نوع مقاله : مقاله پژوهشی
عنوان مقاله [English]
Industrial development increased concerns of heavy metals in wastewater. Heavy metals in the industry have caused many problems. Heavy metals are the first hazardous materials in pollutants, which have hazard for the environment and harmful for Human health. Chromium is a heavy metal could be found in industrial sewage systems like metal polish, loom, leathery, fertilizer manufactures. Chrome, often in the form of hexavalent chromium, is one of the metals in aquatic environments. Chrome is harmful to human health and has adverse effects on human and animal bodies; therefore, the use of absorbent material to remove it from the water and wastewater treatment is essential. Trivalent chromium (Cr(III)) ion is possibly required in trace amounts for sugar and lipid metabolism, although the issue remains in debate. In larger amounts and in different forms, chromium can be toxic and carcinogenic. The most prominent example of toxic chromium is hexavalent chromium (Cr(VI)). Abandoned chromium production sites often require environmental cleanup. Rice is one of the cheapest and frequent absorbent due to its cellulose and carbon fiber capacity. The aim of this study is to evaluate the effect of particle size on the amount of rice chromium removal from aqueous solution. To do so, milled rice stems were collected from rice fields of Lenjan, Isfahan. After that, the particles were sieved with 75, 300, 850 and 1200 µm meshes. In this study, four levels of rice amount including 1, 1.5, 2, and 3 grams with 75, 300, 850 and 1200 ?m were used. To prepare a solution of hexavalent chromium, potassium dichromate was used (95% purity, prepared by Merck Co., Germany). The standard solution for testing, were prepared using the dilute solution. A chromium concentration of 10 mg/lit has been selected for all experiments. In this study, several non-equilibrium models, including Lagergren, Ho and colleagues and the intermolecular distribution and equilibrium models of Langmuir, Freundlich, Redlich- Peterson and Sips were used. To test the results of equilibrium and non-equilibrium models, the nonlinear equations were used (Matlab ver. 7.8.).
pH of the solution is an important parameter influencing chemical reactions and biological process of sewage. Changes in pH affect the absorption of chromium. This response will affect the relationship between absorbent and absorption material. In other words, positive surface absorbent and electrostatic adsorption increases its tendency to absorb anions. Equilibrium time determines the capacity of equilibrium as well as the maximum capacity of absorbent material. For a specific amount of absorbent, equilibrium time is the duration of completing absorbent capacity (unattraction of absorbent element). Absorbent particle size (specific surface area), determines the absorption capacity as the most important feature of absorbent material. In general, when the absorbent material decreases the specific surface area increases exponentially, therefore, their area of irregular shapes are increased and consequently the amount of absorption capacity due to the speed of the process will increase. The results showed a significant (p?0.05) relationship between absorbent particle size and equilibrium time as well as absorbent particle size and maximum absorption capacity.
The maximum adsorption efficiency in 1 gram of rice stem with the 75 micrometer particle size in pH=2 and 98.25 percent was occurred. The synthetic adsorption assessment shows that, none equilibrium model of Lagergren (Correlation coefficient 1) and Ho et al. (Correlation coefficient= 0.999) models were clarified adsorption of chromium by rice stem very good. Also, the balance time was increased logarithmically by increasing in the particle size of rice stem, to which the balance time in the particle size of 75, 300, 850 and 1200 micrometers provided 30, 90, 90, and 120 min, respectively. The isotherm adsorption assessment shows that, in 75 micrometer particle size of rice stem, Langmuir (Correlation coefficient= 0.993), Ferundlich (Correlation coefficient= 0.989), Redlich- Peterson (Correlation coefficient= 0.993) and Sips (Correlation coefficient= 0.994) models were clarifying isotherm adsorption, respectively. Maximum adsorption was decreased linearly by increasing in the particle size of rice stem. Adsorption capacity of chromium by Langmuir model for the rice stem particle size of 75, 300, 850 and 1200 micrometers were shown as 3.096, 2.490, 1.820 and 1.456 (mg/g), respectively. Comparing the maximum capacity of absorption of rice stem in this study with other absorbents in the other studies showed that the gridded rice stem is an elegant absorption material to remove chromium of wastewater.