Adsorption of Congo red by fibrous xonotlite prepared from waste silicon residue

Fibrous xonotlite was synthesized under the conditions of Na₂SiO₃ concentration of 0.05 mol·L^-1, molar ratio of Si/Ca of 1:1, temperature of 220 °C and time of 9 h. It is worth pointing out that the Na₂SiO₃ solution as raw material was obtained from silicon residue through several procedures. The fibrous xonotlite exhibits excellent adsorption capacity for Congo red. 50 mL solution of Congo red with the concentrations of 100, 150 and 200 mg·L^-1 can be almost completely adsorbed by 30 mg of fibrous xonotlite within 10 min, and the adsorption ratios are 94.05%, 95.50% and 94.14%. The Langmuir model describes the adsorption well, indicating the adsorption is monolayer. The adsorption kinetics follows the pseudo-second-order model. The calculated maximum adsorption capacity of fibrous xonotlite for Congo red is 574.71 mg·g^-1 at room temperature. Fibrous xonotlite is a potential efficient adsorbent for Congo red owing to its rapid adsorption, high adsorption capacity and regeneration capacity.