THE ROLE OF PARTICULATE MATTER IN THE MOBILIZATION OF TRACE-METALS DURING ANAEROBIC-DIGESTION OF SOLID-WASTE MATERIAL

Metal ions bound to particulate matter represent the greatest portion (i. e. > 95%) of the total metal content found in leachate from reacto r experiments where solid waste material was anaerobically digested. T his seems true even though strong complexing agents are in solution wh ich increase the solubility of Pb and Cu by a factor up to 10(4)...10( 5) over that theoretically predicted according to the solubility of th e corresponding sulfide mineral. A titrimetric characterization of the metal ion binding sites of the particulate matter suggests that the m etal binding properties of the particulate matter are mainly due to or ganic, amino-acid-type compounds (amines, thio groups, carboxylic grou ps) probably of bacterial origin. The change of the concentration of t he binding sites with time, together with the change of the compositio n of the particulate matter indicates that bacterial flocs are suspend ed in the leachate during the switch from acidogenic to methanogenic c onditions - either due to the detachment of bacterial films from the s olid material by the intensive gas production or due to the formation of syntrophic methanogenic bacterial associations or a combination of both. A combination of the two factors, strong affinity of bacterial m ass to metal ions on the one hand, and suspending of the bacterial mas s in the leachate on the other hand, will therefore imply a great mobi lizing potential for trace metals. Consequently, the highest concentra tions of particulate bound Cd were found in reactor experiments where sewage sludge contaminated with Cd was added to the waste material. An increase of the concentration of dissolved cadmium over the solubilit y of cadmium sulfide, however, could not be observed.