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Impact of Humic-fulvic Acid on the Removal of Heavy Metals From Aqueous Solutions Using Nanomaterials- A Review

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  Review Impactofhumic/fulvicacidontheremovalofheavymetalsfromaqueoussolutions using nanomaterials: A review Wang-Wang Tang, Guang-Ming Zeng ⁎ , Ji-Lai Gong ⁎ , Jie Liang, Piao Xu, Chang Zhang, Bin-Bin Huang College of Environmental Science and Engineering, Hunan University, Changsha, 410082, PR ChinaKey Laboratory of Environmental Biology and Pollution Control, Ministry of Education, Hunan University, Changsha 410082, PR China H I G H L I G H T S ã  The review outlined heavy metals' removal from water by nanomaterials affected by HA/FA. ã  Potential mechanisms involved in the interactions were discussed. ã  Environmental implications of HA/FA to nanomaterials and heavy metals were evaluated. ã  Outlook for further challenges and potential development was also offered. a b s t r a c ta r t i c l e i n f o  Article history: Received 18 August 2013Received in revised form 15 September 2013Accepted 15 September 2013Available online 4 October 2013Editor: D. Barcelo Keywords: NanomaterialsHumic/fulvic acidHeavy metalsInteraction mechanism Nowadays nanomaterials have been widely used to remove heavy metals from water/wastewater due totheir large surface area and high reactivity. Humic acid (HA) and fulvic acid (FA) exist ubiquitously in aquaticenvironmentsandhaveavarietyoffunctionalgroupswhichallowthemtocomplexwithmetalionsandinteractwith nanomaterials. These interactionscannot only altertheenvironmentalbehaviorofnanomaterials, butalsoin 󿬂 uence the removal and transportation of heavy metals by nanomaterials. Thus, the interactions and theunderlying mechanisms involved warrant speci 󿬁 c investigations. This review outlined the effects of HA/FA onthe removal of heavy metals from aqueous solutions by various nanomaterials, mainly including carbon-basednanomaterials, iron-based nanomaterials and photocatalytic nanomaterials. Moreover, mechanisms involvedin the interactions were discussed and potential environmental implications of HA/FA to nanomaterials andheavy metals were evaluated.© 2013 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10152. Heavy metals' removal affected by HA/FA using various nanomaterials and the underlying mechanisms . . . . . . . . . . . . . . . . . . . 10152.1. Carbon-based nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10162.1.1. Carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10162.1.2. Graphenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10162.2. Iron-based nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10172.2.1. Zero-valent iron nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10172.2.2. Iron oxide nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10202.3. Photocatalytic nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10212.3.1. Titania nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10212.3.2. Zinc oxide nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10222.4. Other miscellaneous nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10233. Environmental implications of HA/FA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10234. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1024Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025 Science of the Total Environment 468 – 469 (2014) 1014 – 1027 ⁎  Corresponding authors. Tel./fax: +86 731 88823701. E-mail addresses:  zgming@hnu.edu.cn (G.-M. Zeng), jilaigong@gmail.com (J.-L. Gong). 0048-9697/$  –  see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.scitotenv.2013.09.044 Contents lists available at ScienceDirect Science of the Total Environment  journal homepage: www.elsevier.com/locate/scitotenv  1. Introduction Water pollution has become a critical issue worldwide. The qualityofwaterresourcesisdeterioratingdaybydayduetopopulationgrowth,rapiddevelopmentofindustrialization,agriculturalactivities,andothergeological and environmental changes (Chong et al., 2010; Zeng et al.,2011, 2013a). The continuous release of various contaminants such asheavy metals and organic compounds into the environment is causinggrowing concern to the whole world (Deng et al., 2013; Zeng et al.,2013b;Chenetal.,2013).Heavymetalsareparticularlyproblematicbe-cause, unlike most organic contaminants, they are non-biodegradableand can accumulate in living tissues, posing great threat to bothhuman health and ecological environment (Lesmana et al., 2009).Themostcommonheavymetalsmainlyincludemercury,cadmium,lead, chromium, arsenic, zinc, copper, nickel, cobalt, etc. These metalions can cause toxicities and serious side effects toward human health.For example, copper has universally been considered to be very toxicat high concentration. It can cause copper poisoning in humans suchas gastrointestinal problems, kidney damage, hair loss, nausea, anemia,hypoglycemia, severe headaches and even death (Tang et al., 2012;RahmanandIslam,2009).Cadmiumisatoxicheavymetalofsigni 󿬁 cantenvironmental and occupational concern. It has been identi 󿬁 ed as ahuman carcinogen and teratogen substance severely impacting lungs,kidneys, liver and reproductive organs (Waalkes, 2000; Filipic, 2012).Chromium exists in the environment both as Cr (III) and Cr (VI)forms. However, Cr (VI) is  󿬁 ve hundred times more toxic than Cr (III)and toxicity of Cr (VI) includes skin irritation, lung cancer, as well askidney, liver, and gastric damage (Kumar et al., 2007; Selvi et al.,2001). Consequently, the need for heavy metals' removal has becomea must.To date, various methods have been proposed for ef  󿬁 cient heavymetal removal from waters, including but not limited to coagulation,chemical precipitation, membrane  󿬁 ltration, reverse osmosis, solventextraction,  󿬂 otation, ion exchange and adsorption (Xu et al., 2012b;Chen et al., 2010; Hua et al., 2012; Fu and Wang, 2011; Wang et al.,2003;Ali,2012).Inthelasttwodecades,nanotechnologyhasdevelopedwith its applications in almost all branches of science and technology(KaurandGupta,2009;SavageandDiallo,2005).Withtherapiddevel-opmentofnanotechnology,therehasbeenagreatdealofinterestinen-vironmentalapplicationsofnanomaterials.Nanomaterialsareexcellentadsorbents and catalysts (Khin et al., 2012). Since nanomaterials offersigni 󿬁 cant improvement with extremely high speci 󿬁 c surface area,numerous associated sorption sites, low temperature modi 󿬁 cation,short intraparticle diffusion distance, tunable pore size and surfacechemistry compared to other materials ( Ju-Nam and Lead, 2008; Quet al., 2013; Chen et al., 2007), extensive research have been carriedout to remove heavy metals from wastewater by developing andusing various nanomaterials.Humic substances typically represent a large portion of naturalorganic matter (NOM) distributing in soils, sediments and waters(Morales et al., 2012). They are straw-colored, hydrophobic organicacids that are mainly derived from soil humus and plants. However, ithasbeenalsoreportedthat3 – 28%ofthedissolvedorganicmatterinef- 󿬂 uentsfromwastewatertreatmentplantsarehumicsubstances(Mouraet al., 2007; Imai et al., 2002). Thus, humic substances are very impor-tant compounds often encountered in the environment as a result of their extensive ubiquity (Matilainen et al., 2011). Humic acid (HA)and fulvic acid (FA) are two major components of humic substances.Generally, HA and FA differ in molecular weight, elemental and func-tional group contents. HA is higher in molecular weight and containsless oxygen-containing functional groups compared with FA (Güngörand Bekbölet, 2010; Weng et al., 2006). Despite the differences, theircommon functional groups such as carboxyl, phenol, hydroxyl, amineand quinine groups make it possible that HA and FA in waters causemany serious environmental and health problems. HA/FA has strongcomplexation ability with heavy metals and thus increases theirtransportation in waters. In addition, HA/FA can react with chlorineduring water treatment, thereby producing carcinogenic disinfectionbyproducts such as trihalomethanes (Zhao et al., 2008; Wang et al.,2008, 2010b). Literature reports some approaches for removal of HA/FA from water including coagulation/ 󿬂 occulation, membrane separa-tion, advanced oxidation, ion exchange, adsorption etc. (Imyim andPrapalimrungsi, 2010; Tao et al., 2010).In general, HA/FA and heavy metals exist simultaneously in theenvironment, where they may affect each other's behavior (Li et al.,2012; Yang et al., 2011; Mak and Lo, 2011; Wang et al., 2009a). AsnanomaterialsarewidelyusedtoremovebothHA/FAandheavymetalsfromwater/wastewater,thepresenceofHA/FAmayin 󿬂 uencetheinter-action between heavy metals and nanomaterials. Similarly, the pres-ence of heavy metals may also in 󿬂 uence the interaction between HA/FA and nanomaterials. Meanwhile, it is also noteworthy that withtheir large production and widespread application, nanomaterials willinevitably interact with HA/FA once they are released into theenviron-ment, which may alter the surface properties, stability and transporta-tion of nanomaterials and then affect the mobility and transportationof heavy metals in the environment, thus possibly enhancing the bio-availability and toxicity of heavy metals (Hyung and Kim, 2008; Zhangetal.,2011b;YangandXing,2009).Therefore,itisofconsiderableprac-tical interest to study the effects of HA/FA on the removal of heavymetals by different nanomaterials in waters as well as the potentialenvironmental implications of HA/FA. Recently, extensive relevantstudies have been done. However, to the best of our knowledge, up tonow a review on this topic is still lacking.This review highlights the removal of heavy metals by variousnanomaterials, mainly including carbon-based nanomaterials, iron-based nanomaterials and photocatalytic nanomaterials, as affected byHA/FA in batch and column systems. The phenomena, factors andpotential mechanisms involved in the process are also discussed thor-oughly. Meanwhile, the potential environmental implications of HA/FA to nanomaterials and heavy metals are evaluated. Finally, futureperspectives are offered to inspire more exciting developments in thispromising 󿬁 eld. 2. Heavy metals' removal affected by HA/FA using variousnanomaterials and the underlying mechanisms Nanomaterials refer to materials on the nanoscale level betweenapproximately 1 nm and 100 nm (Stone et al., 2010). Generally,nanomaterials can be categorized into carbon-based nanomaterialssuch as carbon nanotubes and graphenes, and inorganic nanomaterialsincluding the ones based on metal oxides and metals. Combinations of different nanomaterials are also developed ( Ju-Nam and Lead, 2008).Nanomaterials hold great promise of reducing contamination of heavymetals. Meanwhile, the production, use and disposal of nanomaterialswill inevitably lead to discharges to aquatic environment. HA and FA,which are ubiquitous in natural environment, effectively in 󿬂 uence theremoval and transportation of heavy metals. The strong complex abili-ties between metal ions and HA/FA could also in 󿬂 uence the underlyingmechanisms involved in the interaction among the three components(heavy metals, HA/FA and nanomaterials).Presently, numerous analytical techniques are being applied in de-termining the physicochemical properties of NOM and nanomaterialsto elucidate the reaction process. C, H and N contents of NOM can bedetermined using an elemental analyzer. Total acidity values of NOMare determined by the Ba(OH) 2  titration method. The carboxyl groupsare determined by the calcium acetate method. Phenolic alcohol groupcontent is calculated by subtracting the carboxyl content from thetotal acidity (Tian et al., 2012). Molecular weight distribution of NOMcan be obtained with size exclusion chromatography (SEC) (Sun et al.,2012). The redox state changes in NOM can be shown by  󿬂 uorescencespectroscopy (Mak and Lo, 2011). For nanomaterial characterization,many analytical instruments are used. For example, scanning electron 1015 W.-W. Tang et al. / Science of the Total Environment 468 – 469 (2014) 1014 – 1027   microscope (SEM) and transmission electron microscope (TEM) arewidelyusedtoobservethemorphologyandsizeofnanomaterials.Vibrat-ing sample magnetometer (VSM) is utilized to measure the saturationmagnetization of magnetic nanomaterials. Extended X-ray absorption 󿬁 ne structure (EXAFS) and X-ray absorption near edge structure(XANES) are performed to analyze the possible species of sorbed heavymetals on nanomaterials. Fourier transform infrared spectroscopy(FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS),thermogravimetric analyzer (TGA), atomic force microscopy (AFM),dynamic light scattering (DLS), Zeta Meter and Raman spectrometerare employed to analyze chemical surface groups, crystalline structure,surface elemental composition, weight composition, particle surfacemorphology, aggregation condition, zeta potential and binding sites of nanomaterials, respectively (Peralta-Videa et al., 2011).In this part of review, attempts have been made to discuss heavymetals' removal and transportation in waters affected by HA/FA usingvarious nanomaterials and the underlying mechanisms.  2.1. Carbon-based nanomaterials 2.1.1. Carbon nanotubes Carbonnanotubes (CNTs), mainlyincludingsingle-wallednanotubes(SWCNTs)andmulti-wallednanotubes(MWCNTs)(Trojanowicz,2006),havebeenwidelystudiedregardingtheirpotentialenvironmentalappli-cation as superior adsorbents for heavy metals (Perez-Aguilar et al.,2011; Lu and Chiu, 2006; Peng et al., 2005) and organic compounds(Yangetal.,2006;LuandSu,2007;Choetal.,2008;Lietal.,2013a)dur-ing solid-phase extraction and wastewater treatment. However, mostcurrent research is focused on the adsorption of single solute by CNTsinaqueoussolutionandignoresthepotentialinteractionsbetweenmix-tures of metal ions and organic substances that may affect adsorption(Chen et al., 2008; Ren et al., 2011; Li et al., 2013b, 2013c). Since humicsubstances are ubiquitous in the environment, a multiple solute system(HA/FA and heavy metals) may adequately represent the most of mixed contaminant systems commonly encountered in wastewaterand natural water systems. Therefore, it is critical to investigate theadsorption behavior and mechanism of heavy metals by CNTs in theabsence/presence of HA/FA.HA/FAisexpectedtohavesigni 󿬁 cantin 󿬂 uenceonmetalionsorptionbyCNTsbecausethehydrophobicpartsandaromaticmonomersofHA/FA have strong af  󿬁 nity to CNTs through hydrophobic and  π – π  interac-tions, and meanwhile the hydrophilic fractions of HA/FA have variousfunctional groups, such as carboxyl, phenol, hydroxyl, amine and qui-ninegroupsthatcanbindheavymetals(Linetal.,2012).Also,itshouldbenotedthatthedissolvedHA/FAandCNT-boundHA/FAmayhavedif-ferenteffects.Thesurface-boundHA/FAcouldgreatlyincreasemetalionsorption on CNTs because the surface-bound HA/FA could introduceoxygen-containing functional groups and negative charges to theCNTs, and thus increases the apparent sorption of metal ions throughchemical complexation and electrostatic attraction, respectively. Forinstance, the surface-bound HA signi 󿬁 cantly promoted Pb (II) sorptionon MWCNTs although it would decrease surface areas of the MWCNTs(Lin et al., 2012). The mechanismwas that Pb (II)could be electrostati-cally attracted into outer-sphere of the electric double layer of the HA-coated MWCNTs, while a fraction of the loaded Pb (II) would formcoordination complexes with carboxyl groups in the inner-sphere orouter-sphere. Similar results were also observed for Cd (II) sorptiononto HA-coated MWCNTs (Tian et al., 2012).AsfortheeffectsofdissolvedHA/FAonthesorptionofheavymetalson CNTs, the process becomes more complicated. Both HA and FA arenegatively charged in the pH range of 3.0 – 10.0 (Sheng et al., 2010). Inacidic and neutral solutions, dissolved HA/FA could enhance the sorp-tion of heavy metals by CNTs because the negatively charged HA/FAcould be easily adsorbed on the positively charged surfaces of CNTsdue to the electrostatic attraction. Now that most of HA/FA wasadsorbed on CNTs, the subsequent interaction was similar to that of surface-bound HA/FA. The strong complexation ability of surfaceadsorbed HA/FA with metal ions resulted in the increase of metal ionsorption on CNTs. However, in alkaline solutions, dissolved HA/FAcould reduce the sorption of heavy metals by CNTs because of oppositeeffectof electrostatic repulsion.Most of HA/FAwould exist freely in so-lutionandthenformssolublecomplexesofHA/FA-metalions,andthusleadstothedecreaseofmetalionsorptiononCNTs.Forexample,Shenget al. (2010) found a positive effect of HA/FA on Cu (II) adsorption atpH  b  7.5,whereasanegativeeffectatpH  N  7.5byMWCNTs.Inaddition,initial HA/FA concentrations played a role in the adsorption of heavymetals on CNTs. The adsorption isotherms of Cu (II) onto MWCNTs athigher initial HA/FA concentrations were higher than those of Cu (II)at lower HA/FA concentrations (Sheng et al., 2010). Moreover, it wasfound that Cu (II) adsorption by MWCNTs increased with increasinginitial NOM concentrations at both low and high pH values (Sun et al.,2012), which was different from the behavior observed for some sub-stancessuchasaromaticcompounds(Wangetal.,2009b).Inthemean-time, it's noteworthy that there was a higher binding af  󿬁 nity betweenCu (II) and FA compared to HA, due to more functional groups (e.g.,OHand/orCOOH)inFAthatcouldcomplexwithCu(II)aswellashigherpolarities.Toimprovetheadsorptionperformanceandavoidthedisadvantageof CNTs in adsorption process (e.g. easy aggregation and inherentinsolubility), various CNT-based composites have been synthesized toexploretheeffectivenessofmetalions'removalunderdifferentcircum-stances. Speci 󿬁 cally, combining the magnetic properties of iron oxidewith adsorption properties of CNTs is of increasingly environmentalconcern as a rapid, effective and promising technology for removinghazardous pollutants in water and has been proposed for widespreadenvironmental applications in wastewater treatment and potentiallyinsituremediation(Tangetal.,2012).AlthoughCNT-basedcompositeshave better performances compared with CNTs, the in 󿬂 uence of HA/FAontheremovalofheavymetalsbyCNT-basedcompositesmightremainsimilar to that by CNTs (Wang et al., 2011b; Liu et al., 2013; Yang et al.,2011; Hu et al., 2012). For instance, It was found that the presenceof HA/FA enhanced Co (II) sorption on magnetic multiwalled carbonnanotube/iron oxide composites (MWCNTs/IO) at low pH values, butsuppressedCo(II)sorptionathighpHvalues(Wangetal.,2011b).Sim-ilar phenomena were also found in the sorption of Co (II) on MWCNT – hydroxyapatite composites in the presence of HA/FA (Liu et al., 2013)and in the simultaneous removal of Pb (II) and HA by MWCNTs/poly-acrylamide composites from aqueous solutions (Yang et al., 2011). Inthe simultaneous removal of Pb (II) and HA by MWCNTs/polyacryl-amide, adsorption mechanisms were indicated based on the fact thatdifferent effects of HA/Pb (II) concentrations and addition sequencesonPb(II)andHAadsorptionwereobserved.Theproposedmechanismsof Pb (II) and HA adsorption on MWCNTs/polyacrylamide were sche-matically shown in Fig. 1.Additionally, combining the excellent adsorption capacity of CNTswith other remarkable properties such as photocatalysis to removeheavy metals affected by HA/FA is also worth studying. In the ternarysystems HA/FA – Cr(VI) – TiO 2 /MWCNT composites, the increase of HA/FA concentration did not cause any drastic changes in the adsorptioncapacityofCr(VI)inthedark,butaminorincreasingtrendforthepho-tocatalytic reduction of Cr (VI). The presence of HA/FA enhanced thephotocatalytic reduction and adsorption of Cr (III) to TiO 2 /MWCNTcomposites (Tan et al., 2008).  2.1.2. Graphenes Asrelativelynewadsorbents,graphene-basedadsorbentshavebeenproven to possess extremely great adsorption capacity for removingorganic and inorganic pollutants from water/wastewater (Zhu et al.,2012; Yang et al., 2013b; Wu et al., 2011). Recently, many researchershave been focusing their efforts on investigating the potential applica-tion of graphene oxide nanosheets in removing heavy metals fromwaters because unlike CNTs, which require special oxidation processes 1016  W.-W. Tang et al. / Science of the Total Environment 468 – 469 (2014) 1014 – 1027   to introduce oxygen-containing functional groups to improve metalion sorption, graphene oxide nanosheets can contain many oxygen-containinggroupssuchascarboxyland hydroxyl groups onthe surfacewhen they are prepared from graphite using Hummers method (Zhaoet al., 2011). However, the application of graphene oxide nanosheetsin the removal of heavy metals from aqueous solutions in the presenceof HA/FA is still scarce. Only few relevant investigations have been car-ried out. For example, one type of graphene-based material (few-lay-ered graphene oxide nanosheets) has been synthesized and used asadsorbentsfortheremovalofCd(II)andCo(II)fromaqueoussolutionsinthepresenceofHA(Zhaoetal.,2011).Resultsindicatedthatthepres-ence of HA reduced Cd (II) sorption on few-layered grapheme oxidenanosheets. For Co (II) sorption, the presence of HA reduced Co (II)sorption at pH  b  8, while at pH  N  8 no obvious difference was foundin the presence or absence of HA. The strong surface complexationand high surface site density of graphene oxide nanosheets may resultin the decreased adsorption of Cd (II) and Co (II) in the presence of HA at pH  b  8. This revealed that the surface properties of adsorbentsin 󿬂 uenced the effect of HA on the adsorption of heavy metals.Li et al. (2012) studied the simultaneous removal of FA and Cu (II)from aqueous solution by graphene oxide nanosheets decorated withFe 3 O 4  nanoparticles (GO/Fe 3 O 4 ). Results showed that: 1) the presenceofFAledtoastrongincreaseinCu(II)sorptionatlowpHandadecreaseat high pH; 2) FA concentration greatly in 󿬂 uenced Cu (II) sorptionanddifferentFAconcentrationsdisplayedsigni 󿬁 cantlydifferenttrends;and3)additionsequenceshadnoimpactonCu(II)sorptionatpH  b  5.5,butdifferenceswereobservedinthethreedifferentadditionsequencesatpH  N  6.0(Fig.2).Thisresearchcon 󿬁 rmedthatGO/Fe 3 O 4 whichcom-bined the high sorption capacity of GO and the separation convenienceof Fe 3 O 4  can be used as an effective sorbent for the simple and rapidremoval of inorganic and organic pollutants from water samples. Inaddition,thesorption – desorptionprocessindicatedexcellentregenera-tion capacity of GO/Fe 3 O 4  by using HNO 3  (pH 2.0).In summary, there is still much room for further exploration inconsideration of the widespread application of graphene-based mate-rials and, more importantly, the limited number of experiments aboututilizinggraphene-basedmaterials to remove heavy metalsfrom aque-ous solution in the presence of HA/FA.  2.2. Iron-based nanomaterials 2.2.1. Zero-valent iron nanomaterials Among the most relevant zero-valent metal nanomaterials to envi-ronmentalsystems,zero-valentiron(ZVI)nanomaterialshavereceivedgreatattention due to their potential applicationsin theremediation of contaminated groundwater (Elliott and Zhang, 2001; Quinn et al.,2005). Hexavalent chromium and arsenic species are contaminantscommonly found in groundwater, caused by various sources, such aswood preservatives and unlined land 󿬁 lls for construction and de-molitionwastes (Weber et al., 2002). However, Cr (VI)and As (V) con-tamination has raised much health concern because of their toxic,carcinogenic and mutagenic properties (Smedley and Kinniburgh,2002). ZVI has been proven to be capable of removing Cr (VI) and As(V) from groundwater effectively. The removal mechanism of Cr (VI)byZVImainlyinvolveschemicalreductionofCr(VI)toCr(III),withsub-sequentCr(III)precipitationasCr(III)hydroxidesandmixedFe(III)/Cr(III) (oxy)hydroxides (Alowitz and Scherer, 2002), while the removalmechanism of As (V) by ZVI is primarily via the adsorption onto or co-precipitation with the iron corrosion products (Lackovic et al., 2000).Humicsubstancesareubiquitouslypresentingroundwater.Itisimpor-tanttoknowhowtheywouldaffecttheremovalofCr(VI)andAs(V)byZVI.Unfortunately,tosomeextent,studiescarriedoutinthepresenceof HA/FArevealed thattheeffectof humic substances on ZVI treatment inaqueous solutions was controversial.In a recent study, HA was found to have an inhibitory effect on Cr(VI) removal by ZVI nanoparticles, especially at low concentrations Fig. 1.  Proposed schematic mechanisms of Pb (II) and HA adsorption on MWCNTs/polyacrylamide.Adopted from Yang et al. (2011).1017 W.-W. Tang et al. / Science of the Total Environment 468 – 469 (2014) 1014 – 1027 
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