Halloysite-Magnesium Silicate Composites as Adsorbent for Removal of Methylene Blue and Heavy Metals from Aqueous Solution

Objectives: Thermal stability of the halloysite and magnesium silicate is discussed in terms of microstructure and adsorption behavior to optimize their composite as an absorbent for methylene blue (MB) and heavy metal ions removal from an aqueous solution. Methods: Halloysite/magnesium silicate tubular composites with high adsorption capacity of methylene blue and heavy metal ions were prepared with extrusion and consequent firing. To define the firing temperature of the tubular media, The microstructure and the adsorption equilibrium characterized the thermal stability of the halloysite and the magnesium silicate Results and Discussion: The magnesium silicate used in this study shows broad peaks in x-ray diffraction; however, the treatment of 750℃ induces its crystallization. The specific surface area of the magnesium silicate is also gradually decreased along with the firing temperature increase. No significant degradation in methylene blue adsorption capacity for magnesium silicate is observed until 500℃-treatment. However, the higher firing temperature reduces the capacity: 14.0%P, 26.1%P, and 96.3%P decrease for 600℃, 700℃, and 750℃, respectively. Regardless of heat treatment, the Langmuir isotherm equation represents the adsorption equilibria well compared to the Freundlich model. The tubular media of the halloysite – 30 wt.% of magnesium silicate fired at 600℃ exhibits a high specific surface area of 115 m2 g-1 and a relative porosity of 43.5%. Compared to the halloysite-only tubular media, the adsorption capacity of the composites is significantly improved with the incorporation of magnesium silicate. The adsorption capacity of 168h for methylene blue is more than tripled, 29.8 mg g-1, and the adsorption capacity of Cr(III), Cu(II), and Zn(II) is also greatly enhanced by more than 4.8 times. Conclusions: Thermal stability of magnesium silicate is investigated for use as an additive adsorbent in terms of the microstructure and the adsorption capacity. Even at 600℃ of the firing temperature, magnesium silicate shows a higher specific surface area and higher adsorption capacity of MB compared to those of the halloysite; the adsorption fits well into Langmuir behavior. The addition of magnesium silicate into the halloysite media significantly enhances its adsorption capacity for MB and heavy metal ions such as Cr(III), Cu(II), and Zn(II).