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

Abstract

Land application of sewage sludge (SS) is a convenient way of disposal compared to incineration or landfill, but can result in accumulation of potentially toxic elements (PTE) in agricultural soils. The environmental risk associated with PTE accumulation in soils that received several SS applications depends mainly on soil pH and SS treatments. We studied the effects of addition to soil of SS treated and untreated with FeCl3 on the biological availability of Zn, Ni, Cd and Cu to oilseed rape (Brassica napus). The plants were grown on an acid and a neutral soil in a greenhouse incubation experiment. The following treatments were carried out: (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 either untreated SS or FeCl3 treated SS at a rate equivalent to 75td.w.ha−1y−1.Plant growth was enhanced by addition of both untreated SS and FeCl3 treated SS, but the latter produced a lower shoot dry matter than untreated SS, particularly in the acid soil. Evaluation of biological availability of PTE was performed by three different methods: plant shoot analysis, single DTPA-extraction, and sequential fractionation procedure (SEP) in soil. 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 in comparison to untreated SS. Copper was generally unaffected by SS treatment in both soils. Plants significantly decreased the DTPA-extractability of Cd, Ni and Zn in both soils that received FeCl3 treated SS, but they did not change it in soils that received untreated SS.These different behaviors 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 and addition to soils 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.