Solidification/stabilization of Pb (II) and Cu (II) containing wastewater in cement matrix

Abstract

This study reported solidification/stabilization of lead and copper-laden fly ash (adsorbent) utilizing cement as binder for their ultimate disposal. The Pb (II) and Cu (II) loaded fly ash was successfully immobilized within the cement matrix without presence of any chemical agents. A retardation of 80-100 minutes in the setting time of cement paste was noticed on the addition of metal-laden fly ash attributed to the presence of metal ions. However, a gradual decrease in mechanical strength of the mortars was observed with higher amounts of Pb (II) and Cu (II)-loaded fly ash in the mix composition. This gradual decrease is ascribed to the breakdown of calcium silicate hydrate (CSH) gel network in the presence of metal crystallites, as confirmed by scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) analyses. TG-DTG studies also reveal a decrease in CSH (%) from 4.77% (for fly ash cement mortar) to 4.14% and 3.86% for Pb (II) and Cu (II)-loaded fly ash mortars, respectively. X-ray diffraction (XRD) analysis of metal-laden fly ash cement mortars substantiate the immobilization of Pb (II) and Cu (II) metal ions in the cement matrix as peaks for Ca[Pb(OH)3]2 and Ca[CuH2O5Si] are visible in their patterns, respectively. TCLP tests conducted on 56 day cured metal-laden fly ash mortars show leachate concentration not exceeding the discharge standards. Overall, these results indicate that this integrated adsorption- solidification/stabilization process is efficient for safe disposal and utilization of heavy metal-laden fly ash for building and construction related work as a secondary material.