Soil science
H. Hatami; A. Fotovat
Abstract
Introduction
Boron (B) has a dual effect on living systems, so that the concentration range within which B is changed from a nutrient to a pollutant is rather narrow. Although B plays essential roles in all living organisms, its long-term excessive uptake has adverse effects on either human beings or ...
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Introduction
Boron (B) has a dual effect on living systems, so that the concentration range within which B is changed from a nutrient to a pollutant is rather narrow. Although B plays essential roles in all living organisms, its long-term excessive uptake has adverse effects on either human beings or plants and animals. Furthermore, part of the B that can be used as fertilizer is highly soluble and easily leached into the soil profile leadsing to some problems such as decrease of fertilizer efficiency. Therefore, to improve agricultural productivity through its gradual uptake by plants, the increase of B adsorption in the soil solution is necessary. Many adsorbents have been used for the adsorption of B from aqueous solutions; however, layered double hydroxides (LDHs) have been considered as one of the most effective adsorbents as well as slow releaser fertilizers of inorganic anions such as nitrate, phosphate, etc. The formula of LDHs are typically denoted as [M1-x 2+M x 3+ (OH)2]x+ (An-) x/n .m(H2O), where M2+ and M3+ are divalent and trivalent cations, respectively, the significance of x is the molar ratio of M3+/(M3++ M2+) and An- is the intercalated anion. Although LDH materials are commonly prepared by combining two divalent and trivalent metals, more metals can be introduced in the brucite layer to achieve a large variety of composition and higher adsorption capacity. Stability of LDHs in soil can be affected by numerous factors (e.g. low molecular weight organic acids (LMWOAs)) leading to release of structural cations in addition to interlayer anion. However, there are scarce investigations that have evaluated the potential of ternary LDHs (e.g. Zn–Mn–Al LDH) in desorption of B (as interlayer anion) and release of Zn and Mn (as structural anions) in a simulated soil solution. Therefore, the objectives of this study were, i) to compare the desorption of B capacity of binary LDH (Zn–Al LDH) and ternary LDH (Zn–Mn–Al LDH) in the simulated soil solution, and ii) to investigate the effect of three different electrolytes (potassium nitrate, oxalic acid, and citric acid) on the release of Zn and Mn from synthesized LDHs.
Materials and methods
A modified urea hydrolysis method was employed to synthesize Zn–Al and Mn-substituted Zn–Al LDHs with Zn(+Mn)/Al molar ratio of 2. Herein the contents of Mn with respect to Zn corresponded to 2% and 10% molar ratio. Accordingly, the synthesized materials denoted as Zn–Al, Zn–Mn1 and Zn–Mn2 for the samples without Mn, with 2 and 10 mol% Mn with respect to Zn content. For investigation of B desorption at a concentration of 10 mM, 15 mL from equilibrium solutions were substituted with 15 mL of 0.03 M KNO3 and shaken for 240 min. Substitution was repeated four times and A modified urea hydrolysis method was employed to synthesize Zn–Al and Mn-substituted Zn–Al LDHs with Zn (+Mn)/Al molar ratio of 2. Herein the contents of Mn with respect to Zn corresponded to 2% and 10% molar ratio. Accordingly, the synthesized materials denoted as Zn–Al, Zn–Mn1 and Zn–Mn2 for the samples without Mn, with 2 and 10 mol% Mn with respect to Zn content. For investiigatigatingon of B desorption at a concentration of 10 mM, 15 mL from equilibrium solutions were substituted with 15 mL of 0.03 M KNO3 and shaken for 240 min. Substitution was repeated four times and B concentrations in extracts were measured by Azomethine-H method. Furthermore, the supernatant Zn and Mn concentrations were determined by GF-AAS (PG 900). This process was repeated for 1.25 mM oxalic acid and 1.25 mM citric acid to study the effect of these compounds on B desorption as well as release of Zn and Mn. B concentrations in extracts were measured by Azomethine-H method. Furthermore, the supernatant Zn and Mn concentrations were determined by GF-AAS (PG 900). This process was repeated for 1.25 mM oxalic acid and 1.25 mM citric acid to study the effect of these compounds on B desorption as well as release of Zn and Mn.
Results and Discussion
The adsorption and desorption isotherm were carried out to describe the distribution of B between the liquid and adsorbent. The isotherm data of synthesized LDHs were matched with Freundlich model. The values of 1/n in this model were found between 0 and 1 for all LDHs indicating favorable sorption of B on these compounds. The highest adsorption was observed for ternary LDHs (particularly Zn–Mn2) due to their higher specific surface area and also due to the ion exchange mechanism in combination with surface adsorption. However, the results showed that the percentages of B desorption by potassium nitrate, oxalic acid and citric acid were lower for Zn–Mn1 (19.4, 29.1 and 38.2%, respectively) and Zn–Mn2 (18.6, 28.2 and 35.9 %, respectively) than Zn–Al (30.8, 41.2 and 46.2%, respectively). This observation suggests that the type of LDH, B adsorption mechanism and background electrolyte can affect the amount of B desorption. Furthermore, after 4 successive desorption cycles, the concentration of Zn and Mn increased in the supernatants (particularly in organic acid electrolytes) suggesting dissolution mechanism possibility happened for the studied LDHs. Among the background electrolytes, citric acid was the most effective compound in releasing Zn and Mn, followed by oxalic acid and potassium nitrate. A reason for this such observations could be that with respect to chemical structure, citric acid by three carboxyl groups can form more chelate rings compared to oxalic acid, which contain two carboxyl groups. Therefore, it seems that B containing Zn–Mn–Al LDH may have potential to be used as a slow release fertilizer in soils to supply three essential elements, including B, Zn and Mn simultaneously. However, further studies are required to support such a hypothesis.
Mojtaba Moqbeli; Mohsen Farahbakhsh; Naser Boroumand
Abstract
Introduction: Boron (B) is an essential plant micronutrient whose soil availability is influenced by many soil factors.Understanding the processes controling activity of boron (B) in the soil solution is important for soil fertility management. The reaction of adsorption and desorption of boron in soil ...
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Introduction: Boron (B) is an essential plant micronutrient whose soil availability is influenced by many soil factors.Understanding the processes controling activity of boron (B) in the soil solution is important for soil fertility management. The reaction of adsorption and desorption of boron in soil determines the amount of boron that is available to plants. Adsorption–desorption processes play a major role on boron equilibrium concentration and therefore on its bio-availability. Ionic strength, pH and ionic composition in exchangeable phase are among themajor factors affecting B adsorption reactions.Reducedadsorption of boron at high pH is because of a surface potential decrease onminerals with pH-dependent charge. Ionic strength has also a considerable effect on B adsorption.Several studies have been performed inthe adsorption of boron and the effect of factors such as ionicstrength and cations has been understudied, however, the effect of sodium adsorption ratio and itsinteraction with the ionic strength on boron adsorption behavior has not been reported. In thisstudy, the adsorption isotherms of boron in the soils affected by the combined effects of ionic strengthand sodium adsorption ratio were investigated.
Materials and Methods: In order to assess the effects of ionic strength (IS) and Sodium Adsorption Ratio (SAR) on availability of B, the adsorption of B was investigated in a calcareous soil that hadlow levels of electrical conductivity, sodium adsorption ratio and available P. For this purpose, 5 g soil wasequilibrated with 20 mL of B solution (0, 2, 5, 8, 10, 15, 20 mg L-1) in 0.02, 0.06 and 0.12 M background solutions (prepared by NaC1,CaC12.2H2O, MgCl2.6H2O), at two SAR levels (20 and 100).The reaction temperature was 25◦C. The suspension was centrifuged, filtered, and a sample was removed and B was determined by Azomethine-H spectrophotometric method (at a wavelength of 420 nm). B adsorption in Soil was obtained by subtracting B in solution after filtration, from added boron.
Results and Discussion: The Langmuir isotherm waswell fitted to the adsorbtiondata based on the R2 and SEE.At different IS and SAR levels, the soil exhibited different adsorption behaviors. The effect of SAR on the boron adsorption was greater at high concentrations.The results showed the increase in sodium adsorption ratio,increased soil pH and Boron adsorption.An increase in sodium adsorption ratio up to 100 resulted in a small increase in Boron adsorption compared to SAR=20. With sodium adsorption ratio of 100, soil pHincreased from 8.3 to 8.7. At about PH=9.5, maximum adsorption occurs because boron dissociation is greater when pka = pH. Increasing ionic strength increased the boron adsorption; the absorption rate wasmuch higher at higher ionic strength.Model-predicted and experimental parameters obtained using the Langmuir equation pointed to the large effect of salt concentration on the boron adsorption which wasan increase of around 10% and 75% in q max as a result of an increase in salt concentration from 0.02 to 0.06 and 0.12 M respectively. We can ignore the effect ofsalt at very low equilibrium concentration; however, it increases gradually with increasing the equilibrium concentration of boron.
Conclusions: The results of the present study showed that sodium adsorption ratio was low, in low equilibrium concentration related to low boron concentration, but the equilibrium concentration of boron increased with increasing the sodium adsorption ratio.In sodium adsorptionratio of 100, increasing pH increased the adsorption of boron. Boron adsorption was increased with increasing ionic strength; the adsorption rate was muchhigher than the rate of increase in ionic strength.Increasing the ionic strength suppresses the DDL on planar surface and therefore more negative borate ions are able to move close enough to interact with the adsorption sites located on the edge surfaces. Assuming that this phenomenon affects the adsorption of boron, the effect of ionic strength on boron adsorption can be partly dependent on it. Due to the high variability of soil minerals and the differences in their chemical properties, interpretation of the effect of ionic strength on adsorption of boron is not easy, but we can say that it is the sum of the effects of the above-mentioned factors. The positive effect of ionic strength on boron adsorption may suggest that the formation of inner sphere complex is the dominant mechanism for boron adsorption.
F. Zareapour Rafsanjani; M. Hamidpour; Hossein Shirani; M. Heshmati; seyed javad hosseinifard
Abstract
Introduction: Boron is one of the eight essential micronutrients required for plant growth and development. The optimal concentration range (between deficiency and phytotoxicity) for boron is narrower than for other plant essential nutrients. Generally, irrigating water containing concentrations of B ...
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Introduction: Boron is one of the eight essential micronutrients required for plant growth and development. The optimal concentration range (between deficiency and phytotoxicity) for boron is narrower than for other plant essential nutrients. Generally, irrigating water containing concentrations of B greater than 1 mg L-1 would be detrimental for most plants. Although, there are a large number of different studies on the removal of B ions from aqueous solutions using different adsorbents, every special adsorbent material requires individual research. Information about the chemical behavior of muscovite for boron is very limited. Therefore, the objective of this study was to investigate boron adsorption on muscovite as a function of solution pH, ionic strength of the background electrolyte, kinds of cation, and initial boron concentration.
Materials and Methods: The muscovite sample was obtained from a mine near Hamadan city in western Iran. It was powdered in a mortar and sieved before sorption experiment. Boron adsorption experiments were performed in batch systems using 15 mL polyethylene (PE) bottles in 0.01 M Ca(NO3)2 electrolyte solution at a adsorbent concentrations of 10 g L-1, and at room temperature (23±2 ◦C). All samples were prepared in duplicate. Blank samples (without adsorbent) were prepared for all experiments. For pH dependent B adsorption, aliquots of B stock solution (1000 mg L−1) were added to obtain initial B concentrations of 5 and 15 mg L-1. The pH of the solutions were adjusted to values of 6.8, 7.7 and 8.8 by adding negligible predetermined volumes of 0.03M NaOH or 0.03M HNO3 solution. To study the effects of kinds of cation on boron adsorption, samples of adsorbent (0.1 g) were mixed with 10 mL background electrolyte solutions (0.01M Ca(NO3)2, Mg(NO3)2 and NaNO3) in 15 mL centrifuge tubes. Then, predetermined amount of B were added to the centrifuge tubes to obtain final concentrations of 5 mg L-1 B. For determination of boron adsorption isotherm, after 10 ml 0.01 M of Ca(NO3)2 was transferred into 15 ml centrifuge tubes, 0.1 g sample of muscovite was added to obtain adsorbent concentration of 10 g L-1. Then a predetermined amount of boron from the stock solution was added to give final concentration range between 1 and 15 mg B per liter. Initial pH of the solution was adjusted to 8.2 ± 0.1 by predetermined amount of 0.03 M NaOH solution. Suspensions were then shaken for 24h. At the end of equilibrium time, final pH was measured in the suspensions and the tubes were then centrifuged for 10 min at 5000 g. Half of the supernatant volume (5 mL) was pipetted out from each tube and then B in the supernatants were measured using the colorimetric Azomethin-H method. The amount of B adsorbed on the adsorbent was calculated as the difference between the B concentration in the blanks and the concentration in the solution after equilibration. Chemical species in the solutions were also predicted using Visual MINTEQ, a chemical speciation program developed to simulate equilibrium processes in aqueous systems.
Results and Discussion: The effect of pH on the amount of B retained depended on the initial B concentration. The amount of boron adsorption increased with increasing equilibrium pH. Boron adsorption on muscovite increased with increasing ionic strength. Greater adsorption was observed in the presence of Mg2+ as compared with Ca2+ at the same ionic strength. Calculations using Vminteq showed that the concentration of Mg-borate ion pairs (MgH2BO3+) were higher than the concentration of Ca and Na-borate ion pairs (CaH2BO3+ and NaH2BO3°). It thus seems that the much greater loss of B from solution observed in the Mg system was caused by Mg-borate ion pair adsorption. Sorption isotherm of B were well described by the Freundlich, Langmuir and Sips models but the Sips sorption model describes the interaction between B and the mineral better than the Langmuir model. On the basis of n value of Freundlich model, adsorption isotherm of boron on muscovite was classified as L-type (n≤ 1). This kind of adsorption behavior could be explained by the high affinity of the adsorbent for the adsorptive at low concentrations, which then decreases as concentration increases. Maximum sorption capacity (qmax) was obtained to be 13.98 mmol kg-1 for muscovite.
Conclusion: The experimental data showed that less than 5% of initial boron concentration was adsorbed by muscovite, thus this mineral has not a reasonable adsorption capacity for B.
Keywords: Boron, Adsorption, Muscovite, Speciation.
