Study of the removal of heavy metals (Cu,Cd, and Zn) by chemical precipitation

Abstract

In this research the efficiency of chemical precipitation was studied for the removal of toxic heavy metals (copper(II), cadmium(II), and zinc(II)). Lime (Ca(OH2), caustic soda (NaOH), and soda ash (Na2CO3) are the used commercial precipitating agents. In addition, calcined eggshell (CaOEggshells) was also used as a synthetic comparative precipitant to conventional lime. CaOEggshells was produced by calcination of Eggshell powder (CaCO3) at 800°C for 7 h. For the individual metal removal (Cu(II), Cd(II), and Zn(II)), about 99.99 % of total removal was obtained for Cu(II) and Cd(II), and more than 97 % for Zn(II) using 400 mg/L of commercial precipitants. The optimum pH (OpH) slightly varied from one metal to another. Around 9.33 of final pH (pHf) was enough to reach the maximum removal of Cu(II), but for the maximum Cd(II) removal, at least 10.5 of pHf was required. While, raising the pH to 10.86 was needed to reach the maximum removal of Zn. Among all the commercial precipitants, NaOH was the suitable one for the individual metal removal. To make the medium more complex, the heavy metals (Cu(II) + Cd(II) + Zn(II)) were combined. The removal efficiency of combined metals remains significant, with over 99 % reached for all metals by using 400 mg/L of precipitants. Na2CO3 is the best agent for the removal of combined metals at 𝑂𝑝𝐻≤10. It has the advantage of removing metals at lower pH compared to other hydroxide agents. The CaOEggshells is also a good choice to remove the combined heavy metals, especially for Cu(II) by 99 % of removal and about 97 % for Cd(II) and Zn(II). In the combined metals system, it is difficult to control the range of pH in order achieve maximum removal of the three metals simultaneously. The sludge analysis of combined heavy metals indicates that the sludge volume produced by hydroxide precipitants was more significant than carbonate precipitates due to their gelatinous form. In contrast, the mass of carbonate precipitates was higher than that of the hydroxide precipitates. To confirm the good efficiency of chemical precipitation in synthetic solution of distilled water, chemical precipitation was applied at two different industrial wastewaters charged with heavy metals. The ENICAB industrial wastewater (Biskra city, Algeria) contained important concentrations of Cu(II) (18.10 mg/L) and Zn(II) (10.21 mg/L). After the treatment using Ca(OH)2, NaOH and Na2CO3, the process removed more than 99 % of Cu(II) and Zn(II) at pHf = 9.21 and 9.96 using 300 mg/L of precipitating agent. Sludge characterization showed that the average particle size of carbonate precipitates was larger than hydroxide precipitates. However, their sludge volume was smaller. In Galvanic Wastewater (Padova city, Italy), charged largely on Cu(II) (51.55 mg/L) and Ni(II) (36.66 mg/L), 1600 mg/L of Ca(OH)2 and CaOEggshells removed about 93% of Cu(II) and Ni(II) at a very basic pH (>12). While, this removal was not conform to maximum concentration standards. Sludge volume produced by using Ca(OH)2 was higher than that produced by using CaOEggshells. The opposite was happened with precipitant mass. SEM and EDX results were discussed at three different precipitating agent doses, in the purpose of following the chemical precipitation stages by showing the different changes in the structure of precipitates and the appearance of new mineral elements at each stage.