16184437 Existing And Potential Applications Of Zeolites In Water Treatment

Existing and Potential Applications of Zeolites in Water Treatment and Agriculture Raul F. Lobo Center for Catalytic Science and Technology

Dept. of Chemical Engineering University of Delaware

Oct. 16, 2007

Zeolites • Crystalline aluminosilicate with a four connected tetrahedral framework structure enclosing cavities occupied by large ions and guest molecules with considerable freedom of movement, permitting ion exchange and reversible dehydration • Gismondine: |Ca2+4 (H2O)16|[Al8Si8O32]–GIS • Key elements: – Framework – Cations – Guest Molecules

Zeolite Gismondine (GIS)

Large Cages with Small Pore Windows

Zeolite Chabazite

Pore Systems • Dimensionality and size of zeolite pores define many of its properties – Size of smallest window determine size of molecules that can enter/leave the zeolite – Dimensionality affect many properties such as diffusion, pore blocking, etc. – Pore intersections act like cages – Classification: • • • •

Small Medium Large Extra-large pore

8-ring 10-ring 12-ring +12-ring

4Å 5.6 Å 7.8 Å >8Å

Zeolite A ZSM-5 USY UTD-1

Framework Composition • Structure is what make zeolites unique, chemical composition can greatly affect their properties – Silica zeolites (SiO2–CHA). Very hydrophobic, covalent bonding dominates, no other interactions besides dispersion forces – High-silica Zeolites (Si/Al>10). Less hydrophobic, H-bond or eletrostatic forces important – Low-silica zeolites (Na+[SiAlO4]–CHA) Very hydrophilic, bonding more ionic, H-bondelectrostatic interactions dominate

Ion Exchange • Extra-framework cations can be ion exchanged in aqueous solutions – Zeo-(Na+)2 + Ca2+ (aq) ⇒ Zeo-Ca2+ + 2 Na+ (aq)

– Di and trivalent cations are adsorbed preferentially – Large cations are ion exchange preferentially (NH4>Na+)

Natural Zeolites • Long history of use for soil remediation, synthetic soil, amendment for soil, animal food additive • Inexpensive – 0.5-0.12 $/kg

• Available in large quantities in US, Europe and many other countries – Clinoptilolite is the most commonly used zeolite due to large surface deposits of good quality (80%+ zeolite) – Over 40 different structures found in nature

Agriculture • Amendment for sandy and low clay content soils • Quantity:8-15 ton/hectare • Improves retention of nitrogen – Can be pretreated with ammonia for slow release effects

• Reduces leaching of metals from soil • Improves water retention and soil porosity

Phytoremediation • Zeolites adsorb metal cations from soils – Reduction in local concentration allows for plants to grow

• Increase quantity of microorganisms in soil • Accelerate nitrogen fixation by releasing NH4+ slowly

Animal Feed Additive • Dozens of studies documenting the effect of zeolites as food additive – Generally, an improvement in health of the animals is observed (1-3% w/w) – Reduction of odors as zeolites sequester NH3 – Few studies address relationship between molecular structure and physiological effect

Water Purification • Zeolites selectively remove ammonium and 2+ cations from aqueous solutions – Sequestration of Zn2+, Cd2+ and other ions. – It is a one-time use unless expensive equipment is used for re-exchange with Na+. – Purification of mining effluent waters

• Use of zeolites is widespread in municipal facilities to remove ammonium ions to acceptable limits.

Outlook • There appears to be no radical new technologies that are being developed with zeolites that could affect agricultural production in poor countries • Niches of opportunity may be present where local concentrations of metal ions affect agricultural output • Water purification is an area of opportunity but technology and applications already exist