The Impact of Selected Pretreatment Procedures on Iron Dissolution from Metallic Iron Specimens Used in Water Treatment

Abstract

Studies were undertaken to determine the reasons why published information regarding the efficiency of metallic iron (Fe 0 ) for water treatment is conflicting and even confusing. The reactivity of eight Fe 0 materials was characterized by Fe dissolution in a dilute solution of ethylenediaminetetraacetate (Na 2 -EDTA; 2 mM). Both batch (4 days) and column (100 days) experiments were used. A total of 30 different systems were characterized for the extent of Fe release in EDTA. The effects of Fe 0 type (granular iron, iron nails and steel wool) and pretreatment procedure (socking in acetone, EDTA, H 2 O, HCl and NaCl for 17 h) were assessed. The results roughly show an increased iron dissolution with increasing reactive sites (decreasing particle size: wool > filings > nails), but there were large differences between materials from the same group. The main output of this work is that available results are hardly comparable as they were achieved under very different experimental conditions. A conceptual framework is presented for future research directed towards a more processed understanding.Sustainability 2019, 11, 671 2 of 20 organic [27,28], fluoride [29][30][31], heavy metals [32,33], nitrate [34,35], pathogens [36][37][38] and radionuclides [32,39]. The desalination of water using Fe 0 has also been reported [40][41][42][43]. The two key advantages of Fe 0 are its affordability and its universal availability (iron nails, scrap iron and steel wool) [9,10,44,45]. It is commonly reported that the major drawback of Fe 0 -based technologies for water treatment is the low intrinsic reactivity of granular materials [28,33,46]. This situation is said to be aggravated by the inherent generation of an oxide scale at the Fe 0 surface (passivation). Countermeasures to overcome Fe 0 passivation were recently reviewed by Guan et al. [28] and further discussed by Noubactep [47]. It is recalled that, considering passivation as a "curse" contradicts the evidence that Fe 0 -based subsurface permeable barriers have been successfully working for more than one decade [48][49][50]. On the other hand, increased "passivation" should have occurred in the spongy iron filters in Antwerpen as well [2,19]. Clarifying this contradiction is certainly a progress for the whole Fe 0 technology. Clearly, an understanding of the role of "passivation" in the process of decontamination using Fe 0 should be useful for enhancing the system's efficiency in practice [30,31,47].There is an agreement on the evidence that using Fe 0 for water treatment and environmental remediation is "putting corrosion to use" [51][52][53]. There is otherwise a contradiction on practically any other aspect concerning the Fe 0 /H 2 O system, including the reaction mechanisms and factors determining the long-term efficiency of such systems [9,28,47,[54][55][56][57][58]. A key reason for this is the large variability of experimental conditions used in testing Fe 0 materials [58][59][60][61][62]. The main influencing operational parameter seems to be Fe 0 itself [58,61,63,6...

A Novel and Facile Method to Characterize the Suitability of Metallic Iron for Water Treatment

“…The R 2 values for EDTA vary between 0.534 for SW2 and 0.994 for SW3. For Phen all R 2 values were larger than 0.94, confirming the better suitability of the Phen test as already discussed (see also [52]). As concerning the 'b' values, it is seen that for the EDTA test, four from nine values are negative and large in absolute values (b > 1.03 mg).…”

“…The perfect linearity observed for Phen in Figure 1 confirms that using the Phen method practically only Fe 0 oxidative dissolution was characterized [60]. Hu et al [52] recently presented the first application of the Phen test for granular Fe 0 materials. Figure 2 compares changes of iron concentration as a function of time for the 9 tested Fe 0 SW in 2 mM Phen for up to 120 h. For all materials, Fe values increased progressively with the experimental duration, even after 120 h of contact.…”

“…Lastly, amorphous Fe 3 corrosion products are (at least partly) dissolved in EDTA (Equation (6)). Amorphous Fe 2 corrosion products, if available, would dissolve both in EDTA and Phen [52].…”

“…This evidence makes the Phen test more reliable that the EDTA test. In addition to using fewer chemicals than the EDTA test, the Phen test was also more affordable [52,60].…”

“…Nine (9) commercial Fe 0 SW were used. The selection of Phen was justified by the extreme stability of the Fe 2 -Phen complex [53,55]: (1) avoiding any further oxidation of Fe 2 from Fe 0 dissolution and (2) non-addressing any Fe 3 species already available in the system [52]. Parallel experiments using EDTA were performed and the results are comparatively discussed.…”