Chemical fate of arsenic and other metals in groundwater of Bangladesh: experimental measurement and chemical equilibrium model.

The presence of toxic level of inorganic arsenic in groundwater used for drinking in Bangladesh and neighboring India is unfolding as one of the worst natural disaster in the region. The purpose of this work is to ascertain the chemical fate of arsenic and other metals in groundwater of Bangladesh. A combination of techniques was used to measure 24 metals, 6 anions, Eh, pH, dissolved oxygen, conductivity, and temperature to understand the distribution of components which were then used in computational chemical equilibrium model, MINEQL+, for detailed speciation. It was found that the fate of arsenic and its speciation were inextricably linked to the formation of hydrous ferric oxide (HFO) and its kinetic. The HFO induced natural attenuation removes 50-75% of total arsenic in first 24 h through a first order kinetics. Adsorption on HFO is the predominant mode of removal of arsenic, iron, manganese, and most trace metals. The equilibrium model points to the presence of excess active sites for the removal of arsenic. MINEQL+ shows that significantly higher concentration of HFO forming iron is required to remove arsenic below maximum contamination level (MCL) of 50 microg/L than predicted by stoichiometry. The practical implication of this work is the prediction of water quality based on models.

Electrochemical measurement and speciation of inorganic arsenic in groundwater of Bangladesh.

The presence of arsenic in groundwater above the maximum permissible limit of 50 mug l(-1) has threatened the health of more than 50 million people in Bangladesh and neighboring India. We report here the development of an inexpensive anodic stripping voltammetric (ASV) technique for routine measurement and speciation of arsenic in groundwater. The measurements are validated by more expensive atomic absorption, atomic emission and other techniques. To understand the present situation in Bangladesh, we measured As(III) in 960 water samples collected from 18 districts. A random distribution of 238 samples was used to measure both As(III) and As(V). The results from the present study indicate that most toxic form of inorganic arsenic, As(III), has the broad range of 30-98%. It shows 60% of the samples have 10 mug l(-1) and 44% of the samples have 50 mug l(-1) or more As(III). The fractional distribution pattern shows significant skew towards high percent occurrence which may indicate a progressive reduction process with a single source or a single mechanism for the formation of As(III). For direct consumption, this is possibly one of the most toxic groundwater known today. Speciation distribution at groundwater pH value shows H(3)AsO(3) is the predominant species including H(2)AsO(4)(-) and H(2)AsO(4)(2-) whose distribution is significantly pH dependent. This is also supported by E(h)-pH measurements. The depth distribution for Kushtia shows most of the As(III) is located within 100-200 ft deep aquifers. Similar fractional distribution of As(III) is found in deeper aquifers and may indicate contamination by leakage from upper aquifer. This study clearly demonstrates the aquifer environment is reductive and conducive to the formation of As(III) species.