Discerning the role of a site cation in ACoO3 perovskites for boosting Co3+/Co2+ redox cycle for pollutant degradation: DFT calculation, mechanism and toxicity evolution.

The degradation of persistent and refractory pollutants, particularly plastic and resins manufacturing wastewater, poses a significant challenge due to their high toxicity and high concentrations. This study developed a novel hybrid ACoO3 (A = La, Ce, Sr)/PMS perovskite system for the treatment of multicomponent (MCs; ACN, ACM and ACY) from synthetic resin manufacturing wastewater. Synthesized perovskites were characterized by various techniques i.e., BET, XRD, FESEM with EDAX, FTIR, TEM, XPS, EIS, and Tafel analysis. Perovskite LaCoO3 exhibited the highest degradation of MCs i.e., ACN (98.7%), ACM (86.3%), and ACY (56.4%), with consumption of PMS (95.2%) under the optimal operating conditions (LaCoO3 dose 0.8 g/L, PMS dose 2 g/L, pH 7.2 and reaction temperature 55 °C). The quantitative contribution (%) of reactive oxygen species (ROS) reveals that SO4•- are the dominating radical species, which contribute to ACN (58.3% for SO4•- radicals) and ACM degradation (46.4% for SO4•- radicals). The tafel plots and EIS spectra demonstrated that perovskites LaCoO3 have better charge transfer rates and more reactive sites that are favorable for PMS activation. Further, four major degradation pathways were proposed based on Fukui index calculations, as well as GC-MS characterization of intermediate byproducts. Based on a stability and reusability study, it was concluded that LaCoO3 perovskites are highly stable, and minimal cobalt leaching occurs (0.96 mg/L) after four cycles. The eco-toxicity assessment performed using QSAR model indicated that the byproducts of the LaCoO3/PMS system are non-toxic nature to common organism (i.e., fish, daphnids and green algae). In addition, the cost of the hybrid LaCoO3/PMS system in a single cycle was estimated to be $34.79 per cubic meter of resin wastewater.

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

This study reported solidification/stabilisation of lead and copper-laden fly ash (adsorbent) utilising cement as binder for their ultimate disposal. The Pb (II) and Cu (II) loaded fly ash was successfully immobilised within the cement matrix without presence of any chemical agents. A retardation of 80-100 min 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 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 immobilisation 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/stabilisation process is efficient for safe disposal and utilisation of heavy metal-laden fly ash for building and construction related work as a secondary material.