ZnO/Ag6Si2O7, An Efficient Photocatalyst for Malachite green Removal by Activation of Persulfate Under Visible‑Light Irradiation

Currently, there is a growing fondness in removing water pollutants because of the significant threat to ecosystems and human health. Many of these persistent contaminants have been detected in a variety of water bodies and they cannot be effectively eliminated by the conventional water and wastewater treatment procedures. Among advanced treatment technologies, advanced oxidation processes (AOPs) have shown to be high efficiency and effective technologies to the destruction of contaminants mainly due to the generation of high concentration of oxidant radicals (O2⁻•, HO•, SO4⁻•, HO2•) [1]. Sulfate radical-based advanced oxidation process (SR-AOPs) has received remarkable attention as a promising method for wastewater treatment. Sulfate radical has the attributes of high selectivity, long lifetime, and high redox potential with E0 (SO4⁻•/SO42−) = 2.50-3.10 V, which is even premier than hydroxyl radicals with E0 (OH•/H2O) = 1.80-2.70 V. So it could be utilized for destructing contaminants from aqueous solution [2]. Usually, SO4⁻•can be generated from the activation of persulfate (PS) and peroxymonosulfate (PMS) anions [3]. The photocatalytic activation of PS has demonstrated to be an effective method for the elimination of contaminants under visible light. The elimination performance is increased by coupling PS with heterogeneous photocatalysts, owing to the generation of SO4⁻•radicals and efficiently trapping of photogenerated electrons, decreasing the recombination of charge carriers [4]. In general, among various heterogeneous photocatalysts, zinc oxide (ZnO) is a most promising semiconductor with a band-gap of 3.32 eV, and it has numerous advantages such as non-toxicity, strong oxidation ability, low cost, good photocatalytic activity, chemical and thermal stability, and easy to synthesize. However, due to fast recombination of the photoexcited charge carriers and low visible-light utilization in this semiconductor, great efforts are being made to improve the photocatalytic efficiency of ZnO [5]. In recent years, it was found that combination of ZnO with low energy gap semiconductors is an efficacious modification pathway to increase visible-light absorption ability of ZnO [6]. As known, a great number of Ag-based semiconductors with narrow energy gap have displayed substantial photocatalytic performance under visible-light illumination. Recently, silver silicate (Ag6Si2O7) has drawn increasing attentions. Silicates have been shown to be used as photocatalysts for the removal of organic compounds [7].Herein, in this study, ZnO/Ag6Si2O7 was successfully synthesized via a reflux method and then utilize for persulfate activation to produce SO4−• and OH•. The photocatalytic performances were exceptionally improved under visible light in removals of Malachite green. The results showed that among the prepared samples, the ZnO/Ag6Si2O7 (30%) photocatalyst had the greatest photocatalytic ability and the addition of persulfate anions considerably enhanced photocatalytic performance of the nanocomposite. As a result, the removal rate constant of Malachite green by ZnO/Ag6Si2O7 (30%) in the existence of 0.74 mM of persulfate ions was 441.32 × 10-4 min−1, which is almost 60.78, 12.96, and 4.36-folds premier than those of the pure ZnO, ZnO/PS, and ZnO/Ag6Si2O7 (30%) samples, respectively.

Keywords:

ZnO/Ag6Si2O7 , Photocatalysis , Persulfate activation , Visible-light-active photocatalyts