Effects of hydrochar-based nano selenium on methane production and arsenic removal from mesophilic and thermophilic anaerobic digestion of swine manure

Anaerobic digestion of livestock and poultry waste has been confined to the low efficiency and the accumulation of harmful heavy metals. An effective way can be expected for the nanomaterials or biochar-based materials rich in the trace element selenium (Se). Particularly, selenium nanoparticles (SeNPs) have attracted much more interest in recent years, due to their low toxicity, high biocompatibility, and excellent physicochemical properties. Therefore, the SeNPs have been widely used in various fields, such as environmental remediation, agricultural production, clinical medicine, and chemical manufacturing. Generally, the SeNPs can be synthesized by different approaches (e.g., biological, physical, and chemical ones). This study aims to explore the effects of hydrochar-based nano selenium on methane production and arsenic removal from mesophilic and thermophilic anaerobic digestion of swine manure. The Bacillus subtilis subspecies stercoris strain XP from our laboratory was added to the preparation of bio-nano selenium. Bio-nano selenium and garden wastes (i.e., bark, branches, and wood chips of arboreal trees) were applied to prepare the hydrochar-based nano selenium (SeHC) through hydrothermal carbonization. Then the physicochemical properties of SeHC were further analyzed to facilitate the subsequent applications. Scanning electron microscopy (SEM), mapping imaging, energy-dispersive X-ray spectroscopy (EDX) were applied to characterize the resulting materials. Batch experiments of anaerobic digestion were carried out to verify the preparation. Swine manure containing arsenic (4.43 mg/kg) was taken as the research object. The SeHC was also added for the reinforcement. Three treatment groups were also applied, including hydrochar (HC), bio-nano selenium (Se), and SeHC. A systematic investigation was implemented to explore the effects of different treatments on methane production of swine manure and the removal rate of arsenic under mesophilic (37°C) and thermophilic (44°C) anaerobic digestions. Meanwhile, the treatment group without any addition of hydrochar or selenium was considered as the control (CK). The results showed that the SeHC shared a multi-dimensional spatial structure, where the spherical nano-selenium particles with a diameter of 100~200 nm were successfully loaded on the surface of SeHC. The BET (Brunauer-Emmett-Teller) surface area of SeHC was significantly (P<0.05) enhanced by 15.1%, compared with the HC without selenium modification. Fourier Transform Infrared Spectroscopy (FT-IR) indicated that many more types of surface functional groups (e.g., N-H, C=O, C-O, and C-X) were found in the SeHC. Moreover, the Gompertz model fitting showed that the thermophilic fermentation coupled with SeHC treatment group was more conducive to the anaerobic methanization. In terms of the mesophilic and thermophilic anaerobic conditions, the final cumulative methane production (P’) and the maximum methane production rate (Rm) of the SeHC treatment group were significantly promoted by 39.37%, 11.70%, 72.20%, and 75.50%, respectively, compared with the CK group. Meanwhile, the treatment of SeHC shared better efficiency in the removal of arsenic under anaerobic digestion. The removal rate was ranked in the descending order of the SeHC>Se>HC>CK. Specifically, the maximum removal rate was 5.3% and 10.7% in the mesophilic and thermophilic digestion, respectively. In summary, the SeHC coupled with the thermophilic anaerobic digestion can be expected to improve the anaerobic digestion for the rapid removal of harmful substances, such as arsenic. The findings can also provide a theoretical reference to develop the hydrochar-based nano selenium in environmental and bioenergy production.