Experimental assessment of metal complexes toxicity on microorganisms

Several studies performed during the past two decades have assessed that heavy metal extraction from contaminated soil can be successfully performed by the in-situ soil washing technology. This technology involves the leaching of metals and other cations from soil, mainly through the complexation with an extractant agent, followed by metal precipitation from the extracted solution and the reuse of the extractant solution. Since the precipitation step hardly achieve to a total separation of the metal and the complexing agent, at the end of the treatment the spent solution with a residue concentration of both metals and complexing agent must be subjected to a further treatment. Due to organic nature of the complexing agent, this step could be a biological treatment. Since the more recent issues in the field deals with the use of biodegradable complexing agents, in this work a traditional chemical chelant, the EDTA, was compared to Rhamnolipid, a biosurfactant produced by Pseudomonas, already tested as potential extractant agent in the remediation of contaminated soil and sediment. Experimental tests were performed by OUR tests technique at selected Rhamnolipid or EDTA ratio with respect to Cu concentration in the sample in order to assess the toxicity of copper and copper complexes towards a consortium of microorganisms collected from a lab-scale activated sludge plant. Results show that, for both the additives, microbial respiration rate decreased when their concentration increased, though Rhamnolipid always caused a lower inhibition with respect to EDTA (less than 32% against 93% at the higher concentration).