Can fire alter the speciation and bioavailability of Cr and other PTEs in soils polluted by organic amendments?

One of the main causes of farmland pollution by chromium (Cr) and other potentially toxic elements (PTEs) is the use of tannery sludge-containing amendments. This agronomic practice generally is not considered environmentally hazardous since Cr occurs in these matrices as Cr(III), a stable and non-toxic form, additionally it is immobilized by organic matter (OM), which also prevents its oxidation to Cr(VI), a very dangerous form. These apparently safe conditions may become hazardous when extreme events, such as voluntary or accidental fires, happen. Fire, by reducing the soil OM stability, may favour the mobilization of OM-associated PTEs. In this study, we investigated the modifications induced by laboratory-simulated fires at 300, 400 and 500 °C on the distribution and speciation of Cr, Zn, Cu and Pb in a polluted agricultural soil. Such pollution resulted from the long-term amendment with tannery sludge-containing compost. A combination of conventional chemical analyses and X-ray analyses, including synchrotron X-ray analyses (XAS and µ-XANES), were performed to assess the PTEs distribution and speciation in soil. The effect of soil heating on PTEs bioavailability to durum wheat plants (Triticum durum Desf.) was also assessed using RHIZOtest bioassays. Soil heating significantly modified the soil chemical properties and enhanced Cr mobility, whereas no variation or even a further stabilization was observed for Pb, Cu and Zn. Increasing heating temperatures promoted a decrease of Cr-OM associations in favour of Cr-iron (hydr)oxides interactions and CaCrO4 formation. Chromium uptake by plants was considerably enhanced in heated soils, although only a minor fraction was translocated to shoots. Conversely, no risk of phyto-accumulation of Zn, Pb, and Cu occurred. Experimental evidences induced to hypothesize that Cr was taken up as Cr(VI). This study proved that fire can increase the mobility and bioavailability of Cr in organically amended soils, with dangerous consequences for crops and human health.