for a better understanding of the present invention , together with other and further objects , advantages and capabilities thereof , reference is made to the following disclosure and appended claims . metal powder blends and a w / sn compact as well as pure tungsten powder as a control were subjected to one or more leaching tests . in one series of tests , an aqueous buffer solution having a ph of 7 . 2 was prepared by dissolving 4 . 03 mg kcl , 50 . 6 mg caso 4 . 2h 2 o , 123 . 2 mg mgso 4 . 7h 2 o , 96 . 0 mg nahco 3 , and 209 . 3 mg of a noncomplexing tertiary amine , 3 -( n - morpholino ) propanesulfonic acid ( mops ) per liter of water . ten gram amounts of as - reduced tungsten metal powder together with separate amounts of antimony ( 100 mg ), lead ( 100 mg ), pbo ( 200 mg or 500 mg ) or tin metal powder ( 7 . 54 g ) were placed in 500 - ml volumes of the aqueous buffer solution in 1 - liter nalgene © ( pp ) erlenmeyer flasks to observe the leaching characteristics of the tungsten . in another flask , two 2 - g pressed tungsten / tin compacts (˜ 57 wt . % w , ˜ 43 wt . % sn ) were added to 500 ml of the aqueous buffer solution . for a control , 10 g of the above tungsten metal powder alone was also placed in 500 ml of the aqueous buffer solution . in another series of tests , 10 - g amounts of tungsten metal powder were placed in 500 - ml volumes of unbuffered aqueous solutions of lead nitrate ( ph 4 . 4 ) in 1 - liter nalgene © ( pp ) erlenmeyer flasks . in both series of tests , the flasks were loosely covered with an aluminum foil and continuously shaken in a dark , thermostated room ( 72 ° f .) with a lab - line ® force orbital open air shaker , model 4690 , for a period of 28 days . periodic 25 - ml samples of the leachate solutions were taken and analyzed for ph , oxygen content , and tungsten content at 7 , 14 , 21 , and 28 days . a constant oxygen concentration of 8 . 3 ± 0 . 2 mg / liter was observed for the entire testing period of 28 days . the results of these tests are shown in table i . the amount of leached tungsten detected in solution is expressed as a percentage of the original amount of tungsten powder . as can be seen , the concentration of tungsten in the leachate from the control sample increases from 0 . 32 % tungsten at 7 days to 0 . 78 % tungsten at 28 days . when antimony and tin are present , the leachability of the tungsten is greatly suppressed . in the case of tin metal powder , less than one - half of the amount of tungsten was present in the leachate after 28 days as compared to the control . the level of suppression was even greater in the case of antimony powder where less than one - tenth of the amount of tungsten was leached after 28 days . there was no significant level of tungsten leached from the w / sn compact ( detection limit 0 . 4 mg w / l ). the tests with the solid lead oxide and aqueous solutions of lead nitrate showed that lead in the form of pb 2 + is also a very efficient suppressant of tungsten &# 39 ; s leachability . lead metal shows a lesser effect compared to pbo and aqueous pb ( no 3 ) 2 because the lead metal must be oxidized to aqueous pb 2 + prior to its reaction with leached tungsten . the leachate &# 39 ; s lead concentration for the aqueous pb ( no 3 ) 2 tests measurably decreased during the 28 - day test period indicating that tungsten metal ( and aqueous tungstate anions ) could be used to remove lead from lead - contaminated water . table i tungsten content in leachate expressed as a % of initial w amount % w % w % w metal or metal cation starting % w 14 - 21 - 28 - sample additive ph 7 - day day day day control — 7 . 2 0 . 32 0 . 49 0 . 65 0 . 78 w powder w powder sn powder ( 75 wt . %) 7 . 2 0 . 13 0 . 20 0 . 24 0 . 28 w powder sb powder ( 1 wt . %) 7 . 2 0 . 06 0 . 07 0 . 07 0 . 07 w / sn — 7 . 2 0 . 00 0 . 00 0 . 00 0 . 00 compact w powder pb powder ( 1 wt . %) 7 . 2 0 . 28 0 . 47 0 . 59 0 . 68 w powder pbo ( 2 wt . %) 7 . 2 0 . 00 0 . 04 0 . 16 0 . 21 w powder pbo ( 5 wt . %) 7 . 2 0 . 00 0 . 00 0 . 00 0 . 02 w powder 0 . 01m pb ( no 3 ) 2 4 . 4 0 . 00 0 . 00 0 . 00 0 . 00 w powder 0 . 001m pb ( no 3 ) 2 4 . 4 0 . 00 0 . 00 0 . 00 0 . 00 w powder 0 . 0005m pb ( no 3 ) 2 4 . 4 0 . 00 0 . 00 0 . 00 0 . 04 w powder 0 . 0001m pb ( no 3 ) 2 4 . 4 0 . 00 0 . 04 0 . 13 0 . 23 similar tests of the leachability of tungsten in the presence of silver , nickel or copper metal powders showed that the presence of these metals greatly increased tungsten leachability , on the order of 7 to 15 times greater than tungsten metal powder alone . while embodiments of the present invention have been described in the foregoing specification , it is to be understood that the present invention is defined by the following claims when read in light of the specification .
the interaction of sn , sb and pb with tungsten in contact with an aqueous medium has a synergistic effect in which the leachability of each metal is suppressed . this means that tungsten has the ability to suppress the leachability of these same metals , in particular lead . for example , tungsten may be added to the lead - contaminated soil in order to suppress lead contamination of the ground water by forming lead tungstate which has a very low solubility .