Sorption of Strontium onto Bacteriogenic Iron Oxides

Bacteriogenic iron oxides (BIOS) were obtained from a dilute, circumneutral groundwater seep, characterized with respect to mineralogy, and examined for their ability to sorb aqueous Sr 2+ . BIOS were composed of microbial sheaths encrusted in 2-line ferrihydrite. Sorption experiments indicated that Sr remained completely unbound at pH 7.6). EXAFS analysis of Sr-loaded BIOS failed to elucidate whether Sr sorption occurred on sites specific to the mineral or microbial fraction, but indicated that sorption likely occurred by outer-sphere complexation between BIOS and hydrated Sr 2+ . Sorption experiments showed that, at low ionic strength (I = 0.001 M), sorption followed a Langmuir isotherm (S max = 3.41 mol Sr (g of Fe) 1- , K ads = 1.26). At higher ionic strength (/= 0.1 M), there was significant inhibition of Sr sorption (S max = 1.06 mol Sr (g of Fe) 1- , K ads = 1.23), suggesting that sorption to BIOS occurs by outer-sphere complexation. The results suggest that, under dilute circumneutral conditions, BIOS deposits should efficiently sorb dissolved Sr from groundwater flow systems where such deposits exist. This finding has particular relevance to sites impacted by radioactive 90 Sr groundwater contamination.