Long-term Effect of Metallic Iron Sorbents on Arsenic Mobility in an Anoxic Aquifer : An assessment based on long-term column experiments

Abstract: Zero valent iron (ZVI) sorbents have successfully remediated soils burdened with different groups of contaminants. When working to alleviate arsenic (As) contamination, the location-specific subsurface chemistry greatly affects the efficiency of the sorbents as it controls the speciation of As and the transformation of the ZVI, as well as the interaction between these (remediating mechanisms). Hence, planning for such full-scale remediation should suitably be based on lab-scale tests mimicking the in-field conditions of the site in question. In Hjältevad, Sweden, wood impregnation practices with chromated copper arsenate (CCA) has contaminated an anoxic aquifer with As. In-field speciation of As has indicated that the expected form is dominating, which is the more mobile species As(III). This study aimed to test the long-term efficiency of different variants of commercially available ZVI towards As immobilization in the sediment from Hjältevad, mimicking the on-site anoxic conditions and groundwater composition. Additionally, this study included an optimization of a method for analyzing sub μg L-1 concentrations of As with inductively coupled plasma mass spectrometry (ICP-MS), and optimization of the column test design. A long-term bottom-up flow column experiment containing sediment from Hjältevad was performed, testing micro-sized ZVI, nano-sized ZVI and their sulfidated counterparts, each in a separate column. The eluates collected from the respective columns were compared with each other and previously performed batch tests of the same composition. The non-sulfidated sorbents showed better efficiency in the simulated Hjältevad conditions in both column and batch tests. Both nano-sized sorbents initially increased mobilization of As from the column system, before shifting to an immobilizing phase. Increased pH and decreased redox potential in the eluates from all sorbent-treated columns in comparison with non-treated columns indicated corrosion of the ZVI core. This study suggests the micro-sized ZVI (Ferox Target, Hepure) for Hjältevad, as it performed the best in the long-term column tests (99.7 % retention of the column As). Reaction mode with O2 was found as the optimal method to analyze sub μg L-1 concentrations of As with ICP-MS. Column experiments with sandy sediment testing ZVI performance for As was found to work well by replacing the provided filters with 70 μm Nylon mesh filter and without inducing aggregation of the sorbents with NaCl. Keywords: Zero valent iron, ZVI, Remediation, Anoxic, Column tests, ICP-MS