Evaluation of Autoclaved Aerated Concrete Fines for Removal of Cd(II) and Pb(II) from Wastewater

Along with the increase of autoclaved aerated concrete (AAC) production, the amount of scrap waste (industrial by-products) is increasing. AAC waste, however, is not yet being fully reused or recycled and is discarded without any treatment in most developing countries. In this study, AAC fines were tested as effective and economic adsorbents for the removal of ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ from wastewater. A series of batch adsorption experiments were carried out using AAC with three different particle sizes ($<0.105$, 0.105–2, and 2–4.75 mm) to examine effects of the particle size, initial metal concentration ($C_i$), initial pH (${\mathrm{pH}}_i$), ionic strength, contact time, and competitive metals. For comparing the adsorption characteristics and capacities, crushed concrete fines and a fine sand were also used. Results showed that the adsorption kinetic data were well fitted to the pseudo–second-order kinetics model for all tested materials. ${\mathrm{Pb}}^{2+}$ adsorption was independent of ${\mathrm{pH}}_i$ and ionic strength for AAC fines, whereas ${\mathrm{Cd}}^{2+}$ adsorption varied depending on ${\mathrm{pH}}_i$ and decreased drastically at ${\mathrm{pH}}_i\le 7$. Based on the results from adsorption isotherms, the Freundlich model fitted well ($R^2>0.90$) to the data for $300\le C_i\le \mathrm{5,000}\mathrm{mg}/\mathrm{L}$, whereas the Langmuir model was applicable ($R^2>0.99$) to the data for $C_i\le \mathrm{2,000}\mathrm{mg}/\mathrm{L}$. The maximum adsorption capacity of each particle size of AAC exhibited almost the same adsorption capacity for ${\mathrm{Cd}}^{2+}$ ($15\mathrm{mg}/\mathrm{g}$) and ${\mathrm{Pb}}^{2+}$ ($>250\mathrm{mg}/\mathrm{g}$), which are similar to or higher than the values of other types of adsorbents in previous studies. The metal removal by AAC is controlled mainly by ion exchange, surface complexation, and surface precipitation. In particular, the ${\mathrm{Ca}}^{2+}$ ion exchange on the hydrated adsorbent surface is the dominant adsorption mechanism at the early stage of adsorption. The selectivity sequence of metal adsorptions became ${\mathrm{Pb}}^{2+}>{\mathrm{Cu}}^{2+}>{\mathrm{Ni}}^{2+}>{\mathrm{Cd}}^{2+}>{\mathrm{Zn}}^{2+}$ for AAC, and the existence of ${\mathrm{Pb}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ in wastewater highly impeded the ${\mathrm{Cd}}^{2+}$ adsorption onto AAC fines.