Leaching of Arsenic from Wastes of Arsenic Removal Systems
Arsenic in groundwater was first detected in Bangladesh at Chapai Nawabgonj district in 1993. Since then arsenic contamination problem has been reported from almost every parts of the country. It is estimated that approximately 27% of the wells are contaminated with levels above 50ppb, the current drinking water standard for arsenic in Bangladesh (Kinninburg, D. G., et al., 2001). It is also estimated that about 77 million people are at risk of arsenic poisoning. A number of technologies are now available for end-of-the-pipe treatment arsenic present in groundwater. These are primarily based on coagulation and filtration. Some of the technologies use alum or ferric chloride as coagulants followed by filtration through a sand column. Other technologies are based on sedimentation and/or filtration through activated/doubly activated column/granular ferric hydroxide column. Aggressive leaching adopted in the Toxicity Characteristic Leaching Procedure (TCLP) at pH ≤ 5 when applied to the wastes collected from different arsenic removal units has not produced lechate concentrations significant enough to term the wastes toxic as per the USEPA regulatory levels. However, TCLP may not be suitable for assessment of long-term leaching of arsenic from arsenicrich waste, because such leaching may be kinetically restricted. Thus, modification of TCLP to represent the natural leaching environment comparable to real disposal conditions is necessary. Tests under modified TCLP may provide different results. The users are currently practicing a number of arsenic-sludge disposal options. The sub-aqueous disposal 162 Fate of Arsenic in the Environment and buried in mud or mixed with organic matters has been considered for studying the anaerobic leaching of arsenic from solids and multiphasic sludge. This option addresses the situation where the users dispose the sludge/waste into a nearby pond where it is mixed with mud and a small amount of domestic sewage. For both the solid wastes and the mutiphasic sludge, the leaching of arsenic into the overlying aqueous phase showed similar trend where the arsenic concentration peaked within the first week and then tapered off and disappeared after twelve weeks. Absence of residual arsenic in the solids following anaerobic leaching indicate that a significant amount of arsenic is lost through biomethylation induced by the presence of organic matters used in the leaching experiment.
A review of arsenic (III) in groundwater
Abstract Recent improvements in sample collection and analytical techniques have suggested that As(III) is more prevalent in groundwater than previously believed. Indeed, reducing conditions in alluvial aquifers supplying single families may result in significant exposures to naturally occurring As(III). These results are noteworthy because As(III) is both more toxic and more mobile in the environment than As(V). The literature contains contradictory information concerning the appropriate preservation and analytical techniques for determining As(III). It appears that several previously reported occurrences of As(V) may have been predominantly As(III), but the samples were either not preserved or analyzed properly. For example, separation of arsenic species by ion exchange is apparently necessary to obtain reliable analytical results for certain environmental samples. The problems encountered with investigating As(III) in the environment are due to the complex series of geochemical reactions undergone by a...
Observations on the interference by copper(II), cobalt(II) and nickel(II) on the determination of arsenic by arsine generation atomic absorption spectrometry
Previous reports on the interference of copper(II), cobalt(II) and nickel(II) on the determination of arsenic by arsine generation atomic absorption spectrometry have been reviewed and an attempt has been made to rationalise the differences between them by examining the influence of interferent concentration, hydrochloric acid concentration, the amount of sodium tetrahydroborate(III) and the oxidation state of arsenic on the interference pattern. The extent to which the interferents are precipitated during the arsine generation procedure has been examined by quenching the reaction at different time intervals and determining the extent of loss to the solution either by analysis or by visual observation. From the results, conclusions have been drawn on the association between the formation of metal-like precipitates and the degree of interference. These data suggest that the interference mechanism may involve formation of soluble species rather than adsorption of arsine on the precipitated metals. This view is supported by polarographic evidence and analysis of solid material formed during reaction with sodium tetrahydroborate(III). Arsine evolution rate studies showed that when intereference is removed arsine is evolved at the same rate as arsine from arsenic(III).