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APPLICATION OF HARDWOOD BIOCHAR AS A REACTIVE CAPPING MAT TO STABILIZE MERCURY DERIVED FROM CONTAMINATED FLOODPLAIN SOIL AND RIVERBANK SEDIMENTS
Wang, A.O., C.J. Ptacek, D.W. Blowes, B.D. Gibson, R.C. Landis, J.A. Dyer, and J. Ma.
Science of the Total Environment 652:549-561(2019)

Hardwood biochar (pyrolyzed at 700°C), a potential candidate for Hg removal, has been proposed for use as reactive capping mats along groundwater discharge zones or riverbanks to control release of Hg from contaminated riverbank sediments. The effectiveness of Hg removal using hardwood biochar was investigated under hydrogeochemical conditions representative of those present within a reactive capping mat installed in a fluvial setting. Two sets of treatment columns containing 50% v.v biochar and quartz sand were subjected to 100 weekly wetting/drying cycles that included dry air, water-saturated air, and drainage using leachate derived from two source columns as input solutions: (1) passing simulated acid rain water through floodplain soil, and (2) passing river water through riverbank sediment. Results from solid-phase extraction analyses suggest that Hg accumulated near the air/biochar-sand interface (0-2 cm) in the treatment columns at low loadings but was present at greater depths at higher loadings. Results of µ-XRF mapping and µ-XANES for the biochar collected at depths 0-2 cm in treatment columns suggest retention of Hg-bearing particles derived from riverbank sediment and floodplain soil within the biochar pore structure. Sulfur K-edge XANES analysis of unused biochar and post-treatment biochar suggest formation of Hg complexes on the biochar surface. These results indicate the potential for effective use of hardwood biochar in reactive mats for controlling Hg discharging from contaminated riverbank sediments.



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