Synthesis of allophane from rice husk ash and its use as a phosphate adsorbent: A novel approach

There is a need to develop a recycling method for rice husk ash (RHA), as large amount of RHA is generated in rice husk-based power plants. Present study evaluated the RHA discharged from a rice husk-based power plant in Myanmar. It was found that RHA contained 91.65 mass % silica, of which 92.7 % Si could be extracted with 1.0 mol/L NaOH solution at 80 °C. Type I and II allophanes were successfully synthesized by adjusting the pH of the Si extract. The BET specific surface areas of type I and II allophanes were 334.2 and 150.2 m2/g, respectively. Both have bimodal pore structures in the micro- and meso-micro ranges, with the pore volume in the micro region being much larger for type I allophane. Both the adsorption rate and adsorption capacity for phosphate was higher for type I allophane than for type II. Langmuir adsorption isotherms revealed that the maximum adsorption capacities of the type I and type II allophanes were 35.1 and 4.3 mg/g, respectively. [Display omitted] •Allophane, an aluminosilicate material, was successfully synthesized from rice husk ash (RHA) for the first time.•Type I and type II allophanes could be synthesized by changing the pH of Si source solution, respectively.•Maximum phosphate adsorption capacities of type-I and type-II allophanes were 35.1 mg/g and 4.3 mg/g, respectively.•Type-I allophane prepared from RHA can be a promising adsorbent for phosphate.