Surface properties and phosphate adsorption of binary systems containing goethite and kaolinite

In soils goethite and kaolinite are often cemented together as a binary association, which has a significant influence on the physical and chemical properties of soils. In this study, the surface properties and phosphate adsorption of goethite, kaolinite, goethite-kaolinite association (GKA) and goethite-kaolinite mixture (GKM) were investigated. Compared to the average simple sum values of goethite and kaolinite, the pore volume and specific surface area (SSA) of GKA increased, and those of GKM remained almost unchanged. Goethite, kaolinite, GKM and GKA have a pH of point of zero charge (PZC) at around 8.2, 4.1, 6.1, and 7.0, respectively; and their surface charge is 0561, -0.092, 0.041, and 0.097 mmol/g at pH 5.0, respectively. The phosphate adsorption data of goethite and kaolinite could be fitted by one-site Langmuir model (R-2 = 0.963 and 0.956, respectively). Two-site Langmuir model described the phosphate adsorption by GKM and GKA well (R-2 = 0.985 and 0.982, respectively); and the Freundlich correlation coefficients of GKM and GKA were 0.967 and 0.924, respectively. This indicated that GKM and GKA possessed the surface with two types of reactive sites, and the surface heterogeneity of GKM was higher than that of GKA. Compared to the average value of goethite and kaolinite, the adsorption capacities of GKM (q(max) = 198.17 and q(t) = 226.11 mmol/g) increased slightly and those of GKA (q(max) = 230.24 and q(t) = 235.96 mmol/g) significantly increased, indicating that GKA was a highly effective adsorbent for phosphate. (C) 2013 Elsevier B.V. All rights reserved.