ISOLATION OF AMMONIUM FROM FRESH WATERS FOR NITROGEN ISOTOPE ANALYSIS USING AMBERLITE IR120
AND DOWEX 50WX8 ION EXCHANGE RESINS
A COMPARATIVE STUDY *

ALY, A.I.M.⁽¹⁾, BASTAWEESY, A.M.⁽²⁾, AHMED, M.A.⁽¹⁾ and GOMAA, H.E.⁽¹⁾

(1) National Centre for Nuclear Safety and Radiation Control, Atomic Energy Authority, Cairo, Egypt and (2) Chemical Engineering Department, Faculty of Engineering, Elminia University, Elminia, Egypt.

Key words: Ion-exchange, Dowex 50WX8, Amberlite IR120,
Ammonium, Isotope analysis.

فصل الأمونيوم من المياه العذبة للقياس النظائرى للنيتروجين بإستخدام راتنجات
التبادل الأيونى من نوع أمبرلايت- IR120  ودوفكس- 50WX8  
دراسة مقارنة

على إسلام متولى على و على محمد بسطويسى و منى عباس أحمد و حسن السيد محمد جمعة

خلاصـــة

تم فى هذا البحث استخدام راتنجات التبادل الأيونى فى تجميع وتركيز الأمونيا من عينات المياه العذبة. وقد تم تقييم مدى مناسبة نوعين من هذه الراتنجات ودراسة مدى تأثير النقص فى نسبة الإسترجاع على القيم النظائرية التى يتم الحصول عليها. تمت الدراسة على نوعين من راتنجات التبادل الأيونى هما دوفكس- 50WX8وأمبرلايت IR120-.

أظهرت النتائج أن زيادة تركيز أيون الكالسيوم فى المياه يؤدى إلى نقص فى نسبة إسترجاع الأمونيا ويؤدى أيضاً إلى نثرية نظائرية للأمونيا المتبقية فى مياه العينة عند استخدام راتنجات أمبرلايت-IR120 . ومن مميزات هذه الطريقة أنها لا تعانى التداخل من النيتروجين العضوى حيث أنه يتم إسترجاع الأمونيا المحملة على راتنجات التبادل الأيونى بالطرد المباشر بإستخدام طريقة انتشار الأمونيا المعدلة.

ABSTRACT

A procedure for isolating ammonium from fresh water and wastewaters using ion-exchange resins was demonstrated. The method presented consists of two consecutive steps; the first is the collection of ammonium on ion-exchange resins and the second is direct elution-diffusion of the collected ammonium. Two types of ion-exchange resins for nitrogen isotope analysis (amberlite IR120 and dowex 50WX8) were compared. Ammonium collected on the cation-exchange resins was eluted directly using the modified ammonia diffusion method instead of elution using KCl and subsequent diffusion of ammonia from elute. The method was evaluated with simulated fresh water solutions containing varying amounts of standard NH₄⁺-N (δ ¹⁵N = -1.6‰) and potentially interfering cations (Ca²⁺ and Mg²⁺). The presented method has the advantages of: (1) Offering an attractive sampling tool eliminating the need to carry large volumes of water to the laboratory. (2) Isolating ammonium from fresh water samples prior to diffusion and hence samples can be diffused for separating ammonium and nitrate in parallel rather than in series which cut down the diffusion time to less than half. (3) Eliminating the need to pre-treatment steps to minimize the blank from dissolved organic nitrogen (DON) needed when combusting the resin directly with the collected ammonium.

Dowex 50WX8 yielded more accurate and reproducible δ¹⁵N values (SD =
± 0.22‰) than amberlite IR120 (SD = ± 0.35‰).  Calcium and magnesium ions had higher affinities for amberlite IR120 than that for dowex 50WX8. With increasing Ca²⁺ concentrations in solution, the resultant δ¹⁵N values were becoming more depleted in case of amberlite IR120. No expected blank was associated with dissolved organic nitrogen, since the conditions were very mild. To test the analytical variability in natural samples using the presented procedure, δ¹⁵N values of ammonium isolated from samples collected from wastewater drain were measured using the modified ion-exchange resin procedure, the ammonia diffusion method and the vacuum distillation method.