Mechanisms of potentially toxic metal removal from biogas residues via vermicomposting revealed by synchrotron radiation-based spectromicroscopies
Graphical abstract
Introduction
In China, over 3 billion tons of manures and 650 million tons of straws are produced each year (Awasthi et al., 2019, Yu et al., 2011). Approximately 20% of this waste is used to produce biogas. Biogas residues (BR) are subsequently used as organic fertilizers to enhance the moisture-holding and buffering capacities of soils and stimulate the growth of plants and microorganisms (Dahiya and Vasudevan, 1986, Chen et al., 2012, Duan et al., 2012). However, because of the ubiquitous application of feed additives to control diseases (Ju et al., 2007; Zhang et al., 2012, Sahariah et al., 2015, Sarwar et al., 2017), excessive heavy metals in animal manures and thus BR could cause wide public concern for the agricultural safety (Asada et al., 2012, Zhao et al., 2015). It is therefore imperative to decrease the bioavailability and total amount of potentially toxic metals in BR.
Composting is one of the most popular and versatile techniques for safe disposal of BR (Gajalakshmi and Abbasi, 2008, Hussain et al., 2018). Although composting can effectively eliminate pathogens and decrease the bioavailability of heavy metals in BR, it fails to reduce the total concentration of heavy metals in BR (Lim et al., 2016). Similar to composting, vermicomposting is another biological process with adding earthworms to treat organic wastes (Aira et al., 2007, Karmegam and Daniel, 2009, Fornes et al., 2012, Lim et al., 2016, Hussain et al., 2018). Vermicomposting has the benefit that some earthworms, such as Tiger worm (Eisenia fetida), can help to remove or immobilize heavy metals (e.g., Zn, Cu, and Pb) from BR (Hobbelen et al., 2006; Li et al., 2014, Soobhany et al., 2015). However, unlike composting, vermicomposting fails to eliminate pathogens owing to it not acting as an exothermic process (Gajalakshmi and Abbasi, 2008, Lim et al., 2016). Although combined composting and vermicomposting could both kill pathogens and remove considerable amounts of potentially toxic metals from BR, uncertainties remain in the amount of potentially toxic metals that could be removed, the removal mechanisms, and the fate of removed metals.
The objectives of this study were to (i) investigate whether and to what extent combined composting and vermicomposting affects the concentrations of bioavailable and total potentially toxic metals (Cu, Zn, and Pb) in BR, and (ii) examine the microscale binding sites and characteristics of potentially toxic metals in BR and earthworms during vermicomposting. For these purposes, a combined composting and vermicomposting system was developed and built. Throughout our experiments, complementary synchrotron-radiation-based techniques, i.e., micro-Fourier transform infrared (μ-FTIR) and μ-X-ray fluorescence (μ-XRF) spectromicroscopies (Lehmann et al., 2007, Sun et al., 2017), were integrated to identify the binding characteristics of potentially toxic metals in BR and earthworms.
Section snippets
Biogas residues, peanut straw, and earthworms
BR was collected from a pig farm biogas engineering system (Jiangsu Nongle Biotechnology Co., Ltd.). The BR was produced by fresh pig manure under a fully mixed anaerobic reactor at medium temperature (35 °C). To balance the living and composting environment of microorganisms, the peanut straw (2–4 cm) was used to adjust the C/N ratio during composting. During composting and after mixing well every time, three randomly BR samples were collected at 1, 13, 30, 37 and 69 days for sequential
Impacts of composting on different potentially toxic metal fractions in BR
Overall, composting modestly decreased the available fractions (F1 and F2) but increased the total contents of all examined metals in BR (Fig. 1). The highest percentage (51–63%) of Cu and Pb was found in the oxidizable fraction during composting, followed by the residual fraction (36–43%), exchangeable fraction (0.2–4.5%) and reducible fraction (0–1.5%) (Fig. 1B and 1C). During composting, the percentage of available (i.e., sum of exchangeable and reducible fractions) Cu and Pb gradually
Removal characteristics of potentially toxic metals from BR by vermicomposting
pH is essential for the growth of earthworms (Leduc et al., 2008). In this study, the pH of BR was between 7.0 and 7.2, which was almost the optimal pH for earthworm growth. As a result, the BR with a content of OM (188.6 ± 10.6 g/kg, Table S1) was a perfect food for earthworms. Meanwhile, the pH values, OM contents and bioavailability of metals are important factors for the accumulation of potentially toxic metal by both plants and earthworms (Qian et al., 1996, Li et al., 2010, Imseng et al.,
Conclusions
Our results provide direct evidence that composting of BR reduced the potentially toxic metal bioavailability but vermicomposting increased the removal efficiency of Cu (26.4%), Zn (32.3%) and Pb (13.7%) from BR. Since a burst of animal industry and a short of energy sources in China, anaerobic digestion has been fantastically developed. As a result, large amounts of BR with high contents of potentially toxic metals were produced. Therefore, this study provides a new strategy for removing
Acknowledgements
We thank Drs. Xiaojie Zhou at the BL01B of National Center for Protein Sciences Shanghai (NCPSS) and Jichao Zhang at the BL15U1 beamlines at Shanghai Synchrotron Radiation Facility for assistance during data collection. This work was supported by the National Key Research and Development Program of China (2017YFD0800803) and National Natural Science Foundation of China (41977271).
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