Kinetics of Iron Oxide Dissolution as Enhanced by Organic Ligands

Abstract

Previous studies indicate that colloids may be released in the subsurface environment upon the dissolution of cementitious iron oxides (Ryan and Gschwend, 1992). Our field investigation showed that the concentrations of both total and ferrous iron increased during an injection of natural organic matter (NOM), implying an NOM-enhanced dissolution of iron oxide and mobilization of the oxide colloids in s i tu (Liang et al. , 1993). Because of the lack of quantitative data, predictions on the rates and the extent of iron-oxide dissolution in a natural system are hindered. The current laboratory studies are aimed at developing and testing conceptual and mathematical models for iron oxide dissolution. Here experimental work is undertaken to determine the rates of dissolution of ferrihydrite as influenced by organic ligands, such as oxalate, citrate and ascorbate ions. The type of organic ligands, ligand concentrations, and pH were varied in order to determine the effect of each variable on the kinetics of the dissolution reaction. An important application of this project is to assess iron oxide colloid mobilization corresponding to the dissolution of the oxide in groundwater, which is investigated in column flowthrough systems.