Evidence of Hexavalent Chromium Formation and Plant Uptake in Agricultural Soils after Simulated Fires

Controlled fires are widely used in many common agricultural practices to remove the vegetation residues and manage weeds and pests. High temperatures occurring during fire events can cause significant changes in soil physico-chemical properties and element biogeochemistry. Therefore, also potentially toxic elements (PTEs) can change their speciation and hence their bioavailability. In particular, chromium (Cr), depending on the soil redox conditions and the content and type of organic matter (OM), can modify its oxidation state thus forming highly mobile and toxic hexavalent species (Cr(VI)). In the present study, the effect of laboratory-simulated fires on the distribution, speciation, and plant uptake of Cr in highly polluted agricultural soils was investigated. Three thermal treatments (up to 300 ◦C, 400 ◦C, and 500 ◦C) were performed by means of a muffle furnace to simulate a fire event of medium severity. The soil chemical and mineralogicalcharacteristics were assessed before and after each thermal treatment, as well as the Cr fractionation and speciation. The latter were assessed through an integrated approach based on conventional analyses (determination of total Cr, total Cr(VI), and exchangeable Cr(VI), DTPA extraction, BCR sequential extractions), and X-ray based techniques, including synchrotron X-ray absorption spectroscopy (XAS). Heating treatments strongly altered the soil chemical characteristics, especially the OM content and Cr fractionation and availability. After heating, a partial oxidation of Cr(III) to Cr(VI) was observed, and from 21% to 88% of the total Cr(VI) was in the exchangeable form. A combination of bulk and micro X-ray absorption near edge spectroscopy (XANES) analyses allowed the identification of different Cr species in the polluted soil samples before and after the thermal treatments, showing an increase of chromite (FeCr2O4), Cr(III)goethite, and CaCrO4 with increasing temperature, and a decrease of Cr-OM complexes. The formation of Crgoethite and chromite caused a relative immobilization of Cr, while CaCrO4, being slightly soluble, could represent a potential risk for the environment with negative consequences for plants and human health. Indeed, rhizotest experiments with durum wheat plants confirmed an increased Cr accumulation in plant tissues. Cr concentration in roots increase from 34 mg kg−1 DW, in the unheated soil, to 467 and 825 mg kg−1 DW in 300 ◦C and 500 ◦C-heated soil, respectively. Cr was also detected in shoots of plants grown on 300 ◦C and 500 ◦C-heated soils, at concentrations of 26 and 51 mg kg−1 DW, respectively. The Cr accumulation in plants appeared to be related to the exchangeable Cr(VI) amount in soil. The overall results suggest that Cr was preferentially taken up by the plants as Cr(VI), as also supported by micro X-ray fluorescence analyses. Further experiments are needed to investigate Cr speciation inside the plant and Cr-uptake mechanisms underlying its plant acquisition.