Sorption studies of Tetraoxophosphate (v) ions on some agricultural soils in Akwa Ibom State, eastern Niger Delta, Nigeria

The phosphate sorption capacity regulates principally the phosphorus concentration and available phosphorus in any soil solution. This work aims to study the phosphate sorption characteristics of five agricultural soils in Akwa Ibom State, Nigeria. The phosphate adsorption isotherm was obtained by shaking each soil sample with 20, 40, 60, 80, 100 and 120 mgL–1 for 24 h with 20 mL of KH2PO4 in 0.01 M CaCl2 solution. The Langmuir, Freundlich, Van Huay and Temkin isotherms were applied to study the PO43-sorption characteristics of the soil samples. The Freundlich model provided the best choice for phosphate sorption with mean R2 value of 0.9872. Results obtained for the phosphate sorbed (µg g–1soil) with 120 mgL-1 solution, heterogeneity factor of adsorption sites (nF), and the Freundlich constant (KF) (Lg–1) were 101(84.3), 0.515 and 7.355 for Onna soil; 62.8(52.3), 0.641 and 2.464 for Ikot-Amama 2; 60.6(50.5), 0.708 and 3.985 for Ogu-Itumbonuso soil; 59.3(49.5), 0.652 and 2.505 for Ikot-Amama 1 soils; 58.0(48.3), 0.704 and 3.682 for Ikot-Nkon soils, respectively. The equilibrium concentration for phosphates (EC-P) were used as desorption index to determine whether the soils could serve as potential sources for their bioavailability to plants and results ranged from 5.0 to 9.90 mgL–1, indicating high vulnerability to phosphate loss. The relationship between phosphate sorption parameters and soil physicochemical properties showed that the Freundlich adsorption intensity factor, nF, strongly correlated negatively with organic carbon and organic matter (r = -0.953 and -0.947; P<0.01), Fe and Mn (r = -0.911 and -0.812, P<0.05). The EC-P correlated strongly with soil pH(H2O) (r = 0.959, P<0.01), clay (r = 0.913, P<0.05), cation exchange capacity (CEC) (r = -0.824, P<0.05), Mn (r = -0.792, P<0.05) and Fe (r = -0.826, P<0.05). The study illustrated that soil management practices which involves the using organic fertilizers as the basis for PO43- sorption in relation to soil properties can be a useful tool for sustainable crop production and environmental soil-water contamination.