Combined Effect of Weak Magnetic Fields and Anions on Arsenite

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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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Environmental Science & Technology

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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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ACS Paragon Plus Environment


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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

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pH levels,3,

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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

243

the absence (Figure 1(g)) and presence (Figure 1(h)) of WMF was also investigated.

244

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

246

presence of WMF. Another notable feature in Figure 1 was that the sulfate-enhancing

247

effect was much more pronounced than chloride, which was in agreement with the

248

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

259

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

263

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

265

solutions played important roles in the WMF enhancing effect on contaminant

266

sequestration by ZVI. With respect to the mechanism of As(III) removal by ZVI, it is

267

commonly accepted that As(III) is sequestered mainly by adsorption and

268

co-precipitation, resulting from the continuous generation of iron (oxyhydr)oxides due

269

to the ZVI corrosion reaction.51 Hence, it can be concluded that the co-presence of

270

WMF

271

adsorption/co-precipitation process. On the other hand, many studies have shown that

272

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

274

enhanced the conversion of As(III) to As(V) at pH range of 3.0-9.0 with 3 mM CaSO4

275

as background electrolyte.37 Therefore, it is expected that, the anions such as Cl-,

276

SO42-, NO3-, and ClO4- can enhance the As(III) oxidization process during As(III)

277

removal by ZVI with the presence of WMF. To verify this point, taking Cl- as a

278

representative anion, the variation of dissolved As(III) and As(V) concentrations was

and

anions

accelerated

ZVI

corrosion

12


and

the

subsequent

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279

determined in the process of As(III) removal by ZVI with the co-present of WMF and

280

Cl- at pHini 5.0 and 8.0. As demonstrated in Figure S3, as the Cl- concentration

281

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

284

effects of WMF and anions not only enhanced the adsorption/co-precipitation of

285

As(III)/As(V) but also accelerated the transformation of As(III) to As(V) in solution.

286

With regard to the mechanisms involved in the WMF-induced improvement of

287

ZVI reactivity, many efforts have also been made in our previous studies. Two forces,

288

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

290

identify the relative contribution of F∆ and F in the WMF enhancing effect,

291

further investigations were carried out and it was verified experimentally and

292

theoretically that F∆ (see eq. 2) was the major driving force.46

293

F∆ = B ∆B

(2)

294

Hereby, χ denotes the molar magnetic susceptibility of the involved species and c is

295

their concentration, µ0 is the magnetic permeability of vacuum, B is flux intensity of

296

magnetic field, and ∆B is gradient of the magnetic field which mainly arises from the

297

induced magnetic field around the magnetized ZVI particles. Being pulled by F∆,

298

the paramagnetic Fe2+ generated in ZVI corrosion tends to move along the magnetic

299

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

301

ZVI surface.46 This mechanism, highlighting the key role of Fe2+, well explained the

302

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

304

process but may be accompanied with some secondary effects or influenced by some

305

factors, e.g., the presence of oppositely charged anions. Thus, in order to improve our

306

understanding of how the WMF effect works, the Fe2+ transport within the boundary

307

layer should be further discussed by taking the co-existing anions into account.

308

Actually, in the electrochemical processes, the effects of magnetic field have been

309

intensively investigated over the past several decades.47-48, 56, 58-59 More importantly,

310

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


(6)

Page 15 of 35


327

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



351

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


Page 16 of 35

Page 17 of 35


374

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



398

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


Page 18 of 35

Page 19 of 35


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



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


Page 20 of 35

Page 21 of 35


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



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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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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103, (23), 4876-4883.

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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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Smedley, P. L.; Kinniburgh, D. G., A review of the source, behaviour and

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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arsenic(V) reactions with zerovalent iron corrosion products. Environ. Sci. Technol.

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the microbial community of a reservoir enclosure system via in situ oxygen

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Page 31 of 35


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.

31



697 698

Figure 2. Proposed schematic illustration of the combined effects of WMF and anions

699

on ZVI corrosion.

700

32


Page 32 of 35

Page 33 of 35


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

33



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

Page 34 of 35

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

34


Page 35 of 35


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


Combined Effect of Weak Magnetic Fields and Anions on Arsenite

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