Fine-tuning green rust - bone char composite synthesis for efficient chlorinated ethylene remediation

Green rust (GR), a layered Fe-II - Fe-III hydroxide, can fully reduce chlorinated solvents in the presence of bone char (BC), which acts as an electron transfer mediator. However, how different synthesis approaches affect the composite reactivity, electron efficiency and shelf-life is currently unknown, and these are important considerations to help move this dechlorination method toward field applications. In this study, a range of GR and BC composites were produced by co-precipitation applying different GR and BC concentrations, variable Fe-II/Fe-III ratio, and using either air or persulfate as an oxidant in GR synthesis; for comparison, physically mixed GR and BC composites were also prepared. Trichloroethylene (TCE) was used as a probe substance to test the composite reactivity. Results showed that doubling of the composite BC content led to a 2.4-fold increase in the TCE reduction rate, while an increase in GR content and Fe-II/Fe-III ratio had little and no impact, respectively. Using persulfate in the synthesis allowed better control of the Fe-II/Fe-III ratio in GR and the resulting composite showed a shelf-life of at least 10 months. The co-precipitated composite exhibited a 3 times higher TCE reduction rate compared to the physically mixed composite. However, the physically mixed composite that had been aged for 4 h exhibited the same reduction rates as the co-precipitated composite, and both composites likely reach an Fe-II utilization efficiency for dechlorination close to 100 % in pH buffered system. This study showcases the great potential of the GR and BC composite for remediation applications and also a high degree of flexibility for large-scale manufacturing.