Competitive Adsorption of As and P to Goethite, Ferrihydrite and Allophane and their Remobilization by Rhizosphere Processes.
Doris Vetterlein1, Krisztian Szegedi1, Juergen Mattusch1, Heinz-Ulrich Neue1, Reinhold Jahn2, Juliane Ackermann3, Annett Kaufhold4, and Hartmut Tanneberg4. (1) Centre for Environmental Research UFZ, Theodor-Lieser-Str. 4, Halle/Saale, Germany, (2) Institute of Soil Science and Plant Nutrition, Martin Luther University, Halle(Saale)210, D-06108, Germany, (3) Martin-Luther-University Halle Wittenberg, Weidenplan 14, Halle/Saale, Germany, (4) Martin-Luther-University Halle-Wittenberg, Weidenplan 14, Halle/Saale, Germany
The minerals ferrihydrite, goethite and allophane are known to adsorb high amounts of phosphate and of arsenate which is chemically similar to phosphate, showing nearly the same dissociation constants. The minerals differ in their specific surface area and in their affinity for phosphate and arsenate. In addition their sorption capacity is altered to a different extent by changing soil pH. Arsenate and phosphate are taken up by the plants via the same transporter systems, which likewise show different affinities for arsenate and phosphate. Hence, the ratio of phosphate to arsenate in soil solution is important for arsenate uptake into the plant and thus transfers of arsenic into the food chain. In addition, plants have evolved a number of mechanisms to increase P availability in soils like exudation of organic acids for ligand exchange and acidification of the rhizosphere by proton release. Experiments were conducted to evaluate whether plants increase As mobility and uptake in the attempt to mobilise P from soil mineral phases differing in their sorption capacity and affinity to phosphate and arsenate. Experimental findings from a compartment system, which enabled us to follow changes in soil solution composition over time and with increasing distance from the root surface, are compared to calculations of soil solution equilibrium with the geochemical code PHREEQC. Phosphate and arsenate were supplied in a molar ratio of 37.5 to 1 (equivalent to 80 mg P kg-1 and 5 mg As kg-1). The minerals Goethite, Ferrihydrite and Allophane were added to an artificial quartz substrate at rates of 1 and 4 g per kg. The adsorption of arsenate by the minerals decreased in the order ferrihydrite>goethite>allophane with the initial soil pH varying from 5.5 to 6.0. For phosphate the adsorption was similar for ferrihydrite and allophane but lower for goethite. As a result of the different affinities of the minerals for phosphate and arsenate, the ratio of P to As in soil solution increased with increasing amounts of ferrihydrite or goethite application but decreased with increasing amounts of allophane application. Plants were able to mobilize substantial amounts of P from goethite, indicated by the increase of P concentrations with time in the soil solution sampled from the root compartment. Mobilization could hardly be detected from ferrihydrite of allophane. For Arsenate no mobilisation was observed from goethite or ferrihydrite which is in line with the higher affinity of this minerals for As compared to P indicated by the changes in P/As ratio in the soil solution. For allophane As mobilisation was not expected as there was very little adsorption to begin with. The changes in soil solution concentrations of phosphate and arsenate induced by the addition of the minerals were reflected in plant As and P uptake. Small amounts of goethite and ferrihydrite (1g kg-1) alleviated the growth depression induced by arsenate; higher application rates did not further improve (goethite) or even decreased (ferrihydrite) growth due to the induction of P deficiency which could not be overcome by mobilizing some of the phosphate.