1Symbiotic Resources Laboratory Co., Ltd., Sendai, Miyagi 980-0845, 2Graduate School of Environmental Studies, Tohoku University, Sendai, Miyagi 980-8579, Japan
In order to remove cadmium from scallop wastes including mid-gut glands, a novel process using subcritical water treatment was developed. Crushed scallop organs and pure water were charged in a stainless steel reactor tube with Swagelok caps. The reactor tube was then sealed and immersed in an oil bath adjusted to the prescribed temperatures. Experiments were carried out at temperatures ranging from 90 to 200°C and for reaction times of 10 to 30 min. After 10 minutes of subcritical water treatment at temperatures higher than 150°C, most of the charged samples had decomposed and separated into two parts, liquid and solid phases, and most of the cadmium had concentrated into the solid phase. This phenomenon could be interpreted as that cadmium ions were adsorbed by suspended organic adsorbent materials and settled out. The phenomenon strongly depended on the pH of the samples, and the most appropriate pH value for removal of cadmium was 4.5, which was assumed to be the isoelectric point of the suspended organic adsorbent materials. The weight of the solid phase which contained cadmium decreased to less than 20 mass% in comparison with the raw wet material by centrifugal separation after the treatment at 200°C/10 min. In this case, cadmium concentration in the liquid phase decreased to 10 mass ppb (in 4 times diluted samples) or less without pH control.