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نوع مقاله : مقالات پژوهشی

نویسندگان

1 دانشگاه ولی عصر رفسنجان- دانشکده کشاورزی-

2 گروه علوم خاک، دانشکده کشاورزی ، دانشگاه ولی‌عصر (عج) رفسنجان

چکیده

طبیعت خاک‌های آهکی که بخش عمده‌ای از اراضی زراعی و باغی کشورمان را شامل می‌گردد، باعث شده است که کودهای شیمیایی فسفره از کارایی کمتری برخوردار بوده و برای همین منظور امروزه برای افزایش کارایی آنها و هم چنین کاهش آلودگی زیست محیطی ناشی از کاربرد این کودها به ترکیبات جدید کندرها توجه ویژه‌ای شده است. هدف از انجام این پژوهش بررسی اثر pH و نسبت‌های کاتیون دو به سه ظرفیتی بر سینتیک آزادسازی فسفر از هیدروکسیدهای دو گانه لایه‌ای (LDHs) بود. در این پژوهش ابتدا دو نوع Mg-Al-LDH با نسبت کاتیون دو ظرفیتی (Mg) به سه ظرفیتی (Al) سه به یک و چهار به یک با آنیون بین‌لایه‌ای فسفات که به ترتیب LDH (3:1)و LDH (4:1) نامگذاری شدند، سنتز شدند. آزمایشات پیمانه‌ای در الکترولیت زمینه 03/0 مولار نیترات پتاسیم جهت بررسی اثر pH و زمان بر سنتیک آزادسازی فسفر از LDH-P (3:1) و LDH(4:1)انجام شد. نتایج آزمایش نشان داد که افزایش pH از 6 به 8 در حضور الکترولیت زمینه0/03 مولار پتاسیم نیترات، منجر به افزایش فسفر آزاد شده از هر دو نوع LDH شد. به عنوان مثال با افزایش پی‌اچ اولیه سوسپانسیون‌ها از ۶ به ۸ مقدار فسفر آزاد شده از LDH (4:1) از 61/4 میلی‌گرم بر کیلوگرم به 29/9 میلی‌گرم بر کیلوگرم افزایش یافت. درهمه پی‌اچ‌های مورد مطالعه، آزادسازی فسفر از LDH (3:1) در محیط الکترولیت زمینه کمتر از LDH (4:1) بود. به عنوان مثال، در پی‌اچ ۶، 7 و ۸ مقدار فسفر آزاد شده از LDH (4:1) به ترتیب 48/1، 10/2 و 45/2 برابر بیشتر از LDH (3:1) بود. سینتیک آزادسازی تجمعی فسفر از LDHهای مورد مطالعه نشان داد که مقدار آزادسازی فسفر با افزایش زمان تسریع می‌شود. رهاسازی فسفر از LDH در مرحله اول از 0 تا 400 دقیقه، دارای سرعت بیشتر و در طی 1175-400 دقیقه با سرعت سرعت کمتری ادامه یافت. هم‌چنین بر اساس نتایج، در بین معادلات سنتیکی مطالعه شده، معادلات شبه رده دوم و پخشیدگی پارابولیکی بهترین برازش را بر داده‌های رهاسازی فسفر داشتند.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Phosphorus Release Kinetic Of Layer Double Hydroxides: Effect of pH and Divalent to Trivalent Cation Ratios In Mineral Structure

نویسندگان [English]

  • ‌Amir Hasanzadeh 1
  • Mohsen Hamidpour 2

1 Department of Soil Science, Faculty of Agriculture, Vali-e-Asr University of Rafsanjan

2 Department of Soil Science, Faculty of Agriculture, Vali-e-Asr University of Rafsanjan

چکیده [English]

Introduction

Layered double hydroxides (LDH) have gained considerable attention for their potential application in agriculture, serving as a slow-release source of essential nutrients for plants. The appraising of LDH as a favorable fertilizer is in the early development, and more studies on the nutrient release mechanism of LDH is needed to answer the question of how LDH could replace commercial fertilizers for the stable provide nutrients to plants. Although, several studies on the release of P from LDH exist in the literature, no information regarding ratios M2+/M3+ in LDHs on phosphate release from LDHs is available. So, it is important to raise our knowledge about various parameters like pH and time on the solubility of LDHs. This study aimed to investigate the effects of pH and the ratio of divalent (M2+) to trivalent cation (M3+) on the kinetic release of P from Mg-Al-LDH.



Materials and Methods

All the chemical substances in this research, such as magnesium nitrate hexahydrate (Mg-(NO3)26H2O) and aluminum nitrate nonahydrate Al (NO3).9H2O were of analytical grade and obtained from Merk (USA). The solutions were made with decarbonated pure water without impurities (electrical resistivity = 18 MΩcm). Two nitrate form of Mg-Al-LDH were synthesized using the co-precipitation method at constant pH by varying the Mg/Al ratio (3:1 and 4:1) in the precursor solution (Everaert, et al. 2016). Briefly, 50 mL of 1M solution containing nitrate salt of divalent (Mg (NO3).6H2O) and trivalent cations (Al (NO3)3.9H2O) in the appropriate ratio (3:1 and 4:1) were added simultaneously for 2h to 400 mL of 0.01M solution of sodium hydroxide-sodium chloride while being stirred vigorously in a nitrogen atmosphere. The pH was kept at 9.5 by adding volumes of 3 M NaOH. Afterward, the material was ripened in the synthesis mixture for 2 h and centrifuged at 3000 rpm for 20 min. The precipitates were washed by three washing-centrifugation cycles with Milli-Q water and subsequently dried at 70 °C. In this study, LDH-HPO42- was made by ion exchange. The LDH-NO3- were treated with 0.05 M KH2PO4 solutions at pH 7.2. The suspensions were shaken end-over-end for 24h, followed by centrifugation, washing, and drying as described above. After digesting the dried LDHs in aqua regia (3:1 HCl/HNO3), the total P concentration of the LDHs was determined. The chemical composition of the synthesized LDHs was determined by furnace atomic absorption spectrophotometry (SavantAA, GBC) after acid digestion (3:1 HCl/HNO3). Crystallization and morphology of the LDHs were characterized via scanning electron microscopy (SEM) and X-ray diffraction (XRD). The XRD patterns were prepared using an x-ray diffractometer (Panalytical x Pert Pro, Netherlands). The phase purity was surveyed by comparing these XRD diagrams with those found in the literature. The SEM photographs were gained on a scanning electron microscope (Sigma VP, Germany). Fourier Transform Infrared (FTIR) spectrum was done on a Nicolet iS10 FT-IR spectrometer by utilizing KBr pressed disk technique.

A batch study was done to determine the effect of different ratios of M2+/M3+ in LDHs at different pH 6.0, 7.0, and 8.0 on the release of P from LDHs. Briefly, 0.01 g of synthesized LDHs were put in a centrifuge tube mixed with 10 ml of 0.03M KNO3 at initial pH=6, 7 and 8. Suspensions were shaken at a constant temperature (25 ± 0.5 °C) and agitation (180rpm) by using an incubator shaker for 8h. Phosphorus concentration in supernatant solutions was measured by the vanadate yellow method at 470 nm wavelength.

In order to investigate the kinetics of phosphorus release, LDH (3:1) and LDH (4:1) were used at two initial pHs of 6 and 8. First, 0.012 g of LDH sample was placed in 120 ml of KNO3 electrolyte solution (with ionic strength of 0.03 M) in an Erlenmeyer flask. The flasks were shaken for 5 to 1175 min by an incubator shaker at 100 rpm. Then the suspensions were centrifuged at a speed of 4000 rpm for 20 minutes and the phosphorus concentration was determined by the previously described method. All experiments were performed with three repetitions. Two equations (pseudo-second-order and parabolic diffusion) were used to fit the kinetics data.

Results and Discussion

The XRD patterns of the LDHs exhibited a distinct characteristic reflection (003), which indicates that the basal spacing decreased as the Mg/Al ratio decreased (higher AEC). The intercalation of phosphate anions into Mg/Al LDH is in adaptation with the change toward lower 2θ angles of the (001) reflections corresponding to the expansion of the basal distance d003 compared to the host Mg/Al-NO3-.

The results showed that increasing the pH from 6 to 8 in the presence of 0.03 M potassium nitrate background electrolyte led to an increase in phosphorus released from both types of LDH. For example, by increasing the initial pH of suspensions from 6 to 8, the amount of phosphorus released from LDH (4:1) increased from 4.61 mg/kg to 9.29 mg/kg. In all studied pHs, phosphorus release from LDH (3:1) in background electrolyte environment was lower than LDH (4:1). For example, at pH 6, 7, and 8, the amount of phosphorus released from LDH (4:1) was 1.48, 2.10, and 2.45 times higher than LDH (3:1), respectively. The cumulative phosphorus release kinetics from the studied LDHs showed that the amount of phosphorus release accelerates with increasing time. Phosphorus release from LDH continued at a higher rate from 0 to 400 minutes in the first stage and at a slower rate during 400-1175 minutes. Also, based on the results, among the studied kinetic equations, pseudo-second-order equations and parabolic dispersion had the best fit on phosphorus release data.

Conclusions

The results of this research showed that the release of phosphorus from LDH is dependent on time, pH and the type of LDH. Based on the results of fitting the kinetic models to the experimental data, the release rate of phosphorus from LDH (4:1) was higher than that of LDH-P (3:1). Phosphorus release from LDH (4:1) compared to LDH (3:1) was 48, 145% higher at pH 6 and 8, respectively.

کلیدواژه‌ها [English]

  • Kinetic models
  • layer double hydroxide
  • pH
  • slow-release Fertilizer
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