Phosphate adsorption by clays from Brazilian Oxisols: relationships with specific surface area and mineralogy

Oxisols derived from clayey sediments, sandstone, mafic rocks and schist were sampled in the “Triangulo Mineiro” region, Minas Gerais State, Brazil, to study the relationship between the phosphate adsorption and mineralogy and specific surface area. The specific surface area of the iron oxides estimated from N2 adsorption by the soil clays before and after dithionite-citrate dissolution of the Fe-oxides ranged from 45 to 110 m2 g−1. Specific surface area of the iron oxides correlated to the goethite: goethite +hematite ratio (r = 0.694∗). Specific surface area estimated by differential X-ray analysis did not show good agreement with BET-N2 adsorption values. Phosphate adsorption maxima for the soil clays (< 2 μm) ranged from about 70 to 160 μmol P g−1 of clay. Stepwise regression analysis showed phosphate adsorption for the total clays to be primarily related to goethite, gibbsite, the sum of Fe plus Al oxides and also amorphous Al oxides. A mild treatment for concentrating the soil iron oxides showed small effect in the phosphate adsorption for most soils. It showed the importance of gibbsite and crystalline Fe-oxides in the adsorption, with goethite explaining most of the variation occurring in P adsorption after the treatment. Correlation studies showed the adsorption capacity per unit specific surface to be somewhat dependent on variations in the iron oxide mineralogy of the clay samples. Clay color showed good correlation to the goethites: (goethite + hematite) ratio in soils but no direct relationship between phosphate adsorption and clay color was detected