Factors affecting arsenic and uranium removal with zero-valent iron: laboratory tests with Kanchan-type iron nail filter columns with different groundwaters

Abstract

Environmental context Tens of millions of people worldwide depend on groundwater with naturally high arsenic concentrations for drinking and cooking. We studied simple filters built with locally available and inexpensive iron nails, which can oxidise and bind arsenic in forming iron oxides and rust layers. Filters containing iron are being successfully applied in several regions, but efficiencies depend on the type of groundwater, and sufficiently large iron surfaces and contact times with water are needed for good arsenic removal. Abstract Zero-valent iron (ZVI)-based filters are able to remove arsenic and other pollutants from drinking water, but their performance depends on the form of ZVI, filter design, water composition and operating conditions. Kanchan filters use an upper bucket with ZVI in the form of commercial iron nails, followed by a sand filter, to remove arsenic and pathogens. We evaluated factors that influence the removal of arsenic and uranium with laboratory columns containing iron nails with six different synthetic groundwaters with 500μgL–1AsIII, 50μgL–1 U, 2mgL–1 B, and with 0 and 2mgL–1 P (added as o-phosphate), 0.25 and 2.5mM Ca, 3.2 and 8.3mM HCO3–, at pH 7.0 and 8.4 over 30 days. During the first 10 days, As removal was 65–95% and strongly depended on the water composition. As removal at pH 7.0 was better than at pH 8.4 and high P combined with low Ca decreased As removal. From 10–30 days, As removal decreased to 45–60% with all columns. Phosphate, in combination with low Ca concentrations lowered As removal, but had a slightly positive effect in combination with high Ca concentrations. U removal was only 10–70%, but showed similar trends. The drop in performance over time can be explained by decreasing release of iron to solution due to formation of layers of FeIII phases and calcite covering the iron surface. Mobile corrosion products contained ferrihydrite, Si-containing hydrous ferric oxides, and amorphous Fe–Si–P phases. Comparisons with another type of ZVI filter (SONO-filter) were used to evaluate filter design parameters. Higher ZVI surface areas and longer contact times should lead to satisfactory As removal with Kanchan-type filters.