The role of gentisic acid on the Fe(III) redox chemistry in marine environments

Abstract Phenolic compounds excreted by marine microalgae are part of the ligand pool in natural waters. The effect of the polyphenol gentisic acid (GA; 2,5 dihydroxybenzoic acid) on the Fe(III) reduction as a function of organic ligand concentration (100 nM – 1000 nM) and the pH (7.00–8.01) was investigated in seawater . Major seawater ions interactions with both GA and Fe(III) were also considered using 0.7 M NaCl +2 mM NaHCO 3 solutions with each major ions (Mg2+, Ca2+, SO42−, K+, F−, Sr2+) at the same ratio than that in a 35 salinity seawater. The Fe(III)-GA complex in solution was able to produce Fe(II) in a pH-dependent process that was faster in seawater than in NaCl-NaHCO3 solution. The addition of each major ion affected the Fe(III) reduction process. At pH 7.5, the presence of Mg2+ and Ca2+ contributes to accelerate the reduction rate constant of Fe(III) to Fe(II). The K+, F−, Sr2+, and SO42− ions slow down the Fe(III) reduction rate constant. At pH 7.8, the effect of the ions is counteracted. Increasing the pH to 8, the effect on the solubility of Fe(III) is more important, being greater in NaCl and no reduction of Fe(III) was detected, compared with seawater. In this study, the formation and dissociation of the Fe(III)-GA complex in seawater was studied, resulting in kf = 1.19(±0.18)•104 (M−1 s−1), kd = 1.86 (±0.53)•10−4 (s−1), and conditional complexation constant of logKFe3+LCond = 17.81 ± 0.05 in seawater. Accordingly, phenolic compounds like GA influence the Fe marine biogeochemical cycles promoting the formation of the bioavailable Fe(II) in solution. Taking this into account, the Fe(III) reactions with phenolic compounds have to be considered to improve our understanding of the global iron cycle.