Experimental determination of the dissolution kinetics of zero-valent iron in the presence of organic complexants

Abstract

Environmental context. Iron metal is being considered as a material to be used for the treatment of groundwater contaminated with toxic metals and organics. Although time-dependant information is available, predicting the long-term behaviour of this material has been complicated by the build-up of rust or other alteration phases on the surface of Fe metal. In addition to the build-up of rust, changes to important environmental factors also complicate these types of predictions. The research discussed in this paper uses a non-traditional experimental technique to isolate the impact of specific environmental factors (i.e. pH, temperature) and organic complexants on the dissolution of Fe metal. Abstract. The geochemical cycling of iron, the reactivity of iron minerals and, more recently, the reactivity of zero valent iron (α-Fe), have been the subject of numerous investigations for over more than three decades. These investigations provide a wealth of knowledge regarding the effect of pH, temperature, chelating agents etc. on the reactivity and mechanism(s) of dissolution for α-Fe and iron oxide/oxyhydroxide minerals. However, most investigations have been conducted under static conditions that promote the formation of a partially oxidised surface film (e.g. passivating layer). In the presence of a passivating layer, the proposed dissolution mechanisms are vastly different and are based on the composition of the partially oxidised surface film. The objective of this study was to quantify the dissolution of α-Fe under conditions that maintain the pO2 at a relatively constant level and minimise the formation of a passivating layer on the metal surface. Single-pass flow-through tests were conducted under conditions of relatively constant dissolved O2 [O2(aq)] over the pH(23°C) range from 7 to 12 and temperature range from 23 to 90°C in the presence of ethylenediamine tetraacetic acid (EDTA) and ethylenediamine di-O-hydroxyphenylacetic acid (EDDHA) to maintain dilute conditions and minimise the formation of a partially oxidised surface film and Fe-bearing secondary phase(s) during testing. Although more information is needed, these results suggest the adsorption of EDTA and EDDHA, or the diffusion of the oxidised Fe–organic complex from the surface of α-Fe, is the rate-limiting step in the dissolution reaction. Results also suggest that the rate of dissolution is independent of pH, has a non-linear dependence on the concentration of organic complexant, and the forward dissolution rate for α-Fe is as much as three orders of magnitude greater than when a passive film and corrosion products are present.