Rapid Degradation Of Trichloroethylene In Groundwater By Ferrous Hydroxide/Bone Char Composites

Trichloroethylene（TCE）is one of the priority-control pollutants in groundwater due to its strong toxicity and environmental persistence.Nano zero valent iron（nZVI）has been widely used for TCE dehalogenation because of its high reactivity.But its properties of easy aggregation and passivation,leading to hinder its transportation and lower the efficiency of electron utilization.However,Fe（II）has excellent reduction capability due to its low redox potential,and it is easily injected and distributed in the contaminated zone.In this study,Fe（OH）₂ and bone char（BC）composite was prepared by facile mixing of Fe²⁺,NaOH and BC at room temperature,which could be applied for ultra-fast TCE dehalogenation.The major outcomes from this study are:1)The effects of different molar ratios of Fe²⁺/NaOH and the dosage of BC on the removal efficiency of TCE were investigated.The experimental results showed that the fastest TCE removal was recorded under excess Fe²⁺condition,and the optimal molar ratio of Fe²⁺/NaOH was 20:3.With the increase of BC concentration,the removal efficiency of TCE was significantly improved,and the reaction kinetic model shifted from pseudo zero order kinetics to pseudo first order kinetics,indicating that BC provides reaction sites for dechlorination.The optimum reaction conditions were 30m M Fe²⁺,4.5 m M NaOH and 2 g/L BC,where 20μM TCE could be completely degraded within 30 min.The dechlorination rate was 7.81 h^-1,which is much higher than other mainstream iron-based materials.The main dehalogenation product of TCE degradation was acetylene（～75%）,and no other chlorinated products were detected.Then,the environmental impact of the composites during the synthesis process was evaluated by life cycle assessment（LCA）.Compared with other conventional iron based materials（e.g.,nZVI,S-ZVI and green rusts）,the Fe（OH）₂/BC composite has the minimum impact on the environment.2)The effects of the addition sequence of Fe²⁺,NaOH and bone char,the degree of aging,the initial pH and the co-existence of cations and anions on reductive dechlorination were further studied.The experimental results showed that the best dechlorination efficiency was obtained when Fe²⁺and BC were premixed for 12 hours.SEM-EDS results showed that Fe²⁺can enter into the pores of BC during the premixing process of Fe²⁺and BC,thus improving the dechlorination efficiency.The electrochemical properties of the composites were further examined,and the cyclic voltammograms results showed that the reaction system with BC and Fe²⁺mixed first had stronger conductivity.The aging degree of iron hydroxide can affect the removal efficiency of TCE.With the extension of aging time,the dechlorination efficiency decreased slightly.The initial pH and common groundwater ions had limited effect on the TCE dechlorination efficiency,indicating that the composite has a certain buffering capacity against varying conditions.The multiple cycles treatment demonstrated that the Fe（OH）₂/BC composite shows good reusability and sustainability.Under excess Fe²⁺loading,the high TCE removal efficiency could be maintained by multiple base injection.In summary,Fe（OH）₂/BC composite has the advantage of being easy to prepare,having high removal efficiency,and having sustainability.It can be used as an effective material to eliminate chlorinated compounds from groundwater.