Synthesis of zeolites from coal fly ash for the extraction of chrominium from wastewater treatment plants

Abstract

South Africa consumes more than 100 million tons of low-grade coal annually to produce electricity. This process results in generation of vast amounts of waste ash in coal fly ash (CFA) which is voluminous and is a source of toxic particulate matter into the atmosphere. In this work, a zeolite was synthesized from CFA for the selective and efficient extraction of chromium ions from wastewater treatment plants (WWTPs). The first part of the study was directed at characterization of CFA and performing optimization studies for the dissolution of the CFA aluminosilicate matrix. The dissolution conditions studied were concentration of NaOH activating agent (0.5 - 2.5 M), ageing time (24 h - 72 h) and the solid/liquid ratio (15 g/30 mL - 15 g/100 mL). The optimized concentration of activating agent was 2.5 M NaOH, aged for 24 h at a solid/ liquid ratio of (15 g/100 g/mL). The second stage was focused on optimization of crystallization conditions of the CFA/NaOH slurries for the synthesis of zeolite. The effect of changing the water content, crystallization time, and crystallization temperature was studied, and the optimized conditions were H2O/SiO2 molar ratio of 1.81 crystallized at 140°C for 24 h. FT-IR studies revealed the formation of zeolitic bands as a result of asymmetric stretching of Al-O and Si-O, indicating conversion of the CFA aluminosilicate phase. Characterization of CFA using X-ray fluorescence (XRF) analysis indicated that CFA was class C with a Si/Al ratio of 1.19 and a loss on ignition (LOI) of 3.29%. Batch adsorption and kinetic studies revealed that Cr (VI) adsorption onto zeolites particles followed a Freundlich adsorption isotherm as well as a pseudo-second-order adsorption model. This study indicated that the binding surface energies on the zeolites were heterogeneous, resulting in multiple interactions via chemisorption. Removal efficiencies for Cr (VI) during applications studies from Makhado and Rietviei WWTPs using inductively coupled plasma atomic emission spectroscopy (ICP-OES) were found to be 69.25% and 70.02%, obtained for zeolite.