Journal of Applied Solution Chemistry and Modeling, 2013, 2, 165-177 165 E-ISSN: 1929-5030/13 © 2013 Lifescience Global Optimising the Design of Fe0-Based Filtration Systems for Water Treatment: The Suitability of Porous Iron Composites Mohammad Azizur Rahman1,2, Shyamal Karmakar1,3, Heba Salama4, Nadège Gactha-Bandjun5, Brice Donald Btatkeu K.6 and Chicgoua Noubactep1,7,8,* 1 Angewandte Geologie, Universität Göttingen, D - 37077 Göttingen, Germany 2 Institut für Strömungsmechanik und Umweltphysik im Bauwesen, Leibniz Universität Hannover, D - 30167 Hannover, Germany 3 Institute of Forestry and Environmental Sciences, University of Chittagong, 4331 Chittagong, Bangladesh 4 Crop Science Department, Faculty of Agriculture, Alexandria University, Egypt 5 Department of Chemistry, University of Maroua, BP 55 Maroua, Cameroon 6 ENSAI, University of Ngaoundere, BP 455 Ngaoundere, Cameroon 7 Kultur und Nachhaltige Entwicklung CDD e.V., Postfach 1502, D - 37005 Göttingen, Germany 8 Comité Afro-européen - Avenue Léopold II, 41 - 5000 Namur, Belgium Abstract: This study assessed the functionality of metallic iron (Fe0) filtration systems using porous iron composite (PIC) as an alternative to granular Fe0/aggregate mixtures. The usage of PIC for water treatment has many challenges which are related to the well-drained nature of highly porous filters and the corresponding increase in hydraulic conductivity (shorter contact time). In this article, the extent of (i) iron exhaustion and (ii) porosity loss in four filtration systems are critically discussed. The considered filtration systems are: (i) Fe0 alone, (ii) PIC alone, (iii) Fe0/sand and (iv) Fe0/pumice. In all four systems, mono-sized granular spherical particles are assumed. Sand and Fe0 are compact ( = 0 %) whereas PIC and pumice are porous (e.g.  = 40 %). Results demonstrated that under anoxic conditions (Fe3O4 as major corrosion products) Fe0 depletion is possible in all systems except Fe0 alone. Under oxic conditions (e.g. formation of Fe(OH)3), the PIC system exhibited the highest level of Fe 0 depletion (58 %). The increasing order of sustainability was: Fe0 < Fe0/sand < Fe0/PM < PIC. These results suggested that manufacturing PIC with defined porosity and intrinsic reactivity is the key for more efficient usage of Fe0 for environmental remediation and water treatment. Keywords: Porous media, Permeability loss, Reactive filtration, Water treatment, Zero-valent iron. 1. INTRODUCTION The suitability of metallic iron (Fe0) for water treatment has motivated a great deal of work on the development of Fe0-based filtration systems during the past two decades [1-19]. Within the remediation community, metallic iron is commonly termed as zero- valent iron (ZVI). Fe0-based filtration has been demonstrated an affordable, applicable and efficient water treatment system. Fe0 has been successfully used in environmental remediation during the past two decades [3,11,13,20-22]. There are currently more than 200 subsurface Fe0-based permeable reactive barriers (Fe0 PRBs) installed worldwide [11,16]. Despite the large volume of work done on ‘using Fe0 for environmental remediation’, progress towards the understanding of involved processes is slow. Recent progress in understanding the operating mode of Fe0 filtration systems has revolutionized the design of *Address correspondence to this author at the Universität Göttingen, Angewandte Geologie, Goldschmidtstrasse 3, D - 37077 Göttingen, Germany; Tel: +49 551 39 3 3191; Fax: +49 551 399379; E-mail: cnoubac@gwdg.de Fe0-based filters for safe drinking water provision and wastewater treatment [10,16,18,23-33]. The achieved progresses are mainly based on theoretical considerations [8,23,27,34-39]. There is a broad consensus in the technical literature that Fe0 is a reducing agent under experimental conditions [3,12]. However, there is little agreement on the interpretation of experimental data on all the pertinent variables which have decisive influence on the reduction process. Also, because primary Fe0 oxidation products (FeII and H/H2) are reducing agents, the reactions investigated are inherently complex [18, 34-37]. The complexity of the Fe0/H2O system has exacerbated the search for Fe 0 filter designs that provide sustainable water treatment with low maintenance [40-44]. The significant importance of admixing granular compact Fe0 with non expansive materials (e.g. gravel, MnO2, pumice, sand) has recently been demonstrated [37,38,45-51]. The usage of porous, non-expansive materials (e.g. anthracite, pumice) as admixing agents for Fe0 was experimentally demonstrated beneficial for filter’s 166 Journal of Applied Solution Chemistry and Modeling, 2013 Volume 2, No. 3 Rahman et al. sustainability [15,32,52-54]. The sustainability’s characteristic was attributed to the availability of intra- particular spaces (pores) within pumice particles for storage of in-situ generated iron corrosion products [54]. However, in comparison to Fe0/sand systems, more permeable Fe0/pumice systems were less efficient for contaminant removal [32]. This observation demonstrated the crucial challenge of concealing two controversial issues in designing sustainable Fe0 filters: (i) increased contaminant removal efficiency (as much Fe0 as possible) and (ii) increased system permeability (as less Fe0 as possible). The introduction of a Fe0- based porous composite (termed composite iron matrix - CIM) as reactive material [6,17,55-57] in a sustainable filter (SONO filter) suggests that Fe0-based porous composites should be regarded as the next generation filter materials. Another CIM-like material termed as SIM (sulfur- modified iron) has been recently presented by Allred [10,24,25] as reactive material in filters for agricultural drainage water treatment. Direct reduced iron (DRI or ‘sponge iron’) was also tested as an alternative to conventional compact Fe0 for wastewater treatment [19, 58-64]. DRI results from direct reduction of iron ore by a reducing gas produced from natural gas or coal [64]. DRI resembles a honey comb structure and is spongy in texture. DRI may react differently in many ways when compared to compact iron ( = 0). Inherent characteristics of DRI are high porosity (  0), low density and high surface area. Sponge iron is a traditional water treatment material [65-67], but its application in passive filters is regarded as a new research field. For example, the ‘sponge iron’ tested by Yi et al. [19] is a commercial material which is conventionally used as oxygen scavenger for water treatment. This material exhibits a density of 2.2 g/cm3 and a specific surface area of 85 m2/g. In essence, conventional DRI can be properly micro-alloyed to manufacture various CIM-like materials. It should be noticed that the original CIM material is a porous matrix produced by in-situ processing inside the filter. The starting materials termed as CIG (composite iron granules) have a porosity ranges between 8 % and 20 % [56]. In this article, all porous CIM-like materials are collectively termed as PIC (Table 1). This article applies a theoretical approach to discuss the suitability of PIC on the performance of Fe0 filters. The sustainability of Fe0, PIC, Fe0/sand and Fe0/pumice systems are comparatively discussed. Systems are compared in terms of (i) the extent of Fe0 exhaustion and (ii) the extent of porosity loss. 2. FUNDAMENTAL ASPECTS OF CLOGGING OF GRANULAR Fe 0 FILTERS Gradual decrease of the hydraulic conductivity (permeability loss) of granular Fe0 filters as water passes through them has been intensively investigated during the last two decades [1,3,11,20-22,44,68-71]. The reason for the permeability loss is certainly the deposition of precipitates in the voids between granular Fe0 particles (inter-granular porosity). However, a comparative performance analysis of published data is almost impossible because important media characteristics such as shape, surface smoothness and Fe0 intrinsic reactivity have not been well documented or were documented in a limited way [15,30,32,33]. Moreover, the crucial importance of the expansive nature of iron corrosion [72-74] to fill the initial porosity was not properly considered [38,39,45-51]. Accordingly, it is important to consider ‘endogen’ causes (e.g. grain’s shape and surface smoothness, porosity of grains, changes in size of Fe0 grains, nature of packing arrangement) for clogging of granular Fe0 filters in more details. Only once these ‘endogen’ parameters are properly considered, an accurate evaluation of the impacts of external factors (e.g. Table 1: Some characteristics of two compact Fe 0 and three selected porous iron composites from the literature. The paucity of data relevant for proper discussion on Fe 0 reactivity is obvious. The characteristics of the PIC (SMI) introduced by Allred [10,24,25] are not specified. n.s. stands for not specified Material Fe density Mn S P Cu Sn Bi Ref. (%) (g/cm 3 ) (%) (%) (%) (%) (%) (%) CIM 68 - 92 2.4 0.3 - 3.0 <0.002 0.05 - 2.0 <0.002 <0.002 <0.002 [56] DRI 98.0 2.2 n.s. 0.03 n.s. 0.002 0.002 0.002 [19] ZVIa 96.7 n.s. 0.46 0.09 n.s. n.s. n.s. n.s. [58] ZVIb 91.5 n.s. 3.06 0.08 n.s. 0.31 n.s. n.s. [58] SMI n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. [25] Porous Composite Materials Could Revolutionize the Iron Technology Journal of Applied Solution Chemistry and Modeling, 2013 Volume 2, No. 3 167 precipitation of foreign species including CaCO3 and FeCO3) would be adequate. Despite two decades of intensive researches on granular Fe0 filters for water treatment, limited works have been done to understand filter clogging phenomenon as impacted by expansive iron corrosion [15,32,45-54]. The inter-relationship between permeability loss and treatment performance has been mostly investigated on a pragmatic basis. Many researches performed ‘with sufficient Fe0 to remove all of the aqueous contaminants’ until/before system clogging. Evidently this approach is less useful when it comes to search for generalized design criteria like optimal filter depth or Fe0 ratio in a reactive zone. As an example, the following volumetric (v/v) or weight (w/w) Fe0:sand ratios were tested by various investigators such as: 22:78 (w/w) [3], 50:50 (w/w) [29,66], 15:85 (v/v) [31]. The question arises how to compare such results where each study targeted on different contaminants while using columns of different dimensions. Recently, it has been demonstrated that using small Fe0 quantities in laboratory columns would enable the characterization of involved processes within a reasonable experimental time (up to 4 months) [30,33,75-77]. In such type of experiments, the goal is not 'no breakthrough', but rather the characterization of Fe0 reactivity and the resulting efficiency of the reactive system despite breakthrough. The results of such experiments could be ‘transposed’ to cases where a tolerable level of contamination is needed. Additionally, these systems give a realistic image of the kinetics of iron corrosion over the initial stage of increased reactivity. This ‘residual reactivity stage’ is a key stage to assess if reliable results for long term performance prediction are sought. The present study comparatively characterizes the evolution of the porosity in four Fe0-based filtration systems as Fe0 is progressively consumed at the ‘residual reactivity stage’. It is certain that compact Fe0 and (porous) PIC will corrode under different kinetics but these kinetic aspects are not addressed here. 3. THE PROCESS OF POROSITY LOSS IN VARIOUS Fe 0 FILTERS In this section, an evaluation of porosity loss in a series of four Fe0 filters is given. The four reactive systems are: (b) ‘Fe0 alone’, (d) ‘PIC alone’, (e) ‘Fe0/sand’ and (f) ‘Fe0/PM’ (Figure 1). Two mono- aggregate systems ((a) ‘sand alone’ and (c) ‘pumice alone’) are considered as operational references. 3.1. Equation of the Filters The equations of Fe0 filters have been recently established [38,47,48,51]. These equations are based on the volumetric fraction of voids (pores) in a filter and its evolution with time. The fundamental equations are: Vsolid 0 +Vpore 0 = Vfilter (1)  solid + pore = 1 (2) Where V°solid is the volume occupied by compact (non porous) solid particles, V°pore the volume of the Figure 1: Schematic layout of the six systems discussed in this study. All particles are assumed spherical, the resulting columns are of similar compactness. The initial inter-granular porosity (V°pore) is the same for all the systems. Considered materials are: sand (compact), metallic iron (ZVI - compact), pumice (PM - porous), and porous iron composite (PIC). 168 Journal of Applied Solution Chemistry and Modeling, 2013 Volume 2, No. 3 Rahman et al. inter-granular voids, and Vfilter the total volume of the filter (or a reactive zone within a filter). solid and pore are the corresponding volumetric fractions. Per definition, solid = C is the compaction coefficient (compactness) and pore = 0 = 1 - C is the initial porosity. For a centred cubic arrangement of compact (non porous) spherical particles, C = 0.64 and 0 = 0.36. These ideal values will be considered during the estimations of this study. For porous particles with an internal porosity , Vsolid and Vpore values should be corrected. Eq. 1 can be rewritten as follows: (Vsolid 0  *Vsolid0 ) + (Vpore0 +  *Vsolid0 ) = Vfilter (3) If the filter is made up of several solids, their volumes (V°,isolid), their proportions in the solid phase i, and their porosities (i) should be considered. Eq. 3 is then rewritten to: (Vsolid 0  vi *i *Vsolid0,i ) + (Vpore0 + vi *i *Vsolid0,i ) = Vfilter (4) Equation 4 is the general equation of filtration systems of granular particles. For i = 0 (no porous material), the basic form (Eq. 1) is restored. The application of Eq. 4 to the six systems of this study is summarized as follows: System 1: Vsand 0 +Vpore 0 = Vfilter System 2: VZVI 0 +Vpore 0 = Vfilter System 3: VPM 0 PM *VPM0 + (Vpore0 + PM *VPM0 ) = Vfilter System 4: (VPIC 0 PIC *VPIC0 ) + (Vpore0 + PIC *VPIC0 ) = Vfilter System 5: vsand *Vsand 0 + vZVI *VZVI 0 +Vpore 0 = Vfilter System 6: vZVI *VZVI 0 + vPM * (VPM 0 PM *VPM0 ) + (Vpore 0 + vPM *PM *VPM0 ) = Vfilter In the systems 5 and 6, ZVI = PM = sand (= 0.5) is added to account for the 1:1 dual mixture nature. The next important feature is the estimation of the initial volume of Fe0 (V°ZVI) in individual systems. The application of Eq. 4 restricted to Fe0 yields: System 1 and 3: VZVI 0 = 0 System 2: VZVI 0 = (1 0 ) *Vfilter System 4: VZVI 0 = (1 0 ) *Vfilter PIC *VPIC0 System 5 and 6: VZVI 0 = vZVI * (1 0 ) *Vfilter (vZVI = 0.5) The density of Fe0 (ZVI) is supposed invariable in compact ZVI and porous PIC. The mass of Fe0 in each system is deduced from Eq. 5: mZVI = ZVI *VZVI0 (5) For the calculations of this study, the following numerical values are considered: Vfilter = 1000 mL; ZVI = 7.8 g/cm3; C = 0.64; 0 = 0.36; PIC = PM = 0.40. The critical porosity of sand [78] is arbitrarily considered for both porous media. The ZVI value (2.2 g/cm3) of the sponge iron tested by Yi et al. [19] is inferior to the average density of sand (2.6 g/cm3) and shows that a PIC porosity of 40 % is a realistic value. The porosity of PIC used in SONO filters (CIM) varies between 30 and 35 % [57]. 3.2. Descriptive Aspects The initial pore volume corresponding to a filter described above is V°pore = *Vfilter = 360 mL. The corresponding volume of solid is V°solid = 640 mL. For porous materials like PIC and pumice, a fraction of V°solid is also a part of the total porosity. With a granular porosity of 40 % ( = 0.4), the additional pore volume (V’p) in systems containing porous materials is *V°solid = 256 mL. In order words the systems with 100 % PIC or pumice result in an initial total pore volume of 360 + 256 = 616 mL. For dual 1:1 systems involving porous pumice (ZVI = PM = 0.5), the V’p value is one half of 256 mL and the corresponding V°p value is 488 mL. As concerning the solid fraction, Vsolid is maximal in systems with compact materials (Fe0 and sand; V°solid = 640 mL). The Fe0 fraction in the pure PIC system is V’solid = (1 - )*V°solid = 384 mL. In the dual Fe0 systems (Fe0/pumice and Fe0/sand), VZVI is one half of V°solid (VZVI = 320 mL). VZVI is essential to discuss the extent of Fe0 depletion and the corresponding porosity loss. VZVI can be occupied by 2.5 to 5.0 kg of Fe 0 (density: 7.8 g/cm3) (Table 2). The values in Table 2 confirm that the initial porosity is minimal (36.0 %) for systems containing compact particles (Fe0 and sand) and maximal (61.6 %) in systems containing porous particles (PIC and PM). In 1:1 mixing (vol/vol) a compact and a porous material, the initial porosity is 48.8 %. The next section will discuss the extent of Fe0 depletion as expansive iron corrosion is limited by the available pore volume. Porous Composite Materials Could Revolutionize the Iron Technology Journal of Applied Solution Chemistry and Modeling, 2013 Volume 2, No. 3 169 3.3. Fe 0 Depletion and Porosity Loss Section 3.2 has determined the mass of Fe0 contained in individual systems (Table 2). For the economy of the systems, it is essential to know the proportion at which the available Fe0 amount can be depleted. Iron corrosion is a volumetric expansive process. The volume of the corrosion product is higher than that of the original metal. The volumetric ratio () between the expansive corrosion product (Vox) and the iron consumed (VZVI) in the corrosion process is called ‘‘rust expansion coefficient” [73,74]. Basically, iron corrosion stops when there is no free space for expansive iron oxidation. That is when the excess volume (Vexcess = Vox - VZVI) occupied by corrosion products is equal to the initial pore volume (V°pore). The excess volume contributing to filter clogging is given by Vexcess in Eq. 6 [38,48,51]. ( 1)VZVI = Vexcess (6) Where,  (2.08    6.4) is the coefficient of volumetric expansion [73]. The Fe0 filtration system is clogged when the volume Vexcess is equal to the initial inter-granular voids (V°pore). The corresponding Fe 0 volume (VZVI) is given by Eq. 7: VZVI  = Vexcess ( 1) (7) The percentage of Fe0 depletion (PZVI, Table 2) is given by Eq. 8: PZVI = VZVI  Vexcess *100 (8) PZVI  100 % indicates that VZVI  V°p and Fe0 depletion occurs before or just at complete clogging. Table 2 summarized the results of PZVI values for two different conditions: (i) anoxic conditions where Fe3O4 ( = 2.08) is the main corrosion product, and (ii) oxic conditions where Fe(OH)3 ( = 4.2) is the main corrosion product. For reference purpose, the coefficient of relative reactivity under oxic conditions (Rox) is also given. The PIC system served as the operational reference. Results indicate that under anoxic conditions, the pure Fe0 system is clogged when only 52 % of Fe0 is depleted. These results confirm the previous findings of Noubactep and colleagues [38,45-51], and reiterate that pure Fe0 filters are not sustainable under oxic conditions [55]. Accordingly, efforts should be directed at lowering dissolved O2 level on top of the Fe 0 filter (e.g. using a biosand filter). Under oxic conditions, all systems will experience clogging before iron depletion. The increasing order of Fe0 depletion is: Fe0 < Fe0/sand < Fe0/PM < PIC. These results (i) support the suitability of sand to sustain Fe0 filtration efficiency, (ii) corroborate the superiority of PM over sand in sustaining Fe0 filtration efficiency, and (iii) demonstrate the particular suitability of PIC for sustainable Fe0 filters. Actually, various PICs have been presented (Table 1). The PIC (termed as CIM) presented by Hussam and colleagues [6,17,55-57] is clearly the most intensively tested (over the past 8 years). Several other non porous composites have been presented, including nano-scale multi-metallic systems [13,37,79,80]. In particular, Bojic and colleagues [81-84] developed an Al0-based composite (micro-alloyed aluminium composite - MAlC) which was efficient to remove aqueous biological (e.g. Escherichia coli) and chemical (e.g. CrVI, CuII, trihalomethanes) contamination. Table 2: Evaluation of the initial Fe 0 volume and the initial porosity in the five investigated systems. Calculations are made for a filter compactness of 64 % (C = 0.64 or 0 = 0.36) and a granular porosity of 40 % (0 = 0.40) for the composite iron material (CIM) and pumice (PM). R ox is the relative reactivity factor under oxic conditions. System V°solid V°pore V°ZVI 0 m°ZVI VZVI PanoxZVI PoxZVI Rox (mL) (mL) (mL) (%) (kg) (mL) (%) (%) (-) Sand 640 360 0 36.0 0.00 0 - - - ZVI 640 360 640 36.0 4.99 333.3 52.1 17.6 0.35 Pumice 384 616 0 61.6 0.00 0 - - - CIM 384 616 384 61.6 3.00 570.4 100.0 50.1 1.00 ZVI/sand 640 360 320 36.0 2.50 333.3 100.0 35.2 0.70 ZVI/PM 512 488 320 48.8 2.50 541.9 100.0 47.7 0.95 170 Journal of Applied Solution Chemistry and Modeling, 2013 Volume 2, No. 3 Rahman et al. Keeping in mind that Al0 is almost ‘inert’ under environmental conditions (PCO2 = 0.035 %, T < 30 °C), it seems that micro-alloying is the key to manufacture reactive materials with controllable reactivity. The need of materials with controllable reactivity is urgent because most of the available commercial materials are mixture of scrap material (e.g. Connelly, Peerless, Rheinfelden) whose final composition is even unknown from the ‘manufacturers’ [85-89]. 4. DISCUSSION 4.1. Lessons from SONO filters The presentation, until now, has demonstrated the urgent necessity of using highly porous systems for sustainable Fe0-based filtration systems. While highly inter-connected porous systems offer certainly sustained permeability, the efficiency for contaminant removal should be considered as well [15,32,54]. The calculations in Table 2 have rationalized the observed long term functionality (> 8 years) of efficient SONO arsenic filters containing a pure layer of porous CIM (5 to 10 kg making up a material layer of about 13 cm) [17]. Using the same design parameters with only 3 kg of iron chips (pure compact Fe0) lost its porosity within 6 months [6,55]. These results equally question the current rationale behind the functionality of subsurface Fe0 PRBs made up of 100 % Fe0 layers that have been working for more that 8 years [12]. A plausible explanation is the limited level of O2 in the subsurface. Another plausible explanation is the fact that used materials were not spherical and the initial porosity was larger than 36 % used in the calculations of this study [38,51]. For a traceable discussion, the intrinsic characteristics of the used materials, the initial subsurface conditions and their time-dependant variability should have been documented. 4.2. The Porosity of Tested Systems The evolution of the porosity of the six systems [(a) to (f)], presented in Figure 1, is discussed in this section (Figure 2). The first 3 systems (sand, ZVI and PM) are reference systems. The pure Fe0 system (ZVI) is a negative reference aiming at attesting that expansive iron corrosion causes system clogging in the short term (Rox values, Table 2). System (a) and system (c) document the difference in using compact or porous materials. In both cases the particles are inert in water under environmental conditions. A porosity loss can only result from accumulation or precipitation of inflowing species and/or contaminants within the (interconnected) pores. The discussion of porosity loss due to the accumulation of foreign precipitates is out of the scope of this study. Such systems have been used in water and wastewater treatment for decades [90- 100]. The remaining systems [(d), (e) and (f)] are reactive systems in which Fe0 sustainability (long term efficiency) is supported by (i) using granular porous CIM, or (ii) admixing granular compact Fe0 to inert species (compact sand and porous pumice). As discussed above, system (d) (pure PIC) is the most sustainable system in terms of delay in porosity loss. It is essential to notice that the space occupied by sand in system (e) (Fe0/sand) is totally lost as it neither contributes to water flow nor stores fouling agents (iron corrosion products) (Figure 3). The intra-particle pore space in system (f) (Fe0/PM) may be accessibile to store iron corrosion products. However, quantitative transport of dissolved and colloidal iron species into the porous structure of pumice can not be garanteed, even in case they are really interconnected. Therefore, the extent of occupation of the pores of pumice by iron corrosion products is difficult to assess. Another important feature favoring the application of porous PIC is that the internal surface of the grains is in contact with water and do corrode [10,59,60]. This phemenon has two advantages: (i) contaminant removal by size-exclusion occurs also within the intra- 0 20 40 60 80 100 0 20 40 60 80 100 sand ZVI PM PIC Fe0/sand Fe0/PM p o ro s it y / [ % ] Fe 0 consumption / [%] Figure 2: Comparison of the evolution of the relative porosity due to expansive iron corrosion under oxic conditions ( = 4.2). The reference system (sand or pumice) experiences no porosity loss. For the Fe0-based systems, the relative porosity is normalized by the extend of Fe0 depletion (PZVI) using the PIC system as reference. The suitability of porous materials (e.g. PIC, PM) for sustainable permeable systems is corroborated. Porous Composite Materials Could Revolutionize the Iron Technology Journal of Applied Solution Chemistry and Modeling, 2013 Volume 2, No. 3 171 granular porosity and (ii) the probability of porous Fe0 ( = 0.4) depletion increases as the corrosion kinetics are increased comparatively to the case of a compact Fe0 material ( = 0). For this reason, it is likely that the order of sustainability obtained here by comparing the extent of porosity loss (Figure 2) is replicated in contaminant removal experiments. However, the evolution of individual systems will depend on two fundamental parameters: (i) intrinsic reactivity of Fe0 materials, and (ii) the water flow velocity. Hence, the objective of this article is to motivate the manufacture of various porous composites for site-specific applications. While existing approaches mostly collectively earn for more reactive materials (including nano-scale composites) [37,80], the present study advocates for manufacturing appropriate (porous) materials for site- specific applications. To the best of the author’s knowledge, only Li et al. [101] have intentionally alloyed Fe0 to decrease its reactivity. However the objective of the authors [101] was to reduce the iron level in the filter effluent. As experimentally demonstrated by several authors [15,30,32,33] this increased iron level is the expression of initial increased reactivity. The iron level was lowered to values less than 1 mg/L within some 6 weeks. In other words, properly micro-alloying Fe0 alone will not solve the clogging problem. Therefore, reactive porous iron composites are urgently needed. 4.3. Designing the Next Generation Fe 0 Filter Metallic iron is the reactive material in Fe0-based filters. Fe0 oxidizes to produce iron hydroxides and oxides for the elimination of biological and chemical contamination by adsorption, co-precipitation and size- exclusion [79,102-107]. The process of progressive transformation of Fe0 to iron oxides is well-described in the literature and will not be repeated here [8,108-112]. It is just to recall that strictly compact Fe0 is transformed through several steps of porous iron hydroxides (e.g. Fe(OH)2, Fe(OH)3) to almost compact oxides (e.g. Fe3O4, Fe2O3, FeOOH) [34-36]. During this dynamic process nascent iron hydroxides adsorb and co-precipitate contaminants while less reactive oxides just adsorb dissolved species. All types of iron species (reactive and less reactive) contribute to contaminant removal by size-exclusive filtration as their formation occupies the inter-particular voids [34]. The present study has demonstrated the crucial importance of developing more efficient Fe0 materials: porous iron composites. Once a new Fe0 material is developed (e.g. micro-alloyed PIC), four scientific aspects need to be understood: (i) the long-term behaviour of iron corrosion (corrosion kinetics at (a) (b) Figure 3: Schematic layout of water flow: (a) around a compact particle and (b) through a porous particle. The space occupied by a compact particle is ‘lost’ whereas the intra-granular porosity of porous particles can be exploited as space for expansive iron corrosion. 172 Journal of Applied Solution Chemistry and Modeling, 2013 Volume 2, No. 3 Rahman et al. pseudo-equilibrium), (ii) the kinetics of contaminant removal at pseudo-equilibrium of iron corrosion, (iii) the water flow velocity compatible with satisfactorily water treatment, and (iv) the time-dependant evolution of the filter permeability as impacted by expansive iron corrosion. The profound knowledge of these four key aspects determines the mass of Fe0 needed to manufacture a filter with a specific capacity for a certain group of contaminants. The contaminant affinity to iron oxides significantly determines the filter design [32,33,77]. Contaminants with low affinities to iron corrosion products must be removed before the Fe0 filter. The in-depth knowledge of the nature and the extent of contamination determine the physical designs of a treatment system. These design efforts could be supported by modern numerical modelling algorithms and tools, including finite element methods for the solution of mass-transport equations [57]. Fe0-based filters have been suggested as a stand- alone technology for the treatment of waters with unknown chemical and microbial quality [8,23,34,36]. Therefore, beside Fe0, water filters require active adsorbents to accumulate species with low affinity to iron oxides (e.g. cationic dyes) and species exceeding the Fe0 unit. Relevant adsorbents include sand, activated carbon, and wood charcoal. The presentation above suggests that for a sustainable Fe0 unit, the level of dissolved O2 should be considerably reduced. Therefore, a biosand filter or any other O2-scavenging unit is necessary on top of the Fe0-unit. Figure 4 depicts the general sequence of efficient Fe0-based water treatment plants. The Fe0-unit, which is the heart of the filtration system can advantageously contain several inert granular materials including anthracite, blast furnace slug, brick chips, crushed stones, gravel, pumice or sand to impart mechanical stability and/and regulate the hydraulic conductivity. The role of these species in sustaining iron corrosion has already been discussed above, such as sand and pumice as admixing agent or dispersant in Fe0/PM and Fe0/sand systems. Figure 5 presents some schematic representations of possible configurations of the Fe0 unit. It is expected that by varying the granulometry and the thickness of the inert material layer (PM or sand), various levels/scales of the hydraulic conductivities can be achieved. Another Figure 4: Concept of treatment train combining contaminant removal in Fe0/H2O systems and successive systems containing adsorbents. The primary aim of the O2-scavenger is to create anoxic conditions for sustainable Fe 0 filtration. Figure 5: Schematic layout of six possible embodiments of Fe0-based materials for sustainable filtration systems. Depending on the site of the treatment systems individual layers could be contained in separated beds. Porous Composite Materials Could Revolutionize the Iron Technology Journal of Applied Solution Chemistry and Modeling, 2013 Volume 2, No. 3 173 possible advantage of sandwiching pumice layer between reactive zones is the accumulation of iron corrosion. So that short-distance transport of iron (hydr)oxides is more likely to occur than in thicker beds. It must be noted that SONO filters was developed on the basic idea of using Fe0 as generator of soluble Fe species to favour As removal in sand filters [113-116]. 5. CONCLUDING REMARKS There are some evidences that porous composites improve the performance of Fe0-based filtration systems. The extent of material exhaustion is optimal in these systems as well. Whilst the demonstrated principle of the fundamental suitability of porous composites has a universal validity, target experiments with well characterized (new) materials are needed to understand more precisely the clogging phenomenon in granular Fe0-based filters. Relevant experiments will assess the influence of (i) composite type (intrinsic reactivity), (ii) composite porosity, (iii) composite particle size, (iv) general filter design (depth of filter, layering arrangement with particles of different sizes, compaction of the media during construction), (v) chemistry of inflowing water (nature and concentration of dissolved species, turbidity) and (v) the frequency of filtration events (intermittent filters). The overall results of these investigations will improve the understanding of the efficiency of Fe0 filters as impacted by the clogging phenomenon. The ultimate aim is to build a sound basis for a system- independent design of Fe0 filters. Once a reliable design guidance is available, predicting site-specific filter’s hydraulic and treatment performances along with their lifespan could be fine-tuned by pilot studies. 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