In the case of disinfecting underground water sources with bleaching powder, the dose needs to be evaluated with utmost concern, when the water sources are located in alluvial sandy deposits and contaminated with arsenic. Because, excess chlorine may oxidize the alluvial sediment to release the As (V) into the water.
Is there any scope that oxidation through chlorine during disinfection may eventually lead to or aggravate arsenic contamination in water sources from alluvial sandy areas? What could be the precautionary measures to avoid that?
Your main ores of arsenic are sulfides, which like pyrite are very prone to leaching when exposed to air/water.
2 As2S3 + 9 O2 → 2 As2O3 + 6 SO2
The addition of hypochlorite ions would likely accelerate any leaching by being another source of oxygen and making the water acidic by throwing either sulfates or chlorides in. Either way the solubility of arsenic is increased substantially when compared to that of the sulfide.
Manganese greensand (MGS) may be a good material to use to combat any arsenic leaching.
http://mtac.isws.illinois.edu/mtacdocs/finalreports/ArsenicTreatmentMTACFinalReport.pdf
Agree that in aboveground water treatment, MGS is useful because it adsorbs arsenic. If in situ treatment, much needs to be considered. If conditions are already oxidized in groundwater, I don't know of a reaction that would release arsenic. If conditions are reducing and iron is present, such as a sulfide, then adding a strong oxidizing agent will result in dissolution of the sulfide and release of arsenic and iron and reprecipitation of iron as an oxyhydroxide with adsorbed arsenic. However, oxyhydroxides are unstable in reducing conditions so gradual dissolution of the mineral and release of the adsorbed arsenic will occur.
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