Enhancement of methanogenic performance by gasification biochar on anaerobic digestion

Highlights

• The aromatization degree increased with the increase of gasification temperature.

• Biochar with more aromatic groups was studied in the promotion of CH₄ production.

• The potential direct interspecies electron transfer was established.

• Biochar enhanced acetate decarboxylation of the methane metabolism pathway.

Abstract

This work evaluates the performance of different biochar-amended anaerobic digestion systems. The Fourier Transform Infrared analysis showed that more ordered aromatic groups formed and the aromatization degree increased with the rise of gasification temperature. The biochar produced at 900 °C still showed an excellent ability to maintain the stability of anaerobic digestion performance, where the specific methane yield content steadily reached 742 mL CH₄/g ethanol. Besides, the enzymatic activity test indicated an improved performance with the addition of biochar obtained at gasification temperature. The relationship between the microbial community and metabolism pathways result are signified due to the direct interspecies electron transfer among Pseudomonas or Candidatus cloacimonas and Methanosaeta via biochar. These links have promoted the methane metabolism pathway of acetate decarboxylation. Therefore, the current study helps better understand the influence of surface functional groups of biochar at different temperatures on anaerobic digestion performance.

Introduction

Anaerobic digestion (AD) is an established technology to address waste treatment challenges and proper energy recovery (Zhang et al., 2014). During the AD process, the inoculum, which consists of a consortium of microorganisms, converts the complex organic substrates into biogas (CH₄ and CO₂) (Pan et al., 2019). However, due to the complexity of AD reactions, there are many methane production problems (Wang et al., 2017). Several studies have evaluated the use of conductive material to enhance AD performance (Wang et al., 2019, Codignole Luz et al., 2018, Cai et al., 2016). Among the conductive materials, carbon-based materials have been used extensively in improving methanogenesis. To date, granular activated carbon (GAC) has been one of the most investigated carbon-based materials for its high conductivity and surface area (Rasapoor et al., 2020). Many studies have confirmed that biochar has an active redox property, making biochar as effective as GAC in AD enhancement (Shen et al., 2020). Biochar is an environmentally friendly byproduct, that can be obtained from AD digestate (Ayilara et al., 2020). Methane yield is enhanced by 26.7%, 23.0%, and 26.4% following the addition of 3%, 5%, and 7% biochar, respectively, as well as the use of biochar for the treatment of heavy metal pollution in AD systems dedicated for treating pig manure (Wang et al., 2021). The addition of biochar permits the immobilization of microorganisms to promote the growth of biofilm and enhance cation exchange throughout the porous surface structure. Surface functional groups and the structure of biochar have critical roles because they facilitate the immobilization of microorganisms via adsorption on biochar. To improve AD performance, some authors have used the biochar’s ability to enhance the direct interspecies electron transfer (DIET). DIET operates in favour of the substrates and intermediates degradation quicker than mediated interspecies electron transfer (MIET); indeed, the electron transfer velocity is 106 times faster than MIET (Rotaru et al., 2014). The immobilization function is required for DIET while biochar enhances the microorganism colonization on its surface. The internal mechanism of DIET includes resisting acidic shock when the volatile fatty acids (VFAs) accumulation occurs and then changes the electron transfer pathway to promote methane production (Chen et al., 2014). The efficiency of DIET is closely related to the electrical conductivity and the active redox property of biochar (Rasapoor et al., 2020), which positively correlate with surface functional groups and the structures. Biochar possesses three main functional groups which affect AD performance, i.e., aliphatic, oxygenated, and aromatic groups (Xue et al., 2015). The aromatic groups increasing is linked to the biochar’s conductivity improvement (You et al., 2017), promoting DIET efficiency and reducing the AD’s lag phase. Additionally, alkaline functional groups (OH, CC, NH, CO(CO), CO₃²⁻) increase the biochar alkalinity, a buffering property that inhibits the acid caused in AD (Pandey et al., 2020), stabilizes the digestion conditions and shortens the AD stagnation time. Many studies have proved that the surface functional groups, the pore volume, porosity and rough surface morphology of biochar can affect mass transformation and microorganism metabolism during the AD process (Cai et al., 2016, De Vrieze and Verstraete, 2016, Lü et al., 2016). The characteristic of biochar depends mainly on the source of the raw materials and the preparation conditions.

Gasification has become a vital method to achieve biomass conversion and obtain it, due to the high degree of aromaticity and porous structure of this compound (Sajjadi et al., 2018). The selection of parameters in the biomass gasification process includes temperature, heating rate, and reaction residence time, among others. It affects the adsorption and other physic-chemical biochar properties (Shen et al., 2020), which improves the methanogenic ability of AD. Codignole Luz et al., 2018 studied the impact of different gasification temperatures on biochar property and concluded that temperature influences the surface structure, composition, and biochar properties. In this sense, while the gasification temperature increases, the volatiles are continuously separated, leading to the creation of cracks and pores in the microstructure of the residual solid (Paethanom et al., 2012). During the process of increasing the temperature to 700 °C, the functional groups of acidic oxygen decompose into CO₂ or CO to be gradually removed. Alkane such as methyl (CH₃) and methylene (CH₂) disappear, but the aromatic ring structure and the aromatization degree increase, lead to an improvement in stability (McBeath et al., 2011). Moreover, it has been studied that the graphitization degree of the biochar was enhanced when the gasification temperatures continue to rise. It seems quite possible that the graphene structure in biochar enhances the electrical conductivity significantly to augment DIET efficiency. Besides, the π electrons on the basal planes in of graphene also increase the basicity of biochar and facilitate the adsorption function (Sajjadi et al., 2018).

The reaction activity of biochar is affected by its functional groups, aromaticity, conductivity, and other physical and chemical properties, which show significant differences due to different treatment temperatures. Many studies have investigated the effects of biochar to improve the AD system (Rasapoor et al., 2020). However, the connection between properties of biochar and the internal mechanism of AD improvement have not been elucidated clearly. Little is known about the effects of surface functional groups of biochar produced at different gasification temperatures on AD performance. Additionally, the enzyme activity and methanogenic pathways could be affected by biochar in the AD system, ultimately affecting methane production efficiency. In this study, the effect of preparation conditions on the properties of biochar were related to the improvement of AD. The current study helps to understand better the influence of surface functional groups of biochar at a different gasification temperature on methanogenesis as well as providing a better way to improve methane production from the biochar preparation conditions. Furthermore, it seeks to reveal the potential biochar’s DIET mechanism improved by the functional groups.