Advantages of low pH and limited oxygenation in arsenite removal from water by zero-valent iron

Journal of Hazardous Materials 252–253 (2013) 77–82

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Advantages of low pH and limited oxygenation in arsenite removal from water by zero-valent iron Sivan Klas ∗ , Donald W. Kirk Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College St., M5S 3E5, Toronto, ON, Canada

h i g h l i g h t s    

Limited aeration and acidic conditions are advantageous for arsenite removal by ZVI. Higher removal rate and lower ZVI demand, compared with anaerobic conditions. Higher reduction and lower sludge formation, compared with non-acidic conditions. Formation of Fe(II)-intermediate were associated with enhanced performance.

a r t i c l e

i n f o

Article history: Received 22 December 2012 Received in revised form 21 February 2013 Accepted 24 February 2013 Available online 1 March 2013 Keywords: Arsenic Iron Wastewater Green rust Permeable reactive barrier

a b s t r a c t The removal of toxic arsenic species from contaminated waters by zero-valent iron (ZVI) has drawn considerable attention in recent years. In this approach, arsenic ions are mainly removed by adsorption to the iron corrosion products. Reduction to zero-valent arsenic on the ZVI surface is possible in the absence of competing oxidants and can reduce arsenic mobility and sludge formation. However, associated removal rates are relatively low. In the current study, simultaneous high reduction and removal rates of arsenite (H3 AsO3 ), the more toxic and mobile environmentally occurring arsenic species, was demonstrated by reacting it with ZVI under limited aeration and relatively low pH. 90% of the removed arsenic was attached to the ZVI particles and 60% of which was in the elemental state. Under the same non-acidic conditions, only 40–60% of the removed arsenic was attached to the ZVI with no change in arsenic oxidation state. Under anaerobic conditions, reduction occurred but total arsenic removal rate was significantly lower and ZVI demand was higher. The effective arsenite removal under acidic oxygen-limited conditions was explained by formation of Fe(II)-solid intermediate on the ZVI surface that provided high surface area and reducing power. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Arsenic is a toxic element and its presence in natural and drinking water is restricted to very low levels [1]. The two soluble arsenic species commonly found in the environment are arsenite (As3+ -species) and arsenate (As5+ -species), with the former being more toxic and mobile [2,3]. Although several dissolved arsenic removal technologies such as membrane filtration, ion exchange and adsorption on activated carbon or on mineral surface are proven, cost effectiveness remains a major issue, especially in developing countries [4]. The use of elemental iron, also termed zero-valence iron (ZVI), to remove dissolved arsenic species is a promising approach due to its simplicity, low cost, efficiency and applicability in passive treatment (e.g., permeable reactive barriers) [2,5,6]. This method was

∗ Corresponding author. Tel.: +1 647 7781345. E-mail addresses: [email protected], [email protected] (S. Klas). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.02.044

shown to achieve complete arsenic removal [7] and has recently drawn considerable interest as reflected by a fairly large number of publications concerned with kinetic and mechanistic aspects [1–3,6–20]. This literature suggests that the main removal mechanism of both arsenite and arsenate is their adsorption onto the iron corrosion products surrounding the ZVI particle or onto the iron oxy-hydroxides released to the solution [1,19]. The adsorption is affected by pH, due to the weak-acid properties of the As ions [21] and by surface area [13]. The removal capacity may be enhanced by precipitation and co-precipitation of As-Fe compounds on solid surface [2,6,22], but these compounds may re-dissolve [22]. An additional removal mechanism is reduction of arsenic ions to elemental arsenic which is sequestered in the corrosion layer [8,17] or forms an intermetalic phase with Fe0 [20]. This mechanism appears to occur in the absence of competing oxidants such as oxygen, where reduction of As(V) to As(III) and of As(III) to As(0) on the surface of ZVI was recorded [8,17,18]. Arsenic bound to ZVI media, probably in such manner, demonstrated low leaching rates [6,13]. Another advantage associated with anaerobic

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conditions is the absence of iron oxy-hydroxides that produce sludge and further treatment requirement. However, without the iron oxy-hydroxides and with the relatively low surface area of the ZVI particles, significantly lower arsenic removal rates compared with oxygenated conditions were recorded [8,21,23]. In the presence of oxidants such oxygen, ferric oxides, water, carbonate and reactive Fe(II)-Fe(III) corrosion intermediates, As removal was kinetically fast but As(III) was partially oxidized to As(V) [2,9,14,16,19,24,25]. The chemistry of the system, however, was recently shown to be more complex as nano-ZVI particles demonstrated the ability to simultaneously reduce and oxidize As(III) [17]. Using ZVI to remove selenate (SeO4 2− ) form water, we have recently shown that limited oxidation and acidic conditions resulted in surprisingly high removal rates together with low formation of sludge. This was explained by formation of green rust (GR) on the ZVI particles that provided high surface area and reducing power, allowing a large conversion rate of Se ions to the elemental state [26]. Although the effects of oxygen and pH on As removal using ZVI was previously investigated, these parameters were not maintained constant [21] and the removed As distribution was not characterized at different pH values [8,23]. The scope of the current work was to show that under acidic conditions and limited aeration, high arsenite removal rate together with high arsenite reduction and low iron oxy-hydroxides formation, is possible.

2. Experimental 2.1. Procedures Four distinct sets of conditions were tested, namely pH 4.0 and 9.0, each under an anoxic (N2 ) or mildly oxygenated (aerated) environment. Another test under aeration and pH 7 was also performed. Initial arsenite (analytical grade As2 O3 , Fisher Scientific Co.) concentration was 70 mg As L−1 to minimize interferences by impurities and side reactions, to evaluate the effect of As concentration on the removal rate and to enable comparison with published data. The experimental set up was described in detail previously [26]. In short, the system comprised of a 2 L magnetically stirred container from which a 1.8 L solution was circulated at a rate of 1.8 L h−1 through a 30 mL glass column (30 cm long), partially filled by ZVI fillings (40 mesh, Fisher Scientific Co.) which were effectively mixed. Because of the short hydraulic residence time in the ZVI filled volume (<20 s), the system as a whole was considered to be closely analogous to a continuous stirred tank reactor (CSTR). N2 bubbling or aeration (200 mL min−1 ) started approximately 0.5 h prior to addition of 17.2 g ZVI fillings and pH was adjusted using 0.12 M HCl solution. The used aeration rate and Fe0 mass followed a selenate removal study preformed under identical conditions. The system was operated at room temperature for 7 h after ZVI addition and a constant pH was maintained by 0.12 M HCl solution, a pH controller (Eutech 190 series) and a peristaltic pump. The HCl solution was stored in a 50 mL buret and its consumption was monitored intermittently, along with oxidation reduction potential (ORP, Accumet pH meter, model 10). 10 mL samples were drawn from the container using a syringe and filtered using a replaceable membrane (Whatman Nuclepore, 0.45 ␮m pore size) syringe filter. Determination of As and Fe concentrations in the suspended solids (SS) followed dissolution of the solids attached to the membrane, filter plates and O-ring using in 37% HCl solution. At the end of each trial, SS were extracted using centrifuge and dried at 40 ◦ C for Xray diffraction (XRD) and X-ray photoelectron spectrometry (XPS) analyses. The solids in the ZVI column were extracted at the end of each trial by filtration and kept without drying for several days

before XPS analysis of particles taken from the sample interior. A portion of each of these samples was dried at 40 ◦ C for XRD analysis. 2.2. Analyses Total dissolved Fe and As concentrations were determined by inductively coupled plasma atomic emission spectroscopy (ICPAES, Perkin Elmer, Optima 7300 DV). XRD (Philips PW 3710) analyses were conducted using Cu K␣ radiation at 40 mA and 40 kV, at a scanning rate of 0.01 2␪ s−1 . XPS was conducted using a Thermo Scientific Al K␣ spectrometer (ThermoFisher, E. Grinstead, UK) and pass energy of 200 eV, followed by higher resolution (PE = 25 eV) at the As 3d region. 3. Results and discussion Table 1 summarizes the operational conditions employed and the pH and ORP values measured in the mixed container and in the column outlet after 7 h of operation. The thermodynamically stable Fe and As species in solution and in the ZVI column were assessed according to the corresponding pH and ORP values and to Fe-H2 O [27] and As-H2 O [28] Pourbaix diagrams, as indicated in Table 1. The concentration of dissolved As and Fe with time are depicted in Fig. 1. Fig. 1A shows, as expected, that the highest As removal rate was obtained under aeration. The somewhat higher rate at pH 4 and 7 compared with pH 9 was associated with lower concentration of SS in the latter (Fig. 2A), probably as result of uncontrolled lower aeration rate, and corresponding lower removal by adsorption. The possibility that the lower adsorption was a result of different arsenic speciation is not likely, because arsenic adsorption on the SS could not be correlated with pH (Fig. 2B). Fig. 1A also shows that As removal rates were greatly reduced at concentrations below 10 mg L−1 , very similar to selenate removal kinetics under identical conditions [26]. Fig. 1B shows that dissolved iron concentration was increasing only under pH 4, under both aerated and anoxic conditions. The dramatic (∼104 ) oxidation rate increase of dissolved Fe(II) by O2 between pH 6 and 8 [29] explains the accumulation of dissolved Fe(II) only at pH 4, as under pH 7 and 9, Fe(II) is rapidly oxidized and precipitates as ferric oxy-hydroxides (i.e., SS). This also explains the accumulation of SS only under non-acidic aerated conditions (Fig. 2A). Because ferric oxy-hydroxides, unlike Fe(II) solid species, precipitate readily at pH 4 [30], the accumulated dissolved iron under pH 4 was concluded to be predominantly divalent. The iron concentration in the SS and the As to Fe mole ratio in them, are depicted in Fig. 2. The relatively high initial SS concentration was attributed to fine ZVI particles, washed off from the column. Fig. 2A shows a significant accumulation of SS only under aeration and non-acidic conditions, which was comparable in magnitude with the accumulated dissolved iron under acidic conditions (Fig. 1B). With the exception of pH 9.0 anaerobic conditions, where iron release was negligible, iron was released to solution in a similar rate under all conditions, suggesting that a similar amount of ZVI Table 1 Operational conditions, pH, ORP and assessed stable species. Gas

pH (out)a

ORP (out)a (mV)

Stable speciesb

Air N2 Air N2 Air

4.0 (5.9) 4.0 (5.7) 9.0 (9.1) 9.0 (9.2) 7.0 (7.3)

260 (65) 175 (−10) 70 (−10) −400 (−290) −115 (−180)

[Fe2+ ], H2 AsO4 − , (H3 AsO3 ) [Fe2+ , H3 AsO3 ] [Fe2 O3 , HAsO4 2− ] [H3 AsO3 , As, Fe2 O3 , Fe3 O4 ] [Fe2 O3 , H3 AsO3 ]

a Steady-state values measured in the stirred container (out = at ZVI column outlet at t = 7 h) b Thermodynamically stable species in solution (ZVI column) [both].

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N2, pH4

70 60 N2, pH9

50 40 30

O2, pH4

20

O2, pH9

10

A

O2, pH4 N2, pH4 O2, pH9 N2, pH9 O2, pH7

200 Fe in the SS (mg L-1)

Dissolved As (mg L-1)

A

79

150

100 50

O2, pH7

0

0

2

4

0

6

0

2

Time (h) 250 N2, pH4 200

(O2, pH9), (N2, pH9), (O2, pH7)

150 100

O2, pH4

50 0

0

2

4

6

Time (h)

6

Time (h) Fig. 1. Concentration of total dissolved As (A) and iron (B) in time. Initial ZVI mass was 17.2 g.

B As to Fe mole ratio in SS

Dissolved Fe (mg L-1)

B

4

0.3

O2, pH4

N2, pH4

N2, pH9

O2, pH7

O2, pH9

0.2

0.1

0.0 0

2

4 Time (h)

6

Fig. 2. Concentration of Fe (A) and As/Fe mole ratio (B) with time. Initial ZVI mass and arsenite concentration was 17.2 g and 70 mg L−1 , respectively.

was consumed. In light of the lower arsenite removal rate under anaerobic conditions, oxygen addition was concluded to result in lower ZVI demand (i.e., lower ratio between consumed ZVI and removed arsenic). The mole ratio between arsenic adsorbed to the SS and iron in the SS, is presented in Fig. 2B with respect to time. This figure shows a relatively similar As/Fe mole ratio range of 0.1–0.2 under all employed conditions, with no distinct preference to a specific pH. XRD pattern of SS samples obtained in the aerated trials are presented in Fig. 3A and show the presence of only poorly crystalline magnetite and graphite impurity in these samples. This indicates that the SS contained mainly amorphous iron oxy-hydroxides, characteristic to dissolved Fe(II) oxidation products [30]. XRD pattern of ZVI samples obtained under aerated conditions are presented in Fig. 3B and also show the presence of poorly crystalline, probably hydrated, magnetite [31]. The peak at 9◦ 2Theta could not have been fitted to any relevant mineral other than to an As-Fe compound such as tooelerite (Fe8 (AsO4 )6 (OH)6 ·5H2 O) or Kaatialaite (FeAs3 O9 ·8H2 O), although additional peaks associated with such minerals were not observed. Because As oxidation state in these compounds is +5, not observed under XPS examination (Fig. 4B), it was concluded that such minerals may have formed during sample drying. XPS measurements of SS samples produced under aerated conditions at different pH values are presented in Fig. 4A and reveal the presence of both As(III) (20–40%) and As(V) (60–80%) under all pH values. The apparent oxidation of As(III) to As(V) is in agreement with thermodynamical predictions (Table 1) and with previous reports [2,9,14,16,19,24,25].

Fig. 4B depicts the XPS spectra of ZVI samples attained in the aerated trials at pH 4 and 9. Spectra fittings indicated that arsenic oxidation state was around 60% zero-valent and 40% trivalent at pH 4, and 100% trivalent at pH 9. Under anaerobic conditions, As0 and As3+ comprised about 50% each (data not shown). The arsenic oxidation states were integrated in Fig. 5 with the calculated distribution of removed arsenic (i.e., in the ZVI and SS phases) after 2 h operation, during which reaction kinetics were not significantly reduced due to the low arsenic concentrations (Fig. 1A). Fig. 5 shows again the much higher arsenite removal rate under aerated conditions. It also shows that under limited aeration and acidic conditions, 90% of the removed As was retained in the ZVI column compared with only 40% - 60% under non-acidic conditions. Furthermore, it reflects the high (∼60%) reduction of arsenite under aerated acidic conditions, which was similar to that attained under anaerobic conditions. With the exception of adsorption, these outcomes fit the proposed mechanisms of selenate removal by ZVI under identical conditions [26]. Explicitly, the high extent of arsenite removal and reduction under acidic and limited aeration conditions only, could be attributed to formation of reactive GR phase on the ZVI surface which provided high surface area and reduction power. Although GR and As0 appeared to be thermodynamically restricted in the ZVI column under these conditions (Table 1), the major change in pH and ORP values at the column outlet (Table 1) indicated that conditions at the molecular distance near the ZVI surface were thermodynamically sufficient. GR was previously identified during arsenic removal by ZVI [2,19] and under identical conditions in the presence of sulfate ions [26]. Under acidic anaerobic conditions,

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30

40

50

10

Position ( 2Theta)

20

30

40

Position ( 2Theta)

Fig. 3. XRD pattern of (A) suspended solids formed and (B) of the extracted ZVI particles after 7 h reaction under aerated conditions and different pH. M = magnetite (PDF 00-008-0415), G = graphite (PDF 01-085-1436), T = tooelerite (PDF 00-044-1468) Fe = iron (PDF 01-001-1262).

no GR or significant amount of SS can form and the relatively low surface area explains the much lower removal extent. Under aerated and non-acidic conditions, oxidation of GR or other Fe(II)- solid intermediates by O2 is rapid and arsenite can be removed only

400

As0

As3+

As5+

Counts /s

300

A

pH 9 pH 7

200

pH 4

100 0 40

42

46

48

B50

Binding Energy (eV)

200 Counts /s

44

through adsorption. The fact that As(III) adsorption on the ZVI particles was much lower under anaerobic conditions, suggests that the increased uptake under the aerated trials was due to development of a corrosion layer with high surface area and adsorption sites. It is noted that operation under acidic and limited oxygenation should also be advantageous for As(V) removal because adsorption of the dominant H2 AsO4 − species is expected to be high on the positively charged solid surfaces that exist at this pH [21]. With the exception of overall removal rate, the beneficial effects of aeration discussed above are expected to decrease upon aeration rate increase, as this will increase the competition between arsenite and oxygen over the Fe(II)-intermediates and increase the formation of SS. Nevertheless, as shown in the selenate removal case [26], careful increase in oxygen supply can nearly eliminate both acid demand, dissolved iron release and SS formation due to oxidation of the produced Fe(II) by O2 directly on the ZVI surface. The acid demand decrease with aeration increase can be appreciated from

pH 4 pH 9

100

0 40

42

44

46

48

50

Binding Energy (eV) Fig. 4. XPS spectra of SS (A) and ZVI (B) samples in the binding energy range of As 3d.

Fig. 5. Distribution of removed As after 2 h reaction and As oxidation state on the ZVI.

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HCl consumption (mL)

70

Acknowledgements

O2, pH4 N2, pH4 O2, pH9 N2, pH9 O2, pH7

60 50 40

81

We wish to thank Miss T. Amerian for assistance in the laboratory work and to the Lyon Sachs Foundation for the generous financial support of this research.

30

References

20 10 0 0

2

4 Time (h)

6

Fig. 6. Consumption of HCl solution with time. Initial ZVI mass and arsenite concentration was 17.2 g and 70 mg L−1 , respectively.

the pH 4 curves in Fig. 6, which depicts the consumption of acid in time. The acid consumption measurements could also help, to a limited extent, supporting the mechanistic assessment described above. Under anaerobic acidic conditions, the main expected redox reactions were proton and arsenite reduction, as represented by Eq. 1 and Eq. 2, respectively. Under acidic aerobic conditions, the expected reactions were iron oxidation by oxygen, represented by Eq. 3, and iron oxidation by oxygen to GR1 [31] followed by GR1 oxidation to magnetite through arsenite reduction to As0 , as represented by Eq. 4 (overall reaction). Fe◦ + 2H+ → Fe+2 + H2

(1)

H3 AsO3 + 1.5Fe0 + 3H+ → As0 + 1.5Fe+2 + 3H2 O

(2)

Fe◦ + 0.5O2 + 2H+ → Fe+2 + H2 O

(3)

H3 AsO3 + 7.2Fe0 + 4.05O2 → As0 + 2.4Fe3 O4 + 1.5H2 O

(4)

The mean ratio between consumed acid and dissolved iron calculated at each sampling interval in the acidic trials was 1.97 ±0·45 and 1.82 ±0·23 for the anaerobic and aerobic trials, respectively. These values are close to the theoretical value of 2.0 expected from Eqs. (1) and (2) (anaerobic conditions) and Eqs. (3) and (4) (aerated conditions).

4. Conclusions Application of limited aeration and acidic conditions upon reacting dissolved arsenite with ZVI presented the following advantages: (A) significantly increased removal rate and lower ZVI demand compared with anaerobic conditions, (B) significantly higher reduction and retention of arsenic on the ZVI surface, and much lower sludge formation, compared with aerated non-acidic conditions. The effective arsenite removal under acidic and limited aeration conditions, reported here for the first time, was explained by formation of Fe(II)- solid intermediate on the ZVI surface that provided high surface area and reducing power. The results indicate that ZVI treatment would be advantageous in treating arsenitebearing acidic water, and that acid addition should be considered for active treatment of As-bearing water by ZVI. Further work is needed to validate reaction mechanisms under very low arsenic concentrations, such as near the permitted levels in potable water, where the reaction kinetics is significantly reduced.

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