Steel Wool for Water Treatment: Intrinsic Reactivity and Defluoridation Efficiency

Abstract

Studies were undertaken to characterize the intrinsic reactivity of Fe0-bearing steel wool (Fe0 SW) materials using the ethylenediaminetetraacetate method (EDTA test). A 2 mM Na2-EDTA solution was used in batch and column leaching experiments. A total of 15 Fe0 SW specimens and one granular iron (GI) were tested in batch experiments. Column experiments were performed with four Fe0 SW of the same grade but from various suppliers and the GI. The conventional EDTA test (0.100 g Fe0, 50 mL EDTA, 96 h) protocol was modified in two manners: (i) Decreasing the experimental duration (down to 24 h) and (ii) decreasing the Fe0 mass (down to 0.01 g). Column leaching studies involved glass columns filled to 1/4 with sand, on top of which 0.50 g of Fe0 was placed. Columns were daily gravity fed with EDTA and effluent analyzed for Fe concentration. Selected reactive Fe0 SW specimens were additionally investigated for discoloration efficiency of methylene blue (MB) in shaken batch experiments (75 rpm) for two and eight weeks. The last series of experiments tested six selected Fe0 SW for water defluoridation in Fe0/sand columns. Results showed that (i) the modifications of the conventional EDTA test enabled a better characterization of Fe0 SW; (ii) after 53 leaching events the Fe0 SW showing the best kEDTA value released the lowest amount of iron; (iii) all Fe0 specimens were efficient at discoloring cationic MB after eight weeks; (iv) limited water defluoridation by all six Fe0 SW was documented. Fluoride removal in the column systems appears to be a viable tool to characterize the Fe0 long-term corrosion kinetics. Further research should include correlation of the intrinsic reactivity of SW specimens with their efficiency at removing different contaminants in water.

Characterizing the Reactivity of Metallic Iron for Water Treatment: H2 Evolution in H2SO4 and Uranium Removal Efficiency

“…Therefore, "remediation using Fe 0 " is currently regarded as one of the best available technologies in the global effort to achieve Goal 6 of the UN Sustainable Development Goals (SDGs): Ensure availability and sustainable management of water and sanitation for all [14][15][16]. However, the Fe 0 remediation technology as a whole suffers from lack of reliable methods to characterize the intrinsic reactivity of used materials [17][18][19][20][21][22][23].…”

Metallic Iron for Environmental Remediation: Starting an Overdue Progress in Knowledge

“…For this reason, a more rational approach is to consider that all species can be removed regardless of their redox potential, and identify exceptions on a case by case basis. In this regard, recent results by Hildebrant et al [140] have confirmed that fluoride removal is low, but less reactive materials (EDTA test) comparatively remove more fluoride. This last observation corroborates the view that there is no single Fe 0 material for all situations and calls for more systematic investigations to identify appropriate Fe 0 materials for specific remediation applications.…”

“…For example, while treating water in fluidized systems (Anderson Process), dense materials were better than sponge iron [13,168]. Similarly, Hildebrant et al [140] reported that Fe 0 materials of low reactivity according to the EDTA test exhibited a better efficiency for fluoride removal. The question arises how to select the right Fe 0 specimen for a given application?…”

“…This shows a ratio of reactivity of 18.5 for Fe 0 SW specimens which are often tested as a uniform class of materials compared to granular materials for example. Hildebrandt et al [10,140] presented the k EDTA values for 13 reactive Fe 0 SW and one granular Fe 0 as 3.7 ≤ k EDTA (µg h −1 ) ≤130.8. The lowest k EDTA value corresponds to granular Fe 0 , suggesting a reactivity ratio of up to 36 between Fe 0 SW and granular Fe 0 .…”

“…Selected probing agents enable the discussion of the results, and orange II, methyl orange and reactive red 120 were positively tested in this regard [61,62]. Recent results by Hildebrandt et al [140] suggest that fluoride could be the next candidate for affordable probing reagent. In fact, it was found that fluoride removal is more efficient by low reactive Fe 0 specimens.…”