Elsevier

Water Research

Volume 57, 15 June 2014, Pages 31-39
Water Research

Adsorption behavior of tightly bound extracellular polymeric substances on model organic surfaces under different pH and cations with surface plasmon resonance

https://doi.org/10.1016/j.watres.2014.03.018Get rights and content

Highlights

  • The adsorption behavior of TB-EPS was characterized by using SPR in real-time.

  • TB-EPS adsorption on different surfaces was dependent on solution condition.

  • Hydrophobic surface achieved the maximum adsorbed amount at the same pH value.

  • The adsorbance of TB-EPS on surface had a substantial increase by adding Fe3+.

Abstract

Tightly bound extracellular polymeric substances (TB-EPS) play a substantial role on microbial aggregates, which can promote microbial cells to aggregate and adhere onto the carrier in bioreactor. However, the attachment and adsorption of TB-EPS on different surfaces were awaited to be elucidated. In this study, four self-assembled monolayers (SAMs) carrying methyl (CH3-SAM), amino (NH2-SAM), hydroxyl (OH-SAM), and carboxyl (COOH-SAM) terminal groups were prepared to model different surfaces. TB-EPS adsorption on these surfaces under different pH conditions and additional cations were investigated using surface plasmon resonance. The adsorption of TB-EPS dramatically decreased with the decreasing pH values. CH3-SAM surface achieved the maximum adsorption at the same condition. Na+ promoted the TB-EPS adsorbed on COOH-SAM surface. The Ca2+-mediated complexes were attracted by COOH-SAM and repelled by NH2-SAM, respectively. The adsorptions of TB-EPS on the four SAM surfaces were significantly increased by adding Fe3+. These results demonstrated that the TB-EPS adsorption on the organic surfaces were dependent on the pH and cation of solution.

Introduction

Activated sludge floc, granular sludge, biofilm are microbial aggregates that adhere to each other and are typically surrounded by a matrix of extracellular polymeric substances (EPS) in wastewater treatment. These substances have a substantial effect on the formation and growth of microbial aggregates (Sheng et al., 2010). EPS are high-molecular-weight molecules consisting of proteins, polysaccharides, DNA, lipids, and humic acids (Frolund et al., 1996). These substances present both outside of cells and in the interior of microbial aggregates (Flemming and Wingender, 2010). EPS-wrapped cells can increase the aggregation of microbial cells and enhance the cell deposition on surfaces (Liu et al., 2010, Su and Yu, 2005). The adsorption and adhesion of EPS onto the carrier and cell surfaces may alter the physicochemical characteristics of surfaces, thereby affecting the cell deposition and biofilm formation (Omoike and Chorover, 2006). EPS are amphiphilic molecules with positively charged and negatively charged groups, as well as hydrophilic and hydrophobic groups (Sheng et al., 2010). Previous studies have concluded that the solution chemistry, such as the ionic strength, pH, divalent cations, and natural organic matter may have dramatic impacts on EPS deposition or adsorption (Orgad et al., 2011, Tong et al., 2011). Furthermore, EPS adsorption is also controlled by the surface properties of carrier and cell. However, the adsorption behavior of EPS on different surfaces is not known and makes it difficult to evaluate the relative importance of surface property on EPS adsorption (Renner and Weibel, 2011).

Two issues must be addressed for the systematic investigation of EPS adsorption. First, the different surfaces need to be simulated because the surface properties of natural samples cannot be controlled. Self-assembled monolayers (SAMs) can be used to address this problem. SAMs can be formed via the adsorption of alkanethiolates on a metal surface, such as gold, silver, and platinum (Arima et al., 2011). The monolayer forms a densely packed and very stable structure with the contribution of van der Waals interactive forces between the alkyl chains, and another terminal group (–R) is effectively displayed at the surface of the SAM (Arima et al., 2011, Love et al., 2005). It is noted that the alkyl chains in SAMs of alkanethiolates on Au(111) usually are tilted ∼26–28° from the surface normal, and display ∼52–55° rotation about the molecular axis (Ulman, 1996). SAMs with various terminal functional groups can be easily prepared to simulate different surface properties (such as the wettability, surface charge, and morphology), and they have been widely used to study macromolecule adsorption and cell adhesion (Wang et al., 2005, Ostuni et al., 1999, Peelen et al., 2006). Therefore, EPS adsorption on different surfaces can be implemented using SAMs. Second, a free-label, real-time, rapid, and sensitive detection method with minimal sample consumption is required to analyze the EPS adsorption. Surface plasmon resonance (SPR) was used to obtain the information on the binding specificity between two molecules, the target molecule concentration, the kinetic parameters of association and dissociation processes, the binding strength, and so on (Wilson, 2002). SPR is widely used to study the adsorption of proteins or polysaccharide on a sensor surface (Gam-Derouich et al., 2011, Mi et al., 2012). To the best of our knowledge, SPR has actually been utilized to study the role of EPS in bacterial adhesion (Pranzetti et al., 2012).

EPS are often divided into two major fractions: soluble EPS (SEPS) and bound EPS. The inner layer of the bound EPS consists of tightly bound EPS (TB-EPS), whereas the outer layer contains loosely bound EPS (LB-EPS) (Chen et al., 2013, Sheng et al., 2010). The compositions and properties of SEPS and LB-EPS are unstable and easily influenced by external conditions. By contrast, the stable composition and function of TB-EPS can be easily controlled and are less affected by the environment (Ye et al., 2011). TB-EPS has a high molecular weight and has significant functions in activated sludge and biofilms (Liang et al., 2010). They maintain the stability of the sludge (Xu et al., 2010). Gao et al. (2011) reported that TB-EPS have strong biosorption ability; their functional groups have different roles in biosorption. Moreover, TB-EPS performs a high flocculating rate and promotes the aggregation of cells by sweep flocculation and bridging; this aggregation effect depends on the separation distance between sludge cells (Liu et al., 2010, Yu et al., 2009). However, the influence of surface properties on the adsorption and aggregation of TB-EPS has not been elucidated. Therefore, the mechanism of TB-EPS adsorption onto the different surfaces should be understood.

The TB-EPS extracted from a sequencing batch reactor (SBR) with partial nitrification process was used as an example to perform this study. Four SAMs with different terminal functional groups including methyl (CH3-SAM), amino (NH2-SAM), hydroxyl (OH-SAM), and carboxyl (COOH-SAM) were prepared to model different property organic surfaces. SPR was implemented to investigate the TB-EPS adsorption on different surfaces under different pH values and valences of additional cations.

Section snippets

TB-EPS extraction

Activated sludge sample was collected from an SBR at the partial nitrification stage. The reactor was fed with the effluent from an anaerobic sequencing batch biofilm reactor. The effluent contained 420 mg/L to 460 mg/L of COD, 200 mg/L NH4+–N, 20 mg/L to 30 mg/L of total phosphorus, and ∼16.5 g/L of NaCl. The reactor temperature (32 ± 1 °C), DO (0.3–0.5 mg/L), and HRT (24 h) were maintained. The pH was kept at 8.0 ± 0.2 using KHCO3.

TB-EPS was extracted from the activated sludge sample using

Surface characteristics of SAMs

Au–S bond is formed by the interaction of gold and the thiol of alkanethiolates. XPS can reveal the information on the unbound and bound thiol on the gold surface (Li et al., 2003). The S2p3/2 binding energy of 163.5 eV–164 eV was assigned to unbound thiol, whereas that of 161.9 eV or 162 eV was assigned to bound thiol. The high-resolution spectra of the S2p region from the XPS of CH3-SAM, COOH-SAM, NH2-SAM, and OH-SAM are shown in Fig. 2. The S2p spectra could be fitted to a doublet with a 2:

Conclusion

The hydrophobic surface achieved the maximum adsorption at the same pH and additional cations. At pH 4.5, three hydrophilic surfaces obtained the similar adsorption. The COOH-SAM adsorbed the lowest TB-EPS amount at pH 10.5. Na+ promoted the TB-EPS adsorbed on COOH-SAM surface. Ca2+ encouraged the TB-EPS adsorption by forming TB-EPS complexes. Furthermore, the Ca2+-mediated complexes were attracted by COOH-SAM and repelled by NH2-SAM, respectively. The adsorption of TB-EPS on the four SAM

Acknowledgments

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (51108482, 51178484 and 51278509), Chongqing Science and Technology Commission (cstc2012ggB20002) and the Public Experiment Center of State Bioindustrial Base (Chongqing).

References (46)

  • A. Omoike et al.

    Adsorption to goethite of extracellular polymeric substances from Bacillus subtilis

    Geochim. Cosmochim. Acta

    (2006)
  • E. Ostuni et al.

    The interaction of proteins and cells with self-assembled monolayers of alkanethiolates on gold and silver

    Coll. Surf. B-Biointerf.

    (1999)
  • M. Puiu et al.

    Kinetic approach of aflatoxin B1-acetylcholinesterase interaction: a tool for developing surface plasmon resonance biosensors

    Anal. Biochem.

    (2012)
  • K. Raunkjaer et al.

    Measurement of pools of protein, carbohydrate and lipid in domestic waste-water

    Water Res.

    (1994)
  • G.P. Sheng et al.

    Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: a review

    Biotechnol. Adv.

    (2010)
  • M.P. Tong et al.

    Influence of natural organic matter on the deposition kinetics of extracellular polymeric substances (EPS) on silica

    Coll. Surf. B-Biointerf.

    (2011)
  • Z.W. Wang et al.

    Extracellular polymeric substances (EPS) properties and their effects on membrane fouling in a submerged membrane bioreactor

    Water Res.

    (2009)
  • M. Watanabe et al.

    Flocculating property of extracellular polymeric substance derived from a marine photosynthetic bacterium, Rhodovulum sp

    J. Biosci. Bioeng.

    (1999)
  • H.C. Xu et al.

    Enhanced storage stability of aerobic granules seeded with pellets

    Bioresour. Technol.

    (2010)
  • F.X. Ye et al.

    Effect of C/N ratio on extracellular polymeric substances (EPS) and physicochemical properties of activated sludge flocs

    J. Hazard. Mater.

    (2011)
  • G. Yu et al.

    Characteristics of extracellular polymeric substances (EPS) fractions from excess sludges and their effects on bioflocculability

    Bioresour. Technol.

    (2009)
  • Z.Q. Zhang et al.

    A novel biosorbent for dye removal: extracellular polymeric substance (EPS) of Proteus mirabilis TJ-1

    J. Hazard. Mater.

    (2009)
  • Y. Chang et al.

    A systematic SPR study of human plasma protein adsorption behavior on the controlled surface packing of self-assembled poly(ethylene oxide) triblock copolymer surfaces

    J. Biomed. Mater. Res. Part

    (2010)
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