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Show Where As(m) is mobile, but non-volatile arsenic, As(g) is volatile arsenic, A~03(S) is arsenic trioxide in the solid phase, A~03(g) is arsenic trioxide in the gas phase, AsX(m) is an arsenic compound other than arsenic trioxide in a mobile state and AsX(g) is a vapor phase arsenic compound other than arsenic trioxide; where reaction pairs #14 and #15, and #16 and #17 are among the competing reaction sequences to retain arsenic in non-volatile compounds within the solid matrix; and where reaction # 19 consumes some portion of the arsenic trioxide produced by either reaction # 18 or, more likely, from vaporization of arsenic trioxide in the feed. Of these reaction sequences, reactions 14 - 17 are considered -to be the dominant mechanisms associated with the Tacoma Steam Plant #2 fluidized bed system. 5.0. CONCLUSIONS In conclusion, the arsenic contained in the flyash at Tacoma Steam Plant #2 probably came from a single fuel supplier, no longer doing business with Tacoma Public Utilities. Additional sources of arsenic may have been a few random loads of demolition wood. The coal and RDF do not appear to be significant sources of arsenic at the plant. Once the arsenic entered the system it is quite probable that, at the temperatures in the fluidized bed system, the evolution of arsenic from the bed region as a volatile product was less than 50 percent, and could have been as low as 10 percent This governs certain mechanisms associated with arsenic behavior. The arsenic products in the flyash as quantified by ESCA technique were predominantly in the As+ 3 oxidation state, but they had binding energies sufficiently low to conclude that they are not A~03. Although no explicit compound identification was done, it is expected that the arsenic compounds formed were calcium arsenates and calcium-alumina-silica-arsenic complexes. The formation of large polymers and complexes is consistant with the research of Ho and coworkers (1990, 1991), Litt and Tewksbury (1989), Waterland (1991), and other researchers. Further, it is consistant with the TCLP results reported in Table 1. Mechanisms exist which readily document the probability of such reactions. These mechanisms are consistant with experience in glass making and in cement kiln operations. These mechanisms are considered to be the most dominant in the Tacoma fluidized bed system. The complete oxidation of arsenic to arsenic pentoxide is also possible, and there is some suggestion that this occurs to a limited extent. The evidence for this reaction sequence comes largely from arsenic ore roasting. On the basis of this research, we can only conclude that significantly less than half of the arsenic in the flyash at Tacoma Steam Plant #2 probably would be in the form of arsenic trioxide. Measurements of binding energy by ESCA technique never identified A~03 as the product seen. Evaluation of other literature confmns that the most probable products are large polymers and complexes. |