Factors influencing phosphorus adsorption onto sediment in a dynamic environment

The adsorption of phosphorus onto sediment in a batch reactor and a dynamic flume was studied experimentally. The effect of velocity, sediment transport patterns, sediment supply, and phosphorus added to water was investigated in the two systems. Results showed that the equilibrium isotherm adsorption was better described using the Langmuir model. The adsorption amount increased as the initial phosphorus concentration in water increased before reaching saturation state, which was described as maximum amount of adsorption; the rate of increase in adsorption however decreased because of the limit adsorption sites and the increasing desorption. The kinetics of soluble reactive phosphorus (SRP) in water over time in both the batch reactor and the dynamic flume were well described using a pseudo-second order equation that gave good predictions of both the adsorption rate at the initial stage and the amount of adsorption in a state of quasi-equilibrium. The amount of adsorption per unit mass of sediment in the quasi-equilibrium state increased as the initial phosphorus concentration in water increased or as the sediment concentration decreased. In the quasi-equilibrium state, the adsorption amount of the SRP fluctuated within a larger range in the dynamic flume than in the batch reactor, because of continuous exchange between the suspended sediment and the bed sediment in the flume. The adsorption rate of the suspended sediment was greater than that of the bed sediment. The amount of adsorption decreased as the velocity increased when the phosphorus concentration was low, while it increased when the phosphorus concentration was high, as a result of a different concentration gradient at the water-sediment interface. The distribution coefficient and the adsorption rate coefficient both decreased as the initial phosphorus concentration increased when the sediment concentration was fixed. The variation of the distribution coefficient (KD) indicated that the adsorption amount at equilibrium in the dynamic flume was larger than that in a quiescent environment when the phosphorus concentration was low, while the difference between the two systems was small when the phosphorus concentrations were high. However, the KD value in both the flume and in static conditions was smaller than the value for the continuous disturbed environment in the batch reactor, indicating that the adsorption capacity was over-estimated in the latter.