Environmental Science Nano · 2019. 7. 2. · Environmental Science Nano PAPER Cite this: DOI: 10.1039/c8en01289j Received 15th November 2018, Accepted 28th January 2019 DOI: 10.1039/c8en01289j

EnvironmentalScienceNano

PAPER

Cite this: DOI: 10.1039/c8en01289j

Received 15th November 2018,Accepted 28th January 2019

DOI: 10.1039/c8en01289j

rsc.li/es-nano

Enhanced debromination oftetrabromobisphoenol a by zero-valent copper-nanoparticle-modified green rusts†

Liping Fang, a Ru Liu,b Ling Xu,b Ji Li,b Li-Zhi Huang *c and Fangbai Li a

Green rusts (GRs) interlayered with Cl−, SO42−, and CO3

2− were used to reduce tetrabromobisphenol A

(TBBPA), which is the most widely used brominated flame retardant. The modification of GRs by copper

nanoparticles (Cu NPs) greatly enhanced the reductive reactivity of GR. GRĲCl)–Cu NPs with a Cu NPs con-

tent of 0.5% had the highest reactivity toward TBBPA reduction. High pH did not favor TBBPA reduction by

both GR(Cl) and GRĲCl)–Cu NPs. The presence of SO42− and CO3

2− inhibited TBBPA reduction by GR(Cl)

and GRĲCl)–Cu NPs, whereas the inhibition effect of PO43− and humic acid was much greater than those

for SO42− and CO3

2−. The TBBPA reduction by GRĲCl)–Cu NPs could be explained by a galvanic cell model,

where electrons are transferred from GR(Cl) to Cu NPs, where TBBPA reduction occurs. Our findings dem-

onstrate that nature-occurring Fe-bearing minerals may play an important role in the reductive transforma-

tion of TBBPA besides the biotransformation and that GRĲCl)–Cu NPs can be used for the efficient removal

of reducible halogenated pollutants.

1. Introduction

Tetrabromobisphenol A (TBBPA) is the most widely used bro-minated flame retardant in the world. TBBPA is a potentialneurotoxicant and a thyroid endocrine disruptor, whichmeans it poses potential risks to the ecological environmentas well as to human beings.1 As a persistent pollutant, TBBPAhas been detected in soil, sediment, sludge, surface water,etc.2 TBBPA undergoes reductive bio-debromination underanoxic conditions, such as found in sediments and ground-water. The complete bio-debromination of TBBPA tobisphenol A (BPA) with BPA as a final debromination product

has been observed under both methanogenic (within ∼55days) and sulfate-reducing (within ∼112 days) conditions.3 Asa potential sink for TBBPA, anoxic sediments consist of iron-bearing minerals, which play an important role in the reduc-tive transformation of reducible pollutants. Green rusts (GRs)are one of the most reactive iron-bearing minerals found iniron-rich anoxic sediments and soils. GRs are intermediatephases formed by the partial oxidation of FeĲII) minerals orthe partial reduction of FeĲIII) (hydr)oxides.4 GRs are a type ofmixed FeĲIII) and FeĲII) iron minerals with a unique layeredstructure,5 and have the general formula of [FeII(1−x)Fe

IIIx -

ĲOH)2]x+ [(x/n)An−, mH2O]

x−, where An− is the intercalated an-ion (An− = Cl−, SO4

2−, CO32−) and x is the molar fraction of

FeĲIII). As a strong reductant, GRs play an important role inthe transformation of reducible pollutants, such as chlori-nated solvents,6,7 antimony,8 nitrate,9 chromium,10 neptunyl(NpVI),11 uraniumĲUVI),12 and technetium (TcVII).13 The reduc-tive dehalogenation of TBBPA by GRs is rarely studied, al-though GRs may play an important role in the transforma-tion of TBBPA.

Environ. Sci.: NanoThis journal is © The Royal Society of Chemistry 2019

aGuangdong Key Laboratory of Integrated Agro-environmental Pollution Control

and Management, Guangdong Institute of Eco-Environmental Science & Technol-

ogy, No. 808, Tianyuan Road, Guangzhou 510650, Chinab Faculty of Material Science and Chemistry, China University of Geosciences, No.

388, Lumo Road, Wuhan 430074, Chinac School of Civil Engineering, Wuhan University, No. 8, East Lake South Road,

Wuhan, P.R. China. E-mail: [email protected]

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8en01289j

Environmental significance

As one of the most used brominated flame retardants, TBBPA is widely found in the environment. The microbial mineralization of TBBPA is usuallybelieved to be the main reason for TBBPA removal in organic-carbon-rich environments. During the microbial mineralization process, TBBPA is first reduc-tively debrominated to BPA under anaerobic conditions, and is then subsequently mineralized under aerobic conditions. GR is usually found in FeĲII)-containing anaerobic sediment and soils. This work shows that the reductive debromination of TBBPA can also proceed at appreciable rates in abiotic sys-tems in the presence of GRs. The modification of GR by zero-valent Cu NPs can greatly enhance the reductive debromination of TBBPA, and the engineeredGR can thus be used as a potential remediation reagent for TBBPA removal.

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Environ. Sci.: Nano This journal is © The Royal Society of Chemistry 2019

Owing to their high reductive reactivity and the opportu-nity to inject reactive GRs into contaminant plumes,engineered synthetic GRs have been widely used asdehalogenation reductants in environmentalremediation.14–16 The intercalation of surfactant anions intoGR can greatly enhance the reductive dehalogenation due tothe sorption of hydrophobic halogenated pollutants onto thesurfactant interlayered GRs.14 The presence of amino acids,such as glycine, can reduce the formation of toxicbyproducts, i.e., chloroform, during the dehalogenation ofcarbon tetrachloride by GRs.17 Our recent work demonstratedhow the dehalogenation reactivity of GR can be turned off/onby a silicate/glycine switch, which overcome the problems ofthe reactivity loss during the application of GRs for soil andgroundwater remediation.16 The modification of GRs by ametal is another efficient way to enhance the reductive reac-tivity of GRs, and the enhancement options could even becombined, showing a synergistic effect for the significantlyimproved reactivity of GRs.15

The introduction of a second metal, such as Cu, Ni, Pb,Pt, Ag, or Au, to iron-bearing reductants could significantlyincrease the reductive activity of iron-bearing reductant.18–24

A galvanic cell model has been generally accepted to explainthe enhanced reactivity, where iron-bearing reductants areoxidized and transfer electrons to the generated reactivemetal nanoparticles. Oxidant pollutants are reduced muchfaster on the surface of the generated reactive metal nano-particles compared to iron-bearing reductants, which resultsin an enhancement of the reductive reactivity. The introduc-tion of Au or Ni nanoparticles (NPs) to ZVI has beenreported to be an effective way to enhance thedebromination of TBBPA by ZVI.22,23 Cu is one of the low-cost and abundant metals, and Cu-modified iron-bearing re-ductants are promising for real applications. Cu-modifiedZVI bimetallic systems have shown high reductivedehalogenation reactivity.25,26 Cu-modified GR systems oftenuse Cu2+ as a precursor for the in situ generation of zero-valent Cu nanoparticles on the GR surface;27 however, thedirect modification of GRs by zero-valent Cu has never beenstudied. On the other hand, the formation of mono-valentCu rather than the zero-valent form of Cu was observed afterthe addition of Cu2+ into GR suspensions in our recentwork.15 The formation of zero-valent Cu is important forensuring high reductive reactivity because of the higherelectron-donating property and higher electron conductivityof zero-valent Cu compared to mono-valent Cu. Further-more, the in situ generation of active Cu species from Cu2+

reduction by GRs results in the oxidation of GRs to otheriron oxides with less electron-donating capabilities, whichmay suppress the electron transfer from GRs to Cu.

In this work, the influences of interlayered anions, thepH, and background anions on the reductive debrominationof TBBPA by GRs were investigated. Furthermore, Cu NPswere introduced to a GR reductant to enhance the reductivedebromination of TBBPA by GRs. The enhanced reactivity ofGRĲCl)–Cu NPs was systematically compared to GR and GR

modified with Cu2+, and the mechanism for the enhanceddebromination was investigated.

2. Materials and methods2.1 Synthesis and characterization of GRs

GRs interlayered with Cl−, SO42−, and CO3

2− were synthesizedthrough the air oxidization of a ferrous hydroxide [FeĲOH)2]precipitate in aqueous solution.28 The synthesis takes placein a 500 mL beaker. For the synthesis of Cl−-interlayered GR(GR(Cl)), 0.035 mol FeCl2·4H2O was dissolved in 50 mL 1 molL−1 NaOH under magnetic stirring. Then, 150 mL deionizedwater was quickly added to form an FeĲOH)2 suspension. TheFeĲOH)2 suspension was partially oxidized in air with mag-netic stirring at 380–400 r min−1. The GR synthesis was com-pleted when the pH was about 7.8 and the oxidation timewas about 60 min. For the synthesis of SO4

2−-interlayered GR(GRĲSO4)), 0.035 mol FeSO4·7H2O was used and the synthesistime was 90 min. CO3

2−-Interlayered GR (GRĲCO3)) was syn-thesized via vigorous stirring of the as-synthesized GRĲSO4)suspension in the presence of 7.42 g NaCO3 for 3 h. The as-synthesized GRs were transferred to a glovebox, washed withO2-free deionized water, and stored in the dark till furtheruse.

2.2 Synthesis of Cu NPs

The Cu NPs were synthesized using a liquid-phase reductionmethod.18 Briefly, 0.05 mole CuSO4·5H2O was dissolved in 50mL deionized water, followed by the addition of 0.7 mole gla-cial acetic acid under magnetic stirring. The pH of the solu-tion was adjusted to around 10 with ammonium hydroxide.Then, 0.1 mole hydrazine hydrate was added, followed bystirring for 1 h. The Cu NPs were collected via centrifugationat 11 000 r min−1 for 15 min. The as-synthesized Cu NPs werewashed with O2-free ethanol and water, and then stored in aglovebox until further use.

2.3 Synthesis of GRĲCl)–Cu NPs and GRĲCl)–Cu2+

GR interlayered with Cl− anions was used for the synthesis ofGRĲCl)–Cu NPs. First, 0.035 mol FeCl2·4H2O was dissolved in50 mL 1 mol L−1 NaOH under magnetic stirring. Then, 150mL deionized water was quickly added to form FeĲOH)2 sus-pensions. The Cu NPs were sonicated for 10 min and addedinto the FeĲOH)2 suspensions. The mixture of Cu NPs andFeĲOH)2 suspension was partially oxidized in air with mag-netic stirring at 380–400 r min−1. The synthesis was com-pleted when the pH was about 7.8 and the oxidation timewas about 60 min. For the synthesis of GRĲCl)–Cu2+, the as-synthesized GR(Cl) was amended with CuSO4 solution,followed by being well dispersed by stirring for 300 min. Theas-synthesized GRĲCl)–Cu hybrid with different Cu mass load-ings were designated as GRĲCl)–Cu NPsĲX) or GRĲCl)–Cu2+(X),X = 0.2%, 0.5%, 1.0%, 2.0%. For the measurement of the GRand GRĲCl)–Cu NPs concentration, the as-synthesized GRs orGRĲCl)–Cu NPs suspension was magnetically stirred for 10

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min in an anaerobic environment. A certain volume of GRsor GRĲCl)–Cu NPs suspension was transferred to a centrifugetube. The supernatant was removed after centrifugation, andthe remaining sludge was freeze dried for 24 h. The concen-tration of the suspensions was calculated by the dry mass/thevolume of the suspensions.

2.4 TBBPA degradation

The debromination of TBBPA by GR(Cl), GRĲSO4), andGRĲCO3) was carried out in Tris buffer solution (1 mol L−1,pH = 8.0). The GR dosage was 0.5 g L−1 (added as a suspen-sion), while the initial TBBPA concentration was 15 mg L−1,and the reaction temperature was 25 °C. The reaction mix-ture was shaken on a shaking table and sampled at 10, 30,60, 180, 360, and 960 min. The TBBPA in each sampled sus-pension was determined by high performance liquid chroma-tography (HPLC) after filtration using 0.22 μm mixed cellu-lose ester membrane. The removal efficiency for TBBPA bythe reductants was calculated by eqn (1):

Removal efficiency e CC0

100% (1)

The mass normalized removal efficiency of TBBPA (qe) wascalculated by eqn (2):

qC C V

mee

0 100% (2)

where C0 and Ce are the initial and final concentration ofTBBPA (mg L−1), V is the volume of reaction mixture (L), andm is the mass of GR (g).

The effect of Cu NPs content on TBBPA reduction byGRĲCl)–Cu NPs was studied using GRĲCl)–Cu NPsĲX), X =0.2%, 0.5%, 1.0%, 2.0% wt%. TBBPA reduction by GRĲCl)–Cu2+(X) and by pure Cu NPs was conducted as a comparison.The dosage of reductants was 0.5 g L−1, while the initialTBBPA concentration was 15 mg L−1, and the reaction tem-perature was 25 °C. The reaction suspensions were sampledat 20, 30, 60, 120, 300, and 720 min followed by TBBPA deter-mination (see below). The effect of pH on TBBPA reductionby GR(Cl) and GRĲCl)–Cu NPs was studied in Tris buffer withdifferent initial pH values from 8.0 to 10.5. The reaction wascarried out for 720 min, followed by TBBPA determination. Inorder to study the effect of background anions on TBBPAdebromination, different background anions, includingSO4

2−, CO32−, PO4

3−, and humic acid (in terms of carbon con-tent) were added to the reaction suspensions with a final con-centration of 10, 20, 30, 40, and 50 mg L−1. The reaction wascarried out in Tris buffer at pH 8.0. All the experiments werecarried out under strictly O2-free conditions in the glovebox(O2 < 0.1 ppm). Thus, possible ˙OH generation from O2 acti-vation could be excluded.

2.5 Analytical methods

The concentration determination of FeII and FeIII in the GRsand in the solutions were performed in the glovebox using amodified phenanthroline method.29,30 Cu2+ and Cu+ in theGR–Cu NPs samples were determined using the 2,9-dimethyl-1,10-phenanthrolinespectrophotometric method at 457 nm.31

Total Cu (Cu2+ + Cu+) was determined after the reduction ofCu2+ to Cu+ with excess hydroxylamine hydrochloride, andCu2+ was calculated by subtraction. TBBPA was analyzed byan HPLC instrument (Agilent 1260) equipped with an C18column (5 μm, 150 mm × 4.6 mm). The eluent consisted of10 mM phosphoric acid solution (pH = 3) in acetonitrilemixture (1/4, v/v). The injection volume was 20 μL and theflow rate was 0.8 mL min−1. UV-vis detection was carried outat a wavelength of 219 nm. The concentration of Br− ionswas determined by ion chromatography (IC, 882 Compact ICPlus, Switzerland) equipped with a Supp 5 analytical column(100 × 4 mm i.d.). The eluent consisted of 0.30 mol L−1

NaHCO3 and 0.24 mol L−1 Na2CO3 and the flow rate was 1.4mL min−1. The extent of debromination was calculated asper eqn (3):

Debromination extentBr

TBBPA

N100% (3)

where [Br−] is the concentration of Br anions in the reactionsolution, [TBBPA] is the initial concentration of TBBPA, andN is the number of Br atoms in TBBPA (N = 4).

The debromination products of TBBPA were identifiedusing an Agilent 1200 series liquid chromatography instru-ment coupled to an Agilent 6410 electrospray triple quadru-pole mass spectrometer (LC-MS) with an Agilent ZORBAXEclipse Plus C18 reversed-phase column (25 cm × 4.6 mm × 5mm particle size; Agilent, Santa Clara, USA). The mobilephase was a mixture of methanol/water (v/v, 8/2) and the flowrate was 1 mL min−1. The mass spectrometric analysis wasperformed using electrospray ionization (ESI) in the negativeion mode with a scan range from 45 to 700 amu.

X-ray diffraction (XRD) data were collected in the angular2θ range 5–60° using a Bruker AXSD8-Focus X-ray diffractom-eter (Germany). Transmission electron microscopy (TEM) im-ages were obtained using a JEOL transmission electronmicroscope with an accelerating voltage of 200 kV. The reduc-tants were suspended in O2-free water under vigorous stir-ring. Droplets of the suspensions were dried on a microscopecarbon grid, followed by TEM observation. Scanning electronmicrocopy (SEM) images were observed by a Quanta 200microscope (FEI Company, USA) operating at 10 kV.

3. Results and discussions3.1 Characterization of GRs and GRĲCl)–Cu NPs

The XRD patterns of the as-synthesized GRĲSO4), GRĲCO3),and GR(Cl) were in agreement with the observation by others(Fig. 1a–c).28 The XRD patterns of GR(Cl) and GRĲCl)–Cu NPs

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were almost the same, demonstrating the GR structure waskept intact in the presence of the Cu NPs (Fig. 1d). In con-trast, the XRD patterns of GRĲCl)–Cu2+ showed that GR(Cl) istransformed to lepidocrocite and magnetite upon the addi-tion of Cu2+ (Fig. S1†). The XRD patterns of Cu NPs showeddiffraction peaks at 2θ values of 43.6° and 50.8°, correspond-ing to the (111) and (200) planes of fcc structure of Cu(Fig. 1e).32 No diffractions from Cu NPs were observed in theXRD patterns of GRĲCl)–Cu NPs, which may be attributed tothe low concentration of Cu NPs in the GRĲCl)–Cu NPs. Theas-synthesized GR(Cl) particles had a hexagonal morphologywith a lateral dimension of 100–400 nm, as shown by TEM(Fig. 2a). Cu NPs with a particle size of 20–100 nm were lo-cated on the basal plane of GR(Cl) (Fig. 2b). High-resolutionTEM showed the (111) plane of Cu, which resulted in diffrac-tion peaks at 43.6° in the XRD pattern.

3.2 TBBPA reduction by GR(Cl), GRĲSO4), and GRĲCO3)

The three types of GRs all showed debromination activity to-ward TBBPA, indicating GRs may play a role in the transfor-mation and fate of TBBPA in the environment. Thedebromination of TBBPA by GR(Cl), GRĲSO4), and GRĲCO3) allceased after 720 min. The reactivity of the GRs toward TBBPAreduction depended on the intercalated anions, with Cl−-interlayered GR displaying a higher reactivity than GRs inter-layered with SO4

2− and CO32− (Fig. 3a). The removed amount

of TBBPA by GR(Cl) was 21.44 mg g−1 with a removal effi-ciency of 71.47%, which was three times that by GRĲSO4) andaround seven times that by GRĲCO3). The structural FeII inGR with a high electron-donating capability was responsiblefor the reductive activity of the GRs. Thus GR(Cl) with ahigher FeII content could reduce TBBPA faster than GRĲSO4)and GRĲCO3).

33 Despite having the highest reductive strength,GR(Cl) was also less stable and easier to undergo dissolutionin the natural environment compared to GRĲSO4) andGRĲCO3).

16 On the other hand, GRĲCO3) was more stable thanGRĲSO4) and GR(Cl), but had the lowest activity towardTBBPA reduction. Thus, the transformation of TBBPA by thedifferent types of GRs in the natural environment could becomplex.

3.3 Effects of Cu NPs content on TBBPA reduction by GRĲCl)–Cu NPs

The presence of Cu NPs can enhance the reductive activity ofGR(Cl) toward TBBPA debromination. The reactivity ofGRĲCl)–Cu NPs increased with the Cu NPs content increasingfrom 0 to 0.5 wt% (Fig. 3b). GRĲCl)–Cu NPs(0.5%) showed thehighest TBBPA removal efficiency of 92.11%. Further increas-ing the Cu NPs content in GRĲCl)–Cu NPs from 0.5 to 2 wt%resulted in a decrease in the reactivity of GRĲCl)–Cu NPs to-ward TBBPA debromination (Fig. 3b), and the TBBPA removalefficiency dropped from 92.11% to 77.03%. In contrast, pure

Fig. 1 XRD patterns of GRĲSO4) (a), GRĲCO3) (b), and GR(Cl) (c),GRĲCl)–Cu NPs (d), and Cu NPs (e).

Fig. 2 TEM images of GR(Cl) (a) and GRĲCl)–Cu NPs (0.5%) (b). Insert in (b) is high-resolution TEM of Cu NPs in GR(Cl)–Cu NPs.


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Cu NPs showed negligible reductive reactivity toward TBBPAdebromination. These phenomena can be explained by a gal-vanic cell model, in which electrons are transferred fromGR(Cl) to the Cu NPs, where TBBPA reduction occurs. Cu NPsare stronger reductants than GR, which result in a fasterdebromination of TBBPA on Cu NPs than on GR. However,the limited reductive reactivity of pure Cu NPs suggests thepossible fast deactivation of the Cu NPs surface due to corro-sion in water. In the GRĲCl)–Cu NPs system, a galvanic cellmay form where the GR(Cl) dissolves and act as a “sacrificialanode” to avoid corrosion of the Cu NPs. Too high a Cu NPscontent in GRĲCl)–Cu NPs leads to an imbalanced proportionof electron donaters (GR(Cl)) and electron acceptors (CuNPs), which may result in a decrease in the overall reductiveactivity of the system.

The experimental kinetics were fitted to the pseudo-first-order and pseudo-second-order kinetics by a linear and anon-linear method (Tables 1 and 2). The experimental TBBPAremoval amount was very close to the theoretical values

obtained by the pseudo-second-order non-linear method.This suggested that the TBBPA debromination by GRĲCl)–CuNPs followed the pseudo-second-order kinetic model.

3.4 Effects of pH

The reactivity of both GR(Cl) and GRĲCl)–Cu NPs towardTBBPA reduction declined with the pH going from 8 to 10.5(Fig. 4a, S2 and S3†). When the reaction pH increased from8.0 to 10.0, the TBBPA removal efficiency by GR(Cl) decreasedfrom 60.24% to 20.29% and that by GRĲCl)–Cu NPs(0.5%) de-creased from 78.12% to 34.06%. Under alkaline conditions,GRs may be transformed to other types of iron oxides withlower reductive reactivity, and the electron-donating capabil-ity of the transformed GRs to Cu NPs may also besuppressed.34 This will eventually lead to a lower dissolutionof GR during the reaction. We thus determined the releasedfree Fe2+ in the reaction solution as a consequence of the GRdissolution. The concentration of released free Fe2+ in both

Fig. 3 TBBPA reduction by GRs interlayered with Cl−, SO42−, and CO3

2− (a); and the effects of Cu NPs content in GRs on the TBBPA reduction byGRĲCl)–Cu NPs (b). Experimental conditions: GRs and GRĲCl)–Cu NPs dosage = 0.5 g L−1, pH = 8, initial TBBPA concentration = 15 mg L−1.

Table 1 Pseudo-first-order linear kinetics parameters and nonlinear kinetics parameters for TBBPA removal by GRĲCl)–Cu NPs

Material

Pseudo-first-order linear kinetic parameters Pseudo-first-order nonlinear curve kinetic parameters

qexp1 (mg g−1) kobs (min−1) qcol (mg g−1) R2 qexp (mg g−1) kobs (min−1) qcol (mg g−1) R2

GR(Cl) 17.97 0.0032 16.55 0.6180 21.44 0.0018 19.29 0.9443GR–CuNPsĲ0.2%) 19.89 0.0121 20.22 0.8363 22.71 0.0056 21.53 0.9750GR–CuNPsĲ0.5%) 22.95 0.0185 22.27 0.8478 27.63 0.0084 25.36 0.9531GR–CuNPsĲ1.0%) 20.92 0.0163 21.83 0.8418 23.98 0.0073 22.92 0.9871GR–CuNPsĲ2.0%) 20.40 0.0148 21.50 0.8236 23.11 0.0066 22.20 0.9877

Table 2 Pseudo-second-order linear kinetics parameters and nonlinear kinetics parameters for TBBPA removal by GRĲCl)–Cu NPs

Material

Pseudo-second-order linear kinetic parameters Pseudo-second-order nonlinear curve kinetic parameters

qexp (mg g−1) kobs g mg−1 min−1 qcol (mg g−1) R2 qexp (mg g−1) kobs g mg−1 min−1 qcol (mg g−1) R2

GR(Cl) 21.44 0.1108 21.73 0.9996 21.44 0.0141 20.34 0.9755GR–CuNPsĲ0.2%) 22.71 0.1209 22.98 1.0000 22.71 0.0054 22.91 0.9986GR–CuNPsĲ0.5%) 27.63 0.0850 28.00 0.9998 27.63 0.0043 27.09 0.9907GR–CuNPsĲ1.0%) 23.98 0.1007 24.31 1.0000 23.98 0.0038 24.59 0.9985GR–CuNPsĲ2.0%) 23.11 0.1163 23.42 1.0000 23.11 0.0043 23.72 0.9984


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TBBPA–GRĲCl) and TBBPA–GRĲCl)–Cu NPs systems decreasedwith increasing pH (Fig. 4b), which suggested that a highTBBPA removal efficiency is correlated to a high GR dissolu-tion. The higher concentration of released Fe2+ ions in theTBBPA–GRĲCl)–Cu NPs system compared to the TBBPA–GRĲCl) system also confirmed the accelerated GR dissolutionin the galvanic cell model. Furthermore, the high pH resultsin negatively charged TBBPA molecules, which may repel thenegatively charged GR with a point of zero charge of ∼8.5.35

A similar phenomenon was observed for tetrachloroethene re-duction by GR(Cl) in the presence of CuĲII), where alkalineconditions decreased the reactivity of GRĲCl)–CuĲII).36 Acidicconditions were not explored due to the possible dissolutionof GR, which may have led to the release of free FeĲII) ionsinto the reaction solution with low reactivity.37

3.5 Effect of the background anions

The presence of SO42− and CO3

2− inhibited TBBPA reductionby GR(Cl) and GRĲCl)–Cu NPs, whereas the inhibition effectof PO4

3− and humic acid were greater than that of SO42− and

CO32− (Fig. 5). TBBPA (pKa1 = 7.5, pKa2 = 8.5) exists as an an-

ion form at pH 8, at which pH all the experiments were car-ried out. The sorption of negatively charged TBBPA onto thesurface of GRs may favor TBBPA reduction by GRs. Thus, thepresence of anions, such as SO4

2−, CO32−, and PO4

3−, has anegative effect on TBBPA reduction due to the competitionfor both the GRs and Cu reactive sites.

A very similar inhibition effect of SO42− and humic acid on

TBBPA reduction was observed in both the GRs and GR–CuNPs systems. In the presence of 50 mg L−1 SO4

2−, the TBBPAremoval efficiency decreased by 57% for GR(Cl) and 51% forGRĲCl)–Cu NPs. This was supported by the higher Fe2+ con-tent in GR(Cl) and GRĲCl)–Cu NPs after reaction in the pres-ence of SO4

2−. A huge negative effect of HA on TBBPA reduc-tion by GR (decrease of 93%) and GR–Cu NPs (decrease of86%) was observed. This could be attributed to the strong HAadsorption onto the GR surface through complex formation,

such as complexation between iron/iron oxide and the vari-ous types of functional groups in HA (e.g., carboxylate, phe-nolic, and carbonyl groups).38,39 The oxidation of GR by HAmay also inhibited TBBPA reduction by the GRs (seebelow).40,41

For CO32− and PO4

3−, the inhibition effect was greater inGR compared to in the GR–Cu NPs systems due to the al-tered reductive sites from GR to Cu NPs. In the presence of50 mg L−1 CO3

2−/PO43−, the TBBPA removal efficiency de-

creased by 69%/81% for GR(Cl) and 38%/48% for GRĲCl)–CuNPs. For the GR system, CO3

2− could react with Fe2+ andprecipitate as FeCO3 (siderite) on the reductant's surface,which inhibited TBBPA reduction by GR.42 Also, the higheraffinity between the positively charged Fe hydroxide layer inGR and the CO3

2− anions compared to Cl− anions may leadto the formation of GRĲCO3) via ion exchange. The lower re-activity of GRĲCO3) toward TBBPA reduction compared toGR(Cl) led to the decreased reactivity of GR(Cl) in the pres-ence of CO3

2−. In addition, the corrosion/dissolution of GRduring the reaction resulted in the release of ferrous ionfrom the surface of GRs. In the presence of PO4

3−, ferrousions easily precipitate as Fe3ĲPO4)2 on the surface of GRsand greatly inhibit TBBPA reduction.43 The slightly strongerinhibition effect by PO4

3− observed in the GR system com-pared to in the GR–Cu NPs system could be attributed tothe fact that Fe3ĲPO4)2 precipitate formation on Cu surfacemay be less favored compared to on the more ferrous-richGRs surface. This observation is in line with our proposedgalvanic cell model, where there is a shift of TBBPA reduc-tion site from the GR to the Cu surface in the GR–Cu NPssystem. For the GR–Cu NPs system, the inhibition effectonly results from the competitive adsorption for Cu activesites between CO3

2−/PO43− and TBBPA. Thus, the presence of

CO32−/PO4

3− has a greater inhibition effect in GR comparedto in GR–Cu NPs systems.

The effects of anions/humic acid on the change of thecontent and valence of Fe and Cu in the material were inves-tigated. In the presence of SO4

2−, PO43−, and CO3

2−, more Fe2+

Fig. 4 Effects of pH on TBBPA reduction by GR(Cl) and GRĲCl)–Cu NPs (a); and the concentration of Fe2+ in aqueous solution under various pHconditions (b). Experimental conditions: GRs and GRĲCl)–Cu NPs dosage = 0.5 g L−1, reaction time = 12 h, initial TBBPA concentration = 15 mg L−1.


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was left in both GR and GR–Cu NPs after reaction (Fig. 6). Incontrast, the Fe2+ content in GR(Cl) and GRĲCl)–Cu NPs was

slightly lower in the presence of HA (Fig. 6). This demon-strated that the presence of HA leads to the GR oxidation byHA. The oxidation of FeĲII) by HA has also been observed byothers.40,41 The combined effect of adsorption onto GR andthe consumption of electrons in GR by HA greatly inhibitedTBBPA reduction by the GRs. In the GR/Cu NPs system, Cu0

was transformed to Cu+ in the absence or presence of back-ground anions and humic acid (Fig. S4†). This was furtherconfirmed by the formation of Cu2O after GRĲCl)–Cu NPs re-action with TBBPA (Fig. S5†). The presence of background an-ions and humic acid had little influence on the Cu+ produc-tion. This demonstrated that the background anions andhumic acid mainly influence the reaction via interaction withthe GR.

3.6 Debromination products

The reductive debromination of TBBPA by GR(Cl) andGRĲCl)–Cu NPs resulted in the formation of tribromobis-phenol A (tri-BBPA), dibromobisphenol A (di-BBPA), andmonobromobisphenol A (mono-BBPA), which were identifiedby LC-MS. The related mass spectra are shown in Fig. S3.†Two isomers of di-BBPA were observed by LC-MS, which was

Fig. 5 Effects of SO42− (a), CO3

2− (b), PO43− (c) anions, and humic acid (d) on TBBPA reduction by GR(Cl) and GRĲCl)–Cu NPs. Experimental

conditions: GRs and GRĲCl)–Cu NPs dosage = 0.5 g L−1, reaction time = 12 h, pH = 8, initial TBBPA concentration = 15 mg L−1.

Fig. 6 The influence of SO42−, PO4

3−, CO32−, and humic acid on the

change of Fe2+ and Cu+ in GR(Cl) (a) and GRĲCl)–Cu NPs (b) afterTBBPA reduction. Experimental conditions: GR(Cl) and GRĲCl)–CuNPs(0.5%) dosage = 0.5 g L−1, pH = 8, reaction time = 12 h,concentration of anions and humic acid = 50 mg L−1, initial TBBPAconcentration = 15 mg L−1.


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in line with a previous study.23 The released Br− anions wereidentified by ion chromatography, with the extent ofdebromination shown in Fig. 7. The doping of Cu NPs intoGR(Cl) greatly enhanced the debromination rate and in-creased the debromination extent from ∼60% to ∼80%.

3.7 Mechanisms of the enhanced reduction

In order to further reveal the mechanism of the enhancedreduction of TBBPA by GRĲCl)–Cu NPs, the reactivity of theGRĲCl)–Cu NPs system was compared to that of the GRĲCl)–Cu2+ counterparts. The oxidation rate of FeII in GRs and therelease of Fe2+ cations into the reaction solution were alsoinvestigated. A higher reactivity of GRĲCl)–Cu NPs towardTBBPA reduction was observed compared to their GRĲCl)–Cu2+ counterpart (Fig. 8). This demonstrated that the addi-tion of Cu2+ into GR suspensions cannot generate the samesystem with a reactivity as high as the GRĲCl)–Cu NPs. Thiscould be attributed to: i) the copper species formed, i.e.,mono-valent copper is formed instead of zero-valent copper.

There was evidence of mono Cu generated in our GRĲCl)–Cu2+ system (Fig. S4 and S5†). Mono-valent copper has alower electron-donating capability than zero-valent copper,which results in a lower reactivity toward TBBPA reduction.Also, the lower electron conductivity of mono-valent coppermay result in a lower electron-transfer efficiency comparedto zero-valent copper, whereas a fast electron transfer be-tween GR, Cu, and TBBPA is essential for TBBPA reductionin a galvanic cell model; ii) the addition of Cu2+ inevitablyled to the oxidation of GR with the formation of less reac-tive iron oxides, as shown by the XRD data (Fig. S1†). Thisconsumes the electrons in the GR, which acts against theelectron-donating process from GR to Cu and the latter re-duction of TBBPA. This effect may have little impact of thereductivity since the Cu2+ addition was only 0.5%. However,the transformation of GR was evidenced even though theCu2+ dosage was low. This may be attributed to the instabil-ity of the GR structure. The transformation of GR to themore stable iron oxide may occur once the oxidation of GRwas triggered by the Cu2+ addition. On the other hand, we

Fig. 7 Evolution of Br− anions during TBBPA reduction by GR(Cl) (a) and GRĲCl)–Cu NPs(0.5%) (b). Experimental conditions: GR(Cl) and GRĲCl)–CuNPs(0.5%) dosage = 0.5 g L−1, pH = 8, initial TBBPA concentration = 15 mg L−1.

Fig. 8 Comparison of TBBPA reduction by GR(Cl), GRĲCl)–Cu NPs, and GRĲCl)–Cu2+ (a), and oxidation of structural FeII in GR(Cl), GRĲCl)–Cu NPs,and GRĲCl)–Cu2+ during TBBPA removal, respectively (b). Experimental conditions: reductants dosage = 0.5 g L−1, pH = 8, initial TBBPAconcentration = 15 mg L−1.


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did observe a lower reactivity in GR–Cu2+(0.5%) compared toGR–Cu NPs(0.5%), where no oxidation of GR occurred(Fig. 8a). We thus speculate that the formation of less reac-tive iron oxides was the reason for the decreased reactivity.This phenomenon can also be explained by the fact that thegenerated mono-valent Cu in the GR/Cu2+ system is less re-active than zero-valent Cu in the GR/Cu NPs system, butquantification of the contribution from each side is very dif-ficult. Furthermore, the generated Cu active sites are mostlikely in direct contact with the oxidized GR, i.e., less reac-tive iron oxides, which is a poor electron conductor/shuttleand suppresses the electron transfer from GR to Cu.

The oxidation rate of FeII in GR in the different systemsfollowed the same order of the TBBPA reduction rate: GRĲCl)–CuNPs > GRĲCl)–Cu2+ > GRCl. This implied that the electron trans-fer from GR to Cu occurs because the oxidation rate of FeII in GR

should be the same in different systems if GR and Cu both act asdiscrete reductants. GR(Cl) was transformed to magnetite (Fe3O4)and goethite (α-FeOĲOH)) after reaction with TBBPA (Fig. 9). Asimilar oxidation product was also observed after GR(Cl) wasreacted with nitrate.44 TBBPA reduction occurs on Cu NPs sur-face. Subsequently, Cu+ was produced as an intermediate productfrom Cu oxidation (Fig. 6b) and GR transformed to Fe3O4 andα-FeOĲOH) (Fig. 9):

C15H12Br4O2 + 3Cu → C15H15BrO2 + 3Br− + 3Cu+ (4)

5Cu+ + 3FeII3FeIII1 (OH)8Cl → 5Cu + 4Fe3O4 + 3Cl− + 8H+

+ 8H2O (5)

3Cu+ + Fe3Fe1(OH)8Cl → 3Cu + 4FeO(OH) + Cl− + 4H+ (6)

Thus all our evidence supported the galvanic cell mode:electrons were transferred from GR to Cu active sites, whereTBBPA was reduced. The proposed enhanced reduction ofTBBPA by GRĲCl)–Cu NPs is illustrated in Fig. 10.

The microbial mineralization of TBBPA is usually be-lieved to be the main reason for TBBPA removal in organic-carbon-rich environments.3 During the microbial mineraliza-tion process, TBBPA is first reductively debrominated to BPAunder anaerobic conditions, and is subsequently mineral-ized under aerobic conditions.45 GR is usually found inFeĲII)-containing anaerobic sediment and soils. This workshowed that the reductive debromination of TBBPA can alsoproceed at appreciable rates in abiotic systems in the pres-ence of GRs. The toxicity of TBBPA can be greatly decreasedafter debromination. However, the total organic carbon isnot deceased as no mineralization occurs during debromi-nation by GR. The debrominated TBBPA is less toxic andmuch easier to be degraded/mineralized microbially.46 Themodification of GR by zero-valent Cu NPs can greatly

Fig. 9 Oxidation products of GRĲCl)–Cu NPs after reaction withTBBPA.

Fig. 10 Proposed debromination pathway for TBBPA removal by GRĲCl)–Cu NPs.


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enhance the reductive debromination of TBBPA, and theengineered GR can thus be used as a potential remediationreagent for TBBPA removal.

4. Conclusion

GRs interlayered with Cl−, SO42−, and CO3

2− can reduceTBBPA with tri-BBPA, di-BBPA, mono-BBPA, and Br anions asreduction products. GR modified with Cu NPs was found tobe more reactive compared to the traditional Cu2+ modifica-tion. The optimum Cu NPs content in GRĲCl)–Cu NPs was0.5%, which showed the highest reactivity toward TBBPA re-duction. High pH did not favor TBBPA reduction in both theGR(Cl) and GRĲCl)–Cu NPs systems. The presence of SO4

2−

and CO32− inhibited TBBPA reduction by GR(Cl) and GRĲCl)–

Cu NPs, and the inhibition effect of PO43− and humic acid

were much greater than SO42− and CO3

2−. TBBPA reduction byGRĲCl)–Cu NPs could be explained by a galvanic cell model,whereby electrons are transferred from GR(Cl) to Cu NPs,where TBBPA reduction occurs. Our findings demonstratedthat natural-occurring Fe-bearing minerals may play an im-portant role in the reductive transformation of TBBPA be-sides the biotransformation and that GRĲCl)–Cu NPs can beused for the efficient removal of reducible halogenatedpollutants.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The current work was financially supported by the NationalNatural Science Foundation of China (Grant No. 21407131,21876161 and 41807188), and GDAS' Project of Science andTechnology Development (2019GDASYL-0102006).

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Environmental Science Nano · 2019. 7. 2. · Environmental Science Nano PAPER Cite this: DOI: 10.1039/c8en01289j Received 15th November 2018, Accepted 28th January 2019 DOI: 10.1039/c8en01289j