Synthesis of high-quality zeolite LTA from alum sludge generated in drinking water treatment plants

This study demonstrated the conversion of alum sludge produced at water treatment plants to high quality zeolite LTA. Thus, problems with sludge landfilling costs and environmental contamination may be avoided. The methodology involved alkali fusion pre-treatment of alum sludge followed by hydrothermal synthesis to improve the quantity and quality of zeolite LTA made. Alum sludge activation using fusion facilitated formation of high-quality zeolite LTA (97 wt%) which was comparable to commercial material (98 wt%) and superior to samples made using hydrothermal synthesis without fusion pre-treatment (81 wt%). Notably, the alum sludge derived samples contained greater amounts of impurities such as iron, calcium, and magnesium. Preferred synthesis conditions using fusion-pre-treatment were: SiO2/Al2O3 = 2.5; Na2O/SiO2 = 3; H2O/Na2O = 40; reaction time =5 h: reaction temperature = 80 degrees C. The average zeolite LTA particle size ranged from 0.7 (hydrothermal) to 1.1 (fusion pre-treated) to 3.0 (commercial) microns. Kinetic studies indicated that the exchange of calcium ions with all zeolite samples reached equilibrium within 60 min. Equilibrium ion exchange data revealed the maximum loading of calcium ions was 106.6, 83.9, and 64.6 mg Ca2+/g for fusion pre-treated, hydrothermally treated, and commercial LTA, respectively. However, on a dry basis the cation capacity of the commercial and fusion pre-treated samples was similar.

Automated summary

This study demonstrated the successful conversion of alum sludge from water treatment plants into high-quality zeolite LTA, with alkali fusion pre-treatment improving both yield and quality. The samples obtained through fusion pre-treatment exhibited comparable quality to commercial material. Different synthesis methods resulted in variations in impurity content and ion exchange capacity of the zeolite samples.