Soil Factors Affecting Sulfadiazine Sorption in Brazilian Soils

The role of metal (oxy)hydroxides in sulfadiazine (SDZ) sorption is poorly understood despite their high contents in many highly weathered tropical soils. Therefore, this work evaluated soil attributes affecting SDZ sorption to several Brazilian soils having contrasting metal (oxy)hydroxide contents. Samples of 23 soils were collected from the upper soil layer (0-20 cm) in the São Paulo State, Brazil, and characterized according to their texture (silt, sand, and clay), pH-H2O, point of zero charge (PZC), organic matter (OM), exchangeable cations (Ca, Mg, K, and Al), potential acidity (H+Al), CEC, well (Fed) and poorly (Feo and Alo) crystallized Fe and Al (oxy)hydroxides, and amorphous Mn oxides (MnO), respectively. Batch sorption studies were performed and related to the soil attributes to group them according with their soil mobility. SDZ sorption coefficients were high (Kd = 15.7–24.0 L kg-1) in soils having high clay (> 500 g kg-1), OM (> 55 g kg-1), and metal (oxy)hydroxide [Fed > 150; Alo > 5; and MnO > 10.0 g kg-1] contents; but they were low (Kd = 0.7–4.9 L kg-1) in soils having low clay (< 250 g kg-1), OM (< 25 g kg-1), and metal (oxy)hydroxide (Fed < 30 and MnO < 2.0 g kg-1) contents. Kd values could be estimated by a regression equation based on CEC as well as Fe and Mn (oxy)hydroxide contents. Acidic tropical soils favor SDZ-neutral species and formation of neutral or positively charged surface sites on pedogenic (oxy)hydroxides that should favor H-bond and surface complexation.