Interactions of Cd2+, Co2+ and MoO42− Ions with Crushed Concrete Fines

Abstract

Construction and demolition activities generate approximately two thirds of the world’s waste, with concrete-based demolition material accounting for the largest proportion. Primary aggregates are recovered and reused, although the cement-rich fine fraction is underutilised. In this study, single metal batch sorption experiments confirmed that crushed concrete fines (CCF) are an effective sorbent for the maximum exclusion of 45.2 mg g−1 Cd2+, 38.4 mg g−1 Co2+ and 56.0 mg g−1 MoO42− ions from aqueous media. The principal mechanisms of sorption were determined, by scanning electron microscopy of the metal-laden CCF, to be co-precipitation with Ca2+ ions released from the cement to form solubility limiting phases. The removal of Co2+ and MoO42− ions followed a zero-order reaction and that of Cd2+ was best described by a pseudo-second-order model. The Langmuir model provided the most appropriate description of the steady state immobilisation of Cd2+ and Co2+, whereas the removal of MoO42− conformed to the Freundlich isotherm. Long equilibration times (>120 h), loose floc formation and high pH are likely to limit the use of CCF in many conventional wastewater treatment applications; although, these properties could be usefully exploited in reactive barriers for the management of contaminated soils, sediments and groundwater.