Effect Of Humic Acid On Adsorption Efficiency Of Quartz Sands Media

Sand media is usually fouled in the gravity filtration especially in polluted drinking water treatment process with relatively high level of nature organic matter (NOM). The consequent results are the decline of filtration efficiencies in particle and contaminants removal in filtration. The decline of the removal effectiveness caused by NOM cannot be remedied by simply water backwash. To investigate this phenomenon the quartz sand granules were ground to powder to conduct adsorption and desorption tests with humic acid (HA) as the representative of NOM. The results showed that the adsorption and desorption phenomena occurred on the sand media.The results showed that the adsorption and desorption phenomena occurred on the sand media. Adsorption isotherm data are well fitted by the Langmuir equation model, which suggested that a monolayer adsorption process may be the dominant than multiplayer. Adsorption kinetics on the quartz sand surface follows initial diffusion-controlled and then attachment-controlled adsorption kinetics. Humic acid adsorbed on the surface of quartz sands cannot be desorbed completely, and the higher initial concentration of HA could cause lower relative desorption rate. The higher initial concentration of HA adsorbed caused the more negative Zeta potential on the surface of quartz sand. The results of XPS shows that adsorption of HA also changed the oxide state of some active site on the surface of quartz sand by formation of some new bonds such as O-Si-C and Si-O-C bonds. So, the main mechanisms of the adsorption of HA on the quartz sands is supposed as that the surface silanol groups (Si-OH) acted as the main active site for the adsorption of HA molecules, and subsequently formed Si-O-(O=)C bonds with carboxylic or hydroxylic functional groups of HA. The more negative Zeta potential and the newly formed O-Si-C and Si-O-C bonds are supposed to the reduction of efficiencies in particle and contaminants removal in filtration.When kaolin particles and coagulant Al2(SO4)3·18H2O are present in the water, the adsorption is enhanced: the XPS results shows that lower initial concentration of HA is needed to form the chemical adsorption and longer time are needed to destroy the newly formed chemical bonds; the adsorption kinetics data shows that adsorption amount increased and adsorption process accelerated sharply, and the desorption kinetics data shows that relative desorption rate decreased and desorption process decelerated distinctly.