Removal of Cd and Pb from Wastewater through Sequent Addition of KR-Slag, Ca(OH) Derived from Eggshells and CO Gas.

The effect of heavy metals in aqueous solutions has been addressed by several methods. Precipitation using lime, slaked or quick, is one of the commonly used techniques. In this work, KR-slag was used in addition to Ca(OH) that served as a seeding material. Furthermore, CO has been injected into the suspensions for pH stabilization, which further increases the removal efficiency. Accordingly, results have shown a great performance regarding both removal efficiency and reduced sludge production. More than 99% of Cd and Pb was removed with 1 g/L of KR-slag, 0.5 g/L of Ca(OH), and CO injection at a rate of 1 L/min. The effect of carbonation has been evaluated by examining the removal efficiency before and after carbonation. Following the injection of CO, removal efficiency has increased from 58.7 to 99.8 and 71.2 to 99.3% for Cd and Pb, respectively. Moreover, sludge volume from this treatment method was obtained as 103 mL/L, which is much less than the sludge volume obtained from the carbonation of only Ca(OH), that is, 361 mL/L. Leaching of residues was also conducted to evaluate the environmental performance of the removal process. After carbonation, there was a lower concentration of metals when leached out in a wide range of pH solutions. Contrarily, it was observed that a relatively higher concentration of metals was released in acidic solutions due to the substitution of metal ions (Cd and Pb) with H ions. Residues were then characterized by X-ray diffraction and differential thermal analysis/thermogravimetric analysis for phase identification. Both characterizations detected the presence of CaCO, which was an indication of the transformation of Ca(OH) to CaCO.