Study on the influence mechanism of mineral components in biochar on the adsorption of Cr(VI)

Abstract

Cr(VI) is extremely harmful to the ecological environment. The secondary purification method of treating low concentrations is not satisfactory currently. In this paper, wheat straw were pyrolyzed at 800 °C to obtain biochar, which were used to explore the effect of minerals on adsorption effect as a precursor at different pH and 25 °C. The biochar was gradually eluted with HCl and HF to remove the ash, soon loaded with K+, Ca2+, Mg2+ and Fe3+, respectively. The results show that under the action of H+, the CO bonds left after mineral removal have lower potential energy than the original surface functional groups in terms of conversion to –OH and binding to Cr. The order of promotion of Cr(VI) adsorption by the four elements is Fe3+> Mg2+＞ Ca2+> K+. As the radius of metal ions decreases, so does the electronegativity of biochar and the surfactive site increases. The adsorption mechanisms of alkali metals, alkaline earth metals and transition metals to Cr(VI) are mainly based on ion exchange, precipitation and complexation, respectively. The adsorption of Cr(VI) by wheat straw biochar was consistent with the quasi-secondary kinetic model. Combined with the results of intraparticle diffusion model fitting, it shows that this process is a multi-stage adsorption process with the combined effect of physical and chemical adsorption.