| This research examines the retention of Pb and Cd by kaolinite, humic acid (HA) and mackinawite, and mixtures of these three materials. Some aspects of how pH influences metal retention and how the pH is changed by interactions between the sediment components and the metals, are considered. Initial metal concentrations ranged from 0.24 mmol/L to 22 mmol/L. The proportion of solid material to solution for sediments containing kaolinite was 0.1, and for the sediments without kaolinite was 0.001 or 0.002.; It was found that the principal mechanism of metal retention by mackinawite was through lattice exchange, particularly over the lower range of metal concentrations. Up to 33% of the Fe in the mackinawite was replaced by Cd and Pb. Surface adsorption of Cd and Pb was of secondary importance, increased with initial metal concentration, and eventually became constant. After lattice exchange was complete the surface adsorption of Cd and Pb by the mackinawite reached maximum values of 1.90 mmol/g and 0.91 mmol/g respectively. Pb and Cd may have been forming soluble sulfide complexes as no hydroxide precipitation of Pb and Cd was apparent.; At high initial metal concentrations, HA seemed to encourage the growth of three-dimensional Pb and Cd species. This precipitation occurred at pH 2.6 for Pb and at pH 3.9 for Cd, although these pH values are lower than is commonly expected for precipitation of these metals. In addition, the formation of soluble Cd humate species was noticed at pH 2.8. Finally, determination of the cation exchange capacity (CEC) of airdried and undried samples of HA resulted in values of 436 meq/100 g and 806 meq/100 g respectively.; For kaolinite and HA, adsorption of Pb was generally greater than adsorption of Cd. The Pb appeared to be strongly attracted to the face and edge sites of kaolinite, while the Cd was only strongly attracted to the edge sites of kaolinite.; Retention of Pb and Cd by HA was approximately 100 times greater than retention by kaolinite. The maximum retention of Pb and Cd by mackinawite was 4 times greater than by HA, except that mackinawite released Fe into solution in place of some of the Cd and Pb that was taken up.; The pH of a kaolinite suspension was lowered 0.9 of a pH unit by the addition of 1% HA and was raised by 2.3 pH units upon the addition of 0.7% mackinawite. It was also shown that the pHzpc of HA was below pH 0.5, indicating a very negatively charged material. |
