Competitive Sorption of Arsenate and Phosphate onto Synthetic Hematites with Different Particle Morphology.
M. Pigna1, Claudio Colombo2, Giuseppe Palumbo2, and Antonio Violante3. (1) Univ di Napoli, Università 100, Portici (Napoli), Italy, (2) Dip. SAVA Molise Univ, Via De Sanctis, Campobasso, 86100, Italy, (3) Univ of Naples FedericoII, Portici (Napoli) I, Via Università 100, Naples, 80055, Italy
The presence of arsenic in the environment is attributable both to the parent materials and to anthropogenic waste. Arsenic may lead to contamination of both agricultural soils and surface waters and sediments in areas where mining and smelting are present (Smith et al., 1998). Sorption processes of arsenate on different soil components may have a dominant role in regulating arsenic mobility in soils. The nature of inorganic soil components affects the competition in sorption between arsenate (As) and phosphate (P). In previous studies we have demonstrated that Fe-oxides and phyllosilicates particularly rich in Fe (nontronite, ferruginous smectites) were more effective in sorbing As than P. In fact, by adding As and P as a mixture (As/P molar ratio of 1) at 100% of surface coverage and after 24 h of reaction more As than P was usually sorbed on goethite, nontronite and ferruginous smectite, but more P than As was sorbed on noncrystalline Al precipitation products, gibbsite, boehmite, allophane, and kaolinite. The sorbed As/sorbed P molar ratio (rf) indicated the selectivity of each sample to adsorb preferentially one of the two ligands (Violante and Pigna, 2002). In this research, we have studied the competitive sorption of As and P onto synthetic hematites synthesized in the presence of organic and inorganic ligands. Synthetic hematites were obtained at different pH (5.5 to 10.0), temperatures (70-140°C) and in the presence of organic (oxalate, citrate, tartrate) or inorganic (silicate, nitrate) ligands and/or aluminum ions (Colombo et al. 1994). The sorption of As and P added alone or as a mixture (initial As/P molar ratio of 1) was studied at pH 4.0, keeping the suspensions to react from 1h to 20 days. Synthetic hematites showed different morphology, surface properties, and microporosity. In particular, hematite obtained in presence of oxalic acid exhibited particles with irregular ellipsoidal or spherical shapes and high surface area (109 m2 g-1) and porosity, hematite obtained in presence of citric acid exhibited crystals elongated in the c direction and a surface area of 20 m2 g-1, whereas that obtained in presence of tartaric acid exhibited exagonal crystals larger in diameter and a surface area of 29 m2 g-1. Hematite obtained without anion showed hexagonal shapes with well developed euhedral faces. Platy disk morphology with predominant 001 faces was observed in hematites with Al substitution. At a surface coverage of both the ligands of 100% and after 24 h of reaction, rf values were 2.70 for hematite with platy morphology, 2.17 for hematite synthesized in the presence of tartrate, 1.36 for hematite synthesized in the presence of citrate and 1.21 for hematite synthesized in the presence of oxalate. The kinetic studies showed that the rf values decreased up to 1 only after many days of reaction (12-20 days). In conclusion, this work showed that the competitive sorption between As and P were not only affected by the mineralogy of the sorbents, the pH and the reaction time but also by surface properties, morphology and porosity of the samples. Bibliography:(1) Colombo C., V. Barrón and J. Torrent. 1994. Phosphate adsorption and desorption in relation to morphology and crystal properties of synthetic hematites. Geochimica et Cosmochimica acta. 58. 1261-1269. (2) Smith, E., R. Naidu, and A.M. Alston. 1998. Arsenic in the soil environment: A review. Adv. Agron. 64:149–195. Violante A., M Pigna. 2002. Competitive Sorption of Arsenate and Phosphate on Different Clay Minerals and Soils. Soil Sci. Soc. Am. J. 66:1788–1796.