The following dissertation contains a study of the removal of lead, zinc, and cadmium from sampled sediments using a process known as sediment washing. Sediment samples were collected near the Drava River (north side of Lake Ptuj). Two sediment samples were taken: sediment S (old), dredged from the river 15 years ago, and sediment N (new), dredged one year ago. A series of analyses were performed: sediment was characterized by determining water content (S 4.5 % and N 8.9 %) and loss on ignition (LOI) (S 12.6 % and N 10.8 %). Total metal content of lead, zinc, and cadmium was determined by means of acid digestion of the sediment with nitric(V) acid. The approximate values are 400 ppm for lead, 1100 ppm for zinc and 7 ppm for Cd. The same was done for a sediment sample, which was separated by particle size. It was observed that the heavy metal content increases with decreasing particle size. Analyses of metal removal effectiveness were performed using various chemical reagents: HNO3, HCl, citric acid, humic acid, rhamnolipid, saponin, DL-lactic acid, salicylic acid, L-ascorbic acid, succinic acid, DL-malic acid, tartaric acid, and acetic acid. Sediment washing with EDTA, citric acid, and DL-malic acid was optimized. The conditions that were optimized were the concentration the of reagent, the washing time and the ratio between the volume of the washing solution and the mass of the sediment (R/S ratio). Sediment washing with two successive washing cycles was also investigated. All measurements used for the quantification of lead, zinc, and cadmium were done by flame atomic absorption spectroscopy (FAAS).
It was found that all reagents tested were able to remove lead, zinc, and cadmium from the sediment to some degree. EDTA (percentage of metals removed: Pb 40 %, Zn 15 % and Cd 55 %), citric acid (percentage of metals removed: Pb 50 %, Zn 60 % and Cd 75 %), and DL-malic acid (percentage of metals removed: Pb 45 %, Zn 60 % and Cd 75 %) proved to be the most successful. Under certain conditions, the two simple carboxylic acids removed even more of the metals than EDTA. For all three reagents, the dependence of their effectiveness of metal removal on sediment washing conditions was investigated by optimizing the procedure. The optimal conditions were also determined for these reagents: EDTA concentration 0.01 M, washing time 16 h and ratio R/S = 5/1; citric acid 0.2 M, 16 h and R/S = 10/1; DL-malic acid 0.3 M, 16 h and R/S = 10/1. Sediment washing with two successive washing cycles was carried out with citric acid and DL-malic acid and proved to be more effective in removing metals than sediment washing under optimized conditions.
At laboratory scale, the sediment washing process using different chemical reagents was demonstrated to be effective at removing lead, zinc, and cadmium. Further work will include: speciation of present metals by fractional analysis of the sediment, using a combination of two or more reagents for sediment washing, testing new reagents for sediment washing (phytic acid and phytates), applying the developed sediment washing procedures to CRM materials and other sediment types, and possible remediation of the leaching solution, which contains high concentrations of heavy metals after sediment washing.
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