A green route for preparation of low surface area SiO2 microspheres from wheat straw ash with activated carbon and NPK compound fertilizer as by-products

Abstract

In order to make full use of wheat straw ash which is abundant and causes some serious environmental problems in the north of China, an environmentally friendly route is proposed to convert the wheat straw ash to useful materials in this paper. In this route, low surface area SiO₂ microspheres are obtained from wheat straw ash, with the activated carbon and Nitrogen–Phosphorus–Potassium (NPK) compound fertilizer as by-products. KOH is used as the silica extraction reagent and H₃PO₄ is the precipitator. The resulting SiO₂ microspheres with narrow size distribution have an amorphous structure with diameters of approximately 480 nm and a specific surface area of 47.79 m² g⁻¹ under the optimized synthesis conditions with a volume ratio of 1 : 1.5 : 1 for extract/water/ethanol, a pH value of 8 and H₃PO₄ solution dripping rate of 3 mL min⁻¹. In this optimized route, the yields of SiO₂ and activated carbon reach 96.77% and 71.89%, respectively. The waste water produced in this process is used to produce NPK compound fertilizer. The whole route is an environmentally friendly process without any emission of pollution.