When environmentally friendly solutions are worse than the contaminated product: the KEU case study

KEU (Kraftanlagen Energie und Umwelttechnick) is a by-product of a pyrolyzed waste of sewage sludge produced by wastewater treatment from tannery industries. KEU is regarded as an industrial non-hazardous waste after inertization to obtain a sintering granulate and usually mixed with demolition-construction or industrial mud products to produce EoW (End of Waste) material. However, the resulting materials are apparently responsible of the anomalous concentrations of Cr and CrVI and other PTEs (Potentially Toxic Elements), recovered in several domestic wells distributed along the Siena-Empoli motorway (Tuscany, central Italy) since KEU was used as roadbed. Similar concerns were also evidenced in other areas, as in the local groundwater system, where the KEU industrial by-product is stored, anomalous contents of heavy metals were determined.

In this work, an extensive analytical work was conducted to characterize the mineralogical and chemical bulk composition of eight KEU-bearing samples collected from different cumulus stored in an aggregate crushing plant and one pyrolysis char (the KEU) sample. It is remarkable the presence of high Rare Earth Elements (REEs) concentrations. While pure KEU has total REEs of 14 mg/kg, the KEU-bearing materials are up 4300 mg/kg.

To verify whether the KEU-bearing samples were able to release PTEs, three leaching tests (after 1-hour, 1-day and 7-days) were performed by shaking 20 g in 200 mL of MilliQ and 20 g in 200 mL of CO₂-saturated MilliQ water, the latter simulating the interaction between meteoric waters and KEU-bearing materials. The resulting suspensions of all the aliquots were centrifuged and the surnatant was analyzed for pH, electrical conductivity, main composition, CrVI and trace elements. The 1-day post-centrifugation residue was leached and shaken for 7 days to evidence whether the PTEs were still released after a relatively long-term leaching. The analytical results showed that the MilliQ water leachates have high pH values (up to 11.75) whereas those obtained by CO₂-saturated MilliQ partly buffer the pH although moderately alkaline pH values were measured. Our study indicates that, as expected, in most cases the CO₂-saturated MilliQ water is able to more efficiently scavenge PTEs than those solubilized by MilliQ water. Moreover, the 1-day leachates resulted to be enriched in many PTEs with concentrations, in most cases, from hundreds to thousands microg/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 materials is able to transfer PTEs to the groundwater systems. The 1-hour and 1-day leachates showed relatively high concentrations of CrVI (from 20 to 1370 microg/L) while REEs were always approaching the detection limit or below it. Another important aspect is that the investigated samples are chemically heterogeneous, indicating that the inertization process was not performed by using the same amount of demolition and construction materials although the main composition was mostly Ca-SO₄. 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 applied.