Assessing chromium mobility in natural surface waters: Colloidal contribution to the isotopically exchangeable pool of chromium (EwCr value)

Abstract

The labile pool of chromium (Cr), ECr value for soils and EwCr value for suspended particulate matter (SPM) in water, can be efficiently assessed in different environmental compartments through the use of isotopic dilution techniques. The overall objective of this work was to evaluate Cr availability based on the suspended particle matter size range and, more specifically, to determine the role of colloids in the calculation of the EwCr value. Surface water samples were collected from ponds and streams at the active nickel mine of Barro Alto (Goiás State, Brazil). Two isotopic exchange (spiking) experiments were performed: #E1, the water samples were either spiked before cascade filtration (0.7 μm, 0.2 μm, 500 kDa and 1 kDa) or, #E2, according to the range of particle sizes (>0.7 μm, 0.2–0.7 μm, 500 kDa–0.2 μm and 1–500 kDa). This study showed that: 1) mining activities in ultramafic environments locally increase the presence of particulate matter and colloids containing Cr-bearing phases, mainly iron oxy(hydroxides); 2) in the pristine area, chromium was mainly associated with colloids (1 kDa–500 kDa), while mining activities increase the amount of large particles (>0.2 μm) containing chromium in the impacted area, and 3) there is an overestimation of the exchangeable pool of chromium (EWCr value) due to the presence of organic and inorganic colloids containing non-exchangeable chromium. A correction to overcome this discrepancy was proposed.