Mechanisms Accompanying Chromium Release from Concrete

The importance of time and other determinants in the assessment of heavy metals release during solid waste management

One of the parameters affecting the leachability of heavy metals from waste is their contact time with the leachant. In this paper, the leaching behaviour of Zn, Cu, Pb and Ni was evaluated in relation to the liquid to solid ratio (L/S), which is a reflection of time after which a certain volume of water permeates the material, e.g. in slag heaps or landfills. A leaching study was carried out by different leaching methods with using three test materials, i.e. hazardous zinc slag, lump copper slag and mineral-organic composite. It was found that the highest amount of metals leached in the long term in the maximum availability test, under the following leaching conditions: L/S = 50 dm³/kg, reduced pH of the leachant, fragmentation of the materials to particle size < 0.125 mm. Comparing the results obtained in the batch test and the percolation test, no strict trend was observed in the release of a given metal from different test materials. The analysis using the tank test showed that processes controlling leachability can result in the release of the highest metal loads immediately after contact between the material and the leachant, but can also contribute to the release of metals only after prolonged contact.

The effects of soil organic matter on leaching of hexavalent chromium from concrete waste: Batch and column experiments

Concrete is one of the most common building materials in the world and in accordance with the world's shift to a circular economy, there is a need of an increase in concrete reuse and recycling. One of the environmental concerns linked to concrete recycling is the leaching and spread of hexavalent chromium (Cr(VI)). In the present study the Cr(VI) leaching from crushed concrete waste and the effects of soil organic matter (SOM) on chromium (Cr) speciation has been investigated in realistic reuse scenarios by the means of batch shale tests and layered column tests. The effects of concrete properties (pH, grain size and age) on Cr(VI) leaching was also studied. Cr leaching from concrete alone is mainly in the form of Cr(VI), with the pH of the leachate being >10. The smaller the grainsize of the concrete, the higher the Cr(VI) concentration is in the leachate. There was no correlation between the age of the concrete and concrete leaching. When exposed to SOM the Total-Cr concentration in the leachate was reduced. The reduction increased with higher TOC level, with a 99% reduction at very high TOC (25%). The results indicate that Cr(VI) leaching from recycled concrete waste can be mitigated by exposing it to SOM in the desired recycling scenario.