جذب سطحی رقابتی آرسنات و فسفات روی کلسایت

نوع مقاله : مقاله پژوهشی

نویسندگان

1 گروه خاکشناسی، دانشکده کشاورزی، دانشگاه تربیت مدرس، تهران، ایران

2 گروه خاکشناسی، دانشکده کشاورزی، دانشگاه تربیت مدرس، تهران، ایران

3 گروه علوم و مهندسی خاک، دانشکده کشاورزی و منابع طبیعی، دانشگاه محقق اردبیلی، اردبیل، ایران

4 گروه علوم و مهندسی خاک، دانشکده کشاورزی، دانشگاه شهید باهنر کرمان، کرمان، ایران

چکیده

کلسایت، پایدارترین شکل کربنات کلسیم در خاک، یکی از اجزای مهم فاز جامد خاک­های مناطق خشک و نیمه‌خشک است. این کانی بر رفتار شیمیایی یون­ها از جمله اکسی آنیون­ها و فلزات انتقالی در خاک اثرات مهمی دارد. آرسنات و فسفات از اکسی آنیون­های مهم در سیستم­های طبیعی نظیر خاک هستند و به دلیل تشابه یونی، معمولاً برای بار سطحی ذرات کلوییدی خاک از طریق واکنش‌های جذب سطحی و رهاسازی به شدت با هم رقابت می‌کنند. با توجه به اهمیت این واکنش در کنترل غلظت تعادلی آرسنات و فسفات در خاک، در این تحقیق جذب سطحی آرسنات روی کانی کلسایت در غلظت‌های مختلف آرسنات و در قدرت­­های یونی مختلف و در رقابت با فسفات بررسی و با مدلی شیمیایی توصیف شد. همچنین رفتار باری کلسایت به روش تیترسنجی اسید و باز در قدرت­های یونی مختلف اندازه­گیری گردید. بر اساس داده­های تیتراسیون، نقطه صفر بار الکتریکی کانی در pH2/8 تعیین شد. همدما­های جذب سطحی آرسنات نشان داد که جذب سطحی آرسنات متأثر از قدرت یونی و غلظت اولیه آن است. در قدرت­های یونی کم، جذب سطحی آرسنات حداکثر بود و با افزایش قدرت یونی از مقدار آن کاسته شد. جذب سطحی آرسنات با افزایش غلظت فسفات کاهش یافت اما آرسنات تأثیری بر جذب سطحی فسفات نداشت که نشان می‌دهد فسفات قوی‌تر از آرسنات جذب سطحی کلسایت می‌شود. داده­های آزمایشی با مدل شیمیایی CD-MUSIC توصیف و پیش‌بینی گردیدند. مدل داده­ها را با در نظر گرفتن دو کمپلکس درون کره­ای سطحی ≡Ca2O2AsO2 و  ≡Ca2O2PO2  با دقت بالا (R2=0.998) توصیف کرد. به‌طور کلی نتایج آزمایشی و ضرایب مدل نشان داد که برهم‌کنش بسیار قوی‌تر فسفات با کلسایت موجب افزایش زیست‌فراهمی آرسنات و تحرک آن در خاک‌های آهکی می‌شود.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Competitive Adsorption of Arsenate and Phosphate on Calcite

نویسندگان [English]

  • Mojtaba Moghbeli 1
  • Rasoul Rahnemaie 2
  • Esmaiel Goli Kalanpa 3
  • Naser Broomand 4
1 Department of Soil Science,, Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran
2 Department of Soil Science,, Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran
3 Department of Soil Engineering, Faculty of Agriculture and Natural resources, , University of Mohaghegh Ardabili,, Ardabil, Iran
4 Department of Soil Science Enginering, Faculty of Agriculture, Shahid Bahonar University, Kerman, Iran
چکیده [English]

Calcite, the most stable calcium carbonate in soil, is a major part of soil solid phase in arid and semiarid regions. This mineral significantly affects the chemical behavior of ions including oxyanions and transition metal ions in the soil. Arsenate and phosphate are two important oxyanions in natural systems like soil and because of similar ionic properties strongly compete for the colloids surface charge via adsorption and desorption reactions. Because of the importance of this reaction in controlling the equilibrium concentrations of arsenate and phosphate in soil, in this research, arsenate adsorption on calcite was measured and modeled as a function of arsenate concentration and ionic strength and also in competition with phosphate. In addition, charging behavior of calcite was measured by acid-base titration at various ionic strength. Based on the titration data, calcite point of zero charge (PZC) was obtained at pH=8.2. Adsorption isotherms showed that arsenate adsorption is affected by the ionic strength and the initial concentration of arsenate. Adsorption of arsenate was high at low ionic strength and was decreased with increasing the ionic strength. Arsenate adsorption was also decreased with increasing phosphate concentration, but arsenate had no effect on phosphate adsorption indicating phosphate is adsorbed stronger than arsenate on calcite. The experimental data were successfully (R2=0.998) described with a single set of parameters by CD-MUSIC model, considering two inner sphere surface complexes ≡Ca2O2AsO2 and ≡Ca2O2PO2. Overall, the experimental data and model parameters implied that the stronger interaction of phosphate with calcite increases bioavailability and mobility of arsenate in calcareous soils.

کلیدواژه‌ها [English]

  • Adsorption
  • Arsenate
  • Calcite
  • CD-MUSIC model
  • Phosphate
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