M. Mahdizadeh; A. Reyhanitabar; Sh. Oustan
Abstract
Introduction: Sorption and desorption are important processes that influence phosphorus (P) chemistry in soil. Desorption is a process more complex than sorption and usually not all that is adsorbed is desorbed. This indicates that adsorption and desorption mechanisms are not similar and it seems that ...
Read More
Introduction: Sorption and desorption are important processes that influence phosphorus (P) chemistry in soil. Desorption is a process more complex than sorption and usually not all that is adsorbed is desorbed. This indicates that adsorption and desorption mechanisms are not similar and it seems that such reactions are irreversible. Such irreversibility is usually called hysteresis. Major factors such as chemical changes in the structure of minerals, non-equivalent processes, inflation of adsorbent material, changes in the strength of crystals, irreversible fixation of adsorbed molecules in fine pores and equilibrium time less than its true value lead to hysteresis phenomenon. The concentration of phosphate in soil solution and thus its availability for plant are closely related to sorption processes by soil components. This relationship can be explicated by sorption isotherms. Soil organic matter (SOM) especially in arid and semiarid regions is one of the important indices of soil quality and plays important role in phosphate chemistry and fertility. Organic matter could decrease P sorption, maximum buffering capacity, and bonding energy and could increase P concentration in calcareous soils solution. Organic matter and organic acids resulted from its decomposition may coat calcium carbonate surfaces and prevent the formation of apatite precipitation. There are several methods to remove soil organic matter including using hydrogen peroxide and sodium hypochlorite solutions. It has been reported that H2O2 is penetrated into the interlayer spaces of phlogopite and vermiculite through exchange with water and cations and decomposes into H2O and O2. Therefore, this study was conducted to quantify the hysteresis indices, to investigate the effect of organic matter removal on phosphorus (P) hysteresis indices and to evaluate the relationship between hysteresis indices and soil characteristics and selection of index with the close correlation.
Materials and Methods: This study was carried out to obtain soil organic matter (SOM) removal with sodium hypochlorite solution (NaOCl, pH=8) effects on P hysteresis indices in 12 calcareous soils of Iran with different characteristics. For experiment of P sorption, 2 gr of soil subsamples was placed in separate 50 mL centrifuge tubes, to which were added 20 ml of monocalcium phosphate containing 5, 10, 15, 20, 30, 40, 60, 80 and 100 mg P L-1, which had been prepared in 0.01 M CaCl2 solution as background. Centrifuge tubes were shaken in a shaker incubator for 48-hour period to reach an equilibrium. Then, they were centrifuged at 4000 rpm for 5 minutes. The supernatant was filtered through a filter paper and the P concentration of filtrates determined using a spectrophotometer. The difference between initial and final P concentrations was assumed to be the amount of P adsorbed by the soil. Desorption experiments were assumed at the end of sorption experiments at the highest initial concentration of P with 0.01 M CaCl2 solution. The tubes were shaken to reach phosphate desorption equilibrium time (24 hours) at 25 °C in incubator shaker. Then, it was centrifuged for 5 minutes at 4000 rpm and 15 ml of the supernatant solution was pipetted and then 15 ml of solution of 0.01 M CaCl2 was added to tubes and the above steps continued to 9 steps. Freundlich model was used to describe the sorption – desorption isotherms data. DataFit 9.0.59 software (1995-2008) was used for nonlinear fitting of Freundlich to sorption data.
Results and Discussion: According to the results, P sorption and desorption data showed hysteresis which indicates adsorption and desorption mechanisms are not the same. As expected, nonlinear Freundlich equation showed a best fit (R2=0.96) to the data. The mean value of desorbed P in studied soils after SOM removal was decreased by 40%, so it was concluded that P sorption was more irreversible. In NaOCl treated soils, the mean values of seven studied hysteresis indices (HI) increased. Regression analysis indicated that the fourth hysteresis index, obtained from the distribution coefficient (Kd), had close relation with clay (r = 0.69, p < 0.05) and active calcium carbonate (r = 0.7, p < 0.05) concentration. Moreover, this hysteresis index showed significant (p<0.01) positive correlation with Kfsorb and Kfdesorb, which suggests that increasing bonding energy in sorption and desorption isotherms decreased desorption amount due to the strong interaction between adsorbed P and absorbent surface, increasing this hysteresis index.
Conclusion: It was concluded that among seven used hysteresis indices, HI4 can be introduced as the best index for the studied calcareous soils. It is predicted that using organic matter or preventing its reduction in arid and semi-arid calcareous soils may increase the efficiency of P fertilizer, given an increase in hysteresis index after the removal of the organic matter.
Ahmadreza Sheikhhosseini; H. Shariatmadari; M. Shirvani
Abstract
Introduction: Pollution of soil and water environment by release of heavy metals is of great concerns of the last decades. Sorption of heavy metals by low cost materials is considered as an inexpensive and efficient method used for removal of heavy metals from soil-water systems. The presence of different ...
Read More
Introduction: Pollution of soil and water environment by release of heavy metals is of great concerns of the last decades. Sorption of heavy metals by low cost materials is considered as an inexpensive and efficient method used for removal of heavy metals from soil-water systems. The presence of different ligands with various complexing abilities can change the sorption properties of heavy metals and their fate in the environment as well. In order to assess the effect of citrate and arginine as natural organic ligands in soil environment, in a batch study we investigated the effects of these ligands on equilibrium sorption of nickel to sepiolite and calcite minerals and also kinetics of Ni sorption by these minerals.
Materials and Methods: Minerals used in this study included sepiolite from Yazd (Iran) and pure calcite (Analytical grade, Merck, Germany). Sepiolite was purified, saturated with Ca using 0.5 M CaCl2, powdered in a mortar and sieved by non-metal 230 mesh standard wire sieve. For equilibrium sorption study, in a 50-mL polyethylene centrifuge tube,0.3 g sample of each mineral was suspended in 30 mL of a 0.01 M CaCl2 solution containing 0, 5, 10, 20, 40, 60, 80 and 100 mg L-1 Ni (NiCl2) and containing zero (as control) or 0.1mmol L-1 citrate or arginine ligands. The applied concentrationsfor each ligand can naturally occur in soils. Preparedtubes were shaken (180±2 rpm, 25±1oC) for 24 h using an orbital shaker and centrifuged (4000×g for 10 min) and the supernatants were analyzed for Ni concentration using an atomic absorption spectrophotometer (AAnalyst 200 Perkin-Elmer) at a wavelength of 232 nm and a detection limit of 0.05 mg L-1. The quantity of Ni retained by each mineral at equilibrium was calculated using equation qe = (Ci - Ce)V/W where qe was the amount of nickel retained by mineral surface at equilibrium. Ci and Ce were the initial and the equilibrium concentrations (mg L-1) of Ni, respectively, V was the volume (L) of the solution, and W was the mass (g) of the sorbent. The Langmuir, Freundlich and linear isotherm models were fitted to sorption data using Graphpad prism 5.0. For kinetic study,30 mL of 0.01 M CaCl2 solution, with or without 0.1 mM citrate or arginine, containing Ni at a concentration corresponding to the maximum sorption capacity of each mineral (estimated from sorption isotherms) were transferred into 50-ml polyethylene centrifuge tubes containing 0.3 g of sepiolite or calcite. The suspensions were shaken (180±2 rpm, 25 °C) continuously and after 0.5, 1.5, 3, 6, 12, 18 and 24 hours, corresponding tubes were centrifuged (4000×g for 10 min) and supernatants were analyzed for Ni concentration by atomic absorption spectrophotometer. Using Graphpad prism 5.0, kinetic data were fitted to Pseudo-first order, pseudo-second order and power function kinetic models.
Results: With or without ligands, the Langmuir model was the best description of Ni sorption to sepiolite while the linear model was the best fit of calcite data showing the physical nature of Ni sorption by this mineral. Kinetics of Ni sorption to sepiolite and calcite were best described by power function model. In the presence of citrate, both capacity and rate of sorption of Ni to sepiolite decreased. There was no considerable change in sorption of Ni to calcite. In the presence of arginine, however, sorption capacity of minerals for Ni increased. Arginine enhanced the rate of Ni sorption on all three minerals. Citrate showed opposing effects on Ni sorption kinetics depending on the studied minerals. Totally, citrate and arginine had opposite effects on sorption of Ni to sepiolite and calcite.
Conclusion: Organic ligands can change sorption characteristics of the minerals. It seems that citrate decreases sorption of Ni to sepiolite but its effect on Ni sorption to calcite is negligible, while arginine increases Ni sorption to both minerals. Our results suggested that presence of citrate and arginine in soil influence Ni sorption by soil minerals. As in warmer seasons of year,microbial activities due to optimum temperature and moisture result in production of citrate and argininewhich facilitate and suppress uptake of Ni by plants respectively. Production of citrate in soil may increase risk of Ni contamination of underground and surface water sources while arginine can decrease soil solution Ni and in turn the risk of water contamination.
