Enhanced immobilization of arsenic and cadmium in a paddy soil by combined applications of woody peat and Fe(NO): Possible mechanisms and environmental implications.

Organic matter (OM) plays an important role in the mobility of heavy metal(loid)s. Peat containing abundant OM can be used as an organic fertilizer improving physical and chemical properties of soil. Previous studies indicated that the immobilization of heavy metal(loid)s by peat is affected by the presence of metal oxides and/or hydroxides and that Fe-enriched peat is very effective in immobilizing metal(loid)s. Accordingly, we hypothesize that simultaneous application of peat and Fe-containing compounds may pronouncedly immobilize heavy metal(loid)s. In this study, the effects of the combined applications of woody peat and Fe(NO) on As and Cd mobilities and accumulations in rice during the whole growth period were investigated by a pot experiment. The combined applications of woody peat and Fe(NO) significantly decreased As(III) and Cd in porewater due to pH increases induced by applications of Fe(NO), and these decreases were enhanced with increasing Fe(NO). In addition, simultaneous application of peat and Fe(NO) significantly decreased mobile portions of As and Cd but significantly increased their immobile portions. Increasing Fe(NO) increased the amount of As immobilized by poorly crystalline Fe oxides. The formation of Fe plaques and production of poorly crystalline Fe oxides were enhanced by Fe(NO) addition, which also contributed to the immobilization of As and Cd in soil. Overall, the combined applications of woody peat and Fe(NO) provided a strategy for simultaneously immobilizing As and Cd in soils and further alleviating their accumulations from soil to rice plants. In paddy soil, the frequent occurrence of iron redox activity due to the alternating wetting and drying cycles provided favorable conditions for interactions between Fe and OM, and this process and its associated metal(loid) immobilization may be more important than we thought and need further study.