Arsenic(III) Oxidation by Iron(VI) (Ferrate) and Subsequent Removal of Arsenic(V) by Iron(III) Coagulation

Abstract

We investigated the stoichiometry, kinetics, and mechanism of arsenite [As(III)] oxidation by ferrate [Fe(VI)] and performed arsenic removal tests using Fe(VI) as both an oxidant and a coagulant. As(III) was oxidized to As(V) (arsenate) by Fe(VI), with a stoichiometry of 3:2 [As(III):Fe(VI)]. Kinetic studies showed that the reaction of As(III) with Fe(VI) was first-order with respect to both reactants, and its observed second-order rate constant at 25 degrees C decreased nonlinearly from (3.54 +/- 0.24) x 10(5) to (1.23 +/- 0.01) x 10(3) M(-1) s(-1) with an increase of pH from 8.4 to 12.9. A reaction mechanism by oxygen transfer has been proposed for the oxidation of As(III) by Fe(VI). Arsenic removal tests with river water showed that, with minimum 2.0 mg L(-1) Fe(VI), the arsenic concentration can be lowered from an initial 517 to below 50 microg L(-1), which is the regulation level for As in Bangladesh. From this result, Fe(VI) was demonstrated to be very effective in the removal of arsenic species from water at a relatively low dose level (2.0 mg L(-1)). In addition, the combined use of a small amount of Fe(VI) (below 0.5 mg L(-1)) and Fe(III) as a major coagulant was found to be a practical and effective method for arsenic removal.

Ferrate(VI) in the Treatment of Wastewaters: A New Generation Green Chemical

“…demonstrated to be one electron process whereas, the oxy compounds of arsenic, selenium, nitrogen and sulphur are possessed with the two-electron mechanism while these are reacted/degraded with Fe(VI). The reactions of ferrate(VI) with a series of inorganic compounds such as iodide, cyanide, superoxide, sulfide, hydrazine, ammonia, azide and oxy-compounds of nitrogen, sulphur, selenium and arsenite possessed with seconds-order kinetics [38,47,49,51,[57][58][59][60][61][62][63][64][65]. In general, similar to the organic compounds the reaction with inorganic compounds (P) may be demonstrated as equation (26): where k P is the second-order rate constant for the reaction.…”

“…The arsenic (III) oxidation to As(V) and hence, the removal of As(V) by reduced Fe(III) via coagulation process was effectively achieved [65]. The two moles of Fe(VI) required to oxidize three moles of As(III) (reaction 50).…”

Trichloroethylene removal from water by ferrate treatment

“…The applicability of ferrate for removal of heavy metals (Lim et al 2010), cyanide (Lee et al 2009) , hydrogen sulfide (Sharma et al 1997), ammonia (Sharma et al 1998), arsenic (Lee et al 2003) or organic contaminants e.g. biphenol-A (Li et al 2008), carbohydrates (Sharma et al 2012), phenol and chlorophenols (Graham et al 2004), pharmaceutical residues (Sharma et al 2006), personal care products (Yang et al 2012) was demonstrated in the literature.…”

Removal of arsenite by simultaneous electro-oxidation and electro-coagulation process

“…At the meantime, since As(III) is more mobile and more toxic than As(V), it is advantageous to convert As(III) to As(V). Also due to poor As(III) removal from water by conventional processes, oxidation of As(III) to As(V) was generally performed [14][15][16][17].…”