@article{fdi:010048256,
  title = {{A}ssessing chromate availability in tropical ultramafic soils using isotopic exchange kinetics},
  author = {{G}arnier, {J}. and {Q}uantin, {C}. and {E}chevarria, {G}. and {B}ecquer, {T}hierry},
  editor = {},
  language = {{ENG}},
  abstract = {{B}ackground, aim, and scope {T}he presence of labile chromate in the soils is an environmental problem because of its high toxicity. {T}he isotopic exchange kinetics ({IEK}) methods have been shown to be a useful tool to measure the phytoavailability of major ( {P}, {K}) and trace elements ({C}d, {Z}n, {N}i, {P}b) in soils. {T}his study focused on the potential of applying {IEK} for chromate to characterize its availability in two tropical ultramafic {F}erralsols. {M}aterials and methods {T}wo {F}erralsols ({NIQ} {II} and {NIQ} {III}) of the ultramafic complex of {N}iquelandia ({G}oias, {B}razil), known to have a high content of extractable chromate, were investigated. {W}e adapted {IEK} for chromate in order to distinguish different pools of available chromate according to their rate of exchange kinetic. {R}esults {T}he extractable {C}r( {III}) ranged from 9 to 132 mg kg(-1), whereas extractable {C}r( {VI}) ranged from 64 to 1,014 mg kg(-1). {T}he intensity factor, i.e., concentration of soluble {C}r, ranged from 78 to 231 mu g {L}-1 in profile {NIQ} {II} and from 6 to 141 mu g {L}-1 in profile {NIQ} {III}. {T}he highest concentrations were found in both topsoils and in the {NIQ} {II}-5 horizon. {M}ost of the {C}r({VI}) was labile in short ({E}0-1 (min)) or medium-term ({E}-1 (min-24) (h)) in both soils. {T}he {E}0-1 min and {E}-1 (min-24 h) represented 39 to 83% of labile {C}r ({VI}) in {NIQ} {II} and 69 to 80% in {NIQ} {III}. {A} high quantity of {C}r({VI}) was thus extremely labile and highly available, particularly in {NIQ} {II}. {M}oreover, both soils had a high buffering capacity of soluble {C}r({VI}) by labile pools. {D}iscussion {T}he {C}r({VI}) availability is large and varied significantly among the soil profiles. {T}he r(1)/{R} parameter has long been considered as an indicator of the soil "fixing capacity" for ions like {P}. {T}he values of r(1)/{R} for {C}r({VI}) measured on the two studied soils are among the lowest ever reported for any element, especially in the organic matter-poor and iron oxide-rich horizons (r(1)/{R} in the 0.001-0.003 range). {B}ut, considering the high proportion of labile {C}r{VI} in these soils, it is more appropriate to relate r(1)/{R} to the buffer capacity. {T}he latter was extremely high and probably due to labile {C}r({VI}) retained in its majority by low-energy bonds on the surface of colloids. {T}he quantity of readily labile {C}r ({E}0-1 (min)) was significantly correlated (r-0.96, p<0.01) with the quantity of {C}r associated to amorphous or poorly crystallized {F}e-oxides. {T}hus, amorphous {F}e oxides control the {C}r availability in these {F}erralsols. {T}he correlation between {E} parameters and clay content has to be carefully considered. {I}ndeed, these soils contain mainly fine and discrete clay-sized {F}e oxides, particularly goethite. {D}espite different data supporting the idea of the formation of inner-sphere surface complexes of chromate on goethite, the high quantity of readily labile {C}r ({VI}) and the high buffer capacity observed for these soils are consistent with low-energy bonds on the surface of colloids in agreement with the formation of outer-sphere complexes. {C}onclusions {T}he two studied {F}erralsols contain a large quantity of labile {C}r({VI}), which is controlled by amorphous {F}e oxides and p{H}. {IEK} for chromate allows distinguishing different pools of available chromate according to their rate of exchange kinetic. {T}he buffer capacity of these soils is extremely high and probably with a majority of low-energy bonds on the surface of colloids. {T}he study highlighted a high chromate availability in those soils and, consequently, potential chromate toxicity on soils organisms. {R}ecommendations and perspectives {IEK} could be a powerful tool to quantify chromate availability in soils. {O}ur attempt to apply the {IEK} for chromate seems to be a success and the {IEK} for {C}r consequently seems to have future. {H}owever, the robustness and the limit of the {IEK} for chromate have to be examined in more detail for a large diversity of soils.},
  keywords = {{A}vailability ; {B}razil ; {C}hromate ; {I}sotopic exchange ; {L}ability ; {M}etal ; {S}erpentinic soils ; {T}oxicity ; {U}ltramafic soils},
  booktitle = {},
  journal = {{J}ournal of {S}oils and {S}ediments},
  volume = {9},
  numero = {5},
  pages = {468--475},
  ISSN = {1439-0108},
  year = {2009},
  DOI = {10.1007/s11368-009-0062-4},
  URL = {https://www.documentation.ird.fr/hor/fdi:010048256},
}