Phosphorus binding by poorly crystalline iron oxides in North Sea sediments

Abstract

Differential X-ray powder diffraction (DXRD) and extraction procedures were used to characterize the iron oxides present in four sediments from contrasting environments in the North Sea. Stations were located in depositional areas on the southern shelf (German Bight) and on the north-eastern shelf-slope transition (Skagerrak) and in areas with no net deposition in the southern North Sea. Poorly crystalline ferrihydrite and akageneite (extractable with 0.1 M HCI and 0.2 M NH,-oxalate) were identified in the fine sediment fraction ( < 10 pm) of surface samples at all locations. Evidence for the dominant role of these Fe oxides in the binding of phosphorus in North Sea sediments was obtained from the good relationship of both the content of Fe-bound P and the linear adsorption coefficient for phosphate with NH,-oxalate extractable Fe. A tight coupling of pore water Fe*' and HPO$-was observed at 3 stations. Pore water Fe'+/HPOjratios at maximum pore water concentrations of Fe*+ were similar to NH,oxalate Fe/Fe-bound P ratios for surface sediment at these locations, and were in the range known for synthetic poorly crystalline Fe oxides. This suggests that pore water HPOi-production at the time of core collection was dominated by release from poorly crystalline Fe oxides. In contrast, at the German Bight station, much higher HPOi-levels and a decoupling of pore water Fe*+ and HPOi-was observed, suggesting a larger contribution of mineralization of organic matter to pore water HF'Oa-than at the other sites. Solid phase P analyses indicate possible redistribution of Fe-bound P to another inorganic phase at depth at the Skagerrak station, but not at the other stations. The persistence with depth of poorly crystalline Fe oxides and Fe-bound P suggests that these Fe phases can act as both a temporary and permanent sink for P in continental margin sediments.

Dysaerobic conditions during Heinrich events 4 and 5: Evidence from phosphorus distribution in a North Atlantic deep-sea core

“…Therefore, we have also performed phosphorus extractions on bulk sediments from selected HL and ambient glacial samples. Fe-bound P and organic-bound P concentrations in both bulk and fine-sized fraction are in the same range of values, confirming the assumptions that (a) P associated with Fe oxyhydroxides is mainly present in the fine fraction (Slomp et al, 1996), and (b) organic matter is mainly concentrated in this fraction (see discussion above). Detrital P concentrations are slightly higher in bulk sediments than in the Ͻ50-m fraction, but being nonreactive, detrital P is not affected by postburial redistribution.…”

“…during the first step of the SEDEX method (Ruttenberg, 1992;Slomp et al, 1996), exhibit higher values for Holocene samples than for ambient glacial sediments.…”

“…In all cases, as dissolved P is removed either from the water column or from the sediments, we can consider and refer to it as authigenic. Ferric Fe from both amorphous and well-crystallized minerals and Fe from amorphous Fe sulfides have been extracted during the first step of the SEDEX method (for details on the methodology, see Table 1 and Slomp et al, 1996).…”

Modelling the geochemical fate and transport of wastewater-derived phosphorus in contrasting groundwater systems

“…c Assuming a P:Fe ratio of 0.5 (Gunnars et al, 2002) and 10% of Fe(OH) 3 is available for fast sorption (Slomp et al, 1996); d Assuming a P:Ca ratio of 0.6 (Freeman and Rowell, 1981) and 5% of CaCO 3 is available for fast sorption (Cole et al, 1953). solids are denoted in mM (mmol dm − 3 pore volume ) and mmol dm − 3 (mmol dm − 3 total volume ), respectively. The reaction network includes aerobic dissolved organic carbon (DOC) degradation, denitrification, secondary reoxidation reactions, precipitation/dissolution of calcium carbonate and P minerals, acid-base equilibria and reversible P and NH 4 + sorption.…”

“…where P s-fast is the concentration of P sorbed through equilibrium sorption (mol dm − 3 ) and X T is the maximum capacity of the aquifer solids (Fe(OH) 3 or CaCO 3 ) for fast sorption (mol dm − 3 ).In the Fe oxide-rich aquifer, 10% of the Fe(OH) 3 is assumed to be available for fast sorption (Slomp et al, 1996), so that X T = 0.1 ⁎ Fe(OH) 3 (Table 1). Similarly, 5% of the CaCO 3 is available for fast sorption (Cole et al, 1953).…”