Use of modified rice husks as a natural solid adsorbent of trace metals: characterisation and development of an on-line preconcentration system for cadmium and lead determination by FAAS

Abstract

The use of rice husks as an alternative adsorbent in an on-line preconcentration system for Cd (II) and Pb (II) determination by flame atomic absorption spectrometry (FAAS) is described. The potential of rice husks as a natural adsorbent was evaluated as a material modified with 0.75 mol l −1 NaOH solution and in the unmodified form. For this task, several techniques such as spectroscopy and thermogravimetry were used for elucidation of possible functional groups responsible for the uptake of Cd (II) and Pb (II). Furthermore, based on adsorption studies and adsorption isotherms applied to the Langmüir model, it was possible to verify that modified rice husks present a higher adsorption capacity for both metals. After establishing this material as a promising natural adsorbent, it was used for on-line preconcentration of Cd (II) and Pb (II) metals. The multivariate optimisation of chemical and flow variables was performed by using a full factorial design (2 4) including the following factors: preconcentration time, preconcentration flow rate, concentration and volume of eluent. The optimum pH values used for on-line preconcentration were taken from prior univariate experiments. Under optimised conditions for Cd (II) determination (4 min of preconcentration at a 6 ml min −1 preconcentration flow rate, in which comprises 24 ml of preconcentration volume, 200 μl elution volume and 1.0 mol l −1 HNO 3 solution as eluent), the system achieved a detection limit of 1.14 μg l −1 and an enrichment factor of 72.4. Similar conditions were used for Pb (II) determination (4 min of preconcentration, 6 ml min −1 preconcentration flow rate, 300 μl elution volume and 1.0 mol l −1 HNO 3 solution as eluent) from which a detection limit of 14.1 μg l −1 and enrichment factor of 46.0 were achieved. Also, rice husks have been shown to be a homogeneous and stable adsorbent in which more than 100 preconcentration/elution cycles provide a relative standard deviation (RSD) of less than 6.0% on the analytical signal. The satisfactory accuracy of the method developed was obtained by using spiked water samples (mineral water and lake water) and spiked red wine samples. These values were confirmed by electrothermal atomic absorption spectrometry (ETAAS). The certified reference material [pig kidney (CRM 186)] and the reference material [beech leaves (CRM 100)] were also used.