318058 Iron Electro-Coagulation for Simultaneous Arsenic Removal and Microbial Attenuation: Mechanisms of E.Coli Removal and Inactivation
Around 60 million people in South Asia drink groundwater from arsenic contaminated shallow aquifers. Research over the last two decades has focused on arsenic removal alone to mitigate this problem, largely ignoring possible microbial contamination of shallow groundwater. Recently, fecal indicators and pathogens were detected in shallow tubewells in Bangladesh, and diarrheal diseases are still prevalent in the region. Therefore comprehensive treatment technologies addressing both microbial and arsenic contamination of drinking water are needed, and will potentially increase health impacts and social acceptability.
Iron electro-coagulation (EC) is a simple technology relying on the quick dissolution of a sacrificial Fe(0) anode to produce iron precipitates that serve as an adsorbent for arsenic and as a coagulant for microbes. In the process, strong oxidants generated by Fenton-like reactions convert As(III) into As(V), which is more amenable to adsorption. The mechanisms of arsenic removal with iron EC have already been described. In this work, we demonstrate that iron EC can simultaneously remove arsenic and the model organism E.coli in synthetic groundwater similar to contaminated aquifers in South Asia. The iron dosage (in mg-Fe/L) required to achieve 4-log attenuation of E.coli is very sensitive to pH and varies from 25 mg-Fe/L at pH 6.6 to 140 mg-Fe/L at pH 7.5. In this pH range, iron precipitates generated in synthetic groundwater have a negative surface charge, whose variation cannot entirely explain the sensitivity of E.coli attenuation to pH. We find that inactivation as well as coagulation with iron precipitates both contribute to E.coli attenuation. We propose to analyze these two mechanisms.
Fe(II) can cause oxidative stress on bacterial membranes and inside cells due to the production of strong oxidants by Fenton-like reactions. We examine the effect of pH and strong oxidant scavengers on E.coli inactivation during iron EC. In order to probe oxidative stress on E.coli membranes, we quantify membrane permeability changes and lipid peroxidation. We discuss the possible role of anodically produced Fe(II) as a bactericidal agent in our system.
Covalent bonding between bacterial terminal phosphate groups and iron oxides has been previously reported. We use Fourier Transform Infrared Spectroscopy to analyze how E.coli binds to a range of EC-precipitates (amorphous ferrihydrite, magnetite, carbonate green rust, etc) that can be generated by varying EC operating conditions.
A better understanding of the mechanisms of E.coli removal and inactivation by iron EC allows us to discuss the impact of water quality parameters, like pH and natural organic matter. Finally, we discuss the implications of our results on the potential for iron EC to provide a viable solution to contaminated groundwater in South Asia.