The Effect Of Water Quality And Conditions On Coagulation Behavior Of Al₁₃ Species In Poly-aluminum Chloride

In order to study the coagulation efficiency and behavior of the Al₁₃ species in poly-aluminum chloride (PAC), as well as the factors impacting the coagulation efficiency during the coagulation treatment process, the ethanol- acetone separation and purification method was used to prepare the Al₁₃ species from PAC solution. An in-depth study was conducted to compare the difference between Al₁₃ species, traditional aluminum salt AlCl₃ and PAC in coagulation properties at different water quality and coagulation conditions (such as water samples with different pollutants, different ions and hardness, respectively, and coagulation process of different mixing intensity and broken shearing). Then the best coagulation conditions and water quality for Al₁₃ species to exhibit its effectiveness were identified and the coagulation mechanism of Al₁₃ species in the process was explored. The main conclusions are as follows:(1) The three coagulants Al₁₃, AlCl₃ and PAC are proved effective to treat with the water samples of humic acid and humic acid- kaolin. The difference in coagulation efficiency of the three coagulants is little with the treatment of the humic acid-kaolin water samples. While during the coagulation process of humic acid water samples, strict coagulation conditions were required to remain better outflow quality. Especially for the monomer form of AlCl₃, the optimal coagulant dosage and pH range are narrow. PAC has slightly broader optimal dosage range and pH range than Al₁₃ species; on the whole the performance is not very different between the two. But, for Al₁₃ species, a small change (little lower or higher than the optimal coagulant dosage and pH range) in water quality results in a dramatical reduction in coagulation efficiency. That's to say strict coagulant dosage and pH range were required during coagulation process for Al₁₃ species.(2) With the ionic strength increasing, the coagulation efficiency of the three coagulants was significantly decreased, no matter for the treatment of humic acid samples or for humic- kaolin samples. And compared to the other two coagulants, Al₁₃ species can maintain better coagulation efficiency.(3) No matter what water samples were treated, humic acid or humic acid- kaolin, in the ion concentration discussed, cations' presence had little impact on the coagulation efficiency overall, except at the presence of NH₄⁺ the coagulation properties of Al₁₃ species and AlCl₃ were slightly inhibited. Anions could affect the coagulation efficiency dramatically for Al₁₃ species and AlCl₃ especially when water contains SiO₃^2-, HPO₄^2- and H₂PO₄^-, which was much more because of the hydrolysis of anions and aluminum salts.(4) During coagulation process, Al₁₃ species did not show superior properties of charge neutralization than AlCl₃ and PAC, which may because the interactions of coagulants and contamination are determined by the actual Al species in the coagulation process. Charge neutralization is not the only coagulation mechanism, and pollutants may be associated with Al³⁺ and its hydrolyzate to form the insolube chelating complex which is more easily removed.(5) The coagulation efficiency of aluminum salts with different Al species increased with the increase of total hardness. However, AlCl₃ remove humic acid effectively in different total hardness water, especially in high total hardness water. While the coagulation efficiency of Al₁₃ species would be the highest in low total hardness water.(6) Increasing total hardness leads to slower initial reaction rates for all tested coagulants, and lower growth rate for Al₁₃ species, wider flocs size range for AlCl₃. The largest flocs produced by AlCl₃, PAC and Al₁₃ species appears at total hardness of 8,4, 1 mmol·L^-1(1/2 Ca), respectively. For treating water with different total hardness, the reaction rate of monomer Al species is the fastest and the flocs size is the largest, whereas the reaction rate of Al₁₃ species is the slowest and the flocs size is the smallest. However, the flocs size range of Al₁₃ is the narrowest.(7) In Al₁₃ species treatment, although there is a longest lag time at the beginning of coagulation, the floc grew to the steady size with the fastest grow rate and the shortest grow time, and generated smaller flocs; while in AlCl₃ treatment, the floc size grew to the largest value with faster grow rate after the shortest lag time, however, the growth time of AlCl₃ treatment is the largest.(8) At any hydraulic gradient disscussed, the floc size was of the following hierarchy: AlCl₃ > PAC > Al₁₃ species. Fractal dimension in the long shear breakage period was larger than that in the short shear breakage period. Floc structure is independent of the number of breakage and re-formation cycle. The flocs formed by AlCl₃ have the largest fractal dimension, followed by PAC and Al₁₃ species, which means AlCl₃ formed the most compact flocs, while Al₁₃ species formed the loosest flocs.This thesis will provide a theoretical basis for enhancing the coagulation effeciency of aluminum coagulants and optimizing the coagulants selection, and provide a very important theoretical and practical significance for the research and development of high-performance aluminum coagulants.