Impact of fly ash fractionation on the zeolitization process

Improvement in adsorption of Hg2+ from aqueous media using sodium-type fine zeolite grains

To increase the adsorption capability of Hg²⁺ from aqueous media, we prepared sodium-type fine zeolite grains with various particle sizes (denoted as ZE1, ZE2 and ZE3). The particle sizes of ZE1, ZE2 and ZE3 were 16.363 ± 0.365, 1.454 ± 0.357 and 0.607 ± 0.377 μm, respectively. Moreover, the CEC, specific surface area and pore volume were in the order ZE1 (42 mmol/g and 23.5 m²/g) < ZE2 (72 mmol/g and 67.1 m²/g) < ZE3 (135 mmol/g and 176.6 m²/g). Subsequently, the Hg²⁺ adsorption capability was investigated. The performance of tested agents on Hg²⁺ adsorbed was in the order ZE1 (5.0 mg/g) < ZE2 (9.4 mg/g) < ZE3 (20.2 mg/g). It was concluded that fine crystalline zeolite was important in enhancing the adsorption capability of Hg²⁺. In addition, the mechanism of adsorption of Hg²⁺ on the ZE samples was evaluated. Our results suggested that Hg²⁺ was exchanged with sodium ions in the interlayers of ZE samples with correlation coefficients of 0.966-0.979. Our findings revealed that these ZE samples constitute potential agents for the adsorption of Hg²⁺ from aqueous media.

Kinetics Study of Al Extraction from Desilicated Coal Fly Ash by NaOH at Atmospheric Pressure

The most promising source of alumina in the 21st century is the coal fly ash (CFA) waste of coal-fired thermal plants. The methods of alumina extraction from CFA are often based on the pressure alkaline or acid leaching or preliminary roasting with different additives followed by water leaching. The efficiency of the alumina extraction from CFA under atmospheric pressure leaching is low due to the high content of acid-insoluble alumina phase mullite (3Al₂O₃·2SiO₂). This research for the first time shows the possibility of mullite leaching under atmospheric pressure after preliminary desilication using high liquid to solid ratios (L:S ratio) and Na₂O concentration. The analysis of the desilicated CFA (DCFA) chemical and phase composition before and after leaching has been carried out by inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray diffraction (XRD). The morphology and elemental composition of solid product particles has been carried out by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). An automated neural network and a shrinking core model (SCM) were used to evaluate experimental data. The Al extraction efficiency from DCFA has been more than 84% at T = 120 °C, leaching time 60 min, the L/S ratio > 20, and concentration of Na₂O-400 g L^-1. The kinetics analysis by SCM has shown that the surface chemical reaction controls the leaching process rate at T < 110 °C, and, at T > 110 °C after 15 min of leaching, the process is limited by diffusion through the product layer, which can be represented by titanium compounds. According to the SEM-EDX analysis of the solid residue, the magnetite spheres and mullite acicular particles were the main phases that remained after NaOH leaching. The spheric agglomerates of mullite particles with non-porous surface have also been found.

Experimental and Computational Approaches for the Structural Study of Novel Ca-Rich Zeolites from Incense Stick Ash and Their Application for Wastewater Treatment

At present, chemical Si/Al sources are mainly used as precursor materials for the manufacturing of zeolites. Such precursor materials are quite expensive for commercial synthesis. Here, we have reported the synthesis of Ca-based zeolite from incense stick ash waste by the alkali-treatment method for the first time. Incense stick ash (ISA) was used as a precursor material for the synthesis of low Si zeolites by the alkali-treatment method. The as-synthesized zeolites were characterized by various instruments like particle size analyzer (PSA), Fourier transform infrared (FTIR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), electron diffraction spectroscopy (EDS), transmission electron microscopy (TEM), and X-ray fluorescence (XRF). FTIR and XRD helped in the identification of the microstructure and crystalline nature of the zeolites and also confirmed the synthesis of Ca-based zeolite with two thetas at 25.7°. The microscopic analysis by FESEM and TEM exhibited that the size of synthesized Ca-rich zeolites varies from 200 to 700 nm and they are aggregated and cuboidal in shape. Additionally, structural, electronic, and density of states’ characteristics of gismondine (Ca2Al4Si4O16·9H2O) structures were evaluated by computational simulations (first principle, density functional theorem). The structural optimization of structures was carried out in the first stage under the lowest condition of total energy and forces acting on atoms for the lattice constant, as well as the available experimental and theoretical findings. The present research approach predicted the transformation of ISA waste into a value-added mineral, i.e., zeolite, which was further used for the removal of both heavy metals and alkali metals from fly ash-based wastewater using inductively coupled plasma-optical emission spectroscopy (ICP-OES).

Adsorption/Desorption Capability of Potassium-Type Zeolite Prepared from Coal Fly Ash for Removing of Hg2+

The feasibility of using potassium-type zeolite (K-type zeolite) prepared from coal fly ash (CFA) for the removal of Hg2+ from aqueous media and the adsorption/desorption capabilities of various potassium-type zeolites were assessed in this study. Potassium-type zeolite samples were synthesized by hydrothermal treatment of CFA at different intervals (designated CFA, FA1, FA3, FA6, FA12, FA24, and FA48, based on the hours of treatment) using potassium hydroxide solution, and their physicochemical characteristics were evaluated. Additionally, the quantity of Hg2+ adsorbed was in the order CFA, FA1 < FA3 < FA6 < FA12 < FA24 < FA48, in the current experimental design. Therefore, the hydrothermal treatment time is important to enhance the adsorption capability of K-type zeolite. Moreover, the effects of pH, temperature, contact time, and coexistence on the adsorption of Hg2+ were elucidated. In addition, Hg2+ adsorption mechanism using FA48 was demonstrated. Our results indicated that Hg2+ was exchanged with K+ in the interlayer of FA48 (correlation coefficient = 0.946). Finally, adsorbed Hg2+ onto FA48 could be desorbed using a sodium hydroxide solution (desorption percentage was approximately 70%). Our results revealed that FA48 could be a potential adsorbent for the removal of Hg2+ from aqueous media.

Complete Extraction of Amorphous Aluminosilicate from Coal Fly Ash by Alkali Leaching under Atmospheric Pressure

One of the potential sources of alumina and mesoporous silica is the coal-fired thermal plants waste known as the coal fly ash (CFA). The studies of the alumina extraction from CFA are often focused on the preliminary desilication, but the efficiency of the alkali desilication is low due to formation of the desilication product—Na6[Al6Si6O24]·Na2X (DSP). This research is focused on the possibility of CFA desilication without formation of DSP using a leaching process with higher liquid to solid ratios (L/S) and alkali concentrations. The experimental data were analyzed using an artificial neural network (ANN) machine learning method and a shrinking core model (SCM). The investigation of the CFA morphology, chemical and phase composition before and after leaching were carried out by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray diffraction (XRD). The present work shows that it is possible to avoid formation of DSP if using the L/S ratio >20 and concentration of Na2O—400 g/L during CFA leaching. The kinetics analysis by SCM showed that the process is limited by the surface chemical reaction at T <100 °C, and by diffusion through the product layer at T >100 °C, respectively. The SEM images of the solid residue after NaOH leaching under conditions that prevent the DSP formation show mullite particles with an acicular structure.