The migration behavior of heavy metals within flue-gas streams is primarily governed by their volatilization-condensation dynamics. However, the relevant volatility under industrial conditions is determined by the stability and partial pressure of the actual metal-containing species present at a given temperature and gas composition. Consequently, the same element can exhibit markedly different release behavior when its initial occurrence, redox environment, chlorine or sulfur availability, and surrounding mineral matrix change^[25-27].

As the flue gas cools, decreasing vapor pressure can drive gas-to-particle conversion through two distinct routes. Homogeneous nucleation occurs when supersaturated vapors form new ultrafine particles, whereas heterogeneous condensation and surface reaction occur on pre-existing fly ash, mineral aerosols, or reactor surfaces^[6,28,29]. These pathways are particularly important for semi-volatile elements because they control both particle-size enrichment and the subsequent efficiency of particulate capture devices.

Mercury (Hg) serves as the prototypical highly volatile metal. Elemental Hg0 can remain in the gas phase over much of the downstream temperature range, but its actual speciation is strongly affected by halogen chemistry, sulfur-containing gases, particulate matter, catalytic surfaces, and temperature^[30-33]. In chlorine-rich atmospheres, oxidative and chlorination pathways can increase the fraction of Hg(II)-containing species, whereas Hg⁰ can remain important when such conversion is limited. Thus, Hg speciation can differ substantially between coal-derived and chlorine-rich waste-incineration flue gases even at comparable downstream temperatures.

Arsenic (As), cadmium (Cd), lead (Pb), thallium (Tl), antimony (Sb) and zinc (Zn) can undergo appreciable high-temperature release followed by substantial redistribution during cooling^{[28,29,34,35]}. Their behavior is particularly sensitive to chemical form. For example, chlorine can enhance the vapor-phase transfer of Pb and Cd through the formation of chloride species, whereas reactions with mineral matrices or reduced sulfur species can favor condensed or particle-associated forms under suitable conditions^[25,28,36]. By contrast, chromium (Cr), nickel (Ni), copper (Cu), manganese (Mn), cobalt (Co), aluminum (Al) and tin (Sn) are generally more strongly associated with solid residues under typical thermal-treatment conditions, although their partitioning also varies with feed composition and process atmosphere^[2,24,26,27].

In real flue gas, heavy metals exist as oxides, chlorides, sulfides, or particle-bound species, whose volatility can differ sharply from that of the elemental form. The thermal behavior of these species also differs fundamentally: some compounds undergo conventional melting and boiling, whereas others sublime, decompose, or transform before a normal boiling transition is reached. Selected phase-transition data for representative elemental forms, oxides, chlorides, and sulfides are summarized in Table 1. These data provide useful thermophysical context for interpreting metal release and condensation, while actual flue-gas partitioning remains dependent on species partial pressures, gas composition, redox conditions, and mineral or particulate interactions.

The environmental fate and chemical transformation of heavy metals in flue gas are strongly governed by their interactions with reactive nonmetal components, including O₂, HCl, SO₂, CO₂ and H₂O. These gaseous species regulate a series of heterogeneous and homogeneous reactions, such as oxidation, chlorination, sulfation/sulfidation, and mineralization, thereby determining the distribution of metals among the gas phase, fine particulate matter, wet scrubbing solutions, and stabilized solid residues^[48,49].

Oxygen chemistry plays a critical role in controlling the redox conversion and oxide formation of heavy metals in flue gas. The presence of O₂ can promote the oxidation of elemental and low-valence metal species into higher oxidation states or oxide phases, resulting in changes in volatility, polarity, surface affinity, and interaction with particulate matter or sorbents.

For Hg, oxidation of Hg0 to oxidized Hg(II) species, such as HgCl₂ or surface-bound Hg-O/Cl species, generally enhances downstream capture efficiency due to their higher water solubility and stronger adsorption affinity^[16,50]. For As, changes in oxidation state influence its interaction with Fe- and Ca-containing mineral phases, and oxidized As species can be incorporated into condensed mineral phases through heterogeneous reactions^[51,52]. For Pb and Cd, oxide formation may enhance their association with fly ash and mineral surfaces; however, the stability and retention efficiency of these oxide species depend strongly on temperature, gas composition, and subsequent heterogeneous reactions. Importantly, oxidation does not necessarily lead to permanent immobilization, because metal oxides may undergo further chlorination, sulfidation, hydration, or surface-mediated redistribution during cooling and post-combustion processes^[48,49].

Chlorine-containing species represent one of the most influential factors controlling heavy-metal volatility and migration behavior. HCl, Cl₂, alkali chlorides, and chloride-rich fly ash can promote the transformation of metals or metal oxides into chlorides through gas-phase reactions and solid-gas interactions. For mercury, chlorination is generally considered beneficial because oxidized Hg-Cl species are more readily captured by downstream sorbents or wet-cleaning processes than Hg^0[31,53]. In contrast, chlorination of semi-volatile metals, including Pb, Cd, Sb, and As, may increase their volatility by generating mobile chloride-containing species, thereby facilitating vapor-phase transport and enrichment^[28,29,54]. Consequently, chlorine exhibits a dual regulatory role: it can promote downstream Hg capture when suitable conversion and collection pathways are available, but may simultaneously enhance the volatilization or fine-particle enrichment of other metals.

Sulfur-containing chemistry is more complex because sulfidation, sulfation, and sulfur-functionalized chemisorption represent distinct mechanisms. Sulfidation requires a sufficiently low oxygen potential and an available reduced sulfur source; under these conditions, metal sulfides such as HgS, CdS, or PbS can form or become stabilized^[25,36,55]. In an oxidizing flue gas dominated by O₂ and SO₂, however, bulk sulfide formation is not necessarily thermodynamically favored, and sulfate formation or sulfur-induced modification of surface chemistry can become more important. Sulfur-functionalized sorbents provide a further pathway in which reactive surface sulfur species directly bind or transform gaseous metals without requiring precipitation of a bulk sulfide phase^[55]. SO₂ can additionally modify catalyst and sorbent performance by occupying active sites, altering surface redox chemistry, or competing with chlorine-mediated reactions^[56]. Thus, sulfur can either promote retention or suppress capture depending on its chemical form, oxygen potential, and the nature of the reaction surface.

In addition to O₂, HCl, and SO₂, CO₂ and H₂O also participate in the transformation and stabilization of heavy metals. Water vapor can promote hydrolysis reactions, surface hydroxylation, and aqueous-phase transfer during wet scrubbing, whereas CO₂ may facilitate carbonate formation and mineralization of certain metals, particularly in alkaline ash or sorbent systems^[57,58]. Meanwhile, nitrogen oxides (NO_x) may indirectly affect metal transformation by altering oxidation pathways, radical chemistry, and catalyst-mediated reactions^[32,33]. Collectively, these metal-nonmetal interactions establish a dynamic chemical network that governs the persistence, partitioning, and stabilization of heavy metals in flue gas [Figure 1]. The same gas component may promote capture under one set of conditions but enhance volatilization, competitive adsorption, or secondary redistribution under another. This condition-dependent chemistry provides the mechanistic basis for the phase regulation and interfacial control strategies discussed in Section CONTROL PATHWAYS ALONG THE HEAVY-METAL FATE CHAIN.

Gas-phase regulation occurs before heavy metals are ultimately transferred to sorbents, liquid phases, particulate matter, or catalytic surfaces. Its primary function is not necessarily direct removal, but rather the transformation of metal species and regulation of their partitioning among gaseous, particulate, and condensed phases. Representative approaches include controlled cooling and condensation, homogeneous chemical oxidation, and field-assisted activation [Figure 2].

Controlled condensation represents one of the most straightforward physical approaches for regulating the phase partitioning of semi-volatile metals. During flue gas cooling, supersaturation of metal-containing vapors may induce nucleation, condensation, or deposition onto fly ash particles and reactor surfaces [Figure 2A]. This pathway is particularly important for As, Cd, Pb, and Sb, which volatilize under high-temperature conditions and subsequently undergo enrichment on fine particles during cooling^[59,60]. Because cooling can be integrated with existing heat-exchange, quenching, particulate-removal, or wet-cleaning processes, it may reduce the need for an additional dedicated treatment step in some process configurations. However, condensation itself represents a phase-transfer process rather than complete removal. The condensed species may remain in the form of submicron particles, soluble chlorides, or weakly bound surface deposits, requiring subsequent filtration, absorption, or stabilization^[61]. For Hg⁰, simple cooling is generally insufficient because of its high volatility and low condensation tendency under typical downstream conditions^[62].

Homogeneous oxidation regulates heavy-metal behavior through gas-phase oxidants, halogen species, or reactive radicals [Figure 2B]. Hg⁰ is the representative target, whereas Hg(II) species, particularly HgCl₂ and other oxidized compounds, are more reactive and more easily removed downstream^[63]. Halogen chemistry plays a central role in this process. Bromine-containing species generally exhibit higher oxidation activity toward Hg⁰ than chlorine-containing species, whereas chlorine remains highly relevant in HCl-rich flue gases and catalyst-assisted oxidation systems^[64]. More recently, machine-learning analysis has provided a quantitative perspective on how flue-gas composition and operating conditions jointly regulate homogeneous Hg⁰ oxidation. Using gas composition and operating parameters as model inputs, Zhang et al. identified Cl₂, HCl, initial Hg⁰ concentration, temperature, HBr, and SO₂ among the most influential variables governing predicted Hg⁰ oxidation^[65]. These outputs reinforce the view that homogeneous oxidation is a coupled multivariable process governed by oxidant identity, temperature, reactant concentration, and competing flue-gas constituents. In addition, advanced oxidation approaches based on ozone, photochemical activation, and other reactive oxygen species have demonstrated potential for enhancing Hg⁰ conversion while simultaneously affecting NO_x and organic pollutants^[66]. Although homogeneous oxidation can be effective, its low-carbon performance should be assessed together with oxidant consumption, corrosion risk, halogen slip, residual oxidants, and the possible formation of volatile chloride species from semi-volatile metals such as Pb, Cd, As, or Sb^[10,67]. Future low-carbon design should therefore minimize additional chemical input while retaining sufficient pre-conversion to improve downstream capture and avoid secondary metal migration.

Plasma-assisted conversion provides a field-driven approach for low-temperature heavy-metal speciation regulation [Figure 2C]. Nonthermal plasma generates energetic electrons, excited molecules, O·, OH·, O₃, and other reactive species without requiring bulk heating of the entire flue-gas stream^[68]. These reactive species can promote oxidation and may also participate in the simultaneous transformation of multiple pollutants under O₂-, H₂O-, HCl-, NO-, and SO₂-containing atmospheres^[69,70]. Unlike conventional activation by bulk-gas heating, nonthermal plasma uses energetic electrons to generate reactive species at relatively low gas temperatures^[68,69,71]. However, its practical application remains limited by energy efficiency, byproduct formation, humidity effects, and scale-up challenges. Plasma-catalyst coupling can partially address these limitations because catalysts may improve reaction selectivity and reduce the required energy input, whereas plasma supplies reactive intermediates that are difficult to generate under mild conditions. Xie et al. coupled nonthermal plasma oxidation with downstream dynamic-wave wet scrubbing for the simultaneous control of Hg⁰, NO, and SO₂^[71]. The integrated process achieved removal efficiencies of 98% for SO₂, 50.9% for NO, and 51.3% for Hg^0[71]. The markedly lower direct removal of NO and Hg⁰ compared with SO₂ reflects their poor aqueous solubility and the need for prior oxidative conversion. Plasma-generated reactive species first transformed these poorly captured pollutants into more reactive and water-accessible forms, after which the dynamic-wave scrubber intensified gas-liquid contact and transferred the oxidized products from the gas phase into the scrubbing liquid. In the optimized plasma-catalyst-scrubbing system using M(Ce,Cu)-Mn/13X, the reported removal efficiencies of SO₂, NO, and Hg⁰ reached 100%, 98.7%, and 96%, respectively^[71]. These results indicate that effective plasma-assisted control depends on the sequential coupling of gas-phase activation, catalytic conversion, and downstream phase transfer, rather than on plasma oxidation alone. The energy requirement of plasma-assisted systems should also be considered when evaluating their practical applicability. In a separate plasma-only Hg⁰/O₂/N₂ experiment in the same study, approximately 63% Hg⁰ conversion was obtained at an inlet Hg⁰ concentration of 100 μg m^-3, O₂ and N₂ flow rates of 250 and 700 mL min^-1, respectively, and a discharge frequency of 300 Hz, at an specific energy input (SEI) of 640 J L^-1 (~ 0.178 kWh m^-3)^[71]. The 51.3% and 96% Hg⁰ values both describe overall removal through the scrubbing stage, whereas the 63% value measures conversion in the plasma reactor alone under the Hg⁰/O₂/N₂ feed. The 640 J L^-1 benchmark therefore applies to this separate plasma-only test; the energy demand of the optimized 96% removal configuration requires an energy inventory for its reported operating conditions. However, the reported SEI represents only the electrical energy delivered to the discharge zone and does not include additional inputs associated with power-supply losses, auxiliary power, catalyst production, scrubbing reagents and pumping, and treatment of Hg-containing residues. Therefore, the advantage of catalyst-assisted plasma processes should be evaluated based on the reduction in total energy and material requirements needed to achieve a given removal target, together with the downstream management of captured mercury-containing streams.

The removal of volatile heavy metals is primarily governed by reactions occurring at gas-solid or gas-liquid interfaces. From an interfacial perspective, major removal pathways can be categorized into three fundamental processes: adsorption and catalytic conversion at gas-solid interfaces, and absorption at gas-liquid interfaces [Figure 3]^[72,73]. These processes perform different functions along the heavy-metal fate chain: adsorption transfers metals onto solid surfaces, catalytic conversion modifies their chemical speciation, and absorption transfers soluble or reactive species into the liquid phase.

In gas-solid adsorption, gaseous or condensable heavy-metal species are transferred to solid surfaces through external mass transfer and pore diffusion, followed by physical adsorption, chemisorption, or surface reaction. Porous carbon materials provide large surface areas and accessible transport pathways, whereas sulfur-, halogen-, oxygen-, and metal-containing functional groups can strengthen metal-surface interactions or promote transformation into less mobile surface species^[74]. Beyond carbonaceous materials, mineral sorbents, metal oxides, metal sulfides, fly ash-derived materials, and waste-derived porous materials provide additional platforms for heavy-metal capture^[75,76]. Li et al. designed a carbon material, hydrogen-substituted graphdiyne (HsGDY), featuring a hexagonal pore structure (16.3 Å) and an extended π-conjugated carbon skeleton composed of aromatic rings and acetylene linkages^[77]. HsGDY enables highly efficient gaseous Hg⁰ capture via an in situ adsorption-coupled oxidation process, benefiting from its large hexagonal pores and uneven surface charge. These features promote rapid Hg⁰ diffusion and effective electron transfer to form strongly bonded Hg, with a reported adsorption capacity of 0.71 μg/m², which is 23.66 and 17.75 times those of graphdiyne (GDY) and activated carbon (AC), respectively. In the laboratory fixed-bed experiment, the mass-normalized uptake was 113.5 μg g^-1 over 90 min under nitrogen containing 340 μg m^-3 Hg^0[77]. The practical sorbent requirement depends on mass-normalized breakthrough capacity, contact time, and gas composition. The study also examined temperature effects, SO₂ exposure, and eight adsorption-desorption cycles; these results establish laboratory performance, while continuous operation in a complete flue-gas mixture and production-scale synthesis remain to be demonstrated.

Multielement surface chemistry can further strengthen Hg fixation: Se-halogen systems can form highly stable Hg₃Se₂X₂ (X = Cl, Br, or I) phases, while SO₂-driven selenite-chloride surfaces have been shown to promote rapid Hg⁰ deposition and stable Hg₃Se₂Cl₂ formation^[78,79]. For semi-volatile metals such as As, Cd, Pb, and Sb, adsorption commonly occurs together with condensation and enrichment on particulate surfaces. Potential process-level advantages of adsorption include relatively simple reactor configurations and compatibility with existing particulate-control systems. Biomass- and waste-derived carbons provide additional feedstocks for this adsorption strategy^[80,81].

In gas-solid catalytic conversion, solid materials function primarily as reaction platforms rather than passive storage media. Catalysts activate O₂, HCl, H₂O, lattice oxygen, or other flue-gas components to convert poorly capturable metal species into more oxidized, chlorinated, or otherwise capturable forms. Catalytic surfaces can activate O₂ and HCl and promote the participation of surface or lattice oxygen in converting poorly capturable metal species into oxidized or chlorinated forms^{[16,31,50,53]}. H₂O can modify interfacial chemistry and the availability of adsorption sites, with its effect depending on the catalyst and gas composition^[48,57]. Hg⁰ oxidation over SCR catalysts and transition-metal oxides is a representative example because the resulting Hg(II) species can subsequently be captured through wet scrubbing or retained by downstream solids^[82]. Yang et al. developed a single-atom Fe-decorated N-doped carbon catalyst for the simultaneous conversion of NO and Hg^0[83]. The Fe₁-N₄-C sites exhibited favorable adsorption and activation of NO and O₂ while maintaining catalytic activity in the presence of SO₂ under the reported conditions^[83]. In addition, catalytic conversion alone does not constitute permanent removal: the products must ultimately be immobilized on solids, transferred into liquids, or separated with particulate matter. Catalyst poisoning, competitive adsorption, product accumulation, and loss of active surface species must therefore be considered together with apparent conversion efficiency. This single-atom system provides laboratory evidence for site-specific activation and sulfur tolerance, while industrial maturity requires sustained conversion under representative space velocities, combined impurities, and repeated regeneration. Relevant engineering comparisons must also include catalyst preparation and replacement, pressure loss, the downstream Hg(II) capture step, and the final mercury balance.

In gas-liquid absorption, heavy-metal species cross the gas-liquid interface and undergo dissolution, complexation, oxidation-reduction reactions, or precipitation. This pathway is particularly effective for water-soluble oxidized species such as HgCl₂ and can be integrated with wet flue-gas desulfurization and other existing scrubbing systems^[84]. The overall efficiency of absorption depends not only on gas-liquid mass transfer but also on liquid-phase reaction kinetics and product stability. For example, absorbed Hg(II) may be reduced by sulfite or other reductive components and regenerated as volatile Hg⁰, resulting in secondary emission^[85]. Similarly, dissolved As, Cd, and Pb require subsequent conversion into stable precipitates, concentrated products, or recoverable resources^[86,87]. From a low-carbon perspective, absorption systems should also be evaluated according to reagent demand, pumping energy, wastewater generation, sludge production, re-emission control, and downstream treatment requirements.

Collectively, gas-solid adsorption, gas-solid catalytic conversion, and gas-liquid absorption constitute a continuous reaction network rather than isolated treatment technologies. Adsorption provides direct capture, catalytic processes regulate metal speciation and reactivity, and absorption enables transfer into the liquid phase for subsequent chemical treatment. Their effective integration determines whether heavy metals are merely redistributed among different phases or are ultimately stabilized, recovered, or directed toward a controlled final sink within the treatment system.

Co-benefit heavy-metal control is achieved not through a single dedicated device, but through the sequential matching of heavy-metal speciation with the conversion and capture functions already available in conventional air-pollution-control systems. Figure 4 represents a functional sequence: source release, controlled cooling, catalytic conversion, optional sorbent injection, particulate control, wet scrubbing, and residue management, rather than a universal equipment configuration. Although the specific arrangement varies among coal combustion, waste incineration, and nonferrous smelting, the underlying principle remains consistent: volatile metal species are first transformed into more condensable, adsorbable, or soluble forms, subsequently transferred into solid or liquid sinks, and finally stabilized or recovered from the resulting residues^[11,88]. This integrated strategy may reduce additional energy and material requirements by maximizing the utilization of existing APCDs instead of introducing independent energy-intensive units specifically for trace-metal removal.

At the emission source, heavy metals are released in substantially different chemical forms depending on their volatility, feedstock composition, and combustion or thermal-processing conditions. Less volatile metals are preferentially retained in bottom ash or slag, whereas semi-volatile metals may vaporize as elemental, oxide, or chloride species and subsequently redistribute during cooling^[11]. The heavy-metal control pathway should be considered as comprising multiple interconnected streams, including particle-associated metals, semi-volatile metal vapors, and gaseous mercury. Downstream control devices do not remove elements independently of their chemical form; rather, they capture the specific species generated and delivered by upstream thermal processes.

Controlled cooling provides the first opportunity for phase redistribution and partial capture. As temperature decreases, supersaturation promotes nucleation, condensation, and adsorption of semi-volatile metals onto entrained ash particles. As, Cd, Pb, Zn, and related elements may therefore shift from the vapor phase toward fine particulate fractions that are accessible to dust collectors^[59,60]. However, cooling should be regarded as a phase-regulation step rather than as complete removal. Metal enrichment frequently occurs on ultrafine particles, while volatile chlorides and condensable species may remain difficult to collect.

Laboratory and field studies show that SCR-mediated Hg⁰ oxidation can facilitate downstream capture by particulate-control and wet-desulfurization units^[88-90]. Nevertheless, SCR should be considered a speciation-conversion unit rather than a final mercury sink. Its co-benefit performance depends on flue-gas composition, catalyst condition, and operating temperature^[82]. NH₃, SO₂, and H₂O can modify Hg⁰ conversion through competitive adsorption and changes in surface chemistry^[32,82,89].

Sorbent injection combined with particulate control represents the primary solid-phase capture pathway. Activated carbon, functionalized carbon materials, mineral sorbents, or recycled fly ash can be injected upstream of electrostatic precipitators (ESPs) or fabric filters to enhance the adsorption of Hg and other volatile metals^[91,92]. The subsequent dust-collection stage removes fly ash, condensed metal species, and spent sorbents. In a field study of six municipal solid waste (MSW) incinerators, Li et al. estimated that achieving 90% overall Hg removal required increasing activated-carbon injection from approximately 100 mg Nm^-3 to 135 mg Nm^-3 for grate furnaces and 170 mg Nm^-3 for circulating fluidized-bed furnaces^[91]. The corresponding additional sorbent demands of approximately 35 and 70 mg Nm^-3 illustrate how a higher removal target can increase material consumption and metal-bearing solid waste.

Wet scrubbers and wet flue gas desulfurization (WFGD) systems provide the final liquid-phase capture pathway for water-soluble heavy-metal species. Soluble metal chlorides and entrained fine particles can be transferred into the liquid through dissolution, complexation, precipitation, and droplet capture processes^[93].

The ultimate goal of co-benefit heavy-metal control is therefore not simply removal from the gas stream, but effective residue management. Fly ash, spent sorbents, gypsum, scrubber sludge, and wastewater concentrates containing captured metals may remain environmentally mobile or thermally remobilizable if not appropriately treated^[94,95]. Consequently, the optimal APCD configuration should not maximize the performance of individual units independently. Instead, it should minimize total energy consumption, pressure loss, reagent demand, wastewater generation, and secondary waste formation while maintaining a closed heavy-metal mass balance^[18,96].

Heavy-metal control in industrial flue gas should be understood as a continuous fate-regulation process rather than as an isolated removal step. The volatility and chemical speciation established during high-temperature processing determine subsequent condensation, adsorption, catalytic conversion, and absorption, while conventional APCDs redistribute the captured metals into fly ash, spent sorbents, gypsum, sludge, and wastewater. Accordingly, high stack-removal efficiency is only one component of effective control. The additional energy and material inputs required for removal, together with the stability and final destination of the transferred metals, determine whether an apparently efficient process provides a genuine environmental benefit. Existing life-cycle studies already demonstrate that both the choice of front-end control route and the downstream treatment of metal-bearing residues can substantially alter the overall environmental burden^[18,19].

Future control strategies should therefore integrate emission reduction with deliberate regulation of the final metal-containing stream. Where existing APCDs provide sufficient phase conversion and capture, maximizing these co-benefit functions can limit additional energy and material consumption. Where further intensification is required, sorbent, reagent, and external-energy inputs should be justified by the additional removal achieved and by their effects on downstream residue generation. More importantly, capture should be coupled to a defined endpoint. Concentrated and selectively separable streams may support regeneration and metal recovery, whereas dilute or compositionally complex residues may require conversion into low-mobility mineral or chemical forms^[21,106,107]. The appropriate pathway will therefore vary with metal concentration, speciation, toxicity, recoverability, and the energy and reagent requirements of downstream treatment.

To translate this fate-oriented framework into practical decision-making, future studies should establish evaluation protocols that bridge controlled laboratory experiments and realistic operating environments. Validation should progressively move from simplified gas mixtures toward pilot-scale conditions incorporating representative H₂O, SO₂, HCl, NO_x, and particulate components, because these species can simultaneously influence metal speciation, surface reactivity, and phase transfer. Long-term operation should be evaluated not only by removal efficiency but also by breakthrough behavior, catalyst or sorbent deactivation, corrosion, and redistribution of metals across the treatment train. For regeneration-based processes, the recovered capacity, energy and reagent requirements, metal release during regeneration, and material replacement frequency should be quantitatively assessed. Meanwhile, the stability of final metal-containing streams, including fly ash, spent sorbents, gypsum, sludge, and wastewater concentrates, should be verified through leaching and aging assessments. Integrating these measurements with closed metal balances and energy/material inventories will be essential for linking laboratory-scale performance with operational burden, carbon demand, and the ultimate environmental fate of captured metals.

Ultimately, the next stage of heavy-metal control should move from maximizing removal at individual unit operations toward minimizing the combined burden of emission control and final metal management. A low-carbon and fate-oriented system should use the least additional energy and material necessary to transfer metals from flue gas into either a recoverable product or a demonstrably stable sink. Establishing this connection between speciation, capture, cross-media transfer, and final environmental endpoint provides a more meaningful basis for designing heavy-metal control systems compatible with increasingly stringent emission and carbon-reduction targets.