Clam shell waste recycling and valorization for sustainable Hg remediation

Shellfish aquaculture world production is constantly growing due to the increase in demand for seafood and reached over 18 million tons in 2022. The suitable management of the shell waste is one of the main environmental challenging issues as most of this waste is sent to landfills with emanation of foul odors, pathogens proliferation and reduction of available space. However, the conversion of this biowaste to new value-added materials could provide significant environmental and economic benefits. Clam shell waste was the starting material for the synthesis of hydroxyapatite (CSHAP) applied as an adsorbent for Hg²⁺ removal from aqueous solutions. Adsorption experiments were performed in batch using simulated wastewaters prepared from HgCl₂ to investigate the effects of contact time and initial Hg²⁺ concentration on the removal process. Mineralogical composition, morphological features and elemental composition of CSHAP before and after the experiments were investigated by XRPD, SEM-EDS and FTIR analysis. The concentrations of Hg²⁺ and Ca²⁺ in the solutions were analyzed by ICP-AES. The adsorption kinetics of Hg²⁺ was simulated with the pseudo-first-order rate model, the pseudo-second order model and the intraparticle diffusion model. The results of the kinetics study showed that the Hg²⁺ adsorption followed the pseudo-second-order kinetics model and reached equilibrium within 40 min. The Langmuir model fitted the experimental results better than the Freundlich, Temkin and Dubinin-Radushkevic isotherm models, with a maximum adsorption capacity of 65.8 mg/g which is generally higher than other waste-derived adsorbents used for the removal of Hg²⁺ ions from water. The removal mechanism includes rapid surface complexation on CSHAP grains, followed by a slow incorporation of the Hg²⁺ ions in the crystalline structure. The results of this study could contribute to delineate a new research direction for a more sustainable management of clam shell biowaste.