Advanced oxidation of pharmaceuticals with peroxymonosulfate (PMS) activated by novel spinel-type oxides supported on porous clay minerals

Abstract

The presence of pharmaceuticals and their metabolites in waterbodies has been recognized as an emerging environmental issue globally partly due to their easy entry, continuous release, bioaccumulation, considerable toxicity, and possible carcinogenic effects on organisms. However, since conventional wastewater treatment plants (WWTPs) are never designed for the removal of pharmaceuticals, it has in turn become a point source of pollution. Therefore, advanced oxidation processes (AOPs) with the generation of oxidative reactive oxygen species, have experienced remarkably development in the elimination of emerging contaminants including pharmaceuticals due to factors such as simplicity, higher efficiency, cost-effectiveness, and mineralization potential. In this study, novel composite catalysts tailored by CoMn2O4 (CMO) and porous natural clay minerals, e.g. halloysite (Hal) and kaolinite (Kln), were developed to activate peroxymonosulfate (PMS) for efficient degradation of pharmaceuticals in aqueous environment. The results showed that among the series of halloysite (Hal)-based composite catalysts with different CMO loading percentage, 40-CMO/Hal with a moderate CMO content displayed homogeneous dispersion, restricted grain size, and superior catalytic ability with better stability and durability than bare CMO. Therefore, 40-CMO/Hal (Abbrev. CMO/Hal) with the optimal overall performances was chosen to further investigate the effects caused by different clay mineral substrates through comparisons with CMO anchored on kaolinite (Kln), a natural lamellar clay mineral, at the same percentage, 40%. Hal and Kln helped to control the crystallinity of CMO simultaneously with inducing more oxygen vacancies (OVs), which significantly enhanced the working efficiency. The presence of Hal and/or Kln contributed to the composite catalysts with better stability and durability than CMO. In addition, CMO/Hal contained more OVs, while CMO/Kln showed higher structural stability with lower metal leaching. This work shed light on the understanding of unearth surface reaction mechanism during PMS activation, which also provides a new reference in enhancing the catalytic performance of transition metal oxides by immobilizing them on natural clay mineral substrates. What's more, since Kln and Hal are both abundant in China and many other countries, the application of them in wastewater treatment and chemical industries also offers another chance in enhancing regional fundamental researches related to natural resources applications. Since inorganic anions and natural organic matter (NOM) are unavoidable in nature water bodies and wastewaters, their effects towards AOP using activated PMS should be considered, which, however, still remains unknown. Therefore, the degradation efficiency of tetracycline (TC) using PMS activated by LaCoO3 with the presence of some common inorganic anions and NOM (e.g., humic acid (HA)) were investigated. The results showed that the presence of inorganic anions (i.e., H2PO4 −, Cl− and SO42−) and low-concentration of HA promoted the degradation. Among them, H2PO4 - and lower concentration of HA showed the most obvious outcome, while the SO42− can regulate the radical formation, minimize the peak radical level, and therefore promote the overall performance of the process. Generally, the LaCoO3/PMS system was proved as a rapid, economical and stable approach for TC degradation, which provided a novel strategy for pharmaceutical removal during water purification.