Remediation of preservative ethylparaben in water using natural sphalerite: Kinetics and mechanisms

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Abstract

As a typical class of emerging organic contaminants (EOCs), the environmental transformation and abatement of preservative parabens have raised certain environmental concerns. However, the remediation of parabens-contaminated water using natural matrixes (such as, naturally abundant minerals) is not reported extensively in literature. In this study, the transformation kinetics and the mechanism of ethylparaben using natural sphalerite (NS) were investigated. The results show that around 63% of ethylparaben could be absorbed onto NS within 38 hr, whereas the maximum adsorption capacity was 0.45 mg/g under room temperature. High temperature could improve the adsorption performance of ethylparaben using NS. In particular, for the temperature of 313 K, the adsorption turned spontaneous. The well-fitted adsorption kinetics indicated that both the surface adsorption and intra-particle diffusion contribute to the overall adsorption process. The monolayer adsorption on the surface of NS was primarily responsible for the elimination of ethylparaben. The adsorption mechanism showed that hydrophobic partitioning into organic matter could largely govern the adsorption process, rather than the ZnS that was the main component of NS. Furthermore, the ethylparaben adsorbed on the surface of NS was stable, as only less than 2% was desorbed and photochemically degraded under irradiation of simulated sunlight for 5 days. This study revealed that NS might serve as a potential natural remediation agent for some hydrophobic EOCs including parabens, and emphasized the significant role of naturally abundant minerals on the remediation of EOCs-contaminated water bodies.

Introduction

The occurrence of emerging organic contaminants (EOCs) in water and sediments has already become a global concern (Gao et al., 2016; Mitch, 2017; Schwarzenbach et al., 2010). As an important group of EOCs, parabens are often used as additives to induce antimicrobial characteristics in certain streams of pharmaceuticals, cosmetics, and food-stuffs (Brausch and Rand, 2011; Chen et al., 2017; Liao et al., 2013). The annual consumption of parabens was estimated to be about 8000 tons around the world (Ramaswamy et al., 2011). Furthermore, several parabens could inevitably enter water bodies due to incomplete elimination in wastewater treatment plants (WWTPs). As a result, parabens have been frequently detected in the effluents of WWTPs, surface water bodies, as well as sediments, and the corresponding concentrations have been reported to lie within the range of 15–400 ng/L (Cheng et al., 2018; González-Mariño et al., 2011; Kasprzyk-Hordern et al., 2008). Moreover, previous studies have found that parabens have potential adverse impacts on aquatic organisms and human health (An et al., 2014; Gao et al., 2016). For instance, parabens could impose endocrine-disrupting effects on human health, even at concentrations as low as ng/L (Haman et al., 2015), furthering the cancer-causing potential of parabens (Darbre and Harvey, 2008). More importantly, recent studies observed the increased aquatic toxicity and endocrine-disrupting characteristics during their ultraviolet (UV) photochemical degradation (Fang et al., 2013; Gao et al., 2020). Therefore, understanding the transformation mechanism and environmental fate of parabens in natural water bodies and sediments are very important for the risk assessment and protection of human health.

In recent years, several research groups have mainly focused on the degradation of parabens using UV irradiation, photocatalytic, electrochemical and ozone oxidation technologies (Dobrin et al., 2014; Fang et al., 2013; Frontistis et al., 2017; Gao et al., 2020; Petala et al., 2015). Parabens could be degraded relatively easily, however is often accompanied by incomplete mineralization and the consumption of extra energy (González-Mariño et al., 2011). The migration and transformation of various parabens in the natural aquatic environment is rarely studied, particularly with the remediation of parabens-contaminated water using natural matrices, such as natural minerals.

Natural minerals are an essential part of the environment. More importantly, several natural minerals exhibit excellent promise for participating in the transformation and/or purification of pollutants in the environmental field (Zhao et al., 2020), including the adsorption of lead using rock phosphate and that of proteins using montmorillonite (Prasad et al., 2000; Rytwo et al., 2010). Among natural minerals, certain special attention has been paid to natural sphalerite (NS) because of its adsorption capability and visible-light (VL) photocatalytic activity (Li et al., 2020). For instance, NS can inactivate bacteria and harmful algae upon visible light (VL) irradiation (Chen et al., 2011; Shen et al., 2020; Wang et al., 2017). Moreover, NS could perform as visible-light-driven (VLD) photocatalysts for reducing metal ions and degrading azo-dye and carbon tetrachloride (Li et al., 2009; Yan et al., 2006; Yang et al., 2011). In short, NS is a naturally occurring adsorbent and photocatalytic material that has the potential to be used to resolve traditional as well as metal-related environmental pollutions. However, for the increasingly serious EOCs, their transformation and environmental fate over the interface of NS minerals have not yet been studied.

In this study, ethylparaben was selected as a typical preservative of EOCs to evaluate their environmental behavior and final fate on NS minerals. Firstly, the adsorption processes were analyzed based on adsorption kinetics, adsorption isotherms, and adsorption thermodynamics. Total organic carbon (TOC), analysis of the surface area and pore size, fourier transform infrared (FT-IR) spectroscopy, and theoretical calculations were performed to explore the adsorption mechanism. Moreover, the potential secondary release and the stability of adsorbed NS were also measured as a result of the desorption of ethylparaben from the NS. Figuring out the kinetics and mechanism of the remediation of parabens-contaminated water using NS will help understand the environmental fate of parabens and further inspire more dramatic advancements in the field of natural remediation of pollutants.

Section snippets

Materials

Ethylparaben (> 99.0%) was purchased from Tokyo Chemical Industry (Japan). NS was obtained from Huangshaping deposit in Hunan, China. NS used in the study was firstly crushed mechanically and milled at the mine. Then natural sphalerite powder (particle sizes ≤ 40 μm) was obtained by passing it through a 340-mesh sieve as used in earlier references (Chen et al., 2011; Li et al., 2020; Yang et al., 2011). As a contrast in the experiment, the nano-sized ZnS was obtained from Sigma-Aldrich. All

Adsorption kinetics of ethylparaben onto NS

Firstly, the adsorption kinetics of ethylparaben on NS with the concentration of 6 g/L was investigated. As shown in Fig. 1a, the apparent equilibrium was attained at approximately 50 hr. Approximately 78% of the ethylparaben was adsorbed within the initial 12 hr, and the adsorption amount was found to be 0.35 mg/g. Then, the adsorption capacity increased slowly, and 97% adsorption was achieved within 38 hr. Finally, the equilibrium adsorption of 0.45 mg/g was achieved within 50 hr.

Furthermore,

Conclusions

This study shows that ethylparaben can be readily adsorbed on NS minerals. In particular, the adsorption process was a spontaneous process under the water temperature of more than 313 K. The sorption mainly occurred through surface adsorption along with micropore filling. Weber-Morris model satisfactorily explained the three-step adsorption process, whereas the main adsorption occurred in the initial stages of adsorption on the external surface of NS. The adsorption of ethylparaben on the

Acknowledgments

The authors appreciate the financial supports from the National Natural Science Foundation of China (Nos. 41977365 and 41425015), the National Key Research and Development Program of China (No. 2019YFC1804503), the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (No. 2017BT01Z032). Natural Sciences and Engineering Research Council of Canada, the Canada Research Chairs Program, Alberta Innovates, and Alberta Health for their support. YG acknowledges the

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