Impact of soluble humic substance on Cd²⁺ sorption on apatite in aqueous solutions

Abstract

The effect of soluble humic substances (HUM) on Cd sorption and desorption on apatite in aqueous solutions is studied. Batch experiments were carried out using synthetic hydroxyapatite (HAp) and fluorapatite (FAp). Parallel experiments in distilled water, Cd²⁺, Ca²⁺, HUM, Cd²⁺+HUM, Cd²⁺+Ca²⁺ and Ca²⁺+HUM solutions showed that the solubility of apatite did not increase in the initial pH range from 5 to 8 in the presence of dissolved HUM. Dissolved HUM were bound on apatite in suspensions. The pH of suspensions stabilised at 6.4–6.5 in water. The presence of soluble HUM did not change this. Cd uptake resulted in a reduction in pH. The amount of Cd bound on apatite was not changed by HUM, but HUM affected the desorption level of sorbed Cd²⁺.

Keywords:

• humic substance
• apatite
• cadmium
• sorption
• desorption

Acknowledgements

The study was supported by the Estonian Science Foundation (Grant No. 7159) and the target financing by MES of Estonia (Project No SF0140082s08).