Cadmium Binding by Humic Acids: An Experiment in FTIR Spectroscopy and Soil Chemistry

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 9 Issue 5 (2004) pp 317-320

Cadmium Binding by Humic Acids: An Experiment in FTIR Spectroscopy and Soil Chemistry

Armando Carrasquero-Durán* and Iraima Flores

Universidad Pedagógica Experimental Libertador, Departamento de Química. Apartado postal 1017 – Las Acacias, Maracay. Estado Aragua, Venezuela, acarrasquerod@latinmail.com
Received March 16, 2004. Accepted May 13, 2004.

Published online: 23 Sepember 2004

Abstract. Cadmium binding by humic acids (HA) extracted from earthworm compost is studied by FTIR as a didactic experiment in spectroscopy. Using a guided-research technique, students quantify the adsorption of Cd²⁺ by a sample of HA and propose the mechanism, at a molecular level, responsible for this adsorption. FTIR spectra of HA at pH 5.0 show signals of phenolic groups, carboxylic acids, and aromatic and aliphatic carbon, among others. Changes in FTIR spectra are observed at pH 7.0, characterized by a lower-intensity 1725 cm^–1 band, υ_a(COOH), and displacements from 1600 to 1550 cm^–1 of υ_a(COO^–) and from 1338 to 1388 cm^–1 of υ_s(COO^–) due to ionization of the carboxylic acids. When increased amounts of Cd²⁺ are added to HA at pH 5.0, similar changes are observed in the region from 1750 to 1300 cm^–1 indicating that metal ions induce deprotonation of acid groups followed by coordination with carboxylate, which acts as a bidentate ligand, as is demonstrated by the position of the symmetric and asymmetric stretching bands in the chelate as compared with the uncomplexed carboxylate. In this exercise students not only identify functional groups in the spectra but also see how HA helps to control heavy-metal soil contamination. The application of chemistry to study a real-world problem increases student motivation.

Key Words: Research in Teaching and Learning; organic chemistry; stereochemistry; representations; test; research; models; molecular structure; visualization; computers; animation

(*) Corresponding author. (E-mail: acarrasquerod@latinmail.com)

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