Evaluation of the mobility of heavy metals in the sediments originating from the post-galvanic wastewater treatment processes.

The article presents the assessment of heavy metals mobility in sediments from the process of galvanic wastewater treatment (pH 2.5, Co 1.5 mg/L, Cr6+ < 0.02 mg/L, Cr(total) 62 mg/L, Cu 110 mg/L, Ni 129 mg/L and Pb 59 mg/L) based on the use of hydroxides (Ca(OH)2, NaOH) as well as inorganic and organic sulphur compounds (Na2S, sodium dimethyldithiocarbamate (DMDTC), sodium trithiocarbonate (Na2CS3), trimercapto-s-triazine trisodium salt, TMT). The leachability was assessed after 1, 7, 14 and 21 days of sediment contact with the leaching agent (deionized water). FeCl3 was used as a coagulant. The efficiency of metal removal changed within a range of 99.67-99.94% (for NaOH), 98.80-99.75% (for TMT), 99.67-99.92% (for DMDTC), 99.67-99.91 (for Na2CS3). The heavy metal content in the obtained precipitates changed within the following ranges: 0.1-0.2 g/kg (Co), 9.8-14.7 g/kg (Cr), 23.6-39.8 g/kg (Cu) 30.5-43.2 g/kg (Ni), 24.3-33.1 g/kg (Pb) and 12.2-18.7 g/kg (Cd). The leachability tests revealed the release of 34-37% of Cd, 6.4-7.5% of Ni and 0.06-0.07% of Cu after using an excess of Na2CS3 as the precipitant. The use of NaOH resulted in the release of 0.42-0.46% of Cr from the sediment, and the use of TMT 0.03-0.34% of Ni. The best immobilization of heavy metals was observed in the case of the precipitate resulting from the use of DMDTC as a precipitating agent. The findings may be useful for predicting the mobility of heavy metals in the sludge and assessing the risk involved so as to support their removal and management.

Evaluation of the stability of heavy metal-containing sediments obtained in the wastewater treatment processes with the use of various precipitating agents.

The article presents the results of research on the leachability of selected heavy metals (cadmium, nickel, chromium, cobalt, lead, and copper) from solid waste obtained in laboratory processes involved in the industrial treatment of wastewater generated in metal surface treatment plants. The test sludges were precipitated using sodium hydroxide solution, calcium hydroxide suspension, 45% solution sodium trithiocarbonate (Na2CS3), 15% solution trimercapto-s-triazine, sodium salt (TMT), and 40% solution sodium dimethyldithiocarbamate (DMDTC). The precipitates were treated with artificial acid rain and artificial salt water. After 1, 7, 14, and 21 days of leaching, the concentration of Cd, Co, Cr, Cu, Pb, and Ni in the leachate was determined. Artificial acid rain leached Ni and Cd to a maximum concentration of 724 mg/L and 1821 mg/L, respectively, from the sludge obtained after the application of Na2CS3, while artificial salt water leached Ni in the maximum amount of 466 mg/L and Cd-max. 1320 mg/L. When Ca(OH)2/NaOH was used, the leaching of Cr reached a similar level for both leaching agents, i.e., the maximum for artificial acid rain was 72.2 mg/L and the maximum for artificial salt water 71.8 mg/L. The use of Na2CS3 or Ca(OH)2/NaOH poses a risk of some heavy metals entering the environment, which may have a negative impact on living organisms, whereas the sludges obtained with the use of DMDTC and TMT as precipitants were the most stable under the experimental conditions and did not pose a potential environmental hazard.