Characterization of Al-Humic ComplexationCharacterization of Al-Humic Complexationand Coagulation Mechanismand Coagulation Mechanism

Removal of natural organic matter (NOM) by coagulation using metal coagulants (aluminium or ferric salts) is a common practice in surface water treatment. However, there are still many things unknown regarding the interactions between the soluble organic molecules and the metal coagulants. In this study, the authors applied fluorescence analysis and online optical monitoring techniques in a series of jar-tests for coagulation of humic acid (HA) by aluminium sulphate (alum). Attention was mainly paid to the characteristics of aluminium hydrolysis and Al-humic complexation. On this basis the mechanisms of Al-humic coagulation were also discussed.

IntroductionIntroduction

Results & DiscussionResults & Discussion

As shown in Figure 2, the output signal of PDA2000 is expressed as flocculation index (FI curve), and the variation of the FI curve with agitation time reveals the process of alum floc formation resulted from hydrolysis and precipitation. At pH 5.0 with a low alum dose of 0.1 mM, no precipitated aluminium particles are detected in the whole period of agitation. As alum dose increases to 0.2 mM, slight increase is noticed after agitation for 20 min, indicating the formation of small flocs of aluminium precipitates. Contrarily, at pH 7.0 with even a low alum dose of 0.1 mM, noticeable formation of tiny particles are noticed from the beginning of agitation and after 10 min particle growth becomes more apparent. At higher alum dose of 0.2 mM, larger flocs of aluminium precipitates form quickly. By comparing the results of Figure 1 and Figure 2, it can be concluded that at pH 5.0 the aluminium species which can react with HA molecules to bring about Al-humic complexation are soluble aluminium ions, while at pH 7.0 hydrolysis of aluminium ions preferentially occurs and only after they form aluminium precipitates, can reaction begin between these precipitates and HA molecules. Such a reaction is believed to be a process of adsorption or sweep coagulation rather than Al-humic complexation. However, sweep coagulation may also play important role at pH 5.0 as alum dose is very high.

ConclusionsConclusions

Figure 1 is the fluorescence analysis result which shows the condition of Al-humic complexation in a raw water of HA concentration of 10 mg/l as TOC at two typical pH values. At pH 5.0, there is almost a proportional relationship between the dosed Al and unreacted Al, which indicates that the dosed Al ions are reacting with HA molecules at a constant rate. It can be estimated that the ratio of reacted and dosed Al is about 0.88 regardless of the dosed concentration. At pH 7.0, there is firstly a linear relationship with a slope of 1/1 between the unreacted and dosed Al until a concentration about 0.11 mM, and then there is a sudden decrease of the unreacted Al. After the dosed Al increases to 0.3 mM, the unreacted Al reaches an almost constant value of 0.7-0.8 mM. 

Figure 2 FI Curves of Aluminium Precipitates at pH 5.0 and 7.0Figure 3 Characteristic parameters of FI curve

and coagulation zone at pH 5.0

Al-humic coagulation shows different characteristics in different pH ranges: at pH 5.0, soluble Al ions react preferably with HA molecules forming Al-humic complexes, while at pH 7.0, aluminium hydrolysis firstly happens and then adsorption or sweep flocculation occurs to bring about combination of HA molecules with the hydrolysed aluminium precipitates.

Figure 1 Relation between dosed and unreacted aluminium

Al-humic binding at pH 5.0 and 7.0Al-humic binding at pH 5.0 and 7.0

Aluminium hydrolysis at pH 5.0 and 7.0Aluminium hydrolysis at pH 5.0 and 7.0

Mechanisms of Al-humic coagulationMechanisms of Al-humic coagulationBased on the results of a series of experiments at pH 5.0, a diagram was formulated as Figure 3, where s and h are the characteristic parameters of the FI curves – s as the gradient of the rising front of the curve and h as the height of the plateau it finally reached. Correlative relations are apparent among s, h, TOC removal and zeta potential of the coagulated particles. Four coagulation zones can thus be divided according to the destabilization states: Zone I is the stabilization zone where the concentration of the dissolved aluminium species is not enough for Al-humic complexation to bring about sufficient charge-neutralization; Zone II is the destabilization zone where the alum dose is optimum to result in favourable Al-humic complexation and charge-neutralization; Zone III is the restabilization zone where extra alum dose causes charge reversal which hinders Al-humic coagulation; and Zone IV is the sweep coagulation zone where large amount of aluminium precipitates form as the result of extremely high alum dose.

I Stabilization; II Destabilization; III Restabilization; IV Sweep Coagulation

Acknowledgement: This study is supported by the National Natural Science Foundation of China (Grant No. 50278076)

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