Cadmium activities, solution speciation, and solid phase distribution of Cd in cadmium nitrate and sewage sludge-treated soil systems

Experiments were conducted to compare the solution activities, the solution speciation, and the solid phase distribution of cadmium introduced into Domino soil as a Cd nitrate solution and as sewage sludge. The Cd loading rates ranged from 0.125 to 2.5 mg Cd per 2 g soil. A dual chamber diffusion apparatus (DCDA), which has two chambers separated by a nucleopore membrane, was used to bring the introduced Cd and soil into a steady state condition. Cd as Cd nitrate or sewage sludge was added to one chamber and soil to the other in a background solution of 4.5 mM CaSO4 + 9 mM NaCl. Both sides were allowed to equilibrate by diffusion through the membrane. At the end of each experiment, the chamber contents mere collected, and the solution and solid phases were separated and analyzed independently. Cadmium concentrations in the solution phase were measured by flameless atomic absorption spectroscopy (FAAS), and Cd activities and speciation were estimated by GEOCHEM-PC. The solid phase distribution of Cd was determined by a sequential extraction procedure and a mass balance was achieved. The extracting reagents were KNO3/deionized water, NaOH, EDTA and concentrated HNO3, HF and HCl, which represent exchangeable, organic, carbonate mineral and adsorbed, and residual fractions, respectively. The results indicate that (i) When Cd is introduced into the soil as sludge-borne Cd, the majority of the Cd remains in the sludge, and only small amounts of Cd are transferred from the sludge to the solution and the soil solid phases. (ii) When Cd is introduced into the soil as a Cd nitrate solution, the majority of the Cd is adsorbed by the soil solid phase and the Cd concentration in solution remains high. (iii) The solution speciation of Cd was similar in both the Cd nitrate, and sludge-treated systems and was dominated by the free Cd2+ ion (51%) for both systems. (iv) The solid phase distribution of Cd was dominated by the EDTA fraction for all the solid phases including the sludge, the sludge-treated soil, and the Cd-nitrate treated soil, even though the total Cd concentration was an order of magnitude higher in the Cd nitrate-treated soil compared with the sludge-treated soil.