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.
Study on the effectiveness of activated alumina in removing arsenic from groundwater
Elevated level of arsenic in ground water is a major concern for many communities in the world, because long term exposure to arsenic via drinking and cooking water leads to a wide range of health problem. The public health sector of Bangladesh is currently under serious threat from widespread arsenic contamination of shallow groundwater aquifers, as water abstracted from these aquifers is the prime source of drinking and cooking water. An estimated 35 million people in Bangladesh are at risk of arsenic toxicity. Providing safe water to the affected people is a viable solution of the arsenic related health problems and treatment of arsenic contaminated water is one of the options to do this. Among the methods available for removing arsenic from water, adsorption on activated alumina is a promising one for implementing on a small-scale rural community or household levels. Various factors influence the removal efficiency of activated alumina. In this thesis work the effects of a number of parameters including oxidation state of arsenic, pH, iron, chloride, sulfate; phosphate, activated alumina size, empty bed contact time and the regeneration of spent alumina in removing arsenic by activated alumina were evaluated. Laboratory column tests were conducted using synthetic water with different compositions and operating conditions. A method was also developed for easy and effective regeneration of spent alumina. In this method, spent alumina was taken out of adsorption column and acid wash was applied after washing with normal tap water. Then the usual base regeneration and neutralization of the alumina were carried out. The efficiency/effectiveness of activated alumina in removing arsenic were determined in terms of Empty Bed Volume (BV), quantity of adsorbed arsenic and average arsenic removal efficiency up-to 50 ppb arsenic level in the effluent. The results of the study show that pentavalent arsenic is removed efficiently than trivalent arsenic. The bed volume of treated water decreases from 2115 to 375 with the increase of arsenic concentration from lOa ppb to 500 ppb for As(V), whereas, the bed volume decreases from 372 to lOa as the As(III) concentration increases from 100 ppb to 500 ppb. The study reveals that the presence of iron has a significant negative effect on As(V) removal efficiency and some positive effect on As(III) removal efficiency. The bed volume of treated water for As(V) changes from XIV 5287 to 160 as the iron concentration increases from 0.0 mgIL to 5.0 mglL, whereas, the variation for As (III) is from 25 to 316 as the iron concentration increases from 0.15 mglL to 5.0 mg/L. As(V) removal efficiency is better within the pH range of 5.0 to 6.0, whereas, As(III) removal increases at higher pH values. Chloride has no effect on the arsenic removal efficiency but the presence of sulfate and phosphate has significant effect. The results show that the increase of phosphate from 0.68 mg/L to 345 mglL decreases the bed volume oftreated water from 2100 to 70. Arsenic removal efficiency is found better for activated alumina size 28x48 as compared with the size l4x28. The arsenic removal efficiency increases significantly with the increase of the empty bed contact time. The study also shows that the modified method of regeneration of spent alumina is very effective in removing arsenic from groundwater.
Hydridmethode zur Arsenbestimmung durch Atomabsorptionsspektrometrie mit Atomisierung im heißen Quarzrohr
SummaryNickel, cobalt and copper interfere in the quantitation of arsenic by atomic absorption spectrometry with atomisation of arsine in a hot quartz tube. A number of chelating agents was investigated with a view to eliminate the suppressing effect of these metals. Pyridine-2-aldoxime was most efficient: 300 mg of pyridine-2-aldoxime exclude an interference by up to 40 mg of metal oxide in the final solution.ZusammenfassungNickel, Cobalt und Kupfer stören die Arsenbestimmung durch Atomabsorptionsspektrometrie mit Atomisierung von Arsenwasserstoff im heißen Quarzrohr. Der Störeinfluß läßt sich durch Komplexbildner vermindern. Von einer Anzahl untersuchter Chelatbildner zeigte Pyridin-2-aldoxim die größte Wirkung: 300 mg Pyridin-2-aldoxim in der Meßlösung schalten die Störung durch 40 mg Metalloxid aus.