Kinetics Modeling and Isotherms for Adsorption of Nitrate from Aqueous Solution by Wheat Straw

Abstract

Nitrate is a colorless, odorless chemical substance with a chemical formulation of NO3- and average mass of 62.0049 gr/ Mol. According to an announcement of the world health organization (WHO), the standard amount of Nitrate in potable water is at most 50 ml/ lit (based on nitrate). Nitrate enters into the body and is transformed to nitrite by digestive system’s bacteria, then enters to the circulatory system and oxides the exiting iron in Hemoglobin of blood which converts the iron capacity from 2 to 3. As a result of this process Hemoglobin is converted to Methemoglobin which has far more less capacity in oxygen delivery. Therefore, the tissues cannot receive sufficient oxygen and it causes a disease called “Methemoglobinemia”. The objective of this study was to investigate the nitrate removal using wheat straw and determining the adsorption isotherms and kinetics. In this study, nitrate solutions were prepared from potassium nitrate salt. The pH values of the solutions were adjusted by NaOH and HCl at a concentration of 0.1 molar. The pH of the solution was adjusted to different values (4 to 13). Kinetics models of Ho et al and Lagergren were used to describe the data. Isotherm models of Langmuir and Freundlich were used to describe the data. The results showed that the maximum capacity of wheat straw in nitrate adsorption occurred at pH=6 and contact time 140 minutes. Equilibrium models (Langmuir and Freundlich) and non-equilibrium (Ho et al and Lagergren) were used to investigate the adsorption process. Comparing the determination coefficients between measured data and obtained value from Ho’s model (R2= 0.97) and Lagergren model (R2= 0.91) showed that the Ho’s model describes experimental data better. Also, comparing the Langmuir and Freundlich isotherm for nitrate adsorption by wheat straw showed that Freundlich isotherm (R2= 0.98) was more proper than Langmuir isotherm (R2= 0.83) in describing adsorption process.