Microwave Pyrolysis of Spent Activated Carbon-Laden Sludge: A Critical Review of Regeneration, Cross-Phase Pollutant Transfer, and Environmental Reactivity

2.1 Formation Pathways and Operational Definition of SACLS

SACLS is defined here as a carbonaceous composite sludge residue generated after the incorporation, retention, and co-separation of spent activated carbon within wastewater treatment systems. Unlike conventional physical sludge or biological sludge, SACLS possesses a coupled structure consisting of carbon particles, extracellular polymeric substances (EPS), microbial-derived organic matter, mineral phases, and adsorbed contaminants. Depending on the sludge source, SACLS can exhibit substantially different physicochemical characteristics. For example, biological SACLS derived from activated sludge systems is dominated by microbial flocs, EPS, and biogenic minerals, whereas physically generated carbon-containing sludge may contain fewer biological components but higher proportions of mineral particles and carbon fragments^[27]. These differences directly influence dielectric properties, heat transfer behavior, pollutant partitioning, and regeneration performance during thermal treatment. It is commonly formed when powdered activated carbon added to biological or advanced wastewater treatment is co-separated with waste sludge. Other sources include fines from granular activated-carbon systems, carbonaceous adsorbents used in sludge conditioning, and adsorption media blended with sludge before thermal treatment^[28]. Isolated spent activated carbon that can be physically separated and regenerated independently is excluded, because the surrounding sludge matrix is central to the thermal behavior considered in this review. SACLS composition varies with activated-carbon type, dose, particle size, pore structure, contaminant loading, moisture, ash, coagulant-derived minerals, and chloride, sulfur, calcium, phosphorus, iron, and aluminum contents. These differences strongly affect microwave absorption, devolatilization, pore redevelopment, and pollutant release. SACLS characterization should therefore report the carbonaceous fraction, particle-size distribution, proximate and ultimate analyses, ash and mineral composition, moisture basis, and major adsorbed contaminants.

2.2 Composition and Physicochemical Characteristics of SACLS

Unlike engineered carbon-sludge composites, SACLS does not possess a fixed activated carbon-to-sludge ratio because its composition is determined by the wastewater treatment configuration and operational conditions. In PAC-based wastewater treatment, the carbon fraction is controlled by the applied PAC dosage, sludge retention time, and sludge production rate^[29]. Recent full-scale and pilot-scale studies have reported PAC dosages ranging from several mg L^-1 to tens of mg L^-1 for micropollutant removal, indicating that the final carbon content in sludge can vary substantially among treatment systems^[11]. In GAC-based systems, the carbon fraction is additionally affected by carbon attrition, backwashing losses, and particle carryover. Therefore, SACLS should be considered a compositionally variable carbon-organic-mineral matrix rather than a material with a universal carbon/sludge ratio^[30].

Typical physicochemical characteristics of SACLS include a porous carbon phase originating from spent activated carbon, microbial-derived organic matter and extracellular polymeric substances from biological sludge, mineral ash components, and concentrated adsorbed contaminants. Compared with conventional biological sludge, SACLS generally exhibits higher carbon content, stronger hydrophobicity, enhanced adsorption potential, and altered dielectric properties due to the presence of activated carbon^[31]. However, these properties are highly dependent on carbon loading, sludge origin, and wastewater treatment process. A summary of reported characteristics of carbon-containing sludge systems is provided in Table 1.

2.3 Environmental Value and Risks of SACLS

The treatment objective is not simply to maximize char yield. Regeneration should restore accessible porosity, suitable surface chemistry, mechanical integrity, adsorption capacity, and application-specific selectivity while avoiding unacceptable contaminant release or environmental reactivity. Although the embedded carbonaceous fraction retains substantial reuse potential, the surrounding sludge matrix also concentrates organic pollutants, salts, mineral phases, and heavy metals, making SACLS fundamentally different from both conventional sewage sludge and isolated spent activated carbon. Environmental performance therefore cannot be judged from contaminant concentrations in the regenerated char alone. Organic-matter loss may increase the apparent concentration of non-volatile metals without increasing their absolute mass, whereas lower char concentrations may reflect transfer to particles, condensates, deposits, or downstream traps rather than actual destruction. Regeneration performance and environmental safety should consequently be evaluated on a common feed-mass basis and within a system boundary that includes all recoverable and releasable product phases. The distinctive characteristics, treatment objectives, and analytical requirements of SACLS relative to conventional sewage sludge and isolated spent activated carbon are summarized in Table 2.

2.4 Regeneration Mechanisms of SACLS-Derived Activated Carbon

Spent activated carbon progressively loses adsorption performance through pore blockage, competitive adsorption, deposition of inorganic matter, accumulation of strongly retained contaminants, and modification of surface functional groups. Thermal regeneration aims to reverse these effects by removing or transforming retained species and restoring access to the internal pore network. During heating, weakly bound compounds may first be desorbed, whereas more strongly retained organic adsorbates undergo cracking, devolatilization, carbonization, or partial gasification. These reactions can reopen blocked pores, remove deposited organic matter, and reconstruct the surface chemistry of the carbonaceous matrix. At the same time, secondary reactions may form additional carbonaceous deposits, alter micropore dimensions, remove oxygen-containing functional groups, or promote mineral deposition within the pore structure. Regeneration therefore involves competing processes of pore recovery, surface reconstruction, secondary deposition, and carbon loss. The most effective regeneration condition is consequently not necessarily the condition with the highest treatment temperature or the greatest apparent contaminant removal. Rather, it is the condition that sufficiently removes or transforms adsorbates while preserving the carbon framework, restoring application-relevant pore accessibility, and limiting the generation or transfer of secondary pollutants.

2.5 Microwave-Assisted Regeneration and Matrix Effects

Conventional thermal regeneration relies on heat transfer from the reactor wall to the particle surface and interior. Although technically established, it often requires long residence times and may treat wet, aggregated, or heterogeneous materials non-uniformly. Excessive oxidation or gasification can also cause carbon loss, reduce mechanical strength, and alter pore-size distribution, particularly when activated carbon is embedded in a moisture- and mineral-rich sludge matrix. Microwave heating instead deposits electromagnetic energy directly within microwave-absorbing domains. Because activated carbon is an effective microwave absorber, this mode of heating can accelerate adsorbate desorption, cracking, and pore redevelopment. Microwave regeneration should not, however, be regarded simply as faster conventional heating. Ania et al.^[38] showed that microwave treatment rapidly regenerated phenol-loaded activated carbon and produced a pore structure different from that obtained by conventional heating, indicating that spatial and temporal energy deposition affects desorption, vapor transport, and carbon restructuring. Microwave regeneration has also been demonstrated for activated carbons loaded with persistent or strongly retained contaminants. Its performance depends on activated-carbon properties, microwave power and irradiation time, pollutant characteristics, and carbon loss^[32]. Adsorption-microwave treatment restored granular activated carbon used for ester-containing wastewater^[17], while pyrolysis regenerated spent powdered activated carbon and recovered substantial surface area and phosphorus adsorption capacity^[18]. These studies confirm technical feasibility, but regeneration must be assessed together with carbon yield, pore structure, energy use, residual contaminants, and releases to gaseous, particulate, and condensable streams. Selective microwave absorption may improve regeneration while also causing local overheating, carbon burn-off, and contaminant volatilization.

In isolated spent activated carbon, the carbon particle is both the principal microwave absorber and the regeneration target. In SACLS, it is surrounded by water, sludge-derived organic matter, microbial residues, minerals, coagulant-derived phases, and treatment chemicals. This heterogeneous matrix modifies dielectric behavior, heat and mass transfer, vapor-solid interactions, and contaminant fate. During initial heating, moisture may dominate microwave absorption and consume energy through evaporation. As drying and carbonization proceed, carbon-rich domains can heat more rapidly than adjacent mineral-rich regions, producing spatial temperature gradients and localized carbon-organic or carbon-mineral hot spots. Meanwhile, sludge-derived vapors may be readsorbed, cracked on carbon surfaces, or converted into secondary char that blocks pores. Mineral phases may protect the carbon framework, catalyze gasification, alter surface reactions, or deposit within the pore network. These interactions explain why results obtained for isolated spent activated carbon cannot be transferred directly to SACLS. Strong microwave absorption may enhance adsorbate removal and pore recovery, but it may also accelerate carbon loss, pore widening or collapse, mineral volatilization, and fine-particle formation. Regeneration should therefore be evaluated using carbon yield, pore-size distribution, surface chemistry, adsorption capacity and kinetics, mechanical stability, repeated-use performance, leaching, residual organics, and radical-mediated reactivity^[39]. The conceptual mechanism of microwave-induced thermal heterogeneity and interfacial reactions in SACLS is summarized in Figure. 2. The figure highlights selective microwave absorption by carbonaceous domains and the resulting formation of local hot spots, which may simultaneously enhance regeneration and promote secondary pollutant transformation.

3.1 Composition-Dependent Microwave Absorption and Heating

Microwave heating of SACLS depends strongly on feedstock composition and cannot be represented by applied power alone. Early studies of sewage-sludge microwave pyrolysis showed that wet sludge may not reach effective pyrolysis temperatures without an additional microwave absorber. Although moisture initially promotes microwave coupling, much of the supplied energy is consumed by evaporation, after which the remaining organic and mineral fractions may absorb microwaves weakly or unevenly. Domínguez et al.^[40,41] further showed that microwave and conventional heating can produce different liquid, gas, and solid yields even at comparable apparent temperatures. These findings support a central feature of SACLS: the embedded carbonaceous fraction acts simultaneously as the regeneration target, a microwave susceptor, and a reactive surface for vapor conversion.

The microwave response of SACLS reflects the combined effects of moisture, sludge-derived organic matter, activated carbon, ash-forming minerals, and treatment-derived additives. Antunes et al.^[42] showed that moisture content, ash composition, and added susceptors substantially affect the dielectric behavior of sewage biosolids and the properties of the resulting biochar. In SACLS, activated-carbon particles generally absorb microwaves more strongly than surrounding organic- or mineral-rich regions, although the extent of this contrast depends on carbon loading, particle size, pore structure, mineral coverage, contaminant loading, and spatial distribution^[43].

Feedstock composition also governs subsequent heat and mass transfer. Moisture delays pyrolysis through evaporation, carbon-rich domains promote local heating and devolatilization, and mineral phases may catalyze tar cracking, alter gas composition, or retain released inorganic species. Consequently, identical nominal microwave powers can produce markedly different thermal histories, product distributions, and pollutant-release profiles among SACLS feedstocks.

Representative experimental evidence for these feedstock- and heating-mode-dependent effects is compiled in Figure. 3. Oh et al.^[37] demonstrated distinct char, liquid, and gas distributions among raw and digested organic wastes during microwave-assisted pyrolysis and further identified correlations between feedstock composition and pyrolysis-product distribution. Li et al.^[18] further demonstrated that pyrolysis temperature and residence time strongly influenced spent powdered activated-carbon regeneration, with subsequent recovery of adsorption performance. Collectively, these results illustrate that both feedstock characteristics and regeneration conditions govern the effectiveness of carbonaceous-residue valorization (Figure. 3).

3.2 Microwave Energy Deposition and Local Hot Spots

Unlike conventional heating, microwave energy is deposited directly within absorbing domains, allowing rapid internal heating but also creating spatially heterogeneous temperature fields. In SACLS, carbon-rich regions may heat more intensely than adjacent organic- or mineral-rich domains, generating local temperature gradients at carbon-organic and carbon-mineral interfaces. This energy distribution changes continuously during treatment. As water evaporates, organic matter decomposes, and the residual carbon becomes more aromatic, the dielectric properties and microwave penetration behavior of the material also evolve^[44]. Applied power therefore does not correspond to a constant heating rate or a uniform temperature field, and carbon-rich regions may become increasingly dominant absorbers during drying and carbonization.

Interfacial hot spots can simultaneously accelerate adsorbate desorption, devolatilization, tar cracking, aromatization, carbon gasification, and mineral decomposition. They may also promote salt and metal volatilization followed by downstream nucleation or condensation, while increasing carbon burn-off, altering micropore structure, and generating defects that stabilize environmentally persistent free radicals. Reviews of microwave-assisted sewage-sludge pyrolysis identify hot-spot measurement, energy-efficiency assessment, reactor scale-up, and product safety as persistent uncertainties^[45-50]. These issues are particularly important for SACLS because its strongly absorbing carbonaceous phase is heterogeneously distributed.

3.3 Limitations of Apparent Temperature as a Process Descriptor

Microwave sludge-pyrolysis studies commonly report applied power, final reactor temperature, residence time, and carrier-gas flow. Although useful for comparison, these parameters do not fully describe the thermal history of individual activated-carbon particles or carbon-organic and carbon-mineral interfaces. A single measured temperature may therefore conceal substantial spatial and temporal heterogeneity within the reactor bed.

Thermocouple readings depend on probe position, contact with surrounding solids, electromagnetic interference, and local gas flow. Measurements taken in colder regions may underestimate the temperature of nearby carbon-rich domains, whereas probes located close to strongly absorbing particles may overestimate the bulk temperature^[51]. Infrared measurements avoid direct probe interference but generally represent only visible surfaces and may not capture internal gradients in opaque or heterogeneous beds. Because activated carbon absorbs microwaves strongly, individual particles may reach temperatures substantially higher than the measured bulk or surface value. These local differences can alter desorption, devolatilization, vapor-solid reactions, tar conversion, mineral transformation, and particulate formation. Nominal reactor temperature should therefore be regarded as an incomplete indicator of reaction severity. Claims of a distinct “microwave effect” should also be interpreted cautiously. Differences between microwave and conventional pyrolysis may arise from heating rate, spatial temperature gradients, vapor residence time, vapor-solid contact, mineral catalysis, and interfacial temperatures rather than from a universal non-thermal mechanism. Meaningful comparisons therefore require equivalent and, where possible, spatially resolved thermal histories.

3.4 Major Pathways of Particulate Formation

Particulate matter generated during SACLS pyrolysis can be grouped into three overlapping categories: primary entrained particles, organic condensation particles, and inorganic nucleation particles. This mechanistic classification is more informative than total particulate mass because each pathway responds differently to feedstock composition, local temperature, gas flow, vapor residence time, and downstream cooling.

Primary particles arise from fragmentation, abrasion, mineral exfoliation, and entrainment of pre-existing solids. Sludge aggregates may disintegrate during drying and devolatilization, while activated-carbon particles, mineral coatings, ash, and secondary char may detach and enter the gas stream. Their abundance and size distribution are controlled mainly by feed preparation, particle size, gas velocity, mixing, and reactor configuration. Organic condensation particles form when volatile organic compounds and tar vapors cool, become supersaturated, and nucleate or condense onto existing surfaces. They may form in cooler reactor regions, transfer lines, dilution systems, or sampling devices rather than directly in the hot reaction zone. Dai et al.^[52] showed that heating mode and temperature influence the distribution of polycyclic aromatic hydrocarbons among gaseous, liquid, and solid products, indicating that organic contaminants may be redistributed rather than destroyed. Ko et al.^[53] further demonstrated that pyrolysis temperature and vapor residence time strongly affect particulate-bound priority polycyclic aromatic hydrocarbons. Inorganic nucleation particles form when salts, metal compounds, or other mineral-derived species volatilize at elevated local temperatures and subsequently nucleate or condense during cooling. These vapors may form ultrafine particles through homogeneous nucleation or condense heterogeneously onto entrained ash and carbonaceous particles, producing mixed organic-inorganic agglomerates.

Temperature-resolved experiments indicate that lower-temperature particles are often dominated by condensed carbonaceous material, whereas inorganic contributions become more important at approximately 500 °C−700 °C. Pb and Cd concentrations in collected particles have also been reported to increase at higher temperatures^[19]. However, this transition is not universal because feed composition, local hot spots, vapor residence time, transfer-line temperature, and sampling conditions can shift the observed particle distribution.

The major particulate-formation pathways and their principal controlling factors are summarized in Figure. 4. These pathways include direct entrainment of carbon/mineral fragments, condensation of vaporized organics, nucleation of volatile inorganic or metal species, and subsequent coagulation and surface reactions.

3.5 Dual Effects of Spent Activated Carbon on Particulate Formation

Spent activated carbon can either suppress or promote particulate formation during SACLS microwave pyrolysis. Its pore network and external surface can adsorb volatile and semi-volatile compounds, thereby delaying or reducing their immediate transfer into the particulate phase. Carbon surfaces may also catalyze cracking, deoxygenation, condensation, and aromatization reactions, converting condensable vapors into lighter gases or secondary carbonaceous deposits. These interactions can decrease the amount of condensable material leaving the hot reaction zone, but they may also promote secondary char formation on activated-carbon surfaces. When secondary carbon blocks micropores or accumulates on external surfaces, adsorption recovery may be reduced even if total char yield increases. The fate of adsorbed vapors therefore depends on local temperature, pore accessibility, mineral coverage, vapor composition, and vapor-solid contact time. Conversely, activated carbon may increase particulate emissions through several mechanisms. Selective microwave absorption can generate localized overheating and promote the volatilization of mineral- and metal-containing species. Thermal stress and mechanical mixing may fragment carbon particles or detach mineral coatings accumulated during wastewater treatment. Adsorbed contaminants, inorganic salts, and coagulant-derived phases on spent carbon can also act as precursors for organic condensation particles or inorganic nucleation particles. In addition, carbon-rich primary particles can provide surfaces for the downstream condensation of tar vapors and volatilized metals.

Whether spent activated carbon suppresses or enhances particulate formation therefore depends on its loading history, particle size, pore structure, mineral coverage, contaminant inventory, heating rate, local temperature, gas velocity, and vapor-solid contact time. This dual role cannot be resolved by comparing total particulate mass alone. Particle number concentration, aerodynamic size, morphology, elemental composition, organic carbon and elemental carbon contents, mineral phases, surface functionality, and radical activity should also be determined. Size-resolved characterization is particularly important because submicrometer particles have high specific surface areas, long atmospheric residence times, and relatively high mobility. They may preferentially concentrate volatile metals, condensed aromatic compounds, carbon defects, and redox-active mineral sites. However, particulate collection and handling may also cause the loss of semi-volatile compounds or alter radical signals. Analytical procedures must therefore distinguish particles entrained directly from the hot zone from particles formed through downstream cooling and condensation.

4.1 Retention, Enrichment, and Volatilization of Heavy Metals

During pyrolysis, heavy metals undergo competing retention, transformation, and release. Their concentrations in char may rise as organics and carbon are removed, while decomposition of organic complexes, redox changes, and reactions with sulfur-, chlorine-, phosphorus-, silicon-, aluminum-, iron-, and calcium-containing phases alter chemical form. Some metals remain in char or newly formed minerals; others volatilize and later condense in particles, liquids, or deposits^[54].

Volatility alone cannot predict migration. Initial speciation, association with organics or minerals, local redox conditions, and available inorganic ligands determine whether a metal is immobilized, converted into a stable compound, or released as vapor or particulate matter^[20-22]. Local microwave hot spots further weaken the value of nominal reactor temperature as a predictor. Increasing pyrolysis temperature can shift some metals from labile to residual fractions, but responses are element-specific^[55,56]. Consequently, higher char concentrations may reflect mass loss rather than increased risk, whereas lower concentrations may indicate cross-phase transfer rather than detoxification^[54]. Concentrations must therefore be combined with product yields and expressed as absolute recovery relative to the initial feed.

4.2 Distribution of Heavy Metals among Char, Particles, Condensates, and Deposits

Regenerated char is usually the dominant metal-containing product because many non-volatile elements remain associated with ash or enter transformed mineral phases. Metals with strong affinity for phosphates, silicates, aluminosilicates, sulfides, or oxides may be retained even as their concentrations increase through organic-matter loss. Such enrichment does not necessarily imply greater mobility, although a small leachable fraction may still control risk.

More volatile species can enter the gas phase and enrich fine particles. Pb and Cd enrichment at higher sludge-pyrolysis temperatures illustrates this pathway^[19]. Metals may nucleate as ultrafine inorganic particles, condense on ash, or adsorb on carbonaceous aerosols. Although these fractions can contain a small share of total metal mass, their mobility, surface area, and inhalation relevance may be high. Their small aerodynamic diameter also increases residence time and reduces capture by conventional coarse-particle separators, so a minor mass fraction can still dominate exposure-relevant risk.

Aqueous and organic condensates can contain soluble salts, metal-organic complexes, colloids, and entrained particles. Reactor walls, transfer lines, filters, sorbents, and impingers are also important sinks because vapors and particle-bound metals deposit as the stream cools. Excluding these products creates apparent mass deficits and can misclassify redistribution as gaseous loss. Final phase distributions therefore depend on element identity, initial speciation, matrix composition, atmosphere, vapor residence time, and downstream temperature history. Metal partitioning between aqueous and organic condensates may also change during phase separation or storage, making separate analysis of dissolved, colloidal, and suspended fractions preferable. Deposits are operationally important because they can be remobilized during cleaning and maintenance, creating a secondary waste stream that is absent from routine product-yield calculations.

4.3 Whole-Process Mass Balance of Heavy Metals

A whole-process mass balance is essential for distinguishing stabilization from redistribution. For each target metal, the recovered mass in char, size-resolved particles, aqueous and organic condensates, reactor and transfer-line deposits, and terminal traps should be calculated from product concentration and recovered mass or volume. Results should be reported as absolute mass, percentage of the feed inventory, and overall recovery. Where size-resolved sampling is used, each stage should be weighed independently and corrected for substrate blanks and handling losses. The particle enrichment of Pb and Cd reported by Li et al.^[19] would be missed by feed-and-char analysis. Poor recovery should likewise not be assigned automatically to unmeasured gaseous species because metals may reside in deposits, condensates, or downstream sorbents. The sampling boundary should therefore include reactor solids, loose ash, deposits, condensers, filters, impactor stages, impingers, and terminal traps wherever practicable. Analytical methods and blanks must be validated for carbon-rich solids, minerals, water, and tar-containing matrices. Mass-balance closure should be treated as a quality-control criterion; unresolved fractions should be reported rather than interpreted speculatively. Digestion recovery should be checked with matrix-matched reference materials or spike tests because refractory mineral phases and carbon-rich matrices can produce element-specific bias.

4.4 Particle-Size-Dependent Metal Enrichment

Particle size is a critical dimension of metal transfer. Volatilized compounds may nucleate homogeneously, condense on ash or carbonaceous aerosols, or adsorb after cooling. Ultrafine particles can be strongly enriched because of their high surface area, whereas coarse fractions generally originate from feed fragmentation, mineral exfoliation, carbon attrition, and mechanical entrainment. The resulting size distribution determines atmospheric persistence, respiratory deposition, surface reactivity, and the efficiency of downstream control devices. Metal concentration alone is insufficient for comparing size fractions. A small ultrafine fraction may have a high concentration but contain little absolute mass, while a lower concentration in a large coarse fraction may represent a larger inventory. Enrichment factors should therefore be reported with particle mass distributions and the percentage of feed metal in each size class.

Size-resolved information is also required for emission control. Cyclones mainly remove coarse particles, whereas metal-rich submicrometer aerosols require high-efficiency filtration or electrostatic capture. Because coagulation, condensation, and deposition can change particle size during sampling, transfer-line temperature, dilution, residence time, and collection configuration must be documented. These data distinguish mechanically entrained mineral particles from secondary metal-rich aerosols and identify the fractions most relevant to mobility and exposure. Leaching or bioaccessibility should be measured by size class where possible because surface-enriched condensates can behave differently from metals incorporated into coarse mineral particles.

4.5 Controls Imposed by the Carbon-Mineral Matrix

The carbon-mineral matrix controls metal retention through adsorption, oxygen-containing functional groups, mineral coatings, pore confinement, and association with adsorbed organics. Pyrolysis removes some functional groups, increases aromaticity, and creates reducing microenvironments that can alter metal valence and coordination. Mineral phases may immobilize metals by adsorption, encapsulation, or formation of phosphates, silicates, aluminosilicates, sulfides, and oxides; thermal treatment can also convert amorphous associations into more crystalline and less soluble structures^[57].

Chlorine can enhance volatility through metal-chloride formation, whereas sulfur and phosphorus may favor less volatile sulfides or phosphates. The net outcome depends on temperature, atmosphere, competing ions, mineral accessibility, and reaction sequence, with local hot spots creating transient high-temperature conditions. Activated carbon can adsorb released metals and support heterogeneous condensation, but it can also promote reduction, volatilization, or transfer through particle fragmentation. The multiple Pb(II) sorption mechanisms reported for sludge-derived biochar^[22] illustrate why metal retention cannot be attributed to carbon alone: the same multiphase matrix can immobilize metals in char or provide surfaces for metal-rich particle formation.

4.6 Metal Speciation, Leaching, and Environmental Risks

Total metal concentration is only a partial risk indicator. Environmental relevance depends on speciation, mineral association, oxidation state, particle size, leachability, bioaccessibility, and exposure pathway. Pyrolysis may increase metal concentration in char through mass loss while shifting metals into less mobile fractions; conversely, a small mass transferred to fine particles or condensates may be more available. Temperature-dependent stabilization has been reported for several metals, but the response is element- and feedstock-specific^[55,56]. Leaching tests should accompany concentration and mass-balance measurements under pH, ionic strength, redox, and dissolved-organic-matter conditions representative of intended use. Repeated adsorption-regeneration cycles are also important because mineral coatings and previously inaccessible phases can change. Fine particles require separate leaching or bioaccessibility tests because surface-enriched or amorphous metals may dissolve more readily, and coexisting organics or EPFRs may alter redox state and dissolution^[58]. Condensates and periodically removed deposits should be assessed directly because they can contain mobile dissolved, colloidal, or particle-bound metals. For water-treatment reuse, tests should also consider whether target adsorption mobilizes metals through ion exchange, complexation, or pH change. For disposal or incorporation into materials, weathering and abrasion may be more relevant than short-term aqueous extraction.

Risk interpretation should distinguish total inventory, mobility, and exposure. Stable metal-rich char may constrain reuse without presenting high immediate mobility, whereas a low-mass ultrafine fraction or aqueous condensate may require stringent control^[59]. A defensible assessment therefore combines absolute recovery, phase and size distribution, oxidation state, speciation, leaching, bioaccessibility, and exposure-relevant endpoints, avoiding the assumption that apparent char stabilization represents whole-process risk reduction. Regulatory comparison should therefore be linked to the intended product use and exposure pathway rather than applied as a single generic threshold.

5.1 Formation and Stabilization Mechanisms of EPFRs

EPFRs are paramagnetic species that persist far longer than transient aqueous or gas-phase radicals because unpaired electrons are stabilized on conjugated aromatic structures, defects, oxygen-containing groups, and mineral or metal-oxide surfaces. Their occurrence in combustion-derived PM2.5 demonstrates persistence after high-temperature formation^[25]. During SACLS pyrolysis, decomposition and aromatization of sludge organics create conjugated structures and defects, while cleavage of functional groups, incomplete condensation, and secondary carbonization can preserve carbon- or oxygen-centered radical sites. Treatment severity can therefore produce non-monotonic trends: moderate carbonization may maximize defect-associated radicals, whereas extensive graphitization or radical recombination can reduce the measurable population.

Metal oxides provide an additional pathway. Organic precursors can chemisorb on Fe(III) oxide, transfer an electron to the surface, and form stabilized radicals^[26]. Radical formation therefore depends not only on temperature but also on metal identity, oxidation state, coordination, and accessibility. EPFRs have been quantified in municipal and industrial sludge chars^[23], and iron and natural organic matter jointly controlled radical generation in Fenton-conditioned sludge^[24]. Sludge-derived biochars contain carbon-centered, oxygen-centered, and oxygen-adjacent carbon-centered radicals whose distributions vary with temperature, sludge composition, and biomass addition^[61]. Different signatures in incineration ash and pyrolysis biochar further demonstrate the importance of thermal route and the mineral-carbon matrix^[62,63]. EPFR evolution should thus be understood as coupled carbonization, defect generation, mineral transformation, and interfacial electron transfer rather than as a single temperature-controlled reaction. The observed Electron Paramagnetic Resonance (EPR) signal consequently reflects both the number of radical sites and their stabilization environment, and comparisons across studies require attention to feed composition, thermal history, and measurement conditions.

5.2 Phase Distribution of EPFRs

Most studies measure EPFRs only in cooled bulk char, although radical-active material or precursors may also occur in particles, condensates, and reactor or transfer-line deposits. Char stabilizes radicals through aromatic structures, pore surfaces, defects, functional groups, and embedded minerals. Moderate thermal severity may preserve heterogeneous radical sites, whereas extensive carbon ordering can reduce some defect-associated populations. Bulk analysis can therefore mask redistribution from a relatively immobile char matrix to a smaller but more accessible particle fraction.

Fine particles may be particularly important because high surface area, condensed aromatic compounds, carbon defects, minerals, and redox-active metals are co-located. Primary carbonaceous particles may carry radicals formed in the hot zone, while condensation and nucleation particles can acquire aromatic precursors and metal sites during cooling. Condensates and deposits may contain radical precursors or stabilized surface radicals, but their contribution is poorly constrained because oxygen exposure, solvent handling, phase separation, and sampling alter detectability. Phase-specific EPR analysis should therefore distinguish char, size-resolved particles, deposits, and selected condensate fractions; a high radical concentration in a small particle fraction can have different implications from the same concentration in stable bulk char. Reactor-wall and transfer-line deposits deserve attention because repeated condensation, thermal cycling, and contact with metal-containing surfaces may promote secondary aromatization and radical stabilization. They may also become an occupational exposure source during maintenance^[64].

5.3 Effects of Sampling, Storage, and Aging on EPFR Characterization

EPFR measurements are sensitive to collection, moisture, oxygen exposure, storage temperature, and measurement delay. Water can quench or transform surface radicals and affect EPR response; oxygen may either consume radicals or generate new oxygen-associated species while changing metal oxidation state. Continued reactions during storage mean that an EPR spectrum records the population present at analysis, not necessarily that formed during pyrolysis. Storage-induced changes may be phase-specific because condensed organic coatings, mineral surfaces, and pore-confined radicals have different access to water and oxygen^[65].

Reports should therefore provide calibrated spin concentration, g-factor, linewidth, dry-mass basis, moisture, storage atmosphere and temperature, and elapsed time from collection to measurement. Time-resolved analyses under inert, air, humid, and aqueous conditions are needed to determine decay, transformation, or secondary formation^[66]. Collection and preparation should minimize phase alteration because fine particles can lose semi-volatile precursors, aggressive drying can change surface chemistry, and extraction or filtration can redistribute condensate-associated species. Calibration standards, microwave power used during EPR measurement, sample packing, and replicate variability should also be reported where they affect quantitative comparison.

5.4 Functional reactivity, ROS Generation, and Toxicological Implications

EPFR-associated electron transfer can be functionally useful by activating oxygen, hydrogen peroxide, persulfate, or other oxidants and promoting contaminant degradation. The same reactivity can generate superoxide-related species, hydrogen peroxide, hydroxyl-radical-related oxidants, and oxidative stress when materials contact oxygen and water, particularly in the presence of redox-active metals. This dual functionality is important when regenerated material is proposed as an adsorbent-catalyst or catalyst support, because apparent treatment performance may partly arise from oxidative transformation rather than adsorption alone.

Hazard cannot be inferred from spin concentration alone. It depends on radical type, persistence, surface accessibility, moisture, oxygen, coexisting metals, particle size, aging, and reuse scenario. Fine particles may be especially reactive because they combine high surface area with accessible radical and metal sites, whereas radicals buried in stable char may be less available until leaching, abrasion, or repeated use exposes them. Evaluation should combine EPR, decay kinetics, ROS assays, metal leaching, and exposure-relevant biological or chemical endpoints. Reviews of biochar EPFRs accordingly emphasize context-dependent beneficial and adverse effects^[65]. Application-specific tests are therefore essential: catalytic reuse may tolerate controlled radical activity, whereas water-contact or inhalation-relevant applications may require much lower persistent ROS-generating capacity.

5.5 Evidence for Metal-Assisted EPFR Formation

The most direct evidence for metal-EPFR coupling concerns metal-assisted radical formation. Multivalent transition metals can accept or donate electrons while changing oxidation state or coordination, and iron-containing phases are common in sludge as natural minerals, coagulant precipitates, and Fenton-conditioning products. Aromatic or phenolic precursors can chemisorb on metal oxides and transfer an electron to a metal center, producing a resonance-stabilized organic radical; Fe(III) oxide model systems directly support this mechanism^[26]. The interfacial mechanism is favored when organic precursors and accessible metal sites are co-located, but can be inhibited by mineral encapsulation or loss of suitable functional groups.

Fenton-conditioned sludge further shows that iron and natural organic matter jointly govern EPFR formation^[24]. Other transition-metal oxides may promote stabilization, recombination, or quenching depending on identity, loading, dispersion, mineral coating, and accessibility. Heterogeneous metals also regulate persistent radicals that act as redox-active sites in iron-containing biochar during persulfate activation^[63]. These observations support metal-assisted EPFR formation, but correlations between total metal content and EPR intensity do not identify the active phase or prove electron transfer. Oxidation state, local coordination, spatial co-localization, and surface accessibility must also be established. Controlled comparison of metal-free, metal-depleted, and metal-reconstituted matrices would provide stronger causal evidence than correlations among bulk composition, temperature, and radical abundance.

5.6 Possible Effects of EPFRs on Metal Speciation and Migration

The reverse effect of EPFRs on metal behavior is less certain. Radical-mediated electron transfer could change the valence, coordination, surface affinity, or solubility of redox-sensitive metals, while ROS generated during air or water exposure could oxidize sulfides, alter organic ligands and surface charge, or modify carbonaceous coatings around metal-bearing particles. These processes might either increase or decrease dissolution depending on metal identity, mineral phase, pH, oxygen, and radical chemistry. Radical reactions could also alter organic ligands that complex metals or expose previously shielded mineral surfaces, thereby changing bioaccessibility during aging.

Evidence from reactive biochars shows that persistent radicals and metals can participate in coupled redox processes^[63], but it does not demonstrate that EPFRs control metal volatilization, condensation, or particle partitioning during SACLS pyrolysis. These pathways should therefore remain testable hypotheses. Causal evidence requires experiments that vary radical abundance independently of temperature, carbon structure, and mineral content while monitoring metal oxidation state, leaching, and phase distribution. Experiments should include radical quenchers or controlled oxygen exposure and verify that changes are not caused simply by altered pH, mineral dissolution, or carbon oxidation.

5.7 A Testable Three-Stage Metal-EPFR-ROS Coupling Model

A testable metal-EPFR-ROS model comprises three stages. First, co-localization occurs when aromatic precursors, carbon defects, and redox-active metals accumulate on char or particle surfaces during pyrolysis and cooling; activated-carbon pores, mineral coatings, tar deposits, and fine-particle agglomerates can bring them into close contact. Second, interfacial electron transfer produces or stabilizes radicals while changing metal valence or coordination, a pathway most strongly supported for iron-containing systems^[24-26]. Third, exposure to oxygen, humidity, or water converts stored electronic reactivity into ROS generation, metal transformation, dissolution, contaminant conversion, or biological stress. The first and second stages may occur in the hot reactor or during downstream cooling, whereas the third is especially relevant during storage, reuse, or environmental release.

Each stage requires distinct evidence. Microscopy and elemental mapping should establish co-localization; synchronized EPR and oxidation-state or coordination measurements should test electron transfer; and controlled aging, ROS, leaching, and biological assays should evaluate environmental activation. Fine particles may amplify coupling through co-enrichment of metals, aromatic precursors, and defects, but statistical association does not establish causality and some metals or mineral coatings may quench radicals. Time ordering is also important: metal volatilization may precede radical stabilization, while subsequent aging can change both metal accessibility and ROS production. The model should therefore be tested with phase-resolved samples collected at defined positions and aging times. On the basis of the literature evidence discussed above, the proposed coupling among precursor transformation, EPFR formation, metal-mediated electron transfer, ROS generation, and environmental reactivity is synthesized in Figure. 7. Importantly, the arrows in this framework represent testable mechanistic links rather than universally established causal relationships.

6.1 Limitations of Single-Endpoint Optimization

Optimizing only BET surface area, pore volume, or adsorption capacity can favor severe conditions that improve pore accessibility while increasing carbon loss, energy demand, particles, metal volatilization, or oxidative reactivity. Mild treatment can preserve yield but leave strongly retained adsorbates, condensable organics, unstable precursors, and leachable contaminants. Temperature and heating mode also redistribute polycyclic aromatic hydrocarbons (PAHs) among gas, liquid, particles, and char^[52,53], and temperature-dependent PAH leaching has been observed in sludge biochar^[55]. Reuse tests must therefore distinguish adsorption of a new target from release of residual adsorbates, dissolved organic carbon, PAHs, and thermogenic products. The optimum should be defined as a constrained operating window, not a single maximum. This is especially important for SACLS because improved char properties can coincide with increased pollutant loading in a much smaller particulate or liquid stream.

6.2 Indicators of Regeneration Performance

Regeneration performance requires complementary structural, functional, and durability indicators. Char yield and carbon loss should be reported together with pore-size distribution and surface chemistry because increased brunauer-emmett-teller theory (BET) area may reflect pore widening, collapse, or inaccessible secondary deposits rather than restoration of target-selective porosity. Adsorption capacity should be accompanied by kinetics, selectivity, and tests under relevant pH, ionic strength, competing solutes, and dissolved organic matter. Repeated adsorption-regeneration cycles should track mechanical stability, carbon loss, mineral accumulation, contaminant release, and declining selectivity^[67]. Comparisons with fresh activated carbon should use identical target concentrations, contact times, and water chemistry so that apparent recovery is not inflated by different test conditions.

6.3 Indicators of Particulate, Metal, and Organic Chemical Risks

Cross-phase risk assessment should combine particle mass and number, aerodynamic size, morphology, carbon fractions, minerals, metals, and associated organics. Deposits and sampling-line condensation must be included so that primary entrainment is distinguished from secondary particle formation. Metal evaluation requires absolute recovery, phase and size distribution, speciation, leaching, and bioaccessibility rather than char concentration alone. Organic assessment should cover residual adsorbates, PAHs, thermogenic aromatics, leachable organic carbon, tar, and aqueous condensate, identifying whether contaminants were destroyed, transformed, or transferred. Phase inventories should be normalized to initial dry feed and, where relevant, to the initial activated-carbon fraction so that enrichment caused by mass loss is separated from true transfer.

6.4 EPFR and ROS Reactivity Assessment

EPFR assessment should report calibrated spin concentration, g-factor, linewidth, moisture basis, storage conditions, and measurement delay. Because radical populations evolve during storage and exposure, decay and transformation should be measured under inert, air, humid, and aqueous conditions. ROS assays should be linked with metal leaching and normalized to material mass, surface area, or particle number. EPFR concentration alone is not a hazard metric; persistence, accessibility, aging, ROS production, and exposure-relevant endpoints are required. The core indicator set is summarized in Table 3. Measurements should be conducted for regenerated char and the most exposure-relevant particle or deposit fractions, because equal spin concentrations do not imply equal accessible reactivity.

NOTE: BET: Brunauer-Emmett-Teller theory; PAHs: Polycyclic Aromatic Hydrocarbons; EPFR: environmental persistent free radical; ROS: Reactive Oxygen Species.

6.5 Non-Compensable Environmental Safety Thresholds

Some outcomes should be treated as non-compensable constraints. Improved adsorption cannot justify metal leaching above application-specific criteria, uncontrolled submicrometer or metal-rich particle emissions, hazardous residual or newly formed organics, or persistent EPFR/ROS activity under realistic exposure conditions. Excessive carbon loss, mechanical failure, poor mass-balance closure, arcing, or disproportionate energy demand may provide additional exclusion criteria. Thresholds should reflect the intended reuse, applicable regulation, and feasible emission control rather than being converted into penalties that high adsorption performance can offset. These criteria should be specified before optimization to prevent post hoc weighting from masking unacceptable outcomes.

6.6 Definition of a Regeneration-Risk-Constrained Operating Window

A regeneration-risk-constrained operating window should first exclude conditions that fail these safety thresholds and then compare the remainder using adsorption recovery, carbon yield, pore characteristics, energy use, particle generation, residual chemical reactivity, and cycle stability. The trade-off is non-linear: low severity may leave adsorbates and unstable organics; intermediate severity can restore pores while preserving carbon and retaining many metals; high severity may cause burn-off, pore damage, inorganic particles, metal volatilization, altered EPFRs, and greater energy demand. The intermediate region should not be assumed automatically safe; it must be demonstrated by complete product recovery and exposure-relevant testing.

The boundaries are feedstock-specific because moisture, activated-carbon loading, ash, coagulant minerals, chlorine, sulfur, phosphorus, and initial contaminants govern microwave coupling and pollutant fate. No universal optimum temperature or power should be proposed without composition- and phase-resolved evidence. After exclusionary constraints are met, transparent multi-objective or Pareto analysis can compare acceptable conditions for a defined reuse scenario. Sensitivity analysis should be used to show how conclusions change with uncertain weighting, incomplete balances, or alternative reuse scenarios.