Adsorption Features of Cadmium by Humic Acid in Alkaline Conditions

In order to investigate the adsorption mechanism of cadmium on humic acid in alkaline condition and understand cadmium contamination mechanisms and its relationship with the ecological environment in alkaline saline soil, the adsorptions of cadmium on humic acid were studied by experiment in alkaline conditions. The different effect of initial concentration, reaction time, different pH and ionic strength on the adsorption of cadmium by humic acid were investigated though batch adsorption test methods. The results indicated that, humic acid had strong cadmium adsorption ability. Langmuir and Temkin adsorption models were well fitted isotherm adsorption process(r are 0.980 9 and 0.981 6). The adsorption capacities at the rapid response period within 60 minutes and the slow reaction within 60 minutes to 6 hours, are 2.895 mg·g-1 and 3.342 mg·g-1, respectively. The process before the adsorption reaction reached equilibrium state can be well described by the Elovich equation(r = 0.928 5). As the pH increased, the adsorption rate showed a gradual increasing trend, and had two boundaries at pH 4.5 and 8.5, respectively. In low ionic concentration, ionic strength can be increased to promote cadmium adsorption on humic acid. However, the opposite regularity was represented in high ionic concentration. Under the same test conditions, the different ionic strengths affect the abilities of humic acid adsorption of cadmium as follow: calcium chloridemagnesium chloridepotassium chloride sodium chloride. The bioavailability and the ecological risk of heavy metals are increased during the soil salinization process, due to the increase in salt concentration.