Adsorption and desorption are key processes influencing the fate and transport, and bioavailability of heavy metals, and hence their toxicity in the aquatic environment. A fundamental understanding of adsorption and desorption of heavy metals on sediments is a requirement in modelling environmental transport and fate of heavy metals, and to help policy-makers in setting environmental quality standards. In this study, the adsorption and desorption kinetic behaviours of heavy metals on sediments under different redox conditions were examined. The factors influencing the adsorption capacity such as the initial concentration of heavy metal ions in the water, and the competition between heavy metals in adsorption were investigated. In addition, the characteristics such as the bioavailable fraction (by EDTA extraction) of the heavy metals in the sediments, the conditional stability constants for complexation of heavy metals with the surface sites of the sediment particles, and the maximum loading capacity of the sediments for the heavy metals were determined. The desorption kinetic behaviour of heavy metals from sediments under oxic and anoxic conditions was studied in the presence of EDT A - a powerful ligand that can compete with the surface sites of the particles for the heavy metals. The residual concentration of heavy metals with time was observed by collecting the sediment samples at different time intervals for metal analysis. The achieved results illustrate that the heavy metals desorbed strongly under oxic condition by the presence of EDT A and reached equilibrium within several hours up to several hundreds hours, whereas they were not nearly released under anoxic condition. The exceptions were Cr and Mn. Cr did not desorb under both oxic and anoxic condition while Mn was strongly released under both conditions. In contrast, the adsorption kinetic behaviour of heavy metals on sediment under oxic and anoxic conditions was studied by spiking with heavy metals, and monitoring the adsorbed concentration of the heavy metals with time. The investigated heavy metals exhibited a rapid adsorption on sediments under oxic condition from several hours to 30 hours, and a quite slow adsorption under anoxic condition for several hundred hours. However the adsorption capacity of the heavy metals under anoxic condition was much higher than under oxic condition. The influence of the initial concentration of heavy metal ions in the water on their adsorption capacity to sediments were determined by spiking with different initial concentration levels of heavy metals, and observing their adsorbed and dissolved concentrations at equilibrium. The established Langmuir isotherms showed an increase in the adsorption capacity with increasing the initial spiked concentration of the heavy metals. The influence of the competition between heavy metals on their adsorption capacity under oxic condition was examined by spiking with equal concentrations of heavy metals in two situations: a separate situation ( only one heavy metal in each solution) and a combined situation (all heavy metals in one solution). The adsorption capacity was decreased for Ag, and was increased for As and Pb in competition with other heavy metal ions in adsorption, whereas the adsorption capacity of Cd, Co, Cr, Cu, Ni nearly was not influenced. |
