Short-term and long-term leaching of arsenic from wastes of arsenic removal systems

Abstract

Arsenic contamination of groundwater in the alluvial aquifer underlying Bangladesh has been recognized as a major problem of catastrophic proportions. Millions of people are exposed to high levels of arsenic from drinking tubewell water and thousands are suffering from arsenic poisoning but little is known about the fate and transport of arsenic in the environment. In Bangladesh, a number of arsenic removal technologies (both for household and community use) have been developed for getting arsenic-free water. Majority of these technologies are based on either coagulation-coprecipitation (e.g., using alum and ferric chloride) or adsorptive filtration (e.g., using activated alumina, iron coated sand, granular ferric hydroxide, etc.) techniques. Besides, some devices use ion exchange and membrane filtration techniques. All the arsenic removal units generate some form of arsenic-rich wastes, primarily in the form of slurry containing coagulated flocs of alum or iron salt (from coagulation based units), or spent adsorption/ion-exchange media (from filtration based units). In the absence of any clear guidance for safe disposal of wastes, indiscriminate disposal of these spent media or sludge is likely to contaminate the surface water and groundwater sources, as a result of leaching of arsenic from these wastes. In this study short-term leaching characteristics of arsenic-rich wastes have been evaluated through Toxicity Characteristic Leaching Procedure (TCLP). For majority of the slurry samples, leaching of arsenic expressed as percent of initial arsenic present is found to be negligible. The arsenic content in the TCLP extract for slurry waste samples varied from 0.01 I to 1.255 mg/I which is within the range of acceptable limit. In the case of waste samples from spent media, leaching of arsenic expressed as percent of initial arsenic present was also found to be very low. The arsenic content in the TCLP extract for spent media samples varied from 0.172 to 0.654 mg/kg of spent media. The TCLP results confirm that the slurry waste samples and solid waste samples from different arsenic removal units are not "hazardous" as defined by the United States Environmental Protection Agency (USEPA). Column experiments were carried out with spent media samples from "Shapla filters" in order to evaluate the long-term leaching characteristics of the waste. For column experiment, the initial arsenic concentration of the spent filter media was 38.8 mg/kg, and at the end of the experiment, 35.3 % of arsenic was found to have leached out with distilled water; leaching was 28.1 %, 27.4 %, 25.4 %, and 22.7 % with groundwater, rainwater, pond water and extraction fluid # 1 (TCL? extraction fluid), respectively. About 1564 bed volume of distilled water, 1750 bed volume of groundwater, 1658 bed volume of rainwater, 1720 bed volume of pond water and 1929 bed volume of extraction fluid# 1 was passed through the respective columns before the arsenic concentration in the leachate reached below the method detection limit (MOL). Distilled water showed highest leaching, followed by groundwater, rainwater, pond water and finally the extraction fluid # 1. Thus, long-term leaching is much greater than the short-term leaching. Short-term leaching was found to be less than 2% of initial arsenic content, whereas long-term leaching (column leaching) was found to be between 22.7 % and 35.3 %. It takes a maximum of four and a half months for the surface bound arsenic to leach out to a level below the MOL. Extensive research work should be undertaken in order to investigate the mobility of arsenic. It is essential to create awareness among the users of different arsenic removal units about the disposal of waste materials and sludge generated from the units. Potential management options for waste disposal must also be considered prior to distribution of the treatment systems. The protocol for management of sludge generation from the arsenic removal units is currently being developed. It is expected that this study will help the process of protocol development.