Heavy metal removal from aqueous solutions by sorption using natural clays from Burkina Faso
The acid-base properties of two raw and purified mixed clays from Burkina Faso were studied, as well as their potential to remove copper(II), lead(II) and chromium(III), and thereby their ability to be used to purify water from heavy metals. The purification procedure of the clays involved removal of carbonates, iron oxides and organic matter. A determination of the elemental composition of the mixed clays revealed the presence of aluminum, iron and silicon as main constituents. The high alkaline pH in one of the samples is attributable to the presence of carbonate in the raw clay. The point of zero charge (pHpzc) values of the clays, as determined by potentiometric titrations, were 6.79 and 9.52 for the raw clays, while after purification they were 6.87 and 6.76, respectively. Metal adsorption to the clay surfaces started at pH values below pHpzc, strongly indicating formation of inner-sphere complexes. With contact time of 48 h, complete removal of copper(II) was achieved at pH 8 for all samples. More than 90% of the lead(II) removal was attributed to adsorption while for chromium(III), it was 85%. Adsorption to organic matter and iron oxides, and precipitation of metal hydroxides gave significant contributions to the removal of metal ions in aqueous systems.
Key words: Mixed clays, potentiometric titration, heavy metals, pHpzc.