| Poorly-crystalline Al(III)- and Fe(III)-hydroxides have a high capacity to sorb orthophosphate (PO4). Due to the redox-active characteristic of Fe, fluctuation in redox potential might be one of the major factors influencing PO4 fate in soils and sediments. The goal of this research was to determine molecular mechanisms of PO4 retention and dissolution in non-crystalline Al(III)/Fe(III)-hydroxide co-precipitate systems as affected by dissimilatory microbial Fe(III) reduction. The specific objectives were: (1) to quantify the relative distribution of sorbed PO4 bonding between Al(III) and Fe(III) in relation to the structural properties of Al/Fe-hydroxide co-precipitates; (2) to determine how Al co-precipitation affects Fe(III) bioreducibility in relation to the local coordination structure and transformations in Fe-hydroxide domains; and (3) to characterize PO4 re-distribution between solid phases and aqueous solutions during bioreduction of Al/Fe-hydroxide co-precipitates. Phosphate sorption isotherms experiments at PO4 concentrations between 42 and 162 mmol mol-1 Al+Fe were conducted on Al/Fe-hydroxide co-precipitates with Al/(Al+Fe) molar ratios of 0, 20, 50, 75, and 100 mol%. Characterization using x-ray absorption near edge structure (XANES) spectroscopy shows preferential PO4 bonding to Al on co-precipitates containing ≤50 Al mol%, but a non-preferential PO4 distribution to either Al or Fe as Al proportion was increased to 75 mol%. Such PO4 distribution was correlated to the near-surface composition of Al as indicated by x-ray photoelectron spectroscopy (XPS). Along with increasing proportion of co-precipitated Al, trends in decreasing structural ordering and decreasing size of Fe domains were found. After 168 h of bioreduction promoted by Shewanella putrefaciens CN32, total Fe(II) production increased 40-fold as co-precipitated Al increased from 0 to 50 mol%. Over the course of bioreduction, Al tended to impede microbial-induced structural development in Fe domains. Apparently by optimizing surface interactions between bacterial cells and Fe(III)-hydroxides, increasing Al enhanced electron transfer between Fe(III) and microbes. Alternatively, bioreduction of phosphated Al/Fe-hydroxide co-precipitates was not only a function of Al proportion, but also a function of sorbed PO4 concentration. In co-precipitates with ≤20 mol% of Al, the Fe(III) bioreducibility increased with increasing sorbed PO4 concentration. Analogous to co-precipitated Al, PO 4 stabilized Fe domains against polymerization. For samples with ≥50 mol% of Al, however, effects of Al-hydroxide appeared to be greater than that of PO4, resulting in no effect of sorbed PO4 concentration on Fe(III) bioreducibility. During the 168-h bioreduction period, no reductive dissolution of PO4 was detected, contrary to my hypothesis. Availability of Al and Fe binding in residual solids during bioreduction accounted for PO4 retention. Accompanying enhanced Fe(III) bioreduction, an increase in Fe[(II)/(III)]-PO4 bonding was generally responsible for PO 4 sorption in systems with Al ≤20 mol%. As Al(III) became dominated (Al ≥50 mol%), however, a preferential PO4 bonding to Al was found in residual solids. Superior PO4 sorption capacity of Fe[(II)/(III)] and Al(III) polymers retained PO4 against other possible solid-phase retention pathways such as precipitation of biominerals or microbial uptake, and thereby constrained reductive PO4 dissolution. In general, my research showed that enhanced dissolution of PO4 observed under reducing soil conditions depends on processes other than direct release of PO4 into solution during reductive dissolution of Fe(III)-hydr(oxide) minerals. |
