Pedogeochemical characteristics of chromite in a paddy soil derived from serpentinites

• This is the first study on chromite weathering in paddy soil.
• Cr was clearly detectable in the potentially labile fractions.
• Incongruent dissolution of chromite was evident by a preferential release of Cr.
• Chromite acted as a significant source of Cr in the pedon.

Chromite is a primary Cr-bearing mineral in serpentinitic soil; therefore, these soils generally contain a high concentration of Cr, which causes a potential risk to human health after Cr is mobilized by chromite weathering into the environment. This study examined the micromorphological characteristics of chromite and identified chromite weathering as a source of Cr in a paddy soil profile derived from serpentinites in Eastern Taiwan. The soil samples were tested by partitioning Cr in the bulk soil and measuring the element composition in the chromite. The fractionation of Cr in all soil horizons was performed using a selective sequential procedure (SSE). The SSE results demonstrated that the original Cr was primarily fixed in the recalcitrant minerals of the serpentinitic soils. However, labile Cr fractions were also clearly observed in the soils. During chemical weathering, Cr(III) was released from hydrolysis in chromite under the submergence of the pedon, whereas Cr(VI) examined using XANES spectra was absent in the soils. The micromorphological observation using polarized-light microscopic and SEM-BSE images indicated that chromite was altered, which corresponded to the increase of labile Cr toward the surface soil. In the chromite grain, Cr decreased with the increase of Fe and decrease of Al, Mn, Mg, and Zn during pedogenesis. Chromite acted as a source of Cr through dissolution in the pedon.