Environmental Engineering Reference
In-Depth Information
Measurement of pH
Sub-bituminous and lignite coal ashes typically contain relatively high concentra-
tions of calcium, with concentrations exceeding 15 % (expressed as CaO), and pro-
duce alkaline solutions (pH 11-12) on contact with water. Bituminous coal ashes
generally contain much less calcium and yield slightly acidic to slightly alkaline
solutions (pH 5-10) on contact with water (Izquierdo
2012
). To calculate pH 5 g of
sample was taken and was mixed with 5 ml of distilled water and then stirred for
10 s and allowed to stand for 5 min; then pH was measured using pHmeter.
Batch Leaching Test
The leaching test at liquid to solid (L/S) ratio 1:20 was carried out. Leaching was
performed at the natural pH of the samples (i.e. about pH 7-8.1), for 17 h (ASTM
1992
). After the extraction, the extracts were separated from the solid residue by
fi ltration through Whatman fi lter paper no. 42 followed by glass fi bre fi lter of
0.7
m. Few drops of 1 N nitric acid were added and extracts were stored in a refrig-
erator (4 °C) until the element determination. Leachate was analysed using AAS,
Flame Photometry and other methods.
μ
Leachate Analysis
The pH of extracted leachate was determined using pHmeter. Various elements like
Fe, Pb, Cu, Ni, Cd and Cr were analysed by using AAS to understand the leaching
potential of the coal ash samples. Various salts like potassium, calcium and sodium
salts were determined using fl ame photometry. Phosphate and sulphate ions were
determined using molybdate method and turbidity method respectively.
Result and Discussion
Coal Ash Characterization
The major element composition of the respective coal ashes are shown in Table
2
.
The result indicates that samples contain mostly SiO
2
, Al
2
O
3
, Fe
2
O
3
, CaO and
MgO. Silica oxide is the major content found in all the samples i.e. 85-90 %. CaO
content varied from 0.6 to 1.3 % depending upon the parent coal. EDXA analysis
indicated the presence of the following elements: Ca, Mg, Al, Na, K, Ti, Fe, C, O
and Si in the coal ash samples (shown in Fig.
1
). Results show the high content of
oxygen which indicates presence of elements in their oxide form. SEM was used to
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