Research ArticleA low-cost in-situ bioremediation process for perchlorate contaminated aqueous phase
Graphical Abstract
Introduction
Perchlorate (ClO4ˉ) is an endocrine-disrupting toxic oxyanion and emerging environmental contaminant. Its presence in drinking water and food is a public health concern because it can cause hypothyroidism and other health issues, especially in fetuses, infants, and children (Gibbs et al., 1998, Kumarathilaka et al., 2016, Lisco et al., 2020, Maffini et al., 2016, Niziński et al., 2020). Under natural conditions, perchlorate salts are highly soluble in water and resistant to chemical and biological degradation (Stetson et al., 2006). Contamination of groundwater aquifers caused by the leakage of perchlorate from bulk perchlorate handling sites is a well-known environmental issue (Trumpolt et al., 2005). Perchlorate-contaminated water can be treated using physical, chemical, or biological methods (ITRC, 2008, ITRC, 2005, Srinivasan and Sorial, 2009, Urbansky, 1998). Adsorption, reverse osmosis, ultrafiltration, nano filtration and ion exchange are the commonly practiced physical processes (Batista et al., 2002, Heo et al., 2012, Song et al., 2017, Yoon et al., 2009, Yoon et al., 2003). These approaches are less appealing due to their high operating costs and the formation of concentrated brine that requires further treatment. Chemical and electrochemical procedures are sluggish and unsuitable for large-scale treatment facilities (Xie et al., 2018). The most promising option for perchlorate treatment is biological (Choe et al., 2013, Hatzinger et al., 2000, Ye et al., 2012). In the absence of oxygen, and in the presence of a suitable electron donor, perchlorate reducing bacteria (PRB) can use ClO4ˉ as a terminal electron acceptor and reduce it to harmless chloride and oxygen (He et al., 2019, Waller et al., 2004).
For treating perchlorate-contaminated groundwater, both ex-situ and in-situ treatment approaches are reported (Cox, 2009, Hatzinger et al., 2002, ITRC, 2008, ITRC, 2005). Fluidized bed reactors (Hatzinger et al., 2000, Webster et al., 2009), continuous stir tank reactors, fixed bed reactors (Brown et al., 2005, Kim and Logan, 2001, Wallace et al., 1996), and membrane biofilm reactors are used in ex-situ operations (Nerenberg et al., 2002, Sevda et al., 2018, Sutton, 2006). In-situ treatment includes phytoremediation (Nzengung and McCutcheon, 2003, Susaria et al., 1999a, Susaria et al., 1999b), constructed wetlands (Li et al., 2021, Tan et al., 2004), bio-augmentation and bio-stimulation (Fuller et al., 2019). Perchlorate-reducing microbes are ubiquitous, and hence instead of bio-augmentation, the activity of indigenous PRB can be enhanced (bio-stimulated) to degrade ClO4ˉ to below the detection limit by providing nutrient and electron donor sources. (Coates et al., 1999, Höhener and Ponsin, 2014, Wang et al., 2013). In practice, the bio-stimulated zones are created across contaminated plumes for the microbial remediation of the contaminant (Borden, 2007). These bio-barriers can be of three configurations: active, semi-passive, or passive (Borden, 2007, ITRC, 2005, Stroo and Norris, 2009, USEPA, 2005). In an active or a semi-passive bio-barrier, a mobile soluble amendment (substrate for the bio-barrier) is delivered into the contaminated aquifer through injection wells, and the groundwater is mixed with the amendment and is recirculated through extraction–injection wells. In an active system, the groundwater or substrate is recirculated or injected continuously, whereas in a semi-passive system it is done only intermittently. The common substrates used as soluble electron donors and carbon sources are acetate, lactate, citrate, ethanol, etc. (Hatzinger, 2005, Krug et al., 2009, Parr, 2002, Stroo and Ward, 2008, Taraszki, 2009). Alternatively, gaseous electron donors such as hydrogen, ethyl acetate, etc. were also reported in in-situ remediation studies (Cai et al., 2010, Evans et al., 2009, Evans et al., 2011, Evans and Trute, 2006). In passive systems, once the substrate is filled/injected, there is no need for groundwater pumping or recirculation, reducing energy requirements. Passive bio-barrier systems with low-cost substrates are more attractive because they requires low operational and maintenance cost (Stroo and Norris, 2009). However, a key disadvantage of passive systems is that there is no control over the amount of carbon released from the material, which could lead to organic contamination or contaminant breakthrough due to a lack of carbon supply. A summary of different types of bio-barriers reported, their features and disadvantages are presented in Table 1. The selection of biofilm support matrix (filter medium) is another important factor in bio-barriers based perchlorate treatment systems (Okeke and Frankenberger, 2005). The availability, cost, longevity, environmental compatibility, and operational and maintenance cost are factors considered while choosing filter media. The common media reported are granular materials such as gravel, sand, quartz, pumice, perlite, granular activated carbon, etc. In passive trench biowalls, materials such as mulch, compost, etc. are often mixed with the filter media to avoid compaction and for better hydraulic conductivity. The details on bio-barriers studies reported for perchlorate removal are presented in Table 2.
In this context, this study primarily focuses on evaluating the efficacy of a two-stage semi-passive bio-barrier system for perchlorate treatment. Vegetable waste-derived leachate as the sole substrate (carbon source, electron donor, and nutrients) was used for ClO4ˉ reduction. Furthermore, needle felt made from natural fibers with a high lignin content was selected as biofilm support medium. The advantages of the low-cost substrate and filter bed medium, as well as the regulated substrate release in this technique, make it a preferable alternative for in-situ perchlorate remediation practices in the field.
Section snippets
Materials and methods
The bench-scale two-stage treatment system used in this study consisted of two components, (1) An anaerobic leach bed unit for generating leachate from vegetable waste, and (2) the bio-treatment unit for perchlorate degradation.
Performance of Anaerobic Leach Bed (ALB) Unit
In this study, the purpose of the ALB unit was to generate a low-cost source for organic substrate as carbon, nutrient, and electron donor source for the perchlorate-reducing microbial system. During anaerobic digestion of the vegetable waste in the ALB unit, the organic content (TCOD and SCOD) in the leachate was maximum from day 4 to day 6 of the whole digestion period (18 days). When 2 kg of organic waste was leached with 2.5 L of water in recirculation mode for 48 h (one cycle), the SCOD
Discussions
Simple and complex organic substrates have been reported for in-situ perchlorate remediation studies (He et al., 2019, Hatzinger et al., 2009; Okeke and Frankenberger Jr, 2005). The leachate used in this study contained SCOD including total VFAs up to 17 m. eq/L produced from the anaerobic digestion of vegetable waste. Wu et al. have studied the utilization of different substrates such as acetate, lactate, citric acid, and molasses as electron donors for ClO4ˉ reduction. It took seven days to
Conclusions
The combined anaerobic leach bed and bio-augmented anaerobic filter bed system was able to reduce 98.5% of ClO4ˉ (from a starting concentration of 10 mg/L) using the leachate produced by the anaerobic digestion of vegetable waste as the sole substrate. The utilisation of waste-derived leachate as a substrate and needle felt coir fibre as a biofilm support matrix makes the current technique more attractive. The controlled substrate addition will prevent excess residual organic build up,
Statement of Environmental Implication
Perchlorate is a well-known endocrine-disrupting and persistent environmental contaminant. A novel in-situ remediation approach for controlling its widespread contamination is covered in this paper. Controlled delivery of vegetable waste-derived leachate as sole substrate and application of needle felt natural fibre as biofilter medium are the major highlights of this approach. Apart from being more cost-effective and environmentally benign, residual organics can be controlled through this
CRediT authorship contribution statement
Dr. Krishnakumar B. The concept development, project funding, experiment supervision, data analysis, manuscript revision. Dr. Jasmin Godwin Russel Planning and execution of experiments, data collection and analysis, preparation of the manuscript.
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.
Acknowledgments
Infrastructural support from CSIR-NIIST is acknowledged. Dr. Jasmin G. Russel would like to acknowledge CSIR for her Senior Research Fellowship and AcSIR for academic support. The technical support from Mr. Adarsh and Mr. Sreejith in the fabrication of the reactor unit is also acknowledged.
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