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

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 (SO 4 ), oxalate (OX), citrate (CIT), and phosphate (PO 4 ) in binary and ternary systems on synthetic and natural allophanic samples at different pH values (4.0–7.0). The adsorption capacity of SO 4 , CIT, and OX (in the order listed) was much lower than that of PO 4 , but CIT decreased the zero point charge (ZPC) more than OX, meanwhile PO 4 increased the ZPC, whereas SO 4 did not change the ZPC of the sorbents. The adsorption of SO 4 was strongly inhibited in the presence of equimolar amounts of OX, CIT, or PO 4 even at low surface coverage of all the ligands. The lower the final pH of the binary systems, the greater the inhibition of SO 4 adsorption. The capacity of CIT to prevent SO 4 adsorption (ranging from 22–63%) was similar or slightly greater than that of PO 4 (ranging from 25–48%) but clearly higher than OX (ranging from 20–42%). On the contrary, SO 4 very poorly prevented the adsorption of the other ligands (usually lower than 10%). The effectiveness of the other ligands in retarding or inhibiting SO 4 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 SO 4 adsorption more than PO 4 (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 SO 4 and the other ligands. However, competition in adsorption with SO 4 for common adsorption sites was greater for the organic ligands than for PO 4 , because the surface coverage of PO 4 was particularly low with respect to that of SO 4 , CIT, and OX.