Document Type : Research Paper
Department of Reclamation of Arid and Mountainous Regions, Faculty of Natural Resources, University of Tehran, Karaj, Iran
Department of Environmental Engineering, Faculty of Civil Engineering and Environment, Amir Kabir University of Technology, Tehran, Iran
Rivers are considered as one of the main sources of water supply for agriculture, drinking water and industrial use. Water pollution is one of the most important problems in the world, especially in developing countries (Bandpey et al., 2013). Transferring the suspended sediments and pollutants into the outlet of the watersheds by runoff can be classified as an important reason for reducing the quality of water systems (Blanco et al., 2010). Meanwhile, Phosphorus is one of the most important nutrients in aquatic systems and plays an important role in the trophic state of water resources, which need to be managed in order to prevent eutrophication. So, sediments are specified as a main source and factor for the nutrients transferring to rivers, which have a significant impact on factors such as light penetration and water temperature (Eder et al., 2010). Sediments may act as a phosphorus sink due to certain physical, chemical and meteorological conditions that can release a significant amount of phosphorus to the water column, which leads to various problems in water resources (Fytianos and Kotzakioti, 2005). The release of soluble phosphorus in rivers is strongly influenced by the interactions between phosphorus and suspended and bed sediments (House et al., 1995). Therefore, studying of characterizes of phosphorus adsorption by sediment is necessary in order to better understand the interactions between phosphorus and sediments.
Several studies have been carried out on the effects of sediments and various adsorbents on the adsorption of pollutants including heavy metals and etc. in Iran. But those only have focused on the heavy metal adsorption, while nutrients such as nitrogen and phosphorus are also the main source of contaminants in rivers. On the other hand, studying the effect of sediment particles on the adsorption and transfer of nutrients including phosphorus has not been carried out using river sediments and most of them focused on the adsorbents. Therefore, the purpose of this study is to investigate the effect of sediment particles on the absorption and transfer of phosphorus and to determine the kinetics of phosphorus adsorption using natural river sediments.
2. Materials and methods
The natural sediments under 15 cm surface sediment from the several points of the Karaj River were collected and sent to the laboratory. In order to reduce the effect of other sediment components on the physical absorption of phosphorus, all sediment samples underwent a pretreatment process to remove a large number of inorganic, organic, metal ions from natural sediments particles. After removing sediment contaminate, Grinding drum and sieve were used to size the sediment samples. After the clean and sized preparation, four groups of different sediment particle sizes were obtained (Meng et al., 2014) including D1 with size ( Phosphorus adsorption kinetics
Absorption experiments were carried out at different concentrations in order to investigate the rate of adsorption of sediment with time progression. Dried sediments samples (0.2 g) with different grain size D1 to D4 were added in a series of 250-ml beakers with 100 ml phosphate solution (KH2PO4) at various concentrations including 23 and 100 mg/L. Two initial phosphate concentrations were adopted, 23 mg/L for the low concentration and 100 mg/L for the high one. The pH values of the solutions were maintained at 7.5 by adding 0.01 mol/L of NaOH and 0.01 mol/L of HCl. All reaction bottles were agitated at a rotational speed of 190 r/min. The sample solutions were taken at different time (5, 10, 20, 30, 60, 120, 180, 240, 480, and 720 min) and centrifuged immediately at a rotational speed of 5000 r/min for 10 min. The supernatant was immediately filtered through 0.45-ym Whatman GF/C filters for phosphorus analysis. The total phosphorus concentration in the sediment samples was monitored using the molybdenum-blue complex method with a UV/visible spectrometer at the wavelength of 780 nm (Murphy and Riley 1962). Each test was carried out three times, and the average results were recorded if the results of the three tests varied within a certain range. The amount of P adsorbed onto sediment was calculated as the difference in the concentration in the water phase at the beginning and end of the experiment. The difference between the concentration of phosphorus in the initial and final solution was considered to be equal to the amount of phosphorus absorbed (equation 1) (Onyango, 2010).
Qe= ([(C_0-C_e )].V)/M (1)
Adsorption (%) = (C_0-C_e)/C_0 ×100 (2)
Where Qe is the amount of phosphorus absorbed in mg/g, C0 and Ce are, respectively, the initial and equilibrium phosphorus concentration (mg/gL, V is the volume of solution, and m is the mass of sediment (g).
The quasi-first-order adsorption kinetic equation, and quasi-second-order adsorption kinetic equation (Chien and Clayton 1980) were used to model the adsorption kinetic process. They can be expressed, respectively, as follows:
(3) q_t= q_e (1-e^(-k1t))
(4) 〖 q〗_t= q_e q_(ek_2 t)/(1+q_e k_2 t)
Where qe and qt is the amount of phosphorus adsorbed by the sediment sample at equilibrium and time t (mg/g), k1 is the rate constant of the quasi-first-order equation (min-1), and k2 is the rate constant of the quasi-second-order equation (g/(mg•min)).
3. Discussion of Results
Migration and transformation of pollutants in the water environment depend on the adsorption and desorption characteristics of interactions between sediment particles and surrounding water (Wei et al., 2014). The dynamic adsorption process in different sediments showed that there were clear differences in phosphorus adsorption in various particle sizes of sediment. The kinetic process of the phosphorus adsorption appears to occur in three distinct stages: an initial fast adsorption stage, a relatively gradual adsorption stage, and an eventual equilibrium state where the amount of phosphorus adsorbed reaches a maximum. The evaluation of absorbed amount over time indicates that the rapid stage absorption process and the highest absorption amount take place in the first 2 hours and its concentration ranges from 0.04 mg/g to 0.2 mg/g at the concentration of 100 mg /l and from 0/02 to 0/16 mg/g at 23 mg/l concentration and tends to reaches the equilibrium level after 6 hours. Compared to coarse-grained particles, fine-grained particles due to the large specific surface area deserved large amount of phosphate adsorption. Particle with the diameter of 0.05 mm or D1adsorbed much more phosphate than the other ones as it had the smallest size and largest specific surface area. With time increasing, the corresponding adsorption kinetics curves became flatter as the adsorption amount of phosphorus had the tendency to reduce, indicating that the kinetic adsorption process was time-dependent.
Variation in the amount of P adsorption per unit mass of sediment at time t for different sediment concentrations(S0) when the initial P concentration(C0)in the water phase was 100 mg L−1 illustrates that the P adsorption amount increased with increasing sediment concentrations from 1 to 2 g and in this study, it is approximately 1.5 times. Rapidity of adsorption kinetics during the first few minutes can be interpreted by the availability of a significant number of active sites on the surface at the beginning of adsorption, compared to that remaining after a certain time(Mustafa et al., 2010).
Variation of qt over time (t) and its fitting with pseudo-first-order and pseudo-second order equations for the four groups of sediment with different particle size can be seen in Fig.1. The results of two kinetics models were applied the sorption kinetics of sediment samples showed that the kinetics of P adsorption onto sediment are well described by the pseudo-second order model. The quasi second-order equation has the highest correlation coefficient(R2)and it can be concluded that the quasi-second-order equation provides the best representation of the kinetic adsorption process.