吸附在水铁矿,赤铁矿和针铁矿表面的磷酸盐有效性研究

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Guzman et al. Fe (Fet) by dissolving the sample in 11 M HC1. Dissolved Fe was analyzed by atomic absorption spectrophotometry. The products were examined by X-ray powder diffraction as described by Torrent et al. (1990). Specific surface area was determined by N 2 adsorption (BET method) using a Quantasorb Jr. apparatus. The N2-adsorption data were also used to measure microporosity by the t-plot method (Gregg and Sing, 1982). Transmission electron micrographs (TEM) were obtained with a Jeol JEM-200 CX instrument, after diluting a portion of the stock suspension in acetone, and evaporating a droplet of the resulting suspension on a C-coated Cu grid. Phosphate adsorption curves Phosphate adsorption curves were obtained by adding different amounts of KH2PO 4 to 10-12 suspensions of 10 mg (for F) or 60 mg (for H1, H2, G1, and G2) of Fe (hydr)oxides in the nutrient solution described later. The final volume was 30 mL and the suspensions were shaken at 2 Hz and 298 K for 1 day, and, then, 15 min daily for 11 days. After 1, 3, 6, and 12 days, samples of the suspensions were centrifuged, and P in the supernatant was analyzed (Murphy and Riley, 1962). Final equilibrium concentrations ranged between 0.01 and 10mg P L -1. To describe adsorption, the following modified Freundlich equation (Barrow and Shaw, 1975) was used: X = Acbt d (1)
domain arrangement and other factors, which are, in turn, dependent upon the type of mineral. In a study of some Terra Rossa soils, Colombo et al. (1991) found that goethite was more effective than hematite in retaining phosphate against electroultrafiltration. Unfortunately, the behavior of the various Fe (hydr)oxides has not been adequately examined, despite its practical implications for plant nutrition. In this work, the phytoavailability of P adsorbed on synthetic ferrihydrite, hematite and goethite has been studied.
Received 11 June 1993. Accepted in revised form 9 October 1993
Key words:
Abstract
adsorption, availability, ferrihydrite, goethite, hematite, phosphate
Iron (hydr)oxides are important phosphate adsorbents in soils. However, the dependency of phytoavailability of adsorbed phosphate on mineral type and properties has not been established. To examine the influence of mineralogy, one ferrihydrite, two hematites, and two goethites were prepared. They differed widely in morphology (from euhedral crystals to granular aggregates), specific surface area (15-266m 2 g-l) and microporosity (0-87% of total surface area). Their relative affinity for phosphate, which is inversely related to the concentration exponent in the Freundlich equation, b, decreased in the order goethite (b = 0.11, 0.09) > hematite (b = 0.13, 0.17) > ferrihydrite (b = 0.21). Phytoavailability of the adsorbed phosphate was studied by growing sunflower (Helianthus annuus L.) in pots containing suspensions of the phosphated Fe (hydr)oxides in equilibrium with a concentration of 1 mg P L -1. The fraction of the adsorbed phosphate that was phytoavailable, B1, (1) was lower for goethite (B 1 = 0.43, 0.38) than for hematite (B1 = 0.73, 0.49) and ferrihydrite (B 1 = 0.79), (2) was not negatively affected by microporosity, and (3) decreased, in general terms, with increasing relative affinity for phosphate. Similar trends were observed for the production of dry matter. These results, together with previous reports showing that goethites usually have higher relative affinity for phosphate than hematites, suggest that phosphate could be more available to plants in hematitic than in similar goethitic soils.
Introduction It is well established that Fe (hydr)oxides are important phosphate adsorbents in soils (Schwertmann and Taylor, 1989). Parfitt (1978) indicated that phosphate is adsorbed on the hydroxylated surfaces of these minerals as a binuclear bridging complex. This reaction takes place at contiguous OH groups in one coordination with Fe atoms (Type 'A' hydroxyls). Accordingly, the P-adsorption capacity of Fe (hydr)oxides depends on the proportion of faces that possess contiguous A hydroxyls. This model is supported by many P-adsorption measurements on Fe (hy-
dr)oxides that were crystallographically characterized (Barr6n et al., 1988; Torrent et al., 1990, 1992; Willett et al., 1988). Only 30-60% of the phosphate in surface complexes on goethite is isotopically exchangeable (Torrent et al., 1992) suggesting relatively low phytoavailability. Parfitt (1979) showed that the phosphate adsorbed on goethite was not readily available to ryegrass until 40% of the P-adsorption capacity of the goethite had been flled. It is likely that phytoavailability (and desorbability) of phosphate in surface complexes is influenced by the distance between contiguous hydroxyls, electrostatic environment, porosity,