Abstract: It is of significance to select most efficient passivators for remediation of Cd polluted soils. Four conventional passivating agents, namely calcium carbonate, calcium silicate, dihydrogen phosphate and sodium sulfide, were compared for their efficacy to immobilize Cd in soils. Pot experiments were carried out to investigate passivation effects of passivators on fractionation and bioavailability of Cd in a Cd-contaminated red soil. The results indicate that addition of calcium carbonate, calcium silicate or sodium sulfide increased soil pH, whereas a reversal trend was observed with dihydrogen phosphate. At the application rate over 0.5%, soil pH values increased to higher than 8.5 with addition of calcium silicate or sodium sulfide, decreased to below 6.0 with addition of dihydrogen phosphate and maintained stable at 7.6 with addition of calcium carbonate. According to changes of soil Cd forms, the passivation efficiency decreased in the following orders:calcium carbonate, dihydrogen phosphate, sodium sulfide, calcium silicate. The maximum Cd immobilization rate of 10.61% was achieved after application of calcium carbonate in soil. Besides, wheat growth and grain yield were obviously promoted by dihydrogen phosphate and calcium carbonate, but strongly inhibited by sodium sulfide, and barely affected by calcium silicate. Cd contents in grain decreased by 32%, 27% and 19% with the addition of dihydrogen phosphate, calcium carbonate, calcium silicate, respectively, all meet with the food safety standard. The pot experiment demonstrations that 0.5% calcium carbonate is more suitable to remediate Cd-contaminated red soil, because of its higher ability of increasing soil pH and lowering Cd mobility.

Key words: red soil, Cd contamination, passivator, wheat, bioavailability