31 EFFECT OF NATURAL AND SYNTHETIC ZEOLITES ON BACTERIA Jasna Hrenović 1 , Darko Tibljaš 2 , Lavoslav Sekovanić 3 , Tomislav Ivanković 1 , Mirela Rožić 4 1 University of Zagreb, Faculty of Science, Rooseveltov trg 6, 10000 Zagreb, Croatia 2 University of Zagreb, Faculty of Science, Horvatovac bb, 10000 Zagreb, Croatia 3 University of Zagreb, Geotechnical Faculty, Hallerova aleja 7, 42000 Varazdin, Croatia 4 University of Zagreb, Faculty of Graphic Arts, Getaldićeva 2, 10000 Zagreb, Croatia E-mail: jasnah@zg.biol.pmf.hr INTRODUCTION Zeolites display many useful applications in the field of pollution abatement. Uses of zeolite include horticulture product, soil conditioner, animal feed, odour control and hygiene products, air filtration media, cation exchange products, water and wastewater treatment. Currently attention is being drawn to the implementation of zeolites in the wastewater treatment process in order to achieve a higher efficiency of the water purification. Naturally occurring zeolites include tuffs rich in more than 48 known zeolite mineral species such as chabazite, clinoptilolite, erionite, heulandite, mordenite, phillipsite. The natural clinoptilolite is usually chosen in studies on wastewater treatment on the base of its widespread occurrence in the nature, price-easily accessibility and feasibility, cost effectiveness, large surface area, rigidity, surface functionality, thermal, mechanical and radiation stability. Clinoptilolite is a high-silica member of heulandite group of natural zeolites and occurs in abundant and easy mined sedimentary deposits in many parts of the world. However, the composition, purity and mineralogical characteristics of clinoptilolite tuff may vary widely from one deposit to another and even within the same deposit. When putted into contact with microorganisms, natural zeolite tuff influences its metabolic activity positively [1, 2, 3, 4], while synthetic zeolite negatively 5 . This review summarizes these effects. EXPERIMENTAL The zeolitised tuff from Donje Jesenje, Croatia contained more than 50% of clinoptilolite, some quartz and plagioclase and accessory minerals from the mica group (illite- celadonite and biotite). Among the exchangeable cations, potassium was the dominant one in the sample. Four different fractions of particle size of zeolite tuff were tested. The commercial synthetic zeolite of lynde type A (LTA) from Silkem d.o.o., Kidričevo, Slovenia was used. The material was sodium aluminosilicate with average particle size 3-5 m. Zeolite samples were washed with demineralised water and then dried at 105 C/16h before the experiments were to commence. In experiments the pure culture of phosphate-accumulating bacterium (PAB) Acinetobacter junii (DSM 1532), which was isolated from the activated sludge showing the enhanced biological phosphate removal from wastewater, was used. The composition of the simulative wastewater was as follows (in mg/L): Na-propionate 300; peptone 100; MgSO 4 10; CaCl 2 6; KCl 30; yeast extract 20; KH 2 PO 4 44 or 88. The pH of the synthetic wastewater was adjusted to 7.0 before autoclaving (121 C/15 min). The experiments were carried out as batch tests so that the bacterial biomass was suspended in 200 mL of synthetic wastewater. In reactors different concentration of zeolite was added. The reactors left without the addition of zeolite served as control reactors. The