Aqueous extraction of oxalate and other anions from coal

Abstract

A suite of six coals, including three of low-rank, were subjected to aqueous soxhlet extraction and the extract analysed for both cations and anions. In nearly all cases the extracts contained the anions fluoride, chloride and sulphate as well as the organic based anions acetate, carbonate, and oxalate. The oxalate anion was the most prevalent of the extracted organic anions. The extent of oxalate extraction appears to be dependent on coal rank, with substantially more oxalate extracted from the low-rank coals. This is reflected in the cation:anion ratio for the major inorganic ions extracted. The rate of oxalate extraction suggests most, if not all, of the oxalate extracted is due to oxidative breakdown of oxygen containing functional groups in the coals, rather than the intrinsic presence of oxalate itself. The extracts also contained some or all of the cations Na +, K +, Mg 2+, Ca 2+, Al 3+ and Fe 3+. In most cases only minor proportions of the total amounts of the inorganic elements present in the coals were extracted, with the extent of extraction depending on the total element content. For low-rank coals the mole ratio of total extracted cations to the sum of the chloride plus sulphate anions was greater than unity, consistent with the presence of carbon based anions in the extract.