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The existence of heavy metals in sewage is one of the major reasons of water pollution (1). Some heavy metals are toxic such as mercury, lead, cadmium, copper, chromium (Cr) and nickel even in small quantities (2-4). Chromium can be produced from industrial processes such as the leather tanning process, metalworking, manufacturing paints and paper. Mainly there are two valences of Chrome, III and VI (5). Many methods have been reported for restoration and removal of Cr from the aqueous phase such as physical, chemical and biological methods in which the filtration, adsorption, reverse osmosis, ion exchange, electro-dialysis, chemical precipitation and biological absorption were noted (6-8). The US environmental protection agency standard limit has determined that the hexavalent Cr in surface water was 0.1 mg/L and for drinking water 0.05 mg/L (9). Effect of different parameters such as an initial concentration of Cr and nickel, pH, contact time, amount of nanoparticles and temperature has been studied by Akhbarizade et al. (9, 10). An absorption rate decreased with increasing concentration, amount of adsorbent, contact time and temperature. This study showed that by increasing the contact time the absorption was increased due to the increased risk of pollutant contact with the surface adsorbent. The absorption rate was high in the early times, but decreased over time, which indicates that the reaction has reached equilibrium. Results in the study of Singh et al. (2008) on removing Cr (VI) by using iron zero-valent nanoparticles showed that by increasing the contact time between the absorber and Cr (VI), the removal efficiency was increased (11). The results showed that the Cr removal rate increased with decreasing pH due to the increase in Hcro4- and Cro42- ions that increase the ions in the environment, and Cr can be absorbed more easily. Hu et al. in other study achieved similar results (12). The maximum absorption rate was achieved in the initial concentration of 50 mg/L, the amount of adsorbent 0.15 g at 70°C and pH 2.6. Samarghandi et al. studied the decreasing efficiency of Cr (VI) with increasing pH at constant conditions from 57.65% to 30.63% and from 79.5 %to 68.67%, respectively in aerobic and anaerobic conditions (13, 14).
This study aimed to evaluate the absorption of Cr from aqueous solution by iron nanoparticles and RHA and determine the effect of the adsorbent concentration, initial concentration of Cr, pH, and contact time, and finally to determine the suitable adsorption isotherm.
In this study, Cr removal from wastewater by using concrete modified with nanoparticles of iron oxide and RHA was investigated. The 8 molds (Table 3) was dipped in sewage and remained for 6 hours (according to the contact time in an equilibrium tank).
The results of the current study showed that the sample number 4 had a higher absorption. Then, it was selected for the rest of the experiments.
Studies on the laboratory scale showed that the concrete serve as the most widely used building material and as the main materials in water and waste treatment can be a suitable surface for coating iron nanoparticles. Using concrete adhesive as one of the simple and low cost methods compared to other coating processes can be helpful combined with concrete to stabilize the nanoparticles in a long time. The combination of nanoparticles and husk showed well efficiency for Cr absorption. Results of this study showed that by increasing the concentration of Cr from 50 to 100 ppm, the removal rate was decreased. Ghanizadeh et al. showed that the Cr (VI) removal efficiency reduced with increase in the initial concentration of pollutant (20). Owlad et al. found that in Cr removal by activated carbon that increases the initial concentration of Cr led to the saturation of the active sites of absorption and increasing the concentration of pollutants which could lead to a decrease in absorption amount of it (21). This study showed that by increasing the contact time, the absorption was increased (22) due to the increased risk of pollutant contact with the surface adsorbent. The absorption rate was high in the early times, but decreases over time, which indicates that the reaction has reached equilibrium. Results of the Singh et al. study (2008) on removing Cr (VI) by using zero-valent iron nanoparticles showed that by increasing the contact time between the absorber and Cr (VI), the removal efficiency was increased (11). The results showed that the removal rate increased with decreasing pH due to increased Hcro4- and Cro42- ions that increase the ions in the environment, and it can cause the easier absorption. Hu et al. in other study achieved similar results (12). In comparison with other studies conducted in this field, the findings of the current study show that the optimal condition is an acidic environment in which the removal efficiency increases significantly. Also, Freundlich and Langmuir isotherm models are the most appropriate models to demonstrate the adsorption mechanism (23). In a study by S. Hashemian Ghahfarokhi et al. (2012), similar results were obtained (24). It seems that the equilibrium tank that is used in sewage and industrial wastewater treatment using concrete modified by nanoparticles of Fe2O3 and RHA can be suggested as a suitable solution for removing heavy metals such as Cr.
