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April 2009 Williams and Denholm 85 amined the removal of silver from influent water in Publiciy Owned Treatment Works and found that more than 94% of the influent siiver was re- moved during treatment. The percentage of fil- terable silver was directly. related to DOC. The amount of silver passing a 0.4 Nm filter (often regarded as the dissolved fraction) repre- sented just 2% of the total Ag in the sample. They found that 92% of dissolved silver was as- sociated with colioidal particles. In their study, DOC concentrations were typically 4.5 to 11.6 mg/L, and log ICo values ranged between 4.75 and 6.14. A useful illustration of the operational signiflcance of all these factors on the bioavailability of silver, following the release of silver iodide into the envi- ronment over a 40 year period, is shown in the report on the Mokelumne watershed lake water and sediment survey (Stone et aL 1995). This repart showed that, although there were de- tectable concentrations of silver in the lake sedi- ments (average value 0.035 mg/kg), the silver level in the watershed averaged 6.7x10'12 g Ag/ mL. Importantly, no detectable free silver could be measured in leachates of the sediment samples at pH 5 showing that the siiver in the sediments was tightly bound. 6. POTENTIAL ENVIRONMENTAL FATE 6.1 Untake of Siiver bv TeRestrial Plants Accumulation of silver by terrestrial plants is low even when the plants are grown on silver amended sewage sludge or mine spoil (Hirsch, 1998). Where uptake occurs, the silver is found mainly in the root systems (Ratte 1999). As a general rute most of the toxicity testlng on plants, reported in the recent literature, invoNed the use of appropriate concentrations of silver nitrate giv- ing (Ag') concentrations in the mgil (ppm) range. In sensitive plant species, growth and germina- tion were reported to be affected at a concentra- tion of 7.5 mg/kg with germination the most sen- sitive stage. As expected, soluble silver salts were markedly more toxic than insoluble silver saits. Silver uptake by seedlings of perennia{ ryegrass (Lolium perenne L.) and white clover (Tiifolium repens L.) was studied in detail by Ward et al. (1979). They found that some 90% of the silver was immobilised in the root systems of both spe- cies. Uptake was rapid and essentially compiete after 10 days. While the silver concentrations in the roots of both species approached that of the added silver, the aerial parts of the plants were much lower, and seldom exceeded 10% of that in the roots. The silver concentrations used in this study ranged from 0 to 1000 mg/L (ppm) which are sig- nificantly higher than expected for the SPERP. More importantly, they concluded that there would be little danger to stock grazing on pas- tures with these high levels of plant silver. In some much earlier woiic Freeman (1979), looked at silver levels in algae and emergent aquatic plants in an aipine lake in Colorado and found concentrations of the order of 0 to 2.6 mg/ kg in several species. However, this work was well before the introduction of ultra-trace tech- niques and no detail was given of the analytical procedures used. Hirsch (1998) has looked at the germination of a range of crop plants grown on soiis amended with silver-laden sludge derived from photo- graphic industry waste. It was found that gerrni- nation or emergence in all crop specles studied was not adversely affected. And further, for sil- ver levels up to 106 mg Ag/kg in the sludge, the growth and yield of com and oats was not differ- ent from the controls. Hirsch (1998) also reported that yields of plant species such as lettuce, cabbage and spinach were affected at amended soil levels greater than 14 mg Ag/kg. With the exception of lettuce, there was no difference in silver concentrationa in the tissues of the treated crops compared with the controls. Hirsch's conclusion was that land appli- cation of silver rich sludge would not adversely affect plant growth. Sensitive aquatic plants were found to grow poorly at 3.3 to 8.2 Ng Ag/L and died at concen- trations greater than 130 Ng Ag/L (CICAD 44). - Scientific Papers -