Electrolyte and pH effects on Pb(II)-calcite sorption processes: The role of the PbCO_3(aq)⁰ complex

Batch sorption experiments were conducted under conditions of ambient temperature and atmospheric PCO_2(g) to determine the effects of electrolyte type, ionic strength, and pH on Pb(II) interactions with calcite. For 0.15 M nitrate and chloride solutions at pH 8.2, no significant effect of electrolyte type on Pb sorption was observed. Varying ionic strength from 0.15 to 0.5 M produced little effect on Pb sorption in nitrate compared to chloride solutions in which Pb uptake decreased with increasing ionic strength. For a pH range of 7.3-9.4 in 0.15-0.2 M nitrate solutions, Pb sorption increased from pH 7.3 to 8.5 with a subsequent decrease in uptake out to pH 9.4. The trends in electrolyte and pH experiments correlate well with those for PbCO _3(aq)⁰ speciation, indicating that this metal-ligand complex in solution dictates Pb sorption in the system under investigation.