Including sulfate and arsenate

1
Equivalent Balance for Major Cations and Anions
in Leachate for Column A
Molar Balance for Leaching of Calcium in Column
and Batch Sequential Leaching based on Calcium
Compounds in the Ash
Leaching of Arsenic per ml of Leachate after 14,
42 and 56 days
Arsenic in Alkaline Coal Fly Ash Column
Leaching versus Batch Sequential Leaching
Arsenic leached with Time
Student Pratibha Naithani Advisor Prof. Amid
Khodadoust Department of Civil and Materials
Engineering University of Illinois at Chicago

• Research Objectives
• Determine the leaching of arsenic from alkaline
  coal fly ash using column leaching.
• Compare the continuous column leaching with batch
  sequential leaching.
• Determine the effect of column leaching
  characteristics with depth for one particular
  depth, and then apply the results for other
  depths in the same location in the ash
  impoundment.

Leaching of Arsenic and Calcium for PZ8-3
Location with Depth
Cumulative Arsenic Removal
Sample As leached Ca leached
  (mg/kg) (C/Co) (mg/kg) (C/Co)
PZ 8-3 (5-9') 1.96 0.018 182.94 0.034
PZ 8-3 (15-19') 5.49 0.051 285.34 0.053
PZ 8-3 (24-32') 12 0.111 316.95 0.059
PZ 8-3 (30.5-34.5') 19.18 0.177 350 0.065
Properties of Ash Samples for PZ8-3 Location

1. Including sulfate and arsenate
2. Without sulfate and arsenate

Major sources of calcium in ash are CaO, CaSO4,
CaCO3 and Ca3(AsO4)2.
Conclusions
Leaching Parameters for Location
PZ8-3(24-32)Comparison of Column and Batch
Sequential Leaching
CaO, CaSO4, CaCO3 and Ca3(AsO4)2.
CaSO4 Ca2 SO42- CaCO3 Ca2
CO32- CaCO3 H2O Ca 2 HCO3-
OH- Ca3(AsO4)2 3Ca2 2AsO43- Ca3(AsO4)2
2H2O 3Ca2 2HAsO42- 2OH- CaO H2O
Ca2 2OH-
- Column leaching of arsenic is less volume
intensive than sequential batch leaching. -
Tessier extraction shows that most of the arsenic
is either in iron oxide
phase or adsorbed in silicon oxide phase, hard to
leach. - Equivalent plots are based on electcal
neutrality. The ratio of cations and anions
is nearly 1, indicating that major cations and
anions leaching from ash are Ca2, CO32-,
HCO3-, SO42- and HAsO42-. - Long term leaching of
arsenic is limited by leaching of calcium
bicarbonate. - Arsenic leaches from exchangeable
and carbonate phases of the ash. - Calcium
leaches mostly as calcium sulfate and calcium
bicarbonate. - Arsenic leaches mostly as calcium
hydrogen arsenate. - Leaching of arsenic and
calcium increases with depth. - Most of the
arsenic leaches from silt fraction of ash. -
Column leaching results can be applied for other
locations in the ash impoundment.
Solid Phase Analysis of Ash Samples from PZ8-3
Location
Parameters Column A Column B Batch Sequential
As leached (mg/kg) 10.33 12.28 12
As leached (C/Co) 0.0955 0.1135 0.1094
Ca leached (mg/kg) 220.8 249.5 316.9
Ca leached (C/Co) 0.0408 0.0461 0.0585
As / Ca leached 0.0468 0.0492 0.0379
As leached / Silt 0.1806 0.2147 0.2098
As leached / IC 6.887 8.187 8
Ca leached / IC 147.2 166.3 211.3
PZ8-3 PZ8-3 PZ8-3 PZ8-3
Depth (feet) 5-9 15-19 24-32 30.5-34.5
As (mg/kg) 50.7 48 108.19 261
Fe (mg/kg) 28000 25900 38638.26 30400
Ca (mg/kg) 1220 2440 5409.36 5100
Mn (mg/kg) 36.2 35.8 85 78.6
B(mg/kg) ND ND ND 24
Compound Ksp _at_ 25o C
CaSO4 4.9310-5
Ca(OH)2 5.0210-6
CaCO3 3.3610-9
Ca3(AsO4)2 6.810-19