Mobility of trace elements from coal and combustion wastes

Abstract

The mobility of > 30 trace elements from four Spanish coals was investigated using sequential extraction procedures. The total leachable fractions were obtained by comparing the total trace element content in the bulk coal samples with the extractable content. The high-temperature ash of these coals, and the fly ash and slag produced during the combustion of one of the coals in a large power plant, were also investigated to determine the trace element mobilities in the combustion wastes. The results demonstrate that the elements associated with sulfides, sulfates and organic matter in coal show the highest extraction rates (As, B, Be, Mn, Mo, Ni, Pb, Se, Sr, U, V, Y and Zn), while elements with aluminosilicate affinities have lower mobilities. Many elements with organic affinity have a water-leachable fraction. Trace element mobilities in high-temperature ash and in fly ash are very similar. Bearing in mind the conditions used for obtaining the combustion wastes and the behaviour of the trace elements during the sequential extraction, it can be deduced that the most labile elements are associated possibly with oxides (Ge, Mn, Mo, Se, V, W, and Zn) and salts such as sulfates, borates and arsenates. The most refractory phases contain the elements with the lowest extraction rates (Cr, Ga, Rb, Sb, Sn, Th, Zr and REE). Important fractions of the Mo, Sr and V contents are water-extractable and readily exchangeable. The results demonstrate that trace element mobility in coal and combustion wastes depends on the mode of occurrence of these elements and on their behaviour during coal combustion.