. 1995. Weathering of clay minerals induced by fluoride' containing solutions from phosphog:fpsum by-product. . The production ofphosphate fertilizer from phosphate rock results in the formation of phosphogypsum (PG) and HF. Soluble F-ends up in process water and in the pore space of the PG by-product. This study determined changes in the amount, properties and mineral composition of soil clav uoon reaction with acidic PG solutions. Calcareous and non calcareous soils were reacted with PG leachates (PG-L, 30 mg F-'Lt') and process water (PG-PW,6070 mg F-L-') using a sequential batch equilibration method. Phosphogypsum-L did not alter the clay fraction as much as PG-PW, which dissolved as much as two-thirds of the clay fraction. Phosphogypsum-PW reduced surface area ofclay fractions from about 500 to 150 ^' g-'; Cation exchange capacity CEC was decreased from about 60 to 17 cmol" kg-r clay. Reduction in surface area and CEC in PG-PW treated soil corresponded to near complete dissolution of smectite and a negative enrichment of mica. Kaolinite in coarse clay of the PG-PW treaffnent was reduced to 20% Clay Separation and Mineralogical DeterminationClay fractions were separated from the soil samples after the 30th equilibration. Soil samples were subjected to ultrasonic dispersion for 6 min prior to clay separation through successive dispersion and gravity sedimentation cycles (Jackson 1979). The clay samples were further fractionated into coarse (0.2-2 pm) and frne (<0.2 pm) fractions by centrifugation (Jackson 1979). Carbonates in the control clay samples from the Ck horizon were removed by Na acetate-acetic acid treatment (McKeague 1978). All the clay samples were then Caand K-saturated for mineralogical analyses.The surface area of the clay samples was determined by adsorption of ethylene glycol monoethyl ether following the method of Carter et al. (1965). Cation exchange capaciry was calculated from the amount of Ca" displaced by I M NaCl from electrolyte-free Ca-saturated samples.Clay mineral identification was conducted on Ca-and Ksaturated slides by X-ray diffraction (XRD) analysis using a goniometer equipped with a Li-F curved crystal monochromator. Diffraction with Co-Ka radiation generated at 50 kV and 25 mA was carried out by step-scanning at 0.05 '2O intervals every 2 s. Potassium-saturated clays were scanned from 3-36 o2O after equilibration at O% relative humidity (RIf) and from 3 to 19 "2O for 54% RH treatment as well as after heat treatrnent at 300 and 550'C.Calcium-saturated clays were likewise scanned from 3 to 36 "2O at 547o RH and from 3 to 19 "2O after glycerol and ethylene glycol solvations.
Poulsen, L. and Dudas, M. J. 1998. Attenuation of cadmium, fluoride and uranium in phosphogypsum process water by calcareous soil. Can. J. Soil Sci. 78: [351][352][353][354][355][356][357]. Concerns about contaminant migration from phosphogypsum (PG) repositories prompted the present study. A sequential batch procedure, in which acidic process water (PW) from a phosphate fertilizer plant was reacted with samples of a calcareous till, was employed to determine the attenuation of environmentally sensitive elements by reaction with typical subsoils at PG waste repositories. PW leachates were monitored for total soluble fluoride (F) and pH during the experiment. Soil samples were analyzed for pH, F, cadmium (Cd), uranium (U), and calcium carbonate (CaCO 3 ) equivalent at the end of the study. Leachate pH increased from <2 to 5.5 where it stabilized; soil pH stabilized at 6.5. Fluoride solution concentrations were reduced within regulatory limits. Soils accumulated Cd, F, and U in excess of background concentrations (up to 99×, 83×, and 12.5×, respectively). Soil carbonates persisted after leachate was buffered near pH 4.5, and played a major role in Cd attenuation. The results of this laboratory analog of a PW leaching gradient suggest contamination of groundwater is unlikely but accumulation of toxic elements in soil is an issue that may require consideration in the long-term decommissioning of PG waste repositories. Les travaux dont nous faisons état ont été inspirés par les risques de migration des contaminants à partir des dépôts de phosphogypse. Une méthode séquentielle en discontinu, dans laquelle l'eau technique acide (ET) d'une usine d'engrais phosphaté était mise en réaction avec des échantillons de till calcaire, a été utilisée pour déterminer l'atténuation des éléments à risque pour l'environnement par réaction avec les sous-sols typiques des emplacements choisis comme sites de stockage du phosphogypse. Les lessivats d'eau technique étaient suivis quant à leur teneur totale en fluorures (F) et à leur pH durant l'expérience. Au terme de l'expérience, les échantillons de sol étaient analysés sur le pH, les teneurs en F, en cadmium (Cd) et en uranium (U) ainsi que sur l'équivalent en carbonate de calcium (C a CO 3 ). Le pH du lessivat remontait de moins de 2 à 5,5, niveau auquel il se stabilisait. De son côtés le pH du sol se stabilisait à 6,5. Les concentrations en fluorures de la solution étaient ramenées dans les limites réglementaires. Les concentrations de Cd, de F et de U dans les sols excédaient celles des sols naturels environnants dans des proportions allant respectivement, jusqu'à 99, 83 et 12,5 fois). Les carbonates du sol qui persistaient après que le lessivat ait été tamponné aux alentours de pH 4,5, jouaient un rôle de premier plan dans l'atténuation du Cd. À partir des résultats de cette réplique en laboratoire d'un gradient de lessivat de ET, on peut conclure qu'il y a peu de risques de contamination de la nappe mais l'accumulation d'éléments toxiques dans le sol est une question dont il faud...
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