Exploring the Effects of Organic Matter Characteristics on Fe(II) Oxidation Kinetics in Coastal Seawater

[Image: see text] 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, and 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, and 8.1) and temperature (10, 15, 20, and 25 °C). Deviations from the calculated k(cal)(′) at the same T, pH, and S were observed for most of the stations. The measured t(1/2) (ln 2/k′, min) values at the 25 stations ranged from 1.82 to 3.47 min (mean 1.93 ± 0.76 min) and for all but two stations were lower than the calculated t(1/2) 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 and SEE = 0.064 for pH = 8 and T = 25 °C) from a linear combination of three organic variables, k′(OM) = k(cal)(′) −0.11* TDN + 29.9*b(DOM) + 33.4*C1(humic), where TDN is the total dissolved nitrogen, b(DOM) is the spectral peak obtained from colored dissolved organic matter (DOM) analysis when protein-like or tyrosine-like components are present, and C1(humic) is the component associated with humic-like compounds obtained from the parallel factor analysis of the fluorescent DOM. Results show that compounds with N in their structures mainly explain the observed k′ increase for most of the samples, although other components could also play a relevant role. Experimentally, k′ provides the net result between the compounds that accelerate the process and those that slow it down.