Apparent solubilities of schwertmannite and ferrihydrite in natural streamwaters polluted by mine drainage

The apparent solubilities of schwertmannite and ferrihydrite were estimated from the H+, OH-, Fe3+, and SO42- activities of the natural stream waters in Korea and mine drainage in Ohio, USA. Both chemical composition of the s tream waters and the mineralogy of the precipitates were determined for sam ples from two streams polluted by coal mine drainage. This study combines t hese new results with previous data from Ohio, USA to redetermine solubilit ies. The activities of the dissolved species necessary for the solubility d eterminations were calculated from the chemical compositions of the waters with the WATEQ4F computer code. Laboratory analyses of precipitates indicated that the main minerals presen t in Imgok and Osheep creek were schwertmannite and ferrihydrite, respectiv ely. The schwertmannite from Imgok creek had a variable chemical formula of Fe8O8(OH)(8-2x)(SO4)(x). nH(2)O, where 1.74 less than or equal to x less t han or equal to 1.86 and 8.17 less than or equal to n less than or equal to 8.62. The chemical formula of ferrihydrite was Fe2O3. 1.6H(2)O. With known mineralogy of the precipitates from each stream, the activities of H+, OH- , Fe3+, and SO42- in the waters were plotted on logarithmic activity-activi ty diagrams to determine apparent solubilities of schwertmannite and ferrih ydrite. The best estimate for the logarithm of the solubility product of sc hwertmannite, logK(s), was 10.5 +/- 2.5 around 15 degrees C. This value of logK(s) constrains the logarithm of the solubility product of ferrihydrite, logK(f), to be 4.3 +/- 0.5 to maintain the stability boundary with schwert mannite observed in natural waters. (C) 1999 Elsevier Science Ltd.