Kinetic equations for the biological removal of lead from industrial wastewater using indigenous species of aquatic plants in Khuzestan

Introduction
Water pollution by heavy metals is a worldwide environmental problem due to the increasing exploitation of mining activities, industrialization and urbanization has increased around the globe. Lead is the most important environmental pollutants that contaminate soil and water resources from different ways. This element is including the most toxic heavy metals in the United States Environment Organization. Therefore refining of soil and water contaminated with this element is from most important environmental policies of developing countries and Industrial countries. Lead metal is unnecessary for the body, so that trace amount in the body, indicating contamination with this element. Lead substitute for calcium in the cells and disrupt the activities of the body. Also cause liver and kidney dysfunction, genital organs and reproductive system, anemia, loss of intelligence interest and occurrence of the metabolic complications. Remove and control pollution of heavy metals is very difficult due to their multiple and different sources of pollutions. Because each pollutant requires its refining process. Until now, many methods have been developed for the purification of sewage including can be pointed to the chemical precipitation, reverse osmosis and ion exchange of organic. Each of which has its own advantages and disadvantages. Including the optimal method of biological for the removal of pollutants is the use of plants and other expression Phytoremediation.

Materials and methods
Project preparation and cultivation of plants
In order to implement this study, plastic pots prepared with 60 cm in diameter and 40 cm in height. Therefore from sand with grains diameter of 1 to 5 mm and a depth of 30 cm was used as the growth bed in pots that after preparing the pots and planting, were placed a suitable interval each other. Selected plants included Phragmites, Thypha and Cynodon Dactylon selected due to their abundance in the are. Young plant samples were collected from margins of wetlands, open surface drains and rivers and immediately were transferred to the site of the research in experimental field of Islamic Azad University of Dezful. After preparation of bed and adding nutrients to the cultivation medium, each pot was randomly assigned to a treatment in three replicates. The youngest seedlings were selected for cultivation in each medium sample. After gentle washing of seedlings with water, three plant at regular intervals, with the minimum distance of 5 cm from wall of the container and at a depth of 10 cm (density of 25 plants per square meter) were cultivated. After preparing the pots and planting the desired species were irrigated with the common water for 40 days because irrigation with synthetic wastewater in the early stages of cultivation caused stress to the plant due to lack of root stability and compatibility with the new culture medium. So after the elapse of this period time, irrigation with simulated wastewater in different concentrations of 5, 10 and 15 mgr lead/l is performed.

Results and Discussion
Exponential Association regression model
Exponential Association regression model using curvexpert software was used in order to simulate the kinetics of uptake in different treatments. The general form of this model is as . In this equation, t in day and y lead uptake in milligram per kilogram of soil was defined. Lead absorption curves have been showed for Cynodon, Phragmites and Thypha in the treatments of average level of lead concentration. In this diagram, the regression coefficients a and b are variable constants for the different species and under different levels of concentration. According to this model, the kinetics of adsorption was highly correlated. Therefore, these equations can be used to simulate the removal of lead at different concentrations. The results showed that with increasing levels of concentration, the correlation coefficient was reduced. So this model in the medium and high levels had high precision. Comparison of model in the studied species showed a correlation coefficient of reed was much higher than the other two species so had better fit to this plant. Therefore, these equations can be used to simulate the absorption of lead at different times and in different plant species, especially reed with the high correlation coefficients.

Fitting with other models of the kinetics
Due to the lengthy process of phytoremediation, the use of appropriate models to simulate and calculate the necessary time for refining the soil and reach the optimal level is essential.
For this purpose, in addition Exponential Association model, models of zero order kinetics (linear function), first order kinetics and power function were selected and with the data collected were fitted. Results showed the first order kinetic model in all treatments had a correlation coefficient (r) higher and consequently the lower from standard error (s) than the linear model. Comparison first order kinetic model with Power model showed that power function had a higher correlation coefficient. Therefore, it is suggested that from power model to be used simulating kinetic Lead with the high correlation coefficient.

Conclusions
Experimental results showed that the kinetics of the reduction of lead in soil was mainly in the form of an association exponential model and then power model had the best correlation coefficient compared to the other models. The results showed that these two models had better performance and matching during the entire period of experiment. Whereas zero order and first order kinetics models at the initial and final times were not accurate. The results of the calculation of the equilibrium time to achieve half the initial concentration in the soil (y / 2) showed that as the first order kinetics model, times 384, 190 and 100 days respectively by cultivation of Cynodon, Typha and Phragmites required in conditions treatment of the mean levels of wastewater concentration which implies that Phragmites had greater potential for Phytoremediation than the other two species. Therefore bed concentration was reached in less time by half the initial concentration of the soil lead. The results showed that the reaction rate constant for the three species varied in the range 0.0014 to 0.0071 day-1 that this amount was higher for Phragmites than the other species. Thereby reducing rate of soil lead by Phragmites cultivation was faster than the other two species and this constant coefficient in all cases was decreasing with increasing the different concentration levels of wastewater. In the end it was concluded that the kinetics of phytoremediation should be simulated based on plant type, levels of concentration and type of contaminants.
Keywords: Phytoremediation, kinetic model, lead removal, aquatic plants