Study On The Removal Of Trichloronitromethane From Drinking Water By The Carbon-based Nano Zero-valent Iron Composites

The disinfection of drinking water is one of the most effective public health measures during the 20th century,but at the same time some disinfectants(chlorine,ozone,chlorine dioxide,or chloramines)also react with natural organic matters,anthropogenic contaminants and inorganic ions present in source waters to form various disinfection by-products(DBPs).Halonitromethanes(HNMs),as an emerging class of disinfection by-products containing nitrogen(N-DBPs),have been frequently detected in drinking water,sewage and swimming pool water.It has been reported that the average and the highest concentrations of HNMs in water were 1 ?g/L and 10?g/L,respectively.Among the DBPs,halonitromethanes have received special attention due to their higher cytotoxicity,genotoxicity and developmental toxicity than those regulated DBPs,HNMs have been listed in 50 high priority DBPs monitored by USEPA.Therefore,it is of great practical significance to probe and research on investigation of HNMs pollution levels in drinking water,development of efficient removal techniques for the control of HNMs in drinking water.Analysis of HNMs using gas chromatography-mass spectrometry(GC-MS)in traditional split/splitless injection mode could lead to thermal degradation of some HNMs that are thermally unstable.The aim of this study is to develop an analytical method which applies cold on-column(COC)injection coupled with GC-MS system for determination of the nine HNMs in drinking water.Using this method,HNMs levels are investigated in different drinking water plants in Jiangsu Province.Nano zero-valent iron materials(nZVI),a kind of strong reducing agent with standard electrode potential of-0.43 V,have been extensively applied for removal of a variety of pollutants such as chlorohydrocarbon,nitrobenzenes from groundwater,wastewater and drinking water due to its large specific surface area,great surface reactivity and relatively low price.To overcome agglomeration and passivation for better stability,nZVI particles modified by coating or immobilizing on solid supports materials have recently become one of hot research areas.In this work,Fe0 nanocomposites were prepared via facile carbonization and calcinations of glucose and ferric chloride.The structure and morphology of the as-synthesized iron nano-particles were investigated by X-ray powder diffraction(XRD),specific surface area(BET),scanning electron microscope-energy dispersive spectrometer(SEM-EDS),transmission electron microscope(TEM),Raman spectroscopy and X-ray photoelectron spectroscopy(XPS).This study aimed at evaluating the effect of Fe0 nanocomposites on the absorption-reduction of TCNM in water and the removal kinetics under different reaction conditions.The pathways of TCNM removal by the as-prepared Fe0 nanocomposites in water were also investigated.The main results and conclusions are as follows:1)Comparative experiments showed COC had an advantage over the conventional split/splitless injection in minimizing thermal degradation of HNMs,especially dibromochloro-and tribromo-nitromethanes(DBCNM and TBNM).Both debromo-and denitro-products of HNMs were observed in the splitless injection mode under the relatively low temperature such as 117? and 170 ?.It is the first time that COC-GC-MS method combined with liquid-liquid extraction(LLE)sample pretreatment provides a sensitive and reliable quantification method for the simultaneous analysis of nine HNMs in drinking water.The limits of detection for nine HNMs species ranged from 0.06 to 0.9?g/L.Reproducibility and recovery of the nine HNMs in LLE-COC-GC-MS method were evaluated by analyzing 7 individual standard mixtures at two spiking levels in water.The good average recovery(77-90%)and low relative standard deviation(RSD)values(3.4?12%),which represent good reproducibility,were achieved for all nine HNMs.Solid phase extraction(SPE)and LLE procedures were compared for sample pretreatment.SPE provided the good recoveries(>70%)for four commonly detected HNMs in drinking water including dichloronitromethane(DCNM),trichloronitromethane(TCNM),bromochloronitromethane(BCNM)and dibromonitromethane(DBNM),while the recoveries of another five HNMs were below 50%.Concentrations of HNMs were quantified in the distribution system water from 11 surface water treatment plants along the Yangtze River in Jiangsu Province.In these water samples,only one HNM(TCNM)was detected at quantifiable concentrations,the rest eight HNMs were either not found or below detection limits.The concentrations of TCNM which was the most frequently detected in drinking water were 0.41-0.67 ?g/L,which are similar to previously reported values in drinking water.This study also investigated the concentrations of nine HNMs in raw waters,finished waters and distribution system waters from 8 drinking water plants in Jiangsu Province.The results showed the four HNMs(DCNM,TCNM,BCNM and DBNM)were detected and their concentrations were different in finished water and distribution system water,which may be associated with the material of distribution system.2)In this work,the graphene-supported nano-Fe0(G-nZV1)was prepared by facile carbonization and calcinations of glucose and ferric chloride,which was used in the removal of TCNM from drinking water.Compared with the nano-Fe0 composites synthesized via liquid-phase reduction methods with borohydride salt,the as-prepared G-nZVI,when the Fe/C mass ratio was 1:5,has better morphology structure,larger specific surface area(453 m2/g)and smaller Fe0 nanoparticles size(15 nm).Batch experimental results indicated that the as-prepared G-nZVI could effectively remove trichloronitromethane,a dominant in the group of HNMs from drinking water.About 99.2%of initial TCNM could be adsorbed and degraded under 60 mg/L G-nZV1 dosage within 120 min.The effects of various parameters(e.g.nZVI dosage,initial TCNM concentration,reaction temperatures and dissolved oxygen concentrations,etc.)on the removal efficiency of TCNM were investigated.Kinetic studies indicated that the removal of TCNM by G-nZVI followed a pseudo first order rate.The degradation pathways of TCNM by G-nZVI nanocomposites might include dechlorination and denitration of TCNM.In the presence of Fe0,TCNM lost a chlorine atom to form DCNM,and subsequently dechlorinated to chloronitromethane and nitromethane and/or denitrated to methylamine.3)Nano Fe/Cu has been successfully combined into bimetal nanocomposites(G-Fe-Cu)via facile carbonization and calcinations.TEM images showed dearly that the Fe/Cu nanoparticles were well dispersed on the surface of graphite without aggregation and the size of particles rangged from 5-50 nm.The individual nanoparticles were composed by the ?-Fe0 crystalline state and copper alone.Accompany with copper,the reactivity of nano-Fe/Cu bimetallic particles is obviously higher than that of nano-Fe.Mass fraction of metal Fe/Cu affected the removal efficiency of TCNM and the best mass ratio composition of Fe/Cu was 10:1,resulting in the highest removal efficiency and the fastest removal rate.The material with higher or lower Cu mass neither has the better reducing effect on TCNM.The optimal removal efficiency was obtained at G-Fe-Cu nanoparticle addition of 10 mg/L,TCNM initial concentration of 10 ?g/L,initial pH value of 6.0,reaction temperature of 25 ? and no dissolved oxygen or residual chlorine of solution.The bimetal nanocomposites(G-Fe-Cu)can completely remove TCNM in water(99.7%)in 60min and the reaction of TCNM removal followed a pseudo first order equation.4)The commom oxyanions(NO3-,SO42-and HCO3-)and humic acid(HA)could weaken the removal efficiency of TCNM using G-Fe-Cu bimetal nanoparticles.Under the experimental conditions,the effects of anions on TCNM removal followed NO3->SO42->HCO3-.Inhibition of HA showed the trend from increase first and then decreases.Effects of coexisted trichloromethane(TCM)and trichloroacetic acid(TCAA)in reaction system on the removal of TCNM were also investigated.The results showed that either TCM or TCAA could significantly decrease the removal of TCNM because they can compete with TCNM in adsorption and reduction by nanometer Fe/Cu.The influence of coexisted DBPs on TCNM removal followed TCAA>TCM.5)The removal of TCNM in tap water was discussed.The composite materials(G-Fe-Cu)with different amount of iron loading(10 mg/L-200 mg/L)showed good abilities to remove TCNM and the higher yeild of TCNM removal was obtained as the iron loading increased.The iron concentration in water increased significantly with the increase of iron dosage,which could affect the drinking water quality,however,the copper ions were not detected in solution during the reaction.6)XRD images of the used nanocomposite showed that Fe3O4 and Fe2O3 were the corrosion products of Fe0 nanocomposites after reaction.The attenuation of reduction activity of Fe0 nanocomposites could be attributed to excess oxide precipitates on the surface.Iron oxides covered on the surface eventually lead the decrease of reduction reactivity.But the iron oxides in the surface of graphene materials could be restored to Fe0 again via calcinations.These results indicated that the synthesized Fe0 nanocomposites could be a powerful material to remove HNMs from drinking water.