Abstract
The reduction of less stable ferric hydroxides and formation of ferrous phases is critical for the fate of phosphorus in anaerobic soils and sediments. The interaction between ferrous iron and phosphate was investigated experimentally during the reduction of synthetic ferrihydrite with natural organic materials as carbon source. Ferrihydrite was readily reduced by dissimilatory iron reducing bacteria (DIRB) with between 52% and 73% Fe(III) converted to Fe(II) after 31 days, higher than without DIRB. Formation of ferrous phases was linearly coupled to almost complete removal of both aqueous and exchangeable phosphate. Simple model calculations based on the incubation data suggested ferrous phases bound phosphate with a molar ratio of Fe(II):P between 1.14 - 2.25 or a capacity of 246 - 485 mg·P·g-1 Fe(II). XRD analysis indicated that the ratio of Fe(II): P was responsible for the precipitation of vivianite (Fe3(PO4)2·8H2O), a dominant Fe(II) phosphate mineral in incubation systems. When the ratio of Fe(II):P was more than 1.5, the precipitation of Fe(II) phosphate was soundly crystallized to vivianite. Thus, reduction of ferric iron provides a mechanism for the further removal of available phosphate via the production of ferrous phases, with anaerobic soils and sediments potentially exhibiting a higher capacity to bind phosphate than some aerobic systems.
Highlights
Phosphorus is essential for life and is increasingly the limiting nutrient in some ecosystems, as nitrogen pollution becomes widespread [1]
X-ray diffraction (XRD) analysis indicated that the ratio of Fe(II): P was responsible for the precipitation of vivianite (Fe3(PO4)2·8H2O), a dominant Fe(II) phosphate mineral in incubation systems
Reduction of ferric iron provides a mechanism for the further removal of available phosphate via the production of ferrous phases, with anaerobic soils and sediments potentially exhibiting a higher capacity to bind phosphate than some aerobic systems
Summary
Phosphorus is essential for life and is increasingly the limiting nutrient in some ecosystems, as nitrogen pollution becomes widespread [1]. In soils and freshwater sediments, the fate and mobility of phosphorus may be controlled by iron geochemistry, through sorption and desorption, co-precipitation and dissolution with both ferrous (Fe(II)) and ferric (Fe(III)) minerals. Whilst sorption of phosphorous to ferric phases such as ferrihydrite (Fe5O6(OH)9) tends to occur under aerobic conditions, ferrous iron phases are among the most important components to react with phosphate in anaerobic environments. It has been shown that the capacity of soils and sediments to bind phosphate is substantially increased under anaerobic conditions, generally attributed to the formation of ferrous phases [3,4,5,6,7]. The disagreement regarding iron phases binding phosphate in complex environments, no doubt makes it important to understand how ferrous iron phases react with phosphorus
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