Cr(VI) adsorption on activated carbon, sludge derived biochar, and peanut shells derived biochar: Performance, mechanisms during the reuse process and site energy distribution analysis

The reusability of adsorbents for Cr(VI) removal and the related mechanism are important to practical application. In this study, commercial activated carbon (AC), sludge-derived biochar (SBC), and peanut shell-derived biochar (PBC) were reused to the tenth round on Cr(VI) removal and then characterized to analyze the ongoing adsorption mechanisms involved during the reuse process. The site energy distribution theory was used to analyze the changes in site energy during the reuse of synthesized adsorbents. Batch adsorption experiments revealed that Cr(VI) removal efficiency decreased from 99.97 % to 43.37 %, 47.22 %, and 40.13 % when reusing AC, SBC, and PBC, respectively, from the first to the tenth round at optimum conditions. The adsorption capacity decreased with each reuse round due to a decrease in -OH, C=O, O-C-O, and C-O on the surface of adsorbents. The site energy (E*) values decreased as adsorption capacity (q(e)) increased, indicating that HCrO4- and Cr2O72- first occupied high-energy adsorption sites and then the low-energy adsorption sites. Reduction of Cr(VI) to Cr (III) was identified as significant in the elimination of Cr(VI). These findings offer new insights into the reusability behavior and interaction mechanisms of adsorbents during Cr(VI) elimination from wastewater.