Sonochemical Effect Induced by Hydrodynamic Cavitation

8 Chemistry in Inorganic Liquids and Aqueous Electrolytes

In hydrodynamic cavitation literature, chemistry in inorganic liquids most often means water containing acids, bases, salts, dissolved gases, oxidants, or inorganic precursors. These solutes rarely enter the bubble core because they are nonvolatile. Their transformation occurs at the interface or in the bulk through radicals, longer lived oxidants, extreme local supersaturation, and cavitation generated surfaces. Ionic strength also changes gas solubility, vapor pressure, interfacial tension, and bubble coalescence, so salts affect both bubble physics and chemical kinetics.

Figure 8.1. Principal chemical pathways in inorganic aqueous systems are exposed to hydrodynamic cavitation [81].

The Fenton reaction is a common route for converting accumulated hydrogen peroxide into hydroxyl radicals:

Persulfate can be activated thermally, mechanically, photochemically, or catalytically to form sulfate radicals. A simplified activation step is:

Bicarbonate scavenges hydroxyl radicals and forms the less reactive but more selective carbonate radical:

Cavitation generated reducing species can reduce metal ions or metal complexes:

where $e_{{\mathrm{red}}^{-}}$ denotes an effective reducing equivalent supplied by H•, organic radicals, or solvated electron like chemistry depending on the system.

For cavitation assisted crystallization, classical nucleation theory provides a useful baseline:

where J is nucleation rate, ${\gamma }_{\mathrm{sl}}$ is solid liquid interfacial energy, $v_m$ is molecular volume, $k_B$ is Boltzmann's constant, and $S_c$ is supersaturation ratio.

Supersaturation is defined as:

where C is actual solute concentration and C* is equilibrium solubility at the local temperature and pressure.

Cavitation can increase nucleation by several coupled routes: transient cooling or heating, rapid pressure change, formation of clean surfaces, fragmentation of early crystals, removal of adsorbed gas, and intense micromixing. It can also increase secondary nucleation by crystal collision and attrition. Equations (8.5) and (8.6) therefore describe only the thermodynamic contribution; practical controlled flow crystallization also depends on hydrodynamic history and crystal population balance [43-44].

Table 8.1. Influence of common inorganic ions and additives

Species	Primary interaction	Possible benefit	Possible adverse effect
Fe²⁺/Fe³⁺	Fenton and redox cycling	Converts H₂O₂ to •OH and intensifies oxidation	Sludge generation, pH sensitivity, catalyst loss
HCO₃⁻/CO₃²⁻	•OH scavenging and CO₃•⁻ formation	May increase selectivity toward electron rich compounds	Usually lowers nonselective oxidation rate
Cl⁻	Formation of chlorine radicals under strong oxidation	Can assist disinfection in selected conditions	Chlorinated byproducts and radical scavenging
Br⁻	Bromine radical chemistry	Fast electron transfer reactions	Bromate or brominated byproduct risk
SO₄²⁻	Changes ionic strength and interfacial behavior	Can influence coalescence and conductivity	May alter dosimetry and scaling tendency
S₂O₈²⁻/HSO₅⁻	Precursor to sulfate radicals	Extends oxidation and can improve selectivity	Reagent cost, residual sulfate, quenching requirement
I⁻	Sensitive oxidant dosimeter	Quantifies integrated chemical activity	Probe chemistry can perturb the radical field at high concentration
NO₂⁻/NO₃⁻	Products and reactants of nitrogen chemistry	Can indicate high temperature gas reactions	Compete for radicals and affect water quality
Metal salts	Reduction, hydrolysis, precipitation, nucleation	Nanoparticle and catalyst synthesis	Agglomeration, contamination, and uncontrolled redox chemistry

Table 8.2. Inorganic and materials applications of cavitation induced chemistry

Application	Primary chemical role	Important physical role	Key controls
Advanced oxidation	Generate •OH, H₂O₂, SO₄•⁻, or secondary oxidants	Mix reagents and renew interfaces	pH, oxidant dose, ion matrix, energy per order
Fenton and Fenton like treatment	Accelerate Fe redox cycle and peroxide activation	Disperse catalyst and reduce mass transfer resistance	Iron dose, pH, peroxide staging, sludge management
Nanometal synthesis	Reduce metal ions and create unusual phases	Rapid quench, fragmentation, and deagglomeration	Gas atmosphere, precursor concentration, stabilizer, contamination
Metal oxide synthesis	Hydrolysis, oxidation, defect generation	Micromixing and nucleation	Temperature, pH, precursor feed, residence distribution
Crystallization	Local supersaturation and possible radical chemistry	Nucleation, attrition, surface cleaning	Supersaturation, pass number, crystal breakage, scale up
Disinfection	Oxidative damage to cell components	Membrane disruption and microjet action	Water matrix, organism, contact time, residual oxidant
Electrochemical intensification	Remove passivating films and improve redox transport	Boundary layer thinning and gas removal	Electrode spacing, erosion, conductivity, gas evolution

The classic sonochemical synthesis of amorphous iron demonstrates how cavitation can create phases that are difficult to produce under ordinary bulk conditions [45]. Although that work used acoustic cavitation, the mechanism, rapid localized heating followed by quench and radical reduction, is relevant to hydrodynamic reactors when sufficient collapse intensity and precursor access are achieved. Hydrodynamic devices add continuous throughput and potentially larger production rates, but they require careful control of erosion derived metal contamination and residence time distribution.