Abstract:

Complexationultrafiltration process was investigated for selective separation of Hg(Ⅱ), Cu(Ⅱ) and Cd(Ⅱ) from mixture aqueous solutions. This process included the addition of poly(acrylic acid) sodium salt as a water soluble polymer to selectively bind the metal cations, which was followed by an ultrafiltration operation. Effects of pH and loading ratio(LR) on the selective separation were investigated. It was found that controlling pH=5 was helpful to achieve the selective separation. When LR increased from 0.01 to 2, metal separation coefficients(SHg-Cd and SHg-Cu) increased gradually, and then reached maximum values at LR=2. A concentration experiment was carried out at pH=5 and LR=2. Initial mass concentrations for Hg(Ⅱ), Cu(Ⅱ) and Cd(Ⅱ) were all 30 mg/L. Result showed that, when volume concentration factor(VCF) reached 15, mercury mass concentration of mercury(ρr,Hg), copper(ρr,Cu) and cadmium(ρr,Cd) in the retentate were 435.3 mg/L, 42.6 mg/L and 34.2 mg/L, respectively. SHg-Cd, SHg-Cu and SCu-Cd did not change with VCF, and were 229.3, 184.3 and 1.2, respectively. This indicated that Hg(Ⅱ) was retained selectively in the retentate by the ultrafiltration operation while nearly all Cu(Ⅱ) and Cd(Ⅱ) passed through the membrane into the permeate. Then, the final concentrated solution was used to study the washing process. With increasing the washing water volume, ρr,Hg remained nearly constant while both ρr,Cu and ρr,Cd decreased rapidly. After doubling the washing water volume, ρr,Cu and ρr,Cd reached 12.54 mg/L and 4.73 mg/L respectively. Further, another concentration experiment was performed at LR=0.033 and pH=5 by using the permeate solutions containing Cu(Ⅱ) and Cd(Ⅱ). At VCF=16, ρr,Cu increased linearly from the beginning of 27.34 mg/L to the end of 430.9 mg/L, whereas ρr,Cd changed only from 27.83 mg/L to 61.5 mg/L. SCu-Cd was 95.8, therefore copper was selectively concentrated.

Key words: Mercury(II), Copper(II), Cadmium(II), Poly (acrylic acid) sodium salt, Complexation, Ultrafiltration