Removal mechanisms of cadmium and lead ions in contaminated water by stainless steel slag obtained from scrap metal recycling

In this work, steel slag was shown to be a successful filter material for the removal of lead and cadmium from natural waters with a removal efficiency of >95 %, even under acidic conditions (pH 3.5). Overall, precipitation was found to be the main removal mechanism over adsorption with a tight relationship with the final matrix pH. Batch experiments showed fast removal kinetics with >98 % removal efficiency for lead and cadmium in 5 and 15 min, respectively. The removal kinetics followed a pseudo second-order reaction (R2 = 0.999), and the K2 was calculated to be 927 h−1 and 204 h−1 for lead and cadmium, respectively. A strong interference for the removal was observed with ethylenediaminetetraacetic acid (EDTA) but slightly with fulvic and humic acids, and non-significant with polymeric acid e.g. isosaccharinic acid. In addition, the column experiment showed spontaneous removal kinetics for lead and cadmium with a contact time of 3.6–36 seconds depending on loading flow-rates. Column experiments at similar liquid to solid ratios showed a high removal capacity with gradient over isocratic solution loading. The high removal efficiencies in combination with limited hazardous metals leaching led to the conclusion that slag is an appropriate filter material for treating contaminated waters. Moreover, the slag treated solutions with alkaline conditions showed reacted efficiently with CO2 in flue gas. Consequently, the pH of the treated solution is neutralized making it easier to waste and this reaction may contribute to reducing CO2 gas emissions. © 2020 Elsevier Ltd