Mathematics – Logic
Scientific paper
Feb 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.1261c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 4, pp.1261-1269
Mathematics
Logic
4
Scientific paper
Phosphate adsorption and desorption by iron hydr(oxides) are geochemically important processes affecting water quality and soil fertility. Relatively little is known, however, about the interaction between phosphate and hematite, which after goethite is the most abundant iron hydr(oxide) in nature. To study the effect of morphology and other crystal properties of hematite on phosphorous adsorption and desorption, thirty hematites synthesized by different methods and differing widely in properties were used. Phosphate adsorption curves 1 and 124 days after P addition were adequately described by the Freundlich equation. The P adsorbed at an equilibrium concentration of 1 mg P/L (32 M) at 1 and 124 days ( A 1 , A 124 ) ranged between 0.31 and 2.07 , and between 0.54 and , respectively. The values of A decreased when the crystals and the X-ray coherently scattering domains became more platy in the c with respect to the a direction, supporting the hypothesis that the P-adsorbing faces are the nonbasal ones. Exponent b of the Freundlich equation, which gives a measure of the relative affinity of the adsorption sites, increased when domains and crystals became more platy, i.e., the more platy the crystal, the lower the proportion of high-affinity sites. The ratio between slow (1-124 days) and total (124 days) adsorption increased with decreasing affinity (higher b values) and was not related to microporosity. In comparison to goethite, the present results show that hematite has more variability in P-adsorption capacity, adsorbs on average less P per unit surface area, has less affinity for phosphate, and exhibits a more important slow adsorption component. The proportion of the adsorbed phosphate that could be desorbed by an anion-exchange resin or by electroultrafiltration was affected by those hematite properties affecting diffusion (e.g., microporosity). Isotopically exchangeable P did not depend on morphological or crystalline properties. Irrespective of the method, desorbability, i.e., the proportion of adsorbed phosphate that was desorbable or isotopically exchangeable, was positively correlated to parameter b , suggesting that more desorption occurs in samples having a higher proportion of low-affinity sites. Desorbability was also positively correlated to the ratio between slow and total adsorption. It appears, therefore, that slow adsorption and desorption take place preferentially at low-affinity sites.
Barron Vidal
Colombo Camilla
Torrent Jose
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