Are soil amendments able to restore arsenic-contaminated alkaline soils?

International audienceThis study aims to assess the effectiveness of amendments in reducing the mobility and bioavailability of arsenic (As) in alkaline soils compared with acidic soils.An alkaline soil highly polluted with As was amended with marble sludge (limestone), compost (organic matter) and iron oxides in six different combinations. The soils were watered to field capacity and placed in plastic pots, and lettuce (Lactuca sativa L.) bioassays were subsequently carried out. Leachates and pore water were collected, and the pH, Eh, electrical conductivity and As concentrations of both solutions were measured. The As concentration in the leaves and roots of the lettuce was measured, the root and leaf dry weight indices were estimated and sequential As extraction was performed. The results obtained with the unamended and amended soils were compared with those obtained with unpolluted soil.Iron oxide amendments were the most effective in reducing the mobility and bioavailability of As because these amendments significantly increased the As bound to hydrous oxides (non-bioavailable), decreased the As concentration in pore water and decreased the non-specifically and specifically sorbed As (bioavailable). Compost was less effective because it increased the concentrations of As in pore water and non-specifically sorbed As. Marble sludge, although it decreased the concentration of As in pore water and non-specifically sorbed As, was not effective because As bound to CaCO3 (specifically sorbed) was taken up by the lettuce. However, the amendments had an additive effect, and the use of a mixture of the three amendments resulted in the best lettuce development.Although the amendments were effective in reducing the concentration of mobile and bioavailable As, none of the amendment combinations were able to decrease the shoot As concentration to a level below the maximum concentration observed in lettuce growing in unpolluted soils. Thus, the restoration goals for these highly polluted soils should not include future food production