RESEARCH PAPER Synthesis, characterization, and ion-exchange properties of colloidal zeolite nanocrystals Anna Jawor Æ Byeong-Heon Jeong Æ Eric M. V. Hoek Received: 13 January 2009 / Accepted: 23 June 2009 / Published online: 10 July 2009 Ó Springer Science+Business Media B.V. 2009 Abstract Here, we present physical–chemical prop- erties of Linde type A (LTA) zeolite crystals synthesized via conventional hydrothermal and microwave heating methods. Both heating methods produced LTA crystals that were sub-micron in size, highly negatively charged, super-hydrophilic, and stable when dispersed in water. However, microwave heating produced relatively narrow crystal size dis- tributions, required much shorter heating times, and did not significantly change composition, crystallin- ity, or surface chemistry. Moreover, microwave heating allowed systematic variation of crystal size by varying heating temperature and time during the crystallization reaction, thus producing a continuous gradient of crystal sizes ranging from about 90 to 300 nm. In ion-exchange studies, colloidal zeolites exhibited excellent sorption kinetics and capacity for divalent metal ions, suggesting their potential for use in water softening, scale inhibition, and scavenging of toxic metal ions from water. Keywords Nanoparticle Á Zeolite Á Microwave Á Hydrothermal synthesis Á Ion exchange Introduction Zeolites are crystalline aluminosilicates containing pores and cavities of molecular dimensions (Breck 1974). Many natural zeolites exist, but synthetic varieties are among the most widely used as sorbents, catalysts, and molecular sieves (Cundy and Cox 2003). For sorption applications, zeolites are often reported to exhibit high sorption capacity and selec- tivity for divalent elements, which makes the zeolites attractive for environmental applications, such as water softening or removing toxic metals from water (Apiratikul and Pavasant 2008). Recent innovations in zeolite synthesis allow production of sub-micron zeolite crystals and zeolite nanocrystals (i.e., crystal size less than *100 nm). In general, scaling down zeolite crystals from micrometer to nanometer size enhances zeolite sorption capacity and catalytic activity due to higher surface area and shorter diffusion path length (Tosheva and Valtchev 2005; Larsen 2007). The colloidal size of the crystals creates unique properties and expands the potential for zeolites particularly in biomedical and environ- mental applications (Davis 2002; Jeong et al. 2007). Most commercial zeolites are synthesized by conventional hydrothermal synthesis in which an A. Jawor Á B.-H. Jeong Á E. M. V. Hoek (&) Civil & Environmental Engineering Department, University of California, Los Angeles (UCLA), 5732G Boelter Hall, PO Box 951593, Los Angeles, CA 90095-1593, USA e-mail: emvhoek@ucla.edu A. Jawor Á B.-H. Jeong Á E. M. V. Hoek California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA 90095-1593, USA 123 J Nanopart Res (2009) 11:1795–1803 DOI 10.1007/s11051-009-9688-9