Studies On Removal Of Thallium And Residual Aluminum From Drinking Water By Coagulation

Both thallium and aluminium are important metal pollutants in drinking water,their presence could make people poisoned or even thallium can cause fatality if itsconcentration is high. The drinking water standards in China for thallium and aluminumare0.1μg/L and0.2mg/L, respectively. Therefore, to remove thallium and residualaluminum from drinking water is a major issue related to safe drinking water supply. Inthis paper, removals of thallium and residual aluminium from drinking water have beenstudied by coagulation, and the details of the tests and results are listed below:Thallium removal from water by coagulation is investigated with respect to thedependence of both Tl(I) and Tl(III) removal on pH, coagulant type and dosage, theinitial contaminant-loading level and HA. Results show that the optimum pH for Tl(III)with aluminum sulfate and polyaluminium chloride (PACl, B=2.3) was at8, while theoptimum pH with ferric chloride was12, at which the removal rate was more than90%.Ferric chloride has the broadest effective pH range, followed by PACl. Under theoptimum pH, the optimum dosage of ferric chloride to remove Tl(I) and Tl(III) was0.4and0.6mmol/L. For the three kinds of coagulants, initial contaminant-loading levelshad effect on removal of Tl(III), the higher the loading level, the greater was theremoval rate; they had no effect on removal of Tl(I). The removal efficiency of Tl(III)with these three coagulants was better than that of Tl(I), and HA markedly impededTl(III) removal but may promote Tl(I) removal. The characteristics of flocs formed byreaction between ferric chloride and thallium during the coagulation process wereperformed. It can be porposed that the mechanism for Tl(III) removal with ferric chloride was adsorption, and adsorption and coprecipitation for Tl(I) removal.The water turbidity and temperature of a reservoir in Beijing are low in winter, andthis results in excessive residual aluminum in finished water during this period of time.To solve this problem, the performances of polyaluminium chloride (PACl) and newcoagulants consisting of PACl and ferric chloride (FeCl3) were studied by jar tests. Theresults show that the best coagulation performance was achieved at a molar ratio of1/5for Fe/Al, with the lowest residual aluminum and the highest organic matter removalefficiencies. At this ratio, at the dosage of PACl0.03mmol/L, the lowest residualaluminum was at a very low concentration of0.0063mg/L. The turbidity removalefficiencies of iron and aluminum composite coagulants were lower than those of PAClat the same dosage. When the dosage of the composite coagulant with a Fe/Al molarratio of1/5was at0.06mmol/L, the treated water turbidity could be decreased to0.4NTU. In addition, the effective dosage cover range, thus easy for coagulant control intreatment plants; It is also suitable for treating source waters with normal watertemperature with a mild alkalinity. Pilot plant tests with a capacity of5m3/hdemonstrated the effectiveness of the Fe-Al composite coagulant after long time andcontinuous run.