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.

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.

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.

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.

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.

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.