Sorption and desorption of Cd,Co,Cu,Ni and Zn from carbonated Portland cement

A material with potential for use as a sorbent has been prepared by carbonating damp Portland cement in a 100%CO₂ atmosphere at a pressure of 3 bars. Analysis by XRD showed the main crystalline phase present, after eight 1 h cycles of carbonation, to be calcite. A series of batch sorption experiments involving single metal solutions of Cd, Co, Cu, Ni, Zn, at concentrations varying from 200 to 1000 mg L⁻¹, were carried out employing the carbonated media. The solid residues obtained, were then used to examine the desorption of metals over a 24 h timescale. The zeta potential and conductivity of suspensions prepared from the residues were also recorded. The results showed the sorbent could remove significant quantities of metal ions from solution, and up to 176 mg g⁻¹ was recorded for Cd. The desorption experiments showed metal release rates <1% by mass of metal adsorbed. Electrophoretic analysis gave different values for zeta potential and conductivity for each sorbent–metal system, indicating that sorption might take place via different mechanisms including inner- and outersphere complexation.