Effect of humic acids on the kaolin coagulation performance of Moringa oleifera proteins

Cationic proteins from Moringa oleifera seeds are of interest as sustainable coagulants for drinking water treatment in regions with poor access to potable water. They have been shown to reduce turbidity in natural water sources as well as in various model freshwaters. Prior research on kaolin suspension coagulation by M. oleifera proteins demonstrated that coagulation occurs by adsorption and charge neutralization. Natural organic matter may interfere with this mechanism by complexing with proteins or adsorbing to kaolin particles. Electrophoretic mobility measurements of M. oleifera proteins added to mixtures of kaolin and humic acid confirmed that the coagulation mechanism remains neutralization induced by adsorption. Coagulation micro-assays were used to determine the coagulation activity of M. oleifera proteins in United States Environmental Protection Agency model freshwaters (ionic strengths ranging from 1.15 to 9.17 mM) as a function of humic acid concentration. Model freshwaters contained 0.5 g/L suspended kaolin and 2.5 to 10 ppm humic acid. Increasing humic acid concentrations necessitated higher protein concentrations to achieve turbidity reduction. The range of effective coagulating protein doses broadened as humic acid concentration increased and as water hardness increased. Electrophoretic mobility and dynamic light scattering measurements showed evidence of humic acid adsorption to kaolin particles. These coagulation studies reveal the mechanistic roles of particulate and natural organic matter that will likely contribute to the development of methods to treat natural water using M. oleifera seed proteins.