The review presented covers: (a) historical introduction; (b) some analytical comments; (c) some peculiarities of the As geochemistry in environment; (d) an estimation of coal Clarke value of As; (e) some coals enriched in As; (f) mode of As occurrence in coal; (g) factors influencing the As distribution in coal matter and coal bed; (h) genetic topics; (i) some topics related to environmental impact of As by the coal combustion.
The World average As content in coals (coal Clarke of As) for the bituminous coals and lignites are, respectively, 9.0F0.8 and 7.4F1.4 ppm. On an ash basis, these contents are higher: 50F5 and 49F8 ppm, respectively. Therefore, As is a very coalphile element: it has strong affinity to coal matter -organic and (or) inorganic but obligatory authigenic. The coalphile affinity of As is like that for Ge or S.
There is strong regional variability of As distribution due to geologic variability of the individual coal basins. For example, bituminous coals in Eastern Germany, Czech Republic and SE China are enriched in As, whereas the coals in South Africa or Australia are very depleted compared to coal Clarke of As. In general, some relationship exists between As content and its mode of occurrence in coals. Typically, at high As content, sulphide sites dominate (pyrite and other more rare sulphides), whereas at low As content, As org dominates, both being authigenic. A contribution of the terrigenic As (in silicates) is usually minor and of the biogenic As bio (derived from coal-forming plants) is poorly known.
Both organic and inorganic As can exist not only as chemically bound form but also in the sorbed (acid leacheable) arsenate form. With increasing coal rank, sorbed exchangeable arsenate content decreases, with a minimum in the coking coals (German data: the Ruhr coals).
Relations of As content in coal to ash yield (or its partitioning in sink-float fractions) and to coal petrographic composition are usually complicated. In most cases, these relations are controlled by main site (form) of As -As pyr or As org . If As pyr dominates, an As accumulation in heavy fractions (or in high-ash coals) is observed, and if As org dominates, it is enriched in medium-density fractions (or low-and medium-ash coals). Arsenic is in part accumulated in the inertinite vs. vitrinite (As org ?).
There are four genetic types of As accumulation on coal: two epigenetic and two syngenetic: (1) Chinese typehydrothermal As enrichment, sometimes similar to known Carlin type of As-bearing telethermal gold deposits; (2) Dakota type-hypergene enrichment from ground waters draining As-bearing tufa host rocks; (3) Bulgarian type-As enrichment resulting from As-bearing waters entered coal-forming peat bogs from sulphide deposit aureoles; (4) Turkish type-volcanic input of As in coal-forming peat bog as exhalations, brines and volcanic ash.