Comment on “Reductive dechlorination of γ-hexachloro-cyclohexane using Fe–Pd bimetallic nanoparticles” by Nagpal et al. [J. Hazard. Mater. 175 (2010) 680–687] 1 2 3 4 Noubactep C. Angewandte Geologie, Universität Göttingen, Goldschmidtstraße 3, D - 37077 Göttingen, Germany. 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 e-mail: cnoubac@gwdg.de; Tel. +49 551 39 3191, Fax: +49 551 399379 Abstract The author used a recent article on lindane (γ-hexachloro-cyclohexane) reductive dechlorination by Fe/Pd bimetallics to point out that many other of published works in several journals do not conform to the state-of-the-art knowledge on the mechanism of aqueous contaminant removal by metallic iron (e.g. in Fe0/H2O systems). It is the author’s view that the contribution of adsorbed FeII to the process of contaminant reduction has been neglected while discussing the entire process of contaminant reduction in the presence of bimetallics. Keywords: Adsorption; Bimetallic Co-precipitation; Iron corrosion; Zerovalent iron. 1 Introduction Nagpal and his colleagues recently presented a very informative article on the chemical reduction (reductive dechlorination) of lindane (γ-hexachloro-cyclohexane) in aqueous solution using Fe/Pd bimetallic nanoparticles [1]. It is argued in the present work that this study (i) used an inappropriate experimental design/procedure and (ii) failed to recognise the results of several recent publications on contaminant removal using Fe0-based materials as will be shown below. 2 Reactive species present in a Fe0/H2O system The diversity of reactive species in a conventional Fe0/H2O system (micro-sized Fe0) has been extensively discussed during the past 20 years [2-5]. In typical conditions, the Fe0/H2O system is comprised of metallic iron and its associated corrosion products. Iron corrosion products, in dissolved or adsorbed forms, include: (i) primary species, such as FeII, H or H2, 1 which are also reducing agents (co-reductants), (ii) secondary species, such as FeIII or Fe 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 II/FeIII oxide/hydroxides, which may be reducing agents (e.g. green rust) or adsorbents (green rust, FeIII-hydroxides), and (iii) other corrosion products which result from transformations or combinations of primary and secondary corrosion products (FeOOH, Fe3O4, Fe2O3). Other corrosion products include HO• radicals which are known for their oxidative capacity for organic chemicals [4-9]. It can therefore be implied that depending on solution conditions contaminants in the Fe0/H2O system can be either adsorbed, oxidized or reduced [9]. As the Fe0/H2O system can be regarded a dynamically evolving system and it remains highly difficult to separate the relative contributions of each individual reaction [4]. For example: (i) during precipitation, iron hydroxides and oxides will enmesh available (organic and inorganic) contaminants and/or their reaction products (co-precipitation) [10,11], and (ii) iron oxides will serve as adsorption sites for self-catalytic reactions using H2/H and/or FeII [12,13]. Reducible contaminants, such as lindane, could therefore be adsorbed and/or co-precipitated before or after reduction. As a result, it can be stated that the relative contributions of co-reductants, such as adsorbed FeII and H2/H, could be much greater then Fe0 due to their increased accessibility to the aqueous contaminant [5-7,13]. It is argued in the present work that this was not appropriately presented/discussed by Nagpal et al. [1]. The conventional Fe0/H2O system is modified by Nagpal et al. [1] in two ways: (i) by the use of nano-sized particles and (ii) by the use of plating Pd0 (Pd/Fe bimetallic system). While a smaller particle size unequivocally enhances reaction kinetics, the prevailing effects of the presence of Pd0 on Fe0 surface has been challenged [8] but not considered in their presentation by Nagpal et al. [1]. 3 Effects of Pd0 on the performance of Fe0/H2O systems Nagpal et al. [1] attributed enhanced lindane degradation efficiency to “increased catalytic reactivity due to the presence of Pd0 on the surface”. This statement is intrinsically valid but 2 the prevailing mechanism (lindane hydrodechlorination on Pd0 surface) has been challenged [8]. Results from [8] suggest that (i) dissolved Pd 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 II species will be enmeshed in the mass of precipitating iron hydroxides/oxides, and (ii) Pd0 will be covered by surface oxide phases that will retard any potential catalytic activities. In both cases the net result is a decrease in the contaminant degradation efficiency as a function of time. This argument provides strong evidence to suggest that, unlike bimetallic composites, coated bimetallic materials are fundamentally inappropriate for long-term reactivity applications. The fundamental suitability of Fe0 composites for water treatment and their superiority over conventional Fe0 materials has been demonstrated by Hussam et al. [14,15]. These authors replaced conventional iron shavings by a composite iron material (CIM: 92 - 94 % Fe, 4 - 5 % C, 1 - 2 % SiO2, 1 - 2 % Mn, 1–2% S, P) and could transform a highly efficient but not sustainable system (3-Kolshi filter) into a highly functional and sustainable system (SONO filter) [15]. Actually, SONO filters initially designed for arsenic removal has been shown efficient in removing a large spectrum of contaminants including bacteria, nitrates, pesticides and viruses [16,17]. Beside the reaction mechanism for the hydrodechlorination on Pd0 surfaces reported by [1], it can be argued that there is another possible explanation for the enhanced degradation efficiency observed. In the initial stages of the reaction, highly reactive iron hydroxides are produced and these can adsorb dissolved FeII to produce structural FeII (adsorbed FeII or FeII(s)). As the electrode potential for the redox couple FeIII(s)/FeII(s) varies from -0.65 to -0.36 V compared to E0 = -0.44 V exhibited by Fe0 (FeII(aq)/Fe0(s)) it can therefore be stated that the formation of structural FeII on the surface of the material during the initial stages of the reaction could represent a further mechanism for the enhanced (catalytic) reduction of lindane observed. Adsorbed FeII is always superior to molecular hydrogen (E0 = 0.00 V) as a reducing agent [7]. It can also be suggested that the experimental conditions employed by Nagpal et al. [1] were not appropriate for the conclusions drawn. 3 4 Suitability of the experimental design for Fe/H2O reactions 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 Due to the aforementioned complex mechanism of Fe0 oxidative corrosion, research into the Fe0/H2O system is typically characterised by a pragmatic approach, and often a high degree of variation in solution conditions between experiments (Table 1). For example, (i) at present there is no universal Fe0 reference material available; (ii) nano-sized Fe0 materials are used for their increased reactivity, (iii) Fe0 bimetallic systems are also used for their increased reactivity. Accordingly, it can be stated that, when using nano-scale bimetallic Fe0, two variants exist that have strong bearings on nanoparticulate reactivity. Batch experiments of Nagpal et al. [1] were performed with 15 mL solutions of 5 mg/L lindane (C6H6Cl6: 290,83 g·mol-1). The solutions contained 100 to 500 mg/L of Fe/Pd nanoparticles (Fe: 56 g.mol-1). With a Fe0:lidane molar ratio therefore ≥7000 in all systems studied, it can be suggested that this experiment may be inappropriate as a “real-world” analogue, as it would be difficult to maintain such a large Fe0:contaminant molar ratio in the subsurface. The authors also employed an agitator to maintain nanoparticle suspension during the experiment. This poses an uncertainty with respect to ensuring consistent mixing throughout the reaction. Table 1 clearly shows the diversity of used experimental designs on the basis of selected works referenced by Nagpal et al. [1]. In general, the investigation of processes yielding aqueous contaminant removal by Fe0 has been biased by the use of irrelevant experimental conditions [5,22]. In particular, the use of experimental designs disturbing the formation of an oxide scale in the vicinity of Fe0 has significantly disturbed mechanistic investigation and contributed to general acceptance of the view that Fe0 is a reducing agent (direct reduction). However, even though Fe0 could generate reducing species (Fe0, H/H2) that may quantitatively reduce relevant contaminants, the primary objective of water treatment is contaminant removal and not contaminant reduction. Accordingly, even reduced species must be physically removed from the aqueous phase. The next section will give an overview on the 4 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 acceptance on the concept that contaminants are quantitatively removed by adsorption and co- precipitation. 5 Acceptance of the adsorption co-precipitation concept. Although the concept that in Fe0/H2O systems contaminants are quantitatively removed by adsorption, co-precipitation and size-exclusion has been presented in several peer-reviewed communications and has been independently confirmed as summarized in ref. [7], this concept is not yet really accepted in the scientific community [23-25]. Although the concept is based on thermodynamic considerations and has considered several aspects of aqueous iron corrosion, some authors have criticized that mentioned communications are using self-citation [24] or that the concept is lacking experimental support for organic contaminants [24,25]. The argument that organic compounds (known to adsorb very little onto iron oxides) are not going to be sequestered and co-precipitated in the same fashion as metals, is not acceptable for at least four reasons: (i) Organic compounds are in trace in an “ocean” of iron oxides and hydroxides [4]. Despite low affinity, organic compounds will be collected by in-situ generated iron corrosion products and/or removed from the aqueous phase by size exclusion. A direct evidence is given by Lai et al. [26] who reported on quantitative removal of 1,2-dichloroethane and dichloromethane in Fe0 reactive walls although both compounds were “proven to be not treatable by Fe0”. This observation was attributed to enhanced adsorption or microbial degradation. (ii) Organic compounds have been successfully removed by electrocoagulation (using Al or Fe). The similitude between electrocoagulation and contaminant removal in Fe0/H2O system is well-documented [27,28]. For example, Bojic et al. [27] reported that the mechanism of action of alloyed metallic aluminium (Al0) for contaminant removal is based on the “several physico-chemical processes and the in situ formation of the coagulant”. The major processes being adsorption, reduction, hydrogenation, hydrolysis and coagulation, operating synergistically to (degrade and) remove variety of aqueous contaminants. Noubactep [4-7] has 5 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 segregated between removal (adsorption, coagulation, size-exclusion) and transformation (reduction, hydrogenation and hydrolysis) mechanisms and argued that even transformed species are removed by adsorption, co-precipitation or size-exclusion. (iii) The adsorption/co-precipitation concept has been validated by Noubactep using methylene blue (C16H18N3SCl) as model contaminant [29,30]. The model was further independently confirmed using clofibric acid (C10H11ClO3) and diclofenac (C14H11Cl2NO2) [31,32]. (iv) The adsorption co-precipitation concept has been indirectly validated by giving a better interpretation on data on thiobencarb (C12H16ClNOS) [33], triazoles (C2H3N3) [34], Ethylenediaminetetraacetic (EDTA, C10H16N2O8) [35], tartrate (C4H4O62−) and glycine (C2H5NO2) [36]. In conclusion, it is over to the scientific community to disprove (or improve) the adsorption/co-precipitation concept. A recent paper by Gillham et al. [37] has stated that “there are ongoing discussions on the mechanism of contaminant removal processes by metallic iron”. The present work is part of these discussions. 6 Concluding remarks While presenting scientific work, authors should always use objectivity and clarity in formulating their conclusions. Despite supported by a peer review system, this however never ensures 100% validity of published work [38]. The author’s view is that the study by Nagpal et al. [1] has overlooked the role of structural FeII in the chemical transformation and the removal of aqueous contaminants in bimetallic systems [8]. The concept employed by Nagpal et al. [1] has already been proven to be deficient [4-8]. The alternative concept, the adsorption/co-precipitation concept, is largely presented in the Journal of Hazardous Materials and has been demonstrated by independent researchers [13,31,32]. Considering this, the current author would like to appeal to subsequent authors, peer-reviewers, and referees to consider the above arguments. The presented 6 comments with respect to the work of Nagpal et al. [1] can also be applied to several recent publications using Fe 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 0 and Fe0 bimetallic systems for water treatment in various journals. To conclude the author supports the comments of Tien [39] who is paraphrased as “The viability and the acceptability of Fe0-based remediation technology depends largely on the quality of its investigators and the work they produce. The fact that within the Fe0 remediation community, some authors have the choice to: (i) select references to “accurately” present their results, and (ii) possibly select journals to the same purposes must be a concern to all members of the community.” Considering the comments outlined in the current work, it is the author’s view that critical analysis of future and concurrent publications is required in order to ensure the effective advance of research associated within the Fe0/H2O system [40]. Acknowledgments English corrections provided by Richard Crane (Interface Analysis Centre, University of Bristol -UK) on the draft manuscript are gratefully acknowledged. References [1] V. Nagpal, A.D. Bokare, R.C. Chikate, C.V. Rode, K.M. 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Rev. 38 (2009) 203–206. 11 Table 1: Selected experimental conditions of Nagpal et al. [1] and references therein [19-22]. The experimental designs further differ regarding the availability of oxygen (anoxic/oxic), the mixing type and mixing intensity, and experimental duration. V is the used volume of solution and ρ 280 281 282 283 284 Material the material loading. The proportion of plating metal in bimetallic systems has not been considered. Contaminant (X) MX Material V [X] ρMaterial Material/X Ref. (g/mol) (mL) (mM) (mM) (-) Lindane 290.83 nano-Fe/Pd 15 0.017 119 6925 [1] nano-Fe/Pd 15 0.017 595 34623 Pentachlorophenol 266.34 micro-Fe 10 0.038 1786 47619 [18] micro-Fe/Pd 10 0.038 1786 47619 micro-Fe/Pt 10 0.038 1786 47619 micro-Fe/Ni 10 0.038 1786 47619 micro-Fe/Cu 10 0.038 1786 47619 Trichloroethylene 131.39 nano-Fe/Pd 40 0.076 45 587 [19] nano-Fe/Ni 40 3.805 45 12 Trichloroethylene 131.39 micro-Fe 100 0.220 7 32 [20] micro-Fe 100 0.210 7 31 Lindane 290.83 micro-Fe 5 0.024 89 3720 [21] micro-Fe 5 0.024 179 7440 micro-Fe 5 0.024 357 14881 285 12