed directly into the diamond crystal with KRS-5 (mixture of bromide and thallium iodide) at ATR module. For the micrographs, it was used a scanning electron microscope (SEM) Hitachi S-2500 (Tokyo, Japan)coupled to an energy dispersive X-ray microanalyser Thermo Noran for the elemental analysis. XRD was performed using a Philips PW-1250 powder diffractometer equipped with an X-ray tube (Cu-Kα radiation, 40 kV, 25 mA). A small quantity of the sample was ground mechanically in an agate mortar, pestle and mounted on a flat holder covered with a thin layer of grease. Data collection was carried out in the 2θ range 5˚ - 60˚, in steps of 0.02˚ and counting time of 10 s. Phase identification was performed by searching the ICDD powder diffraction file database, with the help of JCPDS (Joint Committee on Powder Diffraction Standards) files for inorganic compounds [33]. The relative intensity yields were obtained from normalized XRD intensities of the major reflection for each material.

2.5. The On-Line Mini-Column Preconcentration/FIA-FAAS

The manifold was operated in a mode based on time control (Figure 1) and the operational sequence is completed in two steps: preconcentration (load) and elution. In the first step (Figure 1(A)), 6 ml of the sample or standard solution containing 5 - 100 µg∙L^{−1 M2+} (at optimum pH for each metal M) were introduced into the system through the sample channel S, while 0.3% w/w APDC solution was through the reagent channel, R. These flows were continuously propelled using a peristaltic pump PP into the system through valve V₂ and mixed in the mixing reactor MR with the APDC, during optimum preconcentration time of each metal. Then the neutral chelate was adsorbed on the zeolite mini-column, located in the loop of valve V₂, and the sample matrix sent to waste. During this period, a stream of water was continuously passed through the nebulizer, so that the matrix of the sample never reached the detector. Meanwhile, the coil, located in the injection valve V₁ is charged with MIBK.

After the loading time, the injection valves were switched automatically by action of the timer to the injection step (Figure 1(B)) and the neutral chelate was eluted from the mini-column with MIBK directly into nebulizer and subsequently the flame. The aspiration flow rate of the nebulizer was 4.0 µL∙min⁻¹.

The peaks heights were used as analytical signals. The injection valves were again switched to initial position in order to have the system ready for the next preconcentration step.

2.6. Isotherms Adsorption Models

The adsorption experiments were performed on line, using the configuration without mini-colum to constant temperature of 22˚C (Figure 2). The calibration curves in organic phase for each of the analytes under study were obtained following the next procedure: 1) Standards were prepared of 1 - 10 mg∙L⁻¹ Pb²⁺, 0 - 3 mg∙L^-1 Cd²⁺, 0 - 7 mg∙L⁻¹ de Ni²⁺ and 0 - 9 mg∙L⁻¹ Co²⁺ in MIBK containing 0.3% w/w APDC. The preparation in an organic solution, involves using small quantities in µL of aqueous solutions with high known concentrations of the analytes in study. 2) These organic solutions were loaded in the coil by the displacement system and then were turning the injection valve (Figure 2) to insert a stream of water that drove the analytes to the detector. Subsequently,

establishing a mathematical relationship where the absorbances obtained by FIA system with columns of Figure 1 (µg∙L⁻¹ aqueous solutions, C_e) are substituted into the linear regression equation obtained with FIA system without column (organic solutions) of the Figure 2, for obtained the equivalent values in ng∙g⁻¹ of zeolite material, called q.

3. Results and Discussions

3.1. Adsorbents Material Analysis

3.1.1. Spectra FTIR and ATR

Figure 3 shows the FTIR-spectra obtained for kaolin. The characteristic peaks at 3478 cm⁻¹ and 3495 cm⁻¹ are corresponding to the OH-stretching vibration. Bands at 1087 cm⁻¹ and 1064 cm⁻¹ were assigned to Si-O bonds in the SiO₄ molecules and Al-OH vibrations. The bands at 787 cm⁻¹ and 641 cm⁻¹ were Si-O symmetric stretching. Absorption at 480 cm⁻¹ was assigned as Si-O-Al stretching vibration where the Al is in octahedral coordination [34]. On the other hand, comparing the FTIR-spectra of the obtained kaolin with the FTIR-spectra of Na-LTA and Na-FAU (Figure 4) one can appreciate differences between the bands of stretching vibrations and deformation of the H-O-H in the region of 3000 - 3700 cm⁻¹ for the adsorbed molecular water which is located in the