The effects of organic matter characteristics on Fe(II) oxidation kinetics in coastal seawater

Abstract

The iron (II) oxidation kinetic process was studied at 25 stations in coastal seawater of the Macaronesia region (9 around Cape Verde, 11 around the Canary Islands, 5 around Madeira). In a physicochemical context, experiments were carried out to study the pseudo-first order oxidation rate constant (k', min-1) over a range of pH (7.8, 7.9, 8.0, 8.1) and temperature (10, 15, 20, 25ºC). Deviations from the calculated k'cal at the same T, pH and S were observed for most of the stations. Measured t1/2 (ln2/k´, min) values at the 25 stations ranged from 1.82-3.47 min (mean 1.93±0.76 min) and for all but two stations were lower than the calculated t1/2 of 3.21±0.2 min. In a biogeochemical context, nutrients and variables associated with the organic matter spectral properties (CDOM and FDOM) were analyzed to explain the observed deviations. The application of a multilinear regression model indicated that k´ can be described (R=0.921, SEE=0.064 for pH=8 and T=25ºC) from a linear combination of three organic variables. k´OM = k´cal -0.11* TDN + 29.9 * bDOM + 33.4 * C1humic where TDN is the total dissolved nitrogen, bDOM is the spectral peak obtained from colored DOM analysis when protein-like or tyrosine-like components are present and C1humic is the component associated to humic-like compounds obtained from the parallel factor analysis (PARAFAC) of the fluorescent DOM. Results show that compounds with N in their structures principally explain the observed k´ increase for most of the samples. Although other components could also present a relevant role. Experimentally, k´ provides the net result between the compounds that accelerate the process and those that slow it down.