Effect of pH and shear force on flocs characteristics for humic acid removal using polyferric aluminum chloride–organic polymer dual-coagulants

doi:10.1016/j.desal.2011.07.065

Desalination

Volume 281, 17 October 2011, Pages 243-247

https://doi.org/10.1016/j.desal.2011.07.065 Get rights and content

Abstract

The growth, breakage, regrowth and fractal nature of the aggregated flocs formed by a new composite coagulant, polyferric aluminum chloride–polydimethyldiallylammonium chloride, with humic acid were investigated. Different solution pH and various shear forces were investigated to evaluate the size, strength and structure of the flocs. The results showed that the flocs generated in acid conditions were more resistant to shear and more recoverable than those did in alkaline conditions, with the results suggesting that the order of the floc strength and recoverability was pH 5 > pH 6 > pH 8. The ability of flocs to resist breakage decreased as the shear rate increased and the reformation of flocs after high shear was limited. Fractal dimension analysis indicated that the flocs formed in acid pHs were more compact than those formed in alkaline pHs.

Highlights

► A new composite coagulant was prepared for experiments. ► The breakage, regrowth and fractal nature of the flocs were investigated. ► The order of floc strength and recoverability was pH 5 > pH 6 > pH 8.

Introduction

Coagulation is considered as an efficient way for removing organic matter in water treatment works [1]. As the primary organic compounds, humic substances are significant in water treatment for several reasons. They are anionic charged colloids that can complex with heavy metals and organics [2], [3]. Most significantly, they are the precursors of carcinogenic disinfection by-products (DBPs) formed after chlorination [4], [5]. In recent years, humic substances and their removal by coagulation have been paid considerable attention [6], [7], [8].

Inorganic and organic coagulants are the most commonly applied coagulants in water treatment. With the advantages of both Fe (III) salts and Al (III) salts, polyferric aluminum chloride (PFAC) has been developed [9].

In comparison with inorganic coagulants, organic polymeric coagulants have inherent advantages of higher molecular weight, less pH dependence and increased aggregation capacity [10], while the high cost of organic coagulants limits their comprehensive application in water treatment. In order to utilize the advantages of both inorganic and organic coagulants, the composite inorganic–organic flocculants has recently become research hotpot. In this paper, a new composite inorganic–organic flocculant, polyferric aluminum chloride–polydimethyldiallylammonium chloride (PFAC–PD), was developed to increase the aggregating ability of the coagulant.

Floc properties, such as floc size, strength and regrowth, play a significant role during coagulation process. Boller and Blaser [11] found that small particles generally have lower removal efficiencies in solid/liquid separation process following flocculation. Smaller particles showed lower removal efficiencies than larger particles of the similar density. Generally, floc would be broken into smaller clusters in the regions of higher shear forces. Flocs must resist these stresses to prevent being broken up. Researchers also reported that broken flocs were difficult to regrow, because their surface charge may be altered and partially re-stabilized [12], [13]. Additionally, the collision efficiency, floc strength and settling rates may be affected by floc shapes [14], which have been described in terms of fractal dimension of the aggregates. Therefore, floc strength, regrowth and fractal structure are important to evaluate the coagulation efficiency of coagulants. Floc characteristics of coagulants were comprehensively influenced by the coagulation conditions such as pH value and shear rates.

In this paper, the major objectives were to: (1) measure the strength and regrowth properties of flocs under different solution pH and shear force by laser diffraction instrument in treating synthetic water; (2) investigate the fractal dimension of flocs under various solution pHs; (3) study floc size distribution of flocs under different shear force; and (4) discuss the relationship between flocs properties and coagulation mechanism.

Section snippets

Preparation of coagulants

The PFAC–PD used in this work was prepared at room temperature. Firstly, a pre-determined amount of FeCl₃·6H₂O (AR) and AlCl₃·6H₂O (AR) was dissolved in distilled water. Then, Na₂CO₃ powder was continuously added to the above solution within 30 min to the desired basicity (B, OH/(Fe + Al)). After that, Na₂HPO₄ (AR) was added to the solution as a stabilizer. Finally, amount of polydimethyl diallylammonium chloride (PDMDAAC) solution (40% w/w aqueous solution, 100% charge density, and 1.21 dL g⁻¹

Floc breakage and regrowth

Fig. 1 demonstrated the floc d₅₀ for flocs formed under different solution pH of 5, 6, and 8 throughout the period of growth, breakage and regrowth. In the growth period, d₅₀ of the flocs decreased as the pH increased. Solution pH 5 gave rise to maximum d₅₀ of about 600 μm. This phenomenon could be attributed to the coagulation mechanism. At acidic solution pHs, polyvalent hydroxyl complexes of PFAC–PD polymerized and formed longer chain polymer than PFAC–PD did. When microflocs formed from

Conclusions

The breakage, regrowth, fractal nature and floc size distribution of the flocs formed by PFAC–PD were investigated with humic acid solution under different solution pH and shear rate. A possible mechanism reason for the irreversible breakage was also provided. For PFAC–PD, the flocs generated in acidic HA solution were stronger and more recoverable than those generated in alkaline solution. Floc strength decreased with the shear force increasing. The flocs generated in acid pHs were more

Acknowledgements

This study was supported by the National Natural Sciences Foundation of China (no. 2008ZX07422003-02, 2006BAJ08B05-2 and 50808114). The advice and kind suggestions of the anonymous reviewers are gratefully acknowledged.

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Effect of pH and shear force on flocs characteristics for humic acid removal using polyferric aluminum chloride–organic polymer dual-coagulants

Abstract

Highlights

Introduction

Section snippets

Preparation of coagulants

Floc breakage and regrowth

Conclusions

Acknowledgements

Chemosphere

Colloids Surf., A

J. Colloid Interface Sci.

Colloids Surf., A

Water Sci. Technol.

Water Res.

J. Hazard. Mater.

Int. J. Miner. Process.

J. Colloid Interface Sci.

Water Sci. Technol.

Environ. Sci. Technol.

J. Colloid Interface Sci.

Water Res.

J. Colloid Interface Sci.

Water Res.

Water Res.

Water Res.