Bulk composition and leaching tests on an environmentally dangerous production residue (KEU)

Abstract: KEU (Kraftanlagen Energie und Umwelttechnick) is a by-product of a pyrolyzed waste of sewage sludge produced by wastewater treatment of. This material is indeed processed after inertization, being mixed with quarry aggregates, to obtain a sintering granulate. It is likely the responsible of the anomalous concentrations of Cr and CrVI and other heavy metals recovered in several domestic wells distributed along the Firenze-PisaLeghorn motorway (Tuscany, central Italy) since KEU was used as roadbed. The same concerns were also recently evidenced a little more to the south, close to Arezzo, as in the local groundwater system high contents of heavy metals were determined and attributed to a different type of KEU, i.e. derived from the inertization of muds from refining industries. In this work, an extensive analytical work was carried out to characterize the mineralogical and chemical bulk composition of seven KEU-bearing samples collected from different cumulus stored in an aggregate crushing plant and one pre-treated KEU sample. Additionally, two distinct leaching tests were performed to evaluate the release of PTEs (Potentially Toxic Elements) by shaking 20 g of each sample in 200 mL of MilliQ and CO2 -saturated MilliQ water, respectively, the latter simulating the interaction between meteoric waters and KEU. After 1 day of leaching, the resulting suspension was centrifuged and the surnatant was analyzed for pH, electrical conductivity, main composition, and trace elements. The post-centrifugation residue was once again leached and shaken for 7 days to evidence whether the PTEs were still released after a relatively longterm leaching. The preliminary results showed that the MilliQ water leachates of the KEU-bearing materials are characterized by high pH values (up to 11.75), whereas the CO2 -saturated MilliQ water partly buffers the pH since it was in most cases <8. Our study indicates that, as expected, in most cases the CO2 -saturated MilliQ water is able to remove more heavy metals than those solubilized by MilliQ water. Moreover, the 1-day leachates resulted to be enriched in many heavy metals with concentrations, in most cases, from hundreds to thousands of mg/L. Despite a general decrease in the 7-days leachates, high contents of some heavy metals were still measured, suggesting that prolonged interaction between meteoric waters and the KEU-bearing material is anyway able to transfer PTEs to the groundwater systems. Another important aspect that was highlighted is that the investigated samples are chemically heterogeneous, indicating that the inertization process was not performed by using the same amount of quarry aggregates. Notwithstanding such an inertization, aimed at stabilizing unwanted toxic elements, its efficiency is rather scarce and, consequently, its use as by-product is strongly discouraged unless a more adequate inertization process is realized.