Numerousinvestigators have found that the slope of a sorption capacity of metal ion versus pH decreases significantly somewhere between pH 6 and 8.5 in the presence of CO2, with geomedia as diverse as quartz, layered silicates, and amorphous iron oxides (e.g.,[30]) due to the formation of aqueous metal complexes with carbonate to a significant degree, whereas this decrease is not observed in CO2-free systems [31,32].