A Study On The Chemical, Morphological Characteristics And Kinetics Of Coagulation Of Humic Substances

Humic acids (HA) affect the color, odor in water and are precursors of THMs. They are key controlling pollutants in drinking water. However, systematic and thoroughly investigation has not been carried out on the chemical, morphological characteristics and kinetics of coagulation of HA. Using a dvanced instrumental and chemical analysis, this paper mainly studied the physical and chemical characteristics of HA, the chemical and morphological properties of Al-humic floes and coagulation kinetics.The element components, molecular weight (MW) distribution, functional groups and structures of HA were studied by element analysis, HPLC, FT-IR and GC-MS techniques. It showed that the main components of HA are C, H, O, N which account for 96 percent, and about 87 percent of HA molecules are below 6000 Da. It was found that the main functional groups on the HA molecules include carboxyl, hydroxyl, ketone, phenol, ether and ester which are linked by benzene and hydrocarbon chains to form the framework of macromolecules.Jartest was conducted for the coagulation of HA with aluminum as coagulant at different pH values. The results showed that at pH 5.0, the removals of TOC and UV₂₅₄ were 50% and 90% respectively. Similar removal could be achieved at neutral pH of 7.0, but the required Al dose was about 5 times of that at pH 5.0. Combining with ζ potential analysis result, it can be concluded that at pH 5, the main hydrolyzed species of aluminum are less polymerized ones with higher electric charge density so that charge-neutralization and co-precipitation becomethe main mechanisms of coagulation, while at pH 7 the main hydrolyzed species are precipitated aluminum hydroxides so that sweep flocculation becomes the main mechanism of coagulation.HPLC analysis results showed that HA with MW higher than 1000 Da could be effectively removed by coagulation. By means of FT-IR, GC-MS and FR analyses, it was further revealed that Al-humic co-precipitation occurred mainly at pH 5.0 while sweep flocculation occurred mainly at pH 7.0. At pH 5.0, almost all organic matters with carboxyl and hydroxyl groups could be removed by coagulation, while at pH 7.0, many carboxyl groups could not be removed. Functional groups containing nitrogen could not be removed at both pH values by alum coagulation. By taking the functional groups on the HA molecules which can react with aluminum as 'activated biding sites', a continuous multiligand distribution model was worked out and quantitative evaluation of the 'reactivity' of HA with aluminum became possible by a comparison of the conditional equilibrium constants at pH 5.0 and pH 7.0. Because the conditional equilibrium constant at pH 5.0 is much larger than that at pH 7.0, it is theoretically proved that aluminum ions tend to combine with the functional groups on the HA molecules and to form Al-humic complexes at pH 5.0, which provides an optimum condition for the coagulation of humic acids.By means of a photometric dispersion analyzer (PDA2000), online monitoring of the Al-humic coagulation process was realized. The output signal of PDA2000 is taken as flocculation index (Fl curve). The variation of the Fl curve with agitation time revealed the process of floc growth, and the parameters of the Fl curve was found to be correlative with floc size. At pH 5.0, the Fl curve varied in different ways in different range of coagulant dosage: at low coagulant dose, the rising velocity and the equilibrium height of the Fl curve increased with alum dose; at higher coagulant dosage, the rising velocity and the equilibrium height decreased with alum dose; and at still higher coagulant dosage, Fl curve rose to a maximum height, then decreased and finally reached a plateau. With a comparison with the ζ potential measurement results, the above mentioned three states were thought to correspond with the states of destabilization, restabilizationand sweep coagulation respectively. On the contrary, at pH 7, the rising speed and equilibrium height only increased with the increase of coagulant dosage, and sweep coagulation...