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Developing the Ascorbic Acid Test: A Candidate Standard Tool for Characterizing the Intrinsic Reactivity of Metallic Iron for Water Remediation

Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany
School of Earth Science and Engineering, Hohai University, Fo Cheng Xi Road 8, Nanjing 211100, China
Department of Sedimentology & Environmental Geology, University of Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany
Grassland Science and Renewable Plant Resources, Faculty of Organic Agricultural Science, University of Kassel, Steinstrasse 19, D-37213 Witzenhausen, Germany
Leibniz Institute for Agricultural Engineering and Bioeconomy (ATB), Max-Eyth-Allee 100, D-14469 Potsdam, Germany
Department of Water and Environmental Science and Engineering, Nelson Mandela African Institution of Science and Technology, Arusha P.O. Box 447, Tanzania
Faculty of Science and Technology, Campus of Banekane, Université des Montagnes, Bangangté P.O. Box 208, Cameroon
Centre for Modern Indian Studies (CeMIS), Universität Göttingen, Waldweg 26, D-37073 Göttingen, Germany
Author to whom correspondence should be addressed.
Water 2023, 15(10), 1930;
Submission received: 29 March 2023 / Revised: 28 April 2023 / Accepted: 17 May 2023 / Published: 19 May 2023
(This article belongs to the Section Wastewater Treatment and Reuse)


Granular metallic iron (gFe0) materials have been widely used for eliminating a wide range of pollutants from aqueous solutions over the past three decades. However, the intrinsic reactivity of gFe0 is rarely evaluated and existing methods for such evaluations have not been standardized. The aim of the present study was to develop a simple spectrophotometric method to characterize the intrinsic reactivity of gFe0 based on the extent of iron dissolution in an ascorbic acid (AA—0.002 M or 2 mM) solution. A modification of the ethylenediaminetetraacetic acid method (EDTA method) is suggested for this purpose. Being an excellent chelating agent for FeII and a reducing agent for FeIII, AA sustains the oxidative dissolution of Fe0 and the reductive dissolution of FeIII oxides from gFe0 specimens. In other words, Fe0 dissolution to FeII ions is promoted while the further oxidation to FeIII ions is blocked. Thus, unlike the EDTA method that promotes Fe0 oxidation to FeIII ions, the AA method promotes only the formation of FeII species, despite the presence of dissolved O2. The AA test is more accurate than the EDTA test and is considerably less expensive. Eight selected gFe0 specimens (ZVI1 through ZVI8) with established diversity in intrinsic reactivity were tested in parallel batch experiments (for 6 days) and three of these specimens (ZVI1, ZVI3, ZVI5) were further tested for iron leaching in column experiments (for 150 days). Results confirmed the better suitability (e.g., accuracy in assessing Fe0 dissolution) of the AA test relative to the EDTA test as a powerful screening tool to select materials for various field applications. Thus, the AA test should be routinely used to characterize and rationalize the selection of gFe0 in individual studies.

1. Introduction

Metallic iron (Fe0) is an effective reactive medium for the remediation of aqueous systems (e.g., groundwater, rainwater, stormwater, wastewater) polluted with numerous species, including chlorinated solvents, trace metals, nutrients and pathogens [1,2,3,4,5,6,7,8,9,10]. The standard redox potential of the FeII/Fe0 electrode reaction (E0 = −0.44 V; Equation (1)) makes Fe0 theoretically an effective reducing agent for many reducible contaminants with E0 > −0.44 V [11]. The introduction of Fe0 as an efficient reactive material for subsurface permeable reactive barriers (PRBs) was based on this premise [11,12,13,14]. However, it was established long before the advent of Fe0 PRBs that under environmental conditions, only protons (H+ from H2O dissociation) can oxidize Fe0 by an electrochemical mechanism (Equation (1)) [15,16].
Fe0 + 2H+ ⇒ Fe2+ + H2
Recent overview articles on “Fe0 for groundwater remediation” point out that further development of this promising technology is impaired by controversies on the operating mode of Fe0 [9,17,18,19]. In fact, the view that Fe0 is a generator of contaminant scavengers (e.g., solid iron corrosion products—FeCPs), and secondary reducing agents (e.g., FeII, H2, Fe3O4), as summarized in Hu et al. [17], and supported by the seminal work of Whitney [15] has been reported to be “isolated misconceptions” [9,18].
Regarding granular Fe0 (micro-scale) as an environmental reducing agent has created a circular reasoning that dragged the research community into an unprecedented confusion [9,17,18,20]. In particular, to enhance the efficiency of Fe0 to remove selected contaminants from the polluted waters, bimetallic materials (e.g., Fe0/Cd0, Fe0/Ni0, Fe0/Pd0, Fe0/Pt0) and nano-scale materials (nano-Fe0 and nano-bimetallics) have been developed [4,10,21,22]. However, the discussion of the reactivity is still based on the value of E0 = −0.44 V for the electrode reaction FeII/Fe0 [18,21,22]. One problem has been that the terms “efficiency” and “reactivity” have been often randomly interchanged [18,23,24,25]. In essence, the reactivity of any Fe0 sample is fixed by the value E0 = −0.44 V, while its efficiency is the “expression” or the “manifestation” of this reactivity (Fe0 type or Fe0 quality) as influenced by operating conditions (e.g., Fe0 dosage and grain size, co-solutes, contaminant concentration, pH value, temperature). Clearly, under appropriate experimental conditions, a double amount (2 m0) of a given Fe0 material (fixed reactivity) is theoretically expected to remove more contaminant from an aqueous phase than a single amount (m0). In other words, in theory, the efficiency of a Fe0 material for water treatment depends (also) on the used dosage. However, in reality, doubling of iron dosage does not double the amount of contaminant removed. Thus, there is no linear relationship between the iron dosage in the system and the amount adsorbed and co-precipitated contaminants.
Enhanced remediation efficiency by varying the Fe0 dosage is justified by variability of the surface area available for Fe0 dissolution which is thought to be coupled to contaminant reductive transformations [11,21,22,26,27]. The higher efficiency of nano-Fe0 relative to granular Fe0 is based roughly on the same principle i.e., larger available surface area, despite difference in intrinsic reactivity [22,28,29,30]. Two questions arise: (i) Why do Fe0 materials of similar sizes (e.g., nano-Fe0) react differently if their reactivity is controlled by the same E0 value? (ii) How can a contaminant (E0 > −0.44 V) be reduced to a certain extent by a given Fe0 sample and not at all by another Fe0 material under the same operating conditions? It seems obvious that these two questions have not yet received the attention they deserve. Certainly, the thermodynamics (relative E0 values) do not have the ultimate control, but the kinetics of iron dissolution is a key factor [31,32]. The fundamentals of electrochemical reactions teach that Fe0 (uncorroded metal) is only one of the four components necessary for electron transfer. The other three components are: (i) an anode where Fe0 is dissolved, (i) a cathode where released electrons are exchanged with a relevant species, and (iii) an electrolyte that transports Fe2+ ions away from the anode. Uncorroded Fe0 ensures the transfer of electrons from the anode to the cathode. There are two reasons why electrons cannot be exchanged between Fe0 and dissolved contaminants: (i) Fe0 is permanently shielded by a non-conductive oxide scale, and (ii) the oxide scale acts as a diffusion barrier for contaminants and a conduction barrier for electrons [17]. The present work focuses on Fe0 as a stand-alone parameter for the efficiency of Fe0/H2O systems.
The characterization of the actual contribution of Fe0 to the process of contaminant removal using Fe0/H2O systems is complicated by the complex interactions between relevant contaminants, operating conditions (e.g., initial concentration, stirring intensity), water chemistry (e.g., availability of co-solutes, contamination level, pH value), and transport phenomena (e.g., advection, diffusion) in the bulk solution and in the vicinity of Fe0 [21,33,34,35]. Despite the wide application of Fe0 (e.g., bimetals, Fe0, nano-Fe0) for the treatment of a variety of pollutants, the evaluation of Fe0 reactivity has not been standardized [21,34,35,36,37,38,39,40]. Of the few characterization tools presented for the Fe0 intrinsic reactivity, only two are truly contaminant independent as they quantify the kinetics of Fe0 dissolution (Equation (1)) by monitoring either dissolved Fe [34,41,42] or generated H2 [38,43]. The most affordable method presented to date is the o-phenanthroline (Phen) method which uses only a spectrophotometer for FeII determination and relies on Fe0 dissolution in a dilute solution of o-phenanthroline (2 mM or 0.002 M) [42]. This simple colorimetric assay is certainly versatile for universal application, even in poorly-equipped laboratories lacking advanced analytical instrumentation. However, the toxicity of o-phenanthroline [43,44] and its higher price (229.00 €/100 g,—accessed on 28 March 2023) compared to ascorbic acid (vitamin C—8.00 €/kg,—accessed on 28 March 2023) have motivated the development of the ascorbic acid test as a low-cost and environmentally friendly alternative.
Ascorbic acid (AA) is a well-known chemical reducing agent [44,45,46,47,48,49] that has also been used in environmental remediation [50,51,52,53,54,55]. AA has also been applied as an alternative to conventional reducing reagents (e.g., hydroxylamine, hydroquinone) in the standard method of converting iron(III) to iron(II) prior to forming a red-orange complex with o-phenanthroline in the spectrophotometric determination of iron [44,46]. FeII-AA complexes are extremely stable at pH values 3.0 to 8.0 [56,57]. In the present study, AA was chosen as a complexing agent to stabilize Fe2+ from Equation (1) and sustain Fe0 oxidative dissolution. The AA test requires one chemical reagent less than the EDTA method. AA is also nontoxic to researchers, environmentally friendly, and less expensive than EDTA or Phen [53,58].
This study presents a simple colorimetric assay to characterize the intrinsic reactivity of gFe0, using ascorbic acid (AA) to complex Fe2+ from Fe0 oxidative dissolution (Equation (1)). Eight different gFe0 samples were tested for Fe dissolution in a 2 mM AA solution to demonstrate the applicability of the method to assess their intrinsic reactivity. In addition, experiments with 2 mM EDTA and 2 mM Phen solutions were performed for comparison.

2. Materials and Methods

2.1. Solutions

Working solutions were prepared from L-hexuronic acid ascorbic acid (AA) (Wasser Hygiene Chemie GmbH, Hilgertshausen, Germany), disodium salt of ethylenediaminetetraacetic (EDTA) (AppliChem GmbH, Darmstat, Germany), and monohydrated 1,10-Phenanthroline (Phen) (Acros Organics, Geel, Belgium). An iron standard solution (1000 mg L−1) (Fisher Scientific UK Limited, Loughborough, UK) was used to calibrate the used UV/VIS spectrophotometer. The Phen solution (0.2 M) and a buffer solution formed by mixing AA and sodium ascorbate (FeelWell GmbH, Gnarrenburg, Germany) were used to determine the aqueous iron concentration. All used chemicals were of analytical grade.
The pH values of the working solutions (2 mM each) were: ascorbic acid 3.1; EDTA 4.7; and Phen 8.3. Tap water from the city of Göttingen (Germany) was used to prepare the solution. Its pH value was 7.8.

2.2. Iron Materials

A total of 8 Fe0 materials were selected and used in this study. The selection was based on their differential reactivity as determined in previous work [42,43]. The used Fe0 materials had different geometrical shapes and size. Three of these were commercially available Fe0 materials for groundwater remediation referred to as: (i) ZVI1 is material from iPutec GmbH Rheinfelden, Germany: (ii) ZVI2 and ZVI3 is directly reduced sponge iron material (DRI) from ISPAT GmbH, Hamburg, Germany; (iii) ZVI6: Connelly Iron from Connelly-GPM Inc., Chicago, IL, USA. ZVI4 and ZVI5 is scrap iron from the metal recycling company Metallaufbereitung Zwickau/Germany. ZVI5 is a mixture of mild steels from various sources; ZVI4 is a mixture of cast irons. ZVI7 and ZVI8 are spherical iron samples from the Chinese company Tongda Alloy Material Factory.
Table 1 summarizes the main characteristics of the 8 Fe0 samples along with their iron content as specified by the supplier. ZVI1, ZVI3 and ZVI5 were tested in column leaching experiments.

2.3. Experimental Methods

2.3.1. Batch Experiments

Iron dissolution experiments were performed using 0.10 g of the ZVI1 sample in 50 mL of the three complexing agent: AA, EDTA, and Phen (each 2 mM) for up to 144 h (6 days). Further experiments using AA were performed for the 8 tested Fe0 samples. All experiments were performed with the conventional quiescent (non-agitated, non-stirred) experimental protocol described in detail in earlier papers [41]. The experimental vessels were protected from direct sunlight and atmospheric dust. Each experiment was performed in triplicate and the average results are presented.

2.3.2. Column Experiments

1.0 g of each Fe0 material (ZVI1, ZVI3, and ZVI5) was placed in a chromatographic column with sand in the lower third and the 2 mM AA solution in the upper two thirds (Figure 1). Fe0 was leached daily for five consecutive days (Monday through Friday) every week with about 180 mL of the AA solution (pH = 3.1) at temperature of 25 ± 4 °C. At each leaching event, the exact volume of the leachate was monitored and its iron concentration determined. The experiment was terminated when the leaching rate of the reactive Fe0 material reached 40%.

2.4. Analytical Method

Analysis for iron was performed by using the phenanthroline method described in detail in earlier papers [42]. Although Fe(AA) exists already in the Fe(II) form, reduction by AA addition was performed just to follow the analytical protocol which includes also the calibrating solutions. Iron concentrations were determined using a Varian Cary 50 Scan UV-VIS Spectrophotometer (Cary instruments, LabMakelaar Benelux B.V., Zevenhuizen, The Netherlands) at a wavelength of 510.0 nm using a 1.0 cm glass cuvette. The instrument has been calibrated for iron concentrations ≤ 10 mg L−1. The pH values were measured with combined glass electrodes (WTW Co., Weinheim, Germany).

2.5. Experimental Results

Given that iron dissolution of Fe0 and of iron corrosion products (FeCPs) is initially a linear function of the time [38] for a given time (t1 > t0) after the start of the experiment (t0 = 0), the total iron concentration at t1 ([Fe]t) is a linear function as defined in Equation (2).
[Fe]t = a × t + b
where a is the slope of the line, t represents the elapsed time since the immersion of Fe0 in the leaching solution, and b the value of [Fe] at t0. Ideally, b approaches 0.
The purpose of this study was to determine the timeframe for the linearity of Equation (2). The regression coefficients ‘a’ and ‘b’ are characteristic of each individual Fe0 sample. In fact, ‘a’ represents the rate of dissolution of Fe from Fe0 while ‘b’ gives an estimate of the amount of FeCPs or the fraction thereof that is dissolved by the used complexing agent (e.g., AA, EDTA and Phen). Note that AA and Phen form stable FeII complexes, whereas EDTA forms stable FeIII complexes [42]. Accordingly, lower b-values are expected in AA tests. The a (mg L−1 h−1) and b (mg L−1) values derived from Equation (2) are converted to μg h−1 and μg, respectively. Dissolution rates (a values equal to kAA, kEDTA, and kPhen) were calculated using Origin software (Version 8.0).

3. Results and Discussion

3.1. Suitability of the Experimental Protocol

Figure 2a compares the extent of iron dissolution of ZVI1 in 2 mM AA, EDTA, and Phen. It can be seen that EDTA dissolves more Fe0 than AA and Phen. The increasing order of performance (i.e., Fe leaching efficiency) is EDTA > AA > Phen. It should be recalled that: (i) EDTA forms very strong complexes with FeIII [59,60], (ii) AA reduces FeIII to FeII and forms very stable complexes with FeII [53], and (iii) Phen also forms very stable complexes with FeII but does not reduce FeIII [42]. Thus, the data indicate that EDTA Fe0 produces by oxidative dissolution FeII (Equation (1)), which is further oxidized to FeIII by O2 in water (Equation (3)). In addition, some corrosion products are dissolved and stabilized in the aqueous phase [61] (Equation (4)). In other words, EDTA supports both the oxidative dissolution of Fe0 and the dissolution of iron corrosion products. On the contrary, since FeII AA complexes are very stable even in the presence of O2, AA induces the oxidative dissolution of Fe0 and the reductive dissolution of iron corrosion products. Finally, since Phen has no reductive power for iron corrosion products, dissolved Fe in its presence only results from oxidative dissolution of Fe0. This reasoning fully justifies the observed order of leaching efficiencies (EDTA > AA > Phen). The corresponding a-values are: kEDTA = 18.6 μg h−1; kAA = 13.2 μg h−1; and kPhen = 8.1 μg h−1 (Table 2).
4Fe2+ + O2 + 2H+ ⇒ 4Fe3+ + 2OH
FeOOH + EDTA + 3H+ ⇒ [FeEDTA]3+ + 2H2O
Previous efforts to characterize the intrinsic reactivity of Fe0 using ligands (e.g., EDTA, Phen) [34,38,41,42] could not explain a negative b-value from Equation (2). It has been postulated that b represents the amount of Fe leached from iron corrosion products, thus, b should be necessarily greater than or equal to zero. Because AA reduces (some) iron corrosion products before stabilizing them as FeII-AA complexes, the AA method is expected to solve this open issue of negative b values.

3.2. Deciphering the Processes of Iron Dissolution in Fe0/AA Systems

Figure 2b compares the time dependent iron dissolution of 4 selected ZVIs in 2 mM AA. As a rule, the more reactive a material is under given conditions, the greater the kAA value. It can be seen that ZVI1 clearly exhibits a higher dissolution rate, while ZVI6 and ZVI7 are very close. ZVI8 shows the slowest iron dissolution. The dissolution of different Fe0 materials in AA (2 mM) is one goal of this research and will be discussed in the next section for the 8 materials tested (Figure 3). This section focuses on the initial phase of iron dissolution (first 72 h) for four selected materials as representatives for different reactivities [42,43].
As discussed in Section 2.5, dissolved Fe (FeII-AA) results from two concurrent processes: (i) oxidative dissolution of Fe0, and (ii) reductive dissolution of FeCPs. In other words, there is competition for AA, but Fe0 is present in large excess [33,34,41] and FeCPs are poorly crystalline in structure [62,63,64,65,66] and thus comparatively readily soluble [67,68,69,70,71]. Accordingly, it can be considered that the reductive dissolution of FeCPs is quantitative in the early phase of the experiments. Operationally, it is considered herein that Fe is quantitatively extracted from FeCPs by 2 mM AA. This approach is borrowed from the sequential extraction methods of soil chemical analysis [72,73,74,75], acknowledging difficulties in reducing iron oxides, even under acidic conditions and in the presence of chelating agents [73]. Clearly, it is considered, that the amount of Fe extractable from FeCPs is dissolved in the early phase of the experiment.
The main feature evident in Figure 2b is that Fe dissolution is very slow during the first day of the experiment (t < 24 h) and then increases progressively. This observation suggests that, under the experimental conditions used in the present study (e.g., 0.1 g Fe0, 2 mM AA), the fraction of Fe resulting from reductive dissolution of iron corrosion products is relatively small. Except for ZVI1, the other three materials did not experience any significant Fe leaching before t = 24 h. For ZVI8, the dissolution rate remains very low until the end of the experiments. Therefore, it can be assumed that the b value corresponds to the level of iron dissolution after about 10 h. In this study, the value obtained for 6 h is tabulated (Table 2) and the a-value or kAA corresponds to the slope of the line [Fe] = f(t) for t ≥ 24 h. In other words, when characterizing the Fe0 intrinsic reactivity using the AA method, the Fe concentration [Fe] after some 4 to 10 h is used to determine the b value (e.g., in μg) while the [Fe] values for t ≥ 24 h are used to determine a or kAA (e.g., μg d−1). This rule is used in the next section.

3.3. Characterizing Fe0 Dissolution in 2 mM Ascorbic Acid (AA)

Figure 3 compares the extent of iron dissolution of all 8 tested ZVIs in 2 mM AA over 144 h (6 days). It can be seen that the intensity of Fe0 dissolution increases slowly from day 1 (24 h) to the end of the experiment. The corresponding dissolution rates (a values from Equation (2) or kAA values) are summarized in Table 2. As explained in Section 3.2, the first two data points (t < 24 h) are applied to calculate the b values, while the remaining 7 points (t ≥ 24 h) are used to calculate the a or kAA values. Table 2 also shows the coefficient of determination R2 for all 9 experimental data points for all ZVIs. R2 is the proportion of the variation in the dependent variable that is predictable from the independent variable. The poorer regression and the associated negative values of b (e.g., μg) justify the use of the data obtained for 4 and 10 h to calculate the b values as shown in Tabel 2. At the same time, this suggests a simplification of the experimental procedure as there is no need for more than one sampling event per day. On the other hand, the last experimental data point can be ignored so that the protocol for Fe0 characterization using the AA method is reduced to one working week of five days. The suggested sampling times during these five days are: 6, 24, 48, 72, 96, and 120 h. The b-values in Table 2 (17 to 127 μg) suggest that less 1% of the different Fe0 materials are iron corrosion products.
A key feature of the AA test is that apart from the two experimental points at t < 24 h, all other points are used as they obey the ‘linearity’ request. Accordingly, all 7 points were used to determine kAA. In the EDTA test on the contrary, only some few points (e.g., four or five) were useful for the KEDTA value of readily reactive materials (e.g., ZVI1). In fact, considering experimental points corresponding to longer duration yielded to poorer R2 values. The findings of this study clearly demonstrated that this ‘disturbance’ is attributed to the presence of air oxygen (and the stability of FeIII-EDTA complexes) [59,60].
Based on Figure 3, ZVI8 is by far the least reactive material of the eight materials tested. Based on the kAA values in Table 2, the reactivity decreases in the following order:
ZVI2 > ZVI4 > ZVI7 > ZVI1 > ZVI5 > ZVI3 > ZVI6 >> ZVI8.
This order of reactivity corresponds to that determined for the same Fe0 materials by the EDTA method [34], the Phen method [41,42] and the H2 method [43]. However, more data are lacking to allow a detailed discussion because the methods involved have not been independently tested or used. Although the EDTA method is already two decades old [76,77], it has never been tested by other research groups. However, this method was not published in a peer-reviewed journal until 2005 [38]. Nearly 18 years passed and several other independent methods have been presented [39,40,68], but are still not universally tested and accepted [21,35,78]. It is very disappointing that the Fe0 research community has been working for 30 years without characterizing the Fe0 materials that are at the center of their systems. Considering the eight materials tested here, the kAA values vary from 17.2 for ZVI2 to 2.8 for ZVI8, giving a reactivity ratio of more than 6. This, means that six times more H2 or Fe2+ is produced in the ZVI2 system than in the ZVI8 system under similar operating conditions. Clearly, one researcher testing ZVI8 may conclude that Fe0 is not suitable, while a colleague testing ZVI1 (under similar conditions) may strongly recommend Fe0 for the same application.
As mentioned above, a deeper discussion is not possible due to the lack of comparable approaches. Unfortunately, research on Fe0 reactivity has mostly been a race for the most reactive material (i.e., bimetallic, nano-Fe0) [22,78]. What is needed, however, are appropriate materials specific for the problems on site. For example, ZVI8, the least reactive material can be the best material for a field situation where aggressive environmental conditions (such as acid mine drainage) sustain iron corrosion in the long term. In such situations, a more reactive material (e.g., ZVI2) would lead to clogging of the system, or at least result in material wastage, since a larger proportion of the corroded Fe0 is not serving the remediation goal. The remainder of this paper discusses the long-term kinetics of Fe0 dissolution (corrosion rate).

3.4. Characterizing the Long-Term Fe0 Dissolution in Column Studies

Figure 4 and Table 3 compare the extent of iron dissolution behaviour of three selected ZVIs (ZVI1, ZVI3 and ZVI5) in 2 mM AA in column experiments for 55 leaching events. Figure 4a shows that 2 to 12 mg of Fe could be leached daily from each column containing 1.0 g of Fe0. Figure 4b shows that up to 530 mg of Fe could be leached after 55 leaching within 129 days. The Fe0 reactivity increases in the order ZVI1 < ZVI5 < ZVI3. The high reactivity of ZVI3 is due to its higher porosity and surface area compared to the other materials. The same order of reactivity was reported in related works [34,43].
For each material, the amount leached was high at the beginning of the experiment, and then progressively decreased with increasing leaching events (elapsed time) until about 70 days (Figure 4a). After the decrease of Fe concentrations between about 70 and 105 days, they increased again to values comparable to the initial values (Table 4). The trend was the same for all Fe0 specimens with relatively little variations between the samples. The cumulative extent of Fe leaching shows that ZVI3 had a slightly higher Fe leaching efficiency over the 55 leaching events than the other two samples. Taken together, Figure 4a,b illustrates clear material-specific trends in the long-term kinetics of iron corrosion that are well known to iron corrosion scientists [79,80,81,82] but has not been really addressed within the Fe0 remediation research community [83,84,85].
A combination of (i) non-constant kinetics of iron corrosion for individual materials, and (ii) different laws of the variation kinetics amount materials make any prediction of the leaching extent challenging (Table 4). Table 4 shows that for the first 10 leaching events, the increasing order of reactivity was ZVI1 < ZVI3 < ZVI5. Between the 10th leaching event and the 52th there is no uniform trend in the variation of the extent of Fe leaching from the three tested materials. Summarized, these results of long-term Fe0 leaching using AA have confirmed that using constant value of corrosion rates in modelling efforts cannot be supported by any assumption [86,87,88].

4. Significance of the Results

4.1. Fe0 Quality as a Stand-Alone Operational Parameter

It is intuitive that different Fe0 materials would provide different results of water treatment under given operating conditions. This is because each Fe0 sample is unique in its intrinsic reactivity [26,27,38,89,90]. Despite this evidence, little attention has been paid to the Fe0 source (Fe0 quality or Fe0 type) as a stand-alone operational variable for the efficiency of Fe0/H2O systems [21,35,39,40,78]. For example, Westerhoff [26] tested six different Fe0 specimens for nitrate removal at an initial pH of 2.0. His results clearly showed that the kinetics and the extent of NO3 removal and pH change varied as a function of the Fe0 source (Fe0 quality), ranging from 20% to 100% after 4 h. For further investigations, the author selected a Fe0 specimen with intermediate efficiency (“partial, but not complete NO3 removal”) in order to study other parameters such as the nature and amount of reaction products. In retrospect, it can be said that such pragmatic approaches have generally been used within the Fe0 research community to select tested Fe0 materials. In support of this statement, while various Fe0 samples have been used in Fe0-based subsurface permeable reactive barriers (PRBs), available models for Fe0 PRBs use a corrosion rate of a single Fe0 sample based on H2 evolution [36,37], although Reardon [36] reported eight different corrosion rates for various Fe0 specimens [85,87]. Obviously, the selection of Fe0 for field applications has not been justified by quality assurance and quality control.

4.2. Other Key Operational Parameters

The performance of Fe0 applications in the field has been shown to depend on: (i) the acidity of the influent (pH value), (ii) the redox conditions (Eh value), (iii) the concentrations of co-solutes (e.g., Ca2+, Cl, Mg2+, NO3, HPO42−, SO42−), and (iv) the Fe0 dosage or Fe0 quantity [4,6,91,92,93,94]. The relative importance of these parameters in determining the performance of Fe0 PRBs has been established since 2007 [1,7,24]. In 2007, it was timely noted that there is a lack of field data to address the long-term performance of Fe0 PRBs in terms of reactivity and permeability. Unfortunately, 15 years later, the results confirming the efficiency of Fe0 PRBs [91,95,96,97] do not address the questions in terms of filling any knowledge gaps in the past [7,10,17,24]. In other words, the intrinsic reactivity of Fe0 (Fe0 quality not Fe0 quantity) has yet to be considered as a key stand-alone variable in the design of Fe0/H2O systems for remediation [35,39,40,42,87]. Factors affecting the intrinsic reactivity of Fe0 include its porosity, shape, size and size distribution, surface layers, surface area, and surface smoothness [78]. Their importance as operational parameters have been demonstrated in various studies [34,35,38,92,96], but not to the extent that sound science-based recommendations can be made to engineers seeking for appropriate Fe0 specimens for site-specific applications [43,98].

4.3. Current Approaches to Address Fe0 Quality

The Fe0/H2O interface is a heterogeneous system in which interacting reactions are of central importance [1,10,22,99]. Contaminant removal in Fe0/H2O systems is largely controlled by processes occurring at two interfaces: (i) Fe0/oxides and oxides/H2O; and (ii) within the oxide scale. Accordingly, various spectroscopic/microscopic techniques have been used in an effort to gain more detailed knowledge enabling to understand the processes that influence the long-term efficiency of Fe0/H2O systems [78,100,101]. For example, modern analytical techniques such as Mössbauer spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), have been used complementarily to characterize samples from laboratory and field investigations [3,78,102]. However, by characterizing the nature or amount of uncorroded Fe0, the generated corrosion products, and the speciation of contaminants at selected times (e.g., at the end of the experiment), these tools provide only a snapshot of the system at those selected times [103,104,105,106]. Moreover, the duration of the experiments is typically too short (e.g., a few days or weeks) to be representative for of water filters and reactive barriers that are expected to operate for years or decades [1,2,3,4]. Clearly, the conventional approach does not account for the documented non-linear decrease of the corrosion rate which is critical to the design of sustainable remediation systems (Section 3). In other words, despite three decades of intensive research in water remediation, there is still little guidance on how to relate the intrinsic properties of a Fe0 sample (Fe0 quality) to its observed long-term field performance. Only eight peer-reviewed publications were found that focus on characterizing the intrinsic reactivity of Fe0 materials (Table 5) with a perspective of introducing a standard protocol.
For the sake of completeness, it should be acknowledged that Fisher and Feinberg [98] recently introduced a new approach to characterize the extent of Fe0 consumption in Fe0/H2O systems. This innovative approach roots on the differential magnetic susceptibility between Fe0 (and magnetite: Fe3O4) and Fe minerals such as goethite (FeOOH) and hematite (Fe2O3). In fact, a decrease in magnetic susceptibility tracks the conversion of high susceptibility materials (Fe0 and Fe3O4) to lower susceptibility minerals (FeOOH and Fe2O3). Accordingly, if one measures the initial magnetic susceptibility of newly installed Fe0 filter, continued measurements will indicate the remaining capacity of the media to provide additional corrodible iron. Remember that, in water remediation Fe0 is converted to high-surface-area iron oxide minerals (FeCPs) that are excellent contaminant scavengers.
The methods in Table 5 are based either on (i) monitoring the formation of primary iron corrosion products (FeII and H2) [36,38,42,43] or (ii) using some easy-to-monitor reactions with some reactive species [35,39,40,106]. However, they have not been routinely used for quality assurance and quality control (QA/QC), and they are not very user friendly. For example, the very first test by Reardon [36] is 28 years old and has been cited only 386 times, an average of 14 times per year. This is practically insignificant in a context where hundreds of articles on Fe0 remediation are published every year [107,108,109,110,111]. The Reardon test [36] requires large quantities of Fe0 (about 500 g) and sophisticated equipment to monitor the H2 evolution [34,42,90,110]. The Phen test using 1,10-Phenanthroline to complex FeII from iron corrosion suffers from the toxicity of this chemical. Therefore, there is still need for safe, affordable and applicable methods for QA/QC of Fe0. The AA method is presented here as a candidate method for routine QA/QC.

4.4. The AA Method as a Quality Control Tool for Fe0 Materials

We have developed a simple tool to characterize the intrinsic reactivity of commercially available granular metallic iron materials (gFe0) by measuring the iron content in a dilute ascorbic acid solution (2 mM) within one week (5 or 6 days). Ascorbic acid (vitamin C) is inexpensive, nontoxic, and readily available worldwide.
The protocol of the AA test can be summarized as follows:
Add 0.1 g of Fe0 to 50 mL of a 2 mM AA solution and monitor the concentration of dissolved Fe for 0.3, 1.0, 2.0., 3.0, 4.0 and 5.0 days.
Use the iron concentration after 8 h to estimate the amount of iron corrosion products and the remaining data to determine the kAA value. kAA is the slope of the line dissolved [Fe] versus time t for t ≥ 24 h.
In future work, each Fe0 material should be routinely presented in the experimental section with its kAA value. Such an approach will be similar to how conventional filter materials such as activated carbons are presented with their iodine number, removal capacity for methylene blue, or specific surface areas (SSA) [112,113,114,115,116]. Within a reasonable period of time, e.g., two years, it will be possible to better discuss the suitability of these materials in terms of the parameters reported by the manufacturers (e.g., iron content, SSA). The task is to validate a simple test for quality assurance and quality control of Fe0 materials. The AA method is valuable for users to effectively optimize designs of remediation systems, from setting technological parameters to budgeting (life cycle analysis LCA). The AA method is also expected to support the development of new Fe0 materials with tailored properties for specific applications, such as “Fe0 for saline wastewaters”, “Fe0 for acid mine drainage”, or “Fe0 for carbonate-rich waters”. This approach will certainly support the design of Fe0 filters for decentralized safe drinking water provision [83,87,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121] and keep the international community on track to achieve Goal 6 (‘Ensure access to water and sanitation for all’) of the Sustainable Development Goals (SDGs) of the United Nations even for poor and vulnerable populations [122,123,124,125].
Activated carbons for specific applications can be developed because a new material is considered fully characterized when the following is specified: (i) carbon content (%), (ii) specific surface area (m2·g−1), (iii) pore volume (cm3·g−1), (iv) pore size distribution (e.g., percent of micropores), and (v) surface functional groups [112,113,114,115,116]. Once these are known, it suffices to roughly consider solubility and molecular size of the pollutant, as well as solution pH and the presence of other species (e.g., co-solutes) to select an appropriate activated carbon for a specific application. Attia et al. [115] summarized the following rule of thumb for selecting activated carbons: “The most widely used activated carbons are microporous and have high surface areas, and as a consequence, show high efficiency for the adsorption of low molecular weight compounds and low for larger molecules. The adsorption of bigger size compounds such as dyes, dextrines or natural organic compounds, requires materials with high mesopore contribution to the total pore volume of adsorbents”. The present work has starting paving the way for such a rule of thumb for the characterization of Fe0 materials for water remediation.

5. Conclusions

The efficiency of Fe0 materials for environmental remediation and water treatment is certainly related to the oxidative dissolution of used samples. In characterizing Fe0 leaching in 2 mM ascorbic acid, this study has demonstrated the complexity of intrinsic reactivity as a stand-alone operational parameter that must be carefully considered in the further development of an already established technology. The results suggest that materials that are efficient in short term laboratory experiments may not continue to react uniformly over the life time of the system. Although some of the observed differences could be explained by some known properties of the Fe0 material (e.g., porosity of ZVI2), it should be explicitly stated that the discussion of material-related properties (e.g., Fe content, particle shape and size) is beyond the scope of this study. The discussion is based solely on the observed extent of iron dissolution observed under laboratory conditions. The situation may change once the Fe0 is placed in the subsurface (e.g., carbonate rich, anoxic, saline environments).
The AA approach presented here is an improved version of a 20-year-old method using EDTA as a complexing agent to specify the oxidative dissolution of Fe0. Systematic research is needed to further develop the AA method into a unified standard protocol for quality control/quality assessment of Fe0 materials. Systematic testing of micro-sized and nano-sized Fe0 as well as Fe alloys and sulfidized counterparts is required. The goal is to create a database of Fe0 materials to select the right Fe product for site-specific applications.

Author Contributions

X.C., R.T., M.X. and C.N. conceived the presented idea and developed the theory. X.C., R.T. and M.X. carried out the experiment. R.H., H.R., W.G. and C.N. supervised this work. C.N. supervised the redaction of the first draft by X.C., R.T. and M.X. All authors discussed the results and contributed to the final manuscript. All authors have read and agreed to the published version of the manuscript.


X.C. is supported by a research grant from the China Scholarship Council (Project Code: 202006710005).

Data Availability Statement

Data are available on request.


The manuscript was improved thanks to the insightful comments of anonymous reviewers from Water. We acknowledge support by the German Research Foundation and the Open Access Publication Funds of the Göttingen University.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. Column experimental set-up for Fe0 leaching by ascorbic acid (2 mM). The photograph was made at the end of the experiments. The spout of the third column was broken during the experiments but this has no incidence on the performance of the system.
Figure 1. Column experimental set-up for Fe0 leaching by ascorbic acid (2 mM). The photograph was made at the end of the experiments. The spout of the third column was broken during the experiments but this has no incidence on the performance of the system.
Water 15 01930 g001
Figure 2. Time-dependent dissolution of Fe0: (a) ZVI1 in 2 mM AA, EDTA or Phen using, and (b) ZVI1, ZVI6, ZVI7 and ZVI8 in 2 mM AA. Experimental conditions: V = 50 mL, mZVI = 0.1 g, T = 25 ± 4 °C. The lines are not fitting functions, rather, they simply connect points to facilitate visualization.
Figure 2. Time-dependent dissolution of Fe0: (a) ZVI1 in 2 mM AA, EDTA or Phen using, and (b) ZVI1, ZVI6, ZVI7 and ZVI8 in 2 mM AA. Experimental conditions: V = 50 mL, mZVI = 0.1 g, T = 25 ± 4 °C. The lines are not fitting functions, rather, they simply connect points to facilitate visualization.
Water 15 01930 g002aWater 15 01930 g002b
Figure 3. Time-dependent concentrations of dissolved Fe of from the 8 Fe0 samples in the presence of 2 mM AA for the 8 ZVIs tested in this works. For kAA values (Table 2), only data for t ≥ 24 h are considered. Experimental conditions: V = 50 mL, mZVI = 0.1 g, T = 25 ± 4 °C, Fe0 mass loading 2 g L−1.
Figure 3. Time-dependent concentrations of dissolved Fe of from the 8 Fe0 samples in the presence of 2 mM AA for the 8 ZVIs tested in this works. For kAA values (Table 2), only data for t ≥ 24 h are considered. Experimental conditions: V = 50 mL, mZVI = 0.1 g, T = 25 ± 4 °C, Fe0 mass loading 2 g L−1.
Water 15 01930 g003
Figure 4. Time-dependent extent of Fe leaching from the three Fe0 specimens tested: (a) mass (mg) released per leaching event, and (b) cumulative mass. Experimental conditions: miron = 1.0 g; [AA] = 2 mM; and T = 25 ± 4 °C. The lines are not fitting functions. They simply connect points for ease of visualization.
Figure 4. Time-dependent extent of Fe leaching from the three Fe0 specimens tested: (a) mass (mg) released per leaching event, and (b) cumulative mass. Experimental conditions: miron = 1.0 g; [AA] = 2 mM; and T = 25 ± 4 °C. The lines are not fitting functions. They simply connect points for ease of visualization.
Water 15 01930 g004
Table 1. Code and main characteristics of tested Fe0 materials according to the supplier. n.s. = not specified; granular = mechanically broken pieces; sponge = particles with pitted surfaces; scrap = waste generated in any form: spherical = standard sphere with a smooth surface.
Table 1. Code and main characteristics of tested Fe0 materials according to the supplier. n.s. = not specified; granular = mechanically broken pieces; sponge = particles with pitted surfaces; scrap = waste generated in any form: spherical = standard sphere with a smooth surface.
CodeShapeSizeColorSpecific Surface AreaFeSupplier
(mm) (m2/g)(%)
ZVI1granular0.05–5.00blackn.s.n.s.iPutec GmbH
ZVI2sponge0.68–1.00blackn.s.90.0ISPAT GmbH
ZVI3sponge1.00–2.00blackn.s.90.0ISPAT GmbH
ZVI4scrap0.05–5.00blackn.s.n.s.Metallaufbereitung Zwickau
ZVI5scrap0.05–2.00blackn.s.n.s.Metallaufbereitung Zwickau
ZVI7spherical0.05–1.00grey0.74–1.2699.99Tongda Alloy Material Factory
ZVI8spherical2.00grey0.3999.99Tongda Alloy Material Factory
Table 2. Statistical parameters (a = kligand, b, R2) for the eight tested Fe0 materials. Experimental conditions: initial ligand concentration 2 mM, room temperature 25 ± 4 °C, and Fe0 mass loading 2 g L−1. The number of experimental data points are: 2 for b, 7 for a, 9 for R2.
Table 2. Statistical parameters (a = kligand, b, R2) for the eight tested Fe0 materials. Experimental conditions: initial ligand concentration 2 mM, room temperature 25 ± 4 °C, and Fe0 mass loading 2 g L−1. The number of experimental data points are: 2 for b, 7 for a, 9 for R2.
(μg)(μg)(μg h−1)(μg h−1)(-)(-)
Using AA
ZVI4118. 916.
ZVI1 using AA, EDTA, and Phen
Table 3. Summary of the amount of Fe leached from the Fe0 specimens tested over 55 leaching events. The average daily leached amount is the sum of the leached mass divided by 129. Experimental conditions: miron = 1.0 g; [AA] = 2 mM; and T = 25 ± 4 °C.
Table 3. Summary of the amount of Fe leached from the Fe0 specimens tested over 55 leaching events. The average daily leached amount is the sum of the leached mass divided by 129. Experimental conditions: miron = 1.0 g; [AA] = 2 mM; and T = 25 ± 4 °C.
Rate (Unit)ZVI1ZVI3ZVI5
Table 4. Event-specific Fe leached mass (mg) from 1.0 g of the tested Fe0 specimens at 8 selected events. The Fe0 specimens are ordered from left to right in the order of increasing value of the mass after the second leaching event, corresponding to day 2 of the experiment.
Table 4. Event-specific Fe leached mass (mg) from 1.0 g of the tested Fe0 specimens at 8 selected events. The Fe0 specimens are ordered from left to right in the order of increasing value of the mass after the second leaching event, corresponding to day 2 of the experiment.
Table 5. Summary information of the peer-reviewed publications on the characterization of the intrinsic reactivity of Fe0 materials and their citation frequency according to Google Scholar until 2023 (accessed on 18 January 2023).
Table 5. Summary information of the peer-reviewed publications on the characterization of the intrinsic reactivity of Fe0 materials and their citation frequency according to Google Scholar until 2023 (accessed on 18 January 2023).
(Total)(per Year)
1995Anaerobic corrosion of granular iron: Measurement and interpretation of hydrogen evolution rates38613.8
2005Testing the suitability of zerovalent iron materials for reactive walls1106.1
2014Standardization of the reducing power of zerovalent iron using iodine303.3
2015Simple colorimetric assay for dehalogenation reactivity of nanoscale
zero-valent iron using 4-chlorophenol
2016A facile method for determining the Fe(0) content and reactivity
of zero valent iron
2019A novel and facile method to characterize the suitability of
metallic iron for water treatment
2020Characterizing the reactivity of metallic iron for water treatment:
H2 evolution in H2SO4 and uranium removal efficiency
2020Cost-effective remediation using microscale ZVI: comparison
of commercially available products
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Cui, X.; Xiao, M.; Tao, R.; Hu, R.; Ruppert, H.; Gwenzi, W.; Noubactep, C. Developing the Ascorbic Acid Test: A Candidate Standard Tool for Characterizing the Intrinsic Reactivity of Metallic Iron for Water Remediation. Water 2023, 15, 1930.

AMA Style

Cui X, Xiao M, Tao R, Hu R, Ruppert H, Gwenzi W, Noubactep C. Developing the Ascorbic Acid Test: A Candidate Standard Tool for Characterizing the Intrinsic Reactivity of Metallic Iron for Water Remediation. Water. 2023; 15(10):1930.

Chicago/Turabian Style

Cui, Xuesong, Minhui Xiao, Ran Tao, Rui Hu, Hans Ruppert, Willis Gwenzi, and Chicgoua Noubactep. 2023. "Developing the Ascorbic Acid Test: A Candidate Standard Tool for Characterizing the Intrinsic Reactivity of Metallic Iron for Water Remediation" Water 15, no. 10: 1930.

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