Reducing cadmium and nickel concentrations in water resources to acceptable is necessary due to their long lasting existence and bioaccumulation in the environment. Available treatment methods for cadmium and nickel removal from water resources include chemical precipitation, electrolysis, membrane filtration, reverse osmosis, distillation, ion exchange, evaporation and adsorption. The selection of the best treatment method depends on concentrations and treatment costs. Considering economical considerations, adsorption is one of the most viable existing treatment method. In addition, there has been an increasing interest in application of low cost and natural adsorbents including industrial and agricultural wastes as well as natural materials. The main objective of this research was to assess the performance of agricultural wastes for adsorption of heavy metals. Rice hull, as an agricultural waste material, was evaluated as a low cost adsorbent for removal of cadmium and nickel. The effect of two variable and two constant factors were assessed. These were concentration of cadmium and nickel (2 to 20 mg/lit), initial pH (3 to 9), adsorbent dosage (7.5 mg/lit), and mixing velocity (150 rpm). In the case of kinetic studies, initial pH was adjusted in neutral level (pH = 7) and 7.5 gr of the absorbent was added to containers. Containers were mixed using a shaker at 150 rpm. Samples were collected between 5 to 360 minutes after the start and analyzed. pseudo-first-order model didn't fit the data but pseudo-second-order order model (with a correlation coefficient of 0.99) described the results well. The results showed that equilibrium was achieved after 90 and 120 minutes for cadmium and nickel, respectively. Analysis of variance showed that all of the mentioned factors had considerable effect on cadmium and nickel removal and that pH had the biggest contribution on adsorption (with 51.6% of response for cadmium and 56.8% for nickel). The maximum removal achieved were 80% and 87% for cadmium and nickel, respectively. Batch adsorption tests were conducted at concentrations of 2, 5, 10 and 20 mg/L and optimum dosage and pH. Langmuir and Freundlich isotherm model was not able to describe the results while BET isotherm model gave good fits and described the results very well. As a general finding, the adsorbent used in this research was able to remove cadmium and nickel from aqueous solutions and can be considered as a low cost adsorbent material for treatment of water resources contaminated with cadmium and nickel. Key Words : Adsorption, Cadmium, Nickel, Aqueous Solution, Rice husk, Isotherm models
