Competitive sorption of arsenate and phosphate on different clay minerals and soils

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.