| Mobilization of phosphorus from land to surface waters frequently results in the nutrient enrichment of aquatic ecosystems, which leads to enhanced primary productivity and water quality degradation. Mobilization of inorganic orthophosphate (PO4) in soils depends on pH, redox potential, dissolved organic carbon (DOC), and concentrations and forms of accessible Fe and Al. Because the roles of these parameters on PO4 retention in soil organic matter (OM) are largely undefined, my goal was to identify and quantify their roles on PO4 retention in soil OM. My research objectives were to: 1. Determine the molecular coordination environment of Fe(III) bound to OM by direct addition of Fe(III), and addition of Fe(II) followed by oxidation, 2. Identify and separate pH and DOC effects on PO4 retention by OM, 3. Model the relative effects of pH and DOC on PO4 retention by OM, 4. Determine the impact of different peat/metal ratios on the microbial reduction rate of Fe(III) bound to peat, and 5. Determine the impacts of bound Fe and Al proportions on mobilization rates of PO4 from OM. Phosphate sorption may vary with the molecular bonding environment of Fe(III) in seasonally inundated organic soils.;In Pahokee peat with 1200 mmol Fe kg-1 peat, extended X-Ray absorption fine structure (EXAFS) spectroscopy indicated the presence of mixed Fe species ranging from mononuclear Fe-NOM to polymeric Fe(III) clusters. The bonding environment of Fe(III) and the PO4 sorption capacity of Fe(III) bound to OM were indistinguishable between samples where Fe(III) was added directly (as Fe3Cl), and where Fe(II) (as Fe2Cl) was added followed by an oxidation period of 39 h. The pH dependent sorption of PO4 to OM was characterized by envelopes analogous to adsorption envelopes for Fe- and Al-oxide minerals, with maximum PO4 sorption of 420 and 380 mmol PO4 kg-1 peat in Fe and Al systems, respectively. At pH 6.0, dissolved PO4 increased at a rate of 0.36 mumol PO4 mg-1 DOC, suggesting that although pH dominantly controlled PO4 sorption, DOC also played a role. The resultant PO4 sorption to Fe-OM was successfully modeled using a surface complexation model in Visual MINTEQ. No trends in Fe(III) reduction rates were observed as OM concentrations were increased from 0 to 3,330 g OM mol-1 Fe. However, as peat increased from 0 to 833 g OM mol-1 Fe, the proportion of dissolved Fe(II) to bound Fe(II) decreased, and as peat increased from 833 to 3,330 g OM mol-1, the proportion of dissolved Fe(II) to bound Fe(II) increased. Phosphate mobilization from organic matter was also correlated with Fe(III) reduction (P < 0.001), and the Pseudo first-order rate coefficients of PO 4 dissolution increased as Fe/Al ratio increased. Overall, PO4 sorption to OM occurred, and was dominantly controlled by pH. Results suggested that maintaining organic soils between pH 4.0 and 5.0 would maximize PO4 sorption in aerobic organic soils. At any given pH, PO 4 sorption was influenced by bound Fe and Al, DOC, and redox state. Iron(III) reduction was closely tied to PO4 mobilization, and the mobility of Fe(II) depended on total bound Fe(III) concentration. Maximum PO4 mobilized during Fe(III) reduction decreased as bound Al(III) increased, suggesting Al addition would enhance PO4 sorption to organic soils. Overall, the interdependent roles of pH, redox potential, DOC, and bound Fe and Al controlled PO4 sorption to Fe- and Al- bound to OM. |
