Long-term influence of aeration on arsenic trapping in a ZVI/sand bed reactor

Abstract

Arsenic toxicity and occurrence in the environment necessitate the development of easy-to-handle and cheap water treatment processes. Zero Valent Iron (ZVI) supports meet these requirements. However many ZVI by-products might be generated in the unit reactor process, mainly influenced by physico-chemical conditions. This work deals with the influence of aeration onto arsenic trapping in ZVI/sand columns. The monitoring of a pilot unit that consisted of four reactors (two aerated and two non-aerated), at different durations (three days, three weeks and three months), enables the characterization of arsenic sorption capacity. In an aerated system, the highest arsenic removal levels were observed at the beginning of the experiment, and the performances greatly decreased with time. In the non-aerated systems, the highest arsenic trapping capacities were obtained (220 mgAs per gFe in the three months of running the system) but the arsenic removal percentage remained under 60%. Support analyses after dismantling were performed to characterize the arsenic sorption and the ZVI by-products generated. They confirmed the influence of aeration on the arsenic distribution along the reactor: a homogeneous repartition in non aerated conditions whereas it was highly heterogeneous under aerated conditions, with far higher concentrations at the inlet. This long-term study highlights that aeration modifies the oxidation of ZVI, resulting in by-products with high arsenic sorption capacities but for a limited duration whereas the absence of aeration limits the oxidation of ZVI into sorbant by-products but it significantly increases the duration of arsenic trapping and consequently the lifetime of the bed reactor.

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Introduction

Sorption mechanisms are widely used for arsenic removal. Zero Valent Iron (ZVI), alone, or in addition to a co-support such as sand, appears to be an interesting support due to its low cost and high sorption capacity.^1–3 The interactions of ZVI or iron oxides or (oxy-)hydroxides with arsenic have been widely studied during the last decade. Most of these studies were based on batch experiments, but several treatments were based on column filtration using ZVI or iron oxides alone^4–6 or mixed with various supporting media^5,7–13 can also be found. The trapping properties of ZVI involve complex mechanisms, including co-precipitation or adsorption¹⁴ onto its corrosion by-products. These by-products can consist of ferrihydrite, green rusts, lepidocrocite, goethite….¹⁵ Their nature and amount strongly depend on the physico-chemical conditions, as it can be seen by comparison of data from the literature dealing with the identification of ZVI by-products generated inside filtrating columns or passive reactive barriers.^7,16–19 These by-products have different sorption properties, thus the efficiency of ZVI-based treatments is strongly influenced by the physico-chemical parameters of the effluent.

The concentration of dissolved oxygen (DO) is one of the key parameter in the production of iron by-products as it contributes in the initial oxidation of the ZVI⁷ into soluble ferrous and ferric ions:

2Fe⁰ + O₂ + 4H⁺ ⇔ 2Fe²⁺ + 2H₂O (1)

4Fe²⁺ + 4H⁺ + O₂ ⇔ 4Fe³⁺ + 2H₂O (2)

Fe²⁺ + 2H₂O ⇔ Fe(OH)₂ + 2H⁺ (3)

Fe³⁺ + 3H₂O ⇔ Fe(OH)₃ + 3H⁺ (4)

These iron hydroxides can then evolve into various iron oxide or (oxy-)hydroxides according to the DO concentration, among other parameters. For instance, Reinsch et al.²⁰ observed the formation of maghemite from ZVI nanoparticles after exposure to O₂-saturated water, but not in anaerobic conditions. Tanboonchuy et al.²¹ compared the arsenic removal efficiency in batch experiments with or without N₂ purging. Their results indicated that the arsenic removal was more efficient under oxic conditions, which was explained by a rapid formation of iron (oxy-)hydroxides from ZVI in the presence of DO. Bang et al.²² studied the evolution of arsenic and iron in solution during batch experiments performed under aerobic or anaerobic conditions and obtained similar results. Only several investigations were also carried out on column systems. Bang et al.⁴ studied the influence of DO on the removal of arsenic by columns filled with Fe⁰. They found that the arsenic concentration in the effluent decreased with increasing DO level. Since this experiment was conducted by applying different DO conditions to a single reactor on a short duration (100 h), it did not allow a complete understanding of the influence of aeration on long-term performances of As removal. Abedin et al.⁷ monitored, amongst other parameters, As and DO during a 180 day experiment on columns filled with a ZVI/sand media. They highlighted that a high DO concentration increased the oxidation of ZVI and thus the As removal efficiency but they did not modify this parameter during their experiments.

Because short-term batch experiments can lead to biased conclusions compared to continuous treatment units, the main objective of this work was to study the long-term influence of DO on the performances of a middle-scale ZVI/sand filtration unit. Experiments were thus carried out over three months to be more representative of the lifetime of real treatment units. The nature of ZVI by-products generated was determined in order to highlight a potential correlation between DO, arsenic trapping efficiency, arsenic distribution along column and the nature of ZVI by-products. Two aerated and non-aerated columns were operated simultaneously in identical conditions. Removal capacities were evaluated through the monitoring of arsenic concentration at the inlet and the outlet of the columns. After three months, ZVI/sand samples were collected all along the columns to study the evolution of the As distribution and to identify the main ZVI by-products.

Materials and methods

Pilot unit

A scheme of the pilot units can be found in ESI (Fig. S1†). Four up-flow reactors, with an 8 cm internal diameter and 40 cm height, were used.³ Reactors were first filled with a 5 cm height layer of sieved sand (d = 3 ± 1 mm), a second 20 cm height layer filled with a homogenous ZVI/sand mixture (1% Fe/sand, w/w; ZVI powder Jeulin, purity > 99.999%) and a third layer (of 15 cm height) with sieved sand alone. The sand alone layers were used to limit ZVI loss from the column. The ZVI–sand layer was prepared with wet sand in order to get a homogeneous repartition of ZVI. The total mass of sand introduced to fill the reactor was 1.8 kg.

Composition of the synthetic water. The pilot was fed with synthetic water containing a calcareous signature²³ spiked with arsenate (Na₂HAsO₄, Aldrich, purity > 99%) and equilibrated at pH 7.9 ± 0.3 (detailed composition available in ESI, Table S1†). The flow rate was set to 9 ± 1 mL min⁻¹ using peristaltic pumps (MasterFlex, L/S model 7519-10) and was regularly controlled. Two columns were aerated through air diffusers located at their bottom (60 NL h⁻¹), while two other columns were not aerated. Experiments were performed at room temperature (20 ± 3 °C). DO was monitored over time using a HQ-30d flexi oximeter.

Three sets of pilot units were used. The first two sets were operated during three days and three weeks respectively in order to study the evolution of the nature of ZVI by-products according to time. The final set, corresponding to the main experiment, lasted until reaching the complete saturation of the ZVI/sand support in order to study the long-term influence of DO on arsenic retention and to evaluate the maximum sorption capacity.

Arsenic and iron analyses

The determination of total As was performed using a Varian SpectrAA 880Z Atomic Absorption Spectrometer equipped with a GTA 100Z graphite furnace, and a Zeeman background correction (GF-AAS). Total Fe concentration was determined using a Varian SpectrAA 220 Atomic Absorption Spectrometer equipped with a flame atomizer and a D2 lamp background correction (F-AAS). The limits of quantification were 5 μg L⁻¹ and 100 μg L⁻¹ for total As and Fe, respectively.

Pilot dismantling

After three days, three weeks or three months, the duplicated aerated and non-aerated columns were dismantled to recover the ZVI/sand support. For each column, three samples were collected at 10, 18 and 27 cm from the reactor inlet, hereafter referred as sampling points P1, P2 and P3 respectively. These samples undergone chemical and solid analysis to evaluate their As and Fe content as well as to identify the ZVI by-products.

Chemical analysis

The total As and Fe amounts in solid support were determined by GF-AAS and F-AAS respectively after a microwave-assisted acid digestion. Briefly, 3 g of wet support were mixed with 5 mL HNO₃ 69% and 5 mL HCl 37% in a closed Teflon vessel, then heated for 30 min by applying a 1400 W power (AntonPaar Multiwave 3000).

Solid analysis

X-ray diffraction. X-ray diffraction (XRD) analyses were performed using a BRUKER D8 advance diffractometer, from 2 to 80° 2θ with a step size of 0.02° and an acquisition time of 2 s per step using CuK_α1 (λ = 1.5418 Å) radiation. Indexation was obtained with the EVA software and ICDD database.

Micro-Raman spectroscopy. Micro-Raman spectroscopy (μRS) was performed using a Jobin Yvon 6400 Raman spectrometer giving a spatial resolution of one micron (microscope uses a ×100 objective). The excitation source was an Ar⁺ laser operating at 514.5 nm. The incident power was limited to 2 to 5 mW using appropriate filters (D1 or D2). The calibration was obtained by means of a silicon standard. Typical spectral resolution for the Raman system with an 1800 mm⁻¹ grating monochromator was about 2.5 cm⁻¹ at the exciting line and the diameter of the laser spot focused on the sample was about 1 μm. Spectra were collected at room temperature from 90 to 1600 cm⁻¹ in two or three spatially and temporarily successive windows with an acquisition time ranging between 100 and 200 seconds, and an iteration number up to 3. Analyses were performed on wet samples, right after the pilot dismantling.

Secondary electron images. Secondary electron images (SE) were obtained with a Philips XL-30 scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDAX) for morphological and semi-quantitative chemical information. Samples were dried at room temperature and an Au–Pt coating was applied at the surface of the grains prior to SEM analyses.

Results and discussion

Pilot monitoring

The detailed results of O₂ monitoring can be found in ESI (Fig. S2†). The oxygen level corresponded to saturation at the outlet of aerated reactors, whereas in the non-aerated columns, oxygen was depleted at the beginning of the experiment and regularly increased from 0.5 to 8 mg L⁻¹ at the outlet after 70 days. According to eqn (1) and (2), the oxidation of ZVI into Fe²⁺ and then into Fe³⁺ consumes O₂. For the non-aerated columns, only the dissolved O₂ present in the feeding solution was available to perform this oxidation. A large amount was consumed through the column, resulting in a low DO concentration at the outlet. After 20 days, the level of DO at the outlet increased slowly, indicating a decrease of ZVI oxidation rate. For the aerated columns, no variation was observed between the DO level at the inlet and the outlet because the amount of O₂ supplied by air bubbling was higher than the amount consumed to oxidize ZVI.

The different behaviors observed between aerated and non-aerated (A and NA) systems indicated different ZVI oxidation processes, which should led to different reactivity towards arsenic.

Arsenic monitoring was performed until reactor saturation was reached. This corresponded to two and three months for aerated and non-aerated systems respectively.

The monitoring of arsenic at the outlet of aerated and non-aerated systems (Fig. 1) highlighted different trends. Since the results obtained for duplicated columns were in accordance, these differences can be attributed to the presence or absence of aeration. For aerated systems, a nearly complete arsenic retention was observed during the first 5 days, followed by a rapid and constant decrease down to 15% at day 25. For the non-aerated units, arsenic removal was lower, close to 50%, but remained stable up to day 25, then it slowly decreased down to about 10% at day 70. Support saturation was reached after 65 days in aerated conditions whereas 90 days were necessary in non-aerated conditions.

Fig. 1 Arsenic concentration monitoring at the inlet and outlet of non-aerated (NA 1 and NA 2) and aerated (A 1 and A 2) ZVI/sand columns.

The aerated systems were thus more efficient to remove arsenic during the first 15 days, but then this trend was inverted. As a consequence, the addition of DO in the treatment unit improved its performances only for a limited duration.

To compare the maximum trapping capacity of aerated and non-aerated systems, the cumulated amount of arsenic trapped inside each column was determined according to:

where [As]_inlet,i and [As]_outlet,i are the arsenic concentrations (mg L⁻¹) determined at a given sampling time at respectively the inlet and outlet of a column, Q_i is the flow rate (L h⁻¹) determined at the corresponding sampling time and t_i is the time interval (h) between two consecutive samplings.

The evolution of cumulated trapped As according to the volume of treated effluent (V_eff = Q_i × t_i) is presented in Fig. 2.

Fig. 2 Cumulated amount of arsenic trapped (mg) into aerated (A 1 and A 2) and non-aerated (NA 1 and NA 2) ZVI/sand columns according to the volume of treated effluent (V_eff).

Results obtained for duplicated columns were in agreement for the non-aerated systems but differences in amounts of trapped arsenic were observed for the aerated ones (Fig. 2). These differences resulted from a higher arsenic load at the inlet of A 1 column, due to a 10% higher feeding flow rate compared to A 2. Indeed, our previous works indicated that an increase of arsenic load at the inlet of an aerated ZVI/sand system induces an increase of sorption capacity.³ By comparing the mean values for both configurations once the saturation was reached, non-aerated systems were able to accumulate almost 60% more arsenic than the aerated one. The mean trapping capacity of both systems was calculated by taking into account the amount of ZVI present in each column (18 g). It reached 140 mg(As) per g(Fe) for aerated columns and 220 mg(As) per g(Fe) for non-aerated systems, which is one of the highest sorption capacities reported.²⁴

Pilot dismantling

After reaching the saturation, columns were dismantled to collect ZVI/sand support at different locations along the reactor. Pictures of ZVI/sand support inside the column at the end of the experiment can be found in ESI (Fig. S3†). For aerated systems, a quite even distribution of orange ZVI by-products was observed along the column whereas a darker brown area was observed in the first third of the non-aerated columns. These simple observations highlight differences in the nature and repartition of ZVI by-products along the column, which could result in a different arsenic trapping. Chemical and solid analysis were then performed to evaluate the amount of As and Fe as well as to identify the ZVI by-products.

Chemical analysis

After mineralization, the total As and Fe concentrations were determined to study the evolution of the As/Fe ratio along each column. The results are summarized in Fig. 3.

Fig. 3 As/Fe ratio, expressed as μg(As) per g(Fe), determined by mineralization of solid support collected along the column after 90 days.

The evolution of As/Fe ratio followed a very different trend for aerated and non-aerated systems. Under aerated conditions, a strong variation of the As/Fe ratio was observed in the ZVI/sand support collected along the column, with a five-fold higher value at the inlet (reaching 300 mg(As) per g(Fe)) than at the upper part. On the contrary, in non-aerated conditions, the As/Fe ratio was similar along the reactor (240 ± 30 mg(As) per g(Fe)). This heterogeneous As repartition has already been observed in aerated columns.^3,7,9 For example, Nikolaidis et al.⁹ found that the removal of As was ten times faster near the inlet than the outlet.

In the case of non-aerated columns, the As/Fe ratio determined by analyzing the solid support collected at the end of the experiment can be easily compared with the value estimated from the reactor-monitoring (220 mg(As) per g(Fe)). The agreement of these two results validates the accuracy of the determination of the amount of arsenic trapped in the column from the difference between inlet and outlet concentrations.

Solid analysis

Whatever the considered parameters, both XRD (not shown) and Raman investigations indicated that lepidocrocite and goethite were the main by-products issued from the ZVI oxidation (Fig. 4).

Fig. 4 Raman spectra of the ZVI by-products recovered after three months from aerated and non-aerated columns.

For both systems, the comparison of support collected at different locations along the column did not highlight significant differences in the nature of ZVI by-products. However, since it was not a quantitative analysis, it is not possible to conclude about the potential variations in proportion between lepidocrocite and goethite along the column.

Additional pilots (operated in the same conditions as the main experiment) were dismantled after three days and three weeks. At three days, lepidocrocite was the first ZVI oxidation by-product identified. Only layer P2 (18 cm from the inlet) showed evidence of goethite with lepidocrocite in both aerated and non-aerated systems. After three weeks, goethite was found with lepidocrocite at the P2 and P3 (27 cm from the inlet) layers. After three months, differences were observed between aerated and non aerated columns: goethite was widely distributed with lepidocrocite all along the column in aerated systems whereas only lepidocrocite was observed in the non-aerated system. Thus, aeration had a significant influence on the nature of ZVI by-products generated inside the treatment unit as well as on their evolution over time.

Lepidocrocite has already been notified as a main ZVI by-product.^25,26 Liu et al.²⁷ showed that, at ambient temperatures and pH close to 7, ferrihydrite evolved mainly into lepidocrocite. They also showed that lepidocrocite was not thermodynamically stable and evolved within hours into more stable iron oxides such as goethite, which is consistent with our observations. A similar evolution was reported by Kanel et al. for nanoscale ZVI: of Fe⁰ with appearance of magnetite/maghemite, lepidocrocite and an amorphous region during the first 7 days, followed by disappearance of the amorphous region at 30 days.²⁸

Rounded aggregates of lepidocrocite were observed, forming spherulites of lamellar minerals (Fig. 5a), probably attributed to the rapid oxidation of the ZVI.¹⁴ Some other rounded-shape aggregates were observed with smoother surfaces that may correspond to less crystalline Fe oxides or smaller Fe oxides crystals (Fig. 5b).

Fig. 5 SEM images of lepidocrocite of three weeks running columns in (a) aerated and (b) non-aerated conditions.

From SEM semi-quantitative analyses, in aerated conditions, it was also possible to notice that As amount was decreasing from external to internal ZVI grain face. This was ascribed to a decrease in the crystallinity of the iron form and highly different As concentrations from one phase to another, whereas homogeneous As distribution was obtained under non-aerated conditions (Fig. 6).

Fig. 6 SEM images of ZVI grains recovered after three days in aerated (a) and non-aerated (b) columns.

Conclusions for DO influence

From the results of O₂ and As monitoring at the outlet of the columns, it clearly appears that the addition of aeration at the inlet significantly modifies the behavior of the ZVI/sand treatment unit. Indeed, aerated systems had a higher trapping efficiency but for a short duration (close to 100% As removal for 5 days for a 10 mg L⁻¹ solution) whereas non-aerated columns had a lower efficiency (around 50%) but for a longer period, leading to a higher cumulative trapping capacity (around two times higher than with aeration). The analysis of As repartition along the columns at the end of the experiment highlighted an heterogeneous repartition for aerated systems (five-fold higher As/Fe ratio at the inlet than at the outlet) whereas it was almost homogeneous along the non-aerated columns.

In aerated systems, the solution was always saturated with O₂, allowing a fast oxidation of ZVI all along the reactor from the very beginning of the experiment. During several days, the high amount of freshly-formed ZVI by-products were able to trap all the incoming arsenic within the first layers of the column. During this period, the ZVI by-products generated in the upper part of the reactor were not exposed to arsenic. Before arsenic reached the upper layers, these unexposed ZVI by-products evolved into less reactive compounds, resulting in a significant decrease in arsenic-trapping and thus in a lower As/Fe ratio. This is in accordance with results from Triszcz et al.,²⁹ who observed that freshly-prepared colloidal particles were more efficient for As removal than aged (∼30 days) particles. Mähler et al. also reported a decrease of available surface sites for aged (6 weeks) ferrihydrite during batch experiments.³⁰

By contrast, in non-aerated conditions, a slow and continuous oxidation of ZVI, and thus by-products formation, could only occur at the lowest part of the column due to O₂ restriction. This is consistent with the results recently published by Trois et al. who observed a decrease of iron concentration from the inlet to the outlet of a ZVI/sand column.¹³ Once the ZVI located at the inlet of the reactor was oxidized, O₂ became available in the next layer of the column. As a result, freshly-formed highly reactive by-products were continuously generated and thus available for arsenic trapping all along the reactor over a longer period of time. As a result, a quite constant sorption capacity was observed up to complete saturation. An improvement of arsenic trapping under O₂-restricted conditions was previously reported by Klas and Kirk,³¹ probably due to the formation of more reactive iron oxides such as green rusts.

The hypothesis of the formation of a highly reactive amorphous iron phase from ZVI followed by its rapid evolution into less reactive phases such as lepidocrocite or goethite is consistent with the results of solid analysis performed on support collected at different durations. It is also consistent with a recent study of Wang et al.³² who found that the retention performances of arsenic were higher for a column filled with ferrihydrite and sand than for a column filled with lepidocrocite and sand. The very high arsenic retention, which reached more than 200 mg(As) per g(Fe), was thus not explained by As sorption onto crystalline ZVI by-products such as lepidocrocite or goethite, but could results from sorption onto the first-generated amorphous by-products and/or coprecipitation of As and amorphous iron oxy(hydro)xides. Indeed, Tokoro et al. observed a higher removal of arsenic during coprecipitation experiments than for simple adsorption experiments. They found that coprecipitation was favored by an increase of As(V)/Fe(III).³³ This is prone to occur in the non-aerated pilot units, where ZVI by-products generation is limited by O₂ restriction.

Conclusions

This study underlines the major influence of the aeration in the design of arsenic treatment units based on ZVI/sand support. The analysis of solid support at the end of the experiments showed that changes in DO content impacts the nature of ZVI by-products generated inside the column. In aerated systems, a large amount of reactive ZVI by-products was generated within a short duration all along the column, leading to a high arsenic removal. However, only the support layers close to the inlet were effective for arsenic sorption. Indeed, the ZVI by-products located in the upper layers were not immediately exposed to As and were thus able to evolve into less reactive by-products. In non-aerated systems, ZVI oxidation and by-products generation occurred slowly. Fresh and highly reactive ZVI by-products were thus continuously produced and immediately exposed to As, leading to a homogeneous As distribution throughout the column. Therefore, according to arsenic removal objectives, the treatment of a high arsenic load in a short time should require aerated conditions with a short lifetime of the support. On the other hand, non-aerated conditions would be more appropriate for the treatment of a moderate arsenic level over a long period of time. A succession of aerated and non-aerated periods could also be way to adapt the reactivity of the ZVI/sand bed reactor to fluctuation on arsenic concentration.

Acknowledgements

The authors would like to thank France's Limousin Regional Council for its financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06565a

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