Study on tobermorite preparation from fly ash aluminum extraction slag and papermaking white mud and its adsorption capacity on Cd2+

In this study, tobermorite was prepared by hydrothermal method using two kinds of typical solid waste as raw materials, i.e., fly ash aluminum extraction slag (by-product in the process of fly ash hydrochloric acid aluminum extraction) and papermaking white mud. The XRD, SEM, FTIR, TG-DSC and Specific surface area and porosity were used to investigate the influence of Ca/Si ratio, alkalinity, hydrothermal conditions and other factors on the synthesis products, and the formation mechanism of tobermorite. The adsorption capacity of tobermorite was investigated by adsorption experiments. The effects of different dosage, initial concentration of Cd2+ and adsorption time on the adsorption behavior were investigated to understand the adsorption mechanism of tobermorite on Cd2+. The results show that the well crystal tobermorite can be synthesized at 180 °C for 6 h with the Ca∶Si molar ratio of 0.8 and the NaOH addition amount of 2.5%−5.0%. The specific surface area of tobermorite is 80.956 m2/g, the total pore volume is 0.218 cm3/g, the pore size is 1.21 nm, and the crystallization of tobermorite is 97%. The microstructure of tobermorite is dominated by short fibers and has a good thermal stability below 650 °C. The formation process of tobermorite can be described that the CaO in papermaking white clay is converted into Ca(OH)2 and the amorphous silicon dioxide is transformed into sodium silicate. The gel blocks material is formed by calcium hydroxide and sodium silicate solution. Tobermorite whiskers are formed, and the whiskers gradually exfoliate, split and grow to form tobermorite fibrous materials. The Cd2+ removal rate of tobermorite can reach 99.99% at pH value of 3, the dosage of tobermorite is 2.5 g/L, and the concentration of Cd2+ is 100 mg/L. The adsorption of Cd2+ by modified tobermorite fits the quasi-second-order kinetic equation and the Langmuir model, and is controlled by membrane diffusion and intra-particle diffusion, and the chemisorption is the main control step. The study indicates that tobermorite can be prepared from aluminum extraction slag with fly ash and white paper mud. The tobermorite prepared had a good adsorption effect on heavy metals and can be used as a high efficient adsorbent for heavy metal ions. Also, the study realizes the collaborative utilization of two typical solid waste resources in different industries, and provides a potential method for the treatment of wastewater containing Cd2+ and other heavy metal ions.