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DIVISION S-2—SOIL CHEMISTRY

Competitive Sorption of Arsenate and Phosphate on Different Clay Minerals and Soils

Dipartimento di Scienze del Suolo, della Pianta e dell'Ambiente, Università di Napoli Federico II, Via Università 100, 80055 Portici (Napoli), Italy

* Corresponding author (violante{at}unina.it)

Sorption and desorption of AsO₄ on or from different soil components may have a dominant role in regulating As mobility in soils. The objectives of this work were to provide information on the factors that influence the competitive sorption of AsO₄ and PO₄ in soil. We studied the competitive sorption of PO₄ and AsO₄ on selected phyllosilicates, metal oxides, synthetic organo-mineral complexes, and soil samples as affected by pH (4.0–8.0), ligands concentration, surface coverage of the oxyanions on the samples and the residence time. We found that Mn, Fe, and Ti oxides and phyllosilicates particularly rich in Fe (nontronite, ferruginous smectites) were more effective in sorbing AsO₄ than PO₄. In fact, by adding AsO₄ and PO₄ as a mixture (AsO₄/PO₄ molar ratio of 1) more AsO₄ than PO₄ was usually sorbed on birnessite, pyrolusite, goethite, nontronite, and ferruginous smectite, but more PO₄ than AsO₄ was sorbed on noncrystalline Al precipitation products, gibbsite, boehmite, allophane, and kaolinite. For example, at pH 5.0 the sorbed AsO₄/sorbed PO₄ molar ratio (rf) was 1.81 for birnessite, 1.05 for nontronite, but was only 0.45 for kaolinite and 0.14 for allophane. For montmorillonite, illite, and vermiculite the rf values were slightly <1. For soil samples, particularly rich in kaolinite, halloysite, allophane, and containing relatively large amounts of organic C, the rf values were usually much <1. For all the samples, the rf values increased by decreasing the pH and with the residence time of the oxyanions. The sorption of AsO₄ (or PO₄) on goethite and gibbsite decreased by increasing the initial PO₄/AsO₄ (or AsO₄/PO₄ molar ratio) up to 2.0. However, PO₄ inhibited AsO₄ sorption more on gibbsite than on goethite, whereas AsO₄ prevented PO₄ sorption more on goethite than on gibbsite. The data reported in this paper suggest that the mobility, the bioavailability, and the toxicity of As in soil environments may be greatly affected by the nature of soil components, pH, presence of anions (PO₄), and residence time.

Abbreviations: Ald, Al extracted by Na-dithionite-citrate • Alo, Al extracted by NH₄-oxalate • EGME, ethyleneglycol monoethylether • Fed, Fe extracted by Na-dithionite-citrate • Feo, Fe extracted by NH₄-oxalate • IEP, isoelectric point • IMt-2, Montana illite • KGa-1, Georgia kaolinite • OOMWW, olive oil mill waste water • PZC, point of zero charge • PZSE, point of zero salt effect • rf, molar ratio • SWy-1,Wyoming montmorillonite • TEM, transmission electron microscopy • XRD, x-ray diffraction

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