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

Abstract

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.

Keywords: Electroplating; Galvanic industry; Galvanic wastewater; Heavy metals; Mobility; Sediments.