Study On The Adsorption Characteristics Of Aluminum-Containing Scales To Heavy Metals In Drinking Water Distribution System

Aluminum-containing scales in water distribution systems are of increasing interest because of their implications in several new areas of regulatory compliance and their impact on distribution system water quality. One beneficial role they can play is that of providing a barrier to corrosion of distribution system materials and to leaching of regulated metals, or they can provide a substrate to which trace metals adsorb. Conversely,unpredictable short term elevations of metal concentrations could result if water quality changes caused them to desorb, or if scale particles were destabilized and released, along with any sorbed or co-precipitated trace metals. This paper aims to determine the adsorption characteristics of typical Al-containing scales to heavy metals on drinking water pipe systems.In the research, modeling pipe systems were built with amorphous Al(OH)3 scale, aluminum phosphate scale and aluminosilicate scale, respectively, to the adsorption of Cd²⁺, Cu²⁺, Mn²⁺, Ni²⁺, Pb²⁺and Zn²⁺. It was found at pH=7.5, 1g Al-containing scale surface could adsorb Cd²⁺0.4mg, Cu²⁺0.4mg, Mn²⁺0.5mg, Ni²⁺0.65mg , Pb²⁺0.7mg, Zn²⁺0.2mg. When the environmental condition of the pipe system changed, the outflow heavy metal concentrations might be affected. Once the inflow heavy metal concentrations increased or decreased, their concentrations in the outflow showed the same variation tendency. When the inflow pH changed from 7.5 to 6.5, the concentrations of Cu²⁺ and Pb²⁺ in the outflow would increase 20%, while other heavy metals were not affected; when the inflow pH increased to 8.5, the concentrations of Cu²⁺ , Pb²⁺ and Zn²⁺ in the outflow decreased 20%, while Cd²⁺, Mn²⁺, Ni²⁺were not affected. Low flow velocity did not affect the adsorption characteristics of the whole system, but high flow velocity coursed both the concentrations of Cu²⁺ and Pb²⁺ in the outflow in the three scales (amorphous Al(OH)3, AlPO4, and aluminosilicate scale), and Ni²⁺ on the aluminosilicate scale increased 32%.In order to observe the adsorption capacity of Al-containing scale, the adsorption characteristics of amorphous Al(OH)3 deposition, AlPO4 deposition and aluminosilicate deposition to Cd²⁺, Cu²⁺, Mn²⁺, Ni²⁺, Pb²⁺and Zn²⁺were investigated at complete mixing condition. Al-containing deposition showed stronger adsorption capacity to heavy metal ions at complete mixing condition than at surface adsorption. At pH=6.5, 1g Al-containing scales could immediately adsorb Cd²⁺1.8mg, Mn²⁺3.0mg, Ni²⁺1.3mg, Zn²⁺2.7mg, Cu²⁺6.0mg, Pb²⁺5.2mg; When the solution pH was7.5 or 8.5, heavy metal concentrations below 1.0mg/L, Cu²⁺, Mn²⁺, Pb²⁺and Zn2 were all adsorbed , while the residual Cd²⁺ and Ni²⁺ concentrations increased the same ratio with the initial ion concentrations changed.Al-containing scales in water supply system have a strong adsorption capacity to heavy metals irons. Low flow velocity, high solution pH and low heavy metal irons concentrations did not have a bad affect on the adsorption of all the selected metal ions on the Al-containing scales; while, their concentrations in the outflow would increase. Considering the Al-containing scale adsorption to heavy metal irons was meaningful for the tap water quality control.