Mechanism,behaviour and application of iron nitrate modified carbon nanotube composites for the adsorption of arsenic in aqueous solutions

Yingli Li ,Zhishuncheng Li ,Guangfei Qu,*,Rui Li ,Shuaiyu Liang ,Junhong Zhou ,Wei Ji,Huiming Tang

1 Faculty of Environmental Science and Engineering,Kunming University of Science and Technology,Kunming 650500,China

2 National Regional Engineering Research Center-NCW,Kunming 650500,China

Keywords:Carbon nanotubes As(V)Adsorption Nanocomposites Impregnation method

ABSTRACT In this study,ferric nitrate modified carbon nanotube composites(FCNT)were prepared by isovolumetric impregnation using carbon nanotubes(CNTs)as the carrier and ferric nitrates the active agent.The batch experiments showed that FCNT could effectively oxidize As(III) to As(V) and react with it to form stable iron arsenate precipitates.When the dosage of FCNT was 0.1 g.L-1,pH value was 5-6,reaction temperature was 35 °C and reaction time was 2 h,the best arsenic removal effect could be achieved,and the removal rate of As(V) could reach 99.1%,which was always higher than 90% under acidic conditions.The adsorption results of FCNT were found to be consistent with Langmuir adsorption by static adsorption isotherm fitting,and the maximum adsorption capacity reached 118.3 mg.g-1.The material phase and property analysis by scanning electron microscopy,Brunauer-Emmett-Teller,Fourier transform infrared spectoscopy,X-ray photoelectron spectroscopy and other characterization methods,as well as adsorption isotherm modeling,were used to explore the adsorption mechanism of FCNT on arsenic.It was found that FCNT has microporous structure and nanostructure,and iron nanoparticles are loosely distributed on CNTs,which makes the material have good oxidation,adsorption and magnetic separation properties.Arsenic migrates on the surface of FCNT composites is mainly removed by forming insoluble compounds and co-precipitation.All the results show that FCNT treats arsenic at low cost with high adsorption efficiency,and the results also provide the experimental data basis and theoretical basis for arsenic contamination in groundwater.

1.Introduction

Among environmental chemical pollutants,arsenic is one of the most common and most dangerous heavy metals to human health.Arsenic (As) and its compounds are highly toxic and harmful substances in the environment.The valence states of arsenic in water are As(III),As(V) (H3AsO4,and[1-5],it has been reported that its oxides are not easy to be destroyed and can only be transferred to different forms or compounds,in which arsenite is much more toxic than arsenic acid.Centers for Disease Control and Prevention(CDC)and International Agency for Research on Cancer(IARC)have classified arsenic as a primary carcinogen [6-8].It can enter the human body along the food chain or water bodies and produces toxicity by affecting metabolic enzymes,lipid peroxidation,gene damage,gene expression and so on,thus causing serious toxicity to human body [9].China’s arsenic reserves account for 70% of the world’s arsenic ore.The remaining unusable waste rock and waste residue after mining are randomly stacked and oxidized,decomposed and leached in the natural environment.The arsenic dissolves into the surrounding air,soil and water,thus endangers the health of animals,plants and humans.

The reported techniques for arsenic removal include precipitation,adsorption,ion exchange,neutralization and oxidation,ion flotation,electroflocculation,extraction,membrane separation,biological methods,etc.[10-14].These mature methods have been applied to the removal of arsenic,and have their own advantages and some inherent limitations,including the production of toxic waste,low efficiency and high cost of arsenic removal.Among them,the adsorption method has the advantages of simple operation,low cost,wide range of adaptation,and high efficiency,and has become one of the most commonly used methods for treating arsenic-containing wastewater.The commonly used adsorbents are biochar [15,16],activated alumina [17,18],clay and rare earth materials [19,20],etc.The static adsorption experiment of arsenic wastewater with modified carbon was carried out by Zhangetal.[21].The result shown that the maximum removal efficiency of arsenic can reach 98.6%.The effect of membrane operating conditions and water quality on the effectiveness of charged ultrafiltration membranes for arsenic removal was investigated using membrane materials by Brandhuberetal.[22].The relationship between arsenic removal efficiency and experimental conditions was discussed and the model was analyzed.The experimental result shown that the regular model of arsenic removal by ultrafiltration membrane is consistent with the Donnan model.With the study of nanomaterials,carbon nanotubes(CNTs)are considered to be the most effective adsorbents for many contaminants due to their large specific surface area,catalytic potential[23].The potential for sidewall functionalization and surface modification has made it more widely applicable in water treatment [24].However,carbon nanotubes have significant limitations in practical applications due to many defects such as solubility and agglomeration.Therefore,the modification of the surface loaded with active groups can effectively avoid the agglomeration of carbon nanoparticles and improve the adsorption performance of pollutants.Studies by Jangetal.have shown that iron compounds can effectively remove arsenic from wastewater [25].Simultaneously,some studies have shown that the double ligand on the surface of ferric chloride adsorbs arsenic by forming As(V)-Fe(III)binuclear complex,and the removal efficiency of arsenic is remarkable [26-28].Since the strong arsenic removal ability of iron salts,the carbon nanotubes were modified with ferric nitrate as the immobilization material.

2.Experimental

2.1.Materials and instrumentation

Arsenic standard (mg.L-1,purchased from Beijing East-West Analytical Instrument Co.,Ltd.,China) mainly containing total arsenic,As(V) and As(III));carbon nanotubes (CNTs,purity ＞95%,purchased from Shanghai Maclean Biochemical Technology Co.,China).Sulphuric acid (H2SO4),hydrochloric acid (HCl),potassium iodide (KI),ascorbic acid (C6H8O6),potassium borohydride(KBH4),etc.,purchased from Tianjin Zhiyuan Chemical Reagent Co.,China.All the substances used in this study were of analytical grade and did not require further purification.Deionised water was used in all experiments.

The instruments used during the experiments included the AA-7003 automatic flame/graphite furnace atomic absorption instrument(Shanghai East-West Analytical Instruments Co.,Ltd.,China),101C-1B constant temperature blast oven (Shanghai Chongming Experimental Instrument Factory,China),DZF-6020 vacuum drying oven (Shanghai Yiheng Scientific Instruments Co.,Ltd.,China),SHY-2 rotary water bath constant temperature oscillator (Jiangsu Jintan Automatic Instrument Factory,China),PHS-3C pH meter(Hangzhou Qiwei Instrument Co.,Ltd.,China),ZSX100e X-ray fluorescence spectrometer (Rigaku,Japan),etc.

Instruments used for experimental characterisation include scanning electron microscopy (SEM,JSM-7500F,JEOL,Japan),energy dispersive spectroscopy (EDS,7021-H,Horiba,Japan),transmission electron microscopy (TEM,JEM2100F,JEOL),Brunauer-Emmett-Teller (BET,ASAP 2460,Micromeritics,USA),Fourier transform infrared spectroscopy (FT-IR,Tensor 27,Brooke,Germany),X-ray photoelectron spectroscopy (XPS,TAXIS-ULTRA DLD-600 W,Shimadzu,Japan),etc.SEM characterisation is used to observe the microscopic morphology,particle size,elemental distribution,elemental valence and bonding,crystal structure,grain boundary structure and composition of the material.SEM is also used in combination with EDS for qualitative and quantitative analysis of the sample material.TEM characterisation is used to obtain the internal or surface microstructure of the sample material before and after modification.BET characterisation is used to obtain the specific surface area,pore volume and pore diameter of the material before and after modification.FT-IR is used to determine the structure of functional groups and unknowns contained in the sample material.XPS is used to measure and determine the elemental composition,chemical and electronic states of the sample material.

2.2.Synthesis of the samples

In the experiment,concentrated sulfuric acid and concentrated nitric acid with a volume ratio of 3:1 were refluxed at room temperature,5.0 g of carbon nanotubes were treated with 100.0 ml of mixed acid,washed to neutral with deionized water,and dried to constant mass at 110 °C.

Next,the pretreated carbon nanotube materials were modified by ferric nitrate impregnation.The pretreated carbon nanotubes were immersed in 5% (mass) ferric nitrate solution with a solidliquid ratio of 1:5,the mixture was immersed in a 35°C water bath for 24 h,and oscillated at a rate of 150 r.min-1to fully mix.Then,filter and dry the mixture in an electric blast drying oven at 120°C until constant mass.Finally,the sample is cooled to room temperature and stored in a dryer.Blank samples are labeled as CNTs,and FCNT represents carbon nanotube composites modified by ferric nitrate.

2.3.Experimental methods

Four groups of experiments were carried out,and the experimental conditions are shown in Table 1.The first group of experiments were carried out at 5,15,25,35 and 45 °C for 1 h respectively,and then the supernatant was collected and the concentration of total As(V) in its water was measured to study the effect of test temperature on adsorption.In the second group of experiments,the effects of different initial pH values on adsorption were studied.In this experiment,5 parts of 0.1 g FCNT were added to 1 L of simulated As(V)containing wastewater at 35°C,the reaction time was 1 h,and the pH values were 2,3,4,5,6,7,8 and 9 respectively.In the third group of experiments,the effects of different reaction times on adsorption were studied.30,50 and 100 mg FCNT were added to a liter of simulated As(V)containing wastewater at 35°C,and the reaction times were 3,5,10,15,20,25,30,40,60,90 and 120 min,respectively.In the fourth group of experiments,the effects of different doses on adsorption were studied.Add 20,40,60,80,100,120 mg of CNTs and FCNT to 1 L of simulated arsenic containing wastewater at 35°C for 1 h,and then collect the supernatant to determine the concentration of total As(V)in the water.

Table 1 The experimental reaction conditions.

After adsorption equilibrium,the supernatant was absorbed and pretreated,and the concentration of residual As(V) was measured by atomic absorption spectrophotometer.The removal efficiency of As(V) (ηAs(V),%) over the catalysts was defined herein using Eq.(1).

whereC0andCe(mg.L-1) are the initial concentration and adsorption equilibrium concentration of the supernatant.

2.4.Adsorption isotherm

The adsorption experiment was carried out at 35 °C and pH=7.0(adjusted by 1 mol.L-1NaOH or HCl).100 ml of As(V)solution with a concentration range of 0.5-20 mg.L-1) was used.Then,10 mg of FCNT were added to the above arsenic solution.The equilibrium experiment was carried out in a constant temperature oscillation chamber for 24 h.Then,the sample solution was separated by an TGL-16 desktop high speed refrigerated centrifuge(3600 r.min-1).The initial and residual concentrations of arsenic were determined with an AA-7003 automatic flame/graphite furnace atomic absorption instrument.The specific amount of arsenic adsorbed was calculated from the following Eq.(2):

whereqe(mg.g-1)is the equilibrium adsorption capacity,C0andCe(mg.L-1) are the initial and equilibrium concentration of the adsorbate in solution,respectively,Vis the volume (ml) of the arsenic aqueous solution andmis the mass (mg) of adsorbent used in the experiments.

3.Results and Discussion

3.1.Synthesis of modified carbon nanotube composites

3.1.1.SEMandEDScharacterization

SEM images of CNTs and FCNTs are shown in Fig.1,with 1 μm and 50 μm images selected for analysis.The SEM photos of the modified carbon nanotubes show that the iron salt nanoparticles are distributed loosely on the paper carbon nanotubes.

Fig.1.SEM images of carbon nanotubes before (left) and after (right) modification: (a) CNTs and (b) FCNTs.

As can be seen in Fig.1,the CNTs have a dense pore-like structure with no fluffy or gelatinous agglomerates on the surface,with severe agglomeration and no obvious delamination.After ferric nitrate,their dispersion becomes better and the voids between individuals become larger.As part of the pore structure was covered or filled by ferric nitrate,there were granular substances on the surface,and the specific surface area and pore capacity were reduced by the previous BET analysis,but the adsorption of arsenic was increased by the loaded ferric nitrate using the physicochemical method.At the same time,there are many stacked fold areas on the surface of the FCNT,which are caused by the stacking of iron salts,indicating that more active groups are added to the surface of the FCNT.In addition,the EDS energy spectrum also confirms the presence of iron salts on the FCNT surface.Also,as can be seen in Fig.2,a higher Fe peak appears on the FCNT surface in addition to C,N and O,indicating that the CNTs were successfully modified and that iron compounds were indeed solidly loaded on their surface.

Fig.2.EDS energy spectrum of FCNT.

3.1.2.TEMcharacterization

TEM was used to investigate the internal and surface microstructure of CNTs and FCNTs.TEM images of CNTs (Fig.3(a)and (b)) show that the agglomeration of unmodified CNTs is consistent with the results of SEM images (Fig.1(a)).TEM images of FCNTs (Fig.3(c) and (d)) show that the flocculation of FCNTs has increased,with more and more uniform pores between individuals and a greater of accessibility.

Fig.3.TEM images of CNTs (a and b) and FCNT (c and d).

3.1.3.BETcharacterization

The adsorption-desorption isotherm of N2is one of the most common methods for characterizing pore structure of porous materials.In general,if the N2adsorption-desorption isotherms of mesoporous materials follow the type IV trend,single molecular layer adsorption occurs mainly in a low relative pressure range,and then the multi molecular layer adsorption.The isotherm will be a leap until the relative pressure enough to make capillary condensation.Normally,the larger the aperture is,the higher the pressure of the capillary condensation.

As can be seen in Fig.4,the isotherms of the CNTs and FCNT follow the type IV trend with a hysteresis loop type H3based on IUPAC.It can be seen that the pore shape of the materials all includes splinting slit structure,crack and wedge structure,etc.From the two curves of adsorption and desorption,the adsorption curves of CNTs and FCNT were relatively flat in the low relative pressure range (0-0.4),which was mainly due to the adsorption of N2as a monolayer in the pore channels of the samples.However,at high relative pressure,between 0.4 and 1.0,the amount of adsorption-desorption varies greatly,which was a typical capillary adsorption phenomenon with homogeneous pore distribution,which indicated that the pore structure is regular.

Fig.4.Adsorption-desorption isotherms and pore size distribution of the CNTs before and after modification.CNTs,blank carbon nanotubes;FCNT,modified carbon nanotube composites.

Table 2 Mesoporous structures of all the CNTs and FCNT.

The micro morphology and data characterized by SEM,EDS and BET proved that CNTs were successfully modified to FCNT by ferric nitrate.The specific surface area and pore volume of CNTs decreased after the modification of ferric nitrate,and the specific surface area decreased by 19.07%and the cumulative pore volume decreased by 8.62%,indicating that there were ferric nitrate and iron oxide loaded on the surface of CNTs,which blocked part of the pores and reduced the contact between the internal pores of the material during the adsorption process,resulting in the decrease of specific surface area.The large pore area of the FCNT was significantly reduced,and the average pore size was reduced by more than 50%,indicating that the iron compounds blocked some of the large pores of CNTs themselves after the ferric nitrate modified,which increased the proportion of pores occupied by micropores and small pores,and the iron compounds on the surface of the modified materials also contributed to the porosity and specific surface area.

3.2.Effect of main environmental factors on the adsorption of As(V)

3.2.1.Effectofthereactiontemperature

The effect of the reaction temperature on As(V) adsorption by the FCNT and CNTs at an initial As(V) concentration of 1 mg.L-1is depicted in Fig.5.

It can be seen from the comparison that the ηAs(V)of the FCNT was obviously higher than that of CNTs in all temperature ranges of the experiments,indicating that the removal of As(V) was improved obviously over the FCNT.In addition,Fig.5 indicated that the ηAs(V)over the FCNT and CNTs increased when temperature increases from 5 to 35 °C [29,30].When the reaction temperature went up to 35 °C,the ηAs(V)of the FCNT and CNTs were 94.5% and 99.1% respectively,and the equilibrium concentrations were 55 and 9 μg.L-1,respectively.As a result,we found that the ηAs(V)of the lower temperature was less than that of the higher temperature.At the lower temperature,the diffusion rate of As(V) was slower,and the effective collisions between the As(V) and the adsorption sites were reduced and insufficient,prolonging the time of adsorption equilibrium.As the reaction temperature increased further,we observed a slightly decrease in arsenic removal.This phenomenon may be attributed that part of the As(V) adsorbed on the surface of the adsorbents were easy to desorb at higher temperature,so the ηAs(V)was decreased.Besides,the mobility of ions would increase with the increase in temperature,as a result surface precipitation would decrease.Therefore,the optimum reaction temperature is 35 °C.

3.2.2.EffectofthepH

The effect of pH on As(V)adsorption is shown in Fig.6.It can be seen from the figure that when the pH is changed from 2 to 7,the removal rate obviously increases and remains basically above 94%,thus it can be seen that when the pH is acidic,it is favorable for the adsorption of arsenic ions by the modified carbon nanotube composites.

Fig.6.Effects of pH on the adsorption of As(V).

This may be due to the H+solution is more conducive to the adsorbent surface to produce more Fe2+,Fe2+can be quickly oxidized to Fe3+,and the redox reaction and the adsorption and coprecipitation of arsenic ion are more intense.However,when the pH is greater than 7,the removal rate decreases markedly with the increase of pH,which may be due to the large amount of OH-in the solution precipitating Fe3+on the surface of the adsorbent rapidly as ferric hydroxide precipitation,greatly reducing the probability of the arsenic ion and Fe3+precipitation,so that the adsorption rate decreased.Therefore,to determine the experimental reaction pH value of 7.At a pH of less than 3,the removal rate of As(V) is more obvious,possibly due to the redox reaction of Fe3+with excess H+,indicating that the coprecipitation of Fe3+with arsenic ions is very weak at low pH,resulting in reducing absorption efficiency.However,under extremely low pH conditions,coprecipitation was very weak and the adsorption rate of pentavalent gods was still over 95%.This also shows that the adsorption coprecipitation has little effect on the removal efficiency of As(V).

Oh, yes, said the little man, You were just about to run away, but you have taken upon you to stand sentinel in the church to-night, and there you must stay

3.2.3.Effectofreactiontime

The effect of reaction time on As(V) adsorption is shown in Fig.7.It can be seen from the figure that under three different dosages,the ηAs(V)is approximately the same with time.

Fig.7.Effects of the reaction time on the adsorption of As(V).

The ηAs(V)can be divided into two stages: rapid adsorption stage and slow adsorption stage.The adsorption time of the rapid adsorption phase was the first 20 min of adsorption,and the ηAs(V)of FCNT rose rapidly to 80%,89.3%,and 98% respectively with the increase of the reaction time.Subsequently,this relatively slow adsorption phase appeared.As the reaction time increased,the ηAs(V)had an insignificant increase and gradually became balance.The ηAs(V)of FCNT reached 81%,91.3%,and 98.7%respectively,and there was almost no change thereafter.This illustrated that the FCNT adsorbents reached the adsorption saturation within 20 min.Considering the experimental adsorption effect and experimental process,1 h was selected as the adsorption time condition for subsequent experiments.

3.2.4.Effectofadsorbentdosage

Fig.8 shows the effect of different adsorbent dosages on As(V)removal efficiency.It can be seen from the figure that the FCNT has a good removal effect on simulated wastewater and a very small amount of the FCNT also has high removal efficiency.

Fig.8.Effects of adsorbent dosage on the adsorption of As(V).

The number of As(V) adsorption sites increased with the enhancement of FCNT adsorbent dosage,the As(V) removal efficiency enhanced,and more As(V) were adsorbed onto the surface of FCNT.At the same time,the amount of Fe2+Fe3+and Fe(OH)3were also enlarged,which made it easier to form FeAsO4precipitate and coprecipitate with As(V).When the dosage of FCNT was 100 mg.L-1,the ηAs(V)reached 98.8%.As the dosage of FCNT was increased,the ηAs(V)tended to maintain stable.Considering the above experimental results and economic costs,100 mg.L-1was defined as the optimum dosage of the FCNT adsorbents.

3.2.5.Adsorptioncycleexperiment

To investigate the regeneration performance of the modified FCNTs,this study carried out a total of eight elution-sorption experiments under identical conditions.As shown in Fig.9,the adsorption efficiency of the pre-modified CNTs was drastically reduced,with their re-effectiveness halved after 5 cycles and only 35% adsorption efficiency after 8 cycles.In contrast,the FCNT’s regeneration efficiency was above 90% after 8 cycles as well.That is,FCNT has excellent regeneration performance.

Fig.9.Results of cycling experiments with CNTs and FCNT (pH=7,T=35 °C,time=2 h,dosage=0.1 g.L-1,V=10 ml).

3.3.Adsorption isotherm

The adsorption isotherms of As(V) for the FCNT indicated that the adsorption capacity of the as-prepared FCNT significantly increased with the increase of initial As(V) concentrations.To further investigate the adsorption type and describe the relationship between adsorbent and adsorbate,the adsorption data was fitted by the Langmuir,Freundlich and Temkin adsorption isotherms.The Langmuir [31],Freundlich [32] and Temkin isotherm are represented by Eqs.(3)-(5) respectively:

whereCe(mg.L-1)andqe(mg.g-1)are,respectively,the equilibrium concentration and capacity.qmandbare the Langmuir constants related to monolayer sorption capacity(mg.g-1)and sorption equilibrium constant (L.g-1),respectively.kfandn(dimensionless) are the Freundlich constant related to the adsorption capacity and affinity.A(J.mol-1) andB(g.L-1) are the Temkin constants.

Fig.10 shows the fitting curves of As(V)adsorption on FCNT by Langmuir and Freundlich models,and the calculated isotherm parameters are shown in Table 3.The Freundlich model is the most suitable to describe the adsorption process of As(V)onto the adsorbent according to the correlation coefficients (R2=0.9702),suggesting that the adsorption of As(V) on FCNT nanocomposites is mainly monolayer adsorption.The maximum adsorption capacity(Qmax) of the FCNT calculated from Langmuir model was 112.4 mg.g-1for As(V),which was remarkably higher than that of many adsorbents such as Zr-loaded resin (11.28 mg.g-1) [33],manganese-incorporated iron (III) oxide-carbon nanotubes(14.42 mg.g-1) [34] and amorphous ZrO2nanoparticles(32.4 mg.g-1) [35].Moreover,the synthesized FCNT has a significant removal effect on As(V) due to its excellent chemical and physical adsorption capacity.The adsorption capacity of the prepared FCNT was higher than that of many reported adsorbents,as shown in Table 4.

Fig.10.Fitting curves of As(V) adsorption by Langmuir,Freundlich and Temkin models (C0=0.5-20 mg.L-1,35 °C,pH=7.0).

Table 3 Adsorption isotherms parameters for As(V) adsorption.

Table 4 Maximum adsorption capacities of various adsorbents for As removal.

3.4.Samples characterization analysis

3.4.1.FT-IRanalysis

In order to confirm the functional groups in absorbents,the infrared spectra of fresh and used absorbents are shown in Fig.11.

Fig.11.FTIR spectra of the (a) fresh and (b) used FCNT adsorbents.

As shown in Fig.11(a) and Table 5,the band at around 3437.97 cm-1is assigned to the C-H vibration of saturated C,the band at 2169.22 cm-1is the N-H stretching vibration of amines and amides,the band at 1631.60 cm-1is the C-O vibration of-COOH located at the edge of FTCN sheets,the band at 1383.99 cm-1can be attributed to the skeletal vibration of C=C in graphene sheets,confirming the existence of the sp2hybrid carbon skeleton,the band at 567.76 cm-1is the C-H surface bending vibration absorption of olefin.Since the reaction time was short(only 20 min),there was no significant change in the infrared spectrum of the fresh and used FCNT adsorbents.

Table 5 FT-IR absorption spectral data of CNTs before and FCNT.

3.4.2.XPSanalysis

The chemical state,surface atomic concentrations and dispersion of the main elements for the fresh and used FCNT were investigated by XPS and the results are summarized in Fig.12.

Fig.12.XPS spectra of the fresh CNTs and used FCNT over the spectral regions of (a) C 1s,(b) N 1s,(c) O 1s,(d) Fe 2p and (e) As 3d.

As shown in Fig.12(a)shows the C 1s spectrum,which is broad in one peaks.The spectrum can be resolved into three individual peaks: the binding energies of 284.8,285.6 and 288.9 eV are assigned to C-C/C-H (denoted as Cα),ether (C-O-C) (denoted as Cβ) and ester (—O—CO—) (denoted as Cγ) respectively.After the reaction of FC with As(V),Cα increased from 51.7% to 52.2%,Cβdecreased from 28.6% to 25.7%,and Cγ increased from 19.7%to 22.1%,which is consistent with the XPS results of O 1s.As shown in Fig.12(b),the spectrum of N 1s is broad into two peaks and divided into two peaks of 400.2 and 406.1 eV,the former peak is assigned to the group of amino (—NH2),the later one is ascribed to the formation ofAfter the reaction of FG with As(V),the formation ofdisappeared,and only left with the group of amino (-NH2).Therefore,it was deduced that the products contained N utterly transformed into the group of amino (—NH2)on the surface of FCNT in the reaction process.As shown in Fig.12(c),the XPS spectra of O 1s could be divided into three deconvoluted peaks at 531.3,532.2 and 533.6 eV respectively.The first peak was attributed to chemisorbed or hydroxyl oxygen(denoted as Oα),the second was assigned to(denoted as Oβ)and the third was corresponded to carbonyl oxygen (-CO-) (denoted as Oγ),in which,the functional groups are ascribed to the partial oxidation of the CNTs during modification.The percentages of Oβincreased significantly,Oα completely disappear and Oγ reduced slightly as the reaction proceeds,indicating that all hydroxyl oxygen is consumed during the reaction with arsenic and lead to moreand moreis released in the reaction of Fe(NO3)3withto form FeAsO4precipitate.Fig.12(d) shows the Fe 2p spectrum.The peaks at 711.49 and 711.56 eV are considered to be Fe2+cations and the peaks at 714.49 and 718.94 eV in the centre belong to Fe3+cations.The percentage of Fe2+(Fe2+/(Fe2++Fe3+))was 33.16% in the unreacted material,increasing to 50.02% in the reacted material.This study shows that Fe3+can trap electrons efficiently,increasing to 16.86% in the reacted material.Due to its radius (0.69°),Fe3+can trap electrons efficiently.Furthermore,the redox properties of the Fe2+/Fe3+combination are much higher.Therefore,Fe2+is easily oxidised to Fe3+by H+or adsorbed O2.Fig.12(e) shows the As 3d spectrum,which is indicates that the arsenic on the surface of the adsorbent is mainly trivalent arsenic after the reaction.

3.5.Discussion

In this experiment,the FCNT is very effective in adsorbing pollutant As(V),and the adsorption process may not only be a simple physical adsorption but also a process of chemisorption and coprecipitation [36-41].The FCNT removes arsenic by both adsorption of surface-loaded iron compounds and adsorption by CNTs.Adsorption mechanism as shown below:

3.5.1.AdsorptionofAs(V)byferricsaltoncarbonnanotubessurface

Fig.13 shows the removal mechanism of arsenic by ferric salt on the surface of the FCNT.When the FCNT is added into the arsenic-containing wastewater,the ferric salt on the surface of the FCNT undergoes a hydrolysis reaction,so that the pH of the solution decreases [42] showed that Fe(OH)3precipitates when the pH was between 1.87 and 3.20:

Fig.13.The removal mechanism of arsenic by ferric salt loaded on the surface of FCNT.

At pH ＜7,trivalent arsenic exists as an arsenic acid molecule,andappears only after pH ＞7.However,ferrous arsenate is very difficult to form,so only the formation of arsenic ferric arsenate:

When pH ＞10,the reaction betweenand Fe(OH)3in solution would proceed in the following form:

In addition,Fe(OH)3precipitation and As(V)coprecipitation can also remove part of As(V).

Fig.14 shows the mechanism of arsenic removal by FCNT.The main mechanism of FCNT removing As(V) in water is due to the interaction between arsenic ions and oxygen-containing functional groups such as carboxyl groups and hydroxyl groups on the surface of FCNT [43,44],as well as electrostatic adsorption,ion exchange and complexation.These surface hydroxyls increase the electron density on the surface of the FCNT to charge its surface and also form hydrogen bonds with the hydrated As(III) and As(V) ions,which are held together by charge attraction and hydrogen bonding As(III) and As(V) ions in aqueous enforcements.The C on the surface of FCNT bonds with O into form C-O-As bidentate coordination mononuclear complex and bidentate coordination dinuclear complex,so As(III)and As(V)is fixed on the surface of FCNT.

Fig.14.The removal mechanism of arsenic over of FCNT.

In addition,the carbon nanotube’s large specific surface area and rich pore structure also were beneficial to adsorb As in water.

3.5.2.Migrationandtransformationofarsenicionsadsorbedon carbonnanotubesurface

The stability of ferric salt to arsenic adsorbed on carbon nanotube surface is mainly sedimentation and adsorption.Adsorption also includes non-specific adsorption (physical adsorption) and specific adsorption (chemical adsorption),pH can both change the surface charge of the particles,but also affect the formation of ferric hydroxide colloid,which in turn affect both non-specific and specific adsorption.Oxygen oxidizes the arsenite ions in the solid phase into arsenate ions,which react with the ferric ions loaded.Ferric hydroxide generated ferric hydroxide,adsorbed arsenate,arsenate,etc.,to replace the ferric hydroxide surface polynuclear ligand complex ions -OH,based ligand,generated by the coordination of the amorphous state of ferric arsenate precipitation.

4.Conclusions

In this work,the mechanism,behaviour and application of carbon nanotubes modified with ferric nitrate for the adsorption of arsenic in aqueous solution were investigated experimentally,FCNT was prepared by impregnation method using ferric nitrate as the active agent.The results of adsorption isotherm model fitting showed that arsenic may be adsorbed as a monolayer cover with energy uniformly distributed on the adsorbent surface.The maximum adsorption of FCNT for As(V) was up to 118.3 mg.g-1,which is higher than most of the adsorbents reported in the literature.Various characterization results showed that the adsorption mechanism of As(V) included the following aspects: (i) chemical reaction between iron ions and arsenate ions to produce iron arsenate precipitates;(ii) electrostatic adsorption,which existed between positively charged arsenic ions and negatively charged adsorbent surface;(iii) co-precipitates,which occurred between iron hydroxide precipitates and As(V);(iv) hydrogen bonding,in which hydroxyl groups on the adsorbent surface and hydrated As(III)and As(V)ions to form hydrogen bonds,which immobilize free As(III) and As(V) ions in water;(v) pore filling originates from the specific surface area of carbon nanotubes,which form a porous structure.In addition,As(V) also undergoes migration and transformation on the adsorbent surface to further remove As(V) from water.Finally,the present work concludes that FCNT has stronger dispersion,larger surface charge area and more surface active sites for arsenic removal by electrostatic adsorption,chelation,redox reaction and co-precipitation together with low dosage,fast adsorption,high resistance to interference (temperature and pH)and easy separation(magnetic properties).At the same time,FCNT also has excellent stability and reproducibility,with good adsorption effect after 8 cycles,indicating that FCNT is a promising adsorbent and has a broad application prospect in the field of arsenic pollution in groundwater.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (NSFC) on the micro behavior of heavy metal migration and transformation in lead-zinc tailings and its nano micro scale high targeted stabilization mechanism (51968033) and the National Key Research and Development Program ‘‘long-term solidification of heavy metal tailings pollution/environmental functional materials,technologies and equipment of stabilizers”(2018YFC1801702).