Effectiveness and potential risk of CaO application in Cd-contaminated paddy soil

In this study, the accurate effectiveness of CaO in mitigating Cd bioavailability in paddy soil-rice system was investigated and moreover, the potential for reversibility of CaO liming process was provided. Increasing soil pH to ≥6.5 by CaO was determined to be the minimal threshold for minimizing Cd transfer into rice in historically contaminated soils across light to severe Cd levels, while an elevated CaO ratio was needed for soil with recent input of Cd. In CaO remediation treatment, a reduced pool of bioavailable Cd in rhizosphere soil coupled with an increased Cd retention by Fe plaque and an inhibited planta Cd transfer was determined consistently and proposed to be largely responsible for the significant reduction in brown rice Cd. Under continuous simulated acid precipitation test, a negligible level of Cd in eluate with ∼1 unit higher pH than control was recorded with CaO. Significant acidification, however, increased Cd solubilization in the limed soil than in unlimited control, notably at pH 5.5-6.5. As indicated by Visual MINTEQ, a higher Cd-carbonate solubility and much decreased Cd complexation on Fe-(hydr)-oxides across this pH range occurred as a result of elevated Ca input. This suggests that a high soil pH (≥6.5) needs to be permanently maintained once liming has been performed to minimize Cd mobilization. Therefore, amending Cd-contaminated soils with CaO is a cost-effective remedial measure for reducing Cd bioavailability to paddy rice, while a high soil pH need to be permanently maintained to sustain this beneficial effect.