Duolite™ GT-73 ion exchange resin routinely reduces the mercury content of a waste water stream to less than the permitted level of 10 ppb. Effluent concentrations from the ion exchange facility (IEF) are consistently between 1 to 5 ppb, even though the feed contains a varying concentration of mercury (0.2 to 70 ppm). Two operational problems have been encountered at that facility, however. Firstly, the stated capacity of the resin for mercury was not being achieved. The abnormally low capacity was traced to analytical laboratory waste which was intermittently treated by the resin. That waste contained hydrochloric acid, stannous chloride, and potassium permanganate, among other chemicals, which presumably eluted sorbed mercury from the resin and also oxidized the thiol (SH) functional groups on the resin and rendered them inactive. The net effect was that the resin had to be replaced more frequently than anticipated. Secondly, the IEF was temporarily shut down because the mercury content of the waste water could not be reduced to below the permitted level, even with fresh resin. That problem was caused by slow settling solids composed mainly of iron which apparently adsorbed some of the mercury and allowed it to pass through the resin untreated. The solids were presumably a result of processing waste water abnormally high in iron which may have co-precipitated with mercury and other elements in the feed and caused a residual buildup of solids throughout the IEF. The problem was remedied by installing a 0.2 µm cartridge filter between the feed tank and the columns.
Hg removal from SRTC laboratory waste using an in-tank ion exchange probe
