The role of NOM in controlling the size and oxidation state of Fe in natural waters Dr John Gaffney SEAES Introduction Fe is both essential nutrient and terminal electron acceptor in respiration. Fe is ubiquitous and insoluble in circum-neutral pH surface waters. Production of particulate Fe in surface waters may be from mechanical erosion. Material of low SSA is likely from oxidation of Fe2+ entering surface waters via a groundwater source. Upon reaching higher pH (circumneutral) surface waters (oxic conditions) the following reactions occur: Oxidation: 4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H2O Hydrolysis: Fe3+ + 3H2O → Fe(OH)3 + 3H+ Coagulation: Fe(OH)3 → (Fe(OH)3)n Fe forms insoluble Fe(OH)3 in surface waters. Insoluble Fe(OH)3 has a surface area, giving it a sorption capacity and hence control on soluble components of a system (e.g. contaminant metals Pb, Cu and Zn). Determination of Fe(OH)3 particle size is therefore essential for understanding contaminant movement and fate. Thus studying the formation of Fe(OH)3 from an Fe2+ starting point is essential. Previous work in this field has stated that the rate of Fe2+ oxidation is affected only by pH (Davison and Seed, 1983). More recently NOM and ionic strength have been found to have an affect (Pullin, 2004). NOM is also used in water treatment processes as a coagulant and has been shown to influence Fe(OH)3 particle size. NOM has the potential to influence the rate of oxidation of Fe2+, thus the rate of formation of Fe(OH)3 and the stability of Fe(OH)3. Aim and objectives To investigate the role of NOM in controlling the rate of oxidation of Fe(II) in natural waters and the end products of the reaction. Objectives Monitor the rate of Fe(II) oxidation in a laboratory simulation HA and AA Characterise the form of Fe present at the end point and determine the influence of HA and AA on these products. Determine the PSD of Fe and NOM at the end point of the reaction. Examine the form of Fe and the PSD of Fe and NOM in a natural system. Compare data from the laboratory and the natural system to check the validity of the simulation. Method In order to simulate natural conditions laboratory simulations were to be constructed: Solution - synthetic river water (SRW). Fe2+ at environmental concentration (5mg/L). NOM – dominant types present in natural waters at environmental concentration. (Humic acid (HA) 30 and 50mg/L and Alginic acid (AA) 30 and 50mg/L). pH 6.5 in sealed vessel. Fe(OH)3 Determined by addition of NaOH. As Fe(OH)3 forms, 3 H+ are released. By maintaining pH 6.5 by addition of NaOH the amount of Fe(OH)3 formed can be quantified. Free Fe2+ Determined by ferrozine assay. A chelator of divalent Fe, which forms a stable magenta complex (Stookey, 1970) absorbs at 562nm. Sample left to stand for 30 seconds prior to measurement. Hidden Fe2+ Ferrozine is a strong chelator of Fe2+ Therefore in this study, ferrozine was left to interact with the sample for prolonged periods of time. This allowed a competitive reaction to occur between ferrozine and other ligands (i.e. NOM). Subsequent colour development during this time period was attributed to Fe2+ stuck on other ligands being released and thus complexed with ferrozine. Results Inorganic 1.0 0.8 0.6 Fe2+ Fe (%) 0.4 Fe(OH)3 0.2 Other Fe 0.0 0 2 6 10 40 971 DG 24hrs pt DG Time (mins) 30mg/L 1.0 Humic acid 0.8 0.6 Fe (%) 0.4 0.2 0.0 0 2 6 10 40 300 DG 24hrs pt DG Time (mins) 50mg/L 1.0 Humic acid 0.8 0.6 Fe (%) 0.4 0.2 0.0 0 2 6 10 40 300 DG 24hrs pt DG Time (mins) The rate of oxidation of Fe2+ not affected by the presence of HA. The rate of formation of Fe(OH)3 and total conversion to Fe(OH)3 is influenced by HA. Other Fe present during the titration may be attributed to Fe2+ being stuck on the HA rendering it not available for oxidation. Inorganic 1.0 0.8 0.6 Fe2+ Fe (%) 0.4 Fe(OH)3 0.2 Other Fe 0.0 0 2 6 10 40 971 DG 24hrs pt DG Time (mins) 30mg/L 1.0 Alginic acid 0.8 0.6 Fe (%) 0.4 0.2 0.0 0 2 6 10 40 1380 DG 24hrs pt DG Time (mins) 50mg/L 1.0 Alginic acid 0.8 0.6 Fe (%) 0.4 0.2 0.0 0 2 6 10 40 2760 DG 24hrs pt DG Time (mins) The rate of Fe2+ oxidation is influenced by the presence of AA, with an increase in the concentration of AA resulting in a decrease in the rate of oxidation. The rate of formation of Fe(OH)3 is influenced by the presence of AA, increase in AA results in a decrease in the rate of formation. No Fe2+ bound to AA at the end point of the titration. Location of Sites on Crowden Great Brook Crowden Great Brook, (Looking down on site 60) Site 30 Conclusions Fe2+ is present in both laboratory simulations. NOM influences both the rate of oxidation of Fe2+, and formation of Fe(OH)3 (AA) and the total formation of Fe(OH)3 (HA). The longevity of Fe2+ present in oxygenated surface waters is dependant on the presence and concentration of NOM. These findings have implications for determination of the particle size distribution of Fe in natural waters and thus on the transport of contaminants such as Cu, Pb and Zn.