ADSORPTION BEHAVIOUR OF ARSENIC IN THE OXIDIZED AQUIFERS IN MATLAB UPAZILA, BANGLADESH

Elevated concentration of arsenic (As) in the groundwater of the shallow Holocene aquifers in Bangladesh is a major health concern. The levels of As in the groundwater is often above the na-tional drinking water guideline value of 50 µg/L. Arsenic is a naturally occurring element in the sediment-water system and is bound to different oxide minerals. Recent studies have revealed that the As concentration of the groundwater is correlated to the colour of the sediment. Red or brownish sediments have been found to have low conentration of As, possibly due to the adsorption of As on Fe(III)-oxyhydroxides (which gives the sediment its colour) and/or that these sediments have been weathered during the late Holocene age. The present study describes the adsorp-tion behaviour of the oxidised sediments as these sediments could be a potential source of As-safe drinking water for millions of rural villagers in Bangladesh. The study area was located in Matlab Upazila, SE Bangladesh, which is one of the hotspots for As-contamination of groundwater. Twenty tube wells were sampled to establish the water chemistry and to determine the age of the water. Two boreholes were drilled to prepare lithologs as well as to collect oxidised sediments. Batch experiments were performed in columns with flushing of reduced water through oxidised sediments to study the adsorption behaviour of the sediments, and the impact of redox reactions was studied though the addition of organic matter (lactose) to some of the columns. The concentration of As in the tubewells ranged between <5-400 µg/L. Groundwater age range be-tween 1 and 12 kyr and reveal a linear correlation, the depth of the tubewells.as well as restricted movement of water both in the reduced and oxidised aquifers. The sediment chemistry analyses revealed a low amount of As, below 5 mg/kg, in the oxidised sediments. The column tests revealed a good adsorption capacity of the oxidised sediments and the maximum adsorption was not reached within eight weeks of time. However, in the columns with added organic matter, the adsorption was reduced as a result of high leaching of iron and manganese due to redox reactions. Sensitivity analysis showed that Fe(III)-oxyhydroxides, Si and PO₄^3- have the major impact on the As adsorption characteristics, whereas the impact of Ca²⁺ is insignificant.