Characterizing the impact of pyrite addition on the efficiency of Fe0/H2O systems

The role of pyrite (FeS2) in the process of water treatment using metallic iron (Fe0) was investigated. FeS2 was used as a pH-shifting agent while methylene blue (MB) and methyl orange (MO) were used as an indicator of reactivity and model contaminant, respectively. The effect of the final pH value on the extent of MB discoloration was characterized using 5 g L−1 of a Fe0 specimen. pH variation was achieved by adding 0 to 30 g L−1 of FeS2. Quiescent batch experiments with Fe0/FeS2/sand systems (sand loading: 25 g L−1) and 20 mL of MB were performed for 41 days. Final pH values varied from 3.3 to 7.0. Results demonstrated that MB discoloration is only quantitative when the final pH value was larger than 4.5 and that adsorption and co-precipitation are the fundamental mechanisms of decontamination in Fe0/H2O systems. Such mechanisms are consistent with the effects of the pH value on the decontamination process.

Metallic iron (Fe 0 ) and iron sulfide (FeS)-based materials (including pyrite-FeS 2 ) are two important components of Fe 0 -based water treatment technology 11,22,[31][32][33] . Both materials are reported to be stand-alone reducing agents that effectively degrade several aqueous contaminants 34,35 . In this context, Henderson and Demond 34,36 have explicitly compared the suitability of both materials, and observed the superiority of FeS (including FeS 2 ) over Fe 0 with respect to the sustainability in terms of loss of permeability. During the past two decades, Fe 0 and FeS 2 have been often mixed in an effort to increase the efficiency of single-Fe 0 systems 22,33,[37][38][39][40][41] . However, FeS 2 is mostly added to avoid the formation of a passive oxide scale (oxide film) which can hinder further reactions between the Fe 0 and pollutants 41,42 . This application contradicts the successful use of FeS 2 to improve the removal of non-reducible contaminants (e.g. As) in Fe 0 /H 2 O systems 22 . Thus, there is a need to understand the real mechanism by which FeS 2 improves the efficiency of Fe 0 /H 2 O systems, irrespective of any redox transformation. The oxidative dissolution of both Fe 0 (Eq. 1) and FeS 2 (Eq. 2) typically releases Fe 2+ , which is also a stand-alone reducing agent for several contaminants 35 . Fe 2+ from Eq. (1) and/or Eq. (2) can be further oxidized to Fe 3+ (Eq. 3). (1) Background to the experimental methodology. At neutral pH values, immersed reactive Fe 0 corrodes and generates solid iron corrosion products (FeCPs), which progressively coat the surface of sand. The process of iron corrosion causes a pH shift to higher values (Eq. 1). The extent of sand coating depends among other factors on: (i) the Fe 0 intrinsic reactivity, (ii) the volume of the solution, (iii) the initial pH value of the solution, (iv) the Fe 0 /sand ratio, and (v) the duration of the experiment. Under given experimental conditions, the removal efficiency of the system for individual contaminants depends on the final pH value, the extent of sand coating, and the availability of "free" FeCPs. The final pH value determines the speciation of the contaminant and the surface charges of sand and FeCPs 50 . When a FeS 2 mineral is added to a Fe 0 /sand system (at a given Fe 0 :sand ratio) a pH shift to lower values occurs. The extent of pH shift depends on the FeS 2 intrinsic reactivity and the amounts added. Lower pH values avoid or delay sand coating and modify the speciation of dissolved contaminants. It then follows that, when FeS 2 is added to a Fe 0 /sand mixture, there are two counteracting processes controlling the pH value of the system [37][38][39] . Previous results observed with the FeS 2 mineral used in the current study 41 suggest that pyrite dissolution occurs with much rapid kinetics than Fe 0 corrosion. Consequently, the system will not achieve a steady state before the initial pH of the FeS 2 -free system is achieved. The larger the pH shift the larger the amount of FeCPs generated, which will in turn precipitate at pH > 4.5 and induce contaminant removal by adsorption and co-precipitation.
The methodology used for characterizing the impact of FeS 2 on the efficiency of Fe 0 /H 2 O systems comprises monitoring the discoloration of a methylene blue solution (MB method) by Fe 0 /sand systems amended with various FeS 2 amounts. Clearly, the availability of FeCPs and their reactivity is modified by lowering the initial pH value to various extents while observing MB discoloration in systems having a final pH value between 4.0 and 5.0. The discoloration of methyl orange (MO) in parallel experiments is used to support the findings based on the MB method. This approach is radically different from the conventional approach testing dyes as model contaminants 33,51 . For example, Chen et al. 33 recently investigated the removal of three different azo dyes (Orange II, Reactive Red X-3B and Amido Black 10B) in the Fe 0 /FeS 2 /H 2 O system. All the three dyes are negatively charged, and were explicitly reported to be removed via reductive transformations. Following the conventional approach, Chen et al. 33 monitored the concentrations of dyes, iron and protons (pH value), and performed solid phase characterizations using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). On the contrary, the MB method does not imply such solid phase . The dyes were selected due to: (i) similarity in their molecular size, and (ii) differences in their affinity to positively charged iron oxides (Table 1) 55 . The initial dye concentration used was 10 mg L −1 , equivalent to 31.5 μM for MB and 30.7 μM for MO. The working solutions were prepared by diluting concentrated stock solutions (3150 μM for MB and 3070 μM for MO) using deionized water. The pH values of the initial solutions were 6.5 (MB) and 7.0 (MO).

Iron.
A standard iron solution (1000 mg L −1 ) from General Research Institute for Nonferrous Metals was used to calibrate the UV/VIS spectrophotometer used for analysis. In preparation for spectrophotometric analysis, ascorbic acid was used to reduce Fe III in solution to Fe II . 1,10 orthophenanthroline was used as reagent for Fe II complexation 48,49,55 . Other chemicals used in this study included L( +)-ascorbic acid and L-ascorbic acid sodium salt. Ascorbic acid also degrades dyes (in particular MO) and eliminates interference during iron determination.
Solid materials. Metallic iron (Fe 0 ). The Fe 0 material was purchased from Shanghai Institute of Fine Technology (China). The material is available as scrap iron with a particle size between 0.05 and 5 mm. Its elemental composition as specified by the supplier was: Fe: > 99.99%; C: < 0.1%; N: < 0.1%; O: < 0.1%. Its k Phen value is 13 mg h −156 . The k Phen value is the kinetic constant of Fe 0 dissolution in a 2 mM 1,10 orthophenanthroline solution, and characterizes the material's intrinsic reactivity 57 . The material was used without any further pretreatment. Fe 0 was proven as a powerful discoloration agent for MB specifically because the discoloration agents are progressively generated in-situ 45,55 . Therefore, the discoloration capacity of the used Fe 0 cannot be exhausted within the experimental duration used in the current study (41 d).
Sand. The sand conformed to the China ISO standard, and was used as received without any further pre-treatment or characterization. The particle size was between 1.25 and 2.00 mm. Sand was used because it is cheap and readily available and is widely used as admixing agent to prevent rapid permeability loss in Fe 0 /H 2 O systems 58 .
Pyrite (FeS 2 ). The FeS 2 mineral was from Tongling City, Anhui province, China. The particle size was between 38 and 48 μm. Its weight composition was 46.0% Fe and 52.2% S, which is equivalent to a purity of 98.2% 41 Table 2 summarizes the aggregate content of the 8 Fe 0 /FeS 2 /sand systems and one operational reference (blank experiment), giving a total of 9 experimental treatments. Note that the pure Fe 0 system (0.1 g of  www.nature.com/scientificreports/ Fe 0 ) is regarded as a Fe 0 /FeS 2 /sand system without FeS 2 nor sand. The addition of sand was meant to avoid the compaction of the materials by gelatinous FeCPs via cementation 55 . The efficiency of individual Fe 0 systems for dye discoloration was characterized at laboratory temperature (about 25 ± 2 °C). The final pH value, the iron concentrations and the residual dye concentrations were recorded. All experiments were carried out in triplicates under laboratory (oxic) conditions. The test tubes were protected from direct sunlight.
Analytical methods. Aqueous dye and iron concentrations were determined by a 752 UV/VIS spectrophotometer (automatic) (Shanghai Jing Hua Technology Instrument Co. LTD). The working wavelengths for MB, MO and iron were 664.5, 464.0 and 510.0 nm, respectively. Cuvettes with a 1.0 cm light path were used. The iron determination followed the 1,10 orthophenanthroline method 60  Presentation of experimental results. In order to characterize the magnitude of the tested systems for dye discoloration, the discoloration efficiency (E) was calculated (Eq. (4)). After the determination of the residual dye concentration (C t ), the corresponding percent dye discoloration efficiency (E value) was calculated as: where C 0 is the initial aqueous dye concentration (about 10.0 mg L −1 ), while C t gives the final dye concentration at sampling time (t). The operational initial concentration (C 0 ) for each case was acquired from a triplicate control experiment without additive materials (blank). This procedure was mainly meant to account for experimental errors due to dye adsorption onto the walls of the test tubes.

Results and discussion
Dye discoloration in single-aggregate and ternary-aggregate systems. Figure 1a compares the extent of dye discoloration in the four investigated systems: (i) single-Fe 0 , (ii) single-FeS 2 , (iii) single-sand, and (iv) Fe 0 /FeS 2 /sand. Figure 1b shows the final pH variation with varying FeS 2 doses in the ternary Fe 0 /FeS 2 /sand systems. Figure 1a clearly shows that there was no MO discoloration in the pure sand system (E = 0%). The E values for both dyes in all other systems were larger than 30%. The uniqueness of the single-sand system (100% sand) relative to the other three systems is that it contains no in-situ generated FeCPs (Table 3). Therefore, only sand with its negatively charged surface 46,50 is available for dye discoloration via pure electrostatic interactions. Strong surface interactions with positively charged species is thus responsible for the observed MB discoloration, but no MO discoloration occurs in the single-sand system 44,61 . As quiescent batch experiments were performed (no advection), diffusive mass transfer in the bulk solution and/or in the pores of generated FeCPs are the ratelimiting steps for the discoloration process.
The absence of MO discoloration in the single-sand system is the most important observation from these experiments. MO discoloration is observed in all other systems and is consistently more intensive than MB discoloration in single-Fe 0 and single-FeS 2 systems ("Dye discoloration in Fe 0 /sand/H 2 O systems"). For the Fe 0 / FeS 2 /sand system there is no pronounced difference in the discoloration of both dyes. It is recalled that the Fe 0 / FeS 2 /sand system contains 5 g L −1 Fe 0 , 25 g L −1 of sand and 20 g L −1 of FeS 2 . Thus, according to Table 2, the extent of dye discoloration depends on: (i) the availability of adsorption sites on inert sand (adsorption on sand), (ii) the extent to which sand is covered by in-situ generated FeCPs (selective adsorption on sand and/or FeCPs), and (iii) the extent to which excess FeCPs can be freely precipitated in the bulk solution (co-precipitation with FeCPs) (see Table 3). In this case, free precipitation herein means FeCPs that are not coating the sand surface ("Background to the experimental methodology") 37,38,44,61 .
The merit of the experimental design is to demonstrate dye discoloration in a Fe 0 /sand/H 2 O systems as the FeS 2 mass loadings vary from 0 to 30 g L −1 . Figure 1b clearly shows that varying the FeS 2 mass loading under the experimental conditions has resulted in various final pH values, ranging from 4.5 to 7.0 ( Table 4). The results summarized in Table 4 also show lower pH values in all FeS 2 -containing systems with the single-FeS 2 having a pH value of 3.3 ± 0.2 for both dyes. Notably, comparison of MO versus MB showed no profound difference in the final pH value (4.7 vs 4.8), the iron concentration (63 vs 65) and the E values (71 vs 73) in the ternary system with a FeS 2 mass loading of 20 g L −1 . An exception was the Fe 0 /H 2 O system without FeS 2 (i.e., [FeS 2 ] = 0 g L −1 ) for which a significant difference in the pH value was observed (6.5 for MB vs. 7.0 for MO). This corresponds to the pH value of the respective initial dye solutions ("Solutions"). The ternary-aggregate system used in this case comprised of 20 g L −1 FeS 2 . For a better illustration of the role of the FeS 2 mineral in the process of dye discoloration in Fe 0 /H 2 O systems, three lower (5, 12 and 18 g L −1 ) and two higher (24 and 30 g L −1 ) FeS 2 doses were also used. Figure 2 compares the iron concentration in the Fe 0 /sand/H 2 O systems as the FeS 2 loadings vary from 0 to 30 g L −1 . Figure 1b and Table 4 have shown a monotonous, but non-linear decrease of the pH value with increasing FeS 2 mass loadings. Contrary, Fig. 2a shows a monotonous and linear increase of the iron concentration with increasing FeS 2 mass loadings. For a constant FeS 2 loading, the final pH values (Fig. 1b) and the [Fe] values (Fig. 2a) were almost the same for both dyes. This observation suggests that the differential behavior of MB and MO in interacting with the involved aggregates, particularly Fe 0 and sand, is not reflected in changes of the pH value. Thus, the final pH value arises from the pseudo-steady state equilibrium between two antagonistic processes: (i) Fe 0 corrosion which increases the pH value (Eq. 1), and (ii) FeS 2 dissolution which decreases the pH value ("Background to the experimental methodology").    62,63 . Again, there is no significant difference evident between both dyes. The fact that iron dissolution from any reactive material increases with decreasing pH value is intuitive (Fig. 2a). However, the extent to which iron is dissolved under any given operational conditions should be   (Fig. 2b), and their impact on the investigated process (dye discoloration in this case) discussed. Past efforts to characterize the Fe 0 /FeS 2 system have not properly considered these issues as the final pH value was not always recorded and/or not used in discussing the results 22,41 . Moreover, in earlier efforts it was commonplace to vary both the initial pH value and the FeS 2 loading (Table 5), thereby making it difficult to determine the effect of each parameter. By using only various FeS 2 loadings to shift the pH value of a Fe 0 /sand/H 2 O system, the present study is an extension of earlier efforts from the early 2000s [37][38][39] . Moreover, the current study applied a recently developed tool using MB as an indicator of Fe 0 reactivity 44,45,47 .   Figure 3a shows the extent of dye discoloration (E values) by the ternary system as the FeS 2 loadings increase from 0 to 30 g L −1 . Figure 3b depicts the variation of E values as a function of the final pH value. It is evident that there is a general linear decrease in E value with increasing FeS 2 loading or decreasing pH values (Fig. 2a) [37][38][39] while investigating U VI removal in Fe 0 /H 2 O systems. Note that the single-FeS 2 S 2 discolored both dyes (Fig. 1a). This observation raises questions about the assertion that FeS 2 increases contaminant removal in Fe 0 /H 2 O systems 22,33,41 .

Iron release in Fe 0 /sand/H 2 O systems.
Issue 2 can be regarded as a striking feature as there is either a larger extent of MO discoloration relative to that of MB ([FeS 2 ] = 5 g L −1 ) or the opposite ([FeS 2 ] > 20 g L −1 ). Note that the ion-selectivity principle of the Fe 0 /H 2 O system implies that in the presence of FeCPs in aqueous systems, the anionic MO is better discolored than the cationic MB. This is obviously the case at [FeS 2 ] = 5 g L −1 where enough FeCPs is generated to cover the surface of sand, thereby inducing a larger E value for MO than for MB. The observation that for Fe 0 /H 2 O systems without FeS 2 (i.e., at [FeS 2 ] = 0 g L −1 ), MO discoloration was only slightly higher than that of MB is also noteworthy. This indicates that for the experimental duration used in the current study (41 d), enough FeCPs were generated to co-precipitate both dyes. For further clarification of this issue, a binary Fe 0 /sand system should have been investigated, but this was beyond the scope of the current study. The higher MB discoloration relative to MO observed at [FeS 2 ] > 20 g L −1 is explained by the formation of complexes between Fe and MO which delay co-precipitation 55,61,65 .
Finally, Issue 3 can also be regarded as a striking observation because despite all differences (solubility, affinity), there is no difference in the E values for the two dyes. It can be assumed that, under the experimental conditions, MB and MO which have almost the same molecular size (Table 1) are both discolored by co-precipitation 66 . This assumption is corroborated by results in Fig. 3b showing clearly that there is no quantitative dye discoloration (E > 60%) at pH < 4.5. This corresponds to the observations of Noubactep et al. [37][38][39] for the Fe 0 / U VI /H 2 O system. The fact that MB, MO and U VI exhibited very similar behaviors in the Fe 0 /FeS 2 /H 2 O system is an indication that contaminant removal might be a pure positive side effect of aqueous iron corrosion. The most tangible proof for this assertion is the kinetics of Fe 2+ oxidation by dissolved oxygen (O 2 ). According to Langmuir 67 , the kinetics of this reaction increases by a factor 65 between pH 4.0 and 5.0. Thus, quantitative dye discoloration is observed only in systems where Fe 2+ oxidation to Fe 3+ was quantitative for the 41-d experimental period. The in-situ generated Fe III precipitates are good contaminant scavengers.

Mechanisms of contaminant removal in Fe 0 /H 2 O systems. This study has investigated the effect of
FeS 2 addition on the efficiency of Fe 0 /sand systems for MB and MO discoloration. No enhanced dye discoloration could be attributed to FeS 2 addition at mass loading of 0 to 30 g L −1 for 41 d. Two questions arise: First, why is there no increased dye discoloration in a context where the expected pH shift and increased iron dissolution are evident? (Question 1). It is noteworthy that each individual aggregate (Fe 0 , FeS 2 , sand) tested herein can achieve MB discoloration as depicted in Fig. 1a. Second, why did the ternary system perform far lower that single-Fe 0 systems? (Question 2). By applying a known experimental approach consisting of varying individual operational parameters to better understand complex systems 17,[68][69][70] , and accounting for the relative slow kinetics of Fe 0 and FeS 2 dissolution [37][38][39] , this study has adopted a novel approach to answer Questions 1 and 2. Specifically, the current study assessed the role of FeS 2 in enhancing contaminant removal in Fe 0 /H 2 O system. MB is used herein as an operational reactivity tracer ("Introduction") and the achieved results corroborated earlier reports on U(VI) removal [37][38][39] , and account for discrepancies and inconsistencies reported in literature 33,41,49,70 .
The evidence that FeS 2 oxidation produces acidity (Eq. 2) is corroborated in the current study (Fig. 2a). By consuming acidity, Fe 0 (Eq. 1) and Fe 2+ (Eq. 3) oxidation are accelerated by Eq. (2) (Le Chatelier's principle). Fe 3+ from Eq. (2) catalyses FeS 2 oxidation and produced less soluble Fe(OH) 3 . Thus, mixing Fe 0 and FeS 2 can be regarded as continuously generating less soluble Fe(OH) 3 , until one of the reactants is depleted or until a pseudo-steady state is established. This work posits that Fe(OH) 3 discolors the dye solutions mainly by coprecipitation 66,70 . Thus, dye discoloration is only quantitative when Fe(OH) 3 precipitation is intensive (pH > 4.5). Having used quiescent systems, various final pH values could be achieved, thereby confirming the pH shifting function of FeS 2 37,41,59 . However, the extent of dye discoloration depends on the amount of free in-situ generated Fe(OH) 3 55,70 which is determined by the kinetics of Fe 2+ oxidation by dissolved O 2 67 . As expected, for a longer experimental duration (t > 41 d), the efficiency of the ternary mixture will surpass that of the single-Fe 0 systems [37][38][39]70 . This answers Question 1, and demonstrates that enhanced dye discoloration needs more time to occur under quiescent conditions [37][38][39] . Accordingly, the documented delay of quantitative dye discoloration is not a negation of the view that FeS 2 addition enhances the efficiency of Fe 0 /H 2 O system 33,40,41,70 . This study aims to better understand why Fe 0 /H 2 O systems are more efficient upon the addition of pyrite (FeS 2 ) relative to those without pyrite.
In a ternary Fe 0 /FeS 2 /sand system, sand is non-reactive (inert) and is in-situ coated by iron oxides from the dissolution of the two other aggregates ("Background to the experimental methodology" and Table 2). This insitu coating of sand delays the availability of free Fe(OH) 3 for dye co-precipitation. Initially, MB and Fe 2+ /Fe 3+ compete for adsorptive removal at the negatively charged sand surface 70,71 . Once the sand surface is completely coated, it will be no longer attractive for MB. This competition for active adsorption sites explains the observations in Fig. 3a. Note that neither Fe 0 nor FeS 2 are the discoloring agents, but rather the products of their oxidative  (Table 3). To completely answer Question 2, the ternary mixture performed less than the single-aggregate systems because: (i) sand is in-situ coated, thereby retarding the availability of free Fe(OH) 3 , and (ii) the synergy of Fe 0 and FeS 2 has not yet produced enough free Fe(OH) 3 . The latter is the case whenever the pH value of the system has not exceeded 4.5 (Fig. 3b). The presentation until this point has not addressed the redox properties of MB and MO. The thermodynamics predict MO reduction by Fe 055,61 . The results reported herein demonstrate that even the ion-selective nature of the individual dyes was not the key factor accounting for dye discoloration when the pH was lower than 4.5. Thus, regardless of any redox properties, the current work has demonstrated that Fe 0 -based systems are only efficient when the final pH value is larger than 4.5. Unlike the current study, several previous works have mostly failed to record the final pH values of their systems and use them in their discussion ( Table 5).
Significance of the results. Fe 0 -based systems have been important components of the water treatment industry for the past 170 years 8,27,70 . Research reported before 1990 is not really considered by current active scientists whose starting point is the advent of in-situ permeable reactive barriers (PRBs), and the premise that Fe 0 is an environmental reducing agent 18,23,72,73 . Conventional PRBs use micro-scale or granular Fe 0 specimens (g Fe 0 ). During the past two decades, some tools have been developed to improve the efficiency of g Fe 0 . In this regard, the following three tools have been introduced: (i) using nano-scale Fe 0 , (ii) alloying g Fe 0 with metals such as Pd or Ni (also at nano-scale), and (iii) admixing another aggregate with g Fe 011,24,25 . The Fe 0 /FeS 2 /sand system investigated herein is part of the third category. It has been reported that in sulfide-containing environments, using g Fe 0 results in the formation of iron sulfides which are conductive and sustain electron transfer from Fe 0 to the contaminant 22,[74][75][76] . On the other hand, such iron sulfides are stand-alone reducing agents for the reductive transformations of many contaminants 70,77,78 . Because Fe 0 and FeS 2 have in common the release of Fe II species, it can be assumed that the material containing more Fe will be first passivated by Fe III species. However, when both materials are mixed, FeS 2 accelerates Fe 0 corrosion and none of both materials is really available for quantitative reductive transformation of other foreign species, including contaminants. Consequently, any observed enhancement of contaminant removal in a Fe 0 /H 2 O system by virtue of the presence of FeS 2 is an indirect process. This assertion was elegantly demonstrated in the present study by slowing down the process of iron precipitation via addition of various FeS 2 doses to the same Fe 0 /sand system for 41 d. It then follows that, FeS 2 is mostly a pH shifting agent for the Fe 0 /H 2 O system 37,59,70 . Table 5 reveals that all other investigations on the Fe 0 /FeS 2 system were performed under shaken/stirred conditions. However, under such conditions, the target FeS 2 intrinsic properties (including semi-conduction) are undermined. For example, how can FeS 2 act as a 'mediator' for electron transfer' from Fe 0 to contaminants (Fig. 4) when the whole system is mechanically stirred at 400 rpm? Such a high stirring speed was explicitly selected to ensure that both Fe 0 and FeS 2 could be uniformly dispersed in the reaction solution 22 . This example clearly shows that using FeS 2 to enhance the efficiency of Fe 0 /H 2 O systems is a simple tool to design more sustainable Fe 0 -based systems. However, current rationalization efforts are not really based on scientific principles 22,33 . Thus, only when the scientific principles are well-understood can better systems be designed 8,27,70 . A typical design problem is how to cope with the increased Fe 0 dissolution specifically in column operations intrinsically prone to clogging 79,80 . Thus, in solving the enigma of the Fe 0 /FeS 2 /H 2 O system, this work leads to several avenues for sustaining the efficiency of conventional Fe 0 /H 2 O remediation systems. This result is especially important as Fe 0 -based (filtration) systems are an excellent candidate to help the international community to solve the longlasting issue of universal safe drinking water 8,14,15,27,29,30,81,82 .   (Fig. 4) is given by recent investigations in efforts to suppress FeS 2 oxidation under environmental conditions [83][84][85][86] . For example, Seng et al. 84 reported that Fe 0 is able to stop FeS 2 oxidation, and thus remediate acid mine drainage. In essence, Seng et al. 84 investigated a contaminant-free Fe 0 /FeS 2 /H 2 O system ( Table 6) and concluded that from the intrinsic properties the addition of Fe 0 selectively suppress pyrite oxidation. Table 6 shows that the experimental conditions of Seng et al. 84 are very close to those of remediation Fe 0 /FeS 2 /H 2 O systems. The only two distinct differences are: (i) the higher FeS 2 mass loading (FeS 2 : Fe 0 = 10), and (ii) the longer experimental duration (41 days versus < 10 h). By using an even longer experimental duration (41 days) and quiescent conditions (0 rpm), the present work has demonstrated the essential virtue of working under near-field conditions. In other words, it is fair to state that the Fe 0 /FeS 2 literature is full of possibly reproducible results, but with low practical value. As already shown in Table 5, the variability of the operational conditions is a major issue and the significance of results of solid phase characterization is questionable. In fact, as seen in Table 6, a myriad of characterization tools were used to "confirm" the reducing properties of Fe 0 for dyes 33 . In such studies species like methylene blue 70 used herein as a 'tracer' of reactivity or arsenic 22 , and proven to be non-reducible in Fe 0 /H 2 O systems are quantitatively removed. The first merit of the MB method is to uncover these controversial views without solid phase analysis.
The conclusion of Seng et al. 84 supports the view presented herein that the relative kinetics of Fe 0 and FeS 2 oxidation determinate the preponderance of processes in Fe 0 /FeS 2 /H 2 O systems 70 . However, the reported selectivity of the process is questionable as sand and other natural minerals are also covered with FeCPs under similar conditions 29,87-89 . As an example, Song et al. 87 reported on increased Cr VI reduction in Fe 0 /sand/H 2 O systems compared to Fe 0 /H 2 O ones. The extent of coating of each aggregate (e.g. gravel, peat, pyrite, sand) depends on both the intrinsic reactivity of used Fe 0 and the relative proportion of available materials. In the light of the kinetic arguments given herein, a re-evaluation of published works is possible, for example, the data of Sheba et al. 86 discussing the extent of degradation of chlorinated organic compounds (RCl) by Fe 0 /FeS 2 /H 2 O systems and reporting on differential mechanisms at different Fe 0 :FeS 2 ratios. The discussion given herein clearly suggests that if there are differential removal mechanisms, it is due to the differential extent of pH shift. Future research should be designed based on the chemistry of the systems 89 .

Concluding remarks
The concept that adsorption and co-precipitation are the fundamental mechanisms of contaminant removal in Fe 0 /H 2 O systems is consistent with many experimental observations. In particular, quantitative dye discoloration was only observed for pH values corresponding to iron precipitation (hydroxide formation) (pH > 4.5, Fig. 3b), while selective dye discoloration promoted adsorptive removal. Further, while the role of the redox-mediated reactions in the discoloration of both dyes can only be speculatively discussed based on one's results, it is established that the role of FeS 2 is as follows: (i) shifting the pH to more acidic values, and (ii) enhancing contaminant removal by adsorption and co-precipitation during the subsequent pH increase by virtue of iron corrosion. Finally, negatively charged methyl orange (MO) showed no significant increase in discoloration relative to positively charged methylene blue (MB). Both MB and MO have a similar molecular size. This observation is consistent with the role of Fe 0 as a generator of contaminant scavengers, and not as a reducing agent. This observation could explain why various As (As III and As V ) 90 or Se (Se IV and Se VI ) 91 species are quantitatively removed in Fe 0 / H 2 O systems, but not by aged iron oxides. Further research is needed to investigate the phenomena highlighted in the current study using a wide range of contaminants commonly occurring in drinking water and wastewaters.