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Communication

The Suitability of Hybrid Fe0/Aggregate Filtration Systems for Water Treatment

by
Ran Tao
1,
Huichen Yang
1,
Xuesong Cui
1,
Minhui Xiao
1,2,
Nadège Gatcha-Bandjun
3,
Joseline Flore Kenmogne-Tchidjo
4,
Mesia Lufingo
5,6,
Bernard Konadu Amoah
2,
Raoul Tepong-Tsindé
1,7,
Arnaud Igor Ndé-Tchoupé
2,7,
Marquise Touomo-Wouafo
8,
Brice Donald Btatkeu-K
8,
Willis Gwenzi
9,
Rui Hu
2,
Jean Bosco Tchatchueng
8,
Hans Ruppert
10 and
Chicgoua Noubactep
1,2,6,11,12,*
1
Angewandte Geologie, University of Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany
2
School of Earth Science and Engineering, Hohai University, Fo Cheng Xi Road 8, Nanjing 211100, China
3
Faculty of Science, Department of Chemistry, University of Maroua, Maroua B.P. 46, Cameroon
4
Department of Chemistry, Faculty of Sciences, University of Yaoundé I, Yaoundé B.P. 812, Cameroon
5
Faculty of Science and Technology, Teofilo Kisanji University, Mbeya P.O. Box 1104, Tanzania
6
Department of Water and Environmental Science and Engineering, Nelson Mandela African Institution of Science and Technology, Arusha P.O. Box 447, Tanzania
7
Department of Chemistry, Faculty of Sciences, University of Douala, Douala B.P. 24157, Cameroon
8
National School of Agro-Industrial Sciences, University of Ngaoundere, Ngaoundere B.P. 455, Cameroon
9
Biosystems and Environmental Engineering Research Group, Department of Agricultural and Biosystems Engineering, University of Zimbabwe, Mt. Pleasant, Harare P.O. Box MP167, Zimbabwe
10
Department of Sedimentology & Environmental Geology, University of Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany
11
Centre for Modern Indian Studies (CeMIS), University of Göttingen, Waldweg 26, D-37073 Göttingen, Germany
12
Faculty of Science and Technology, Campus of Banekane, Université des Montagnes, Bangangté B.P. 208, Cameroon
 
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Author to whom correspondence should be addressed.
Water 2022, 14(2), 260; https://doi.org/10.3390/w14020260
Submission received: 12 December 2021 / Revised: 12 January 2022 / Accepted: 14 January 2022 / Published: 17 January 2022
(This article belongs to the Topic Emerging Solutions for Water, Sanitation and Hygiene)
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Abstract

:
Metallic iron (Fe0) corrosion under immersed conditions (Fe0/H2O system) has been used for water treatment for the past 170 years. Fe0 generates solid iron corrosion products (FeCPs) which are known to in situ coat the surface of aggregates, including granular activated carbon (GAC), gravel, lapillus, manganese oxide (MnO2), pyrite (FeS2), and sand. While admixing Fe0 and reactive aggregates to build hybrid systems (e.g., Fe0/FeS2, Fe0/MnO2, Fe0/sand) for water treatment, it has been largely overlooked that these materials would experience reactivity loss upon coating. This communication clarifies the relationships between aggregate addition and the sustainability of Fe0/H2O filtration systems. It is shown that any enhanced contaminant removal efficiency in Fe0/aggregate/H2O systems relative to the Fe0/H2O system is related to the avoidance/delay of particle cementation by virtue of the non-expansive nature of the aggregates. The argument that aggregate addition sustains any reductive transformation of contaminants mediated by electrons from Fe0 is disproved by the evidence that Fe0/sand systems are equally more efficient than pure Fe0 systems. This demonstration corroborates the concept that aqueous contaminant removal in iron/water systems is not a process mediated by electrons from Fe0. This communication reiterates that only hybrid Fe0/H2O filtration systems are sustainable.

1. Introduction

The eradication of waterborne diseases (e.g., cholera, typhoid fever) through proper water and waste management was achieved in industrialized nations during the 19th century [1,2]. The paradigm accounting for this achievement can be summarized in three points: (i) protect still clean water sources, (ii) avoid any further water pollution, and (iii) treat/reuse polluted waters [2]. For more than 170 years, it has been well established that the provision safe drinking water and the management of wastes eliminate waterborne diseases [2,3]. This understanding of how to eliminate waterborne diseases virtually represents the greatest achievement for public health [3]. However, access to safe drinking water and proper sanitation is still a dream for millions of people worldwide as aptly captured in the United Nations Sustainable Development Goals [4]. In particular, Goal 6 strives to “ensure availability and sustainable management of water and sanitation for all”. Actually, it is questionable whether Goal 6 will be achieved within the remaining period of less than a decade, particularly in the developing world [5,6], and there is a broad consensus that the main challenge is financial in nature [7,8].
Arguably, safe drinking water provision cannot be achieved without money. However, the question is how much money and which technologies should be used? Available cost estimations are essentially rooted in the “big is beautiful” paradigm, which is based on centralized systems, while regarding decentralized systems as low-technology and/or short-term bridging alternatives [9]. On the contrary, the “small is beautiful” or decentralized approach [10] has not yet received due attention [11,12,13,14]. The decentralized approach, comprising do-it-yourself (D-I-Y) solutions, advocates for the use of local materials, labor, and skills (not only) for safe drinking water supply [10,15,16,17]. Using the wording of Roy and Hartigan [16], it is about “empowering the rural poor to develop themselves”. This effort includes using harvested rainwater (arsenic free) instead of buying reportedly affordable commercial filters for arsenic removal [15]. At the start of the SDGs era, Hering et al. [18] published a timely “call for synthesis of water research to achieve the Sustainable Development Goals by 2030”. The present communication strives for such a synthesis for the design of metallic iron-based filtration systems (Fe0 filters). The presentation is limited to the sustainability in terms of long-term permeability.
Metallic iron (Fe0) has been industrially used for water treatment since the 1850s with the first patent secured in 1857 [3,19,20,21,22]. However, over the years, Fe0 filters have been designed on a pragmatic basis, often without critical evaluation of the published works to enable the advancement of the technology [23,24,25,26,27,28,29,30,31]. Accordingly, although it was known before 1883 that Fe0 filters clog because of cementation between gravel/sand and Fe0 particles [3,19,32], the post-1990 research on Fe0 filtration continued using pure Fe0 beds (100% Fe0) [33,34]. Moreover, pretreatment zones with up to 50% Fe0 (w/w) were tested as oxygen scavengers [34,35,36]. This occurred despite the fact that, under field conditions, quantitative contaminant removal in a field Fe0 reactive permeable reactive barrier containing only 22% Fe0 (w/w) was reported [23,37,38]. This confusion motivated the first efforts to systematically test hybrid Fe0/aggregate systems [39]. Mixing sand with Fe0 was regarded as “Fe0 dilution” with potential negative impacts on the extent of decontamination [24,39,40,41]. Interestingly, some “diluted systems” performed better than pure Fe0 systems [24,39,42,43]. For example, Song et al. [42] reported on the enhancement of Cr(VI) reduction in Fe0/sand systems relative to Fe0 alone. Obviously, the presence of sand and its subsequent coating by nascent iron hydroxide resulted in the removal of Cr(VI) via adsorption. Thus, the results of Song et al. [42] can be regarded as an unexploited opportunity to question the importance of direct reduction reactions (electrons from Fe0) in the process of contaminant removal in Fe0/H2O systems. An understanding of the real reaction mechanism is critical for the discussion of changes in the hydraulic conductivity, hereafter referred to as permeability loss. This is because the stoichiometry of the decontamination process has been used to model the service life of Fe0 filters [44]. Accordingly, assuming effective reductive transformations, it makes a significant difference whether the reducing electrons (for CrVI, for example) come from Fe0 (Equation (1)), FeII (Equation (2)), or H2 (Equation (3)).
3 Fe0 + 2 CrO42− + 16 H+ ⇒ 3 Fe2+ + 2 Cr3+ + 8 H2O.
3 Fe2+ + CrO42− + 8 H+ ⇒ 3 Fe3+ + Cr3+ + 4 H2O.
3 H2 + 2 CrO42− + 10 H+ ⇒ 2 Cr3+ + 8 H2O.
For example, comparing Equations (1) and (2) shows that the reduction of 2 mol of CrO42− requires 3 mol of Fe0 (Equation (1)—electrochemical reaction) or 6 mol of Fe2+ (Equation (2)—chemical reaction). However, the electrochemical reaction is impossible under environmental conditions because the electron transfer is hindered by the nonconductive oxide layer on Fe0 [31,45]. In other words, using reactions similar to Equation (1) to predict the service life of Fe0 filters [44,46] is faulty [29,30]. When it is additionally considered that Fe0 is corroded only by H+ [29,45,47,48,49], it becomes evident that the proper discussion of the permeability loss of Fe0 is yet to be started [29,30,31]. Admixing Fe0 and various aggregates is part of these efforts [25,27,34,50,51].
This work presents the suitability of Fe0/aggregate systems, including the fundamental principles, achievements, and potential applications, as a prerequisite for sustainable Fe0 filters. Specifically, the value of Fe0/sand as a reference system in assessing the suitability of other aggregates (e.g., FeS2, MnO2, pumice, wood chips) is discussed. Lastly, future research directions using appropriate characterization tools including the MB method are highlighted.

2. The Remediation Fe0/Aggregate Filtration System and Its Proper Investigation

2.1. Historical and Fundamental Aspects

Water treatment by filtration is a very old technology [3,19,20,21,22,52,53,54]. The five main operational parameters for the success of a filter to treat a given polluted water are (i) the volume and quality of water to be produced per unit time (e.g., day), (ii) the nature (e.g., charcoal, Fe0, gravel) and the characteristics of the filtering media (e.g., form, reactivity, porosity, size), (iii) the used amount of filtering media (e.g., size of filter, number of filters in series), (iv) the nature and extent of pollution (e.g., co-solutes, contaminants of concern, pH value), and (v) the water flow velocity, which in turn determines the contact time. Additionally, it is preferable that filter media are cost-effective and capable of being easily recycled [23,52,55]. Over the decades, the technology of filtering polluted water through porous beds filled with adsorbents has been established in many areas of environmental engineering, including drinking water provision [11,56,57], mine water treatment [58,59,60], and stormwater treatment [61,62,63]. A common feature for all filtration systems is clogging (permeability loss) resulting from the accumulation of colloids, contaminants, and suspended particles [36,64,65,66]. Fe0 filters are characterized by the in situ generation of colloids [31], meaning that, even without any inflow of contaminants and suspended particles, Fe0 filters may clog, depending on the (kinetics and) extent of Fe0 corrosion by water [67,68,69].
Notter [52] stated that spongy iron (a porous Fe0) was undoubtedly the best medium for water filters and added that “its action is not so rapid as charcoal”. Baker [3,19] recalled that Fe0 generates coagulants or flocs for contaminant scavenging. This then implies that the efficiency of Fe0 filters for water treatment depends on the kinetics and extent of flocs generation within the porous structure. The same flocs fill the initial porosity and are responsible for reported permeability loss [68,70]. For this reason, the discussion of the process of permeability loss of Fe0 filters should have started by addressing the time-dependent reduction in the initial porosity as Fe0 corrodes [30,31,68,70].
A survey of the history of Fe0 filters reveals tangible observations made before 2010 that would have led to the proper consideration of the contribution of iron corrosion to the process of permeability loss (Table 1). In particular, Oldright et al. [54] recommended many thin Fe0 beds in series, rather than a thicker one with the same amount of sponge iron, to avoid material wastage. They rationalized their recommendations by the larger volume of lead (Pb0) replacing iron (Fe0) in the porous system. These authors could have considered the volumetric expansive nature of iron corrosion at pH values > 4.5 as reported 25 years before by Whitney [45].
Table 1 shows that, in addition to sand and gravel, which are considered standard admixing agents [33], researchers have considered other aggregates including jute, peat, and pyrite. It is certain that peat was used as another potential remediation material for PO43−. However, the aspect of sustained permeability should have received more attention. This is particularly true because systems using pelletized jute [76] and peat [75] experienced no permeability loss. It can be postulated that bio-aggregates (e.g., jute, peat, wood chips) are better than compact minerals (e.g., gravel, sand) for sustained permeability. This is because bio-aggregates are likely to undergo decomposition over time, thereby improving the porosity and permeability of the system and offsetting the effects of cementation.
However, a number of knowledge gaps still exist on the use of bio-aggregates in Fe0/H2O systems. First, it is unclear whether the oxidation of bio-aggregates will generate adequate porosity to sustain the Fe0/H2O system in the long term. Second, limited data are available on the depletion of O2 concentration in water passing through a column packed with bio-aggregates/Fe0 mixture. Thus, the impacts of the depletion in O2 concentration, coupled with the potential release of organic compounds on the organoleptic properties (e.g., taste, odor) of the treated water, are currently unknown. These gaps call for systematic studies to investigate and identify the appropriate bio-aggregate and their mixing ratios.
Table 1 is limited to the period before 2010 as it corresponds to the date that the research group of Dr. Noubactep started its systematic investigation on hybrid Fe0 systems. Two very comprehensive review articles were also published in 2010 [79,80], retrospectively demonstrating the confusion on the suitability of hybrid Fe0/aggregate systems for sustainable systems. In particular, Fe0/aggregate systems were used as “equalization zones” [80,81,82,83], “pretreatment zones” [33,35,81], or “sacrificial pretreatment zones” [80] to optimize the efficiency of the pure Fe0 “reactive zone”. The very first result of Noubactep and colleagues published in 2010 was that using porous Fe0 materials like sponge iron or porous composites yields more sustainable systems [84,85]. For the same reasons, using pumice instead of sand yields more sustainable filters. The rationale is that the internal pores of individual grains are good reservoirs for in situ generated FeCPs [84,86]. On the other hand, the rationale for Fe0/sand being more sustainable than pure Fe0 is that there are less expansive Fe0 particles in the system compared to the pure Fe0 system (a fraction is replaced by non-expansive sand), thus delaying clogging (Figure 1). Upon clogging, the residual amount of unreacted Fe0 is pure material wastage [68,70]. In other words, the expression “Fe0 dilution” for hybrid systems is a thinking mistake. Of course, there are fewer Fe0 particles in hybrid systems, but it is rather a prerequisite for sustainability and optimal material usage [68,70]. This means that using pretreatment Fe0/sand zones [79,80,81,82,83] was a thinking mistake as quantitative contaminant removal has been reported in such systems for more than one century, even when porous Fe0 (e.g., spongy iron) was used [3,10]. Figure 2 summarizes the types of filters that have been tested, including the layered hybrid system (Figure 2b).
The presentation at this point recalls that only mixed layer hybrid systems (Figure 2c) are sustainable; however, as pointed out by Domga et al. [68], pure Fe0 filters can still be designed for particular uses, for instance, in an emergency. In such cases, modular designs comprising at least one Fe0 filter are recommended, and selected Fe0 units can be regularly replaced, for example, on a monthly basis. For water supply of households and small communities, however, more sustainable systems are required; a rule of thumb is to develop systems capable at operating for 6 to 12 months without maintenance [57,87,88,89]. The remaining discussion focuses on the suitability of admixing aggregates.

2.2. Investigating the Remediation Fe0/Aggregate Filtration System

The mechanisms of contaminant removal in Fe0 filters are adsorption, coprecipitation, and size exclusion [90,91,92]. No mechanistic discussion is considered herein as the focus is on the extent (and the kinetics) of pore filling by in situ generated FeCPs. The presentation in Section 2.1 indicated that the efficiency of Fe0 filters with regard to contaminant removal varies as a function of several interdependent experimental setups, including the properties of used Fe0 materials and the nature of admixing aggregates. Although the kinetics and extent of FeCPs generation in each system depend on the intrinsic reactivity of used Fe0 materials, this section is virtually limited to the nature of the aggregate. In other words, it is virtually considered that Fe0 and aggregates are from the same sizes (Figure 1), and that polluted water has the same chemistry. Thus, only the nature of the aggregate is discussed. Table 2 gives an overview of the main aggregates that have been used to date.
Table 2 lists 12 different materials that demonstrated in individual studies a significant impact on the efficiency of hybrid systems relative to pure Fe0. Discrepancies between studies are obvious because the employed Fe0 materials were not characterized, although testing methods were made available for decades [103,104,105,106]. On the other hand, each aggregate is tested as an independent success story without any effort to have an operational reference system (e.g., Fe0/sand). To the best of the authors’ knowledge, only Ndé-Tchoupé et al. [106] used Fe0/sand systems as a reference to discuss the suitability of Fe0/pozzolan. Therefore, it is a tangible fact that no normalization of published data is possible. This sad situation is illustrated herein by the Fe0/FeS2 system from a historical perspective.
Pyrite (FeS2) was introduced in the Fe0 remediation literature as a pH-shifting agent by Lipczynska-Kochany et al. [107]. The objective was to address “the deactivation of the metal surface” which was reported to be a barrier to the practical application of the then considered new technique. Between 1995 and 2005, many other researchers reported on the improved efficiency of Fe0/FeS2 system relative to pure Fe0 [23,79,80,81,108,109,110,111]. In particular, Wolfe and Cipollone [23] secured a patent for remediation Fe0/FeS2 systems. Later on, Henderson and Demond [65] used a pure Fe0 bed length of 25 cm to investigate permeability and concluded that Fe0 filters are not sustainable. Henderson and Demond [112] concluded that FeS-based filters are better than Fe0 ones with regard to permeability loss. This conclusion was based on four lines of evidence: (i) their excellent literature review [36], (ii) the results with the pure Fe0 filters [65], (iii) results of a 25 cm bed of FeS-coated sand [112], and (iv) geochemical modeling. However, the same authors [65,112] considered inflowing groundwater and not expansive iron corrosion as the cause of permeability loss. Additionally, no Fe0/FeS system was investigated.
Since 2018, more systematic investigations on the suitability of the Fe0/FeS systems were made available [95,113,114,115]. However, they have not considered past efforts, the duration of the experiments was too short, and the used Fe0 and FeS materials were poorly characterized [116]. Additionally, the discussion of the operating mode of FeS2 in improving the efficiency of Fe0/H2O systems is biased by considering Fe0 as a reducing agent under operational conditions [116,117,118]. The results of Hu et al. [116] support that FeS2 (i) delays (not suppresses) the pH increase in Fe0/H2O systems, and (ii) boosts the production of reactive Fe2+. However, their results clearly demonstrated that any discussion supporting the electron transfer from Fe0 is highly speculative and even wrong because, despite pH decrease, quantitative contaminant removal was only observed when the final pH value was not higher than 4.5 [117,118]. Thus, arguments such as “FeS2 activates the Fe0 surface through replacing partially the passive oxide film with iron sulfide (FeS)” [113] are not acceptable. There is even evidence that the addition of Fe0 to pyrite can suppress its oxidation over the time [119,120]. In other words, the documented enhanced efficiency of FeS-amended Fe0/H2O systems should be further investigated to optimize their use in modular systems. The frequency of exchanging Fe0/FeS units depends on the characteristics of used aggregates.
To sum up, despite 30 years of intensive investigations on the remediation Fe0/FeS system, available knowledge is largely fragmented [116,117,118]. The situation is not better for all other aggregates, including sand. Therefore, real systematic investigations with well-characterized Fe0 and aggregates are needed to optimize the design of efficient Fe0 filters for safe drinking water provision, wastewater treatment, and environmental remediation [30,121].

2.3. Investigating Fe0/Aggregate Systems

The presentation until now has intentionally paid little attention to the nature of contaminants. This is because, regardless of the nature of contaminants, water treatment in Fe0/H2O systems entails Fe0 corrosion in porous systems under aqueous conditions [122,123]. Thus, the nature and the level of contamination are just other operational parameters [25,30,121]. Cognizant of this fact, the methylene blue method (MB method) was introduced one decade ago to characterize the extent of Fe0 corrosion as reflected by the availability of its corrosion products [43,124]. The MB method exploits the differential adsorptive affinity of positively charged MB onto sand (negatively charged) and iron oxides (FeCPs positively charged; charge exclusion). Accordingly, when two Fe0 specimens are to be compared for their intrinsic reactivity, it suffices to take a certain amount of each (e.g., 100 g) mix it to the same volume of sand and investigate the extent of MB discoloration in both systems. The more reactive material is the one that exhibits the lowest MB discoloration efficiency. The rationale is that the more the surface of sand is coated with in situ generated FeCPs, the lower the extent of MB discoloration (charge exclusion) will be [25,26,27,28].
The MB method, in its current form, will certainly work for SiO2-based aggregates (e.g., gravel, pozzolan, pumice) [99]. The MB method was also successful applied for FeS2 [111,112,113] and MnO2 [98,125,126,127]. However, particularly for carbon-based aggregates, the MB method should be assessed and, eventually, appropriate amendments or alternatives could be developed. Unlike minerals (e.g., gravel, sand), carbon-based aggregates do not induce MB discoloration via purely electrostatic interactions [128]. However, in the case of strong affinity, two simple complementary tools can be adopted: (i) lengthening the experimental duration to investigate the system upon the exhaustion of the adsorptive capacity of the tested aggregate, and (ii) using lower amounts of the aggregate to achieve early contaminant breakthrough. In all cases, long-term experiments have to be performed [29,30]. Because only column tests are of concern, tested systems are relevant for household water filters with the results being immediately transferable to commercial filters (prototypes).
Many household Fe0 filters have been presented during the past 20 years, including the Kanchan arsenic filter (KAF) in Nepal [129,130,131,132]. KAF designs can be used to test the concepts presented herein and facilitate the transfer of achieved results. While the KAF is still using up to 5 kg of iron nails, a recent investigation found out that only 300 g of steel wool could provide safe drinking water for 1 year without significant permeability loss [57,89]. One important result from Tepong-Tsindé [57] is that suitable small-size Fe0 materials dispersed in a sand matrix can corrode to exhaustion. One objective of outstanding investigations is to assess to what extent the combination of reactive additives and coarser Fe0 materials would enable comparable efficiency. In these efforts, the investigation of the probability that biodegradable aggregates (e.g., wood chips) degrade in the long term and leave behind room or pores for expansive FeCPs should be an explicit objective. The additional synergetic effects of Fe0 and microorganisms on the decontamination process is well documented [133]. One important result of the proposed investigations will be testing various materials under comparable experimental conditions. Provided that the tested materials are thoroughly characterized, this will ensure that a database for relevant materials will be available soon. From this database, a range of hybrid Fe0/aggregate systems for various applications will be made available, for example, Fe0/MnO2 for drinking water, Fe0/FeS2 for wastewater, and Fe0/peat for phosphate removal. Perhaps none of the resulting efficient systems will be really new, but identifying appropriate Fe0/aggregate systems for specific remediation applications will be a major achievement. However, the rationale for their design, including their service life, will need to be established in a systematic manner, i.e., the path to transform the current drawbacks of this affordable technology to its established limit. For example, Fe0 filters which are efficient only for 6 months [134] should be replaced. Such a filter working only for 6 months is no longer a drawback. Much labor is required to replace an exhausted filter, which is rather a limitation. For example, by using a different grade of steel wool, i.e., a coarser one, the filter of Bradley [134] could work for 1 year [57,89]. In a modular system [12,13], this means that the Fe0 unit is replaced yearly instead of twice a year.

3. Economics of Fe0 Filters

In concerted efforts for self-reliance in sustainable water management worldwide with respect to using affordable and efficient green technologies, Fe0 filters potentially have a significant role to play. Over the past 170 years, Fe0 filters have been successfully used for safe drinking water provision at micro (household), medium (small community), and large (city) scales [22]. They have also been successfully used for the on-site treatment of stormwater, as well as agricultural, domestic, and industrial wastewaters [135,136,137,138,139,140,141]. Their ability to perform well under environmental conditions coupled with the universal availability of cheap Fe0 materials (e.g., iron nails, steel wool) [87] makes them an affordable option that is significantly different from conventional water filters (e.g., charcoal, zeolite). The major difference is that, unlike other filter systems, the central component of the Fe0 filter system (i.e., Fe0) does not need to be locally activated/fabricated or specially imported. The realization of the potential of Fe0 filters has been hampered by current trends to obtain faster results while using as few resources as possible [142].
The aforementioned trends have misled scientists to overlook the paramount importance of the nonconstant and nonlinear kinetics of iron corrosion (corrosion rate). Thus, the net result is that several systems were proven efficient in short-term laboratory and pilot tests, but failed in field applications. The lack of long-term testing is also the reason why permeability loss due to iron corrosion has largely been underestimated.
Despite their promise, Fe0 filters have so far failed to attain the expected widespread adoption within low-income communities [29,143,144,145,146,147]. To be commercially viable, the costs of producing, marketing, and distributing Fe0 filters should be covered by the purchase price paid for the product by the consumer. Bretzler et al. [148] recently gave an overview of the state-of-the-art knowledge on the development of Fe0 filters for safe drinking water provision for low-income communities. The most widely used Fe0 filters so far are the SONO arsenic filters (SAFs) developed in Bangladesh employing a proprietary porous composite iron matrix (CIM) [78,146,149,150] and the Kanchan arsenic filter (KAF) developed in Nepal employing commercial iron nails [129]. SAFs have been more successful than the KAF and are often considered as the basis for designing alternative Fe0 filters [146,148]. Reproducing SAFs without using a porous Fe0 material like CIM is challenging [86]. Iron coils, iron filings, iron nails, iron shavings/scrap, and steel wool have been tested and used as alternatives [57,89]. Iron nails and steel wool are the most widely used Fe0 alternative sources. One ton (1000 kg) of iron nails costs up to 1000 USD [148], meaning that, if 5 kg of iron nails are used for each alternative Fe0 filter, one ton will enable manufacturing 200 Fe0 filters. Thus, 1000 USD is the money expense (not the price) for 200 Fe0 filters in the “small is beautiful” approach advocated herein. With 1000 USD, just some 20 SAF filters can be bought (50 USD /unit) [146]. In other words, by adapting the DIY approach, 10 times more filters are produced, with the additional advantage that the maintenance is operated locally. Tepong-Tsindé [57] recently designed Fe0 filters containing only 0.3 kg of a commercial steel wool, but these were more efficient for drinking water treatment than systems containing 5 kg iron nails. These two examples clearly show that there is room to optimize Fe0 consumption and water treatment.

4. Conclusions

This communication demonstrated that considering the intrinsic nature of Fe0 (volumetric expansion at pH > 4.5), only hybrid Fe0/aggregate systems are sustainable from a purely thermodynamic perspective (filling the initial porosity). For the kinetics of the process (time to permeability loss), the long-term reactivity of relevant Fe0 must be investigated. This task is yet to be started as all known Fe0 characterization tools last for just some few days or weeks. This short timeframe solely corresponds to the initial corrosion rate, but provides no insights into long-term performance and permeability loss. However, the technical expertise from 170 years and the newly established science of Fe0 filters, together with the opportunity to exchange Fe0 units in modular water treatment trains, make the immediate design of efficient Fe0 filters possible. It is predicted that the number and adoption of decentralized Fe0-based water treatment systems will increase exponentially once an improved understanding of the long-term corrosion rate of readily available materials is achieved. To achieve this, more collaborative work is needed among the research community, funding agencies, practitioners such as nongovernmental organizations, and decision- and policymakers in the water, sanitation, and hygiene (WASH) sector.

Author Contributions

Conceptualization, R.T., H.Y., A.I.N.-T. and C.N.; methodology, R.T., A.I.N.-T. and C.N.; literature review, R.T., H.Y., X.C., M.X., N.G.-B., J.F.K.-T., M.L., B.K.A., R.T.-T., M.T.-W. and B.D.B.-K.; writing—original draft preparation, R.T., H.Y., A.I.N.-T. and C.N.; writing—review and editing, supervision, C.N., R.H., W.G., H.R. and J.B.T. All authors read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

We would like to thank the peer reviewers for their valuable suggestions and comments on improving this paper. 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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Water 14 00260 g001 550
Figure 1. Schematic representation of the cross-section of three different filters containing (a) sand alone, (b) Fe0/sand (1:1), and (c) Fe0 alone. The pure Fe0 systems completely clog when the Fe0/sand system has just experienced 50% porosity loss. In all three systems, permeability loss due to inflowing colloids and suspended particles is also possible.
Figure 1. Schematic representation of the cross-section of three different filters containing (a) sand alone, (b) Fe0/sand (1:1), and (c) Fe0 alone. The pure Fe0 systems completely clog when the Fe0/sand system has just experienced 50% porosity loss. In all three systems, permeability loss due to inflowing colloids and suspended particles is also possible.
Water 14 00260 g001
Water 14 00260 g002 550
Figure 2. Sketch of layered and mixed Fe0/sand filters for water treatment: (a) sand alone (reference), (b) layered Fe0/sand, (c) mixed Fe0/sand, and (d) Fe0 alone (negative reference).
Figure 2. Sketch of layered and mixed Fe0/sand filters for water treatment: (a) sand alone (reference), (b) layered Fe0/sand, (c) mixed Fe0/sand, and (d) Fe0 alone (negative reference).
Water 14 00260 g002
Table
Table 1. Timeline of the main experimental observations relating the importance of porosity loss due to iron corrosion before 2010.
Table 1. Timeline of the main experimental observations relating the importance of porosity loss due to iron corrosion before 2010.
YearMain ResultsReferences
1882Spongy iron filters are clogged at the water works of Antwerp (Belgium) because of cementation of iron and gravel.[71]
1903Whitney reported on the expansive volumetric nature of iron corrosion.[45]
1923Pilling and Bedworth established the rule of the volumetric expansive nature of iron corrosion.[72]
1928Bed clogging is attributed to Fe replacement by Pb.[54]
1951It is observed that column clogging occurs rapidly if fine iron filings are used in place of the steel wool. Using extremely fine grade of steel wool should also be avoided.[73]
1986Filtration beds containing 100% iron filings are very efficient at removing selenium from drainage water, but clogging occurs very rapidly.[74]
1992Using steel wool as Fe0 source for phosphate removal, it is demonstrated that Fe0/peat performed better than Fe0/sand.[75]
1993Filtration systems containing 10% to 25% Fe0 particles (iron fillings) mixed with pelletized jute do not experience any permeability loss.[76]
2001Fe0/pyrite filters are essentially more efficient for water treatment than pure Fe0 filters.[24]
2007Filtration systems containing less than 5% Fe0 (steel wool) do not experience any permeability loss.[77]
2000–2009Household arsenic filters with pure Fe0 layers are mostly efficient but not sustainable due to clogging. Only filters using porous materials (CIM = composite iron material) were sustainable.[78]
2009TCE removal rates are higher in an 85% Fe0 filter than in the 100% system.[39]
Table
Table 2. Representative materials used as aggregates in Fe0/aggregate systems.
Table 2. Representative materials used as aggregates in Fe0/aggregate systems.
AggregateRationale for UseCommentsReference
BiocharAdsorbs and accumulates contaminantsAlso used as support for nano-Fe0[93]
Fe oxidesAdsorbs and accumulates contaminantsFe3O4 is used the most[94]
Fe sulfidesShifts pH to lower valuesFeS2 is used the most[95]
GACBuilds galvanic cells with Fe0GAC coating with FeCPs will hinder electron transfer[96]
LapillusStores FeCPsPores are not interconnected[97]
Mn oxidesSustains Fe0 corrosionExtends Fe0 filter’s service life[98]
PeatAccumulates contaminantsMore efficient than sand[75]
PozzolanStores FeCPsMore efficient than sand[99]
PumiceStores FeCPsPores are not interconnected[100]
SandReduces the Fe0 costReference additive[39]
Wood chipsAccumulates contaminantsMostly used in PO43− removal[101]
ZeoliteAccumulates contaminantsAlso used as support for nano-Fe0[102]
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Tao, R.; Yang, H.; Cui, X.; Xiao, M.; Gatcha-Bandjun, N.; Kenmogne-Tchidjo, J.F.; Lufingo, M.; Konadu Amoah, B.; Tepong-Tsindé, R.; Ndé-Tchoupé, A.I.; et al. The Suitability of Hybrid Fe0/Aggregate Filtration Systems for Water Treatment. Water 2022, 14, 260. https://doi.org/10.3390/w14020260

AMA Style

Tao R, Yang H, Cui X, Xiao M, Gatcha-Bandjun N, Kenmogne-Tchidjo JF, Lufingo M, Konadu Amoah B, Tepong-Tsindé R, Ndé-Tchoupé AI, et al. The Suitability of Hybrid Fe0/Aggregate Filtration Systems for Water Treatment. Water. 2022; 14(2):260. https://doi.org/10.3390/w14020260

Chicago/Turabian Style

Tao, Ran, Huichen Yang, Xuesong Cui, Minhui Xiao, Nadège Gatcha-Bandjun, Joseline Flore Kenmogne-Tchidjo, Mesia Lufingo, Bernard Konadu Amoah, Raoul Tepong-Tsindé, Arnaud Igor Ndé-Tchoupé, and et al. 2022. "The Suitability of Hybrid Fe0/Aggregate Filtration Systems for Water Treatment" Water 14, no. 2: 260. https://doi.org/10.3390/w14020260

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