Schwertmannite Synthesis through Ferrous Ion Chemical Oxidation under Different H2O2 Supply Rates and Its Removal Efficiency for Arsenic from Contaminated Groundwater

Abstract

Schwertmannite-mediated removal of arsenic from contaminated water has attracted increasing attention. However, schwertmannite chemical synthesis behavior under different H2O2 supply rates for ferrous ions oxidation is unclear. This study investigated pH, ferrous ions oxidation efficiency, and total iron precipitation efficiency during schwertmannite synthesis by adding H2O2 into FeSO4·7H2O solution at different supply rates. Specific surface area and arsenic (III) removal capacity of schwertmannite have also been studied. Results showed that pH decreased from ~3.48 to ~1.96, ~2.06, ~2.12, ~2.14, or ~2.17 after 60 h reaction when the ferrous ions solution received the following corresponding amounts of H2O2: 1.80 mL at 2 h (treatment 1); 0.90 mL at 2 h and 14 h (treatment 2); 0.60 mL at 2, 14, and 26 h (treatment 3); 0.45 mL at 2, 14, 26, and 38 h (treatment 4), or 0.36 mL at 2, 14, 26, 38, and 50 h (treatment 5). Slow H2O2 supply significantly inhibited the total iron precipitation efficiency but improved the specific surface area or arsenic (III) removal capacity of schwertmannite. For the initial 50.0 μg/L arsenic (III)-contaminated water under pH ~7.0 and using 0.25 g/L schwertmannite as an adsorbent, the total iron precipitation efficiency, specific surface area of the harvested schwertmannite, and schwertmannite arsenic(III) removal efficiency were 29.3%, 2.06 m2/g, and 81.1%, respectively, in treatment 1. However, the above parameters correspondingly changed to 17.3%, 16.30 m2/g, and 96.5%, respectively, in treatment 5.

Schwertmannite Adherence to the Reactor Wall during the Bio-Synthesis Process and Deterioration of Its Structural Characteristics and Arsenic(III) Removal Efficiency

“…According to PDF 47-1775 [33] and the results of XRD and SEM investigated here, schwertmannite is the only mineral synthesized in this system that has a typical spiny spheroidal structure similar in appearance to a hedgehog. This structure of schwertmannite has also been reported in previous studies [20,21]. …”

“…Furthermore, the specific surface areas of adhered-sch and suspended-sch were 4.36 m 2 /g and 9.62 m 2 /g, respectively. The suspended-sch in the present study had a relatively low specific surface area compared to that observed by Bigham et al [39] but was comparable to values recently reported by Liao et al [28] (3.42-23.45 m 2 /g) and Liu et al [20] (10.66 m 2 /g). Overall, the bio-synthesized schwertmannite that adhered to the reactor wall exhibited a significantly different structure with a much smaller specific surface area to that of the spiny spheroidal structure exhibited by bio-synthesized schwertmannite that remained suspended in the reactor system.…”

“…However, when the amount of added schwertmannite was 26.7 g/L, the pH decreased more slowly than in the treatment consisting of 13.3 g/L of added schwertmannite, reaching 2.19 after 60 h incubation. The pH changes in different treatments were governed by ferrous ion oxidation and ferric ion hydrolysis processes [20,34]. Ferrous ion bio-oxidation efficiency in the presence of added schwertmannite increased sharply during 0-36 h and, thereafter, increased gradually to 100% at 60 h in the CK treatment (Figure 2b).…”

“…Adsorption technology is an adopted technique for arsenic(III) removal from arsenic(III)-bearing water due to its easy operation, high capacity, fast kinetic reactions, and mild regeneration condition [15][16][17]. Schwertmannite, an amorphous iron oxyhydrosulfate mineral, is commonly synthesized in an iron sulfate-rich acidic environment, has proved to be a good scavenger of arsenic(III) [18][19][20]. Liao et al [21] observed that arsenic(III) adsorption by schwertmannite improves with the increase of the solution pH in the range of 3-9 and the maximum removal efficiency of arsenic(III) occurs at solution pH 7-9.…”

“…Paikaray et al [24] found that an amount of SO 4 2− can be released from schwertmannite into the solution during arsenic(III) adsorption. Therefore, schwertmannite, with a large specific surface area, exhibited a high removal efficiency for arsenic(III) from groundwaters [20,25]. Schwertmannite can be synthesized by biogenic oxidation of FeSO 4 by Acidithiobacillus ferrooxidans (A. ferrooxidans) followed by subsequent ferric ion hydrolysis [21,26].…”