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.
The Disposing Techniques Of Water Treatment Wastes Containing Arsenic — A Review
Solid waste management in developing countries is often unsustainable, relying on uncontrolled disposal in waste dumps. Particular problems arise from the disposal of treatment residues generated by removing arsenic (As) from drinking water because As can be highly mobile and has the potential to leach back to ground and surface waters. This paper reviews the disposal of water treatment wastes containing As, with a particular emphasis on stabilisation/solidificati on (S/S) technologies which are currently used to treat industrial wastes containing As. These have been assessed for their appropriateness for treating As containing water treatment wastes. Portland cement/lime mixes are expected (at least in part) to be appropriate for wastes from sorptive filters, but may not be appropriate for precipitative sludges, because ferric flocs often used to sorb As can retard cement hydration. Brine resulting from the regeneration of activated alumina filters is likely to accelerate cement hydration. Portland cement can immobilise soluble arsenites and has been successfully used to stabilise As-rich sludges and it may also be suitable for treating sludges generated from precipitative removal units. Oxidation of As(III) to As(V) and the formation of calcium-arsenic compounds are important immobilisation mechanisms for As in cements. Geopolymers are alternative binder systems that are effective for treating wastes rich in alumina and metal hydroxides and may have potential for As wastes generated using activated alumina. The long-term stability of cemented, arsenic -bearing wastes is however uncertain, as like many cements, they are susceptible to carbonation effects which may result in the subsequent re -release of As.
HYPERGOLIC REACTION MECHANISMS OF CATALYTICALLY PROMOTED FUELS WITH ROCKET GRADE HYDROGEN PEROXIDE
Abstract The coupling of the physical and chemical processes controlling the ignition delays (τ ign ) of hypergolic propellants complicates the direct analysis of the transient ignition processes. The transport properties of both the propellants and the ambient gas, and the heat of vaporization and reaction affect τ ign and must be considered experimentally. Furthermore, since hypergolic ignition occurs, by definition, on contact, the mechanics of initial contact and subsequent mixing can affect τ ign and must also be considered. Experimental results from a study of the hypergolic ignition of a catalytically promoted fuel with rocket grade hydrogen peroxide (RGHP) are presented. Statistical analysis of the experimental data was conducted to evaluate the relative effects of RGHP concentration, initial ambient pressure, and the ambient gas on τ ign . At RGHP concentrations of less than 92% in water, stable ignition was difficult to achieve. At sufficiently high Lewis number, an oscillatory ignition process is observed. A phenomenological model comprising mixing, RGHP decomposition, fuel heating and vaporization, and auto-ignition is presented and compared to the experimental results.