Oxalic Acid-Induced Photodissolution of Ferrihydrite and the Fate of Loaded As(V): Kinetics and Mechanism

Article Oxalic Acid-Induced Photodissolution of Ferrihydrite and the Fate of Loaded As(V): Kinetics and Mechanism

Hai-Tao Ren 1,2,3, Jing Han 1, Ting-Ting Li 1,2, Qi Lin 4,*, Jia-Horng Lin 1,3,5,6,7,8,9,* and Ching-Wen Lou 3,7,8,10,11,*

1 Innovation Platform of Intelligent and Energy-Saving Textiles, School of Textile Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China 2 Tianjin and Ministry of Education Key Laboratory for Advanced Textile Composite Materials, Tianjin Polytechnic University, Tianjin 300387, China 3 Fujian Key Laboratory of Novel Functional Fibers and Materials, Minjiang University, Fuzhou 350108, China 4 Fujian Engineering Research Center of New Chinese Lacquer Material, Minjiang University, Fuzhou 350108, China 5 School of Chinese Medicine, China Medical University, Taichung 40402, Taiwan 6 Laboratory of Fiber Application and Manufacturing, Department of Fiber and Composite Materials, Feng Chia University, Taichung 40724, Taiwan 7 Ocean College, Minjiang University, Fuzhou 350108, China 8 College of Textile and Clothing, Qingdao University, Qingdao 266071, China 9 Department of Fashion Design, Asia University, Taichung 41354, Taiwan 10 Department of Bioinformatics and Medical Engineering, Asia University, Taichung 41354, Taiwan 11 Department of Medical Research, China Medical University Hospital, China Medical University, Taichung 40402, Taiwan * Correspondence: [email protected] or [email protected] (Q.L.); [email protected] (J.-H.L.); [email protected] (C.-W.L.); Tel.: +886-424-518-661 (J.-H.L.) Received: 2 July 2019; Accepted: 6 August 2019; Published: 9 August 2019

Abstract: The fate of arsenic in the water environment is of great concern. Here, the inﬂuences of oxalic acid and UV light illumination on the dissolution of naked ferrihydrite (Fhy), Fhy loaded with As(V) [Fhy*-As(V)], as well as the fate of As(V) at pH 3.0 were studied. With the assistance of oxalic acid, complexes of Fe(III)-oxalic acid produced on Fhy/Fhy*-As(V) were reduced to Fe(II)-oxalic acid by photo-induced electrons under UV light irradiation. UV light has nearly no impact on the release of As(V) in the system of Fhy*-As(V) without the assistance of oxalic acid. Nevertheless, in the existence of oxalic acid, UV light illumination resulted in the contents of liberated As(V) decreased by 775–1300% compared to that without light. Considering the coexistence of As(V), oxalic acid as well as iron oxides in aquatic environments, the present study revealed that UV illumination could enhance the retention of As(V) on Fhy in the acidic water environment containing oxalic acid.

Keywords: ferrihydrite; As(V); photodissolution; oxalic acid; fate

1. Introduction In aquatic environments, arsenic (As) is an omnipresent toxic pollutant and mainly occurs as arsenite [As(III)] and arsenate [As(V)] with inorganic forms [1]. It has been reported that cardiovascular diseases and kidney, skin, liver, bladder, and lung cancers are associated with chronic exposure to As [2,3]. The presence of As in drinking water is a result of the migration of As always caused by weathering of As-bearing minerals, geochemical reactions, volcanic emissions, microbial processes, and other anthropogenic factors containing waste discharge, mining, and use of corresponding

Nanomaterials 2019, 9, 1143; doi:10.3390/nano9081143 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 1143 2 of 12 pesticides [4]. On account of the serious toxicity of As, the United States Environmental Protection 1 Agency (USEPA) in 2001 lowered the guideline from 50 to 10 µg L− for total As in drinking water [5]. To attain this stringent threshold, several technologies have been used to remove As from water, and these contain coagulation–flocculation [6], precipitation [6], ion exchange [6], membrane separation [6], electrochemical methods [6], photocatalysis [7,8], and adsorption [9,10]. Out of these techniques, adsorption is considered one of the most promising techniques due to its low processing cost, simple operation, and high efficiency at different pH of water [6,10]. Ferric (oxyhydr)oxides are considered as the most important adsorbents for fixing As due to their high natural abundance and good binding affinities to As [11–13]. It is obvious that the migration of As is dominantly regulated by the dissolution of ferric (oxyhydr)oxides [14]. A previous study demonstrated that ferric (oxyhydr)oxides could be reduced by Fe(III)-reducing bacteria, which led to the dissolution and subsequent liberation of As fixed on those Fe minerals [15]. Additionally, abiotic factors which triggered dissolution of ferric (oxyhydr)oxides makes a contribution to As migration. In general, photo-triggered reductive dissolution of ferric (oxyhydr)oxides contains two key reaction stages. Firstly, Fe(III) on the (oxyhydr)oxides surface is photoreduced to Fe(II). Secondly, the formed surface Fe(II) is liberated into solution, which determines the overall photodissolution rate [16–20]. Two different mechanisms have been verified to explain the emergence of surface Fe(II) in photic water environments: (a) UV light-triggered charge transfer between lattice O(-II) and Fe(III) in ferric (hydr)oxides produces holes and electrons, and then the latter will cause the reduction of surface Fe(III); (b) the production of surface Fe(II) derives from LMCT (ligand-to-metal charge transfer) reaction in light-reactive surface Fe(III)-organic acid complexes [18]. Dissolved organic matters (DOM) could significantly improve the light-triggered dissolution rate of ferric (hydr)oxides [16,18–20]. For example, Borer et al. [20] have found that siderophores [aerobactin and desferrioxamine B (DFOB)] accelerated the light-triggered dissolution rate of lepidocrocite (γ-FeOOH), and the value increased 91-fold at pH 3.0 with the assistance of DFOB [20]. Oxalic acid is widely distributed in natural water and exhibits a prominent contribution to the dissolution of minerals containing Fe [21]. Considerable attention has been paid to the irradiated oxalic acid–iron (hydr)oxide systems in nature, because pollutants can be degraded via the photo-assistant Fenton-like reaction in such systems [22–26]. Wu et al. [26] revealed that at pH 4.0 dissolved Fe(II) 1 with concentration of 25.0 mg L− was yielded by UV illumination for 5 min in the system of oxalate 1 (2.0 mM) and schwertmannite (0.2 g L− ). After 40 min of UV illumination, the removal percentage 1 of 50.0 mg L− methyl orange was 97%. Nevertheless, very limited attention has been focused on photo-assistant chemical interactions of the ternary ferric (oxyhydr)oxides-oxalic acid-As(V) systems. Meanwhile, under UV irradiation, the impact of oxalic acid on the fate of As(V) fixed on ferric (oxyhydr)oxides has not been thoroughly examined, and the corresponding mechanism for retention of the liberated As(V) by the residual or newly-produced minerals needs to be fully comprehended. Ferrihydrite (Fhy, Fe HO 4H O) is an amorphous to semiamorphous iron mineral mainly ranging 5 8· 2 in size from 3.0 to 10.0 nm. It is omnipresent in nature and has been recognized as one of the most important adsorbents for anions. Our previous study suggested that Fhy has an excellent adsorption 1 capacity for As(V) (160 mg g− ), owing to its larger specific surface area and higher density of reactive hydroxyl groups [27]. Herein, the present study focused on understanding the impacts of oxalic acid and fixed As(V) amount on the light-triggered dissolution of Fhy, and the mechanism of As(V) migration involved in this photodissolution process. Nanomaterials 2019, 9, 1143 3 of 12

2. Materials and Methods

2.1. Synthesis of Fhy and Fhy*-As(V) Fhy used in the present work was the 2-line ferrihydrite, and it was prepared by applying the method developed by Cornell and Schwartzman [12]. In a typical synthesis, twenty gram of Fe(NO ) 9H O (Aladdin Chemicals, Shanghai, China) was dispersed in 250 mL deionized water and 3 3· 2 then 1.0 M NaOH (Aladdin Chemicals, Shanghai, China) was introduced into the solution by vigorous stirring at room temperature to maintain the pH at 7.0. Afterwards, the suspension was aged for 12 h under the static condition. The resulting brown precipitates were concentrated by centrifuging and washed repeatedly with deionized water for four times. In terms of our previous study [27], for the synthesis of Fhy loaded with As(V), the corresponding adsorption kinetics were operated at pH 3.0. NaOH (0.1 M) and HCl (Aladdin Chemicals, Shanghai, China) (0.1 M) solution were employed to adjust the pH. Speciﬁcally, Fhy was ﬁrstly dispersed in NaCl (Aladdin Chemicals, Shanghai, China) solution (0.1 M, 100 mL) at pH 3.0 for 24 h to achieve adsorption equilibrium. Afterwards, NaCl solution (0.1 M, 50 mL) including predetermined contents of As(V) with pH 3.0 was blended intensively with the above Fhy suspension. Initial concentration of Fhy in the 1 obtained suspensions was 2.0 g L− , and the respective concentrations of As(V) were 200, 300, and 1 500 mg L− . After shaking continuously for 24 h with speed of 150 rpm on a shaker, the mixture was centrifuged for 10 min at 4200 rpm, and then washed four times with distilled water. The obtained samples [Fhy loaded with As(V)] were labeled as Fhy*-As(V). In addition, the collected supernate after centrifugation was employed to measure the residual aqueous As(V) contents. Apart from characterization analysis (the obtained solid was freeze dried), the synthesized Fhy and Fhy*-As(V) were directly stored in deionized water.

2.2. Photodissolution Kinetics In this study, dissolution kinetics were conducted in a Model XPA-VII photocatalytic apparatus (Xujiang Electromechanical Plant, Nanjing, China). During the experimental process, the 300 W Hg lamp was turned on 5 min in advance to provide stable illumination intensity. Fhy was ﬁrstly dispersed in NaCl solution (0.1 M, 15 mL) with pH 3.0. To investigate the dissolution of Fhy, oxalic acid in NaCl solution (0.1 M, 15 mL) with equal pHs was mingled with the above suspension. The respective 1 concentration of oxalic acid and Fhy was 1.0 mM and 0.1 g L− . The obtained suspensions were continuously agitated at a speed of 800 10 rpm with a magnetic stirrer during the dissolution process. ± Circulating cooling water was employed to provide a stationary temperature of 25 1 C. For an ± ◦ overall comparison, photo-triggered dissolution kinetics of Fhy and Fhy*-As(V) were carried out as well without the assistance of oxalic acid. During the dissolution process, 1.0 mL samples were taken regularly and centrifuged in 1.5 mL Eppendorf tubes (12,000 rpm, 5 min), and the obtained supernatant was applied to detect the concentrations of aqueous Fe(II), total Fe (denoted as TFe) and As(V). All kinetics were repeated twice and the average data were recorded.

2.3. Solution Analysis and Characterization Methods

1 The modified molybdate-blue method [28], with the detection limit of 0.005 mg L− , was employed to determine the concentration of aqueous As(V). Contents of aqueous Fe(II) and TFe were determined by an improved ferrozine method [29]. X-ray diffraction (XRD; D8 Discover, Bruker, Germany) patterns of ferric (hydr)oxides were analyzed with CuKa radiation (λ = 1.5406 Å). The morphology of Fhy was analyzed using a high-resolution transmission electron microscopy (HRTEM; Tecnai G2F20, FEI, Hillsboro, OR, USA) before and after the dissolution process. The Fourier transformation infrared spectroscopy (FTIR; Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA) was utilized to detect ferric (hydr)oxides. Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 12

3. Results and Discussion

3.1. Characterization of Fhy and Fhy*-As(V) The curve a in Figure 1 manifested the XRD result of the synthetic Fhy, in which all diffraction peaks are in conformity to 2-line Fhy (JCPDS, No. 29-0712). In detail, the respective peak position at 2θ of 35.9° and 62.7° was indexed to the (110) and (106) planes of naked 2-line Fhy [12]. In this study, Fhy*-As(V)-i (i = a, b or c) represented Fhy loaded with different amounts of As(V). Results shown in Table 1 suggested that the respective contents of As(V) over Fhy surface were 99.9, Nanomaterials 2019, 9, 1143 4 of 12 146.8, and 196.8 mg g−1 for Fhy*-As(V)-a, Fhy*-As(V)-b, and Fhy*-As(V)-c. Raven et al. [11] demonstrated that adsorption maxima of 0.25 molAs(V) molFe−1 was achieved by Fhy at pH 4.6. 3. Results and Discussion Table 1. The loaded As(V) amounts on Fhy at pH 3.0. 3.1. Characterization of Fhy and Fhy*-As(V) No. As(V) Loading (mg g−1) The curve a in FigureFhy*-As(V)-a1 manifested the XRD result of the synthetic99.9 Fhy, in which all di ﬀraction peaks are in conformity toFhy*-As(V)-b 2-line Fhy (JCPDS, No. 29-0712). In detail,146.8 the respective peak position at 2θ of 35.9◦ and 62.7◦ was indexedFhy*-As(V)-c to the (110) and (106) planes of naked196.8 2-line Fhy [12].

FigureFigure 1. 1. XRDXRD patterns patterns of of Fhy Fhy ( (aa)) before before and and ( (bb)) after after the the photodissoluti photodissolutionon with oxalic acid.

3.2. EffectIn this of Oxalic study, Acid Fhy*-As(V)-i on the Photodissolution (i = a, b or of c) Fhy represented Fhy loaded with different amounts of As(V). Results shown in Table1 suggested that the respective contents of As(V) over Fhy surface were Without the existence of1 oxalic acid, the dissolution of naked Fhy was investigated in the dark 99.9, 146.8, and 196.8 mg g− for Fhy*-As(V)-a, Fhy*-As(V)-b, and Fhy*-As(V)-c. Raven et al. [11] or under UV illumination, and contents of aqueous Fe(II), Fe(III),1 and TFe were showed in Figure 2. demonstrated that adsorption maxima of 0.25 molAs(V) molFe− was achieved by Fhy at pH 4.6. Aqueous TFe and Fe(II) in Fhy suspension increased at the beginning stage and then leveled off at −1 2.07 and 0.93 mg L after 4Table h of UV 1. The illumination, loaded As(V) respectively amounts on (Figure Fhy at pH 2a). 3.0. Nevertheless, it is observed from Figure 2b that Fhy dissolution only caused aqueous TFe of 0.18 mg L−1, without the occurrence 1 of aqueous Fe(II) in the dark. WithoutNo. the assistanceAs(V) of oxalic Loading acid, (mg theg appearance− ) of aqueous Fe(II) by UV illumination can merelyFhy*-As(V)-a be clarified via the photo-triggered 99.9 reduction of Fe(III) over the Fhy surface. This further demonstratedFhy*-As(V)-b that photodissolution of 146.8Fhy can occur under acidic conditions (e.g., pH = 3.0) without the assistanceFhy*-As(V)-c of DOM, even though with 196.8 small reaction rate. Borer et al. [20] indicated that after 360 min of UV illumination of Fhy suspensions with contents of 25 mg L−1, 8 μM dissolved3.2. Effect ofTFe Oxalic was Acidproduced on the at Photodissolution pH 3.0 and 68% of Fhyof TFe existed as Fe(II). Without the existence of oxalic acid, the dissolution of naked Fhy was investigated in the dark or under UV illumination, and contents of aqueous Fe(II), Fe(III), and TFe were showed in Figure2. Aqueous TFe and Fe(II) in Fhy suspension increased at the beginning stage and then leveled off at 2.07 1 and 0.93 mg L− after 4 h of UV illumination, respectively (Figure2a). Nevertheless, it is observed 1 from Figure2b that Fhy dissolution only caused aqueous TFe of 0.18 mg L − , without the occurrence of aqueous Fe(II) in the dark. Without the assistance of oxalic acid, the appearance of aqueous Fe(II) by UV illumination can merely be clarified via the photo-triggered reduction of Fe(III) over the Fhy surface. This further demonstrated that photodissolution of Fhy can occur under acidic conditions (e.g., pH = 3.0) without the assistance of DOM, even though with small reaction rate. Borer et al. [20] 1 indicated that after 360 min of UV illumination of Fhy suspensions with contents of 25 mg L− , 8 µM dissolved TFe was produced at pH 3.0 and 68% of TFe existed as Fe(II). Nanomaterials 2019, 9, 1143 5 of 12 Nanomaterials 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 55 ofof 1212

((a)) ((b))

Figure 2. Dissolution kinetics with suspensions of 0.1 g L−−11 FhyFhy ((aa)) underunder UVUV illuminationillumination andand ((b)) inin Figure 2. Dissolution kinetics with suspensions of 0.1 g L− Fhy (a) under UV illumination and (b) in thethe dark dark at at pH pH 3.0 3.0 without without oxalicoxalic acid.acid.

By contrast, Fhy dissolution with the assistance of oxalic acid resulted in the production of dissolved TFe with a significant significant amount (Figure 3 3).). OnceOnce FhyFhy waswas mingledmingled withwith oxalicoxalic acidacid atat pHpH 3.0,3.0, −−111 thethe aqueous aqueous TFe TFe attained attained 17.73 17.73 mgmg LL− ,, , indicatingindicating indicating FhyFhy Fhy cancan can bebe be quicklyquickly quickly interactedinteracted interacted withwith with oxalicoxalic oxalic acidacid acid [26].[26]. [26]. WhenWhen thethe suspensions were illuminatedilluminated by UV light, a dramatic decline of aqueous TFe was noticed −−11 1 within the first first 0.5 0.5 h h (Figure (Figure 3a),3a), after which which it it stayed stayed constant constant at at 1.13 1.13 mg mg L L.. − InIn. addition,addition, In addition, pHpH pHofof thethe of suspensionsthesuspensions suspensions roserose rose fromfrom from 3.03.0 3.0toto 6.06.0 to 6.0 afterafter after 44 hh 4 hofof of UVUV UV illumination.illumination. illumination. LanLan Lan etet et al.al. al. [25][25] [25 ] figuredfigured figured outout out thatthat the the aqueous Fe(III)/Fe(II) Fe(III)/Fe(II) would all significantly significantly decline in the irradiated systems of goethite and oxalic acid. They They owed owed this this observation observation to the quick jump of pH pH from from 3.5 3.5 to to 6.0. 6.0. ItIt cancan bebe thereforetherefore deduced deduced thatthat a a large large amount amount of of OH OH-- producedproduced duringduring thethe light-triggeredlight-triggered dissolutiondissolution ofof Fhy,Fhy, which which resulted resulted in in thethe hydrolysis hydrolysis of of aqueous aqueous Fe(III) Fe(III) and the decline of aqueousaqueous TFe. However, However, in the dark, contents of −−11 dissolved Fe(III) varied withwith thethe oppositeoppositete trend.trend. ItIt roserose tototo 29.2429.2429.24 mgmgmg LLL− andand remained remained constant constant after after 120120 min reaction, confirmingconfirming thethe keykey rolerole of oxalic acid in promoting the dissolution of Fhy. Aqueous Fe(II) could hardly be found during the total experimental period (Figure 33b),b), showingshowing thatthat aqueousaqueous TFe existed mainly as Fe(III) specie species.s.s. After After 240 240 minmin reaction,reaction, pH pH valu valuevaluee of of FhyFhy suspensionssuspensions in in the the dark dark remainedremained 3.0. 3.0.

((a)) ((b))

Figure 3. Dissolution kinetics with suspensions of 0.1 g L−−11 Fhy at pH 3.0 (a) under UV illumination Figure 3.3. Dissolution kinetics with suspensions of 0.10.1 gg LL− FhyFhy at pH 3.0 (a) under UVUV illuminationillumination andand ( (b)) in in the the dark dark in in the the presencepresenpresencece ofof 1.01.0 mMmM oxalicoxalic acid.acid.

To betterbetter comprehendcomprehend the photo-triggered dissolution of of Fhy Fhy with with the the assistance assistance ofof oxalicoxalic acid,acid, morphological changes of Fhy and newly-produced se secondarycondarycondary ironiron mineralsminerals werewere investigated.investigated. XRD XRD analysis of Fhy after 4 h of UV illumination with oxalic acid showed patterns similar to the synthetic 2-line Fhy, suggestingsuggesting thatthat mostmost ofof ironiron oxideoxide remainedremained FhyFhy (Figure(Figure 11).). However,However, anan additionaladditional newnew peak at 33.2° 33.2◦ waswas attributed attributed to (130) plane of goethite (JCPDS, No. 810464) 810464) [30]. [30]. HRTEM HRTEM image image further further foundfound (111)(111) latticelattice fringesfringes withwith aa spacingspacing ofof 0.2450.245 nm,nm, manifestingmanifesting thethe productionproduction ofof goethitegoethite inin thethe

Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 12 photo-inducedNanomaterials 2019, dissolution9, 1143 process of Fhy with oxalic acid, which was in line with the XRD result6 of 12 (Figure 1 and 4). FTIR analysis of Fhy after 240 min of UV illumination with oxalic acid showed peaks at 794 and 890found cm (111)−1 (Figure lattice 5), fringes which withis assigned a spacing to OH of 0.245 bending nm, manifestingmode in goethite the production [31,32], further of goethite verified in the Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 12 photo-inducedtransformation dissolutionof Fhy to goethite. process Cornell of Fhy withand Sc oxalichwertmann acid, which [12] suggested was in line that with Fhy the is XRDmetastable result (Figures1 and4). andphoto-induced as a precursor dissolution to goethite. process of Fhy with oxalic acid, which was in line with the XRD result (Figure 1 and 4). FTIR analysis of Fhy after 240 min of UV illumination with oxalic acid showed peaks at 794 and 890 cm−1 (Figure 5), which is assigned to OH bending mode in goethite [31,32], further verified the transformation of Fhy to goethite. Cornell and Schwertmann [12] suggested that Fhy is metastable and as a precursor to goethite.

(a) (b)

Figure 4. TEMTEM analysis analysis of Fhy ( a)) before before and ( b) after the photodissolution with 1.0 mM oxalic acid.

FTIR analysis of Fhy after 240 min of UV illumination with oxalic acid showed peaks at 794 and 1 890 cm− (Figure5), which is assigned to OH bending mode in goethite [ 31,32], further veriﬁed the transformation of Fhy to goethite.(a) Cornell and Schwertmann [12] suggested(b) that Fhy is metastable and as a precursor to goethite. Figure 4. TEM analysis of Fhy (a) before and (b) after the photodissolution with 1.0 mM oxalic acid.

Figure 5. FTIR analysis of Fhy and Fhy*-As(V) after the photodissolution with 1.0 mM oxalic acid.

3.3. Effect of UV/Oxalate on the Dissolution of Fhy*-As(V) and the Mobilization of As(V) Obviously, As(V) had an adverse impact on the photodissolution of Fhy. When Fhy*-As(V)-a, Fhy*-As(V)-b and Fhy*-As(V)-c suspensions were illuminated by UV light without the assistance of oxalic Figureacid, the 5. FTIRcontents analysis of aqueous of Fhy and TFe Fhy*-As(V) increased after slightly thethe photodissolution within the first with 90 min 1.0 mMand oxalic oxalicthen leveled acid.acid. off at 0.60, 0.45, and 0.15 mg L−1, respectively (Figure 6a). Dissolved Fe(II) changed in a similar way as the3.3. dissolvedEffect Effect of UVUV/Oxalate TFe,/Oxalate and ontheon thethe final DissolutionDissolution concentr ofofation Fhy*-As(V)Fhy*-As(V) reached and 0.45, the 0.15, Mobilization and 0.15 ofof mg As(V)As(V) L−1 for Fhy*-As(V)-a, Fhy*-As(V)-b,Obviously, and As(V)As(V) Fhy*-As( hadhad ananV)-c adverseadverse systems, impactimpact respectively onon thethe photodissolutionphotodissolution (Figure 6b). Nevertheless, of Fhy. When nearly Fhy*-As(V)-a, no Fhy*- As(V)Fhy*-As(V)-b dissolution and was Fhy*-As(V)-c observed suspensionsin the dark without were illuminated the assistance by UVof oxalic light withoutacid (data the not assistanceassistance shown). of oxalic acid, the contents of aqueous TFe increased slightlyslightly within the firstfirst 90 min and then leveled ooffff 1 at 0.60,0.60, 0.45,0.45, andand 0.150.15 mgmg LL−−1,, respectively (Figure 66a).a). DissolvedDissolved Fe(II)Fe(II) changedchanged inin aa similarsimilar wayway asas 1 thethe dissolveddissolved TFe,TFe, andand thethe finalfinal concentrationconcentration reachedreached 0.45,0.45, 0.15,0.15, andand 0.150.15 mgmg LL−−1 for Fhy*-As(V)-a, Fhy*-As(V)-b, andand Fhy*-As(V)-c Fhy*-As(V)-c systems, systems, respectively respectively (Figure (Figure6b). 6b). Nevertheless, Nevertheless, nearly nearly no Fhy*-As(V) no Fhy*- dissolutionAs(V) dissolution was observed was observed in the darkin the without dark without the assistance the assistance of oxalic of acidoxalic (data acid not (data shown). not shown).

Nanomaterials 2019, 9, 1143 7 of 12 Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 12

−11 FigureFigure 6.6. DissolutionDissolution kineticskinetics underunder UVUV illuminationillumination withwith suspensionssuspensions ofof 0.1 0.1 g g L L− Fhy*-As(V)Fhy*-As(V) atat pHpH 3.0.3.0. Dissolved TFe ((aa),), dissolveddissolved Fe(II)Fe(II) ((bb),), andand As(V)As(V) ((cc)) werewere measured.measured.

TheThe mixmix ofof Fhy*-As(V)Fhy*-As(V) suspensionssuspensions andand oxalicoxalic acidacid immediatelyimmediately resultedresulted inin thethe occurrenceoccurrence ofof 1−1 aqueousaqueous TFe.TFe. TheThe respectiverespective concentrationsconcentrations of of TFe TFe were were 14.49, 14.49, 8.91, 8.91, and and 8.30 8.30 mg mg L− L for for Fhy*-As(V)-a, Fhy*-As(V)- Fhy*-As(V)-b,a, Fhy*-As(V)-b, and and Fhy*-As(V)-c Fhy*-As(V)-c (Figure (Figure7a). 7a). In the In dark,the dark, both both the dissolved the dissolved TFe and TFe Fe(II) and Fe(II) released released from Fhy*-As(V)from Fhy*-As(V) increased increased first and first then and leveled then leveled off after of 3 hf after reaction. 3 h reaction. The final The contents final ofcontents aqueous of TFe aqueous were 1 −1 32.45,TFe were 31.70, 32.45, and 31.7031.70, mg and L −31.70for Fhy*-As(V)-a,mg L for Fhy*-As(V)-a, Fhy*-As(V)-b, Fhy*-As(V)-b and Fhy*-As(V)-c, and Fhy*-As(V)-c systems, respectively systems, (Figurerespectively7a). The (Figure change 7a). of The aqueous change Fe(II) of aqueous was di ff Fe(IerentI) fromwas different the result from of Fhy the ( Figuresresult of6 bFhy and (Figure7b). The 5b 1 −1 respectiveand 7b). The contents respective of aqueous contents Fe(II) of aqueous were 14.49, Fe(II) 8.91, were and 14.49, 8.30 mg8.91, L −andfor 8.30 the mg system L for of Fhy*-As(V)-a,the system of Fhy*-As(V)-b,Fhy*-As(V)-a, andFhy*-As(V)-b, Fhy*-As(V)-c and after Fhy*-As(V)-c 4 h reaction. after 4 h reaction.

Nanomaterials 2019, 9, 1143 8 of 12 Nanomaterials 2019, 9, x FOR PEER REVIEW 8 of 12

−1 1 Figure 7. DissolutionDissolution kinetics kinetics in in the the dark dark with with suspensions suspensions of of 0.1 0.1 g gL L −Fhy*-As(V)Fhy*-As(V) at pH atpH 3.0 3.0with with 1.0 1.0mM mM oxalic oxalic acid. acid. Dissolved Dissolved TFe TFe (a), (dissolveda), dissolved Fe(II) Fe(II) (b), ( andb), and As(V) As(V) (c) were (c) were measured. measured.

A downtrenddowntrend was was found found for dissolvedfor dissolved TFe withTFe thewith assistance the assistance of oxalic acidof oxalic under acid UV illumination under UV 1 withinillumination the first within 120 the min, first after 120 which min, after it leveled which o itff leveledat 0.60, off 0.45, at 0.60, and 0.450.45,mg and L 0.45− for mg Fhy*-As(V)-a, L−1 for Fhy*- Fhy*-As(V)-b,As(V)-a, Fhy*-As(V)-b, and Fhy*-As(V)-c and Fhy*-As(V)-c systems, respectivelysystems, respectively (Figure8a). (Figure It could 8a). be It interpreted could be interpreted that As(V) occupiedthat As(V) surface occupied sorption surface sites sorption on Fhy sites and on oxalic Fhy acidand didoxalic not acid touch did with not touch Fhy readily, with Fhy thus readily, caused thus less dissolutioncaused less dissolution of Fhy*-As(V) of Fhy*-As(V) [33]. In Fhy*-As(V)-a, [33]. In Fhy* aqueous-As(V)-a, Fe(II) aqueous changed Fe(II) in changed a similar in trend a similar to aqueous trend 1 TFe.to aqueous While forTFe. Fhy*-As(V)-b While for Fhy*-As(V)-b and Fhy*-As(V)-c, and Fhy*-As(V) dissolved-c, Fe(II) dissolved contents Fe(II) lifted contents to 2.39 lifted and 2.69 to 2.39 mg and L− in2.69 the mg first L−1 0.5in the h,respectively. first 0.5 h, respectively. Afterwards, Afterwards, the concentration the concentration of dissolved of dissolved Fe(II) had Fe(II) a sink had of a about sink 1 2.30of about mg L2.30− between mg L−1 between 0.5 h and 0.5 2.0 h hand (Figure 2.0 h8 (Figureb). 8b). As(V) fate during the photodissolution of Fhy*-As(V)Fhy*-As(V) without the assistance of oxalic acid was presented inin FigureFigure6 c.6c. For For Fhy*-As(V)-a, Fhy*-As(V)-a, Fhy*-As(V)-b, Fhy*-As(V)-b, and and Fhy*-As(V)-c Fhy*-As(V)-c systems, systems, As(V) As(V) contents contents rose 1 androse attainedand attained the peak thevalues peak values at 0.6, 1.0,at 0.6, and 1.0, 1.6 mgand L −1.6in mg the L first−1 in 2 the h of first UV illumination,2 h of UV illumination, respectively. Withrespectively. the assistance With ofthe oxalic assistance acid, two of oxalic stages acid, were two found stages for the were migration found offor As(V) the migration(Figures7c of and As(V)8c) . 1 The(Figure released 7c and As(V) 8c). The into released solution wasAs(V) 1.8, into 2.0, solution and 3.6 mgwas L 1.8,− after 2.0, theand mix 3.6 mg of oxalic L−1 after acid the and mix Fhy*-As(V) of oxalic suspensionsacid and Fhy*-As(V) for Fhy*-As(V)-a, suspensions Fhy*-As(V)-b, for Fhy*-As(V)-a and Fhy*-As(V)-c,, Fhy*-As(V)-b, respectively. and Fhy*-As(V)-c, Oxalic acid respectively. complexed withOxalic Fe acid atoms complexed and dragged with them Fe outatoms from and the dragged surface of them Fhy, andout As(V)from the would surface not be of liberated Fhy, and until As(V) the Fewould atoms not that be heldliberated As(V) until were the dissolved. Fe atoms Fromthat held Figures As(V)7c and were8c, dissolved. the mobilized From As(V) Figure leveled 7c and o 8c,ff after the 30mobilized min reaction As(V) both leveled under off UVafter irradiation 30 min reaction and in both the dark, under which UV irradiation is attributed and to in the the rapid dark, reaction which betweenis attributed oxalic to the acid rapid and reaction Fhy*-As(V). between However, oxalicthey acid hadand aFhy*-As(V). different change However, trend they in the had first a different 30 min, withchange the trend aqueous in the As(V) first increase30 min, with in the the dark aqueous and decrease As(V) increase under UVin the illumination. dark and decrease The final under contents UV ofillumination. As(V) in the The dark final were contents 2.6, 4.8, of As(V) and 6.2 in mgthe Ldark1, 7.75were13.0 2.6, 4.8, times and as 6.2 that mg under L−1, 7.75 UV− illumination,13.0 times as − − forthat Fhy*-As(V)-a, under UV illumination, Fhy*-As(V)-b, for Fhy*-As(V)-a, and Fhy*-As(V)-c Fhy*-As(V)-b, systems, and respectively Fhy*-As(V)-c(Figures systems,7c and respectively8c) . A pH increases(Figure 7c of theand suspension 8c). A pH from increases 3.0 to ~6.0 of wasthe foundsuspension in the photodissolutionfrom 3.0 to ~6.0 process was found of Fhy*-As(V) in the withphotodissolution oxalic acid. process Fe(III) hydrolysisof Fhy*-As( atV) highwith pHoxalic might acid. result Fe(III) in hydr the generationolysis at high of pH goethite, might whichresult wasin the conducive generation to of the goethite, re-sorption which of was aqueous conducive As(V). to As the a result,re-sorption oxalic of acid aqueous only resultedAs(V). As in a 0.2, result, 0.4, oxalic acid only resulted in 0.2, 0.4, and 0.8 mg L−1 mobilized As(V) under UV light illumination for Fhy*-As(V)-a, Fhy*-As(V)-b, and Fhy*-As(V)-c systems, respectively (Figure 8c).

Nanomaterials 2019, 9, 1143 9 of 12

1 and 0.8 mg L− mobilized As(V) under UV light illumination for Fhy*-As(V)-a, Fhy*-As(V)-b, and Fhy*-As(V)-cNanomaterials 2019 systems,, 9, x FOR respectivelyPEER REVIEW (Figure8c). 9 of 12

−11 Figure 8. Dissolution kinetics under UVUV illuminationillumination withwith suspensionssuspensions ofof 0.10.1 gg LL− Fhy*-As(V) at pH 3.0 with 1.0 mM oxalic acid.acid. Dissolved TFe (a), dissolved Fe(II)Fe(II) ((b),), andand As(V)As(V) ((cc)) werewere measured.measured.

To clarifyclarify thethe compositioncomposition changeschanges ofof ironiron mineralmineral duringduring thethe dissolutiondissolution ofof Fhy*-As(V),Fhy*-As(V), XRDXRD analysis was conducted. However, no other peaks could be observed from XRD results of Fhy*-As(V) after 4 h ofof UVUV illuminationillumination with the assistanceassistance of oxalic acid, probably due to the suppression of As(V) on the transformation of FhyFhy or low contents of newly-formed iron minerals. A previous study suggested thatthat As(V)As(V) adsorbedadsorbed on on Fhy Fhy signiﬁcantly significantly inhibited inhibited the the transformation transformation of of Fhy Fhy to goethiteto goethite as wellas well [27 [27].].

3.4. Mechanism of Photodissolution and As(V)As(V) Fate in the System of Fhy*-As(V) On accountaccount ofof the the above above points points and and published published works works [7, 18[7,18,23,26,27],,23,26,27], a possible a possible mechanism mechanism of Fhy of dissolutionFhy dissolution triggered triggered by oxalic by oxalic acid andacid As(V)and As(V migration) migration in Fhy*-As(V) in Fhy*-As(V) with with and withoutand without UV light UV III III (2n 3(2n-3)) islight illuminated is illuminated (Figure (Figure9). Oxalic 9). Oxalic acid ﬁrstlyacid firstly forms forms[ Fe [≡( FeC O(C)O] )−] − complexes complexes with with Fe(III) Fe(III) on ≡ 2 4 n theon the surface surface of Fhy of Fhy via sorptionvia sorption interaction interaction [Equation [Equation (1)]. Herein,(1)]. Herein, the symbol the symbolrepresents ≡ represents the surface the III(2n 3) (2n-3) ≡ surface site of an iron (hydr)oxide. Subsequently,III [≡ Fe (C−O−)] is excited by UV light to site of an iron (hydr)oxide. Subsequently, [ Fe (C2O4)n] is excited by UV light to produce produce surface Fe(II) complexes of [≡ FeII≡(C 4O 2n) ]4-2n and some radicals containing oxalic acid surface Fe(II) complexes of [ FeII(C O ) ] − (n-1)and some radicals containing oxalic acid radical • 2 4 (n 1) • • radical [(CO) ], carbon dioxide≡ anion radical− [(CO) ], and superoxide radical (O ) [Equations [(C O )•−], carbon dioxide anion radical [(CO )•−], and superoxide radical (O ) [Equations (2–4)]. 2 4 • 3+ 2 2+ 2•− (2–4)]. O can further react3 +with Fe to generate2+ Fe and O2 [Equation (5)]. Fe(III)-oxalic acid O2•− can further reactIII with Fe3-2n to generate Fe and O2 [Equation (5)]. Fe(III)-oxalic acid complexes complexes of [Fe3 2n(CO)] in the suspension could be reduced as well under UV illumination [ III( ) ] − ofaccordingFe C2 toO4 Equationn in (6). the suspensionWithout the could assistance be reduced of UV as light, well the under adsorbed UV illumination As(V) on accordingFhy releases to Equation (6). Without the assistance of UV light, the adsorbed As(V) on Fhy releases into the solution into the solution during the dissolution process, with the peak As(V) concentration of 6.2 mg L−1 1 during(Figure the7c). dissolution However, process,with the with assistance the peak of As(V)UV light, concentration As(V) migration of 6.2 mg in Lthe− Fhy*-As(V)(Figure7c). However,system is with the assistance of UV light, As(V) migration in the Fhy*-As(V) system is inhibited apparently [the inhibited apparently [the peak As(V) concentration is only 0.8 mg L−1 (Figure 8c)], which might be 1 peakascribed As(V) to some concentration liberated is As(V) only 0.8in suspension mg L− (Figure being8c)], re-fixed which on might residual be ascribed Fhy and to newly-produced some liberated goethite [27]. As a result, the effect of UV light should be considered when evaluating the fate of As(V) in water environments including iron minerals and organic acids.

Fhy + n H C O ↔ [≡ FeIII (C O ) ](2n-3)- 2 2 4 2 4 n (1)

Nanomaterials 2019, 9, 1143 10 of 12

As(V) in suspension being re-ﬁxed on residual Fhy and newly-produced goethite [27]. As a result, the eﬀect of UV light should be considered when evaluating the fate of As(V) in water environments including iron minerals and organic acids.

Nanomaterials 2019, 9, x FOR PEER REVIEW i(2n 3) 10 of 12 Fhy + n H C O [ FeIII (C O ) − − (1) 2 2 4 ↔ ≡ 2 4 n III (2n-3)- II 4-2n •- [≡ Fe (C O ) ] + hv → [ ≡ Fe (C O ) ] + C O (2) 2 i4(2nn 3) 2 4 (n-1) 4 2n 2 4 III − − II − [ Fe (C 2O4)n + hv [ Fe (C 2O4)(n 1) + C2O4•− (2) ≡ •-→ ≡ •- C O → CO + CO − (3) 2 4 2 2 C2O4•− CO2 + CO2•− (3) •- → •- CO + O → CO + O (4) CO •−2 + O 2 CO 2+ O •−2 (4) 2 2 → 2 2 • - + 3+3+ → 22++ + O O•− + Fe Fe Fe + OO (5)(5) 2 2 → 22 III 3 2n3-2n II 44-2n •- [FeIII (C O )−] + hv → [FeII (C O ) ] − + C O [Fe (C 2O24)n4] n + hv [Fe (C 2O2 4)4(n(n1-1)) + C2 O4•− (6)(6) → −

FigureFigure 9.9. ProposedProposed mechanismsmechanisms ofof FhyFhy photodissolutionphotodissolution andand As(V)As(V) migrationmigration inin Fhy*-As(V).Fhy*-As(V). 4. Conclusions 4. Conclusions This study investigates the impacts of oxalic acid and UV light illumination on the dissolution This study investigates the impacts of oxalic acid and UV light illumination on the dissolution of naked Fhy and Fhy loaded with As(V). In the dark and under UV illumination, roles of oxalic of naked Fhy and Fhy loaded with As(V). In the dark and under UV illumination, roles of oxalic acid acid are diﬀerent for the dissolution of Fhy. Fhy dissolution triggered by oxalic acid was principally are different for the dissolution of Fhy. Fhy dissolution triggered by oxalic acid was principally mediated by the ligand-facilitated process in the dark. Complexes of Fe(III)-oxalic acid produced on mediated by the ligand-facilitated process in the dark. Complexes of Fe(III)-oxalic acid produced on Fhy were transformed into Fe(II)-oxalic acid by UV light. For Fhy*-As(V) systems, UV illumination has Fhy were transformed into Fe(II)-oxalic acid by UV light. For Fhy*-As(V) systems, UV illumination almost no impact on the migration of As(V) without the existence of oxalic acid. Nevertheless, with has almost no impact on the migration of As(V) without the existence of oxalic acid. Nevertheless, the assistance of oxalic acid, UV light illumination resulted in the released As(V) contents decreased by with the assistance of oxalic acid, UV light illumination resulted in the released As(V) contents 775% 1300% compared to that in the dark. Considering the coexistence of As(V), oxalic acid and iron decreased− by 775%−1300% compared to that in the dark. Considering the coexistence of As(V), oxalic oxides in acidic water environments, this study revealed that solar light (containing UV) illumination acid and iron oxides in acidic water environments, this study revealed that solar light (containing could improve the retention of As(V) on Fhy in the aquatic environments including oxalic acid. UV) illumination could improve the retention of As(V) on Fhy in the aquatic environments including Authoroxalic Contributions:acid. In this study, the concepts and designs for the experiment are supervised by J.-H.L. and Prof. C.-W.L. Experiment and data processing are conducted by H.-T.R. and J.H. Text composition and results analysisAuthor areContributions: performed by In T.-T.L.this study, The experimentalthe concepts and result designs is examined for theby experiment Q.L. and H.-T.R.are supervised by J.-H.L. and Funding:Prof. C.-W.L.This Experiment work was supported and data byprocessing the Opening are conducted Project of Green by H.-T.R. Dyeing and and J.H. Finishing Text composition Engineering and Research results Centeranalysis of are Fujian performed University by T.-T.L. [No. 2017001A,The experiment 2017001B,al result 2017002B is examined and 2017004B]; by Q. L. and the H.-T.R. Open Project Program of Fujian Key Laboratory of Novel Functional Fibers and Materials (Minjiang University), China (No. FKLTFM 1722); theFunding: National This Natural work Sciencewas supported Foundation by the of Opening China [No. Project 21806121]; of Green the Dyeing Natural and Science Finish Foundationing Engineering of Tianjin Research [No. 17JCQNJC08000Center of Fujian and University 18JCQNJC03400]; [No. 2017001A, the Natural 2017001B, Science 201 Foundation7002B and 2017004B]; of Fujian Province the Open [No. Project 2018J01504 Program and of 2018J01505]Fujian Key andLaboratory the Program of Novel for Innovative Functional Research Fibers and Team Materi in Universityals (Minjiang of Tianjin University), [No. TD13-5043]. China (No. FKLTFM 1722); the National Natural Science Foundation of China [No. 21806121]; the Natural Science Foundation of Tianjin [No. 17JCQNJC08000 and 18JCQNJC03400]; the Natural Science Foundation of Fujian Province [No. 2018J01504 and 2018J01505] and the Program for Innovative Research Team in University of Tianjin [No. TD13- 5043].

Conflicts of Interest: The authors declare no conflict of interest. References

Nanomaterials 2019, 9, 1143 11 of 12

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Smedley, P.L.; Kinniburgh, D.G. A review of source, behaviour and distribution of arsenic in natural waters. Appl. Geochem. 2002, 17, 517–568. [CrossRef] 2. Chen, Y.; Ahsan, H. Cancer burden from asenic in drinking water in Banladesh. Am. J. Public Health 2004, 94, 741–744. [CrossRef][PubMed] 3. Duker, A.A.; Carranza, E.J.; Hale, M. Arsenic geochemistry and health. Environ. Int. 2005, 31, 631–641. [CrossRef][PubMed] 4. Mohan, D.; Pittman, C.U., Jr. Arsenic removal from water/wastewater using adsorbents-A critical review. J. Hazard. Mater. 2007, 142, 53. [CrossRef][PubMed] 5. Ogata, F.; Ueta, E.; Kawasaki, N. Characteristics of a novel adsorbent Fe–Mg-type hydrotalcite and its adsorption capability of As(III) and Cr(VI) from aqueous solution. J. Ind. Eng. Chem. 2018, 59, 56–63. [CrossRef] 6. Siddiqui, S.I.; Naushad, M.; Chaudhry, S.A. Promising prospects of nanomaterials for arsenic water remediation: A comprehensive review. Process Saf. Environ. Prot. 2019, 126, 60–97. [CrossRef] 7. Ren, H.T.; Ji, Z.Y.; Wu, S.H.; Han, X.; Liu, Z.M.; Jia, S.Y. Photoreductive dissolution of schwertmannite induced by oxalate and the mobilization of adsorbed As(V). Chemosphere 2018, 208, 294–302. [CrossRef] [PubMed] 8. Ren, H.T.; Han, J.; Li, T.T.; Sun, F.; Lin, J.H.; Lou, C.W. Visible light-induced oxidation of aqueous arsenite

using facile Ag2O/TiO2 composites: Performance and mechanism. J. Photochem. Photobiol. A Chem. 2019, 377, 260–267. [CrossRef] 9. Asere, T.G.; Stevens, C.V.; Laing, G.D. Use of (modiﬁed) natural adsorbents for arsenic remediation: A review. Sci. Total Environ. 2019, 676, 706–720. [CrossRef] 10. Hao, L.L.; Liu, M.Z.; Wang, N.N.; Li, G.J. A critical review on arsenic removal from water using iron-based adsorbents. RSC Adv. 2018, 8, 39545–39560. [CrossRef] 11. Raven, K.P.; Jain, A.; Loeppert, R.H. Arsenite and arsenate on ferrihdrite: Kinetics, equilibrium, and adsorption envelopes. Environ. Sci. Technol. 1998, 32, 344–349. [CrossRef] 12. Cornell, R.M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses; Wiley-VCH: Weinheim, Germany, 2003. 13. Han, C.Y.; Liu, H.; Chen, H.R.; Zhang, L.M.; Wan, G.P.; Shan, X.; Deng, J.S.; Luo, Y.M. Adsorption performance and mechanism of As(V) uptake over mesoporous Y-Al binary oxide. J. Taiwan Inst. Chem. E 2016, 65, 204–211. [CrossRef] 14. Erbs, J.J.; Berqerquó, T.S.; Reinsch, B.C.; Lowry, G.V.; Banerjee, S.K.; Penn, R.L. Reductive dissolution of arssenic-bearing ferrihydrite. Geochim. Cosmochim. Acta 2010, 74, 3382–3395. [CrossRef] 15. Fendorf, S.; Michael, H.A.; Van Geen, A. Spatial and temporal variations of ground water arsenic in South and Southeast Asia. Science 2010, 328, 1123–1127. [CrossRef][PubMed] 16. Banwart, S.; Davies, S.; Stumm, W. The role of oxalic acid in accelerating the reductive dissolution of hematite

(α-Fe2O3) by ascorbate. Colloids Surf. 1989, 39, 303–309. [CrossRef] 17. Sulzberger, B.; Laubscher, H. Reactivity of various types of iron(III) (hydr)oxides towards light-induced dissolution. Mar. Chem. 1995, 50, 103–115. [CrossRef] 18. Borer, P.M.; Sulzberger, B.; Reichard, P.; Kraemer, S.M. Eﬀect of siderophores on the light-induced dissolution of colloidal iron (III)(hydr)oxides. Mar. Chem. 2005, 93, 179–193. [CrossRef] 19. Borer, P.; Sulzberger, B.; Hug, S.J.; Kraemer, S.M.; Kretzschmar, R. Photoreductive dissolution of iron(III) (hydr)oxides in the absence and presence of organic ligands: Experimental studies and kinetic modeling. Environ. Sci. Technol. 2009, 43, 1864–1870. [CrossRef] 20. Borer, P.; Kraemer, S.M.; Sulzberger, B.; Hug, S.J.; Kretzschmar, R. Photodissolution of lepidocrocite (γ-FeOOH) in the presence of desferrioxamine B and aerobactin. Geochim. Cosmochim. Acta 2009, 73, 4673–4687. [CrossRef] 21. Reichard, P.U.; Kretzschmar, R.; Kraemer, S.M. Dissolution mechanisms of goethite in the presence of siderophores and organic acids. Geochim. Cosmochim. Acta 2007, 71, 5635–5650. [CrossRef] 22. Balmer, M.E.; Sulzerger, B. Atrazine degradation in irradiated iron/oxalate systems: Eﬀects of pH and oxalate. Environ. Sci. Technol. 1999, 33, 2418–2424. [CrossRef] Nanomaterials 2019, 9, 1143 12 of 12

23. Lei, J.; Liu, C.S.; Li, F.B.; Li, X.M.; Zhou, S.G.; Liu, T.X.; Gu, M.H.; Wu, Q.T. Photogradation of orange I in the heterogeneous iron oxide-oxalate complex system under UVA irradiation. J. Hazard. Mater. 2006, 137, 1016–1024. [CrossRef][PubMed] 24. Li, F.B.; Li, X.Z.; Liu, C.S.; Li, X.M.; Liu, T.X. Effect of oxalate on photodegradation of Bisphenol A at the interface of different iron oxides. Ind. Eng. Chem. Res. 2007, 46, 781–787. [CrossRef] 25. Lan, Q.; Liu, H.; Li, F.B.; Zeng, F.; Liu, C.S. Effect of pH on pentachlorophenol degradation in irradiation in irradiated iron/oxalate systems. Chem. Eng. J. 2011, 168, 1209–1216. [CrossRef] 26. Wu, Y.; Guo, J.; Jiang, D.; Zhou, P.; Lan, Y.; Zhou, L. Heterogeneous photocatalytic degradation of methyl orange in schwertmannite/oxalate suspension under UV irradiation. Environ. Sci. Pollut. Res. 2012, 19, 2313–2320. [CrossRef][PubMed] 27. Yu, B.; Jia, S.Y.; Liu, Y.; Wu, S.H.; Han, X. Mobilization and re-adsorption of arsenate on ferrihydrite and hematite in the presence of oxalate. J. Hazard. Mater. 2013, 262, 701–708. [CrossRef][PubMed] 28. Dhar, R.K.; Zheng, Y.; Rubenstone, J.; van Geen, A. A rapid colorimetric method for measuring arsenic concentrations in groundwater. Anal. Chim. Acta 2004, 526, 203–209. [CrossRef] 29. Stookey, L.L. Ferrozine—A new spectrophotometric reagent for iron. Anal. Chem. 1970, 42, 779–781. [CrossRef] 30. Burton, E.D.; Bush, R.T.; Sullivan, L.A.; Mitchell, D.R.G. Schwertmannite transformation to goethite via the Fe(II) pathway: Reaction rates and implications for iron-sulfide formation. Geochim. Cosmochim. Acta 2008, 72, 4551–4564. [CrossRef] 31. Cambier, P. Infrared study of goethites of varying crystallinity and particle size: I. Interpretation of OH and lattice vibration frequencies. Clay Miner. 1986, 21, 191–200. [CrossRef] 32. Bhandari, N.; Hausner, D.B.; Kubicki, J.D.; Strongin, D.R. Photodissolution of ferrihydrite in the presence of oxalic acid: An in situ ATR-FTIR/DFT study. Langmuir 2010, 26, 16246–16253. [CrossRef][PubMed] 33. Eick, M.J.; Peak, J.D.; Brady, W.D. The effect of oxyanions on the oxalate-promoted dissolution of goethite. Soil Sci. Soc. Am. J. 1999, 63, 1133–1141. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).