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 (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 PO, 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 SO, 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 SO, 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.