ReviewElemental sulfur as electron donor and/or acceptor: Mechanisms, applications and perspectives for biological water and wastewater treatment
Graphical abstract
Introduction
Elemental sulfur (often written as S0, S8) is one of the predominant sulfur forms (i.e. sulfide, S0, and sulfate) in the terrestrial crust (Rabus et al., 2013), and it is a central intermediate in the geochemical sulfur cycle (Hao et al., 2014). For instance, S0 is a crucial intermediate during biological sulfide oxidation to sulfate (Klok et al., 2012). S0 can also be a final product of biological sulfide oxidation by phototrophic bacteria (Lin et al., 2018) or sulfide-oxidizing bacteria (SOB) with O2, nitrate, or ferric ions as electron acceptors (Di Capua et al., 2019). In other words, S0 can be bio-utilized as either electron acceptor (S0 reduction) or electron donor (S0 oxidation) in biological reactions. This suggests S0 may have a broad application in water and wastewater treatment processes.
Applying biological S0 reduction and/or oxidation in water and wastewater treatment has received increasing attention in the past few decades. Various S0-based biotechnologies, such as sludge reduction, denitrification and metal-laden wastewater treatment, have been developed to solve complicated environmental problems that occur within current water and wastewater treatment processes in a more cost-effective approach (Sahinkaya et al., 2015; Sun et al., 2018; Zhang et al., 2018b). However, there is still a lack of comprehensive summary and discussion of the key advances in the emerging S0-biotechnologies. On one hand, there are substantial distinctions among these S0-based bioprocesses, such as microbial communities (e.g. S0-reducing bacteria, S0-oxidizing bacteria), fundamental mechanisms (e.g., S0 reduction, oxidation), and scenarios employed (e.g., treatment of domestic wastewater, industrial wastewater, groundwater, agricultural wastewater, metallurgical wastewater). It is thus necessary to have a clear picture on the fundamental knowledge of respective S0-based biotechnologies.
On the other hand, there are a number of reviews related to sulfur oxidation metabolism (e.g., sulfide, S0, sulfite and thiosulfate) (Frigaard and Dahl, 2008; Ghosh and Dam, 2009; Gregersen et al., 2011). However, we notice that sulfur metabolism, especially S0 metabolism, has not been used to explain and link with process design and optimization. It is known that the water solubility of S0 is extremely low (5 μg/L at 25 °C) (Boulegue, 1978), which limits the bioavailability of S0. This could be a bottleneck for its scale-up and wide applications in water and wastewater treatment. Thus, systematical assessment on accessibility of microorganisms to S0 and their corresponding metabolism would help develop feasible strategies that could facilitate S0 bio-utilization efficiency, thereby substantially improving process performance.
This review summarizes and critically discusses the S0-related microbiology, and the mechanisms how sulfur-respiring bacteria access and metabolize the almost insoluble sulfur. The present applications of S0-based biotechnologies for water and wastewater treatment are comprehensively reviewed. The chemical and biochemical mechanisms involved in specific scenarios and their process optimization are reported. This review is expected to enrich the knowledge on the emerging S0-based water and wastewater treatment biotechnologies, and to provide technical guidelines for their potential engineering applications. The insights gained are used to define a research agenda.
Section snippets
S0-reducing bacteria
S0-reducing bacteria (S0RB) oxidize organic matter using S0 as electron acceptor with sulfide being the by-product (Eq. (1)). As such, S0 reduction can be employed for organic removal from wastewater. The biogenic sulfide can precipitate metals via forming insoluble metals sulfides, indicating its feasibility for metal-laden wastewater treatment. In fact, S0RB thrive in a variety of environments from acidic to halo-alkaline as well as saline and thermophilic conditions (Table 1), suggesting
S0-based biotechnologies for water and wastewater treatment
Over the past few decades, biotechnologies driven by S0 reduction and/or oxidation have been fruitfully developed for water and wastewater treatment. It is vital to notice that such applications are cost-efficient and practically feasible. These S0-based biotechnologies can be categorized into three groups: a) S0 used as electron acceptor for organic removal; b) S0 used as electron donor for nitrate, perchlorate, and oxidative metals removal; and c) S0 used to generate sulfide for heavy metal
Pathways of S0-respiring bacteria accessing S0 and strengthened strategies
The extremely low water solubility of S0 suggests a poor bioaccessibility, thereby impairing the applicability of S0-based biotechnologies. Understanding how microbes access S0 is beneficial for process optimization and scale-up. So far, there are four putative possible mechanisms for making S0 bioavailable to microorganisms (Fig. 5): (1) direct cell-S0 contact (pathways 1 and 2); (2) polysulfide involvement (pathway 3); and (3) extracellular electron transfer (EET) (pathways 3 and 4). It is of
Future perspectives
Although the applications of S0 during water and wastewater have drawn increasing attention and substantial progresses have been achieved in recent years, many issues related to the basic mechanisms and the applications of S0-based biotechnologies remain unanswered and should be a goal of future research.
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Increasing S0 bioavailability is the key step to broaden the applications of S0-based biotechnologies. The low bioavailability of S0 due to its extremely low aqueous solubility limits the
Conclusions
Elemental sulfur, a non-toxic, cheap, insoluble, and easily available chemical, has been used as electron donor and/or acceptor in S0-based biotechnologies for water and wastewater treatment. This study systematically reviews the principles, microorganisms, mechanisms and applications of S0-based biotechnologies. S0 can undergo oxidative and reductive conversion by a wide array of organisms. This has been recently explored in water and wastewater treatment targeting various contaminants.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the support from the National Natural Science Foundation of China (51978289 and 51638005), the Hong Kong Innovation and Technology Commission (ITC-CNERC14EG03) , the Hong Kong Special Administrative Region, China (T21-604/19-R), and the Fundamental Research Funds for the Central Universities (20lgzd24). The authors would like to thank Dr. Jianliang Sun from South China Normal University for his constructive advice to this review.
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