60    REMOVAL OF CADMIUM AND CHROMIUM FROM
WASTEWATER USING FLY ASH
T. Viraraghavan, Professor
Faculty of Engineering
Univeristy of Regina
Regina, Saskatchewan
Canada S4S OA2
Ganesh A. K. Rao, Engineer
Tata Chemicals Limited
Bombay, India
INTRODUCTION
The heavy metals discharged in the wastewaters can be toxic to aquatic life and cause natural waters
to be unsuitable as potable water sources. The ubiquitous nature of heavy metals, their extreme
toxicity even in trace quantities, their tendency for bioaccumulation in the food chain, and the stricter
environmental regulations related to heavy metal discharges make it necessary to develop schemes for
the removal of heavy metals from wastewaters.
In spite of the effectiveness of activated carbon in the removal of heavy metals from wastewaters,
the high cost of activated carbon has restricted its more widespread use. Since adsorption is one of the
more effective methods for removing heavy metals from wastewaters, an inexpensive and more easily
available adsorbent would make the removal of heavy metals from wastewaters by adsorption an
economically viable alternative. It has been demonstrated that inexpensive adsorbents such as tree
bark, discarded automotive tyres, human hair, sawdust, corncobs, peanut hulls, straw, coffee
grounds, coconut shells, and rice husks are effective in removing heavy metals from wastewaters. The
abundance of fly ash and its easy availability makes it an economical adsorbent for removing heavy
metals from wastewaters.
Fly ash is defined as the finely divided residue that results from the combustion of ground coal. Fly
ash is a particulate substance ranging from 0.5 to 100 microns in size and composed primarily of silica
(Si02) and alumina (A1203). Other macroelements present to a greater or lesser degree include carbon,
calcium (as lime or gypsum), magnesium, sulfur (as SO42 and SO32), sodium and potassium.
The use of fly ash in wastewater treatment has been studied in recent years and experimental results
have shown fly ash to be a suitable adsorbent for the removal of heavy metals from wastewaters.
Gangoli et al.2 examined the suitability of fly ash as an adsorbent for the removal of chromium from
aqueous solutions. They concluded that the removal of hexavalent chromium using fly ash involved a
chemisorption mechanism associated with the bonding between active alumina sites and the chromate
anion. The experimental evidence from their study indicated that chromate ions are partially removed
from aqueous solutions by fly ash. The removal of hexavalent chromium by adsorption on fly ash was
also studied by Grover and Narayanaswamy.3 They concluded that fly ash with a fairly good adsorbent for the removal of hexavalent chromium from aqueous solutions and that the adsorption was
probably due to pore diffusion. Yadava et al.4 investigated the removal of cadmium by fly ash at
different conditions by varying the contact time, temperature and pH. Their results showed that an
increase in temperature from 20°C to 40°C decreased the adsorption of cadmium on fly ash from
96.07% to 83.78%. The removal of cadmium by fly ash increased from 11.85% to 89.82% on
increasing the pH of the cadmium solution from 4.0 to 8.5.
This study was therefore initiated to evaluate the effectiveness of locally available fly ash in the
removal of cadmium and chromium from municipal wastewater. Laboratory batch kinetic and isotherm studies were conducted to evaluate the adsorption capacity of fly ash in respect of cadmium and
chromium in wastewater. The effects of contact time, pH and temperature on the adsorption capacity
of fly ash were also investigated.
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118
Printed in U.S.A.
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