Interaction of synthetic sulphate "Green rust" with phosphate and the crystallization of vivianite

Because layered Fe(II)Fe(III)-hydroxides (Green rusts, GRs) are anion exchangers, they represent potential orthophosphate sorbents in anoxic soils and sediments. To evaluate this possibility, two types of experiments with synthetic sulphate-interlayered GRs (GR(SO4) = Fe42+Fe23+(OH)(12)SO4 xH(2)O) were studied. First, sorption of phosphate in GR(SO4) was followed by reacting suspensions of pure GR(SO4) synthesized by oxidation of Fe(II) with an excess of Na2HPO4 (pH 9.3). Second the possible incorporation of phosphate in GR during formation by Fe(II)-induced reductive dissolution of phosphate-containing ferrihydrites was examined in systems containing an excess of Fe(II) (pH 7). With excess phosphate in solution, GR(SO4) initially sorbed phosphate in the interlayer producing a basal layer spacing of 1.04 nm, but only similar to 50% of the interlayer sulphate was exchanged with phosphate. This GR slowly transformed to vivianite within months. In the Fe(II)-rich systems, reaction with synthetic ferrihydrites produced GR(SO4) similar to that produced by air oxidation. Reaction of Fe(II) with phosphate-containing ferrihydrites initially produced amorphous greenish phosphate containing precipitates which, after 3-4 h, crystallized to GR(SO4) and vivianite. In these solutions, stable phosphate-free GR(SO4) can form since precipitation of vivianite produced low phosphate activity. Consequently, in both systems GR or amorphous greenish precipitates act as reactive intermediates, but vivianite is the stable end-product limiting phosphate concentration in solution. It is also inferred that Fe(OH)(2) is an unlikely phosphate sorbent in mixed Fe(II)-Fe(III) systems because GR phases are more stable (less soluble) than Fe(OH)(2).