The effect of sediment particle size on the characteristics of ‎phosphorus adsorption ‎

‎1-Introduction 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.‎