The inhibiting effect of PS-SRMs concentration changes on the biodegradation of BTX in karst groundwater

In-situ chemical oxidation (ISCO) combined with enhanced bioremediation (EBR) is a viable strategy for remediating organic contaminants in groundwater. Nevertheless, the inhibitory effect of oxidant injection dosages on microbial communities remains an understudied. This study employed a microcosm experiment to remediate karst groundwater contaminated by fuel hydrocarbons, combining slow-release materials (SRMs) with the release of the oxidant persulfate (PS). It mainly investigated the release performance of PS-SRMs, the responses of hydro-chemical indicators and electron acceptor concentrations, the degradation of benzene, toluene, and xylene (BTX), and compositional shifts in microbial communities at different PS target slow-release concentrations. The results showed that the concrete-based PS-SRMs reached peak concentration within 20-28 days, possibly reaching 34 %-100 % of the target concentration, and followed by a diffusion-erosion mechanism. The structural integrity of PS-SRMs with a mass ratio of 2/1 was compromised after 85 d. The concrete matrix increased pH to 11, while limestone buffered the acidity generated by the PS decomposition and created a weakly alkaline environment. A PS target slow-release concentration range of 500-1000 mg/L was identified as optimal for the combined remediation. Within this range, both dissolved oxygen (DO) and nitrate served as electron acceptors, and both aerobic biodegradation and nitrate reduction occurred simultaneously. However, when the PS concentration exceeded 5000 mg/L, DO decreased below 4.0 mg/L, and nitrate reduction was negligible. Microbial analysis revealed that the dominant phylum for BTX degradation were Proteobacteria, Actinobacteriota, and Firmicutes. Although nitrate reduction also contributed, aerobic processes were the primary degradation pathway. Adding limestone and sand significantly enhanced biodegradation efficiency, while adding organic-containing lime soil stimulated nitrate consumption and improved microbial diversity. Future research should prioritize evaluating the impact of PS concentration on biodegradation dynamics in flowing karst groundwater.