Insights into the interactions between triclosan (TCS) and extracellular polymeric substance (EPS) of activated sludge

Highlights

• EPS of activated sludge could bind with TCS spontaneously.

• The proteins components of EPS dominate the binding process.

• LB-EPS play more crucial roles in the binding process compared to TB-EPS.

• TCS could be released from EPS with environmental variation.

Abstract

Triclosan (TCS) contaminant has aroused wide concerns due to the high risk of converting into toxic dioxin in aquatic environments. During the wastewater treatment process, considerable amounts of TCS are accumulated in activated sludge but the mechanisms are still unclear. Especially, roles of extracellular polymeric substances (EPS), the main components of activated sludge, in TCS removal have never been addressed. In this work, the binding properties of loosely-bound EPS (LB-EPS) and tightly-bound EPS (TB-EPS) of activated sludge to TCS are investigated by fluorescence quenching approach. The influences of aquatic conditions including solution pH, ionic strength and temperature on the interactions between EPS and TCS are explored. Possible interaction mechanisms are discussed as well as the corresponding environmental implication. Results indicate that binding processes of EPS to TCS are exothermic mainly driven by the enthalpy changes. The proteins components in EPS dominate the interactions between EPS and TCS by hydrogen bond and hydrophobic interaction. The binding strength could be improved under the condition of weak alkaline and relative high ionic strength. Generally, LB-EPS exhibit stronger binding ability to TCS than TB-EPS under neutral environment, playing more crucial roles in the binding process. This work highlights the important contributions of EPS to TCS removal, that is beneficial to comprehensively understand the migration of TCS in activated sludge system.

Introduction

Triclosan (TCS) is a broad-spectrum antibacterial agent applied in pharmaceutics and personal care products such as soaps, toothpastes, lotions and shampoo (Van Wijnen et al., 2018). With ubiquitous use triclosan is widely found in the environments. Triclosan and its methylated form tend to bioaccumulate, that have been detected in water organisms and even in human milk (Bever et al., 2018, Huo et al., 2018). High environmental risks arise due to the presence of TCS residues, as it has been demonstrated that TCS could be degraded into the toxic dioxin in aquatic systems (Dou et al., 2018, Zhu et al., 2018).

TCS is discharged into the sewage and consequently enters wastewater treatment plants (WWTP). As a hydrophobic organic pollutant with high octanol/water partition coefficient, about 90% TCS can be removed in WWTP employing conventional activated sludge process, of which 30–40% is due to biodegradation while the remainder is adsorbed by the sludge (Chen et al., 2011). Typical concentrations of triclosan in sludge are at 0.028–37.189 mg/kg level (Orhon et al., 2018), revealing that activated sludge has the ability of reserving these compounds through adsorption.

For the previous studies on TCS adsorption, the activated sludge is considered as a whole unit. Actually, as a complex system, the contribution of the sub-fractions of activated sludge to TCS removal are different. Extracellular polymeric substances (EPS), mixture of bio-macromolecular polymers secreted by microorganisms, are considered as the major component of activated sludge (Sheng et al., 2010). Various organic contaminants such as phenanthrene, benzene and dyes can be adsorbed by EPS through complicated interactions (Pan et al., 2010a, Sheng et al., 2008, Zhang et al., 2009). Our previous work demonstrates that EPS of activated sludge could bind with sulfonamides through hydrophobic interaction, influencing the distribution of contaminants in activated sludge system (Xu et al., 2013). Winkler et al. reported that TCS bound to the EPS of sludge in the oxidation ditch interfered the effluent quality, as the bound TCS might be released from EPS in the final clarifier (Winkler et al., 2007). Thus, the binding interactions between EPS and TCS directly influence the removal of TCS. However, the binding properties of sludge EPS to TCS have never been addressed until now.

The binding process between EPS and contaminants could be influenced by many factors, leading to variation of adsorption ability of activated sludge. It was found that the adsorption of activated sludge to humus enhanced with the decreased pH value, attributing to the hydrophobic interaction between humic acid and EPS. With the addition of Ca²⁺, the adsorption rate can be improved by Ca²⁺ bridging between EPS and humus (Sheng et al., 2010). In addition, different sub-fraction of EPS including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) might have different contributions to the binding process. LB-EPS is a loose and dispersible slime layer without obvious edge, while TB-EPS form a certain shape, tightly binding with the microbial cells (Sheng et al., 2010). Compared with TB-EPS, the contaminants bound to LB-EPS might be more easily released with the solubilization of LB-EPS into the surrounding environment (Pan et al., 2010b). Therefore, it is necessary to understand the differences between LB-EPS and TB-EPS in their binding properties to the contaminants.

In this work, the binding properties of LB-EPS and TB-EPS of activated sludge to TCS are investigated by fluorescence quenching approach. The influences of aquatic conditions including solution pH, ionic strength and temperature on the interactions between EPS and TCS are explored. Possible interaction mechanisms are discussed as well as the corresponding environmental implication. This work would highlight the important roles of EPS in TCS removal, that is beneficial to comprehensively understand the migration of TCS in activated sludge system.