بررسي عملكرد زيوليت‌هاي طبيعي در كاهش ميزان شوري آب‌هاي زيرزميني

Performance of Natural Zeolites for Reducing the Amount of Groundwater Salinity

زيوليت‌ها، به تركيبات شيميايي آلومينو سيليكات‌هاي بلوري و هيدراته‌ي فلزات قليايي و قليايي خاكي، كه حدي از عناصر سديم، كلسيم، منيزيم، استرانسيم و باريم و شبكه‌هاي سه بعدي متشكل از چهاروجهي‌هاي -4 ]4 [SiOو-4] 4[AlO را دارد، اطلاق مي‌شود. قابليت بارز زيوليت‌ها عبارت است از خاصيت آنها در تبادل يون‌ها- به‌خصوص يون‌هاي محلول در آب-، و از اين جهت براي نرم‌كردن آب (سختي‌زدايي) بسيار و براي شوري‌زدايي كمتر استفاده شده‌اند. به‌ همين منظور و با استفاده از زيوليت‌هاي طبيعي معادن ايران به‌صورت اوليه و همچنين زيوليت‌هاي اصلاح‌شده، آزمايش‌هايي براي بررسي امكان كاهش شوري آب انجام شده است. آزمايش‌ها در بين سال‌هاي 1391-1389روي نمونه‌هاي زيوليت، كه از يكي از معادن معروف كشور در منطقه‌ي سمنان تهيه شده بودند، با دانه‌هاي به قطر موثر (5/0 1)، (1 5/3) و (1 2) ميلي‌متر انجام شده است. در اين آزمايش‌ها زمان تعادل، ميزان جذب، غلظت ماده‌ي جاذب (زيوليت)، به‌عنوان متغيرهاي فرايند جذب استفاده شده‌اند، و نمونه‌هاي زيوليت طبيعي و همچنين زيوليت اصلاح‌شده به ميزان (5/0، 1، 2، 3، 5، 10، 20 و 50) گرم در زمان‌هاي تعادلي (15- 30- 40 - 45- 60- 90 و120) دقيقه مورد بررسي قرار گرفته‌اند. نتايج اين پژوهش نشان داد كه عملكرد زيوليت كلينوپتيلولايت، كه مي‌تواند عامل كاهش سختي باشد، سبب افزايش شوري و املاح در آب مي‌شود؛ در حالي كه با استفاده از زيوليت‌هاي اصلاح‌شده، امكان كاهش شوري آب وجود دارد و لذا توصيه مي‌شود كه از زيوليت اصلاح‌شده براي استفاده‌ي مكمل در صنعت تصفيه‌ي آب استفاده شود.

Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents and catalysts. Zeolites have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and others. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Their well-defined pore structure and adjustable acidity make them highly active in a large variety of reactions. The ion exchange properties of zeolites are widely exploited in water industry to remove certain cations including Ca2+ and Mg2+ in water softening processes. In this study the application of modified zeolites for removing water salinity was investigated. The raw mineral zeolite used in this research was from mineral sites of Semnan province. The ore contained three-dimensional network structure consisting of tetrahedral [SiO4] 4, and [AlO4] 4 compound. The raw mineral rock was grinded and sieved to prepare samples of granular sizes of 0.5 -1 , 1-2 and 1-3 mm diameters. A part of the prepared samples was chemically treated by washing with hydrochloric acid to increase its activity. Activated zeolite was compared to untreated samples in removing water hardness and salinity. The activation process dissolves and removes some of the salts contained in the mineral structure and increases the adsorption and exchange quality of the mineral. The experiments were carried out with saline water with known salt concentration. The zeolite concentration ranged from 0.5, 1, 2, 3, 5, 10, 20 and 50 gram per liter. The effect of equilibrium time of 15, 30, 40, 45, 60, 90 and 120 minutes were experimented. As expected lower doses of added zelolite has little effect in salt removal due to low adsorption/exchange capacity of the mineral. It was concluded that equilibrium time of 40-60 minutes is adequate and longer contact time has little effect on the removal efficiency. The results indicated that zeolites used were of clinoptilolite variety. The untreated samples were found to be ineffective in reducing water salinity and in some cases increased chloride content of the solution. However treated samples showed good quality in removing water hardness and to some extent has the ability to reduce salinity. In this respect untreated zeolite should not be used for water purification but acid-activated zeolite can be used for moderate desalination.