Removal of phosphate and hexavalent chromium from aqueous solutions by engineered waste eggshell

In this work, a novel adsorbent derived from waste eggshell (ES) was used for phosphate and hexavalent chromium adsorption. EDS, XRD, and FTIR analyses demonstrated that α-FeOOH was successfully loaded onto the eggshell, thus increasing the available sorption sites and facilitating the adsorption abilities of phosphate and hexavalent chromium. The maximum phosphate and hexavalent chromium adsorption capacities on α-FeOOH modified eggshell (F-ES) were 248.73 and 41.57 mg g−1, which were much greater than that of the original eggshell 89.74 and 11.81 mg g−1, indicating that the modification process significantly improved the phosphate and hexavalent chromium removal abilities. Besides, the kinetics data were both well fitted with pseudo-first-order, pseudo-second-order, and Richie kinetics models, which proved that the adsorption process onto the eggshell was controlled by multiple mechanisms. In addition, the removal rates of phosphate and hexavalent chromium increased with increasing adsorbent dosage. However, the removal rates showed downward trends with pH increased from 3 to 9 and ionic strength increased from 0 to 0.1 M.