Suppressive effects of thermal-treated oyster shells on cadmium and copper translocation in maize plants

The effect of varied concentrations of thermal-treated oyster shells (TOS) on the suppression of cadmium (Cd) and copper (Cu) uptake and translocation into the shoots of maize plants was examined. Maize plants were grown in Cd- and Cu-contaminated Andosol for 70 days. The concentration of mobile Cd (extracted with 1 M NH₄NO₃) decreased with increasing TOS applications, whereas an increase in the concentration of mobile Cu in soil resulted from cumulative TOS additions. The addition of 2% TOS had no prohibitive effects on Cd uptake in maize shoots, but the 4 and 8% TOS treatments decreased Cd accumulation in shoots by 41 and 59%, respectively. The possible mechanisms underlying Cd suppression in maize shoots were the enhanced Cd adsorption caused by pH-induced increases in the negative charge of the soil and the antagonistic effects of Ca resulting from competition for exchange sites at the root surface. Cu accumulation in maize shoots increased by 34, 51, and 53% with the addition of 2, 4, and 8% TOS, respectively, but this increase was not observed for Cd accumulation. These results suggested that, in multi-metal-contaminated soils, attention should be paid to the potential mobility of target metals and the pH of the contaminated soil. From a plant physiological perspective, contaminated soils slightly reduced photosynthetic performance. However, the addition of TOS to the soil at levels higher than 4% substantially decreased photosynthetic performance, indicating that CaO-based suppressants at critical loads might damage the net photosynthetic rates of sensitive maize plants.