Synthesis of zeolite/hydrous metal oxide composites from coal fly ash as efficient adsorbents for removal of methylene blue from water

Abstract

Zeolites are useful crystalline aluminosilicate minerals, and zeolite synthesis from coal fly ash (ZFA) has been investigated widely to recycle solid waste. This synthesis process produces waste alkaline solution as a by-product. To date, research has focused mainly on ZFA synthesis and use; the problematic waste alkaline solution has rarely been addressed. In this study, we developed two composites of zeolite/hydrous iron oxide (ZFA/HIO) and zeolite/hydrous zirconia (ZFA/HZ) by adding, after zeolite synthesis, a step in which waste alkaline solutions are neutralized with an iron or zirconium salt. The composites were characterized by X-ray fluorescence, X-ray diffractometry, scanning electron microscopy, acid–base neutralizing ability, specific surface area and pore size distribution, and their performance as adsorbents for the removal of methylene blue (MB) from water was investigated. MB adsorption by ZFA/HIO and ZFA/HZ was much higher than that by ZFA, and increased with increasing pH. This behavior was explained by the increased variable charge on the metal oxides. The adsorption was rapid, and nearly complete removal could be achieved at a sufficiently high dose. Material heating resulted in a decrease in pH and adsorption, but heating with pH adjustment could regenerate adsorbents for repeated use in MB removal. Zeolite/hydrous metal oxide synthesis from coal fly ash was environmentally friendly and products exhibit a high potential as reusable adsorbents for MB removal from water.