Green and efficient: investigating modified and non-modified walnut shells for heavy metal ion removal in clinical dental sewage treatment

The adsorption of four metals—Pb 2+ , Zn 2+ , Cu 2+ and Fe 3+ —in the wastewater from the Clinical Dental Sewage Unit was investigated using the walnut shell (WTS) and its modified forms by activations with acid and base (WTSA and WTSB). The fine fraction of WTS, WTSA and WTSB was characterized by using EDX-RF, FTIR, SEM–EDS and BET. The specific surface area and pore volume were both increased when WTS was activated by acid or base (WTSA or WTSB). The experiment’s optimal pH value was 8, and the adsorption process took place after 300 min. The highest possible adsorption capacities for Pb 2+ , Zn 2+ , Cu 2+ and Fe 3+ by WTSA and WTSB reached (37.89, 33.97, 47.14 and 33.65) mg L −1 , and (38.93, 33.02, 43.02, and 32.53) mg L −1 . The endothermic and spontaneous nature behavior of HMs adsorption on the adsorbents is confirmed by the positive values of the enthalpy changes (ΔH°) and the negative value of the Gibbs free energy (ΔG°). FTIR and SEM–EDS measurements taken before and after adsorption were compared, and the results showed that there was considerable complexation between the heavy metal ions and the surface functional groups, which were primarily alkyl and alcoholic groups. According to the pseudo-second-order kinetic model and Langmuir isotherm, which are based on physical and chemical prosperity, the second-order chemisorption or chemical action (electron exchange) played a significant role in the adsorption process. WTS, WTSA and WTSB which have been synthesized using inexpensive and environmentally acceptable techniques may be candidates for use as an adsorbent for the treatment of wastewater contaminated with heavy metals. Graphical abstract