Investigation into heavy metal uptake by waste water sludges

Preliminary studies conducted in our laboratory with composted sludge have shown that this material has a high affinity for metals. This metal uptake property appears to be particularly pronounced for lead; for which the material showed a retention capacity of 15% of its weight. The metal uptake process is accompanied by the release of an equivalent amount of calcium. Based on the results of this preliminary work, it appears that metal uptake by composted sludge involves phosphates. These phosphates may be associated with minerals, such as apatite, present in the sludge, and/or dead or living microorganisms, such as bacteria, also present in the sluge. It appears that calcium in the apatite is displaced by lead, forming lead phosphate mineral. On the other hand, the involvement of microorganisms can occur in three possible ways. It can involve phosphates associated with the biomass of dead bacteria and other organisms. It can also involve phosphates occurring in the system following microbial detoxification, a process which is believed to be accompanied by the release of inorganic phosphates. Lead uptake by bacteria can also be a genetic process.