3.3. Time-dependent distribution of Hg
In order to track the variation tendency of Hg in fly ash, sequential
samples were collected in chosen MSWI plant 8 and 11.
The Fig. 2. The percentage of Hg in each fraction of 15 fly ash samples (F1: watersoluble
Hg, F2: ‘human stomach acid’ soluble Hg, F3: organo-chelated Hg, F4:
strongly-complexed Hg and F5: mercuric-sulfide).
As shown in Fig. 3, the concentration of HgT in fly ash samples
from MSWI 8 ranged from 5 to 27 mg/kg, and 14 mg/kg on average.
The concentration of HgT in fly ash samples from MSWI 11 ranged
from 4 to 27 mg/kg, and 15 mg/kg on average. Approximately 60%
of the samples contained HgT from 9.2 to 18 mg/kg for MSWI 8,
corresponding from 9.5 to 22 mg/kg for MSWI 11.
As shown in Fig. 4, strongly-complexed Hg was the main speciation
in fly ash, accounting for 26–79% and 30–82% of the HgT for
MSWI 8 and MSWI 11, respectively. Approximately 60% of the
samples contained F4 fraction from 47% to 73% for MSWI 8, corresponding
from 45% to 71% for MSWI 11.
3.4. Leaching behavior of Hg
US EPA TCLP test was used in this study to predict possible
leaching of Hg in environment media. Fig. 5 shows the leaching
concentration of Hg in different fly ash samples by TCLP. As shown
in Fig. 5, leaching contents of Hg ranged from 0 to 170 lg/L, which
were lower than the USEPA TCLP regulatory level of 0.2 mg/L. This
indicates that the low leaching concentration of Hg in most cases
was contributed to the high percentage of F4 content. It is noting
that FA4 provided a relative high leaching concentration of Hg.
This is probably contributed to the lower percentage of F4 content
in the sample. In addition, the Hg leaching concentration was high
in FA7 and FA12 in spite of the low amount of HgT. This may be relative
to the Ca content in fly ash. Low Ca amount in FA7 and FA12
provided more lower final pH (4.2 and 4.5) in TCLP leaching solution
than that in other cases (pH 6.0–7.0). Such low pH may lead to
the conversion of Hg in F4 fraction to F1 or F2 fraction.
3.3. Time-dependent distribution of HgIn order to track the variation tendency of Hg in fly ash, sequentialsamples were collected in chosen MSWI plant 8 and 11. The Fig. 2. The percentage of Hg in each fraction of 15 fly ash samples (F1: watersolubleHg, F2: ‘human stomach acid’ soluble Hg, F3: organo-chelated Hg, F4:strongly-complexed Hg and F5: mercuric-sulfide).As shown in Fig. 3, the concentration of HgT in fly ash samplesfrom MSWI 8 ranged from 5 to 27 mg/kg, and 14 mg/kg on average.The concentration of HgT in fly ash samples from MSWI 11 rangedfrom 4 to 27 mg/kg, and 15 mg/kg on average. Approximately 60%of the samples contained HgT from 9.2 to 18 mg/kg for MSWI 8,corresponding from 9.5 to 22 mg/kg for MSWI 11.As shown in Fig. 4, strongly-complexed Hg was the main speciationin fly ash, accounting for 26–79% and 30–82% of the HgT forMSWI 8 and MSWI 11, respectively. Approximately 60% of thesamples contained F4 fraction from 47% to 73% for MSWI 8, correspondingfrom 45% to 71% for MSWI 11.3.4. Leaching behavior of HgUS EPA TCLP test was used in this study to predict possibleleaching of Hg in environment media. Fig. 5 shows the leachingconcentration of Hg in different fly ash samples by TCLP. As shownin Fig. 5, leaching contents of Hg ranged from 0 to 170 lg/L, whichwere lower than the USEPA TCLP regulatory level of 0.2 mg/L. Thisindicates that the low leaching concentration of Hg in most caseswas contributed to the high percentage of F4 content. It is notingthat FA4 provided a relative high leaching concentration of Hg.This is probably contributed to the lower percentage of F4 contentin the sample. In addition, the Hg leaching concentration was highin FA7 and FA12 in spite of the low amount of HgT. This may be relativeto the Ca content in fly ash. Low Ca amount in FA7 and FA12provided more lower final pH (4.2 and 4.5) in TCLP leaching solutionthan that in other cases (pH 6.0–7.0). Such low pH may lead tothe conversion of Hg in F4 fraction to F1 or F2 fraction.
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