Mechanistic insights into synergistic facilitation of copper/zinc ions and dewatered swine manure-derived biochar on anaerobic digestion of swine wastewater

Abstract

Biochar-assisted anaerobic digestion (AD) has been proposed an advanced system for swine wastewater (SW) management. However, the effects of metallic nutrients in SW, such as copper/zinc ions (Cu²⁺/Zn²⁺), on the biochar-assisted AD of SW are not well understood. This study investigated the influences of individual Cu²⁺/Zn²⁺ or dewatered swine manure-derived biochar, as well as their combined additions, on the AD of SW. The results showed that exposure to 50 mg/L Cu²⁺/Zn²⁺ temporary inhibited methane production, but the addition of 20 g/L biochar alleviated this inhibition by shortening the methanogenic lag time and increasing methane yield. Following a period of acclimation, both Cu²⁺/Zn²⁺ and biochar promoted methane production, although metagenomic analysis revealed distinct mechanisms underlying their promotion. Cu²⁺/Zn²⁺ enhanced ATP processing, including electron exchange between NADH/NAD⁺ and succinate/fumarate transformation, by 26.0-35.8%. Additionally, the gene encoding Coenzyme M methylation was upregulated by 36.2% along with enrichments of Methanocullus and Methanosarcina, contributing to accelerated hydrolysis and methanogenesis rates by 54.7% and 44.8%, respectively. On the other hand, biochar mainly stimulated bacterial F-type ATPase activities by 28.4%, likely facilitating direct interspecies electron transfer between Geobacter and Methanosarcina for syntrophic methanogenesis. The combined addition of Cu²⁺/Zn²⁺ and biochar resulted in "win-win" benefits, significantly increasing the maximum methane production rate from 40.3 mL CH₄/d to 53.7 mL CH₄/d. Moreover, the introduction of biochar into AD of SW facilitated the transformation of more Cu²⁺/Zn²⁺ from a reducible Fe-Mn oxides form to a residual form, which potentially reduced the metallic toxicity of the digestate for soil amendment. The findings of this study provide novel insights into understanding the synergistic impacts of heavy metals and biochar in regulating SW during AD, as well as the management of associated digestate.

Keywords: Biochar-assisted anaerobic digestion; Cu(2+)/Zn(2+); Methane production; Methanogenic pathway; Swine wastewater.