In a lysimeter set-up, compost addition to an industrial contaminated soil slightly reduced phytotoxicity to bean seedlings. The “Phytotoxicity Index” (on a scale from 1 to 4) decreased from 3.5 to 2.8. The same treatment also reduced metal accumulation in grasses: mean Zn, Cd and Pb concentrations decreased respectively from 623 to 135, from 6.2 to 1.3 and from 10.7 to <6mgkg−1 dry weight. When combined with inorganic metal immobilizing amendments, compost had a beneficial effect on plant responses additional to the inorganic amendments alone. Best results were obtained when using compost (C)+cyclonic ashes (CA)+steel shots (SS). The “Phytotoxicity Index” decreased to 1.7, highest diversity of spontaneously colonizing plants occurred, and metal accumulation in grasses reduced to values for uncontaminated soils. Based on the first year evaluation, C+CA+SS showed to be an efficient treatment for amendment assisted phytostabilization of the contaminated Overpelt soil.

A lysimeter approach (under natural climatologic conditions) was used to evaluate the effect of four metal immobilizing soil treatments [compost (C), compost+cyclonic ashes (C+CA), compost+cyclonic ashes+steel shots (C+CA+SS)) and cyclonic ashes+steel shots (CA+SS)] on metal leaching through an industrially contaminated soil. All treatments decreased Zn and Cd leaching. Strongest reductions occurred after CA+SS and C+CA+SS treatments (Zn: -99.0% and -99.2% respectively; Cd: -97.2% and -98.3% respectively). Copper and Pb leaching increased after C (17 and >30 times for Cu and Pb respectively) and C+CA treatment (4.4 and >3.7 times for Cu and Pb respectively). C+CA+SS or CA+SS addition did not increase Cu leaching; the effect on Pb leaching was not completely clear. Our results demonstrate that attention should be paid to Cu and Pb leaching when organic matter additions are considered for phytostabilization of metal contaminated soils.

In order to evaluate the possible use of hydroxyapatite (HA) as a soil additive for the in situ remediation of metal contaminated soils, the immobilizing capacity of this product was investigated. Three different concentrations of HA (0.5%, 1%, and 5% by weight (w/w)) were applied to a metal (Zn, Pb, Cu, Cd) and As contaminated soil originating from an old zinc smelter site in Belgium. After a three weeks equilibration period, exchangeable metal concentrations of the soils were determined using 0.1 M Ca(NO3)2 extraction. Test plants (Zea Mays cv. Volga and Phaseolus vulgaris cv. Limburgse vroege) were grown on all soils. Growth parameters were determined and mineral analysis (Cu, Zn, Pb, Cd, Ni, Mn, Mg, Ca, K, As and P) of plants was performed. Exchangeable metal contents in soil decreased with increasing HA application. Plant growth was partly restored on the 0.5% and 1% HA treated soils. However, at the 5% HA application rate growth was inhibited again. Plant mineral analysis showed that concentrations of `toxic' metals in the leaves of the test plants decreased after HA application. However, the uptake of essential trace elements also decreased and probably led to Mn-deficiency in maize. In bean, addition of 0.5% and 1% HA resulted in a gradual decrease of metal uptake. At the 5% application level an increase of Zn, Cu, and Ni uptake was observed compared to the 0.5% and 1% application rate. In contrast to metal uptake, As uptake was found to increase after HA treatment. The increased PO42− concentration in the soil may be responsible for this. These results illustrate that HA application for the remediation of metal contaminated soils can be effective, but is not self evident. Strong immobilization of essential nutrients may lead to deficiency problems and mobilization of As may lead to an increased transfer to plants and animals and to an increased percolation of this element to the ground water.

INRA, Bordeaux Aquitaine Res Ctr, Agron Unit, F-33883 Villenave Dornon, France. Limburgs Univ Ctr, Dept SBG, B-3590 Diepenbeek, Belgium. Univ Kuopio, Dept Ecol & Environm Sci, FIN-70211 Kuopio, Finland. Univ Tras os Montes & Alto Douro, P-5001 Villa Real, Portugal.Boisson, J, INRA, Bordeaux Aquitaine Res Ctr, Agron Unit, BP 81, F-33883 Villenave Dornon, France.