Separation of Mercury from Liquid Media
Mercury is found in both anionic and cationic forms as well as organic mercury compounds.
As an extremely toxic metal, mercury is coming under more and more scrutiny, and is being regulated to extremely low levels in waste and discharge water from wet flue gas desulfurization (FGD) blowdown or the ash ponds which collect FGD blowdown along with other waste streams from a coal-fired power plant.
Mercury is a very reactive molecule that can exist in multiple forms. As the volatile zero-valent Hg(0), as the charged ion of Hg2+, and as organo-mercury compounds such as methyl mercury. Because it is so reactive, mercury can also form complexes with many other ions that are present in FGD wastewater. Since multiple forms of mercury can be present in the same stream, multiple removal techniques may need to be employed in series.
AmberSorb™ L493 Polymeric Adsorbent works well for the removal of zero-valent mercury, and methyl mercury or other organo-mercury complexes. To remove Hg2+, a thiol- or thiouronium-functionalized chelating resin that is selective for mercury is recommended. AmberSep™ GT74 Resin and AmberSep™ 43600 Resin have both been tested and proven for their ability to remove ionic mercury.
To remove colloidal complexes, filtration is the recommended option. DuPont™ Ultrafiltration SFP Modules will remove colloidal particles greater than 0.03 µm. For smaller colloidal material, FilmTec™ NF90-400/34i Nanofiltration Elements will provide better removal.
In the production of chlorine and caustic soda (sodium hydroxide), the diaphragm and mercury amalgam processes are increasingly being replaced by membrane technology because it consumes less energy and is more environmentally benign. Nevertheless, some of the world’s chlor-alkali processes are still using the mercury amalgam process. Both processes may co-exist for many years and, in fact, may share the same brine feed circuit. The mercury found in the brine circuit of the mercury cell process is detrimental to membrane cell electrodes and irreversibly fouls the brine softening ion exchange resins.
AmberSep™ GT74 Resin is a mercury-selective resin that removes mercury from sodium chloride and sodium hydroxide with very low mercury leakage, typically down to levels of 1-2 ppb. Using AmberSep™ GT74 in front of the brine softening resin protects both the membrane cell electrodes and the brine softening ion exchange resins.
AmberSep™ GT74 has excellent mercury capacity (about 60 g Hg per liter of resin). Regenerable with concentrated HCl, it is the only mercury-removal resin that can be efficiently regenerated and reactivated.
An extremely toxic metal, mercury is regulated to extremely low levels in drinking water. Mercury can exist in water as an organic complex called methyl mercury, as a colloid, or as an inorganic complex typically mixed with sulfides, making it difficult for a single unit operation to meet mercury removal targets successfully.
When the mercury exists in a colloidal complex, DuPont™ Ultrafiltration SFD Modules can remove mercury-containing particles in the final drinking water, and they have been tested and certified by NSF International under NSF/ANSI Standard 61 and PDWE Certifications to ensure that you will meet your drinking water standards. (This option is not recommended for drinking water where dedicated operators are not available.)
Ion exchange resins can be used to remove other forms of mercury. For example, for methyl mercury and zero-valent mercury, AmberSep™ Optipore™ L493 Polymeric Adsorbent has been shown to successfully remove the mercury to very low levels; and when Hg2+ is present, thiol-functionalized chelating ion exchange resins work well, but since none of DuPont’s mercury-removal resins are presently certified by NSF/ANSI Standard 61, they cannot at this time be recommended for drinking water applications requiring this certification.