Dynamics of Zn, Cu, Ni, Pb and Cd extraction from industrial and municipal sewage sludge using EDTA and citric acid solutions

Sewage sludge generated in wastewater treatment plants accumulates varied contaminations of inorganic as well as organic character. Among them, heavy metals constitute a group of chemical substances which are highly dangerous for natural environment, because of the possibility of their mobilization and migration in environment as a result of changes in surrounding system. Moreover, they are pollutants toxic for humans, non-biodegradable and accumulate in living organisms. In recent years, there has been a great interest in the removal of heavy metals from sewage sludge with different chemical compounds solution and especially metal chelating agents. The goal of this work was to determine the optimal time for extraction of selected metals (Zn, Cu, Cd, Ni, Pb) from contaminated industrial and municipal sewage sludge by using 0.1M solutions of EDTA (disodium salt of ethylenediaminetetraacetic acid), as well as citric acid. The analyzed wastewater sludge was collected from metallurgic industry treatment plant and from municipal wastewater treatment plant. In sewage sludge samples, the principal parameters were determined and after mineralization with mixture of concentrated mineral acids: HNO3 and HCl (1 : 3) the total content of analyzed heavy metals was detected using atomic absorption spectrophotometry. Chemical extraction of metals from sewage sludge 5g samples was carried out with 50cm(3) portions of 0.1M EDTA and citric acids solution for different periods of time, i.e. from 0.5 to 24 hours. After given time of shaking, the liquids were decanted, filtered through a filter paper, and finally the extracts were analyzed for heavy metals content with AAS method. It was observed that both applied solutions effectively washed metals from studied sewage sludge samples. For the period of time from 0.5 up to 5.0 hours, the concentration of almost all metals increased linearly and in the 6th hour of extraction the process was limited and finally for the period of time from 8.0 up to 24 hours, the detected metal contents in extracts were practically the same. The analysis of dependences of metal concentration versus time of extraction yielded in the optimal time of process, what value was determined as equal to 6 hours. The efficiencies of the metals removal from the studied sewage sludge by mans of the analyzed solutions of extractants were compared. Copper and lead reached higher concentration in liquids after EDTA extraction. It indicated that EDTA solution was a more effective extractant for the afore-mentioned metals. The concentration of zinc was higher in citric acid solution and citric acid was determined as more effective compound for zinc removal. For nickel and cadmium, the concentrations of metals were similar in both type of washing solution, which indicated that the efficiencies of both solutions were comparable.