Synergetic effects of hydrothermal treatment on the behavior of toxic sludge-modified geopolymer: Immobilization of cerium and lead, textural characteristics, and mechanical efficiency

Abstract

Geopolymers have been subjected as a promising strategy for heavy metals immobilization due to their sustainability (environmental and energy saving). This study proposed a novel hydrothermal technology for the solidification/stabilization of lead and cerium present in a sludge resulting from the glass polishing process. Slag and fly-ash (50wt.%slag + 50wt.%fly-ash) activated with 6 wt% NaOH solution were used as base materials for geopolymer fabrication. The sludge was incorporated in the fabricated geopolymeric composite as a fly-ash replacement (10, 20 and 30 wt%). The impact of sludge incorporation and different curing regimes (humidity at 20 °C and 80 °C for 28-days and hydrothermal treatment at 3 and 6 bar for 1, 2, 6, 12 and 24 hrs) on the mechanical and textural characteristics as well as the formed phases were investigated via compression test, N2-adsorption/desorption technique, SEM/EDX, TGA/DSC and XRD. Moreover, the efficiency of the immobilization process was examined by determining the concentration of lead and cerium in the leachate solution (0.1 M HNO3 and 0.1 M CH3COOH) using Inductively Coupled Plasma Mass Spectroscopy to ensure the safety of developed composites. The results indicated that autoclaving at 6 bar/6hrs greatly returned the strength values for all hardened geopolymers, particularly the composite containing 50wt.%slag + 40wt.%fly-ash + 10wt.%sludge; its strength values were 22, 46 and 60 MPa at 20 °C/28 days, 80 °C/28 days and 6 bar/6hrs, respectively. The enthalpy change calculated by the DSC technique has the highest value (122.2 J/g) at 6 bar/6hrs, indicating the formation of additional hydration products. Hydrothermal treatment technology has proven its worth in immobilizing heavy metals in geopolymeric materials; curing at 6 bar/6hrs for specimen containing 30wt.%sludge decreased lead concentration by 95.2 and 97.8 % and cerium by 98.2 and 99.6 % in leachate solutions (HNO3 and CH3COOH, respectively). This may be attributed to the transformation of lead and cerium into insoluble Ce2Si2O7 and Pb3SiO5 as detected by XRD that are physically encapsulated in a cross-linked geopolymer structure. Furthermore, due to pore structure rearrangement that shifted to micro/mesoporous (2–50 nm) under the steam condition with a high BET surface area (31.7 m2/g) and low BJH average pore diameter (10.01 nm).