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Rapid Ferric Transformation by Reductive Dissolution of Schwertmannite for Highly Efficient Catalytic Degradation of Rhodamine B Jingyu Ran 1, * 1 2

*

ID

and Bo Yu 2

School of Chemical Engineering, Guizhou Institute of Technology, Guiyang 550003, China State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China; [email protected] Correspondence: [email protected]; Tel.: +86-851-8834-9057  

Received: 31 May 2018; Accepted: 5 July 2018; Published: 9 July 2018

Abstract: In this study, reductive dissolution of iron oxides was considered for the acceleration of the transformation from Fe(III) to Fe(II) to improve the degradation of rhodamine B (RhB) by potassium persulfate (PS) activation on schwertmannite. The addition of hydroxylamine (HA) showed an enhancement effect on the degradation at pH 3 and 5, but insignificant efficiency of the addition was obtained at pH 9. The surface reduction from Fe(III)-OH to Fe(II)-OH by HA was considered dominant for the acceleration of PS activation through the reductive dissolution process, and the hydroxyl and sulfate radicals generated by the decomposition of surface complexes were main primary reactive oxidants that contributed to the degradation of RhB. Keywords: ferric transformation; reductive dissolution; schwertmannite; heterogeneous

1. Introduction Advanced oxidation processes (AOPs) are considered efficient for the abiotic degradation of organic pollutants in water treatment due to the high oxidation activity of radicals towards organics, especially biological toxic and non-degradable organics, which are difficult to be degraded by conventional or biological oxidation methods [1]. Recently, sulfate radicals (SO4•− ) based AOPs has attracted considerable attention due to the high standard redox potential of SO4•− (2.6 V) [2], which is comparable to that of hydroxyl radicals (•OH, 2.8 V) [3]. As the important way to generate SO4•− , the activation of peroxymonosulfate (PMS) and persulfate (PS) has been widely studied via heating, UV irradiation and transition metals activated [4–12]. PS or PMS activation can also be achieved by transition metals [4,5,11]. Considering the environmental friendly nature and the advantages of cost effectiveness, Fe(II) has always been selected as the activator of PS or PMS for the generation of SO4•− [13–15]. However, Fe(II)/PS process shows slow transformation from Fe(III) to Fe(II), which further results in low efficiency for PS or PMS activation and radicals generation [11,15]. Although many efforts have been made to accelerate the transformation from Fe(III) to Fe(II) by employing UV irradiation [16], electrochemistry [17] and reducing agents [15,18], the accumulation of ferric oxide sludge and acidic pH conditions still limit the widespread application [19]. To avoid the accumulation of ferric oxide sludge and extend the condition to neutral pH, iron oxides such as ferrihydrite, lepidocrocite, goethite and hematite have been widely investigated as catalysts for heterogeneous AOPs [20–24]. However, the outer Fe(III) layer formed by the oxidation on iron oxides will passivate the surface of catalysts and inhibit the catalytic efficiency toward AOPs. The slow transformation of Fe(III) to Fe(II) still limits the peroxide or persulfate activation and radical generation. Schwertmannite (Sch) is a poorly crystalline Fe(III)-oxyhydroxysulfate mineral, which always precipitates in sulfate-rich acid mine drainage (AMD). It is always represented as Fe8 O8 (OH)8−2x (SO4 )x , Materials 2018, 11, 1165; doi:10.3390/ma11071165

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where x varies from 1 to 1.75 [25,26]. Reductive dissolution is a dissolution process of ferric minerals with the release of Fe(II) from Fe(III) oxides to aqueous solutions. As hydroxylamine (HA) adsorbs onto the surface of iron oxides, surface complex of [Fe(III)-HA] forms followed by electron transfer between Fe(III) and HA, which subsequently results in the formation of Fe(II) and semi-stable state surface complexes. Fe(II) will be directly released through the decomposition of surface complexes. Therefore, addition of HA enhanced the transformation from Fe(III) to Fe(II) on the surface, which results in good efficiency for the improvement of PS activation on schwertmannite. Although schwertmannite has been investigated as a new Fenton-like catalyst in the oxidation of phenol by hydrogen peroxide (H2 O2 ) [23], to the best of our knowledge, activation of PS on schwertmannite for heterogeneous AOPs has never been reported. Therefore, the objective of this study was to investigate the heterogeneous catalytic oxidation of rhodamine B (RhB) by PS and the effect of reductive dissolution of schwertmannite on the heterogeneous advanced oxidation process. 2. Experimental Section 2.1. Materials and Chemicals Ferrous sulfate (FeSO4 ·7H2 O), ferric chloride (FeCl3 ·6H2 O), hydrogen peroxide (H2 O2 ), potassium persulfate (PS), hydroxylamine (HA), rhodamine B (RhB), tert-butyl alcohol (TBA) and methanol were of analytical reagent grade and purchased from Tianjin Guangfu Technology Development Co., Ltd. (Tianjin, China) All chemicals were used without further purification, and Milli-Q water was used throughout this study. 2.2. Synthesis of Schwertmannite Schwertmannite was synthesized via Fe(II) oxidation method [27]: 16.45 g FeSO4 ·7H2 O was dissolved in 1 L deionized water and reacted with 5.3 mL 30% H2 O2 . The solution became dark red, and a red-orange material precipitated immediately with the final pH of 2.5 after 24 h. The solid was then centrifuged, washed three times and freeze-dried for further use. 2.3. Catalytic Oxidation of RhB All experiments were performed in 150 mL triangular flasks with a constant stirring at 25 ◦ C. Solutions with desired concentrations of RhB (0.5 M), PS (0.5 M) and HA (10 mM) were prepared before the reaction, respectively. In a typical experiment, 50 mg schwertmannite solid was hydrated in deionized water for 1 h at a given pH, which was adjusted to desired values by using 0.5 M HCl or NaOH solution during the equilibration. After that, the prepared RhB, PS and HA solutions was added, and the desired initial pH was adjusted rapidly by using 0.5 M HCl or NaOH solution. After the addition of RhB, PS and HA, the final volume of the solution was 100 mL, and the initial concentrations of iron oxides, RhB, PS and HA were 0.5 g/L, 25 mM, 5 mM and 0–1 mM, respectively. Furthermore, Fe(III) as FeCl3 was employed and substituted for schwertmannite to investigate the effect of dissolved iron on the degradation process, the concentration of Fe(III) was 25 µM after mixing. Besides, to understand the activation mechanism of PS on schwertmannite, methanol and TBA were employed as scavengers for radicals formed by the PS activation. After mixing of RhB, PS, and HA, methanol or TBA were added in to the system, and the concentrations were changed from 0 to 1 mM, respectively. During the oxidation experiments, samples were withdrawing regularly and centrifuged for 2 min at 12,000 rpm. The supernatant was then obtained for the determination of RhB and dissolved iron concentrations. All data were determined and collected by dual experiments. 2.4. Characterization Morphology of the synthesized schwertmannite was characterized by field emission scanning electron microscopy (SEM, S4800, Hitachi, Tokyo, Japan) and high-resolution analytical transmission electron microscopy (TEM, Fei Tecnai G2 F20, FEI, Hillsboro, OR, USA). The crystalline structure of

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2.4. Characterization Morphology of the synthesized schwertmannite was characterized by field emission scanning 3 of 13 electron microscopy (SEM, S4800, Hitachi, Tokyo, Japan) and high-resolution analytical transmission electron microscopy (TEM, Fei Tecnai G2 F20, FEI, Hillsboro, OR, USA). The crystalline structure of schwertmannite was by characterized by diffraction powder X-ray diffraction patternwas (XRD), which schwertmannite was characterized powder X-ray pattern (XRD), which employed was employed using Cu Ka on (1.54) radiation2500 on aX-ray D/MAX 2500 X-ray diffractometer at 40 thekV voltage by using Cu Ka by (1.54) radiation a D/MAX diffractometer at the voltage of and of 40 kV and current of 100 mA. Samples were gently ground and analyzed over a diffraction-angle current of 100 mA. Samples were gently ground and analyzed over a diffraction-angle (2θ) range of (2θ)◦ ,range 2–90°, with scanning of 8°/min. Concentration of RhB was at determined 550 nm 2–90 with of scanning rate of 8◦ /min.rate Concentration of RhB was determined 550 nm byatUV–Vis by UV–Vis spectrophotometer (UV-6300, Mapada, Shanghai, China). iron by was measured spectrophotometer (UV-6300, Mapada, Shanghai, China). Dissolved iron Dissolved was measured inductively by inductively coupled plasma–mass spectrometry (ICP-MS, 7700x, Agilent, Santa Clara, CA, USA). coupled plasma–mass spectrometry (ICP-MS, 7700x, Agilent, Santa Clara, CA, USA). Materials 2018, 11, 1165

2.5.Computation ComputationDetails Details 2.5. Orbitalenergy energyanalysis analysis intermediates was achieved by DMol3 module of Materials Studio Orbital ofof intermediates was achieved by DMol3 module of Materials Studio 8.0, 8.0,the and the highest occupied molecular orbital (HOMO) energy level and lowest unoccupied and highest occupied molecular orbital (HOMO) energy level and lowest unoccupied molecular molecular orbital (LUMO) level were studied to evaluate energy fortoelectrons orbital (LUMO) energy levelenergy were studied to evaluate the energy the needed forneeded electrons cross theto cross the energy barrier based on the frontier molecular orbital analysis [28]. energy barrier based on the frontier molecular orbital analysis [28]. 3.3.Results Resultsand andDiscussion Discussion 3.1. 3.1.Characterization CharacterizationofofSchwertmannite Schwertmannite Schwertmannite Schwertmannite was was aa Fe(III)-oxyhydroxysulfate Fe(III)-oxyhydroxysulfate oxide oxide with with poorly poorly crystalline crystalline structure. structure. Morphological Morphologicalanalysis analysisachieved achievedby bySEM SEMand andTEM TEMsuggests suggeststhat thatthe thesynthesized synthesizedschwertmannite schwertmannite shows showsaaspherical sphericalmorphology morphologywith withthe thediameter diameterofof500 500nm nm(Figure (Figure1a,b). 1a,b).XRD XRDanalysis analysisshows shows diffraction peaks as (201), (310), (212), (302), (522), (004) and (542)/(204) (Figure 1c), which are in good diffraction peaks as (201), (310), (212), (302), (522), (004) and (542)/(204) (Figure 1c), which are in agreement with schwertmannite (JCPDS, No. 47-1775). good agreement with schwertmannite (JCPDS, No. 47-1775).

Figure1.1.SEM SEM(a); (a);TEM TEM(b); (b);and andXRD XRD(c) (c)analysis analysisofofsynthesized synthesizedschwertmannite. schwertmannite. Figure

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3.2. Effect of HA on Degradation of RhB by PS on Schwertmannite As shown in Figure 2, RhB could be oxidized and degraded by PS activation on schwertmannite at pH 3, 5 and 9, and the degradation of RhB was more efficient in acid conditions. Concentrations of inin 360 min at at pHpH 3, 53,and 9, respectively (Figure 2a). RhB decreased decreased from from25 25mM mMtoto0.5, 0.5,3.0 3.0and and7.8 7.8mM mM 360 min 5 and 9, respectively (Figure This suggests thatthat schwertmannite shows good catalytic 2a). This suggests schwertmannite shows good catalyticactivity activityfor forradical radicalgeneration generation from from PS activation in 90%, 84% and 37% of RhB waswas degraded in 30in min in heterogeneous heterogeneousAOPs. AOPs.More Morethan than 90%, 84% and 37% of RhB degraded 30 with min the addition of HAofatHA pH at 3, pH 5 and respectively (Figure(Figure 2b). Concentrations of RhB of decreased rapidly with the addition 3, 59, and 9, respectively 2b). Concentrations RhB decreased from 25 from mM to 0.6toand 180 mininat180 pHmin 3, 5 at and 9, 3, respectively. The rapid decreasing rapidly 250.5, mM 0.5,9.7 0.6mM andin9.7 mM pH 5 and 9, respectively. The rapid concentration of RhB with the addition of HA suggests an enhanced activation of PS by the acceleration decreasing concentration of RhB with the addition of HA suggests an enhanced activation of PS by of Fe(III)-Fe(II) [15,18], which attributes the reducing capacity HA for capacity the transformation the accelerationrecycle of Fe(III)-Fe(II) recycle [15,18],to which attributes to the of reducing of HA for from Fe(III) to Fe(II) [29,30]. the transformation from Fe(III) to Fe(II) [29,30].

Figure 2. Degradation of RhB in the absence (a) and presence (b) of HA under various pH conditions. Figure 2. Degradation of RhB in the absence (a) and presence (b) of HA under various pH conditions. Concentrations: Concentrations: [Schwertmannite] [Schwertmannite] == 0.5 0.5 g/L, g/L,[RhB] [RhB]==25 25mM, mM,[PS] [PS]==5 5mM, mM,and and[HA] [HA]==0.5 0.5mM. mM.

Although HA can accelerate the transformation from Fe(III) to Fe(II) and contribute to the Although HA can accelerate the under transformation from Fe(III) Fe(II) and contribute to the increasing degradation rate of RhB acid conditions, high toconcentration of HA will be increasing degradation rate of RhB under acid conditions, high concentration of HA will be counterproductive [15]. It was found that the degradation rate of RhB increased rapidly with the counterproductive [15]. Itofwas themM degradation ratethe of concentrations RhB increasedof rapidly with the increasing concentrations HAfound from 0that to 0.5 at pH 5, and RhB decreased increasing concentrations of HA from 0 to 0.5 mM at pH 5, and the concentrations of RhB decreased from 25 mM to 11.5, 7.0 and 1.8 mM in 90 min, respectively (Figure 3). However, the degradation fromof25RhB mM to 11.5, 7.0 and 1.8 mM 90 min, respectively (Figure 3). However, degradation rate increased slightly when theinconcentration of HA increased from 0.5 to 1.0the mM (Figure 3). rate slightly of RhB increased whenrate the of concentration of HA increased from 0.5 to 1.0 (Figure 3). The increasedslightly degradation RhB under high HA concentration might bemM caused by the The slightly increased degradation rate of RhB under high HA concentration might be caused by the removal of radicals by excess NH3OH+ [2,3,15], which is the main form of HA (pKa 5.96 at 25 °C) at + [2,3,15], which is the main form of HA (pKa 5.96 at 25 ◦ C) at removal by excess NH[31]. 3 OHConsidering pH 3 andof5 radicals in aqueous solutions the rate constant of 5.0 × 108 M−1∙s−1 for •OH and −1 ·s−1 for •OH and ●‒ pH 3 and 5 in aqueous solutions [31]. Considering the rate constant of and 5.0 ×SO 10●‒8 M 7 −1 −1 + 1.5 × 10 M ∙s for SO4 , •− NH3OH would be the scavenger for •OH radicals, as shown in 4 7 − 1 − 1 + ·s (2), 1.5 × 10 M(1) and for when SO4 , the NHconcentration for •OH and SO radicals, as shown in 3 OH would be Equations of the NHscavenger 3OH+ is high enough [2,3]. Equations (1) and (2), when the concentration of NH3 OH+ is high enough [2,3]. (1) NH3 OH +• OH → OH + nitro-products + − NH3 OH + •OH → OH + nitro − products (1) SO• → SO + nitro-products NH OH + NH3 OH+ + SO4•− → SO24− + nitro − products

(2) (2)

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Figure 3. 3. Effect Effect of of HA HA concentration concentration on on RhB RhB degradation degradation at at pH pH 5. 5. Concentrations: [Sch] [Sch] = 0.5 0.5 g/L, g/L, Figure Figure 3. Effect of HA concentration on RhB degradation at pH 5. Concentrations: Concentrations: [Sch]= = 0.5 g/L, [RhB] = 25 mM, [PS] = 5 mM, and [HA] = 0–1 mM. [RhB] = 25 mM,[PS] [PS]==55mM, mM,and and[HA] [HA] = = 0–1 [RhB] = 25 mM, 0–1 mM. mM.

3.3. Release Release Files Files of of Dissolved Dissolved Iron Iron from from Schwertmannite Schwertmannite 3.3.3.3. Release Files of Dissolved Iron from Schwertmannite Figure 44 shows shows the the release release of of dissolved dissolved iron iron from from schwertmannite schwertmannite during during the the degradation degradation of of Figure Figure 4 shows the release of dissolved iron from schwertmannite during the degradation of RhB RhB at at pH pH 33 and and 5. 5. The The dissolved dissolved iron iron was was released released slowly slowly from from schwertmannite schwertmannite without without the the RhB at pH 3 andof5.HA Thedue dissolved ironefficiency was released fromand schwertmannite without the addition addition to the the low low of PS PSslowly activation, 1.6 and and 0.7 0.7 µM µM iron released released in 120 120 of addition of HA due to efficiency of activation, and 1.6 iron in HA due to the low efficiency of PS activation, and 1.6 and 0.7 µM iron released in 120 min at pH 3 min at at pH pH 33 and and 5, 5, respectively. respectively. However, However, more more Fe(III) Fe(III) was was dissolved dissolved and and released released from from min and 5, respectively.inHowever, more Fe(III) wasthe dissolved andofreleased from in the schwertmannite the presence presence of HA HA during degradation RhB. After After theschwertmannite addition of of HA, HA, 26.7 26.7 schwertmannite in the of during the degradation of RhB. the addition presence of HA during the degradation of RhB. After the addition of HA, 26.7 and 20.0 µM iron was and 20.0 µM iron was released in 120 min at pH 3 and 5, respectively. Considering the higher and 20.0 µM iron was released in 120 min at pH 3 and 5, respectively. Considering the higher released in 120 min at pH 3 and 5, respectively. Considering the higher degradation rate of RhB degradation rate rate of of RhB RhB in in the the presence presence of of HA HA (Figure (Figure 2b), 2b), the the increasing increasing release release of of dissolved dissolved iron iron in degradation theindicates presencethe of HA (Figure 2b), the increasing release of dissolved the acceleration indicates the acceleration of transformation transformation from Fe(III) Fe(III) to Fe(II) Fe(II)iron andindicates the improvement improvement of PS PS of acceleration of from to and the of activation on schwertmannite. transformation from Fe(III) to Fe(II) and the improvement of PS activation on schwertmannite. activation on schwertmannite.

Figure 4. 4. Release Release files files of dissolved dissolved iron iron from from schwertmannite schwertmannite during during the degradation degradation of RhB RhB under under Figure Figure 4. Release files ofofdissolved iron from schwertmannite duringthe the degradationofof RhB under various pH pH conditions. conditions. Concentrations: Concentrations: [Sch] [Sch] == 0.5 0.5 g/L, g/L, [RhB] [RhB] == 25 25 mM, mM, [PS] [PS] == 55 mM, mM, and and [HA] [HA] == 0.5 0.5 various various pH conditions. Concentrations: [Sch] = 0.5 g/L, [RhB] = 25 mM, [PS] = 5 mM, and [HA] = 0.5 mM. mM. mM.

3.4.3.4. Effect of of Fe(III) Effect Fe(III)onon onthe theDegradation Degradationofof ofRhB RhB 3.4. Effect of Fe(III) the Degradation RhB ToTo investigate the effect (FeCl3∙6H was 3 ·6H 2 O) investigate the effectofof ofFe(III) Fe(III)on onthe thedegradation degradation of of RhB, RhB, ferric ferric chloride chloride (FeCl (FeCl 2O) was To investigate the effect Fe(III) on the degradation of RhB, ferric chloride 3∙6H2O) was employed and substituted for schwertmannite during the catalytic process. As shown in Figure employed and and substituted substituted for for schwertmannite schwertmannite during during the the catalytic catalytic process. process. As As shown shown in in Figure Figure 5, 5, 5, employed under the restriction of PS activation, concentrations of RhB decreased slowly from 25 mM to 8.6 and under the the restriction restriction of of PS PS activation, activation, concentrations concentrations of RhB RhB decreased decreased slowly slowly from from 25 25 mM mM to to 8.6 8.6 and and under 12.8 mM in 90 min at pH 3 and 5, respectively (Figure 5). However, it showed higher degradation

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12.8 mM in 90 min at pH 3 and 5, respectively (Figure 5). However, it showed higher degradation rate rate of of RhB RhB after after the the addition addition of HA: HA: only 1.7 and 3.0 mM RhB remained at 90 min at pH 3 and 5, respectively. The addition of HA contributed to the acceleration of transformation from from Fe(III)Fe(III) to Fe(II) respectively. The addition of HA contributed to the acceleration of transformation to due to due the reducing capacity, capacity, which further improved generation radicals from PS activation Fe(II) to the reducing which further the improved the of generation of radicals from for PS the degradation [15]. process [15]. activation for theprocess degradation

Figure 5. 5. Effect Effect of ofFe(III) Fe(III)on onRhB RhBdegradation degradationunder undervarious variouspH pHconditions. conditions. Homogeneous Homogeneous catalytic catalytic Figure degradation of RhB at initial pH 3 and 5. Concentrations: [Fe(III)] = 25 µM, [RhB] = 25 mM, =5 degradation of RhB at initial pH 3 and 5. Concentrations: [Fe(III)] = 25 µM, [RhB] = 25 mM, [PS] =[PS] 5 mM, mM,[HA] and [HA] = 0.5 mM. and = 0.5 mM.

3.5. Determination Determination of of Kinetic Kinetic Parameters Parameters 3.5. The initial initial kinetic kinetic constants constants of of RhB RhB degradation degradation were were quite quite different different in in homogeneous homogeneous and and The heterogeneous processed (Figure 6). The initial kinetic constants were obtained by first-order kinetic heterogeneous processed (Figure 6). The initial kinetic constants were obtained by first-order kinetic equation (Equation (Equation (3)), (3)), which which can can also alsobe beexpressed expressedas asEquation Equation(4): (4): equation (3) ⁄ =− dC/dt = −k1 C

ln( /

0

)= −

(3) (4)

where C is the concentration of RhB (mM);ln(C/C C0 is the = −kt concentration of RhB (mM); t is time (min); (4) 0 ) initial and k is the initial kinetic constant of RhB degradation (min−1). The linear relationship between where C is the concentration of RhB (mM); C0 is the initial concentration of RhB (mM); t is time (min); ln(C/C0) and t in heterogeneous and homogeneous process are shown in Figure 6a,b, and the and k is the initial kinetic constant of RhB degradation (min−1 ). The linear relationship between absolute value of linear slope represents the initial kinetic constant k. As shown in Figure 6c, the ln(C/C0 ) and t in heterogeneous and−2 homogeneous process are shown in Figure 6a,b, and the absolute initial kinetic constants are 1.3 × 10 and 7.8 × 10−3 min−1 for heterogeneous process and 1.3 × 10−2 and value of−3linear−1 slope represents the initial kinetic constant k. As shown in Figure 6c, the initial 6.9 × 10 min for homogeneous without the addition of HA at pH 3 and 5, respectively. −2 andprocess kinetic constants are 1.3 × 10 7.8 × 10−3 min−1 for heterogeneous process and 1.3 × 10−2 and −2 and 6.0 × 10−2 min−1 for heterogeneous process and 5.7 × 10−2 and 2.7 × However, they are 9.8 × 10 6.9−2× 10−−13 min−1 for homogeneous process without the addition of HA at pH 3 and 5, respectively. 10 min for homogeneous process in the presence of HA at pH 3 and 5, respectively. Compared However, they are 9.8 × 10−2 and 6.0 × 10−2 min−1 for heterogeneous process and 5.7 × 10−2 with the homogeneous process, more significant improvement of the initial kinetic constant at pH 3 and 2.7 × 10−2 min−1 for homogeneous process in the presence of HA at pH 3 and 5, respectively. and 5 in heterogeneous process suggests that the activation of PS enhanced by HA is mainly Compared with the homogeneous process, more significant improvement of the initial kinetic constant achieved through heterogeneous catalysis at acid conditions despite the existence of homogeneous at pH 3 and 5 in heterogeneous process suggests that the activation of PS enhanced by HA is mainly process. The homogeneous kinetics and iron releasing kinetics can be described as Equations (5) and achieved through heterogeneous catalysis at acid conditions despite the existence of homogeneous (6): process. The homogeneous kinetics and iron releasing kinetics can be described as Equations (5) (5) ⁄ =− 1 ∙ and (6): (6) (5) ⁄ dC/dt = 2( = *−−k1 C)·m r where m is the concentration of released (µM); is the maximum of released iron dm/dt = iron −k2 (m* − m)m* (6) concentrations (µM); k1 and k2 are kinetic constants (min−1); and r is the activity of iron oxides in the where m is the concentration of released iron (µM); m* is the maximum of released iron concentrations reductive dissolution process. As shown in Figure 6d, degradations were 16.1% and 5.8% at 30 min (µM); k1 and k2 are kinetic constants (min−1 ); and r is the activity of iron oxides in the reductive by the released iron at pH 3 and 5, respectively. Removal of RhB increased with the increasing dissolution process. As shown in Figure 6d, degradations were 16.1% and 5.8% at 30 min by the released concentration of released iron based on time. However, degradations were 89.5% and 83.7% at 30

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iron at pH 3 and respectively. Removal of RhB increased with the increasing concentration of released Materials 2018, 11, x 5, FOR PEER REVIEW 7 of 12 iron based on time. However, degradations were 89.5% and 83.7% at 30 min by schwertmannite at pH andschwertmannite 5, respectively. at This indicates dominating degradation on the min3 by pHsignificant 3 and 5,difference respectively. This the significant difference indicates the surface of heterogeneous schwertmannite. dominating degradation on the surface of heterogeneous schwertmannite.

Figure 6. 6. First-order kinetics of of RhB RhB degradation degradation in in homogeneous homogeneous and and heterogeneous heterogeneous oxidation oxidation Figure First-order kinetics processes under various pH conditions. Initial RhB degradation rates in: (a) Sch/PS and Sch/PS/HA; processes under various pH conditions. Initial RhB degradation rates in: (a) Sch/PS and Sch/PS/HA; and (b) (b) Fe(III)/PS Fe(III)/PS and and Fe(III)/PS/HA. Fe(III)/PS/HA. (c) Effect of of and (c)Effect Effectof ofHA HA on on initial initial RhB RhB degradation degradation rates. rates. (d) (d) Effect released iron by the reductive dissolution during the heterogeneous oxidation process. released iron by the reductive dissolution during the heterogeneous oxidation process.

3.6. Comparison of Heterogeneous Process and Homogeneous Process 3.6. Comparison of Heterogeneous Process and Homogeneous Process To further investigate advantages of heterogeneous process compared with homogeneous To further investigate advantages of heterogeneous process compared with homogeneous process, possible homogeneous and heterogeneous intermediates were provided, as shown in process, possible homogeneous and heterogeneous intermediates were provided, as shown in Figure 7, where Fe(II)-S2O8 and FeO4-S2O8 represent homogeneous and heterogeneous intermediates, Figure 7, where Fe(II)-S2 O8 and FeO4 -S2 O8 represent homogeneous and heterogeneous intermediates, respectively. The activation of persulfate would then be considered as the transition of electron from respectively. The activation of persulfate would then be considered as the transition of electron from HOMO (Highest Occupied Molecular Orbital) to LUMO (Lowest Unoccupied Molecular Orbital) HOMO (Highest Occupied Molecular Orbital) to LUMO (Lowest Unoccupied Molecular Orbital) inside intermediates. Orbital energy analysis of intermediates is listed in Table 1. The energy inside intermediates. Orbital energy analysis of intermediates is listed in Table 1. The energy difference difference between HOMO and LUMO indicates the reactivity of intermediates, and they are 4.361, between HOMO and LUMO indicates the reactivity of intermediates, and they are 4.361, 2.678 and 2.678 and 0.404 eV for H2S2O8, Fe(II)-S2O8 and FeO4-S2O8, respectively. It suggests that the 0.404 eV for H2 S2 O8 , Fe(II)-S2 O8 and FeO4 -S2 O8 , respectively. It suggests that the heterogeneous heterogeneous intermediate shows the lowest orbital energy for the electron transition, which means intermediate shows the lowest orbital energy for the electron transition, which means higher reactivity higher reactivity than that of homogeneous intermediate. Therefore, the enhanced activation of than that of homogeneous intermediate. Therefore, the enhanced activation of persulfate is dominated persulfate is dominated by heterogeneous process. by heterogeneous process.

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Figure 7. HOMO and LUMO orbital analysis of possible intermediates in homogeneous and Figure 7. HOMO and LUMO orbital analysis of possible intermediates in homogeneous and heterogeneous process. heterogeneous process. Table 1. Orbital energy analysis of homogeneous and heterogeneous intermediates. Table 1. Orbital energy analysis of homogeneous and heterogeneous intermediates.

Species

EHOMO/eV

Species H2S2O8

EHOMO /eV −11.417

H Fe(II)-S 2 S2 O8 2O8 Fe(II)-S O82O8 FeO42-S FeO4 -S2 O8

− 11.417 −11.745 −−9.611 11.745 −9.611

ELUMO/eV E−2.084 LUMO /eV −2.084 −2.543 −2.543 −1.372 −1.372

ΔE/eV ∆E/eV 9.333 9.333 9.202 9.202 8.239 8.239

3.7. Effect of pHpzc on the Heterogeneous Process 3.7. Effect of pHpzc on the Heterogeneous Process The pH point of zero charge (pHpzc) of iron oxides catalysts always plays an important role to Thethe pHpH point of zeroand charge (pHpzc ) of iron oxides catalystsinalways plays an important role to change in system provide acidic microenvironment the heterogeneous AOPs [23,32]. change the pH in system and provide acidic microenvironment in the heterogeneous AOPs [23,32]. As shown in Figure 8a, the pH decreases from the initial 3, 5 and 9 to the final 2.6, 3.3 and 6.2 in 360 As Figure 8a, the pHrespectively. decreases from 3, 5pH and the final 2.6, 3.3(3.05) and 6.2 in minshown duringinthe degradation, Duethe to initial the low pzc 9oftoschwertmannite [23], 360 min during the degradation, respectively. Due to the low pH of schwertmannite (3.05) [23], pzcenvironment, where the pH is schwertmannite could show the potential to release proton into the schwertmannite could show potential the environment, wheredue the to pHthe is higher than the pH pzc of ironthe oxides. The to pHrelease of theproton systeminto would therefore decrease higher than the pH of iron oxides. The pH of the system would therefore decrease due to the pzc released proton and intermediates formed during the degradation of RhB [23,33,34]. released protonin and intermediates formed during theofdegradation RhB [23,33,34]. As shown Figure 8b, the increasing addition HA finally of caused the significant decrease of As shown in Figure 8b,final the 3.4, increasing finally caused the significant pH from the initial 5 to the 2.8 andaddition 2.6 with of theHA increasing HA addition of 0.1, 0.5decrease and 1.0 of pH from the initial 5 to the final 3.4, 2.8 and 2.6 with the increasing HA addition of 0.1, 0.5 and mM, respectively. Although increasing addition of HA accelerated the transformation from Fe(III) to 1.0 mM, respectively. Although increasing addition of HA accelerated the transformation from Fe(III) Fe(II) for the PS activation, it also resulted in the reductive dissolution of schwertmannite during the to Fe(II) for the PS activation, it also resulted in decrease the reductive dissolution of by schwertmannite degradation process. Therefore, the significant of pH is achieved the release of during proton the degradation process. Therefore, the significant decrease of pH is achieved by the release of proton from schwertmannite during the dissolution, and the new-formed acid microenvironment exposed from during the dissolution, anddegradation the new-formed microenvironment exposed by by theschwertmannite dissolution process resulted in the higher rate acid of RhB. the dissolution process resulted in the higher degradation rate of RhB.

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Figure ChangesofofpH pHduring duringthe the RhB RhB degradation degradation in HA Figure 8. 8. Changes in the the absence absence(a) (a)and andpresence presence(b)(b)ofof HA under various pH conditions. Concentrations: [Sch] = 0.5 g/L, [RhB] = 25 mM, [PS] = 5 mM, and [HA] under various pH conditions. Concentrations: [Sch] = 0.5 g/L, [RhB] = 25 mM, [PS] = 5 mM, and Figure 8. Changes of pH during the RhB degradation in the absence (a) and presence (b) of HA = 0–1 mM. [HA] = 0–1 mM. under various pH conditions. Concentrations: [Sch] = 0.5 g/L, [RhB] = 25 mM, [PS] = 5 mM, and [HA] = 0–1 mM. Mechanism of PS on Schwertmannite in the Presence of HA 3.8. Activation 3.8. Activation Mechanism of PS on Schwertmannite in the Presence of HA To understand the activation mechanism of schwertmannite, methanol and TBA were 3.8. Activation Mechanism of PS on Schwertmannite in PS the on Presence of HA To understand the activation mechanism schwertmannite, methanol and TBA were employed as scavengers for radicals formed by of thePS PSon activation. Figure 9 shows the inhibitory effect To understand the activation mechanism of PS on schwertmannite, methanol and TBA were employed as scavengers for radicals formed by the PS activation. Figure 9 shows the inhibitory of methanol and TBA on the degradation of RhB by PS on schwertmannite in the presence of HA. employed as scavengers for on radicals formed by the PS activation. 9 shows the inhibitory effect effect of methanol thefrom degradation RhB by PSmM onFigure schwertmannite inaddition the presence Concentrations ofand RhBTBA decreased 25 mM toof8.0 and 13.8 with the increasing of 0.1 of of methanol and TBA on the degradation of RhB by PS on schwertmannite in the presence of HA. Concentrations of RhB decreased from 25 mM to 8.0 and 13.8 mM with the increasing addition and 1.0 M methanol, respectively. However, the concentrations of RhB decreased to 4.5 and 11.9HA. mM Concentrations decreased from respectively. 25 However, mM to 8.0Considering and 13.8 mMthe with the increasing addition of 0.1 and 1.0 M of methanol, respectively. the concentrations of RhB decreased to of 4.50.1 and in the presence ofRhB 0.1 and 1.0 M TBA, high rate constants for •OH (9.7 ●‒ and 1.0 methanol, the concentrations ofConsidering RhB decreased 4.5 and mM 8 MM −1∙s −1) [3] 7 M −1)TBA, 11.9 mM in the presence 0.1 ×and 1.0−1∙sM respectively. thetohigh rate11.9 constants × 10 andrespectively. SO4of(2.5 10However, [2], methanol is considered as an effective scavenger for ●‒ − presence of and 1.0 respectively. the highmethanol rate ×constants •OH •− 80.1 1 ·s− 1TBA 7 M−1 · s− 1 ) •OH −1) [3] both SO , while is only effective scavenger for (6.0 108 M ∙sfor but(9.7 notan forin •the OH•OH (9.7 and × 10 )M [3]TBA, and SOan (2.5 ×Considering 10 [2], is −1considered as 4M ●‒ 7 M−1∙s−14) [2], methanol is considered as an effective ●‒∙s−1) [3] and SO4 ●‒ ×for 108SO M−1 (2.5 × 10 scavenger for 5 M−1∙s−1) [2] due to its •− (8.0 × 10 much slower rate constant for SO . According to effective scavenger for both •OH and SO4 , while TBA is only an effective4 scavenger for the •OH 4 ●‒ −1) [3] but not both •OH and SO scavenger for •OH (6.0 × of 108RhB M−1∙swas 4 , while TBA is only •− an effective 8 − 1 − 1 5M −1●‒ −1the et not al. [15], contribution of SO degradation excluded. 5 ·on (6.0investigation × 10●‒ M ·sby )5Zou [3] −1 but for SO (8.0 × 10 s ) [2] due to its much slower rate constant 4 its much slower rate constant for SO●‒ . According to the for SO (8.0 10 Mefficiency ∙s−1) [2] of due to 4 4•− the× lower TBA than for thecontribution inhibition of of RhB indicates forTherefore, SO4•− . According to the investigation by methanol Zou et al.●‒ [15], SOdegradation on the degradation ●‒ [15], contribution of SO 5 on the degradation of 5 investigation by Zou et al. RhB was excluded. that both •OH and SO are the reactive oxidants that formed by the activation of 4 of RhB was excluded. Therefore, the lower efficiency of TBA than methanol for the inhibitionPSof on RhB Therefore, the lower efficiency TBA than methanol for the inhibition of RhB degradation indicates •− schwertmannite described as•ofOH Equations (7)–(12). The possible activation mechanism ofactivation PS on ●‒ degradation indicates that both and SO are the reactive oxidants that formed by the 4 oxidants that formed by the activation of PS on that both •OH in and are of theHA reactive 4 schwertmannite theSO presence is proposed in Scheme 1. of schwertmannite PS on schwertmannite described as Equations (7)–(12). The possible activation mechanism of PS described as Equations (7)–(12). The possible activation mechanism of PS on on schwertmannite in the presence of HA is proposed in Scheme 1. schwertmannite in the presence of HA is proposed in Scheme 1.

Scheme 1. Activation of mechanism of PS on schwertmannite in the presence of HA. Scheme 1. Activation of mechanism of PS on schwertmannite the presence HA. (8), while, in In strongly solutions,of persulfate decomposes according toinin Equations (7)ofand Schemeacid 1. Activation mechanism of PS on schwertmannite the presence of HA. alkaline, neutral and dilute acid solutions, the decomposition follows Equation (9) [35]. Previous In strongly acidthat solutions, persulfate decomposes according to Equations and (8), in studies suggested persulfate decomposition in aqueous solutions is first(7)order andwhile, that the In strongly acid solutions, persulfate decomposes according to Equations (7) and (8), while, in alkaline, neutral and dilute acid solutions, the decomposition follows Equation (9) [35]. Previous reaction is catalyzed by hydrogen ion [35,36]. Meanwhile, there are many hydrogen ions on the alkaline, and that dilutepersulfate acid solutions, the decomposition follows Equation (9) order [35]. Previous studiesneutral suggested decomposition in aqueous solutions is first and thatstudies the reaction is catalyzed by hydrogen ion [35,36]. Meanwhile, there are many hydrogen ions on the

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surface of schwertmannite due to low pHpzc (3.05) [23], which results in the surface of iron oxides existing as the form of Fe-OH2+ [37]. This contributes to the decomposition of persulfate according to suggested (7) thatand persulfate in aqueous solutions3 is first order that the reaction to is Equations (8), anddecomposition then the forming of the Fe-OOSO peroxo metaland complex according catalyzed by hydrogen ion [35,36]. Meanwhile, there are many hydrogen ions on the surface of Equations (11) and (12). schwertmannite due to low pHpzc (3.05) [23], which results in the surface of iron oxides existing as the 2 2 (7) and Othe → SO 2Hpersulfate according to Equations (7) S2 O28 + H2to 5 + SO4 + of form of Fe-OH2 + [37]. This contributes decomposition (8), and then the forming of the Fe-OOSO 3 peroxo metal complex according to Equations (11) and (12). (8) SO25 + H2 O → SO24 + H2 O2 S2 O28− + H2 O → SO25− + SO24− + 2H+ S2 O28 + H2 O → 2HSO4 + 1⁄2 O2 SO25− + H2 O → SO24− + H2 O2 → ≡Fe(II) + nitro-products ≡Fe(III) + NH3 OH S2 O28− + H2 O → 2HSO4− + 1/2O2 2 + +)S+2 ONH →OH ≡Fe(II)-OOSO + nitro SO24 − +products 2H ≡Fe(II)-OH 8 3 ≡ Fe(2III →≡ Fe(II)3 +

+ 2 − 2− + SO2− + 2H+ ≡ Fe(II) − OH + S2→ O28≡Fe(III)-OOSO →≡ Fe(II) − OOSO 22 O 3 ≡Fe(III)-OH 2 +S 8 3 + SO4 + 2H 4

(7)

(9) (8)

(10)

(9)

(11)(10) (12)(11)

≡ Fe(III) − OH2+ + S2 O28− →≡ Fe(III) − OOSO3− + SO24− + 2H+ ≡Fe(II)-OOSO + H O → ≡Fe(II)-OH + SO• + OH ≡ Fe(II) − OOSO3− + H2 O →≡ Fe(II) − OH + SO4•− + OH− ≡Fe(II)-OOSO3 + H−2 O → ≡Fe(II)-OH +• OH + SO24 ≡ Fe(II) − OOSO3 + H2 O →≡ Fe(II) − OH + •OH + SO24−

(12) (13) (13) (14) (14)

− 2− + ≡ Fe(III) − OOSO + H2 O →≡+FeO(2II+ )+ ≡Fe(III)-OOSO SOO242 + + SO 2H4 + 2H 3 + H32 O → ≡Fe(II)

(15)(15)

Previous studies suggested that decomposition of H2O O22 on on transition transition metal metal oxide oxide surface was the the formation of the of peroxo complex to theSimilar decomposition always achieved achievedbyby formation the metal peroxo metal[23,38–41]. complex Similar [23,38–41]. to the mechanism of H O on transition metal oxide surface, the surface hydroxyl group (Fe(III)-OH) on decomposition mechanism of H 2 O 2 on transition metal oxide surface, the surface hydroxyl group 2 2 schwertmannite can be substituted groupby (Operoxo PS3S-OO-SO to form Fe(II)-OOSO (Fe(III)-OH) on schwertmannite canby beperoxo substituted group 3) of PS to form 3 S-OO-SO 3 ) of(O 3 or Fe(III)-OOSO3 3or peroxo metal complex (Equations (11) and(Equations (12)). After(11) that,and the complex decomposes Fe(II)-OOSO Fe(III)-OOSO 3 peroxo metal complex (12)). After that, the to form radicals (Equations and the Fe(II)-OH formed byFe(II)-OH Equation formed (10) willbytransfer to complex decomposes to form(13)–(15)), radicals (Equations (13)–(15)), and the Equation Fe(III)-OH again on surface for heterogeneous process aqueous Fe(III)process released solutions for (10) will transfer tothe Fe(III)-OH again on the surface for or heterogeneous orinto aqueous Fe(III) the homogeneous process. released into solutions for the homogeneous process.

Figure 9. Inhibition effect of radical scavengers on RhB degradation in the presence of HA at pH 5. Figure 9. Inhibition effect of radical scavengers on RhB degradation in the presence of HA at pH 5. Concentrations: Concentrations: [Sch] [Sch] == 0.5 0.5 g/L, g/L,[RhB] [RhB]==25 25mM, mM,[PS] [PS] ==55mM, mM, [HA] [HA] == 0.5 0.5 mM, mM, [TBA] [TBA] == 0–1 0–1 mM, mM, and and [Methanol] = 0–1 mM. [Methanol] = 0–1 mM.

4. Conclusions 4. Conclusions It was found that the addition of HA was more for the degradation of RhB under acid It was found that the addition of HA was more for the degradation of RhB under acid conditions. conditions. Significant release of dissolved iron from schwertmannite was obtained due to the Significant release of dissolved iron from schwertmannite was obtained due to the reductive dissolution reductive dissolution process. Although the aqueous Fe(II) could be achieved by the involved HA, process. Although the aqueous Fe(II) could be achieved by the involved HA, the transformation from the transformation from Fe(III) to Fe(II) was accelerated through the reduction of Fe(III)-OH to Fe(III) to Fe(II) was accelerated through the reduction of Fe(III)-OH to Fe(II)-OH on the surface of Fe(II)-OH on the surface of schwertmannite. The newly formed Fe(II)-OH2+ contributed to the

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schwertmannite. The newly formed Fe(II)-OH2 + contributed to the improvement of PS activation for the generation of •OH and SO4•− radicals, which dominated the degradation process. Author Contributions: Conceptualization, J.R and B.Y.; Methodology, B.Y.; Software, B.Y.; Validation, J.R.; Formal Analysis, J.R. and B.Y.; Investigation, B.Y.; Writing-Original Draft Preparation, B.Y.; Writing-Review & Editing, J.R.; Visualization, B.Y.; Supervision, J.R.; Project Administration, J.R.; Funding Acquisition, J.R. Funding: This work was supported by the Guizhou Province Science and Technology Department-Guizhou Institute of Technology Joint Fund (Guizhou Science and Technology Agency LH [2015] 7097). Acknowledgments: This work was supported by the Guizhou Province Science and Technology Department-Guizhou Institute of Technology Joint Fund (Guizhou Science and Technology Agency LH [2015] 7097). Conflicts of Interest: The authors declare no conflict of interest.

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