Water resources Removal of Arsenic from Water Resources
Arsenite - AsIII, which is hard to oxidize to arsenate - AsV, is the main problem. Removal of AsV by common water treatment processes (coagulation, precipitation, or adsorption on to hydrated oxides of Fe3+ or Al3+) makes no problem. Attention is focussed on ferric and aluminium coagulants usage, optimisation of the doses and pH in the process of coagulation. Significant improvement in flock separation is reached by using coagulant aid Praestol 2530. Removal efficiency of AsV for initial concentration up to 10 mg/l is up to 95-98%. In case of AsIII maximum efficiency is only about 50%. Two other oxidation methods of AsIII to AsV by MnO2 and sand bed Birm are also described here. In these cases the uptake of AsIII is increased about 10%. Arsenic is a carcinogen causing skin cancer in humans . Base compounds of arsenic are poisonous and long-term use of water with concentrations exceeding the permitted limits may lead to chronic diseases. On the other hand, arsenic is an inhibitor of chemical oxidation short-term application of arsenic mineral water supports the generation of red blood cells and deposition of protein in body. The highest permissible content of arsenic in drinking water is 10 µg, 50 µg in water resources and 100 µg in other water courses. With respect to these permissible limits, it is necessary to pay attention to the removal of arsenic, particularly the toxic AsIII. There are several methods for the removal of arsenic from water resources; however, not all of them are technologically applicable under standard circumstances. It is obvious that the selected method will depend not only on the initial concentration of As but also on the subsequent utilization of the treated water. The following methods were used for the removal of As under laboratory conditions:
Precipitation with the use of calcium hydroxide and calcium polysulfide,
Clarification with ferric sulphate and polyaluminium chloride,
Oxidation to pentatomic arsenic with the use of MnO2 .
The technological testing was carried out on underground water and surface water from the Vltava river. The concentration of As was artificially increased to a maximum amount of 10 mg per litre. Arsenic was added in the form of AsV and AsIII According to the results, extremely high pH is required for the removal of both forms of As. Out of many tests, the method of coagulation with ferric or aluminium ions appears to be the most effective. The best results were achieved during the process of clarification with ferric coagulant Prefloc and an aluminium coagulant PAX-18. The recommended optimum pH is 6-8. During the coagulation testing, sludge was generated at a quantity of up to 2% of the overall volume of purified water.
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Doc. Ing. Nina Strnadova UTVP VSCHT Technicka 1905 166 28 Praha 6
