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.
Leaching of Arsenic from Iron Oxide Impregnated Brick Sands ( Shapla Filter Media ) Using Common Chemicals and Water
Leaching of Arsenic from high Arsenic loaded iron oxide impregnated brick sands (IOI CBCM) was investigated using common household chemicals like common salt and soda ash, the industrial chemicals caustic soda and the phosphates used as fertilizers such as potassium hydrogen phosphate and TSP. Leaching by ordinary ground water was also investigated on the arsenic saturated iron oxide impregnated brick sands from filters. A 10% caustic soda solution could leach ~95% arsenic, 20% sodium chloride solution could leach ~17.28%, 10% sodium carbonate solution 11.34%, 10% potassium dihydrogen phosphate solution 11.85%, and a 10% TSP solution could leach 34.56% arsenic from high arsenic rich (~1000 ppm As) iron oxide impregnated brick sands. Since ordinary water in the environment rarely contain such high concentration of chemicals, very little dissolution of As from arsenic loaded brick sands is expected. Natural groundwater showed negligible or no leaching from waste arsenic-rich iron oxide impregnated brick sands from filters both in flow and static tests at pH 4 and 7. 182 Fate of Arsenic in the Environment
Arsenic in rain and the atmospheric mass balance of arsenic
An attempt to construct a mass balance of arsenic in the world atmosphere showed that the published data on arsenic concentrations in rain were not compatible with measured values of atmospheric concentrations at remote sites and with estimates of arsenic fluxes into the atmosphere. To resolve this problem, samples of rainwater and snow from eight sites in California, Washington, and Hawaii were analyzed for arsenite, arsenate, and methylated forms of arsenic. The inorganic species were detectable in most samples, but no methylated forms were present above the detection limit of 0.2 ppt. Between October 1976 and March 1978, 43 samples of rain were collected at three locations near the coast in La Jolla. No significant differences between these sites were evident. The average concentration, weighted for rainfall amounts, was 0.007 ppb arsenite and 0.012 ppb arsenate, giving a total concentration of 0.019 ppb As. The samples from Kauai gave an average total arsenic identical to that from La Jolla. This suggests that the La Jolla samples, most of which were collected during strong onshore flow of air from the Pacific, represent very clean air. During some periods of pollutant buildup, values up to 0.59 ppb were found in La Jolla. In a few samples, on the other hand, the arsenic concentrations were below the detection limit of 0.004 ppb. Comparable values were also found in samples of snow from Norden, California, a site at 2225 m elevation in the Sierra Nevada. These values fit well with concentrations modeled on the basis of aerosol analyses from remote sites. The average arsenic concentration at Anacortes Island, Washington, was significantly higher: 1.06 ppb with 88% of the arsenic in the form of arsenite. This value can be explained by a Gaussian plume model with the Tacoma smelter at its origin. This plant, which is 154 krn from the sampling site, emits ∼180 kg of arsenic per day in the form of arsenic trioxide, which is transported northward by the prevailing winds. The ratio of arsenite to arsenate in rain varies over a wide range. This is interpreted as a result of different oxidation states of arsenic in the source emissions and to redox reactions within atmospheric particulates and hydrometeors. By using the data from this study, a mass balance can be constructed for arsenic in the atmosphere which does not require biogenic methylation or hitherto unknown processes as a source for atmospheric arsenic.