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Soil Chemistry

Mutual Interactions of Sulfate, Oxalate, Citrate, and Phosphate on Synthetic and Natural Allophanes

^a Dep. de Ciencias Químicas, Univ. de La Frontera, Temuco, Chile
^b Dip. di Scienze del Suolo, della Pianta e dell'Ambiente, Univ. di Napoli Federico II, Napoli, Italy

^* Corresponding author (aljara{at}ufro.cl)

In soil environments, particularly at the soil–plant interface, organic and inorganic ligands may compete with each other for sorption sites onto soil components. The interaction between two or more ligands has a great importance for understanding the adsorption/desorption processes of a given ligand in soil. We studied the mutual interaction of sulfate (SO₄), oxalate (OX), citrate (CIT), and phosphate (PO₄) in binary and ternary systems on synthetic and natural allophanic samples at different pH values (4.0–7.0). The adsorption capacity of SO₄, CIT, and OX (in the order listed) was much lower than that of PO₄, but CIT decreased the zero point charge (ZPC) more than OX, meanwhile PO₄ increased the ZPC, whereas SO₄ did not change the ZPC of the sorbents. The adsorption of SO₄ was strongly inhibited in the presence of equimolar amounts of OX, CIT, or PO₄ even at low surface coverage of all the ligands. The lower the final pH of the binary systems, the greater the inhibition of SO₄ adsorption. The capacity of CIT to prevent SO₄ adsorption (ranging from 22–63%) was similar or slightly greater than that of PO₄ (ranging from 25–48%) but clearly higher than OX (ranging from 20–42%). On the contrary, SO₄ very poorly prevented the adsorption of the other ligands (usually lower than 10%). The effectiveness of the other ligands in retarding or inhibiting SO₄ adsorption on an Andisol sample containing 40% allophane decreased with increases in the reaction time. However, even after 15 d of reaction CIT (38%) prevented SO₄ adsorption more than PO₄ (27%) or OX (12%) did. Our results seem to demonstrate that changes in the electric potential of the surfaces after the addition of anions, as well as competition for adsorption sites, affected the reduction in adsorption of SO₄ and the other ligands. However, competition in adsorption with SO₄ for common adsorption sites was greater for the organic ligands than for PO₄, because the surface coverage of PO₄ was particularly low with respect to that of SO₄, CIT, and OX.

Abbreviations: AlSi, aluminosilicate • AlSiFe, allophane coated with iron oxide • CIT, citrate • HPLC, high performance liquid chromatography • LMMOAs, low-molecular mass organic acids • OX, oxalate • PO₄, phosphate • SO₄, sulfate • ZPC, zero point charge