Soil science
Shabnam Jalilian; Faranak Ranjbar
Abstract
IntroductionThe relative preference and the cation exchange capacity of the exchanger are among the important and determining factors in the adsorption and retention of cations. Studies have shown that factors such as valency, the size of the hydrated radius or the relative hydration energy of ions, ...
Read More
IntroductionThe relative preference and the cation exchange capacity of the exchanger are among the important and determining factors in the adsorption and retention of cations. Studies have shown that factors such as valency, the size of the hydrated radius or the relative hydration energy of ions, the type of clay mineral, the concentration of the solution phase, the amount of organic matter, the structural characteristics, and the charge density of the exchanger determine the preferential adsorption of cations in the soil. The aims of this study were: 1) to investigate the effect of contact time, adsorbent dose, and pH on potassium selectivity by bentonite in binary systems including K-Ca, K-Mg, and K-Na based on Gapon, Vanslow, and Gaines-Thomas equations and 2) to investigate the potassium adsorption isotherms by bentonites saturated with calcium, magnesium, and sodium.Materials and MethodsTo saturate bentonite, 1 M solutions of calcium, magnesium, and sodium chloride were separately used. The effects of contact time (10-1440 min), adsorbent dose (0.1-2 g), and pH (3-9) on potassium adsorption and selectivity by bentonites saturated with calcium, magnesium, and sodium in binary systems were investigated. In these experiments, 20 mL of a solution containing 24 meq L-1 of potassium and 6 meq L-1 of the competing cation (Ca, Mg, or Na) were added to the adsorbent. The selectivity coefficients of Gapon, Valselow, and Gaines-Thomas were calculated. Isotherm experiments were also performed to evaluate the effect of different equivalent fractions of potassium (0.1, 0.2, 0.4, 0.6, 0.8, 0.9, and 1) and the competing cation in the solution phase on potassium adsorption. Simple linear, Freundlich, and Temkin equations were fitted to the isotherm data.Results and DiscussionThis study results showed that the adsorption of potassium by Ca-, Mg-, and Na-bentonites increased with increasing contact time and reached its highest value in 24 hours. The pseudo-second-order kinetic equation was better able to describe the process of potassium adsorption by bentonites over time than the pseudo-first-order equation. Potassium adsorption by Mg- and Na-bentonites had a downward trend with increasing the absorbent dose in the range of 0.1-2 g, while Ca-bentonite showed the highest adsorption of potassium in the dose of 0.2 g. With increase in pH, the percentage of potassium adsorbed from the solution phase increased; and reached its maximum value at pH 9. The amount of potassium adsorption by Mg- and Na-bentonite in all pHs was almost the same and at the same time more than Ca-bentonite. The interesting result of this research was that the behavior of Mg-bentonite was more similar to Na-bentonite than to Ca-bentonite. An increase in the negative charge of aluminosilicates with an increase in pH can occur due to the loss of protons by silanol and aluminol groups. The selectivity coefficients of Gapon, Vanslow, and Gaines-Thomas changed under the influence of contact time, adsorbent dose, and pH. Comparing the results of the investigation of the mentioned factors with the results of the selectivity coefficients showed that these coefficients cannot be a definitive criterion for judging the preference of one cation over another cation for adsorption in exchange sites. The isotherm experiment indicated that the amount of potassium adsorption in the solid phase increased with the increase of the potassium equivalent fraction in the solution phase; so the maximum adsorption was observed at the highest initial concentration of potassium (30 meq L-1, which corresponded to the equivalent fraction of 1). The linear adsorption coefficient in the simple linear equation (Kd) showed that potassium adsorption by Na-bentonite was higher than the two others. The highest amount of Kd, 56.0 L kg-1, and the lowest value, 11.9 L kg-1, were obtained for bentonites saturated with sodium and calcium, respectively. The parameter bT, the heat of exchange in the Temkin equation, was estimated to be 4.5, 5.0, and 19.1 (J mol-1) for bentonites saturated with sodium, magnesium, and calcium, respectively. Three simple linear equations, Freundlich, and Temkin were able to describe the adsorption process well. However, based on the highest value of the coefficient of determination (R2) and the lowest value of the standard error (SE), it can be said that the Freundlich equation showed the best fit to the data.ConclusionThe highest adsorption of potassium occurred at a contact time of 24 h, a dose of 0.1 g for Mg- and Na-bentonite and 0.2 g for Ca-bentonite and pH 9. The pseudo-second-order equation described well the kinetics of potassium adsorption by bentonites over time. The results showed that the behavior of Mg-bentonite was more similar to Na-bentonite than Ca-bentonite. The selectivity coefficients of Vanslow, Gaines-Thomas, and Gapon changed under the influence of contact time, adsorbent dose, and pH. The results revealed that it is not possible to definitely determine the preference or non-preference of a cation based only on selectivity coefficients. The isotherm experiment showed that the amount of potassium adsorption increased with the increase of the initial equivalent fraction of potassium in the solution. The highest value of R2 and the lowest value of SE were obtained for simple linear and Freundlich equations, respectively.
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.
Hadiseh Rahmani; Amir Lakzian; Ali reza Karimi; Akram Halajnia
Abstract
Introduction: Laccases are potent enzymes that are capable of oxidizing various phenolic and non-phenolic compounds as well as resistant environmental pollutants. One of the most effective methods for improving their properties, such as increasing the stability of these enzymes and even increasing their ...
Read More
Introduction: Laccases are potent enzymes that are capable of oxidizing various phenolic and non-phenolic compounds as well as resistant environmental pollutants. One of the most effective methods for improving their properties, such as increasing the stability of these enzymes and even increasing their activity, is the immobilization of laccases on different carriers. In the process of immobilization, the enzyme is bonded to a solid carrier which is insoluble in the reaction mixture. In this process, the movement of the enzyme in space is severely restricted, while its catalytic activity is still maintained. One of the carriers used to create recyclable biocatalyst systems is mineral. Minerals as inorganic carriers are inexpensive, abundant in nature, readily available, and also have high biocompatibility. The objective of the present study was to investigate the adsorption properties of Laccase enzyme from T. versicolor fungus on montmorillonite K10 and zeolite minerals using Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms.
Materials and Methods: For this study, the pure laccase enzyme (> 10U mg-1), 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid (ABTS) substrate and montmorillonite K10 mineral (with a specific surface area of 220-270 m2/g and a cation exchange capacity (CEC) equal to 30 meq 100 g-1) were purchased from Sigma-Aldrich. Zeolite mineral was provided from a mine located in southeast Semnan province. Scanning electron microscopy (SEM) images of both minerals, CEC of zeolite with sodium acetate solution (pH=8.2) and zeolite surface area were determined. X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) analyzes of zeolite mineral were also done. In order to immobilize laccase on the minerals, 200 mg of both minerals were activated by shaking with 0.5N HNO3 for 2 hours and a solution of 2% 3-aminopropyltriethoxylane in acetone. The activated minerals were treated by a 5% solution of glutaraldehyde in a 0.1M sodium acetate buffer (pH=5) and were shaken for 24 hours with 0.25-2.0 mg of the laccase dissolved in the buffer. Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms were determined. The experiment was carried out at a constant temperature of 20°C. The results were analyzed using the MSTATC software and the means of the data were compared using Duncan’s multiple range test.
Results and Discussion: Based on the results, the zeolite type was clinoptilolite with a chemical formula of (Na,K,Ca)2.5Al3(Al,Si)2Si13O36.12H2O. Moreover, BET Surface Area, Langmuir Surface Area, t-Plot Micropore Area and t-Plot External Surface Area of zeolite were 40.2712, 645.4780, 3.5188 and 36.7524 m2/g, respectively. Laccase absorption on montmorillonite K10 showed the highest compliance first with the Dubinin–Radushkevich model (R2=0.97) and then with the Langmuir adsorption isotherm model (R2=0.96). Based on the D-R model, the theoretical monolayer sorption capacity (qm) and the constant of the sorption energy (ß) of montmorillonite K10 were 3 mg/g and 0.62 (×103 mol2/J2), respectively. According to the Langmuir isotherm, there was probably a homogeneous distribution of active sites on the montmorillonite K10 mineral surface. On the other hand, laccase adsorption on zeolite showed the best compliance with the Freundlich model (R2=0.87). Accordingly, sorption capacity (KF) of zeolite was 0.05 mg/g (L/mg)1/n. The amount of n parameter as an indicator of the favorability of sorption process was 1.49 demonstrating favorable absorption condition. The values of R2 obtained for Temkin isotherm model were, however, equal in both minerals (R2=0.62 for montmorillonite K10 and R2 = 0.61 for zeolite), and based on this model, the adsorption process was likely to be exothermic. According to the values of the equilibrium parameter (RL) of montmorillonite K10, the absorption was favorable. However, with increasing the initial concentration of laccase, the amount of RL approached zero indicating the laccase adsorption on the mineral is more favorable at higher initial concentrations of laccase. Based on % Removal parameter, the highest percentage of laccase adsorption on montmorillonite K10 and zeolite was related to concentrations of 250 and 125 mg/L, respectively, which showed a statistically significant difference with other concentrations.
Conclusion: In general, laccase absorption on montmorillonite K10 showed the best fit with Dubinin–Radushkevich and Langmuir adsorption isotherm models. On the other hand, adsorption of laccase on zeolite mineral showed the best fit with Freundlich model. A higher degree of steric hindrance and conformational changes in the enzyme structure is likely to occur and subsequently, the catalytic efficiency of the enzyme complexes may decrease. Therefore, montmorillonite is more suited to be used as a carrier of laccase enzymes. However, complementary studies such as kinetic tests will help to make final decisions.
F. Zadjanali Choubari; M. Navabian; M. Vazifehdust; M. Esmaeili Varaki
Abstract
Phosphorus is one of the main pollutants of agricultural surface drainage; that over standard doze of it cause eutrophication in surface water resource. This study investigates evaluation the ability of Semnan's zeolite, that modified by hexadecyltrimethylammonium, to removal phosphorus under flow conditions ...
Read More
Phosphorus is one of the main pollutants of agricultural surface drainage; that over standard doze of it cause eutrophication in surface water resource. This study investigates evaluation the ability of Semnan's zeolite, that modified by hexadecyltrimethylammonium, to removal phosphorus under flow conditions in agricultural drainage. In this regard, after modifying natural zeolite, at first was absorption kinetics, adsorption isotherms, affecting of pH and temperature experiments on phosphorus removal conducted by modified zeolite in laboratory scale. The experiments were done in 3-5 mm size of zeolite for investigating effect of zeolite size on phosphorus removal. In order to assess the flow conditions on phosphorus removal physical model of agricultural drainage and the modified zeolite filter ponds were constructed in length of 2, 3 and 5 cm. The both discharge 0.05 and 0.1 lit/s were applied in drainage and changes of phosphorus concentrations were measured at various times after passing through the filter ponds. The sorption kinetic experiments showed maximum phosphorus removal by modified zeolite occurred in the first 2 hours. Moreover, percent phosphorus removal with increasing the pH from 3 to 8 decreased and increased from 8 to 12. Also percent removal with increasing temperature in the range of 17 to 30 °C decreased. The results showed Langmuir isotherm had good agreement for explan phosphorus removal by modified zeolite. The results of physical model showed that higher percent phosphorus removal occurred in higher discharge and lower length of filter pond due to more contact drain water with the modified zeolite.
