Powered by abundant South African clay, a newly engineered geopolymer traps toxic crystal violet dye in minutes, offering a scalable and affordable path to safer water systems.
Study: Adsorption of Crystal Violet Using Kaolin-Based Geopolymer. Image Credit: Peter Milto/Shutterstock.com
A recent study in Chemistry reports the development of a kaolin-based geopolymer (KBG) for removing crystal violet (CV), a toxic cationic dye, from wastewater.
The researchers used South Africa’s abundant kaolin resources to produce an efficient, sustainable adsorbent. The KBG exhibited high dye-removal capacity under simple processing conditions, revealing a low-cost approach for creating environmentally friendly materials that support cleaner water systems and sustainable waste management.
Effluents from the textile and paper industries release large amounts of synthetic dyes that are difficult to degrade and pose serious toxicity risks. Crystal violet, a widely used cationic dye, is carcinogenic, environmentally hazardous, and highly persistent.
Traditional methods, such as chemical precipitation, advanced oxidation, and membrane separation, can remove CV from wastewater; however, they are costly and produce secondary pollutants.
This study addresses these challenges by developing KBG as an adsorbent for CV removal from water, focusing on the gap in applying geopolymers to complex cationic dyes.
Although geopolymers have been explored for the removal of heavy metals and simpler dye molecules, systematic research on CV adsorption remains limited, despite its frequent occurrence in industrial effluents.
This approach uses kaolin’s abundance and aluminosilicate-rich composition to achieve low-cost geopolymer synthesis with a smaller environmental footprint.
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Methodology: Refining the Approach
The researchers examined KBG’s adsorption performance after alkaline activation under various conditions.
They first synthesized KBG by activating South African kaolin clay with sodium hydroxide, followed by the addition of hydrogen peroxide to enhance pore formation and remove residual organics.
The resulting material was dried, ground, and characterized using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy with Energy-Dispersive Spectroscopy (SEM-EDS), Brunauer-Emmett-Teller (BET) surface area analysis, and X-ray Diffraction (XRD) to establish structure-property relationships.
FTIR identified changes in aluminosilicate bonding, SEM-EDS visualized the porous network and elemental distribution, BET quantified surface area and porosity, and XRD confirmed the transformation from crystalline kaolin to amorphous geopolymer. The BET surface area decreased from 18.58 m2/g in raw kaolin to 11.18 m2/g in KBG, a result the authors attributed to incomplete dissolution of quartz and the formation of secondary minerals.
Batch adsorption experiments were conducted to evaluate KBG’s CV uptake under varying conditions. Parameters such as contact time (5-180 min), adsorbent dose (0.05-1.0 g), pH (2-12), temperature (30-50 °C), and initial dye concentration (10-150 mg/L) were used to determine removal rate, capacity, and stability.
Parallel tests in distilled and natural river water assessed the influence of realistic water chemistry on KBG adsorption performance. Kinetic data were fitted to pseudo-first-order, pseudo-second-order, and intra-particle diffusion models.
Equilibrium isotherms (Langmuir, Freundlich, Temkin) and thermodynamic parameters indicated adsorption spontaneity and endothermic behavior. This framework revealed KBG’s microstructure and surface chemistry control CV adsorption, confirming its potential as a sustainable wastewater adsorbent.
Results and Discussion
FTIR spectra showed the disappearance of kaolinite hydroxyl bands and the formation of new Si-O-Al and C=O bonds, indicating structural reorganization. XRD patterns revealed partial amorphization, with the emergence of illite and magnesite, confirming the formation of a geopolymeric gel.
SEM imaging displayed a transition from smooth surfaces to irregular, porous KBG structures, ideal for dye capture, while EDS detected dominant Si, Al, Na, and O, signifying active geopolymerization.
Adsorption studies demonstrated rapid CV removal, reaching 80 % uptake within 5 min and equilibrium at 30 min. The optimal KBG dosage was determined to be 0.03 g, with efficiency rising and then leveling off at higher concentrations due to site saturation.
Superior removal was observed in distilled water compared to river water, attributed to the interference of Ca2+ and Mg2+ and enhanced activity at an alkaline pH ≈of approximately 10.26. The interaction between the developed adsorbent and dye involved electrostatic attraction between negatively charged KBG and cationic CV.
The Langmuir isotherm (R2 = 0.93) best fit the equilibrium data, indicating monolayer adsorption, while kinetic results followed the pseudo-second-order model (R2 = 1), confirming that chemisorption governed the process. Thermodynamic analysis (ΔG < 0, ΔH = +4.7 kJ/mol) revealed a spontaneous and endothermic mechanism, with improved dye adsorption at elevated temperatures.
A maximum adsorption capacity of 7.27 mg/g according to the Langmuir model (with an experimental maximum of 9.72 mg/g) was achieved, surpassing reported clay-based (0.69 mg/g) and fly ash-based (6.0 mg/g) geopolymers.
However, several other geopolymer systems reported in the literature - such as metakaolin-, rice husk-, and pozzolan-charcoal–derived materials - demonstrate much higher adsorption capacities, providing broader context for evaluating KBG’s performance.
Desorption experiments demonstrated partial reusability, with hydrochloric acid exhibiting superior efficiency in regenerating the adsorbent compared to NaOH or deionized water. Collectively, these findings highlight the structural stability, rapid kinetics, and strong dye affinity of KBG, establishing it as a durable and environmentally sustainable material for advanced wastewater purification.
Remediating in the Future
This study establishes kaolin-based geopolymers as efficient, low-cost, and environmentally friendly adsorbents for CV removal from aqueous media.
The synthesized KBG exhibits distinct structural transformations and enhanced surface reactivity, leading to high adsorption efficiency via synergistic electrostatic interactions, hydrogen bonding, and π-π stacking. Optimal performance under alkaline conditions and elevated temperatures further confirms the endothermic and spontaneous nature of the adsorption process.
The present study advances adsorbent-based environmental remediation by showcasing the conversion of traditional clay resources into high-performance geopolymers suitable for wastewater treatment applications.
While KBG performs competitively within the category of clay-based geopolymers, its adsorption capacity is modest compared with other geopolymer systems with more reactive precursor materials. It establishes a framework for scalable, circular-economy-driven approaches to clean water production by integrating materials chemistry, sustainability principles, and environmental engineering.
Journal Reference
Edokpayi, J. N. (2025). Adsorption of Crystal Violet Using Kaolin-Based Geopolymer. Chemistry, 7(6), 189. DOI: 10.3390/chemistry7060189
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