Research article
Bioassessment of heavy metal toxicity and enhancement of heavy metal removal by sulfate-reducing bacteria in the presence of zero valent iron

https://doi.org/10.1016/j.jenvman.2017.07.075Get rights and content

Highlights

  • A simple and valid evaluation method of heavy metal toxicity on SRB was proposed.

  • The heavy metal mixture exhibited a synergistic toxic effect on SRB.

  • Addition of Fe0 enhanced the SRB activity and heavy metal removal.

  • Heavy metal precipitates were mainly sulfide for Zn, and hydroxide for Mn, Cr.

Abstract

A simple and valid toxicity evaluation of Zn2+, Mn2+ and Cr6+ on sulfate-reducing bacteria (SRB) and heavy metal removal were investigated using the SRB system and SRB+Fe0 system. The heavy metal toxicity coefficient (β) and the heavy metal concentration resulting in 50% inhibition of sulfate reduction (I) from a modeling process were proposed to evaluate the heavy metal toxicity and nonlinear regression was applied to search for evaluation indices β and I. The heavy metal toxicity order was Cr6+ > Mn2+ > Zn2+. Compared with the SRB system, the SRB+Fe0 system exhibited a better capability for sulfate reduction and heavy metal removal. The heavy metal removal was above 99% in the SRB+Fe0 system, except for Mn2+. The energy-dispersive spectroscopy (EDS) analysis showed that the precipitates were removed primarily as sulfide for Zn2+ and hydroxide for Mn2+ and Cr6+.The method of evaluating the heavy metal toxicity on SRB was of great significance to understand the fundamentals of the heavy metal toxicity and inhibition effects on the microorganism and regulate the process of microbial sulfate reduction.

Introduction

The heavy metals owing to their toxicity and persistence nature in environment are considered to be as severe pollutants (Krishna and Mohan, 2016). Certain heavy metals, such as zinc (Zn2+) and cadmium(Cr6+), are commonly found in untreated wastewaters (Ochoaherrera et al., 2011), which are not degraded but may be strongly accumulated in water, soils and the food chain, resulting in sublethal effects or death for plants or animals (Yi et al., 2011). Further, exposure of humans to high levels of risk elements through their contact with contaminated waters, soils and the food chain can lead to serious chronic even carcinogenic diseases (Krishna and Mohan, 2016). Sulfate is not a very harmful pollutant (Blázquez et al., 2016), but the discharge of excessive loads of sulfate can affect public water supplies and disrupt the natural sulfur cycle. Hence, it is of utmost importance to remove heavy metals and sulfate from wastewater before discharge. Several methods including chemical precipitation (Iakovleva et al., 2015), ion-exchange (Mahmouda and Hoadley, 2012), reverse osmosis (Ricci et al., 2015), electrolysis process (Curteanu et al., 2011), etc. are available, most of which are relatively expensive, complex processes and inefficient for heavy metal removal (Kurniawan et al., 2006). It has been validated that microbial remediation by sulfate-reducing bacteria (SRB) under anaerobic conditions was efficient for removing heavy metals and sulfate from wastewater (Bai et al., 2013a, Bai et al., 2013b, Kieu et al., 2011). Using sulfate as an electron acceptor, SRB can digest substrates (such as hydrogen and lactate) to generates sulfide, which could react with heavy metals and remove them from solution as insoluble metal sulfides (Kiran et al., 2017).

Zero valent iron (Fe0) is expected to lower the oxidation-reduction potential (ORP), which is conducive to the formation of enhanced anaerobic environment for SRB (Kumar et al., 2015, Yao et al., 2008, Xiao et al., 2013). Also, Fe0 can release from Fe0 to Fe2+ (Fe0+2H+=Fe2++H2) and thus buffer the solution pH (Liu et al., 2015). When utilized in sulfate-rich wastewater, Fe0 serves as an electron donor to weaken the competition between SRB and methanogens for electrons (Liu et al., 2015). Additionally, Zhang et al. (2011) determined that Fe0 can reduce the toxicity of H2S by increasing pH or the precipitation. Previous study found that Heavy metal removal using Fe0 is based on the contaminant reduction through electron transfer during Fe0 oxidation (Kumar et al., 2015). Bai et al. (2012) and Lee et al. (2003) demonstrated that Fe0 is an efficient reductant which can lower mobility and toxicity of some heavy metals (Cu2+ and Cr6+). Hence utilizing SRB enhanced by Fe0 has been developed to improve the performance of the anaerobic process, resulting in better sulfate reduction and heavy metal removal (Jong and Parry, 2003, Ayala-Parra et al., 2016).

In case of insufficient removal, the residual heavy metals might induce the denaturation and inactivation of enzymes, rupture of organelles and membrane integrity (Alexandrinoa et al., 2011), and subsequently inhibit the growth and metabolism of SRB. To better solve the above problem, it is essential to fully understand and evaluate the heavy metals toxicity on SRB. Traditionally, the toxic effects of heavy metal are reflected by the decrease of chemical oxygen demand (COD), metabolic activity of enzymes and the concentration of bacteria or biofilm thickness, etc. (Cabrero et al., 1998, Stasinakis et al., 2002). Hao et al. (1994) described the heavy metal toxicity in virtue of the inhibition of sulfate reduction by some typical heavy metals. Cabrera et al. (2006) studied the toxic effect of heavy metals though evaluating the precipitation of heavy metals on SRB. However, few reports were available on the modeling of the heavy metals toxicity on the SRB performance.

This paper was designed to record the SRB activity and the heavy metal removal in the SRB system and the SRB+Fe0 system under various conditions. A simple and useful approach has been proposed to better understand the toxic effects of Zn2+, Mn2+, Cr6+ and their mixture on SRB using the heavy metal toxicity coefficient (β) and the heavy metal concentration resulting in 50% inhibition of sulfate reduction (I). To effectively evaluate the toxicity of heavy metals, two mathematical models were developed and modified to profile the sulfate reduction rate (V) or sulfate reduction efficiency (Rs) with different heavy metals concentrations, in which β and I were successfully fitted in the SRB system and the SRB+Fe0 system. Subsequently, the heavy metal removal mechanism in the SRB+Fe0 system was investigated and discussed. This study was conducted at a laboratory scale using synthetic medium to facilitate a modeling method, which was expected to evaluate effects of the heavy metal toxicity on SRB and reference values for large scale commercial process.

Section snippets

Microorganisms and wastewater

The seed sludge containing SRB was initially collected from a wastewater treatment plant at Hua-Bei pharmaceutical factory (Hebei, China). 3% (v/v) inoculum sludge was added into 1.0 L medium, which was modified according to our previous research (Bai et al., 2013a, Bai et al., 2013b), mainly including sodium lactate (4000 mg L−1), sodium sulfate anhydrous (1690 mg L−1), yeast extract (1000 mg L−1), ammonium chloride (1000 mg L−1), potassium phosphate dibasic (500 mg L−1), magnesium sulfate

Individual heavy metal

In this study, the SRB exposed to synthetic wastewater containing separate heavy metals (Zn2+, Mn2+ and Cr6+) were tested for changes of the SRB activity. The sulfate reduction efficiency represented the activity of SRB. Effects of different heavy metal concentrations on SRB activity were investigated in the SRB system and the SRB+Fe0 system, respectively. It was found that the sulfate reduction efficiency decreased as the heavy metal concentration increased. The addition of Fe0 appeared to

Conclusions

A modeling approach was proposed to evaluate the heavy metal toxicity on SRB. The heavy metal toxicity order on SRB was Cr6+>Mn2+>Zn2+ according to toxicity evaluation parameters (β and I), which were obtained from the simple models in this study. The sulfate reduction was inhibited by heavy metals which resulted in lower Rs and V in the SRB system than the SRB+Fe0 system. Moreover, the heavy metal mixture exhibited a synergistic effect on SRB activity. All the heavy metal removal ratios were

Acknowledgements

The authors are grateful for the financial support of National Nature Science Foundation of China (Project No. 21077075) and the Program of Introducing Talents of Discipline to Universities (No. B06006).

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