Evidence of hexavalent chromium formation and changes of Cr speciation after laboratory-simulated fires of composted tannery sludges long-term amended agricultural soils

Controlled or accidental fires can impact agricultural soils amended with composted organic materials since high temperatures cause fast organic matter (OM) mineralization and soil properties modifications. During these events, potentially toxic elements (PTEs) associated with OM can be released and change their distribution and speciation thus becoming a threat to the environment and to crops. In this study, we investigated the changes of distribution and speciation of chromium in soils long-term amended with compost obtained from tannery sludges, after simulating fires of different intensity (300, 400 and 500 degrees C) likely to occur on agricultural soils. A combination of conventional soil chemical analyses and bulk and (sub)micro X-ray analyses allowed the observation of the formation of hexavalent chromium and changes of chromium speciation. Specifically, a strong decrease of Cr-OM associations was found with increasing temperature in favour of Cr-iron (hydr)oxides interactions and CaCrO4 formation. These data provide first evidence that fires can transform OM-stabilized Cr into more mobile, available and toxic Cr-forms potentially accessible for plant uptake, thus posing a risk for the food chain and the environment.