Effect of Organic Matter on Reductive Phosphate Dissolution from Poorly-Crystalline Fe/Al-Hydroxide Coprecipitates.

Under microbial reduction conditions, dissolved phosphate from soils may increase by up to one order of magnitude. The objective of this research was to characterize the effect of organic matter on sorbed phosphate dissolution during microbial reduction of Fe/Al-hydroxide coprecipitates (Al-FH). Aqueous suspensions of Al-FH with 0% to 100% of Al/Al+Fe mole ratios containing 1000 mmol kg^-1 of sorbed phosphate were microbially reduced using Shewanella putrefaciens CN32 (CN32). After 42 h, phosphate adsorption on Al-FH (Al-FH-PO₄), anthraquinone-2,6-disulfonate (AQDS), a well-known surrogate for the semiquinone electron shuttles in humic substances, and formate, as a electron donor, were added to the suspensions of Al-FH-PO₄. Controls without AQDS were also prepared. The Fe reduction was initiated by inoculating CN32 into Al-FH-PO₄ under anaerobic environment and reacted over time. Initial results from 4 days of incubation only showed Fe(II) production to be greater for the 0% Al-FH with addition of AQDS compared with Al-FHs with greater Al. Iron(II) production from Al-FHs with 50 or 75% were almost the same as the control without CN32 inoculation for the short-term incubation. Although around 40% of total Fe was reduced from 0% Al-FH with addition of AQDS, essentially no PO₄ dissolved. The results of this research on interaction among organic matter, microorganisms, Fe/Al, and PO₄ will provide insights on retention and release of PO₄ in organic matter rich soils like those found in the Coastal Plain of North Carolina.