SULPHATE SORPTIVITY IN RELATION TO LAND USE TYPES OF SOILS OF THE HUMID TROPICAL RAINFOREST, SOUTHEASTERN NIGERIA

Abstract

Sorption phenomenon plays relevant role in the estimation of the solubility, availability, mobility and tendency of ion pollution of the environment. Sulphate sorptivity in relation to selected land use types of soils of the humid tropical rainforest, southeastern Nigeria was evaluated by equilibrating 2 g fine earth soil samples with 20 ml of 0.2 N ZnS04 containing graded (0, 5, 10, 15 and 20 mg kg-1) concentrations of S in a 50 ml centrifuge tubes at 25oC for 2hrs. Sorption data was fitted and sorption parameters especially sorption maximum (b), affinity constant (k), maximum buffering capacity (MBC), distribution coefficient (kf), sorption intensity (n), native S (So), native sorbed S (kd) and equilibrium solution concentration (EPCo) calculated. Also thermodynamic parameters particularly the separation coefficient (KR), Gibbs free energy (ΔG0), enthalpy (ΔH0) and entropy (ΔS0) were determined. Equally the relationship between sorption parameters and soil properties were determined. The b, k, MBC, kf, n, So, kd and EPCo ranged from 10.64-20.75 and 11.49-17.04 mg kg-1, 1.11-5.07 and 0.33-6.46 L mg-1, 5.62-87.41 and 10.80-83.66 mg kg-1, 1.14-6.81 and 1.02-3.53 mg g-1, 0.28-4.51 and 0.38-1.20 mg g-1, 0.28-2.86 and 0.76-3.71L mg-1, 1.94-15.06 and 2.00-5.29 mg g-1 and 0.02-0.68 and 0.24-1.86 L mg-1in the top and sub soils respectively, with the former better than the later for all parameters except So and EPCo. Sulphate sorption capacity using the b decreased in the order fallow > cassava > rice > oil palm in the top and rice > fallow > cassava > oil palm in the sub soils. The KR, ΔG0, ΔH0 and ΔS0 ranged from 0.216-0.267 and 0.212-0.373, -4752.92- -324.63 and -3124- -49.06 J mol-1k-1, -68.76- -4.70 and -45.21- -0.71 J mol-1k-1 and 1.07-15.72 and 0.16-10.33 Jmol-1 k-1in the top and sub soils respectively. Sorption parameters correlated with selected soil properties especially sand, silt, clay, OM, pH, ECEC, N and P. In general, sulphate sorption process was favourable, spontaneous, exothermic and controlled by dissociative mechanism

Keywords: Humid Tropics And Rainforest, Sorptivity, Sulphate, land use


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