COD reduction from aqueous solutions contaminated with petroleum derivatives using red mud as adsorbent

Contamination of water resources with petroleum products is an environmental problem in the recent decades. There are many different methods for remediation of aquatic environments. Adsorption is considered one of the most effective water remediation techniques. The availability and cost of absorbent is very important factor in the absorption process. In recent years, a wide range of materials were used as adsorbent for remediation of contaminated water. This study was performed to investigate the use of raw red mud as an adsorbent to remove COD from the soluble fraction of gasoline in aqueous solutions. The effects of different parameters including pH, contact time, adsorbent dose and initial concentration of gasoline on the removal process were investigated. It should be noted that all tests were repeated three times. The results showed that increasing pH, from 3 up to 11, increases the efficiency of COD reduction by the sorbent. According to the results, pH of 7 was selected as the optimal pH. The efficiency of COD reduction in contact times of 2.5, 5, 10, 15, 20, 25, 30, 60 and 120 minutes were studied. The COD reduction by raw red mud occurred quickly at the first step, then proceeds at a slower rate until equilibrium achieved within 15 minutes. Therefore, 15 minutes was considered as the optimum contact time. The effect of adsorbent dosage on the removal of COD of the solution for the values of 1, 2, 5, 10, 15, 20, 50 and 100 g/L was studied. The results showed that with the increasing of adsorbent dosage, the removal rate was increased. Based on the results, the optimal dosage of 10 g/L of raw red mud adsorbent was selected. The COD removal experiments were performed for the initial concentrations of 1%, 2%, 5%, 10% and 20% of gasoline. Removal of COD at low concentrations (1%) was close to 100%. Increasing the initial concentration of pollutant in the aqueous solution decreased the removal rate. Maximum removal of COD (%100) was occurred at the optimum conditions of pH = 7, contact time = 15 min, adsorbent dose = 10 g/L, and initial concentration of pollutant = %1. In order to determine the mechanism of adsorption of pollutants to the surface of raw red mud adsorbent, Langmuir, Freundlich and D-R isotherm models were evaluated. Results of the isotherm studies showed a high consistency (R2 = 0.98) with Langmuir isotherm model. It means that adsorption of gasoline on the surface of raw red mud is occurred as single layer, on the homogeneous surface of adsorbent and limited number of adsorbent active sites. Zero order, Pseudo first order and Pseudo second order kinetic models were investigated to determine the rate of the gasoline adsorption on the adsorbent surface. The kinetics of adsorption process was followed a pseudo-second-order model with a correlation coefficient of 0.99. Parameters such as short contact time, appropriate range for pH, acceptable absorption capacity, low cost and easy accessibility of the absorbent were the advantages of using red mud as an adsorbent to remove COD from soluble fraction of gasoline.