Coagulation by hydrolysing metal salts

doi:10.1016/S0001-8686(02)00067-2

Advances in Colloid and Interface Science

Volumes 100–102, 28 February 2003, Pages 475-502

https://doi.org/10.1016/S0001-8686(02)00067-2 Get rights and content

Abstract

Aluminium and iron salts are widely used as coagulants in water and wastewater treatment and in some other applications. They are effective in removing a broad range of impurities from water, including colloidal particles and dissolved organic substances. Their mode of action is generally explained in terms of two distinct mechanisms: charge neutralisation of negatively charged colloids by cationic hydrolysis products and incorporation of impurities in an amorphous hydroxide precipitate (‘sweep flocculation’). The relative importance of these mechanisms depends on factors such as pH and coagulant dosage. Alternative coagulants, based on prehydrolysed forms of aluminium and iron, are more effective than the traditional additives in many cases, but their mode of action is not completely understood, especially with regard to the role of charge neutralisation and hydroxide precipitation. Some basic features of metal hydrolysis and precipitate formation are briefly reviewed and the action of hydrolysing coagulants is then discussed, with examples from the older literature and from some recent studies on model systems. Dynamic monitoring of floc formation and breakage can give useful insights into the underlying mechanisms. Although the results can be reasonably well explained in terms of established ideas, a detailed understanding of the ‘sweep flocculation’ mechanism is not yet available. There are also still some uncertainties regarding the action of pre-hydrolysed coagulants.

Introduction

Hydrolysing metal salts, based on aluminium or iron, are very widely used as coagulants in water treatment. ‘Alum’ or aluminium sulfate has been used for water purification since ancient times and was first mentioned by Pliny (approx. 77 AD). An interesting historical account has been given by Cohen and Hannah [1]. Hydrolysing coagulants have been applied routinely since early in the 20^th century and play a vital role in the removal of many impurities from polluted waters. These impurities include inorganic particles, such as clays, pathogenic microbes and dissolved natural organic matter. The most common additives are aluminium sulfate (generally known as ‘alum’), ferric chloride and ferric sulfate. Other products based on pre-hydrolysed metals are also now widely used, including a range of materials referred to as polyaluminium chloride.

Nearly all colloidal impurities in water are negatively charged and, hence, may be stable as a result of electrical repulsion. Destabilisation could be achieved along DLVO lines, either by adding relatively large amounts of salts or smaller quantities of cations that interact specifically with negative colloids and neutralise their charge. Highly charged cations such as Al³⁺ and Fe³⁺ should be effective in this respect. However, over the normal range of pH values in natural waters (say, 5–8), these simple cations are not found in significant concentrations, as a result of hydrolysis, which can give a range of products. Many hydrolysis products are cationic and these can interact strongly with negative colloids, giving destabilisation and coagulation, under the correct conditions of dosage and pH. Excess dosage can give charge reversal and restabilisation of colloids.

At around neutral pH both Al(III) and Fe(III) have limited solubility, because of the precipitation of an amorphous hydroxide, which can play a very important role in practical coagulation and flocculation processes. Positively charged precipitate particles may deposit on impurity particles (heterocoagulation), again giving the possibility of charge neutralisation and destabilisation. A further possibility is that surface precipitation of hydroxide could occur, with similar consequences. More importantly in practice, hydroxide precipitation leads to the possibility of sweep flocculation, in which impurity particles become enmeshed in the growing precipitate and thus effectively removed.

These additives can also remove dissolved natural organic matter (NOM), either by charge neutralisation to give insoluble forms, or by adsorption on precipitated metal hydroxide.

As well as simple hydrolysing salts, a range of commercial pre-hydrolysed coagulants is available. These contain cationic hydrolysis products and are often more effective than aluminium or iron salts.

Although the broad principles of action of these coagulants are reasonably well understood, there are still some uncertainties regarding the nature of the active species, the role of other salts, especially anions, in water, and the nature of the aggregates formed. The mode of action of pre-hydrolysed agents is not yet fully understood. A review of the current state of knowledge will be given, with some examples of recent experimental results on model systems.

Section snippets

Monomeric hydrolysis products

All metal cations are hydrated to some extent in water. It is reasonable to think in terms of a primary hydration shell, where water molecules are in direct contact with the central metal ion, and more loosely held water in a secondary hydration shell. In the cases of Al³⁺ and Fe³⁺, it is known that the primary hydration shell consists of six water molecules in octahedral co-ordination [2]. Owing to the high charge on the metal ion, water molecules in the primary hydration shell are polarised

General

Natural waters contain a very wide variety of particulate impurities. These include inorganic substances such as clays and metal oxides, various organic colloids and microbes such as viruses, bacteria, protozoa and algae. Aquatic particles cover a broad range of particle size, from nm to mm dimensions and present a significant challenge in water treatment technology. For smaller particles, separation efficiency can be greatly enhanced by aggregation to give an increased size

Pre-hydrolysed coagulants

As well as traditional coagulants, based on Al and Fe salts, there are now many commercial products that contain pre-hydrolysed forms of the metals, mostly in the form of polynuclear species (see Section 2.2). In the case of Al, most materials are formed by the controlled neutralisation of aluminium chloride solutions and are generally known as polyaluminium chloride (PACl). It is believed that many of these products contain substantial proportions of the tridecamer Al₁₃. Some information on

Interaction with dissolved organic matter

So far, we have only considered the removal of particles from water by hydrolysing coagulants. However, many natural waters contain dissolved organic substances, which also need to be removed. Natural organic matter (NOM) in water may impart undesirable colour to water and some constituents can form carcinogenic substances when water is chlorinated. NOM consists of a huge variety of organic compounds including simple sugars, amino acids, organic acids, proteins and many others. In most cases,

Conclusions

Although the broad principles governing the action of hydrolysing coagulants are reasonably well understood, there are several important gaps in knowledge, which are of both fundamental and practical interest. For simple aluminium and ferric salts at low dosages, it is well established that charge neutralisation can be an effective means of destabilising colloidal particles. The precise nature of the cationic species is not known in detail, but it is likely that some form of surface

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Coagulation by hydrolysing metal salts

Abstract

Introduction

Section snippets

Monomeric hydrolysis products

General

Pre-hydrolysed coagulants

Interaction with dissolved organic matter

Conclusions

Geochim. Cosmochim. Acta

Water Res.

J. Inorg. Biochem.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Water Res.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Water Res.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

J. Colloid Interface Sci.

Water Res.

J. Colloid Interface Sci.

Colloids Surf.

Chem. Eng. Sci.

J. Coll. Sci.

J. Inorg. Biochem.

Colloids Surf.

Colloids Surf. A

Water Res.

The Chemistry of Aqua Ions

Chem. Rev.

J. Chem. Soc. Dalton Trans.

J. Phys. Chem.

Acta Chem. Scand. A

Soil Sci. Plant Nutr.

J. Phys. Chem.

ACS Adv. Chem. Ser.

J. Am. Water Works Assoc.

Water Res.

Langmuir

Langmuir