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The combined effect of weak magnetic field and anions on arsenite sequestration by zero-valent iron: Kinetics and mechanisms Yuankui Sun, Yihong Hu, Tinglin Huang, Jinxiang Li, Hejie Qin, and Xiaohong Guan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06117 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017
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The combined effect of weak magnetic field and
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anions on arsenite sequestration by zero-valent iron:
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Kinetics and mechanisms
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Yuankui Sun,†, ‡ Yihong Hu,†, ‡ Tinglin Huang,†, ‡ Jinxiang Li,§ Hejie Qin,§ Xiaohong
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Guan*, §
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†
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of Education, Xi’an University of Architecture and Technology, Xi’an 710055, P. R.
Key Laboratory of Northwest Water Resources, Environment and Ecology, Ministry
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China
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‡
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Architecture and Technology, Xi’an 710055, P. R. China
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§
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Environmental Science and Engineering, Tongji University, Shanghai 200092, P. R.
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China
Shaanxi Key Laboratory of Environmental Engineering, Xi’an University of
State Key Laboratory of Pollution Control and Resources Reuse, College of
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*Author to whom correspondence should be addressed
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Xiaohong Guan, email:
[email protected]; phone: +86-21-65980956; fax:
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86-21-65986313.
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ABSTRACT
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In this study, the effects of major anions (e.g., ClO4-, NO3-, Cl-, and SO42-) in water on
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the reactivity of zero-valent iron (ZVI) toward As(III) sequestration were evaluated
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with and without a weak magnetic field (WMF). Without WMF, ClO4- and NO3- had
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negligible influence on As(III) removal by ZVI but Cl- and SO42- could improve
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As(III) sequestration by ZVI. Moreover, the WMF-enhancing effect on As(III)
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removal by ZVI was minor in ultrapure water. A synergetic effect of WMF and
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individual anion on improving As(III) removal by ZVI was observed for each of the
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investigated anion, which became more pronounced as the concentration of anion
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increased. Based on the extent of enhancing effects, these anions were ranked in the
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order of SO42- > Cl- > NO3- ≈ ClO4- (from most to least enhanced). Furthermore, the
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inhibitory effect of HSiO3-, HCO3- and H2PO4- on ZVI corrosion could be alleviated
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taking advantage of the combined effect of WMF and SO42-. The coupled influence of
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anions and WMF was associated with the simultaneous movement of anions with
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paramagnetic Fe2+ to keep local electroneutrality in solution. Our findings suggest that
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the presence of anions is quite essential to maintain or stimulate the WMF effect.
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INTRODUCTION
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Zero-valent iron (ZVI) has been shown to be a promising technology for the
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remediation/treatment of groundwater and wastewater contaminated with halogenated
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solvents,1 nitroaromatic compounds,2-3 dyes,4 heavy metals,5-7 metalloids,8-10 and so
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on. Nonetheless, there are still several intrinsic limitations of ZVI technology
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inhibiting its application.11 Among the limitations of ZVI technology, the low
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reactivity of ZVI has arisen many concerns and many efforts have been made to
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increase the ZVI reactivity.12
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There are many factors determining the reactivity of ZVI under realistic
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conditions, one of the most widely recognized and thoroughly studied being solution
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chemistry, especially the co-existing anions.13-16 Quite a few studies have shown that
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the identity and concentration of major anions present in natural water would dictate
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the performance and longevity of ZVI, which can enhance or reduce the ZVI
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reactivity. For example, nitrate, as a reducible solute, was reported to inhibit the
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contaminants reduction by competing reactive sites and/or passivating the ZVI
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surface.17-19 Silicate is also a known corrosion inhibitor that adsorbs readily onto iron
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oxides and thus prevents ZVI from sustained anodic dissolution.20-21 In contrast,
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chloride and sulfate have been reported to enhance iron reactivity since they are
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capable of destabilizing passive surface films.22-24 Carbonate has also been
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extensively investigated, and it was found to be corrosive in the short term but
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passivating in the long run, depending on the types and amounts of corrosion
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products.25-27
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As mentioned above, the roles of anions as inhibitors or promoters in ZVI system
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have been widely investigated and identified, but there are still many contradictory 3
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results in the literatures. For example, Su and Puls18 have systematically compared the
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effects of major anions on the removal of both arsenate (As(V)) and arsenite (As(III))
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from groundwater by granular iron. They found that all the tested anions decreased
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arsenic removal rates relative to chloride (10 mM), and the anions were ranked PO43- >
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HSiO3- > NO3- > CO32- >SO42- (from most to least inhibited). Liu et al.28 evaluated the
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effects of common groundwater solutes (5 mN) on trichloroethylene (TCE) reduction
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by nanoscale ZVI and found that they inhibited TCE reduction in decreasing order of
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NO3- > H2PO4- > HCO3- > SO42- > Cl- (from most to least inhibited). Undoubtedly, the
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different findings can be attributed to the differences in the experimental conditions,
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such as target pollutants,29 anion concentrations,26,
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stirring methods and rates, and so on.23, 30
30
pH levels,3,
31
iron types,32
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In our recent studies, we have demonstrated that the application of a weak
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magnetic field (WMF) can accelerate the ZVI corrosion and thus enhance the removal
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of many toxic ions (e.g. Se(IV),33-34 Se(VI),35 Sb(V),36 As(V)/As(III),37 Cu(II),38
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Cr(VI)39) by ZVI and improve the removal of organic contaminants by H2O2 or
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persulfate activated by ZVI.40-41 Following our study, Xu et al. also reported that the
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removal rates of Sb(III)42 and Orange I43 by ZVI were promoted by 6-72 fold due to
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the application of a WMF, and Xiang et al. showed that WMF improved the
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degradation of 4-chlorophenol (4-CP) in ZVI/H2O2 Fenton-like system.44 However,
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none of the above studies have recognized and addressed the WMF effect on the
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reactivity changes of ZVI in the presence of different anions.45 In other words, the
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major anion effects on ZVI reactivity with the presence of a WMF may be quite
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different from that without. However, this hypothesis has not been validated, to the
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best of our knowledge. Devlin et al.23 investigated the individual effects of SO42-, Cl-, 4
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NO3-, HCO3- (8 mM ionic strength solutions) on the reactivity of granular iron toward
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4-chloronitrobenzene at initial pH (pHini) 10.0 using a glass-encased magnet reactor.
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A magnet was used in their reactor to hold the iron stationary, thus the magnetic field
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effect should exist in their system although the authors did not recognize it. The
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effects of SO42-, Cl-, NO3-, and HCO3- on the reactivity of ZVI toward
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4-chloronitrobenzene were ranked SO42- > Cl- ≥ NO3- > HCO3- (from most to least
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reactive), which may be associated with the magnetic field effect. Therefore, the
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effects of major anions on the reactivity and performance of ZVI in the presence of
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WMF need to be better understood. On the other hand, regarding to the mechanisms
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of the WMF enhancing effect, it has been confirmed that the magnetic field gradient
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force (F∆ ) was the major driving force, which can lead to an uneven distribution of
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paramagnetic Fe2+ and eventually localized distribution of corrosion products in the
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vicinity of the ZVI sphere.46 Considering that Fe2+ is positively charged, its movement
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in boundary layer may be influenced by the negatively charged anions.47-48 Thus, from
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the perspective of better understanding how WMF effect works, the role of
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co-existing anions in ZVI corrosion with WMF should also be clarified in more detail.
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Arsenic is a contaminant of particular concern due to its complex chemistry,
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chronic toxicity, and widespread presence in groundwater across the globe.49-50 Many
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studies have shown that both As(V) and As(III) can be effectively sequestered by ZVI
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through adsorption and co-precipitation.51 In this study, As(III) was selected as the
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target pollutant and its sequestration rate was employed as an indicator of ZVI
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reactivity. In sum, the main objectives of this work were to 1) quantitatively compare
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the effects of major anions in groundwater (including NO3-, SO42-, Cl-, HSiO3-, HCO3-,
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and H2PO4-) on the kinetics of As(III) sequestration by ZVI with and without WMF; 2) 5
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investigate the potential mechanisms of coupled effects of anions and WMF on
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enhancing the ZVI reactivity. Since perchlorate (ClO4-) was generally considered to
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be inert and interact minimally with ZVI surface, the influence of ClO4- on the
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reactivity of ZVI was also examined in this study as the benchmark for comparisons.23,
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52
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performance of ZVI.
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EXPERIMENTAL SECTION
This study will also provide some deep insights into how the WMF influences the
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Materials. All chemicals employed in this study were of analytical grade and
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used as received. The sodium salts of the anions were used. The iron particles were
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obtained from the Shanghai Jinshan reduced iron powder factory (China), which had a
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mean diameter of 40 µm and a BET surface area of 0.76 m2/g. All experiments were
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conducted using the ultrapure water (UP water) produced by a Milli-Q Reference
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water purification system.
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Bach Experiments and Analytical Methods. The experimental setup described
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in our previous study was also used in this study.37 Briefly, a permanent magnet with
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a diameter of 20 mm and thickness of 1 mm was placed under the reactor, which
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could provide a maximum magnetic field flux intensity of ~15 mT at the bottom of
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the reactor. The working solutions (1000 µg/L As(III), Se(IV), Sb(V), or Cr(VI)
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solutions with different co-existing anions of different concentrations) were freshly
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prepared for each batch test. If it was not otherwise specified, the pHini value of the
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working solution was adjusted to 7.0 by dropwise addition of NaOH after the dosing
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of ClO4-, NO3-, Cl-, SO42-, and/or H2PO4- to avoid introducing other anions.
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The influences of HSiO3-, H2PO4-, and HCO3- on As(III) removal by ZVI are
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much more complicated than those of ClO4-, NO3-, SO42-, and Cl- because HSiO3-, 6
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H2PO4-, and HCO3- may compete with As(III)/As(V) for the adsorption sites and form
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soluble Fe(II)/Fe(III) complexes or Fe(II)/Fe(III) precipitates.53 On the other hand, the
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pHini levels of the working solution were elevated to 10.5 and 8.9, respectively, when
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0.5 mM HSiO3- and 5.0 mM HCO3- were dosed to the working solution. The major
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species of silicate at pH 10.5 is HSiO3- (or H3SiO4-) and that of carbonate is HCO3- at
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pH 8.9.18, 25 Thus, when the influence of HSiO3- or HCO3- of lower concentration was
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investigated, the pHini levels were accordingly adjusted to 10.5 and 8.9, respectively,
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with NaOH after the addition of HSiO3- and HCO3-. Since the removal of As(III) at
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pH 7.0, 8.9, and 10.5 by ZVI in UP water was minor even with the presence of WMF,
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1 mM SO42-, 3 mM SO42-, and 3 mM SO42- were employed as the background
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electrolyte to manifest the influence of HSiO3-, H2PO4-, and HCO3- on As(III) removal
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by ZVI with or without WMF. It should be specified that the variation of pH in most
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of the experiments was lower than 0.5. Moreover, the influence of chloride on the
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depletion of total arsenic and As(III) as well as the evolution of As(V) in the process
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of As(III) removal by ZVI with the presence of WMF at pHini 5.0 and 8.0 was
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examined to clarify the influence of chloride on each stage of As removal by
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WMF/ZVI.
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Batch tests were initiated by adding 0.1 g ZVI into a 500 mL solution, and the
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solution was mixed by a mechanical stirrer at 400 rpm to keep ZVI in suspension. All
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experiments were carried out open to the air and run in duplicate. At fixed time
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intervals, approximately 5 mL of suspension was sampled, filtered through a 0.22 µm
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membrane filter, and acidified for analysis of total As, Se, Sb with ICP-MS. The
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speciation of As(III) and As(V) in solution was determined by HPLC-ICP-MS
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following the method set up in our previous study.37 The concentration of Cr(VI) was 7
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analyzed with colorimetric methods using a UV-visible spectrophotometer by
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monitoring the absorbance at 540 nm after its reaction with diphenylcarbazide. The
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concentrations of NO3- and NH4+ were determined by the hydrochloric acid
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photometry and Nessler’s reagent spectrophotometry, respectively.54
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At the end of the experiments examining the influence of HSiO3-, the precipitates
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were collected on membrane filters (0.22 µm), washed with UP water, freeze-dried
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under vacuum, and put into zipper bags before being subjected to Fe K-edge X-Ray
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Absorption Fine Structure (XAFS) analysis. The XAFS spectra were recorded at room
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temperature using a 4 channel Silicon Drift Detector (SDD) Bruker 5040 at beam line
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BL14W1 of the Shanghai Synchrotron Radiation Facility (SSRF), China. The Fe
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K-edge XAFS spectra were recorded in transmission mode, and then processed and
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analyzed by the software codes Athena.55 The major species of Fe in reacted ZVI
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samples were determined by linear combination fitting (LCF) using the collection of
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reference materials including metallic Fe (Fe0), magnetite (Fe3O4), maghemite
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(γ-Fe2O3), lepidocrocite (γ-FeOOH), and goethite (α-FeOOH).
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RESULTS
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In our previous study, when the working solution contained 3 mM SO42- and
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1000 µg L-1 As(III), 50.4% of arsenic was removed by 0.1 g L-1 ZVI at pHini 7.0 in 3 h
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without WMF and 92.7% of arsenic was removed in 1.5 h with WMF.37 However,
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only 13.1% and 35.8% of arsenic was removed by 0.2 g L-1 ZVI at pHini 7.0 in 24 h
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without and with WMF, respectively, if the working solution only contained 1000 µg
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L-1 As(III), as illustrated in Figure 1. Obviously, sulfate greatly affected the reactivity
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of ZVI without WMF and it also had great influence on the WMF-induced
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improvement on As(III) removal by ZVI. This phenomenon may be not restricted to 8
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sulfate, and a synergistic effect of WMF and various anions on enhancing ZVI
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reactivity toward As(III) removal was expected.
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Effect of Perchlorate. It was observed that, without WMF, the presence of 1-30
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mM ClO4- did not exhibit considerably promotive or inhibitive effect on the kinetics
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of As(III) sequestration within 24 h, as illustrated in Figure 1(a). This suggested that,
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in the tested concentration range, ClO4- exerted little influence on the ZVI corrosion
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without WMF. However, as shown in Figure 1(b), it was surprised to observe that
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ClO4- remarkably enhanced both the sequestration rate and efficiency of As(III) upon
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the introduction of a WMF. The dosing of 1 mM ClO4- greatly improved the removal
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of As(III) by ZVI with WMF from 35.8% in 24 h to 53.6% within 180 min (Figure
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1(b)). To better describe the influence of anions on the reactivity of ZVI toward
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As(III), the As(III) disappearance kinetics were simulated with the pseudo-first-order
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reaction model (eq. 1), and the corresponding observed rate constants (kobs, min-1)
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were listed in Figure S1 and Table S1 of Supporting Information.
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d[As(III)] = −kobs [As(III)] dt
(1)
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Compared to the kobs of As(III) removal obtained in UP water (3.0×10-4 min-1) with
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WMF, the rate constant of As(III) removal was increased to 3.6×10-3 min-1 in the
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presence of 1 mM ClO4-. The increase in ClO4- concentration from 1 to 10 mM
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progressively accelerated As(III) sequestration by ZVI with WMF and the rate
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constant was further increased to 14.2×10-3 min-1 when 10 mM ClO4- was added.
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These results indicated that, although perchlorate itself interacted minimally with ZVI,
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its presence could significantly impact the ZVI reactivity when WMF was applied.
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Effect of Nitrate. NO3- has very different influence on the kinetics of As(III)
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removal by ZVI with or without WMF, as demonstrated in Figure 1(c-d). Figure 1c 9
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reveals that, without WMF, NO3- dosed at 1-30 mM did not induce noticeable
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influence on the As(III) removal rate and efficiency within 24 h. However, upon the
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application of WMF, 1 mM NO3- dramatically elevated the kobs of As(III) removal by
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ZVI from 3.0×10-4 (UP water) to 3.9×10-3 min-1 (see Figure 1(d) and Table S1).
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Further increasing the nitrate concentration to 2.5, 5, 10 mM progressively increased
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the rate constants to 4.9×10-3, 13.2×10-3, 15.4×10-3 min-1, respectively, which was
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similar to the trend observed in ClO4- solutions. Obviously, this over one-order
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promoting effect of nitrate on the rate of As(III) removal by ZVI in the presence of
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WMF was inconsistent with the conventional wisdom that nitrate was an iron
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corrosion inhibitor.17-19 Considering that nitrate can be reduced by ZVI to ammonia,
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additional iron corrosion products may be provided for As(III) sequestration due to
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nitrate reduction. However, negligible NO3- removal and NH4+ generation were
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observed in the tested time scale (see Figure S2). Hence, the improved ZVI corrosion
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caused by NO3- reduction should be ignored in this work. For other reducible anions
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such as ClO4- and SO42-, considering they are more difficult to be reduced by ZVI
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than NO3- in short-term, it can be expected that the influence from their reduction
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should also be negligible.
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Effect of Chloride. The influences of various concentrations of chloride on the
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As(III) sequestration kinetics with and without WMF are shown in Figure 1(e-f).
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Compared to the case in UP water, the presence of Cl- enhanced both As(III) removal
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rate constants and efficiencies significantly, regardless of the presence of WMF.
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Without the presence of WMF, ∼18.0% of As(III) could be removed in 180 min and
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the corresponding rate constant was increased to 9.0×10-4 min-1 (Table S1) due to the
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dosing of 1 mM Cl- to the working solution. As shown in Figure 1(e), the 10
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chloride-induced enhancement became more pronounced with increasing chloride
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concentrations from 5 to 50 mM (the rate constant was as high as 1.5×10-2 min-1 with
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50 mM Cl-). Likewise, with WMF, the presence of Cl- enhanced the ZVI reactivity
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markedly. As illustrated in Figure 1(f), with the addition of 1 mM Cl-, the rate
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constants of As(III) removal by ZVI was determined to be 8.6×10-3 min-1 in the
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presence of WMF, which was about one order of magnitude higher than that in the
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absence of WMF (9.0×10-4 min-1). In other words, dosing of 1 mM Cl- enhanced the
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rate constants of As(III) sequestration by ZVI by 11.1 and 28.6 times without and
239
with WMF, respectively. Obviously, the chloride-induced enhancement effect on ZVI
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reactivity was amplified with the aid of WMF.
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Effect of Sulfate. Sulfate is another common anion that can enhance the
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reactivity of ZVI. The impact of SO42- on the kinetics of As(III) removal by ZVI in
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the absence (Figure 1(g)) and presence (Figure 1(h)) of WMF was also investigated.
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Obviously, similar to chloride, the presence of SO42- remarkably enhanced ZVI
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reactivity toward As(III) over the tested concentration range, regardless of the
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presence of WMF. Another notable feature in Figure 1 was that the sulfate-enhancing
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effect was much more pronounced than chloride, which was in agreement with the
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observations reported by Devlin et al.23 Without WMF, the addition of 0.25 mM SO42-
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into the suspensions yielded a kobs value as high as 1.0×10-3 min-1, which was more
250
than one order of magnitude higher than that in UP water (8.1×10-5 min-1). As the
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concentration of SO42- was increased to 10 mM, almost complete As(III) sequestration
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(96.7%) was achieved within 180 min, and the corresponding rate constant of As(III)
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removal was determined to be 9.3×10-3 min-1. The enhancing effect of SO42- was
254
greatly amplified due to the introduction of WMF. As shown in Figure 1(h), in the 11
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presence of WMF and 0.01 mM SO42-, 80.4% of As(III) could be removed from water
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within 300 min and the rate constant reached 6.7×10-3 min-1. As the concentration of
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SO42- was further elevated to 1 mM, As(III) was almost completely removed from
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water within 60 min. Correspondingly, the rate constant of As(III) removal
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dramatically increased to as high as 3.9×10-2 min-1, which was even larger than the
260
case with the presence of WMF and 10 mM Cl- (2.4×10-2 min-1).
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DISCUSSION
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Role of Co-existing Anions in ZVI Reactivity with WMF. According to the
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aforementioned findings, all the four examined anions played a coupled enhancing
264
effect with WMF on As(III) removal by ZVI. In other words, the co-existing anions in
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solutions played important roles in the WMF enhancing effect on contaminant
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sequestration by ZVI. With respect to the mechanism of As(III) removal by ZVI, it is
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commonly accepted that As(III) is sequestered mainly by adsorption and
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co-precipitation, resulting from the continuous generation of iron (oxyhydr)oxides due
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to the ZVI corrosion reaction.51 Hence, it can be concluded that the co-presence of
270
WMF
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adsorption/co-precipitation process. On the other hand, many studies have shown that
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the transformation of As(III) to As(V) occurred in the process of As(III) removal by
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ZVI.49 Moreover, our previous study confirmed that the application of WMF greatly
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enhanced the conversion of As(III) to As(V) at pH range of 3.0-9.0 with 3 mM CaSO4
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as background electrolyte.37 Therefore, it is expected that, the anions such as Cl-,
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SO42-, NO3-, and ClO4- can enhance the As(III) oxidization process during As(III)
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removal by ZVI with the presence of WMF. To verify this point, taking Cl- as a
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representative anion, the variation of dissolved As(III) and As(V) concentrations was
and
anions
accelerated
ZVI
corrosion
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determined in the process of As(III) removal by ZVI with the co-present of WMF and
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Cl- at pHini 5.0 and 8.0. As demonstrated in Figure S3, as the Cl- concentration
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increased from 1 mM to 10 mM, the accumulation of As(V) became much quicker at
282
pHini 5.0 and 8.0, which indicated that the co-presence of WMF and Cl- enhanced the
283
formation of oxidizing agents. Accordingly, it can be inferred that the combined
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effects of WMF and anions not only enhanced the adsorption/co-precipitation of
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As(III)/As(V) but also accelerated the transformation of As(III) to As(V) in solution.
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With regard to the mechanisms involved in the WMF-induced improvement of
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ZVI reactivity, many efforts have also been made in our previous studies. Two forces,
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including the magnetic field gradient force (F∆) and the Lorentz force (F ) were
289
believed to contribute to the WMF-enhancing effect. Very recently, in order to
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identify the relative contribution of F∆ and F in the WMF enhancing effect,
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further investigations were carried out and it was verified experimentally and
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theoretically that F∆ (see eq. 2) was the major driving force.46
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F∆ = B ∆B
(2)
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Hereby, χ denotes the molar magnetic susceptibility of the involved species and c is
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their concentration, µ0 is the magnetic permeability of vacuum, B is flux intensity of
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magnetic field, and ∆B is gradient of the magnetic field which mainly arises from the
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induced magnetic field around the magnetized ZVI particles. Being pulled by F∆,
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the paramagnetic Fe2+ generated in ZVI corrosion tends to move along the magnetic
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lines to the place with higher magnetic field flux intensity, thereby resulting in an
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uneven distribution of Fe2+ and eventually localized distribution of oxide films on
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ZVI surface.46 This mechanism, highlighting the key role of Fe2+, well explained the
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WMF-induced improvement of ZVI reactivity. However, considering the F∆ is a 13
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body force,56-57 the movement of Fe2+ driven by F∆ should not be an isolated
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process but may be accompanied with some secondary effects or influenced by some
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factors, e.g., the presence of oppositely charged anions. Thus, in order to improve our
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understanding of how the WMF effect works, the Fe2+ transport within the boundary
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layer should be further discussed by taking the co-existing anions into account.
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Actually, in the electrochemical processes, the effects of magnetic field have been
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intensively investigated over the past several decades.47-48, 56, 58-59 More importantly,
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based on the numerical and experimental studies, a mathematical model for the effect
311
of magnetic field on the transport of paramagnetic ions in a solution has been
312
developed.47-48 In brief, this model is established on the energy density of a magnetic
313
field (eq. 2), the Navier-Stokes equation describing solution flow (eqs. 3 and 4), the
314
Nernst-Planck equation describing electro-diffusion in the solution (eq. 5), and finally
315
the condition of local electroneutrality of the solution as a simplified form of the
316
Poisson equation (eq. 6).48
317
+ ∇ = ∇ −
∇
+ +
∆
(3)
318
Here, F∆ is taken into the Navier-Stokes equation, denotes the velocity of
319
the solution, P is pressure, g is acceleration of gravity, ρ is the solution density. The
320
equation of continuity is written in the form:
321 322 323 324 325 326
∇ = 0
(4)
Equations for ionic transport for ith component of the solution can be written as: ∇i = 0, i = 1, 2, 3 …
(5)
where, J denotes the flux density. Finally, a condition of local electroneutrality is added in solution: ∑) () *) = 0 14
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where, z is the number of electrical charges and C are their corresponding ion
328
concentrations.
329
According to this model, F∆ could not only act on the paramagnetic ions (i.e.,
330
Fe2+ herein) but also induce additional convection of all components of the
331
solution.47-48 Given that, we tried to use this model to explain the role of anions in
332
WMF-induced improvement of ZVI reactivity as follows.
333
Firstly, the principle of local electroneutrality (eq. 6) should be emphasized,
334
despite it was listed lastly in the abovementioned model. Being driven by F∆ , as
335
stated earlier, Fe2+ moves toward the ends of ZVI sphere which are near the magnetic
336
poles and have the highest |B∆B| . Inevitably, this process will lead to an
337
accumulation of positive charges in these regions. However, in a steady state, these
338
positive charges must be balanced with negative charges to keep local charge
339
neutrality. Otherwise, the movement of Fe2+ would be inhibited or even stopped. It is,
340
therefore, expected that there should be sufficient negatively charged anions in
341
solution and these anions should move together with Fe2+ toward the regions with
342
high |B∆B|. This hypothesis about the role of anions in WMF effect is schematically
343
present in Figure 2. In particular, with regard to the anion-containing scenario, since
344
the co-existing anions such as ClO4-, NO3-, Cl-, and SO42- can balance the positive
345
charges, the transport of Fe2+ inside the boundary layer can be maintained and thereby
346
the ZVI corrosion process can be sustained. However, for the anion-free cases, due to
347
electrostatic exclusion, the accumulation of Fe2+ (positive charge) would suppress the
348
further movement of Fe2+ from the low |B∆B| regions. Hence, the localized
349
distribution of passive oxide film with the presence of WMF and anions would be
350
mediated under the anions-free conditions, and thereby a significant WMF enhancing 15
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effect on ZVI corrosion could not be obtained (e.g., the case of UP water in Figure 1).
352
On the basis of the attractive interaction between oppositely-charged ions, it is
353
expected that the flow of Fe2+ driven by F∆ could induce a secondary flow of
354
co-existing anions, as sketched in Figure 2. Indeed, it had been experimentally shown
355
that paramagnetic metal ions and water molecules move together in a magnetic field
356
due to the F∆.56-57 Additionally, taking into account that electrolyte ions are tightly
357
bound with water due to the polar-polar interactions in aqueous solutions,60 the
358
assumption of the simultaneous movement of Fe2+ and anions is quite reasonable.
359
Secondarily, the continuous flow property of fluid (as reflected by eq. 4) is
360
essential to allow the combined effect of WMF and anions to work properly. It is
361
believed that the magnetic field tends to cause an additional convective transfer of all
362
components of the solution (Fe2+, anions, O2, H+, and contaminants), which will be
363
generated in the vicinity of ZVI surface.47 Therefore, in sum, the co-existing anions
364
appear to serve as “carriers” of Fe2+ to the adjacent regions of high magnetic flux
365
density. It should be noted that, with the presence of WMF, these co-existing anions
366
should still play their basic roles of supporting electrolytes to transfer electron in
367
solutions, although their potential functions are far more than this.
368
Comparison of the Enhancing Effects of Different Anions. In the above
369
section, the combined effect of WMF and various anions has been generally explained
370
in terms of electroneutrality and flow continuity. However, as depicted in Figure S1,
371
the enhancing effect of anions on ZVI reactivity toward As(III) varied with anion
372
types and concentrations. To provide a generalized basis for the comparison among
373
different anions (i.e., ClO4-, NO3-, SO42-, and Cl-), we calculated the ratio (Ranion, the
16
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promotion factor) of kinetic constants (see Table S1) measured with specific anions
375
and without (UP water) in the presence of WMF, according to eq. 7.
376
,-./0. =
1234 56726 1234 89
(7)
377
As demonstrated in Figure 3, for ClO4- and NO3-, similar Ranion values were obtained
378
over the concentration range of 1-10 mM, which fell in the range of 12-51. Whereas
379
for 0.5-10 mM Cl-, Ranion varied from 23 to 79, which was always higher than its
380
counterparts of ClO4- and NO3-. Compared to ClO4- and NO3-, Cl- is believed to be
381
much more aggressive towards passivating oxides because they diffuse readily into
382
the film and form strong complexes with iron centers. These complexes can enhance
383
the dissolution of iron oxide and often cause pitting corrosion.61 Accordingly, it was
384
not unusual to observe a better ZVI performance with Cl- irrespective of the presence
385
of WMF. Although many studies have reported that SO42- could improve the ZVI
386
performance,3, 43, 62 it was surprised to find that the promotion factor was as high as
387
22-129 with the presence of 0.01-1 mM SO42- in the presence of WMF. As a bivalent
388
anion, SO42- possesses more charges per ion than a monovalent anion. This feature
389
may partially explain the excellent performance of SO42- in improving As(III) removal
390
by ZVI with WMF, since electroneutrality is assumed to play an important role in the
391
combined effect of WMF and anions. Nevertheless, further investigations are
392
undoubtedly needed before the exact mechanism can be conclusively established.
393
In order to further examine the relationship between Ranion and the specific anion
394
concentrations, correlation analysis was conducted and the results were plotted in
395
Figure 3. It was found that, for ClO4-, NO3- and Cl-, the Ranion values increased linearly
396
with the initial anion concentrations over the tested range (0.5-10 mM, typical
397
groundwater concentration range). These strong positive correlations further 17
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confirmed the key role of anions in WMF enhancing effect on ZVI reactivity.
399
Whereas for SO42-, Ranion exhibited a hyperbolic behavior as a function of the initial
400
SO42- concentration (insert of Figure 3), which is very different from that of ClO4-,
401
NO3- and Cl-. Ranion of SO42- also experienced an initial linear increase at low
402
concentration but eventually stabilized at relative high concentrations. It can be
403
expected that the linear-dependence of Ranion on anion concentration will become
404
minor if the concentrations of ClO4-, NO3- or Cl- are further elevated, which will be
405
verified in our further study.
406
The Influence of HSiO3-, H2PO4-, and HCO3- on As(III) Removal by ZVI.
407
Given that the typical concentration of silicate in groundwater was in the range of
408
0.12-0.75 mM,20 two concentration levels (0.1 and 0.5 mM) were selected to study its
409
effect on As(III) removal by ZVI at pH 10.5. As expected, an inhibitory effect was
410
observed and little As(III) was sequestrated from water with or without WMF due to
411
the addition of 0.1 or 0.5 mM HSiO3- within 24 h (Figure S4). This inhibitory effect
412
could not be well manifested within the analyzed timescale since the ZVI reactivity
413
was very low in UP water at pH 10.5. Therefore, to better unravel its adverse effect, 3
414
mM SO42- was added as the background solute and the effect of silicate was
415
re-evaluated. As shown in Figure 4(a), the presence of 0.5 mM HSiO3- decreased the
416
As(III) removal from 74.9% to 21.3% without WMF at 5 h, or from 63.2% to 30.1%
417
with WMF at 3 h. Based on the best LCF results of the EXAFS spectra of the
418
As(III)-treated ZVI samples (see Figure S5 and Table S2), the consumed Fe0 fractions
419
during the reaction process were obtained and depicted in the insert of Figure 4(a).
420
With WMF, 89.7% of the metallic iron was transformed to iron (hydr)oxides in
421
silicate-free system within 3 h, while this fraction decreased to 48.2% after being 18
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422
exposed to 0.5 mM HSiO3-. Likewise, without WMF, the consumed Fe0 within 5 h
423
declined from 22.2% to 3.6% when 0.5 mM HSiO3- was present. Therefore, the
424
depressed As(III) removal caused by silicate should be mainly ascribed to the
425
inhibitory effect of silicate on the ZVI corrosion regardless of the application of
426
WMF.
427
Similar to the case of silicate, the influence of H2PO4- on As(III) removal by ZVI
428
with or without WMF was investigated with 1 mM SO42- as background electrolyte at
429
pH 7.0 and the results are demonstrated in Figure 4(b). The application of WMF
430
remarkably increased the removal of As(III) from ~10% to ~100% within 90 min
431
when there was no H2PO4-, arising from the synergistic effect of WMF and SO42- on
432
ZVI corrosion. The dosing of H2PO4- at either 0.01 or 0.1 mM significantly slowed
433
the removal of As(III) with the presence of WMF. Considering that phosphate has the
434
similar structure with arsenite/arsenate, competitive adsorption would inevitably occur
435
between phosphate and arsenite/arsenate, resulting in a drop in arsenic removal.18 The
436
decrease in arsenic removal may also be attributable to the passivating effect of
437
phosphate on ZVI, especially at high phosphate concentration, even in the presence of
438
WMF. This can be evidenced by the fact that the color of ZVI suspension with 0.01
439
mM H2PO4- was much darker than that of 0.1 mM H2PO4-, as depicted in Figure S6.
440
Previous studies have shown that the effect of carbonate (and bicarbonate) is
441
strongly dependent on its concentration.25,
30
With 3 mM SO42- as background
442
electrolyte, the addition of 1 mM HCO3- accelerated the removal of As(III) by ZVI
443
while the dosing of 5 mM HCO3- depressed As(III) removal, irrespective of the
444
presence of WMF (as shown in Figure 4(c)). A comparison of ZVI suspension colors
445
at different HCO3- concentrations reveals that bicarbonate dosed at 1 mM accelerated 19
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446
ZVI corrosion while that dosed at 5 mM retarded ZVI corrosion (Figure S7). It had
447
been documented that when the HCO3- concentration was relatively low, amorphous
448
Fe(OH)2 was formed preferentially relative to passive FeCO3, allowing HCO3- to
449
promote iron corrosion.30 However, with the HCO3- concentration increasing, FeCO3
450
would form preferentially, which could passivate the iron surface.30
451
A close inspection of the data present in Figure 4 revealed that As(III) was
452
always removed with a greater rate with WMF than its counterpart without WMF,
453
ascribed to the coupled influence of WMF and sulfate on the reactivity of ZVI.
454
Although the dosing of 0.5 mM HSiO3-, 0.1 mM H2PO4-, or 5 mM HCO3- greatly
455
depressed As(III) removal by ZVI in the presence of WMF and SO42-, the
456
sequestration rates of As(III) removal by ZVI under these conditions were greater
457
than or comparable to the case without WMF and these anions. This suggested that,
458
the coupled effects of WMF and sulfate (perhaps also perchlorate, nitrate and chloride)
459
could be employed to alleviate the adverse influences induced by such iron corrosion
460
inhibitors as silicate, phosphate and bicarbonate on contaminants removal by ZVI.
461
Furthermore, the experiments examining the effects of H2PO4-, HCO3-, and HSiO3- on
462
As(III) removal by ZVI with SO42- as background anion were conducted at pHini 7.0,
463
8.9, and 10.5, respectively, implying that WMF and anions had coupled effect on
464
As(III) removal by ZVI over a wide pH range. This was further verified by the fact
465
that increasing the Cl- concentration from 1 mM to 10 mM enhanced both the
466
depletion of As(III) and the accumulation of As(V) at pHini 5.0 and 8.0, as illustrated
467
in Figure S3.
468
Environmental Implications. Many studies have shown that the removal of
469
target contaminant by ZVI is influenced by the co-existing anions. The results of this 20
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470
study demonstrates that the effects of major anions on ZVI reactivity vary widely due
471
to the application of WMF. These findings improved our understanding of the effects
472
of anions on ZVI performance and thus could benefit the design and operation of a
473
ZVI based technology. Another implication of this work should be a new method by
474
taking advantage of the combined effect of WMF (or pre-magnetization) and anions
475
(e.g., SO42-) can be developed either to enhance iron reactivity or to overcome the
476
adverse effect of co-solutes. It should be noted that, the introduction of WMF/anions
477
could enhance the ZVI reactivity towards many other oxyanions besides As(III). For
478
example, as shown in Figure S8, the co-presence of WMF and 1 mM SO42-
479
significantly enhanced the removal of Se(IV), Sb(V) and Cr(VI) by ZVI compared to
480
the cases where there was only WMF or 1 mM SO42-. Nevertheless, there is also a
481
need to further investigate the coupled impact of WMF and anions on the
482
immobilization of heavy metals (e.g., Cu2+ and Pb2+) and transformation of organic
483
contaminants (e.g., trichloroethylene and nitrobenzene) by ZVI.
484
AUTHOR INFORMATION
485
Corresponding Author
486
Email:
[email protected], Phone: +86-21-65980956, Fax: +86-21-65986313
487
Notes
488
The authors declare no competing financial interest.
489
ACKNOWLEDGMENT
490
This work was supported by the National Natural Science Foundation of China
491
(Grants 21522704, 51478329, 51608431, and U1532120), the China Postdoctoral
492
Science Foundation (2015M580824), the Fundamental Research Funds for the Central
493
Universities, the Shaanxi Postdoctoral Science Foundation (2016BSHYDZZ45), the 21
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494
Research Project of Shaanxi Education Department (16JS060), and the Research Fund
495
of Xi’an University of Architecture and Technology (QN1616). The authors thank the
496
beamline BL14W1 (Shanghai Synchrotron Radiation Facility) for providing the beam
497
time.
498
Supporting Information Available
499
Eight figures and two tables are provided in the Supporting Information. This material
500
is available free of charge via the Internet at http://pubs.acs.org.
501
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persulfate by Fe0 coupling with weak magnetic field: Performance and mechanism.
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Water Res. 2014, 62, 53-62.
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Xiong, X.; Sun, Y.; Sun, B.; Song, W.; Sun, J.; Gao, N.; Qiao, J.; Guan, X.,
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Enhancement of the advanced Fenton process by weak magnetic field for the
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degradation of 4-nitrophenol. RSC Adv. 2015, 5, (18), 13357-13365.
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Xu, C. H.; Zhang, B. L.; Zhu, L. J.; Lin, S.; Sun, X. P.; Jiang, Z.; Tratnyek, P.
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G., Sequestration of antimonite by zerovalent iron: Using weak magnetic field effects
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to enhance performance and characterize reaction mechanisms. Environ. Sci. Technol.
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2016, 50, 1483-1491.
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Xu, C.; Zhang, B.; Wang, Y.; Shao, Q.; Zhou, W.; Fan, D.; Bandstra, J. Z.;
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Shi, Z.; Tratnyek, P. G., Effects of sulfidation, magnetization, and oxygenation on azo
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dye reduction by zerovalent Iron. Environ. Sci. Technol. 2016, 50, (21), 11879-11887.
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Xiang, W.; Zhang, B.; Zhou, T.; Wu, X.; Mao, J., An insight in magnetic
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field enhanced zero-valent iron/H2O2 Fenton-like systems: Critical role and evolution
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of the pristine iron oxides layer. Sci. Rep. 2016, 6, 24094.
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Kim, D. H.; Kim, J.; Choi, W., Effect of magnetic field on the zero valent
iron induced oxidation reaction. J. Hazard. Mater. 2011, 192, (2), 928-931. (46)
Li, J.; Qin, H.; Zhang, W.-x.; Shi, Z.; Zhao, D.; Guan, X., Enhanced Cr(VI)
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removal by zero-valent iron coupled with weak magnetic field: Role of magnetic
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gradient force. Sep. Purif. Technol. 2017, 176, 40-47.
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Waskaas, M.; Kharkats, Y. I., Magnetoconvection phenomena: A mechanism
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for influence of magnetic fields on electrochemical processes. J. Phys. Chem. B 1999,
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Waskaas, M.; Kharkats, Y. I., Effect of magnetic fields on convection in
solutions containing paramagnetic ions. J. Electroanal. Chem. 2001, 502, (1), 51-57. (49)
Leupin, O. X.; Hug, S. J.; Badruzzaman, A. B. M., Arsenic removal from
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Bangladesh tube well water with filter columns containing zerovalent iron filings and
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sand. Environ. Sci. Technol. 2005, 39, (20), 8032-8037.
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distribution of arsenic in natural waters. Appl. Geochem. 2002, 17, (5), 517-568. (51)
Manning, B. A.; Hunt, M. L.; Amrhein, C.; Yarmoff, J. A., Arsenic(III) and
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groundwater using zerovalent iron: Laboratory column tests on combined effects of
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Zhou, S.; Huang, T.; Ngo, H. H.; Zhang, H.; Liu, F.; Zeng, M.; Shi, J.; Qiu,
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1.0
1.0
0.8
0.8
0.6
0.6
1 mM 2.5 mM 5 mM 10 mM
DI water, w/o WMF DI water, w/ WMF 1 mM 10 mM 30 mM
w/o WMF 0.4
-
ClO4
0.2 0.0
a 0
C/C0
C/C0
w/ WMF -
ClO4
0.4 0.2 0.0
200 400 600 800 1000 1200 1400
b 0
50
100
Time (min)
150
200
1.0
0.8
0.8
1 mM 2.5 mM 5 mM 10 mM
C/C0
C/C0
w/ WMF
w/o WMF
0.4
1 mM 30 mM
c 0
NO3
0.6 0.4
NO3-
0.2 0.0
0.2 0.0
200 400 600 800 1000 1200 1400
d 0
50
100
Time (min)
Cl
0.6 1 mM 5 mM 20 mM 30 mM 50 mM
0.4 0.2
e 0
100
150
200
250
-
Cl
0.6
0.2
300
f 0
50
100
1.0
1.0
0.8
0.8
0.6 0.25 mM 1 mM 2 mM 5 mM 10 mM
SO420.2
g 0
50
100
150
200
200
250
w/ WMF
250
300
0.01 mM 0.05 mM 0.1 mM 0.5 mM 1 mM
2-
SO4
0.6 0.4 0.2 0.0
300
h 0
50
100
Time (min)
692
150
Time (min)
C/C0
C/C0
Time (min)
w/o WMF
300
0.4
0.0 50
250
0.5 mM 1 mM 5 mM 10 mM
w/ WMF 0.8
-
C/C0
C/C0
0.8
0.0
200
1.0 w/o WMF
0.4
150
Time (min)
1.0
0.0
300
Time (min)
1.0
0.6
250
150
200
250
300
Time (min)
693
Figure 1. Effect of ClO4-, NO3-, Cl-, and SO42- on As(III) removal by ZVI in the
694
absence or presence of WMF (ZVI = 0.20 g/L, As(III) = 1000 µg/L, pHini = 7.0, T =
695
25 oC). The lines are the results of simulating the data with pseudo-first-order rate
696
law.
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697 698
Figure 2. Proposed schematic illustration of the combined effects of WMF and anions
699
on ZVI corrosion.
700
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140
Ranion
120
Ranion
100 80
ClO4-
140 120 100 80 60 40 20 0
NO3Cl-
SO42-
SO4
Ranion = 137C/(0.085+C) 2
R = 0.99 0.0
0.2
0.4
2-
Ranion = 5.86C + 21.83 0.6
0.8
Concentration (mM) 60
2
R = 0.99
1.0
Ranion = 4.51C + 10.31 2
R = 0.85 40
Ranion = 3.91C + 8.92 R2 = 0.99
20 0 0
2
4
6
8
10
Concentration (mM)
701 702
Figure 3. Correlation of the promotion factors of different anions (Ranion = kobs anion/kobs
703
UP)
704
shows the same values of SO42--promotion factors but has an enlarged scale. The solid
705
lines in this figure are the results of simulating the data with the listed equations.
with their corresponding initial concentrations in the presence of WMF. The insert
706
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b
1.0
.8
.8
.6
.2 0.0
Consumed Fe0 (%)
C/C0
.6
0.5 mM HSiO3-
100 80 60 40 20 0
0
707
0 mM HSiO3-
.4
-
0 mM H2PO4
-
0.01 mM H2PO4
.6
-
0.1 mM H2PO4 -
0 mM H2PO4
-
0.01 mM H2PO4
.4
-
0.1 mM H2PO4
-
w/ WMF
0 mM HSiO3
0.5 mM HSiO3-
.2
.2
-
w/o WMF w/ WMF
50
100
w/o WMF
150
200
Time (min)
0 mM HSiO3
-
0.5 mM HSiO3
250
300
0.0
0.0 0
10 20 30 40 50 60 70 80 90
Time (min)
w/ WMF
w/o WMF w/ WMF
.8
.4
c
1.0
0 mM HCO3-
w/o WMF
a
1.0
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0 mM HCO3
-
1 mM HCO3
-
5 mM HCO3
-
1 mM HCO3
-
5 mM HCO3
0
10 20 30 40 50 60 70 80 90
Time (min)
708
Figure 4. Effect of (a) HSiO3-, (b) H2PO4-, and (c) HCO3- on As(III) removal by ZVI
709
in the presence of 3.0, 1.0, and 3.0 mM SO42-, respectively (ZVI = 0.20 g/L, As(III) =
710
1000 µg/L, T = 25 oC). The pHini values of working solutions containing HSiO3-,
711
H2PO4-, and HCO3- were 10.5, 7.0, and 8.9, respectively. The insert of (a) represents
712
the consumed Fe0 fractions under different conditions, which were determined by
713
LCF of the Fe k3-weighted EXAFS spectra (see Table S2).
714 715 716 717
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w/ WMF
An- Fe2+
An- Fe2+
Fe00 Fe
Fe2+ Fe2+
The promoting effect Bappl. of anions on the transport of Fe2+ Anfollows this order: An-
SO42->Cl->NO3-≈ClO4-
An-
Co-existing anions
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