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An extract from the seeds of the Moringa oleifera tree that is principally a low molecular mass protein is known to be efficient as a coagulating agent for water treatment. The present paper investigates the adsorption of the purified protein to silica interfaces in order to elucidate the mechanism of its function as a flocculent. Neutron reflection permits the determination of the structure and composition of interfacial layers at the solid/solution interface. Dense layers of protein with about 5.5 mg m(-2) were found at concentrations above 0.025% wt. The overall thickness with a dense layer in excess of 60 A at 0.05 wt % suggests strong co-operative binding rather than single isolated molecules. An ionic surfactant, sodium dodecyl sulfate, was also seen to coadsorb. This strong adsorption of protein in combination with the tendency for the protein to associate suggests a mechanism for destabilizing particulate dispersions to provide filterable water. This can occur even for the protein that has previously been identified as being of low mass (about 7 kDaltons) and thus is unlikely to be efficient in bridging or depletion flocculation.

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Protein extracted from Moringa oleifera (MO) seeds has been advocated as a cheap and environmental friendly alternative to ionic flocculants for water purification. However, the nature and mechanism of its interaction with particles in water, as well as with dissolved surface-active molecules, are not well understood. In this article, we report studies of the protein and its interaction with four surfactants using dynamic light scattering (DLS), zeta-potential and turbidity measurements. Zeta-potential measurements identified points of charge reversal and the turbidity and DLS measurements were used to characterize the microstructure and size of protein-surfactant complexes. From the points of charge reversal, it was estimated that 7 anions are required to neutralize the positive charges of each protein molecule at pH 7. For protein mixtures with sodium dodecyl sulfate and dodecyl di-acid sodium salt, the peak in turbidity corresponds to concentrations with a large change in zeta-potential. No turbidity was observed for protein mixtures with either the nonionic surfactant Triton X-100 or the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate. Changes of pH in the range 4-10 have little effect on the zeta-potential, turbidity, and the hydrodynamic radius reflecting the high isoelectric point of the protein. Addition of small amounts of salt has little effect on the size of protein in solution. These results are discussed in the context of the use of the MO protein in water treatment.

Hexavalent chromium, Cr(VI), is a toxic metal present in industrial effluents. The study was carried to test the use of green Moringa leaves biomass as adsorbent for Cr(VI) from aqueous solutions. Batch adsorption method was used and the concentration of Cr(VI) measured using an ultraviolet-visible (UV-Vis) spectrophotometry. The effects of the adsorption contact time, adsorbent dosage, pH, initial adsorbate concentration and temperature were studied. Results show maximum removal of Cr(VI) of 99% ± 1%, with maximum adsorption capacity of 33.9 mg/g at a pH 2, 60 minutes contact time and 100 mg/l initial Cr(IV) concentration. The Langmuir and Freundlich adsorption isotherm models were used to fit the experimental data. The data showed that adsorption on green Moringa oleifera leaves tea biomass fitted well to Freundlich isotherm (r 2 = 0.9432) compared to the Langmuir isotherm (r 2 = 0.9122). M. oleifera leaves biomass can be used in water purification systems. The sludge of M. oleifera leaves is biodegradable, cost effective and environmentally friendly and therefore attractive in hexavalent chromium removal in water.