Arsenic and heavy metal mobility in iron oxide-amended contaminated soils as evaluated by short- and long-term leaching tests

Several iron-bearing additives were evaluated for their effectiveness in the attenuation of arsenic (As) in various contaminated soils. These were selected for their known or potential ability to adsorb As anions, thus changing the speciation of As in a soil system. Three soils with different sources of As contamination were investigated (canal dredgings, coal fly ash deposits, and low-level alkali waste). The amendments used were goethite (alpha-FeOOH), iron grit, iron (II) and (III) sulphates (plus lime), and lime, applied to the soils at a rate of 1% w/w. A series of leachate extraction tests (UKEA, ASTM and modified Dutch column leaching test) were conducted on the equilibrated amended soils. These were used to firstly evaluate the potential of the amendments as immobilising agents, and secondly to compare the short- and long-term durability of their effects. Column tests demonstrated the efficiency of iron oxides over the longer time scale; these treatments significantly reduced concentrations of arsenic in leachates from all treated soils. Amended soils were also observed to contain higher levels of lead (Pb) and cadmium (Cd) in their leachates, signifying that certain Fe-oxides potentially increased heavy metal mobility in treated soils. The conclusions were that whilst Fe-oxides may be used as effective in situ amendments to attenuate As in soils, their effects on other trace elements, such as Pb and Cd, require careful consideration.