Manure inoculation shapes arsenic speciation and phase partitioning during biodegradation of Pteris vittata biomass

Arsenic-hyperaccumulator P. vittata has been used in phytoremediation of As-contaminated soil, yet As speciation and phase partitioning among solid, liquid and gas during degradation of As-rich biomass remain poorly understood. Here, P. vittata biomass containing 125 mg kg^-1 As was degraded in a bioreactor for 5 weeks, with pig manure as a microbial inoculant. Manure addition enhanced biomass reduction by 57% from 47% in the control. While initially As was in the solid phase, estimated As was accumulated primarily in the gas and solid phases in the control after degradation. Manure addition mainly directed As to liquid phase at 26% by reducing solid-phase As to 17%. While inorganic arsenate (AsV) was dominant in solid phase, organic monomethylarsonic acid (MMA; 10%) and dimethylarsinic acid (DMA; 44%) were dominant in liquid phase, with organic DMA and TMAO being the dominant species in the gas phase via estimation. This shift corresponded to reducing conditions, alkaline pH at 8.2, and elevated temperature. Manure also increased microbial α-diversity (Shannon 5.69 vs. 4.56), Clostridium and Terrisporobacter bacteria (LDA scores 4.5 and 4.1), chemoheterotrophy (62 vs. 51%) and fermentation (28 vs. 17%). Overall, manure inoculation reduced solid-phase As by shifting to a collectable liquid pool while reducing gaseous As loss via estimation. Our data suggest that As can be directed to different phases and speciation during biodegradation for efficient management of As-rich biomass.