عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Many of heavy metals discharge to water resources through wastewater. One of these heavy metals is Chromium. Two types of Chromium remain stable in the environment. Chromium(III) is a necessary ingredient for human body. But Chromium(VI) is toxic and even causes lung cancer. Industrial activities like plating, cement, steel, cannery and etc. produce large amount of contaminated sewage, so make the wastewater infected. The amount of Chromium(VI) allowed for agricultural water is 0.1 mg/lit. Many experiments include electrochemical deposition, ultrafiltration, ion exchange, reverse osmosis and natural biopolymers have been used to remove Chromium(VI) from sewage. Chitosan has the highest capacity to remove various metal ions. In this study, adsorption of Chromium(VI) ions from aqueous solution using chitosan biological adsorbent was investigated. In these experiments different doses of chitosan 1, 2, 3, 4, 5, 10, 15 and 20 gr/lit were mixed with the different concentration of Chromium(VI) 12.5, 20, 30 and 40 mgr/lit and were shaken for two hours with 450 rpm. The top layer of the solution was transferred to another container and every 20 minutes, concentration of metal ions was measured by an atomic absorption device. Experiments were performed 5 times and then average of the results was used in calculations.
All experiments were carried out at room temperature. The effect of different doses of chitosan at pH 4, 6, 8 at different concentration of Chromium(VI) is 150 minutes has been investigated. After this amount of absorption, no significant effect on removal efficiency of Chromium(VI) ions has been observed. This is due to the coincidence of the active sites in higher concentrations. Hence, some of the ions are affected by adsorbents and some other substances remain in the solution even if more adsorbent is added. There are more ion exchange locations at the higher concentrations of chitosan. The NH3+ chitosan group is primarily responsible for the interaction of the onions and negative charge levels. Therefore according to the above texts, 10 mg/lit is determined as the optimum concentration of chitosan. The pH of solution, because of the metal ions potential, has a significant effect on absorption of heavy metal. Also, the pH affects the concentration of opposite ions in the adsorbent group and ionization degree during the adsorption reaction. The pH of 4-8 was investigated for studying the effect of pH on adsorption of Chromium(VI) ions. The result of experiments showed while the maximum removal occurred at pH 4,the minimum removal of Chromium(VI) ions with chitosan happened at pH 8 . The absorption of Chromium(VI) and other metals, such as Arsenic known as anion, depend on pH. At pH less than 4, the amino group protonated chitosan in varying degrees. The NH3+ group on chitosan is mainly responsible for the removal of Chromium(VI). At higher pH, oxygen functional groups negatively affect the absorbent surface; therefore an electrostatic interaction is produced between the adsorbent and the onions. The effect of contact time on the removal of metal ions with water-soluble chitosan powder was investigated. The removal efficiency reached its maximum value at 140 minutes and thereafter no further deletion occurs until 150 minutes. This may be due to the fact that at the beginning, all occupied areas and the gradient of Chromium(VI) concentration were high, therefore based on these results, 140 minutes were selected for isotherm studies. Quickly removing of Chromium(VI) was increasing from 20-140 minutes. Because at first steps most of sites are potentially empty, as soon as the contact time increase, most of the ions absorb to empty sites. Therefore, finding rest of empty sites is difficult for metal ions. For this reason, the ratio of absorption is decreasing. The presence of most functional group on the surface of chitosan, which requires the interaction between the anions and cations, significantly improves with the bonding capacity, and this process proceeds rapidly. So, contact time is an important parameter treatment system that is cost-effective, therefore, an optimal time of 140 minutes was considered for purification. Absorption isotherms are obtained by plotting the absorbed metal graph versus the equilibrium concentration in the solution. Three model of Temkin, Langmuir and Freundlich were investigated in this paper by chitosan for removing Chromium(VI). Freundlich isotherm is used to absorb the single layer absorbing chemical and the absorption of several layers of physical absorption is based on the assumption of absorbent adsorption on homogeneous surface. According to the result of experiments, the absorption data with Freundlich isotherm was the better fitted with higher correlation coefficient than the other isotherms. This means that the metal has also penetrated the underlying layers and absorption takes place uniformly, therefore, the Freundlich isotherm is a suitable model for describing this absorption behavior. The process to remove Chromium(VI) ions from the liquid phase by absorbing the kinetic models and test speed control mechanism of adsorption, such as chemical reaction, mass transfer and playback controls are explained. Kinetic studies were carried out on various kinetic models such as pseudo-first order, pseudo-second order and intraparticle diffusion model. Pseudo-second order model describes the absorption rate based on the square of the number of occupied spaces on the adsorbent. This kinetic has a higher correlation coefficient than the previous two preceding ones; therefore the absorption rate follows the pseudo-second-order.
In this study, it was concluded that this method is possible to investigate the possibility of using chitosan in water purification and heavy metal removal of Chromium(VI). However, for practical use in the process of water and wastewater treatment for irrigation, performing a technical and economic assessment is necessary and comparing this method with others is recommended.