2.1 Coagulation Chemistry & Colloidal Destabilization

2.1 Coagulation Chemistry & Colloidal Destabilization

Coagulation is the chemical conditioning process that alters colloidal and suspended surface charges, converting non-settleable particles into destabilized micro-flocs capable of forming macro-flocs during flocculation. For water treatment operators, mastering colloidal chemistry is critical for maintaining compliance with the Surface Water Treatment Rules, minimizing chemical costs, and preventing unprecipitated coagulant carryover into distribution systems.

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Colloidal Properties and Suspension Stability

Particulate impurities in raw surface waters are classified into three distinct size categories that dictate their physical behavior and separation mechanics:

1. Dissolved Solids: Particles smaller than 0.001 µm (1 nanometer), including simple inorganic salts, minerals, and low-molecular-weight organic compounds.
2. Colloidal Solids: Particles ranging from 0.001 to 1.0 µm in diameter, including fine clays (kaolinite, illite, montmorillonite), metal oxides, color-causing natural organic matter (humic and fulvic acids), bacteria, and viruses.
3. Suspended Solids: Particles greater than 1.0 µm, such as coarse silts, sands, algae, protozoan cysts (Giardia, Cryptosporidium), and microscopic debris.

Why Colloids Do Not Settle Under Gravity

In an unconditioned aqueous environment, colloidal particles remain suspended indefinitely due to two interacting physical phenomena:

• Brownian Motion: Sub-micron colloids possess negligible mass. Random thermal collisions from surrounding water molecules impart sufficient kinetic energy to overcome downward gravitational forces, keeping the particles in perpetual chaotic motion.
• Electrostatic Repulsion: Natural colloids carry a net negative electrical surface charge. As two colloids approach each other due to Brownian movement, their surrounding negative electrostatic fields generate repulsive forces that push them apart before attractive forces can take effect.

According to Stokes' Law, settling velocity ($v_s$) is directly proportional to the square of particle diameter ($d_p^2$):

$v_s = \frac{g (\rho_p - \rho) d_p^2}{18 \mu}$

Where $g$ is acceleration due to gravity, $\rho_p$ is particle density, $\rho$ is water density, and $\mu$ is dynamic water viscosity. Under quiescent gravitational conditions:

• Coarse sand (1.0 mm) settles 1 foot in roughly 3 seconds.
• Fine silt (0.01 mm / 10 µm) settles 1 foot in roughly 2 hours.
• Clay colloid (0.1 µm) requires approximately 20 to 50 years to settle 1 foot.
• Color colloids (0.001 µm) would require hundreds of years without chemical coagulation.

Origins of Colloidal Surface Charges

Aquatic colloids acquire their negative surface charges through three primary geochemical pathways:

• Isomorphous Substitution: In clay mineral crystal lattices, higher-valence cations are replaced by lower-valence cations without changing crystal structure (e.g., $Al^{3+}$ substituted for $Si^{4+}$, or $Mg^{2+}$ substituted for $Al^{3+}$), resulting in a permanent structural negative charge imbalance.
• Ionization of Surface Functional Groups: Organic macromolecules, bacteria, and algae expose carboxyl ($-COOH$), hydroxyl ($-OH$), and phenolic groups. At typical natural water pH levels (6.0 to 8.5), these acidic groups dissociate ($-\text{COOH} \rightarrow -\text{COO}^- + \text{H}^+$), imparting negative charges.
• Preferential Adsorption: Colloidal surfaces selectively adsorb natural organic anions (humic acids) and phosphate ions from solution.

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The Electrical Double Layer (EDL) and Zeta Potential

When a negatively charged colloid is immersed in water, it attracts dissolved cations (counter-ions) from the surrounding solution while repelling dissolved anions (co-ions). This electrostatic attraction creates a structured boundary known as the Electrical Double Layer (EDL).

Structure of the Electrical Double Layer

1. The Stern Layer (Fixed Layer): A compact, inner monolayer of dehydrated or partially hydrated positive counter-ions held tightly against the negatively charged colloidal surface by electrostatic and chemical adsorption forces. The electrical potential drops sharply across this fixed layer.
2. The Diffuse Layer (Gouy-Chapman Layer): Extending outward from the Stern layer into the bulk solution, this zone contains an excess of positive counter-ions whose concentration gradually diminishes with distance until reaching equilibrium with the bulk fluid.
3. The Shear Plane (Slipping Plane): The hydrodynamic boundary separating ions that travel with the moving colloid from the bulk water. As the particle moves, the water and ions within the shear plane move with it, while ions outside remain in the bulk fluid.

Zeta Potential Definition and Practical Significance

Zeta potential ($\zeta$) is defined as the electrical potential difference between the shear plane and the electroneutral bulk solution, measured in millivolts (mV). It is calculated from electrophoretic mobility using the Helmholtz-Smoluchowski equation:

$\zeta = \frac{4 \pi \mu U}{\varepsilon}$

Where $\mu$ is dynamic viscosity, $U$ is electrophoretic mobility, and $\varepsilon$ is dielectric constant.

• Natural Surface Water Range: Raw, untreated surface water colloids typically exhibit zeta potentials ranging from -15 mV to -30 mV.
• Agglomeration Threshold: When zeta potential is more negative than -15 mV, electrostatic repulsion creates an energy barrier that prevents particles from approaching close enough for short-range attractive van der Waals forces to take effect.
• Coagulation Objective: Coagulant addition compresses the double layer and neutralizes negative charges, lowering the absolute zeta potential toward zero—typically targeting -5 mV to +5 mV (or slightly negative, e.g., -3 to -8 mV, to avoid charge reversal). At this depressed potential, van der Waals attractive forces dominate upon collision, allowing particles to bond.

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The Four Colloidal Destabilization Mechanisms

Water treatment relies on four distinct physical-chemical mechanisms to overcome colloidal stability:

1. Charge Neutralization (Adsorption-Neutralization)

• Mechanism: Positively charged metal hydrolysis species (such as monomeric $Al^{3+}$, $Al(OH)^{2+}$, $Al(OH)2^+$ and polymeric complexes like $Al{13}O_4(OH)_{24}^{7+}$ or $Fe_3(OH)_4^{5+}$) adsorb directly onto the negatively charged colloidal surface.
• Kinetics: Extremely rapid microsecond reactions occurring within 10 to 100 milliseconds (0.01 to 0.10 s) of coagulant introduction.
• Operational Characteristic: Requires stoichiometric coagulant dosing proportional to colloidal surface area and charge concentration. If an operator over-doses coagulant, excess positive ions saturate the colloidal surface, shifting the zeta potential positive (+5 to +15 mV) and causing charge reversal (restabilization), where particles repel each other again and remain suspended as pin floc.

2. Compression of the Electrical Double Layer

• Mechanism: Increasing the ionic strength of the water by introducing dissolved electrolyte ions compresses the volume of the diffuse layer, drawing counter-ions closer to the particle surface.
• Schulze-Hardy Rule: The coagulating power of an electrolyte is proportional to the sixth power of the counter-ion valence ($z^6$):

$\text{Coagulating Power} \propto z^6$

For monovalent ($Na^+$, $z=1$), divalent ($Ca^{2+}$, $z=2$), and trivalent ($Al^{3+}$ or $Fe^{3+}$, $z=3$) cations:

• Monovalent: $1^6 = 1$
• Divalent: $2^6 = 64$
• Trivalent: $3^6 = 729$

Trivalent cations ($Al^{3+}$, $Fe^{3+}$) are approximately 700 to 1,000 times more effective at compressing the double layer than monovalent ions like sodium ($Na^+$), and roughly 50 to 80 times more effective than divalent ions like calcium ($Ca^{2+}$).

3. Sweep Coagulation (Enmeshment in Precipitate)

• Mechanism: When metal coagulant is dosed in excess of the solubility limit of the metal hydroxide at the operating pH, rapid precipitation of amorphous, gelatinous aluminum hydroxide ($Al(OH)_3(s)$) or ferric hydroxide ($Fe(OH)_3(s)$) occurs.
• Application: Essential in low-turbidity, low-particulate waters (< 5 NTU) where colloidal particle collisions are too infrequent for charge neutralization alone to create agglomerations. The sticky hydroxide precipitate creates a massive physical blanket that traps, enmeshes, and sweeps colloids out of suspension.
• Operational Characteristic: Requires high coagulant dosages and adequate raw water alkalinity. Overdosing does not cause restabilization in sweep coagulation mode, but it does dramatically increase chemical sludge production.

4. Interparticle Bridging

• Mechanism: Synthetic or natural high-molecular-weight polymers (polyelectrolytes) attach to multiple colloids simultaneously. A single long polymer chain adsorbs onto an active site on one colloid, while its loops and tails extend into the bulk water to anchor onto adjacent colloids, forming a physical structural bridge.
• Application: Typically utilized with high-molecular-weight nonionic or anionic polymers as coagulant aids or filter aids to produce large, dense, shear-resistant flocs.

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Influence of Raw Water Parameters on Coagulation

pH Operating Windows

Coagulant metal ions participate in complex hydrolysis equilibria governed strictly by solution pH:

• Aluminum Sulfate (Alum): Effective coagulation window is pH 5.8 to 7.5. Below pH 5.8, aluminum remains largely in soluble cationic forms ($Al^{3+}$, $Al(OH)^{2+}$) that fail to precipitate. Above pH 7.8, soluble aluminate anions ($Al(OH)_4^-$) predominate, causing elevated dissolved aluminum residuals that penetrate filters and cause post-precipitation turbidity in the distribution system.
• Ferric Salts (Ferric Chloride / Ferric Sulfate): Wide operating window of pH 4.0 to 9.0. Insoluble ferric hydroxide ($Fe(OH)_3$) precipitates across an extensive pH spectrum, making iron salts advantageous in low-pH, soft waters and high-pH softening plants.

Water Temperature Effects

Cold water (< 4°C / 39°F) significantly impairs coagulation efficiency:

• Viscosity Dampening: Water viscosity increases by ~38% from 20°C (1.00 mPa·s) to 4°C (1.57 mPa·s). Increased viscosity retards settling velocities and dampens micro-turbulent shear in rapid mix basins.
• Retarded Hydrolysis Kinetics: Chemical precipitation and floc formation reactions proceed much slower in cold water, requiring extended detention times or higher mixing energy.
• Operational Remedies: Plants utilize pre-hydrolyzed coagulants (PACl/ACH), introduce polymer coagulant aids, increase rapid mix velocity gradients, or optimize chemical dosing via frequent jar tests.

Turbidity and Total Organic Carbon (TOC) Dynamics

• Low Turbidity / High TOC: The most challenging water to treat. Dissolved organic matter consumes coagulant stoichiometrically through complexation before charge neutralization of mineral colloids can occur. Requires enhanced coagulation at depressed pH (5.5 - 6.3) to precipitate organic precursors.
• High Turbidity / Low TOC: Readily treated with moderate coagulant dosages via charge neutralization, as abundant particulate collisions accelerate floc formation.

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Coagulation Reference Tables

Table 1: Colloidal Destabilization Mechanisms

Table 2: Raw Water Parameters and Coagulation Performance