Effects of Ni2+ on aluminum hydroxide scale formation and transformation on a simulated drinking water distribution system

Abstract

Observations of aluminum containing sediments/scales formed within the distribution pipes have been reported for several decades. In this study, the effect of Ni2+ on the formation and transformation processes of aluminum hydroxide sediment in a simulated drinking water distribution system were investigated using X-ray diffraction spectrum (XRD), Fourier transform infrared spectrum (FT-IR), scanning electron microscope (SEM), and thermodynamic calculation methods. It was determined that the existence of Ni2+ had notable effects on the formation of bayerite. In the system without Ni2+ addition, there was no X-ray diffraction signal observed after 400d of aging. The presence of Ni2+, however, even when present in small amounts (Ni/Al=1:100) the formation of bayerite would occur in as little as 3d at pH 8.5. As the molar ratio of Ni/Al increase from 1:100 to 1:10, the amount of bayerite formed on the pipeline increased further; meanwhile, the specific area of the pipe scale decreased from 160 to 122m2g−1. In the system with Ni/Al molar ratio at 1:3, the diffraction spectrum strength of bayerite became weaker, and disappeared when Ni/Al molar ratios increased above 1:1. At these highs Ni/Al molar ratios, Ni5Al4O11⋅18H2O was determined to be the major component of the pipe scale. Further study indicated that the presence of Ni2+ promoted the formation of bayerite and Ni5Al4O11⋅18H2O under basic conditions. At lower pH (6.5) however, the existence of Ni2+ had little effect on the formation of bayerite and Ni5Al4O11⋅18H2O, rather the adsorption of amorphous Al(OH)3 for Ni2+ promoted the formation of crystal Ni(OH)2.