A comparative study of oxidation of Cr(III) in aqueous ions, complex ions and insoluble compounds by manganese-bearing mineral (birnessite)

Decolorization of methylene blue by δ-MnO2-coated montmorillonite complexes: Emphasizing redox reactivity of Mn-oxide coatings

“…As far as abiotic oxidation is concerned, Mn oxides are among the strongest natural oxidants in soils and sediments with reducing potentials between 1.27 and 1.50 V. They are capable of oxidizing many inorganic contaminants [5][6][7], and a wide spectrum of natural and xenobiotic organic compounds such as catechol [1,8], quinines [9], substituted phenols [10], aromatic amines [11,12], phosphonates [13], antibacterial agents [14], antibiotics [15], brominated flame retardant [16], and steroid estrogens [17,18]. It has been reported that abiotic oxidation of a number of refractory organic contaminants by Mn oxides and subsequent polymerization are important pathways of their natural attenuation and immobilization in soils and sediments [8,12].…”

“…As oxidants, however, only discrete particles of Mn oxide have been tested to oxidize inorganic/organic contaminants [5,28,29]. Up to date, there is only limited information on redox reactivity of Mn-oxide coatings, though their redox importance in bulk soils and sediments has long been recognized [12,18].…”

Cr(VI) removal by biogenic schwertmannite in continuous flow column

“…Chromium exists under natural conditions mainly in two oxidation states; namely, Cr(III) and Cr(VI). Generally, Cr(III) species have lower toxicities and are poorly mobile, readily precipitating in the forms of Cr(OH) 3 , CrFe(OH) 6 , and CrPO 4 , or adsorbed by soils (Dai et al, 2009). In contrast, Cr(VI) species are known to be highly toxic and readily mobile in biological systems, causing serious health problems such as liver damage and pulmonary complications (Eary and Rai, 1988;Yurik and Pikaev, 1999).…”

Photo-Oxidation of Cr(III)−Citrate Complexes Forms Harmful Cr(VI)

“…The former hypothesis is easy to understand, as we know that the increase in pH decreases the reduction potential thus favors Cr(III) oxidation based on energetics. The latter hypothesis is supported by the distribution of Cr(III) species simulated by Visual MINTEQ with a span of pH from 0.5 to 14 (17). Cr(III) ions are mostly distributed as free Cr(III) and [CrOH] 2+ at low pH, as Cr(OH) 3 at medium pH, and as [Cr(OH) 4 ] -at high pH.…”