Immobilizing of heavy metals in sediments contaminated by nonferrous metals smelting plant sewage with sulfate reducing bacteria and micro zero valent iron
Introduction
Xiawangang River region is one of the most seriously heavy metals polluted areas in China because of long-term inputs of nonferrous metal industrial wastewaters and municipal sewage. Min Jiang and his co-worker reported that this area was severely polluted by Cu, Cd, Zn and Pb, and the total concentrations of these metals all highly exceeded the Chinese environmental quality standard, especially for Cd [1]. Recently, local government has carried out a series of environmental dredging projects to get rid of the heavy metals pollution and protect the water quality of Xiangjiang River. However, how to safely and economically dispose the huge number of dredging sediment became another environmental issue [2].
Various techniques have been developed for the remediation of heavy metals in sediments, including soil washing, thermal extraction, ion exchange, electrokinetic treatment, reverse osmosis, membrane technology, evaporation recovery, solidification, plasma vitrification and bioremediation [3]. Comparing with other techniques, bioremediation technology is more economically feasible, easier to apply to contaminated sites and causes less secondary pollution [4]. And it is able to applied both ex-situ and in-situ to the design of economical bioremediation process, because microorganisms have a generic character for survival strategies in heavy metal polluted habitats, their specific microbial detoxifying mechanisms such as bioaugmentation, biotransformation, biomineralization or biosorption [5], [6], [7]. Nowadays many studies have demonstrated the feasibility and efficiency of heavy metal remediation by microorganism [8], [9], [10]. However, biological process can also mobilize metal by increasing its solubility. A newly research found that the microbial sulfate reduction played a key role in the mobilization of As from Fe-rich aquifer sediment under anoxic conditions, because most of As initially present in the sediment had been leached out in the form of mobile thio-As species [11], [12]. To prevent leaching of metals from sediments into the water phase by microorganism, metals should be immobilized by biosorption, forming of metal-binding molecules, reductive precipitation, sulfide precipitation or phosphate precipitation [13]. Sulfate-reducing bacteria (SRB) are the most commonly microorganisms that have been used for bioremediation [14], [15], [16]. There are more than 30 genera of SRB having been reported, including gram-negative mesophilic bacteria, gram-positive mesophilic bacteria, and members of other thermophilic groups [17]. SRB are widely distributed in the environment, including paddy soils, sea water, thermal springs, oil field water and sediments. Biological treatment with SRB has been considered as the most promising alternative for heavy metals remediation due to its low cost and high efficiency [18]. The mechanisms of heavy metals remediation by SRB include precipitation of metal with H2S produced by SRB and biosorption of metal onto SRB culture surface by cell wall and extracellular polysaccharides (EPS) [19]. SRB are chemoautotrophic bacteria, they use sulfate as the terminal electron acceptor and consequently convert sulfate to sulfide [20]. Although, SRB have been testified efficiently for heavy metal remediation in laboratory [8], there were still little cases of successful field trials [21]. An important reason of this is that microbial processes generally require optimal environmental parameters (pH, redox potential, sulfate, temperature, suitable electron donor) for sustainable growth, which often do not meet at natural conditions. In that case, additional reactive materials need to be provided to maintain a suitable growth environment for the bacteria [22], [23]. Using Fe0 for dissolved water contaminant remediation has been demonstrated in both laboratory and field tests [24], [25], [26]. The preliminary idea of using metallic iron for the oxidative contaminants was based on the contaminant reduction through electron transfer between Fe0 with oxidation [27]. Toxic metal should be remedied by Fe0 through a combination of surface adsorption, precipitation and co-precipitation with iron oxides [28]. In addition to direct contaminant reduction at the surface, Fe0 particles may also be able to stimulate SRB by depleting O2, lowering the redox potential, and producing water derived H2 via corrosion reactions which can also be used as an electron donor for SRB [23], [29], [30], [31]. Formation of stable metal-sulfide precipitates through sulfate reduction has been well documented [32], [33], [34]. Hence, an integrated Fe0 + SRB process could be of greater interest in heavy metal remediation field. The particle size and dosing of Fe0 are highly important for optimizing the remediation process. Kumar and his co-worker reported that, micro sized Fe0 particles (mFe0, with an average particle size of 20–40 μm) in combination with glycerol were found to enhance microbial sulfate reduction more than granular sized Fe0 particles (gFe0, with an average particle size of 0.25–2 mm) and nano sized Fe0 particles (nFe0, with an average particle size of 70–100 nm) [22]. Based on the research results, this study was to explore the feasibility and efficiency of combining SRB with micro zero valent iron for immobilizing of multi-heavy metals in sediments contaminated by nonferrous metals smelting plant sewage. The objective of this research was to discover an economical and effectively immobilizing technique for actual heavy metals polluted sediment. The results of this study may provide a new way for dealing with Xiangjiang River sediments contaminated by multi-heavy metals.
Section snippets
Fluvial sediment
The sediment samples used in this study were collected from wastewater drainage outlet at Xiawangang Port, Xiangjiang River, Hunan Province, China. This area is one of the most serious heavy metals polluted areas in China. Sediment samples were acquired using a shovel and transported to the laboratory in four sealed polyethylene bags. In laboratory, the sediments were air dried naturally for four months, then grinded and sieved through a 100 mesh sieve before it stored for subsequent
SRB identification
The sequencing information of PCR-DGGE product indicated that the SRB contained in repeating cultured microorganisms were mainly composed of the two species: Desulfobulbus propionicus and Desulfovibrio vulgaris. Based on this identification, authentic cultures of these two species were used in all further studies. These cultures had been previously characterized for their ability to generate Methylmercury [41], [42], [43].
Chemical analysis
Fig. 1 is the variation of pH, which presents the increase in pH (almost
Conclusions
The objective of this experiment is to develop an economical and effectively immobilizing technique for actual heavy metals polluted sediment. The results of this study may provide a new way of safely disposing of the dredging sediments contaminated by multi-heavy metals. The study demonstrates that there is obvious advantage of mFe0 + SRB integrated systems over conventional mFe0 or SRB based system in terms of precipitate stability for heavy metals. The precipitates of Cu, Cd, Zn and Pb are
Acknowledgements
This project was supported by the National Natural Science Foundation of China (Grant No. 51521006, 51579097).
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