LONG -TERM LEACHING OF ARSENIC
4.6 MASS BALANCE
After the completion of the column experiment i.e. when the arsenic concentration in the leachate coming out of the column after passing through the sample in the column was found to be below the method detection limit (MDL) indicating complete stabilization of the sample, a mass balance analysis was performed. Mass balance analysis was done mainly for two purposes, first, to see whether there was any variation of weight of sample after the completion of experiment, and second, to see whether there was any variation of leaching of arsenic from various depth of the column. For these purposes, the columns were divided into four segments along the depth of the column. The total depth of each column was 380 mm of which upper 75 mm was taken as the first segment and second, third and fourth segment consist of 75 mm, I 15 mm and I 15 mm, respectively along the column depth. The columns were broken segment-wise in order to collect the samples (filter media) from different segments. Care was taken during the collection of samples from the segments so that there was no loss of sample. The sample from each of the segments was weighed and was compared to the initial weight of the sample in the column. Table 4.3 shows depths, weights, arsenic content, arsenic leaching, etc. of the sample in different segments of column I. A total of 80 gm of sample was taken in each of the columns, but from Table 4.3, it is seen that the sum of the weights of samples in four segments of column I is 79.523 gm. Segment-wise depth, weight, arsenic concentrations, arsenic leaching, etc. of other columns are presented in Tables 5 to 8 in Appendix D, which show that the sum of the weights of samples in four segments of column 2, column 3, column 4 and column 5 stands to 79.264 gm, 78.915 gm, 78.994 gm and 79.134 gm, respectively. So, there was a little variation in the weight of sample. This
79
variation may be due to inaccuracy of the balance which was used for taking weight of the sample or due to loosing a small amount of sample somehow during the entire period of experiment.
The total arsenic content of the sample in different segments was determined in a procedure described in article 4.2.3. The data were compared to the initial arsenic concentration. For example, in the case of column I, the initial arsenic concentration of sample in all the segments was 38.80 mg/kg, but after the completion of the experiment the arsenic concentration was found to be 19.08, 24.25, 23.00, 19.20 mg/Kg in segment I, segment 2, segment 3 and segment 4, respectively.
Table 4.3: Segment wise depth, weight, arsenic concentration, arsenic leaching etc. of Column 1 (Fluid: Distilled Water)
Total As at
Calculated Segment Depth Weight As Cone. the end of
Initial As %As
column
No. (mm) (gm) (mg/kg)
experiment As (/-lg) Leached Leached
(U!!) (/-lg)
1 2 3 4 5 6 7 6.5 8
I 0-75 15.618 19.08 298
606 308 50.8
2 75-150 16.028 24.25 389 622 233 37.5
3 150-265 24.130 23.00 555 936 381 40.7
4 265-380 23.747 19.20 456 921 465 50.5
Total
-
79.523-
1698 3085 1387 45.0Arsenic concentration of four segments of other columns (column 2, column 3, column 4 and column 5) is shown in Tables 5 to 8 in Appendix D. It can be seen from Table 4.3 that 1698 Ilg of arsenic remained in the waste sample after the end of column experiment. So, the rest (1387 Ilg) of the arsenic is supposed to leach out with the flowing fluid (distilled water in this case) during the time of column experiment.
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60 50.6
50.5
40 50 30
Percent Leaching 10 20
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Figure 4.15: Percent leaching of arsenic from various segments of column 1 (Fluid: Distilled Water)
Calculated percent leaching of arsenic from various segments of column j is presented in Figure 4.15. It can be seen from Figure 4.15 (also from Table 4.3) that there was a little variation in leaching of arsenic from various depths of the column.
Leaching of arsenic from the first segment was 50.8 % and that from second, third and fourth segments were found to be 37.5 %, 40.7 % and 50.5 %, respectively.
However, the reason for leaching of arsenic from various segments in such a pattern is not clear. Calculated percent leaching of arsenic from various segments of other columns is shown in Figures 45 to 50 in Appendix C.
The arsenic content in the sample was also determined in order to perform a mass balance. A total of 3104 J.lgarsenic (80 gm of sample with concentration 38.8 J.lg/gm) was taken in the column. After the end of the column experiment i.e. when the arsenic concentration in leachate reached below the MOL, a total of 1097 J.lg(35.3 % of initial arsenic present) of arsenic, which was measured by determining the arsenic concentration of the leachate collected during the period of column experiment, leached out from column I.
Mass balance error 10.0%
Figure 4.16: Total amount of arsenic leached, arsenic remaining and mass balance error of arsenic in colnmn 1 (Fluid: Distilled Water).
From Figure 4.16, mass balance error for column 1, in which distilled water was allowed to pass, was about 10%. The Tables in Appendix D show that the cumulative leaching of arsenic from column 2, column 3, column 4 and column 5 are 871 j1g
(28.1 %),851 j1g (27.4 %), 787 j1g (25.4 %) and 726 j1g (22.7 %), respectively. The mass balance error for these columns are found to be 12.8 %, 16.6 %, 18.8 % and 18.9%, respectively (These are shown in Table 3 in Appendix D and also in Figures 51 to 55 in Appendix C).
4.7 AN OVERVIEW: SHORT-TERM AND LONG-TERM LEACHING _ A