The mobility of trace elements in soils is often discussed because of consequences for their bioavailability and thus their potential ecological toxicity. Numerous studies showed that oxides and oxi-hydroxides could be an important sink for trace elements [1,2]. In many soils, trace elements tend to accumulate in oxides and oxi-hydroxides that act as scavengers [7,9,10]. Indeed, as a recent example, extraction for As speciation in contaminated soils showed that As originating from irrigation was partly retained by Feoxi- hydroxides [3]. Other experiments also showed that adding Fe-bearing compounds to contaminated soils reduces As mobility, but increases Pb and Cd mobility [8], thus indicating that further investigations are required to elucidate trace-element mobility in soils. However, soil oxides and oxi-hydroxides play a significant role in controlling trace element mobility and new conditions leading to dissolution of oxides and oxi-hydroxides should be carefully considered.
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