Flocculation of sewage sludge with FeCl3 modifies the bioavailability of potentially toxic elements when added to different soils

The effect of sewage sludge (SS) flocculation with FeCl3 on biological availability of Zn, Ni, Cd and Cu in oilseed rape (Brassica napus) plants grown on a acid and a neutral soil were studied during a greenhouse incubation experiment. The experiment included the following treatments: (i) unamended control soil, (ii) sewage sludge amended soils kept bare throughout the experiment and (iii) sewage sludge amended soil planted with oilseed rape. Amended soils received untreated or FeCl3 treated SS at a rate equivalent to 75 t d. w. ha-1 y-1. Plant growth was enhanced by addition of both untreated and FeCl3 treated SS, but sludge treated with FeCl3 produced a lower shoot dry matter than untreated SS, particularly in the acid soil. Evaluation of biological availability of potentially toxic elements (PTE) was performed by three different methods: plant shoot analysis, single DTPA-extraction and sequential fractionation procedure (SEP). After 50 days, accumulation of PTE was considerably larger in plant shoots grown on the acid soil and amended with FeCl3 treated sludge. Conversely, the FeCl3 treated SS reduced Cd, Ni and Zn accumulation in shoots in the neutral soil. Copper was generally unaffected by SS treatment in both soils. Plants decreased significantly the DTPA-extractability of Cd, Ni and Zn in both soils that received Fe-SS, but they did not change it in soils that received untreated SS. These different behaviours may be explained by the different mechanism involved in metals stabilization in the two soils: adsorption on Fe and Mn oxides in the neutral soil, precipitation and organic complexation in the acid soil. The chemical flocculation treatment of waste water with FeCl3 reduces PTE availability in neutral soils amended with SS, but markedly increases their solubility and bio-availability in acid soils. Therefore, the application of SS obtained from flocculation with FeCl3 should be restricted to neutral or calcareous soils.