Preparation of diatomite/corn straw woodceramics and its adsorption kinetics for tetracycline in wastewater

Abstract: In order to use agricultural wastes as resources, the efficient utilization of corn straw was explored. Taking corn straw as the main raw materials, diatomite modified corn straw woodceramics were prepared by carbonizing phenolic resin-corn straw powder composite under vacuum. The structure and properties of as-prepared materials were characterized by modern test methods such as scanning electron microscope, X-ray diffraction, fourier transform infrared spectroscopy and mercury injection. Some properties of as-prepared materials such as pore size distribution, porosity, adsorption capacity and so on were tested in the same time. Using tetracycline as the target pollutant, the adsorption process of tetracycline on samples was studied. Results indicate that woodceramics are mainly amorphous and a small amount of quartz crystal phase and crystalline graphite. Woodceramics contained a large number of holes, and the holes formed by the high temperature calcining of wood fiber in corn straw were wrapped and bridged by the glass carbon. With the original pore structure of diatomite, the interlocking pores were uneven on the surface. The porous size range, the porosity and the specific surface area of the woodceramics were 1 000- 3 800 nm, 48.6 %, and 7.83 m2/g respetively. Woodceramics was a typical non-graphitizable carbon containing C-O-C bonds, C=C bonds and C-H bonds' structure. The adsorption capacity of materials to organic pollutants was improved obviously. According to the correlation coefficient R2, Langmiur isothermal adsorption model could describe the adsorption process of tetracycline on woodceramics better than Frendlich model (0.999>0.975). Adsorption kinetics of tetracycline (initial concentration 5 mg/L) on woodceramics were studied at pH values of 3, 5, 7 and 9. Experimental results show that pH value of the solution increased from 3 to 9, the adsorption rate constant k increased from 8(10(3 to 1.3(10(2 mg /(g·min). Because of different pH values, the existing forms of tetracycline (TCH3+, TCH20, TCH-) were different, and the surface charge of SiO2 in woodceramics also changes. In the lower pH value, the adsorption point was easier to reach saturation. With the increase of pH value, k value reduction indicated that the exchange-adsorption rate was also decreases, which was caused by the change of surface electrical properties of woodceramics and the different forms of tetracycline in different pH values. As the diffusion progresses, the surface of the adsorbent was occupied by a large number of adsorbents, and part of the charge was neutralized. With the Gibbs free energy of materials increasing, the driving force of diffusion decreased, and the adsorption reached to equilibrium. The double constant adsorption model in the decrease of the free energy and entropy increase was the main reason for the adsorption. The decrease of free energy and the increase of entropy in the double constant adsorption model were the main reasons to promote the adsorption. The Pseudo secondary kinetic model was a kind of chemical adsorption based on the limitation of adsorption rate. Results of the kinetic model fitting showed that the quasi-secondary kinetic model parameter (R2 > 0.98) could better fit the adsorption process of tetracycline. This study realized the deep utilization of waste materials, treated effectively tetracycline wastewater, and provided a new way for the utilization of corn straw.