Effects of organic ligands, phosphate and Ca on the structure and composition of Fe(iii)-precipitates formed by Fe(ii) oxidation at near-neutral pH

Abstract

The oxidation of dissolved Fe(II) in near-neutral natural waters leads to the formation of Fe(III)-precipitates. Organic ligands, PO₄, calcium (Ca) and other solutes affect the composition, bulk and nanoscale structure and colloidal properties of Fe(III)-precipitates and their impacts on co-precipitated compounds in interdependent ways. In this study, we quantified the effects of four low molecular weight organic acids (LMWOAs) with different Fe(III) complexation strengths (2,4-dihydroxybenzoic acid (2,4-DHB) ∼ galacturonic acid (Galact) ≪ 3,4-dihydroxybenzoic acid (3,4-DHB) < citric acid (Citr)) and of leonardite humic acid (LH) in combination with PO₄ and Ca on Fe(III)-precipitate structure and composition in a multifactorial experiment. Precipitates were synthesized by oxidation of 0.5 mM Fe(II) in bicarbonate-buffered solutions at pH ∼ 7, at molar (OC/Fe) ratios of 0.1 to 9.6, at molar (PO₄/Fe) ratios of 0.05 and 0.25, and without/with 4 mM Ca. OC-free controls consisted of amorphous (Ca)Fe(III)–phosphate and ferrihydrite (Fh) aggregated in core–shell nanoparticles with lepidocrocite (Lp) crystallites attached to their surface, reflecting their formation sequence. The LMWOAs promoted the formation of OC-loaded Fh at the expense of Lp and thereby led to more effective PO₄ retention and lower residual dissolved PO₄. This effect increased with ligand concentration and Fe(III) complexation strength. At a higher PO₄ level, the co-precipitation of all LMWOAs was reduced, reflecting the stronger affinity of PO₄ for Fe(III). At higher (OC/Fe), the strong ligands 3,4-DHB and Citr increasingly co-precipitated with PO₄ into (Ca)–Fe(III)–PO₄–OC nanoparticles and led to the formation of Fe(III)-colloids and Fe(III)–organic complexes that passed 0.2 μm filter membranes at the highest tested (OC/Fe) levels. Macromolecular LH (humic acid) had similar effects on Fe coordination and PO₄ uptake to 2,4-DHB and Galact, but distinct effects on the nanoscale precipitate structure and colloidal properties. Ca enhanced the co-precipitation of PO₄ and OC with Fe(III), the aggregation of Fe(III)-precipitate particles, and LH coagulation. Collectively, the insights presented in this study highlight the need to take the type of OC, its concentration relative to Fe, and interdependent effects of OC, PO₄, SiO₄, and Ca into account when assessing the formation of Fe(III)-precipitates by Fe(II) oxidation in near-neutral natural waters and their impacts on the cycling of PO₄, OC and other co-precipitating compounds.