The effect of hydroxide solutions on the structural stability and saturated hydraulic conductivity of four tropical soils

Abstract

Industrial and domestic effluents are widely applied to soil as a way of dumping, and/or source of nutrients and water for plant growth. The potential effects of sodium hydroxide (NaOH) and potassium hydroxide (KOH), which are common hydroxides found in industrial effluents/sludge, on aggregate stability, clay dispersion and saturated hydraulic conductivity (Ks) of four tropical soils were studied. For aggregate stability and clay dispersion, the soils were pre-treated with NaOH and KOH solutions (0.003–0.3M, pH = 11.5), and for Ks the NaOH and KOH solutions were used as influent solutions. Aggregate stability and Ks decreased with increased concentration of hydroxide solutions, while clay dispersion increased as concentration of the hydroxide increased. NaOH, which is more commonly used for industrial cleaning, had more negative effects on soil structural stability compared to KOH. Long-term application of a mixture of sewage sludge and effluent to Pension farm soil reduced Ks compared to Churu farm soil (control). Ks decreased to less than 20% for Pension farm soil compared to 40% for Churu farm soil when 0.3M NaOH was used as influent solution, despite having much higher organic matter content in the former. It was concluded that disposal of sewage sludge/effluent high in hydroxides, such as those of sodium (Na) and potassium (K)1 would result in reduced structural stability and Ks. Therefore there is a need to establish critical levels of hydroxides in effluent/sludge that can be disposed onto Zimbabwean soils.