A retrospective study on arsenic pollution in BDP areas - reasons and remedies

Department of Chemistry. Kalyani University, Kalyani-741 235, West Bengal, India soil Mechanics Section, River Research Institute, West Bengal, P.O. HRRI, Nadia. West Bengal, India central Forensic Science Laboratory, 30, Gorachand Road, Kolkata-700 014, India E-mail : sujitclahiri@yahoo.com Manuscript received 29 November 2010, accepted 30 November 2010 The biogeochemical proccsses causing As-distribution in ground water of Bengal Delta plain (BDP) areas are not well understood but As-mobilization in ground water are well known. Increased As-mobilization in ground water is due to increased manmade activities like mining, greater use of ammonium and particularly phosphates (as fertilizers, detergents, animal and plant wastes) and excess withdrawal of water causing depletion of water table. Po43- and also NH4 + ions with minerals may cause displacement of Asv or AsIII from arsenic bearing minerals. However, processes leading to the increase in As-mobilization in ground water (even in surface waters of ponds etc.) are expected to increase with increase in P and N-budget in soils and water and with heavy withdrawal of ground water to meet the demands of increased industrialization and population. Intake of As-polluted water is the destiny for rural people where under-ground arsenic pollution is known unless proper remedial measure like use of filters for As-removal from the polluted waters is undertaken. Most of the filters contain granular ferric hydroxides/activated alumina or aluminium silicate with or without metal oxides/ion-exchange resins. The working principles arc the selective adsorption of AsO33-/AsO43-. oxidation of AsO43- Aso4J- by iron hydroxides/oxides. The filtering capacity is dependent on pH but the capacity is affected by the presence of other ingredients like Po43-, F-, Cl- ions etc. However, As-content in wells and As-removing ability of the filters are not uniform. Some aspects of As-removing capability together with the effects of interfering ions are discussed. The high Fe content at pH> 7 causes precipitation of Fe as colloidal ferric hydroxide having better As-adsorption capability. This helps greater removal of As but is associated with clogging of filter beds having the opposite effect affecting the discharge of water. Frequent cleaning helps the restoration As-removal capability to a greater extent but shortens the life span and reduction of As-removal capability leading to ultimate breakdown of the filters at the vulnerable points. Suitable measures must be taken to reduce the gradual clogging and reduction of As-removal capability in designing the filters. Attempts should be made to find alternative methods for low cost As-removal process from ground water