The Role of Naturally Occurring Fe(II) in Removing Arsenic from Groundwater: Batch Experiments and Field Studies

Abstract

Higher levels of arsenic (As) and iron (Fe) in groundwater have been reported globally. This study aims to enhance our understanding of the role of naturally occurring dissolved Fe(II) in removing As from groundwater. Field experiments were conducted using five clay filters to investigate As and Fe removal from contaminated groundwater. The field results revealed a wide range of arsenic removal (7.3% to 80%) using the clay filters. The filter with the highest Fe concentration (14.5 mg/L) exhibited the highest As removal, while the lowest Fe concentration (2.2 mg/L) resulted in the lowest percentage of As removal. A direct correlation was observed between effluent As levels and the Fe/As molar ratio. An Fe/As molar ratio of 40 or more was identified as necessary to achieve effluent As concentrations below 50 µg/L. Laboratory batch experiments revealed that Fe(II) was more effective than Fe(III) in removing both As(III) and As(V) from contaminated groundwater. As(V) removal was consistently higher than As(III) removal, regardless of whether Fe(II) or Fe(III) was used. The results suggested that the oxidation of As(III) and the subsequent in situ formation of Fe(III) hydroxide were more efficient in As adsorption than direct Fe(III) treatment. The X-ray absorption fine structure (XAFS) analysis of the floc samples confirmed the dominant peaks of As(V), indicating that most of the As(III) oxidized to As(V) in the As(III)-Fe(II) system. The use of natural Fe(II) in groundwater, possibly supplemented with additional sources of Fe(II), is suggested as a promising, cost-effective, and efficient method for As(III) and As(V) removal.