Acidic leachate

Median leachate water chemistry during periods of sulfide oxidation was calculated for each parameter analyzed, by humidity cell. These data, for parameters that show distinct variation between cells, are presented as Table 1. HC2-SQFP, which contains more pyrite and very low carbonate content, rapidly generates acidic leachate causing its leachate to be higher in all parameters except Mo. 

Apart from salinity of acidic leachates at Shilhottle Grange, no problems w ilh nnn-acidic salinity in this coalfield... 

Mining wastes and overburden. Estonian kukersite oil shales occur in limestone strata (Schmidt 1858, 1881), yielding a carbonate-rich spoil with significant potential for acid neutralisation. Other oil shales, for example, the siliceous Estonian Dictyo-nema shales, which contain only traces of carbonate compared with the kukersites, are associated with sulphides, such as pyrite, and may thus generate acidic leachate due to pyrite oxidation (Puura Pihlak 1998 Puura et al. 1999). 

Fig. 1. Schematic diagram showing major pathways, reactions and stable weathering products for CCB alteration. Heavy solid arrow indicates changes caused by progressive natural weathering of alkaline CCBs at high L/S (liquid/ solid) ratios. Heavy dashed arrow indicates changes caused by progressive alteration of CCBs by acidic leachate solutions at low L/S ratios.

The ability of mineralized soil to control the migration of aluminum was observed in another study. Acidic leachate from coal waste containing aluminum was percolated through soil containing varying amounts of calcium carbonate (Wangen and Jones 1984). Soluble aluminum was found to decrease dramatically as the pH of the percolating leachate increased and aluminum oxide precipitates formed at pH 6, no dissolved aluminum was measured. The authors concluded that alkalinized carbonaceous soils provide the best control material for acidic leachates from coal mineral wastes. 

Wangen LE, Jones MM. 1984. The attenuation of chemical elements in acidic leachates from coal mineral wastes by soils. Environ Geol Water Sci 6 161-170. 

Pettersson, C., Hakansson, K., Karlsson, S. and Allard, B. (1993) Metal speciation in a humic surface water system polluted by acidic leachates from a mine deposit in Sweden. Water Res., 27, 863-871. 

It can be seen from Table II that the fly ashes produced in the A boiler essentially form acidic leachates even though the major element composition of these fly ashes were comparable to the amounts found in the boiler C and boiler D fly ashes (see Table III). The Militant, Deep Hollow, Upshur and Badger fly ashes produced in this boiler encountered flame and boiler temperatures that are below even the initial deformation temperatures of the fly ash. Hence, the fusion reactions probably did not occur that might have led to formation of products that generate an alkaline leachate. 

Acid leaching treatments of subsamples were used to establish whether the carbonates held any Cs activity and how easily the Cs activity could be removed from the silicate fraction (Fig 5). The results showed that the biogenic carbonates and the acid leachate had virtually no Cs activity the silicates are therefore the candidate phases that hold the radio-caesium strongly. 

Alkalinity releases from the solidified wastes controlled the final leachate pH by neutralizing the acetic acid leachate. The pH of the acetic acid eluant before addition of the solidified samples was 3.4. In the first 3 sequential extractions, all the acid was neutralized the final pH was above 10.0 in all of these samples. During the next 3 extractions the pH dropped from above 10.0 to below 6.0 in all samples. The pH of the subsequent 9 extractions decreased in a uniform manner to 4.0. The alkalinity releases also decreased with each extraction. 

G. Bartzas, K. Komnitsas, and I. Paspaliaris, Laboratory Evaluation of Ee Barriers to Treat Acidic Leachates, Minerals Engineering, 19(5), 505-514 (2006). 

The pH of the ash leachate is one parameter that controls the migration potential of the elements leached from the ash. Fig. 5 shows the variation in pH that exists between the various leachates collected. Due to the high concentrations of Ca and Mg in most ashes, the majority of leachates are alkaline. Notable exceptions are the slightly acidic leachate of the EKY-FA sample and the acidic leachate of the NEMO-FA and SI-BA samples. These acidic leachates will allow metals leached from the ash to remain in solution, increasing the mobility of these elements. This mobility contributes greatly to the potential for contamination of groundwater aquifers by such solutions. 

Of the nine coal ash samples subjected to leaching, one bottom ash (SI-BA) and one fly ash (NEMO-FA) were found to produce a leachate that would be capable of degrading groundwater quality. Both these ashes produced acidic leachates, which would enable metals to remain in solution and migrate from the disposal site. 

Table IV. Total Acid Leachate for Stone Samples...

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