Phosphate Binding and Iron(III) Reduction as Affected by Iron(III) and Organic Matter Interactions

Being a vital element for life, phosphorus (P) is usually added to soils in the forms of P fertilizer and animal waste (manure) to enhance plant growth. However, due to the imbalance between P input and uptake by plant produce, excess P accumulates in soils. This surplus P will post an environmental threat if P runs off soils to an aquatic system because the elevated P could deteriorate the ecosystem in the aquatic system. As important soil components, organic matter (OM) and Fe-oxide minerals both play critical roles in nutrient retention and release for plant growth. Understanding the interactions between OM and Fe-oxides is crucial for soil remediation or effective P fertilizer application. The goal of this research is to determine at a mechanistic level how interactions between soil iron minerals and organic matter affect phosphate (PO4) binding and reductive dissolution. The specific objectives are: (1) determine whether simple organic acids (OA) compete or corporate with phosphate to bind with Fe and the conditions in which a ternary OA-Fe-PO4 compound forms; (2) synthesize an organo-Fe(III)-PO4 ternary compound and determine its XAS (X-ray absorption spectroscopy) spectral features that can be applied to characterize PO4 binding to Fe-OM complexes; (3) determine how interactions between organic matter and ferrihydrite with time affects PO4 sorption and elucidate the underlying mechanism; (4) determine the redox properties of particulate organic matter, Pahokee peat; (5) determine the influence of Pahokee peat on ferrihydrite chemical and microbial reduction. Coordination between Fe and PO4 dominates the reactions of Fe(III), PO4, and each of the six organic acids, including oxalate, citrate, malonate, succinate, tartrate, and benzoate at pH 5.5. With a structural directing template, 1,3-diaminopropane, a new mixed anion Fe(III) phosphate/oxalate ternary compound, [C3H12N2]2[Fe5(C 2O4)2(HxPO4)8] (I), was synthesized. EXAFS (extended X-ray absorption fine structure spectroscopy) analysis of I illustrates that Fe-phosphate bonding is unambiguously differentiated from Fe-oxalate bonding, which confirms the feasibility of deciphering the coordination structure of Fe in complex matrices by EXAFS spectroscopy. Ferrihydrite (FH) was mixed with various amount of organic matter and the mixtures were let age for 55 days. All OM/FH mixtures show decreased PO4 sorption upon aging. With controlled constant total Fe and peat, a peat/FeCl3/FH mixture is more effective for PO4 sorption than a peat/FH mixture. Fe-peat complexation illustrates characteristic EXAFS spectral features which are missing in the EXAFS spectra of aged peat/FH mixtures. The decreased PO4 sorption in the aged OM/FH mixtures and the lack of EXAFS spectral features for Fe-peat complexation confirm no Fe migration from the mineral to organic matter. We conclude that the sorption interaction between OM and ferrihydrite during aging increases OM stability so that it possesses stronger ability to resist the competition with PO4 for sorption sites. Pahokee peat, was reduced chemically and tested for its reducing capacity with respective to ferrihydrite, Fe-citrate (Fe-Cit), and FeCl3. The amount of electron transferred from H 2 reduced peat to Fe(III) compounds follows the order of FeCl3 >Fe-Cit>FH. The kinetics of peat chemical reduction is well fit by a diffusion limited rate law proceeding in spherical particles. Two distinct redox reservoirs exist in peat: one assigned to quinone functional groups and the other to reducible acid and neutral carbonyl functional groups. Chemical reduction of peat/FH mixtures clearly demonstrates that ferrihydrite and peat are reduced independently and peat does not act as an active shuttle between H2, the electron source and ferrihydrite. Moreover, peat inhibits ferrihydrite chemical reduction. Kinetic modeling of all chemical reduction data reveals that the interactions between peat and ferrihydrite regulate the mechanism by which peat influences ferrihydrite chemical reduction. By contrast, peat acts as an electron shuttle in ferrihydrite microbial reduction and enhances the reduction. However, data of dissolved Fe(II) and organic carbon suggest that organic matter also act as an Fe(II) complexation agent which facilitates biogenic Fe(II) transport and eliminates the formation of mixed valence secondary Fe mineral.