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

Abstract

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 Cd2+ and Pb2+ 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 (Ci), initial pH (pHi), 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. Pb2+ adsorption was independent of pHi and ionic strength for AAC fines, whereas Cd2+ adsorption varied depending on pHi and decreased drastically at pHi≤7. Based on the results from adsorption isotherms, the Freundlich model fitted well (R2>0.90) to the data for 300≤Ci≤5,000 mg/L, whereas the Langmuir model was applicable (R2>0.99) to the data for Ci≤2,000 mg/L. The maximum adsorption capacity of each particle size of AAC exhibited almost the same adsorption capacity for Cd2+ (15 mg/g) and Pb2+ (>250 mg/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 Ca2+ 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 Pb2+>Cu2+>Ni2+>Cd2+>Zn2+ for AAC, and the existence of Pb2+ and Cu2+ in wastewater highly impeded the Cd2+ adsorption onto AAC fines.