Dissolved Organic Carbon and Mercury Treatability in FGD Wastewater

摘要

Overall, the research has shown that mercury is associated with organics in the gelbstoffe, fulvic and humic acid size ranges, although for only for one or two samples by each of two measurement techniques. Specifically, results obtained with the AC-HTC/NDIR-CVAFS method indicate that, for one sample, a significant fraction of the mercury present in the FGD liquor (-47%) was associated with fulvic acids present in the liquor. Note that these results were obtained after multiple pH adjustments, filtrations, and adsorption chromatography, which could have affected the mercury associations. Results generated by the SEC-UV/Vis-ICP/MS method indicate that organic compounds were present in both the gelbstoffe/fulvic acid size range and possibly in the humic acid range and more specifically, that the sizes of most interest are 240-5500 amu and 119,000 amu. Approximately 20 % of the mercury in the parent liquor sample for one site was found to be associated with the gelbstoffe/fulvic acid organics and approximately 5-6% of the mercury in samples from two different sites was associated with a compound or complex with a MW in the humic acid range. The treatability studies conducted in the earlier phase of this project indicate that the ability to remove dissolved mercury decreases with increasing concentration of DOC. The data obtained to date do not allow identification of the specific chemical compound(s) responsible for the effects observed on mercury treatability. The mercury may be strongly complexed to only one or perhaps many specific organic molecules present in the FGD liquor and these specific compounds may or may not be present in FGD systems at all sites. In other words, one or more specific organic compounds, as opposed to the overall concentration of organic species in a particular size class could be responsible for the observed FGD chloride purge water treatability issues experienced by some utilities. It also remains possible that the 'bad actor' is a non-organic acid molecule. Therefore, current work is being performed to scale up the AC-HTC/NDIR-CVAFS methodology to isolate enough material to use in a series of beaker studies. The end goal for separating and isolating the different species of dissolved organic molecules in actual FGD chloride purge water samples is to create a "standard" that contains the actual species present in FGD liquor. This standard will be spiked into synthetic FGD liquor samples in laboratory beaker-scale tests. The purpose of the beaker tests will be to determine if the different species of organics bind with mercury and impede the effectiveness of conventional wastewater treatment chemicals to remove mercury from the wastewater. Different concentrations of the organics and organics from different sites will be tested in an attempt to determine if there is a concentration effect or if there is a bad actor among the dissolved organic species. Different pH values will be tested, to mimic different steps in the wastewater treatment process.