Chemical form of soluble cadmium and copper in arable soils and its implication to their mobility

To clarify the chemical form of soluble cadmium and copper in arable soils, the molecular weight distribution and ion composition of water-soluble cadmium and copper in soils were investigated. The solution extracted with water from soil was fractionated by ultrafiltration and then cadmium and copper in 4 molecular weight size classes were determined. About 50% of water-soluble cadmium was present in the fraction with a molecular weight of 500 or less, 12-25% in the fraction with a molecular weight of 500-300,000 and 25-37% in the fraction with a molecular weight of 300,000 or more. For copper, 4-19% was present in the fraction with a molecular weight of 500 or less and 40-75% was present in the fraction with a molecular weight of 10,000-300,000 and 300,000 or more. The water-soluble heavy metals were also fractionated with ion exchange resin. More than 90% of cadmium was detected in the cation fraction. Conversely, approximately 80% of copper was detected in the anion fraction. These results indicate that water-soluble cadmium in soil is present as divalent cations, and copper is dissolved in a form bonded with substances such as organic matter in soil. To understand the difference in the mobility of cadmium and copper in soils, a plastic container was filled with moist soil and the soil moisture was allowed to evaporate only from the upper part of the container. Water-soluble cadmium was concentrated to a level of 68.5 times the background concentration at the top of the soil and copper was concentrated to 6 times the background concentration. Such a difference in the mobility was discussed in relation to the chemical form of soluble cadmium and copper.