Sorption and sorption reversibility of cadmium on calcite in the presence of phosphate and sulfate

The influence of phosphate and sulfate on the sorption of cadmium on calcite was studied in laboratory experiments at I = 0.7, pH 7.4–9.5 and PCO2 = 10−3.5 atm. The fast initial sorption of cadmium decreased with increasing phosphate and sulfate concentrations. Since total phosphate and total sulfate sorption increase with decreasing pH, due to lower carbonate concentrations, the effects on the sorption of cadmium are most pronounced in the pH range 8.3–7.4. Phosphate and sulfate also affect the sorption reversibility of cadmium. The short term (t < 24 h) sorption reversibility at pH 8.3 decreases, because a relatively greater percentage of sorbed cadmium is present at the high affinity sites in the presence of phosphate. The short term reversibility at pH 7.4 does not change, because, both in the absence and in the presence of phosphate, the greater amount of cadmium is sorbed at the high affinity sites. The long term (t > 144 h) sorption reversibility of cadmium increases in the pH range 7.4–8.3 and with increasing phosphate (or sulfate) concentrations. The main cause for this is a decreased recrystallization rate of calcite and therefore a smaller amount of cadmium is lattice bound. Based on the reduction of the Cd/Ca ratios at the calcite surface and in the ripened calcite exposed to increasing phosphate concentrations in our sorption experiments, the relationship between Cd/Ca ratios in benthic foraminifera and phosphate concentrations in deep ocean water appears to result from biochemical, rather than chemical processes.