Mechanisms of potentially toxic metal removal from biogas residues via vermicomposting revealed by synchrotron radiation-based spectromicroscopies

Highlights

• Biogas residues contain high contents of heavy metals.

• Vermicomposting decreases the content of heavy metals from biogas residues.

• SR-FTIR and µ-XRF are integrated to identify the binding sites of heavy metals.

• The removal mechanisms are revealed by synchrotron radiation spectromicroscopies.

• Vermicomposting provides a safe utilization of biogas residues.

Abstract

Biogas residues (BR) contaminated with potentially toxic metals pose environmental risks to soils and food chains, and strategies are needed to decrease the concentration and bioavailability of potentially toxic metals in BR. Here, metal fractions and removal mechanisms were quantified by synchrotron radiation-based Fourier transform infrared and micro X-ray fluorescence spectromicroscopies on BR and earthworms subject to vermicomposting. Vermicomposting resulted in decreases in concentrations of potentially toxic metals in BR and increases in metal removal efficiencies due to uptake by earthworms. Prior to vermicomposting, Zn, Cu and Pb were associated with N-H, O-H, aromatic C, aliphatic C, and amide functional groups, but following maturation during vermicomposting, metals were associated with N-H, O-H, aliphatic C and polysaccharide functional groups. Following vermicomposting, Zn and Cu were mainly distributed in the dermal portions of earthworms, whereas Pb was more homogeneously distributed among the inner and outer portions of the earthworms, revealing that different metals may have different uptake routes. These findings provide a new strategy for safe utilization of BR by using earthworms via vermicomposting to remove potentially toxic metals and in situ insights into how metals binding and distribution characteristics in BR and earthworms during compost and vermicomposting processes.

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.