The specific contaminated wastewater like metal
The specific contaminated wastewater, like metal finishing wastewater (MF-WW), is produced by the metal finishing industry, where plating and rinsing processes usually use BAM 22P solutions, toxic metals, solvents and cyanides to provide desired surface properties. However, the intermediate processes led to high concentration of metal ions in MF-WW, which has a low pH, high levels of total dissolved solid and contaminations from chromium (Cr), copper (Cu), lead (Pb) and zinc (Zn) (Sthiannopkao and Sreesai, 2009). Metal hydroxide precipitates could be generated through precipitating process of heavy metal ions in alkaline conditions by the addition of strong alkaline–LMP (pH 9.7–13.5). Moreover, amounts of small precipitate particles are freshly formed, and they have relatively high specific surface area (Zhang et al., 2011). Multiple irregular cavities gradually develop to cover on hydroxide particles in different diameters during the growing process of hydroxide particles, and they connected together to form network structure (Zhang et al., 2011). The growth process of hydroxide precipitates would also have the potential to facilitate adsorption and precipitation synergistically to remove heavy metals in wastewater compared with that to remove dyes (Zhang et al., 2011). The reaction can be expressed as shown in Eq. (7), where M extensively represents heavy metals, and x represents 2 or 3 ( Table 3). Meanwhile, a slight amount of soluble sulphides could be obtained by water extraction. Sulphide (S2−) precipitating process is usually induced to remove heavy metal ions such as Pb, Cu, Cr, Ag, Cd, Ag and Hg at pH range of 7.0–9.0 ( Crear, 2001), according to Eq. (8) ( Table 3). Although the sulphide content of LMP is low compared to the content of Ca (35%), O (18%) or Na (5%), sulphides are potential to precipitate metals according to the solubility product constant (Ksp) theoretically. Some heavy metal ions like Pb and Cu, for instance, could be reacted to form sulphide precipitates, CuS (Ksp = 1.2 × 10−37) and PbS (Ksp = 7.0 × 10−29) ( Crear, 2001).