@article{PAR00004144,
  title = {{XANES} evidence for oxidation of {C}r({III})) to {C}r({VI}) by {M}n-oxides in a lateritic regolith developed on serpentinized ultramafic rocks of {N}ew {C}aledonia},
  author = {{F}andeur, {D}. and {J}uillot, {F}. and {M}orin, {G}. and {O}livi, {L}. and {C}ognigni, {A}. and {W}ebb, {S}. {M}. and {A}mbrosi, {J}ean-{P}aul and {F}ritsch, {E}mmanuel and {G}uyot, {F}. and {B}rown, {G}. {E}.},
  editor = {},
  language = {{ENG}},
  abstract = {{A}lthough several laboratory studies showed that {M}n-oxides are capable of oxidizing {C}r({III})) to {C}r({VI}), very few have reported evidence for such a reaction in natural systems. {T}his study presents new evidence for this redox reaction between {C}r({I}ll) and {M}n-oxides in a lateritic regolith developed on ultramafic rocks in {N}ew {C}aledonia. {T}he studied lateritic regolith presents several units with contrasting amounts of major ({F}e, {A}l, {S}i, and {M}g) and trace ({M}n, {C}r, {N}i, {C}o) elements, which are related to varying mineralogical compositions. {B}ulk {XANES} analyses show the occurrence of {C}r({VI}) (up to 20 wt % of total chromium) in the unit of the regolith which is also enriched in {M}n (up to 21.7wt% {M}n{O}), whereas almost no {C}r({VI}) is detected elsewhere. {X}-ray powder diffraction indicates that the large amounts of {M}n in this unit of the regolith are due to the occurrence of {M}n-oxides (identified as a mixture of asbolane, lithiophorite and birnessite) and {M}n {K}-edge {XANES} data indicate that {M}n occurs mainly as {M}n({IV}) in this unit, although small amounts of {M}n({III})) could also be detected. {T}hese results strongly suggest a direct role of the {M}n-oxides on the occurrence of {C}r({VI}) through a redox reaction between {C}r({III}) and {M}n({IV}) and/or {M}n({III}). {O}wing to the much larger toxicity and solubility of {C}r({VI}), such a co-occurrence of {C}r and {M}n-oxides in these soils could then represent an important risk for the environment. {H}owever,the significant amounts of {C}r({VI}) released after reacting the samples from the studied sequence with a 0.1 {M} ({NH})(4){H}2{PO}4 solution, designed to remove tightly sorbed chromate species, suggest that {C}r({VI}) mainly occurs as sorption complexes. {T}his hypothesis is reinforced by spatially resolved {XANES} analyses, which show that {COO} is associated with both {M}n- and {F}e-oxides, and especially at the boundary between these two mineral species. {S}uch a distribution of {C}r({VI}) suggests a possible readsorption of {C}r({VI}) onto surrounding {F}e-oxyhydroxides (mainly goethite) after oxidation by the {M}n({IV})-oxides, {T}hese results, added to leaching tests with a 0.01 {M} {C}a{C}l2 solution indicative of low exchangeability of {C}r in the investigated samples, suggest that secondary sorption reactions onto {F}e-oxides might significantly decrease the environmental impact of the oxidation of {C}r({III}) to {C}r({VI}) by {M}n-oxides.},
  keywords = {},
  booktitle = {},
  journal = {{E}nvironmental {S}cience and {T}echnology},
  volume = {43},
  numero = {19},
  pages = {7384--7390},
  ISSN = {0013-936{X}},
  year = {2009},
  DOI = {10.1021/es900498r},
  URL = {https://www.documentation.ird.fr/hor/{PAR}00004144},
}