Fine chemical speciation and environmental impact capacity of trace elements with different enrichment levels in coal

Abstract

Enriched trace elements in coal are considered to have a high environmental impact, but the extent of the influence of the enrichment level is unclear. To study the chemical speciation and environmental behavior of trace elements in coal at different enrichment levels, representative coal samples from multiple provinces in China were collected, including bituminous coal I-L2 from Inner Mongolia with high concentrations of Be, Y, Zn, Tl, U, Er, and Yb, and 72-9 coal from Anhui enriched with Cu, Cd, Pb, V, and Zn. The chemical speciation of trace elements in coal was analyzed using a variety of techniques, including X-ray Photoelectron Spectroscopy (XPS), Near Edge X-ray Absorption Fine Structure (NEXAFS), and sequential chemical extraction procedures. Cluster analysis was used for grouping the coal samples based on the enrichment coefficients of trace elements. Coal samples with similar genesis and in closer regions were more likely to be grouped. Metal carbonates and metal sulfate were observed in coals through XPS analysis. The main C species in coal were identified as phenolic C, carboxylic C, unsaturated C, and O-alkyl C/carbonyl C through NEXAFS. The amplitude variation of peaks for the fly ash was smaller than that for the feed coal, which showed that the structure of carbon became homogeneous after high-temperature combustion. It was difficult to identify the chemical speciation difference of trace elements with different enrichment degrees in coals through XPS and NEXAFS, but the results of the sequential chemical extraction could compensate. Several enriched trace elements in coal were relatively high in the chemical fractions (exchangeable, carbonates and monosulfides associated, or FeMn oxide bound) that were easy to extract and relatively low in the less insoluble chemical fractions (organic matter-bound, disulfides associated, or silicates associated), indicating that enriched trace elements in coal had higher environmental impact capacity.