Migration of phosphate into aggregated particles of ferrihydrite

123

Abstract--We investigated the crystallization of ferrihydrite prepared by hydrolysis of Fe(NO3)3 solutions containing phosphate. Crystallization was studied at different pH (3-9), temperatures (298, 323, and 373 K), and initial P/Fe atomic ratios for periods to 730 d. Generally, crystallization was inhibited or only poorly crystallized lepidocrocite was formed at P/Fe > 2.5%. Phosphate favored the formation of hematite over goethite at all temperatures for most of the pH and P/Fe ranges investigated. This result is consistent with a model in which phosphate acts as a template for hematite formation, in analogy with other anions, such as oxalate. However, goethite was preferentially formed at alkaline pH and P/Fe > 1%, probably because high phosphate concentration resulted in a large increase in the negative charge of the ferrihydrite particles. This resulted in turn in less aggregation, a process that is known to precede dehydration to hematite. Phosphate greatly influenced the morphology of hematite and goethite. Hematite was often ellipsoidal or spindle-shaped. Twinned goethite crystals with a hematite core were formed at alkaline pH at P/Fe > 1%. Both hematite and goethite particles incorporated phosphate in an occluded form not desorbable by repeated alkali treatments.

Section: Discussionmentioning

confidence: 96%

Effect of Phosphate on the Crystallization of Hematite, Goethite, and Lepidocrocite from Ferrihydrite

Gálvez

Barrón

Torrent

1999

123

“…By comparison, an E M P A of phosphate adsorption on ferrihydrite indicated that slow adsorption (after an initially rapid reaction) was the result of the time required for the phosphate to gain access to surface sorption sites located within aggregates of ferrihydrite particles (Willett et al 1988). The authors proposed time-dependent phosphate adsorption as a result of diffusion within aggregates rather than solid-state diffusion.…”

Section: Metal Solubility and Distribution During Ferrihydrite Phase mentioning

confidence: 99%

Coprecipitates of Cd, Cu, Pb and Zn in Iron Oxides: Solid Phase Transformation and Metal Solubility after Aging and Thermal Treatment

Martı́nez

1998

Abstract--Solid phase transformation and metal solubility were monitored after coprecipitation of Cd 2+, Cu 2+, Pb 2+ and Zn 2+ with Fe 3+ to form ferrihydrite by titration to pH 6. The (co)precipitates were aged at room temperature for up to 200 d and subsequently heated for 60 d at 70 ~ The mode of (co)precipitate formation, rapid and slow titration, was also investigated. Metal solubility was measured by anodic stripping voltammetry. Surface area, Fourier transform infrared (FTIR) and X-ray diffraction (XRD) analysis were used to follow the transformation of ferrihydrite after initial (co)precipitation. Electron microprobe analysis (EMPA) was used to show the distribution of metals within ferrihydrite aggregates. Thermal treatment produced a reduction in soluble Cd 2+ and Zn 2+, whereas Pb 2+ appeared to be expelled from the solid phase. The more stable coprecipitate (formed by slow titration) maintained a constant Cu z+ solubility after thermal treatment. Characterization of the solid phase by XRD indicated that the presence of low levels of metals did not affect the initial or final transformation products, although metals present during the slow titration seemed to stabilize a higher surface area material. The rapid titration resulted in a less ordered (I-line) ferrihydrite than the slow titration (9-line). Furthermore, FTIR analysis indicated that the presence of metals promoted the formation of mixed (microcrystalline) end-products. The initial coprecipitation products seem to determine the final thermal transformation products. These transformation products include ferrihydrite, hematite (Hm), and goethite (Gt)-and lepidocrocite-like microcrystalline structures. Although experimental conditions were favorable for the homogeneous distribution of metals throughout the coprecipitate, EMPA suggests that Cu and Zn segregation within aggregates of Fe oxides occurs.

“…The rapid approach to equilibrium was related to the well crystallized goethite used. For phosphate adsorption on well crystallized goethite, Strauss et al (1997a, b) and Willet et al (1988) found that an adsorption equilibrium was obtained within 1-3 d. The attainment of adsorption equilibrium indicates that the porosity of the goethite is negligible, and that only surface sites are involved in the adsorption process. The only exception was with phosphate in 0.01 M CaC12 electrolyte solutions, where adsorption was initially rapid followed by a slow precipitation reaction.…”

Section: Adsorption/desorption Processesmentioning

confidence: 99%

Effect of KCl and CaCl₂ as Background Electrolytes on the Competitive Adsorption of Glyphosate and Phosphate on Goethite

Gimsing

Borggaard

2001

Abstract--Competitive adsorption between glyphosate and phosphate on goethite was evaluated. The influence of background electrolyte on the adsorption of glyphosate and phosphate was also investigated by using 0.01 M KC1, 0.1 M KC1 and 0.01 M CaCI 2 as background electrolytes. Experiments showed that phosphate displaced adsorbed glyphosate from goethite, whereas glyphosate did not displace phosphate. Results also showed that the background electrolyte had a strong effect on phosphate adsorption, but little effect on glyphosate adsorption. Thus, there are differences between the adsorption of glyphosate and phosphate. The study also showed that 0.01 M KCI caused dispersion of goethite, resulting in inefficient filtering, and that phosphate precipitated as calcium phosphates in 0.01 M CaC12 background electrolyte solutions. The results suggest that 0.1 M KC1 is a more suitable background electrolyte to determine competitive adsorption processes involving glyphosate and phosphate.