The role of humic acid on the adsorption of trace metals at the fresh-salt water interface
Description
The fate and distribution of heavy metal pollutants in estuary regions is of great concern in environmental studies due to their toxicity and cumulative properties. The interactions between the metal and sediments, especially the adsorption/desorption process, are the most important mechanisms which control the metal concentration in aqueous systems. The presence of natural organic materials such as humic and fulvic acid may alter the surface properties of sediments and suspended particles, as well as the speciation of metals in both the dissolved and adsorbed states In this study, a system consisting of Pb, Cu and Cd as typical heavy metal pollutants, montmorillonite as a model sediment and suspended particles together with purified humic acid as a representative of natural organic materials were selected as prototype materials to model the environmental system. The effect of environmental factors such as pH, salinity and concentrations on the interaction between metals, montmorillonite and humic acid were investigated in detail. A theoretical model featured by surface complexation reactions and triple layer theory combined with the Poisson-Boltzmann equation was developed and used to simulate the experimental data. The model was also used to predict surface properties such as potentials and the species of metals adsorbed both on the montmorillonite and humic acid The experimental results have shown that pH and ionic strength are the most important variables in controlling metal adsorption on either montmorillonite or humic acid. While a small increase in the adsorption of Cu and Cd on montmorillonite was observed in the presence of humic acid, the humic acid did not show as much of an effect as expected on metal adsorption in the concentration range tested in this study. The results also shown a high complexation capacity of humic acid for the metals, especially Cu and Pb. This might be the most significant role that humic acid plays in the environmental system Good agreement between theoretical modeling results and the experimental data proved the applicability of the theoretical model to this system. All the parameters in the model can be obtained experimentally. Given these parameters, theoretical calculations obtained using the model proposed in this study can give a detailed picture under actual environmental conditions. Also, the model calculations will help in the evaluation of the actual toxicity of heavy metal pollutants in the aquasystem