Utilizing high pH of geopolymers for activation of persulfates in aqueous phenol abatement

Abstract

Persulfates have received increasing attention in advanced oxidation processes. This study investigates a new concept for the activation of peroxydisulfate (PDS) and peroxymonosulfate (PMS) using geopolymer foams as a flow-through media supplying alkalinity. In the experiments, model solutions or real wastewater containing 10 µM phenol were distributed through geopolymer foams during which the high pH of the foams enhanced the reactive species formation. PDS achieved only maximum ~30% phenol abatement (dose 2.4 mM, pH of ⁓12, and empty bed of contact time of ~30 min), while PMS could achieve ~100% phenol abatement under identical conditions. The efficient phenol abatement was due to the high pH of geopolymer (in the range of 8–12) while PMS alone, without pH increase, could reach only max. ~20% phenol abatement and geopolymer foam alone max. ~16% phenol abatement, likely due to adsorption. In municipal tertiary wastewater spiked with phenol, ~98% phenol abatement was achieved using the 1.2 mM dose of PMS when treating 10 bed volumes of water. In a longer experiment (72 bed volumes), a decline in abatement was observed from ~97% to ~50% as pH decreased from 11.9 to 7.3 after 48 bed volumes of treated water. Radical scavenging experiments indicated that singlet oxygen (¹O₂) is the most important reactive species in the geopolymer-PMS system. Also, electron paramagnetic resonance (EPR) spectroscopy measurements were conducted but they faced challenges at highly alkaline conditions possibly due to spin trap decomposition. Overall, geopolymer foam effectively activated PMS, offering a promising and easy-to-set-up advanced oxidation process not requiring expensive or toxic metals as catalysts.