A long-term perspective on coal combustion solid waste interacting with urban soil

Abstract

Diverse wastes are produced during coal combustion and part of it is recycled worldwide in construction, agriculture, and other industries. As the waste becomes part of the environment, both intentionally and unintentionally, it is important to understand its long-term impact on surrounding soils. In this study, we characterized the situation where such waste, composed of mixed slag, fly ash, gangue, and coke material, was deposited 10–15 years ago as a layer within the urban soil. Two types of soil profiles were identified characterized by a different emplacement of the waste, which either occurred as a 10-cm thick layer or thin seams. Combined chemical and mineralogical analyses of the buried waste showed that slag fragments were affected by the dissolution of primary phases and precipitation of secondary carbonates, Fe-(hydr)oxides, and silica. The impact of the waste was detected in the underlying soils using ratios of trace elements enriched in the waste versus the uppermost soils (e.g., Ni/Pb, V/Zn, Ba/Pb) showing that it might be possible to recognize areas affected by similar deposited waste products. Combined leaching and mineralogical observations showed that V was released, but most probably as particles of unreactive V-rich sulfides, and Ba was preferentially leached from Ba-rich slag glass. Moreover, a combination of mineralogical observations, trace element ratios, and Pb isotopes showed that the thick layer of the waste may have acted not only as a source of potentially toxic elements but also as a trap (insulator) that stopped contamination coming from other sources (e.g., atmosphere).