Effect of copper on the removal of tetracycline from water by Myriophyllum aquaticum: Performance and mechanisms

Highlights

• Adsorption is the main mechanism of TC removal by Myriophyllum aquaticum in short time.

• OH, COOH, and NH₂ are involved in TC adsorption through ion exchange.

• Cu(II) may act as a bridge during TC adsorption on M. aquaticum.

• Competitive adsorption of Cu(II) and TC on M. aquaticum occurs under high Cu(II).

• M. aquaticum is an important bioresource for effectively removing TC from water.

Abstract

Pollution with antibiotics and heavy metals necessitates efficient approaches for their removal. This study was conducted to investigate the role of Cu in the tetracycline (TC) removal potential of the floating plant Myriophyllum aquaticum and determine the underlying mechanisms. Myriophyllum aquaticum exhibited high TC removal potential from water (60% at 50 mg·L⁻¹ TC and 10 g·L⁻¹ M. aquaticum). Adsorption was the main mechanism of TC removal within 2 h, accounting for over 75% and 90% of total TC removal with and without Cu(II), respectively. Fourier-transform infrared spectroscopy and functional group identification showed that OH, COOH, and NH₂ were involved in TC adsorption through ion exchange. Cu(II) may act as a bridge during TC adsorption with M. aquaticum, but competitive adsorption of Cu(II) and TC on M. aquaticum occurs in case of excessive Cu(II). Myriophyllum aquaticum can serve as an important bioresource for effectively removing TC and Cu(II) from aquatic environments.

Introduction

As unique medicines, antibiotics have been widely used to prevent and treat bacterial diseases for decades. They play important roles in human and animal health (Yan et al., 2019). However, the metabolism of antibiotics by animals is difficult. Furthermore, antibiotics have high water solubility and low biodegradability, causing them to persist in the environment (Chen et al., 2017, Karpov et al., 2018, Rosi-Marshall and Kelly, 2015). Thus, environmental contamination with antibiotics raises multiple concerns. As feed additives, heavy metals are also widely used to control diseases and promote animal growth (Jia et al., 2016). Interestingly, these metals can form complexes with several functional groups of antibiotics, such as carboxyl, hydroxyl, and amino groups (Huang et al., 2017a, Jia et al., 2016, Tabrizian et al., 2019). The complexation of the pollutants alters their environmental effects, such as inhibition of enzyme activity and microbial activity, antagonistic effects on plants, and the evolution of antibiotic and heavy metal cross-resistance genes in microbes (Huang et al., 2015, Wang et al., 2019, Zhou et al., 2019). Thus, it is vital to develop efficient and low-cost technologies for removing antibiotics and heavy metal pollutants from the environment.

Among the several methods available (e.g., adsorption, biological treatment, electrochemical methods, advanced oxidation) for removing antibiotics, phytoremediation is considered as a simple, economical, and efficient method (Ngigi et al., 2019, Zhang et al., 2019, Zhou et al., 2019). Phytoremediation is a technology in which plants are used to remove pollutants from water and other environments such as soils, sediments, and even the atmosphere (Carvalho et al., 2014). Various species of plants such as vetiver grass, Brassica juncea, and Lemna minor have been applied for antibiotics removal (Datta et al., 2013, Gahlawat and Gauba, 2016, Iatrou et al., 2017). Efficient removal performance was obtained in these studies; however, these species are mainly emergent plants, which have limited contact area with pollutants in water, resulting in low efficiencies of antibiotic removal, particularly adsorption potential, which occurs relatively quickly.

Compared to emergent plants, submerged and floating plants grow below the water surface and may have more opportunity for contact with pollutants; thus, they may have a relatively higher potential for adsorbing antibiotics. Myriophyllum aquaticum is a floating plant growing in tropical and sub-tropical regions worldwide (Torres Robles et al., 2011). Myriophyllum aquaticum can provide a large contact area, as its stem, branches, and roots can extend over 1 m in length under water (Rehman et al., 2018, Torres Robles et al., 2011). Thus, this plant can offer sufficient contact areas for phytoremediation in water (Guo et al., 2019). Because of potential to take up pollutants, M. aquaticum has been widely used in wastewater treatment (Luo et al., 2017, Zhang et al., 2017). Although the phytoremediation potential of TC by Myriophyllum has been evaluated (Brain et al., 2005a, Brain et al., 2005b, Gujarathi et al., 2005), the adsorption and uptake processes remain unclear. Furthermore, few studies have focused on the influence of heavy metals on TC removal by M. aquaticum, and the underlying mechanisms have not been examined.

To evaluate antibiotic removal by M. aquaticum, the most commonly used antibiotic and heavy metal, tetracycline (TC) and Cu were chosen, respectively. The removal potentials and mechanisms were also investigated using different approaches, including evaluation of the role of plant activity in TC removal, isotherms and kinetic model studies, and characterization and identification of potential functional groups on M. aquaticum by Fourier-transform infrared (FT-IR) spectroscopy and chemical modifications. The aims of this study were to (i) evaluate the TC removal potential by M. aquaticum and identify the roles of adsorption and uptake in TC removal, particularly over a short reaction time; (ii) evaluate the effect of Cu(II) on TC adsorption by M. aquaticum; and (iii) investigate the adsorption mechanism of TC by M. aquaticum as influenced by Cu(II).