A Study On The Characteristics Of Coagulation And Metal-Humic Co-precipitation Using Different Coagulants

Humic acids lie everywhere in terrestrial and aquatic environments, which have been recognized as vehicles of metal ions and different organic pollutants. Researchers in the field of aquatic chemistry pay more and more attention to remove HA efficiently. This paper mainly studied the characteristics of coagulation and metal-humic co-precipitation of three coagulants reacting with humic acid.The jar-tests were conducted respectively for the coagulation of HA with alum sulfate, ferric chloride and polyaluminum chloride (PAC) as coagulants at different pH values. Combining the removals of TOC and UV₂54 with C, potential analysis results, it can be concluded that at pH 5.0, it was same that the optimal dose as mol of three coagulants. But the ratio of HA removal followed the order ferric chloride > alum sulfate > PAC. At neutral pH of 7.0, PAC had higher removal of HA than that of ferric chloride and alum sulfate, but the required PAC and ferric chloride dose (as mol) was about 1.5 times of aluminum dose in order to achieve the optimal removal of HA. HPLC analysis results showed that HA with MW higher than lOOODa could be removed most effectively when the coagulant was ferric chloride at pH 5.0 and pH 7.0, while aluminum was a little greater than ferric with MW under lOOODa at pH 7.0. At pH 5.0 or 7.0, ferric chloride and aluminum sulfate were greater than PAC for the removal of HA especially with MW in the range of 3000-6000Da. By means of a photometric dispersion analyzer (PDA2000), online monitoring of the Me-humic coagulation process was realized. Through comparing and analyzing the results of FI curve, the removal of TOC and C, potential, it was founded that aluminum had four obvious areas at pH 5.0 and ferric had the same trend, while FI curve of PAC varied in limited range of coagulant dosage. At neutral pH of 7.0, the change rule of FI curve of three coagulants was almost accordant, the sedimentation rate of Fe-humic was optimal and the floc size of PAC reacting with HAwas biggest.The co-precipitation characteristics of coagulation of HA and three coagulants were studied by fluorescence analysis, GC-MS and nuclear magnetic resonance (NMR) techniques. Combining with the results of hydrolysis process of three coagulants, FR analysis results demonstrate that the coagulation of HA by three coagulants at low pH is mainly due to co-precipitation. With the pH value increasing, the hydrolysis reaction of metal salt is more competitive than the complex-formation one, so the removal of HA is mainly caused by absorbing and sweep flocculating onto the precipitated metal hydroxides resulted from three coagulants hydrolysis. GC-MS analysis revealed that almost all organic matters with carboxyl and hydroxyl groups could be removed by coagulation at pH 5.0, while at pH 7.0, many hydroxyl, amidocyanogen groups could not be removed. ¹H NMR analysis results further proved that at the acidic condition, co-precipitation was the main mechanism of alum coagulation and Al-complex resulted from alum ion reacting with HA was very stable. At the neutral pH, the carboxyl and hydroxyl groups of HA or Al-complex was adsorbed by the floe of the alum hydroxide, some of which reacted again and formed the hexagonal ring which was more stable.