HETEROGENEOUS Fe(II) OXIDATION AND ZETA POTENTIAL

Zeta potential was used as a tool alongside kinetic experiments to delineate between three plausible mechanisms for the heterogeneous oxidation of Fe(II) by dissolved oxygen. One of these mechanisms is dependant on the positive surface charge that exists on Fe(III)oxyhydroxide surfaces at pH values below the Iso-Electric Point (IEP). However, this mechanism can be disputed as, in the presence of bicarbonate alkalinity typical of many UK mine waters, catalysis is still observed at pH 6.5 despite an IEP shift from pH 8 to 4. As well as an IEP shift an overall reduction of the zeta potential is observed that may have beneficial effects to particle coagulation, settling and permeability characteristics in passive mine water treatment systems. Introduction The abiotic oxidation of Fe(II) by dissolved oxygen can proceed along two parallel pathways, firstly homogeneous oxidation in a solution containing no surfaces and secondly via heterogeneous oxidation in a suspension. In a homogeneous solution the Fe(II) oxidation rate greatly increases with hydrolysis of the Fe(II) ion due to an increase in the reduction potential (see Table 1). For this reason the rate of homogeneous oxidation is highly pH dependant, (Millero, 1985; Wherli, 1990). Table 1. Second order rate constants and redox potentials for Fe(II) species (after Wherli, 1990). Fe(II)i Species pH range at which rate dominated by species E (V) 2 order rate constant (ki) M sec Composite homogeneous rate Law Fe <3.5 0.771 7.9x10 FeOH 3.5 – 5 0.34 25.1 Fe(OH)2 >5 -0.02 7.94x10 R = d[Fe(II)] = (Σki [Fe(II)]i)[O2] dt [ ] is the molar (M) concentration, R is the Fe(II) oxidation rate in M sec , ki is the 2 order rate constant for the oxidation of Fe(II) by O2 in M sec , t denotes the time in sec and E is the standard reduction potential. In a heterogeneous suspension, Fe(III)oxyhydroxide surfaces are well known to catalyse the oxidation of Fe(II), (Tamura et al., 1976; Sung and Morgan, 1980; Tufekci and Sarikaya, 1996; Park and Dempsey, 2005). However, the mechanisms behind the observed catalytic behaviour are not fully understood; three reaction models have been presented in the literature and are described below. 1) Counter-ion mechanism (Tufekci and Sarikaya, 1996): Fe(III)oxyhydroxide surfaces are amphoteric in nature and carry a surface charge dependant on specifically chemisorbed ions. Electric Double Layer (EDL) theory describes how this charge generates an electrostatic potential in contact with a solution that declines rapidly away from the surface. EDL theory also assumes that an excess of counter-ions (ions of opposite charge) exist near the surface (Langmuir, 1997). Below a pH of about 8, Fe(III)oxyhydroxides typically have a positive surface charge (Dzombak and Morel, 1990). Therefore an elevated concentration of OH counter-ions will form near the surface, locally shifting the Fe(II) speciation equilibrium in favour of the faster oxidising FeOH and Fe(OH)2 species. 2) Reactive surface species mechanism (Tamura et al., 1976; Wherli, 1990; Liger et al., 1999): Chemisorbtion of Fe(II) at the Fe(III)oxyhydroxide surface enhances its reactivity in electron transfer reactions in the same way that hydrolysis promotes electron transfer in dissolved Fe(II). 3) Semiconductor mechanism (Park and Dempsey, 2005 and references therein): Fe(III)oxyhydroxide can act as a semiconductor in the presence of adsorbed Fe(II) due to electron transfer with the underlying Fe(III). The surface then acts as an electrode where surface sites with strongly adsorbed Fe(II) become electron rich (anodes) and sites without adsorbed Fe(II) become electron-poor (cathodes). O2 is thus reduced at anode sites whilst Fe(II) is oxidized at cathode sites. In this mechanism both dissolved and adsorbed Fe(II) is required to produce a catalytic effect and the adsorbed portion of the Fe(II) remains un-reacted. The aim of this study is to distinguish between the applicability of each of these mechanisms via kinetic and zeta potential (ζ) analysis. ζ is the potential drop that exists within the EDL between the shear plane and the bulk solution, (Eckenfelder, 2000). Due to a number of assumptions that must be invoked and problems with results IMWA Symposium 2007: Water in Mining Environments, R. Cidu & F. Frau (Eds), 27th 31st May 2007, Cagliari, Italy