Microstructural Characterisation of Adsorbent Ash with Potentially Toxic Elements in a Mortar

Abstract

Potentially toxic elements (PTEs) in ecosystems and construction materials pose a significant environmental concern. Various qualitative and quantitative techniques are employed to analyse PTEs in a sample. This study explores an innovative approach that incorporates PTE (Cd, Zn, Cu, Pb) adsorbent ash, specifically adsorbed paper ash (APA) and adsorbed mulch ash (AMA), into mortar composites. This approach offers several advantages, including reduced reliance on waste landfills, energy recovery during the ashing process, and immobilisation of PTEs within a cement matrix. This study evaluated the elemental and microstructural characteristics of mortar composites incorporated with adsorbed ash by using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), followed by analysing the elemental maps with ImageJ software version 1.8.0. Parallel experiments were conducted to measure the leaching of mortar composites. The total elemental content of PTEs in the leachate solutions was quantified using an inductively coupled plasma-optical emission spectrometer (ICP-OES). The differences in adsorption capacity and leaching of PTEs observed between paper and mulch can be ascribed to their distinct affinities, which are influenced by the recorded pH levels. The examined elemental mapping revealed a consistent distribution across the APA and AMA mortar matrix structures, with greater intensity than the blank mortar sample. Furthermore, there is a reverse correlation between the order of percentage area coverage of the immobilised elements and the order of leaching, indicating that the PTEs were successfully immobilised. The percentage of PTE area coverage within AMA mortar composites followed the subsequent order: Pb > Cd > Zn > Cu, constituting 32.1 %, 28.6 %, 13.8 %, and 12.4 %, respectively. This order was also observed in the blank mortar composite, with percentages of 12.5 %, 8.6 %, 4.5 %, and 4.2 %, respectively. In the case of the APA mortar composite, the percentage of area coverage followed a different sequence: Cd > Pb > Zn > Cu, representing 27.7 %, 26.6 %, 14.5 %, and 14.1 %, respectively. The results also demonstrated notable improvements in the microstructure of the mortar when AMA and APA are incorporated, which is attributed to the ash additives’ micro-filling capacity. The findings contribute to advancing environmentally sound construction practices, with implications for sustainable waste management and pollution mitigation.