بررسی رهاسازی پتاسیم غیرتبادلی از برخی کانی های پتاسیم دار توسط عصاره‌گیرهای مختلف

به منظور مقایسه سینتیک رهاسازی پتاسیم از کانی های پتاسیم دار و نیز انتخاب بهترین معادله سینتیکی توصیف کننده روند رهاسازی پتاسیم توسط عصاره گیرهای آلی و معدنی، آزمایشی در قالب طرح کاملا تصادفی با آرایش فاکتوریل در 3 تکرار اجرا گردید. فاکتورهای آزمایش شامل 3 نوع عصارهگیر (اسید اگزالیک 01/0 مولار، کلرید کلسیم 01/0 مولار، شاهد (آب مقطر))، 3 کانی پتاسیم دار (فلدسپار، ایلیت و فلوگوپیت) و 10 زمان (1، 2، 4، 8، 12، 16، 24، 32، 48 و 64 ساعت) بودند. مقدار پتاسیم آزاد شده با استفاده از دستگاه فلیم فتومتر طی زمان های مختلف در شرایط آزمایشگاهی اندازه‌گیری شد و داده ها به معادلات سینتیکی مرتبه صفر، مرتبه اول، مرتبه دوم، تابع توانی، پخشیدگی پارابولیک و الوویچ برازش داده شد. نتایج نشان دهنده ی اثر معنی دار نوع عصاره گیر بر سینتیک رهاسازی پتاسیم بود، بطوریکه میزان پتاسیم آزاد شده در نمونه های عصاره گیری شده با اسید اگزالیک در مقایسه با نمونه های عصاره گیری شده با کلرید کلسیم و شاهد (آب مقطر) به ترتیب 1/48 و 2/35 برابر بیشتر بود. کانی های مختلف نیز مقادیر متفاوتی از پتاسیم را آزاد نمودند. رهاسازی پتاسیم از فلوگوپیت نسبت به فلدسپار و ایلیت به ترتیب 1/99 و 2/95 برابر بیشتر بود. بیشترین غلظت پتاسیم نیز (440 میلی گرم بر کیلوگرم) در تیمار عصاره گیری شده با اسید اگزالیک و در حضور کانی فلوگوپیت مشاهده گردید. بطوری‌که مقدار پتاسیم در این تیمار 3/15 برابر نسبت به شاهد افزایش یافت. برازش معادلات سینتیکی نشان داد که معادلات تابع توانی و پخشیدگی پارابولیک با بیشترین ضریب تبیین (r^2) و کمترین خطای استاندارد (se)، بهترین مدل برای برازش داده ها بودند این در حالیست که مدل مرتبه دوم قادر به توجیه رهاسازی پتاسیم نبود. چنین استنباط می گردد که سینتیک رهاسازی پتاسیم توسط عوامل مختلفی از جمله نوع کانی و نوع عصاره گیر تحت تاثیر قرار می گیرد و عصاره گیر آلی توانایی بالاتری در استخراج پتاسیم غیرتبادلی از ساختار کانی ها دارد. همچنین تطبیق نتایج این تحقیق با معادلات مرتبه اول، پخشیدگی پارابولیک و تابع توانی حاکی از آن است که رهاسازی پتاسیم غیرتبادلی از کانی ها می تواند متاثر از فرآیند پخشیدگی باشد و بعبارت دیگر پخشیدگی پتاسیم به خارج از توده کانی، کنترل کننده سرعت آزاد شدن پتاسیم می باشد.

Evaluation of NonExchangeable Potassium Release from KBearing Minerals by Different Extractants

Introduction: Potassium is one of essential nutrients for plants and its importance in agriculture is well known. Nonexchangeable potassium that is mainly placed with in layers of Kbearing minerals, such as Kfeldspar and mica, is considered as an important source of potassium for plant growth in most soils. Regarding that low molecular weight acids (LMW) play an important role in improving the bioavailability of soil nutrients such as nonexchangeable K (NEK), and the release rate of NEK plays a significant role in supplying necessary K for plants, the purpose of this study was comparison of potassium release kinetic from Kbearing including feldspar, illite as well as phlogopite minerals and choose the best kinetic equation describing potassium release process, influenced by organic as well as mineral extractants.Material and Methods: The experiment carried out in a completely randomized design with three replications. Experiment factors were including extractant type (0.01 mol l1 oxalic acid, 0.01 mol l1 calcium chloride, control (deionized water)), potassium mineral type (feldspar, illite and phlogopite) and incubation time (1, 2, 4, 8, 12, 16, 24, 32, 48, and 64 hours). Elemental composition of minerals identified by Fluorescence spectroscopy device (S4 Pioneer). Used minerals in the experiment including feldspar, phlogopite and illite were ground and filtered through a 230 mesh sieve. In order to remove exchangeable K, samples were saturated by calcium chloride solution (with a ratio of 2:1), after washing with HCl, samples were dried at 105 °C for 48 hours. 100 mg of washed minerals, was weighed carefully and transferred to centrifuge tubes. Then 20 ml of each of extractants (oxalic acid and calcium chloride 0.01M) was added to the tubes. After 15 minutes shaking, tubes containing a mixture of mineralsextractants was carried out in a controlled incubation chamber for periods of 1, 2, 4, 8, 12, 16, 24, 32, 48 and 64 hours at 25 °C. After each period, samples were centrifuged at 3000 rpm for 10 minutes and filtered using Whatman paper (No. 41). pH and potassium concentration of samples were measured by pH meter and flame photometer, respectively. Data related to potassium release was fitted by zero order, first order, second order, power function, parabolic diffusion and ellovich equations.Results and Discussion: Results showed that the effect of extractant type was significant on kinetic of potassium release, so that potassium release amount in samples extracted with oxalic acid was 1.48 and 2.35 times higher than samples extracted with calcium chloride and control (deionized water), respectively. Also, different minerals released various amounts of potassium. K release from phlogopite was 1.99 and 2.95 times higher than feldspar and illite, respectively. The maximum potassium concentration (440 mg kg1) was seen in phlogopite which was extracted with oxalic acid. So that, amount of potassium in this treatment was 3.15 times higher than control one. Furthermore, the effect of extraction time on K release was significant. So that, at the beginning of incubation period the release of potassium by different extractants was more, but its amount decreased over time and finally continued with a constant speed. Kinetic equation fitting showed that zero order, first order, power function, parabolic diffusion and ellovich equations are able to describe potassium release but second order model cannot justify it. Among these five equation, the power function and parabolic diffusion equations with the maximum coefficient of determination (R2) and the least standard error of estimate (SE), could reasonably describe the K release kinetics, so they are introduced as the best models for data fitting. The slope (b) and interception (a) of ellovich equation indicate interlayer and initial K release, respectively. Oxalic acid and phlogopite had the most amount of interception, it means that the impact of oxalic acid on initial and interlayer release rate of K in phlogopite, is more effective than calcium chloride. Conclusions: It is concluded that different factors like mineral and extractant type influence kinetic of potassium release and organic extractant have more ability in extracting nonexchangeable potassium from minerals structure. Also, the adjustment of the results of this study with first order, parabolic diffusion and power function equations suggest that nonexchangeable potassium release from minerals can be affected by diffusion process from the surface of the study minerals, indicating that NEK release rate is controlled by K diffusion out of the mineral interlayer.