Preconcentration of Cd(II) and Pb(II) Using Humic Substances and Flow Systems Coupled to Flame Atomic Absorption Spectrometry

Abstract.

This paper demonstrates the potential of two natural adsorbents, vermicompost and humic acid, for preconcentration of cadmium(II) and lead(II) using flow systems coupled to flame atomic absorption spectrometry (FAAS). The procedure involves the adsorption of Cd(II) or Pb(II) on these materials (using columns containing 25 mg) and subsequent elution for determination by FAAS. Cadmium(II) was preconcentrated for 4 min (flow rate of 4.0 mL min⁻¹) and eluted with 220 µL of 3.0 mol L⁻¹ HNO₃. Under these conditions, preconcentration factors of 46 and 27 were obtained for vermicompost and humic acid, respectively. Except when using 1.0 mol L⁻¹ nitric acid (for humic acid), all conditions for lead(II) preconcentration were identical to those for cadmium(II), and preconcentration factors of 62 and 83 were obtained when vermicompost and humic acid, respectively, were used. The systems were stable with only slight variations in the slopes of the analytical curves (ca. 5% after 8 h working period). The long-term stability shows that a minimum of 120 and 100 cycles, respectively, can be run using the same masses of vermicompost or humic acid. The detection limits for Cd(II) were 0.4 and 0.8 µg L⁻¹ for vermicompost and humic acid respectively, while the detection limits for Pb(II) were 8.8 and 12.1 µg L⁻¹, also for vermicompost and humic acid. The accuracy of the methods was checked by using spiked and real (certified and reference) samples. Due to the concomitant sorption of other metals leading to variable slopes for lead and cadmium determination, it was necessary to adopt the standard addition method for calibration purposes.