Adsorption of Cu²⁺ and Zn²⁺ by extracellular polymeric substances (EPS) in different sludges: Effect of EPS fractional polarity on binding mechanism

Highlights

• Anaerobic granular sludge adsorbed more Zn and Cu than other two sludges.

• Hydrophilic EPS of anaerobic granular sludge adsorbed more Zn and Cu than HPO-A.

• Freundlich model for hydrophilic EPS adsorption while Langmuir equation for HPO-A EPS.

• Amides 2 group was essential for heavy metals adsorption of anaerobic granular sludge.

Abstract

Extracellular polymeric substances (EPS) in sludge samples played a major role in heavy metals removal during wastewater treatment. In this study, the binding quality, adsorption mechanism, as well as the chemical fractional contribution of the sludge EPS from activated sludge, anaerobic granular sludge and anaerobic flocculent sludge to the adsorption of Zn²⁺ and Cu²⁺ was investigated. For all three sludge samples, Cu²⁺ could be more easily adsorbed than Zn²⁺, and EPS extracted from the anaerobic granular sludge exhibited a relatively higher adsorption capacity than that of anaerobic flocculent sludge and activated sludge. Specifically, hydrophobic EPS of the activated sludge and anaerobic flocculent sludge was more efficient in adsorbing Cu²⁺ and Zn²⁺ than that of the hydrophilic EPS. However, hydrophilic EPS in anaerobic granular sludge played a greater role in heavy metals removal. The adsorption of those two heavy metals onto the unfractionated and hydrophobic EPS could be better described by the Langmuir isotherm, while Freundlich models fitted hydrophilic EPS. In addition, the effect of the heavy metals adsorption on the spectrum characteristics of the sludge EPS was also explored by analysis of FT-IR and fluorescent spectra.

Introduction

Ecotoxicological risks of heavy metals are widely studied and have been a global concern [1]. In China, the continuous development of pulp and paper, refineries, petrochemicals, steel and automobile industries result in increasing discharges of large quantities of toxic heavy metals into drainage systems and subsequently into wastewater treatment plants [2]. Among those hazardous heavy metals, Zn²⁺ and Cu²⁺ are among the metals of most immediate concern according to the World Health Organization (WHO) [3]. For their considerable toxic effects on the receiving environment, those hazardous heavy metals have to be properly managed to meet ever increasing legislative standards [4]. Due to their non-biodegradable characteristics, heavy metals are always adsorbed and removed from wastewater along with the wasted sludge [5].

In biological wastewater treatment systems, extracellular polymeric substances (EPS) are microbially produced from active bacterial secretion, cell surface material shedding, and cell lysis, and accumulate in the sludge. [6], [7]. EPS accounts for 50–90% of total organic carbon (TOC) in a sludge biomass and is primarily composed of carbohydrates, proteins, nucleic acids, and heteropolymers [8], [9], which can be present either as capsular/free EPS or associated with hydrophobic/hydrophilic EPS. Previous studies reveal that sludge EPS has a polymerized three-dimensional structure and exhibits a relative high capacity to bind heavy metals [10], [11], [12], which plays an important role in protecting the biomass against potential inhibition by heavy metals.

The work of Chen et al. demonstrated that more than 90% of heavy metal removal by biofilms could be attributed to the existence of sludge EPS [13]. Wang et al. revealed that Zn²⁺ and Cu²⁺ could be efficiently adsorbed by the EPS extracted from Desulfovibrio desulfuricans, and the adsorption was well represented with Langmuir and Freundlich isotherm models [14]. Similarly, Song et al. recognized that the presence of proteins of Chroococcus sp. related EPS enhanced the adsorption of Hg²⁺ [15]. Liu et al. measured a maximum adsorption capacity of 1587.3, 1470.6 and 1123.6 mg/g for Pb²⁺, Cd²⁺ and Zn²⁺, respectively, in aerobic granular sludge, and the adsorbed heavy metals could be desorbed under acidic conditions [16]. Zhang et al. also found that a right shaking speed would ensure enough contact frequency between the aerobic digested activated sludge and the adsorbate (Cu²⁺ and Cd²⁺). Moreover, a natural pH value at about 5.5 benefited for a high adsorption capacity [17].

Generally, the binding characteristics of heavy metals onto the sludge EPS relates to physicochemical/biological processes including complexation, ion exchange, surface micro-precipitation, etc., due to abundant existence of various binding sites on the surfaces/pores of EPS flocs [18]. Pagnanelli et al. demonstrated that heavy metals could be easily bound to the polysaccharides, phospholipids and hydroxyl, carboxyl, phosphoric amine related proteins in sludge EPS, and formed organo-metal complexes [19]. Similarly, Kantar et al. and Sheng et al. reported that the carboxylic/phosphoric groups played a major role in heavy metals complexation [11], [12]. Moreover, Spath et al. and Hou et al. also stated that heavy metals could be preferentially embedded via hydrophobic interactions [20], [21]. Recently, some of the researchers already focuses on the effect of EPS components (humic acid/proteins/polysaccharides) on the adsorption of heavy metals, however, limited information is available about the fractional EPS characteristics (hydrophobic/hydrophilic polarity) on the binding mechanisms of typical heavy metals.

Here, we extracted three typical sludge EPS (activated sludge, anaerobic granular sludge and anaerobic flocculent sludge) using NH₄OH and subsequently fractionated using XAD-8/XAD-4 resins. Our principal objective was to evaluate which EPS fractions were responsible for heavy metal adsorption. Adsorption kinetic characteristics of the different EPS fractions were comparably studied. In addition, possible changes of the chemical structures in EPS during heavy metals adsorption were examined.