Mercury emissions from six coal-fired power plants in China

Mercury emission field measurements based on the Ontario Hydro Method (OHM) were conducted for six coal-fired power plants in China. The mercury mass balances for the six power plants varied from 100.3% to 139.5% of the input coal mercury for the whole system. About 0.02%–1.2% of the mercury remained in the bottom ash. In the first five power plants equipped with pulverized coal boiler, most of the mercury was emitted from the stack to the atmosphere. The plants with Electrostatic Precipitator (ESP) system emitted more Hg⁰ than Hg²⁺, while the plants with the Fabric Filter (FF) emitted less Hg⁰ than Hg²⁺. Virtually all of the Hg^P enter the ESP or the FF was removed. The FF systems had better Hg⁰ and Hg²⁺ removal efficiencies than the ESP systems. The flue gas desulfurization (FGD) system removed up to 78.0% of Hg²⁺ and only 3.14% of Hg⁰ in the flue gas, while 8.94% of the original mercury in the coal was removed by the FGD system. The average mercury removal efficiencies of the ESP systems was 11.5%, that of the FF systems was 52.3% and that of the combined ESP + FGD system was 13.7%, much lower than the average removal efficiencies of pollution control device systems in US plants which have been used in previous studies of Chinese mercury emission inventory. Hg⁰, rather than Hg²⁺ as assumed in previous estimates, has been found to be the dominant species emitted in the atmosphere. The average emission factor was found to be 4.70 g/TJ (10.92 bl/Tbtu), which is much higher than for US plants burning bituminous coals due to the high mercury content in the Chinese coal and the low mercury removal efficiency of air pollution control devices of power plants.     

Mercury speciation and emission from the coal‐fired power plant filled with flue gas desulfurization equipment

Mercury speciation and emission from two Chinese coal‐fired power stations equipped with flue gas desulfurization device were investigated. Research results reveal that Hg⁰ is the main form in the flue gas in Plant 1; Hg²⁺ is the main form in the flue gas in Plant 2. Most of mercury was emitted to the atmosphere, which was about 77–98%, and the elemental mercury released to atmosphere ranged 73–94% approximately. A pot of mercury is adsorbed by bottom ash, electrostatic precipitator (ESP) ash, and gypsum in Plant 1. However, most mercury, the scale of which is 75–83.2%, is collected by ESP ash, and only 7.0–12.2% mercury is emitted to the atmosphere in Plant 2. The mercury removal by NID semi‐desulfurization system is higher than wet flue gas desulfurization (WFGD) desulfurization system.     

Elimination of inorganic mercury from waste waters using crandallite‐type compounds

The removal of inorganic mercury from waste water streams arising from mines, using an artificial amorphous compound of the crandallite type synthesized in our laboratory, Ca_0.5Sr_0.5Al₃(OH)₆(HPO₄) (PO₄), has been investigated. This compound exhibits an extremely wide range of ionic substitutions: Ca²⁺ and Sr²⁺ were interchanged with Hg²⁺, so the mercury content of the waste water, ranging from 70 to 90 ppm, was reduced to less than 0.1 ppm. The process has been studied under batch conditions. The crandallite showed a high capacity for the exchange of mercury from mercuric nitrate solutions, 1.555 meq g^?1. The ion‐exchange equilibrium isotherms for Hg²⁺ were correlated by the Langmuir equation. The recovery of mercury from Hg‐crandallite using HCl solutions and thermal treatment was also studied. Optimum recuperation of mercury is achieved by chemical reaction with HCl solution (pH 2.25). At these conditions, 75% of the mercury is recovered as the HgCl₄^2? complex in a simple batch process, and the crandallite (in the protonic form) can be reused. © 2003 Society of Chemical Industry     

Speciation of mercury in fly ashes by temperature programmed decomposition

Mercury (Hg) is a toxic trace element which is emitted mostly in gas phase during coal combustion, although some Hg compounds may be retained in the fly ashes depending on the characteristics of the ashes and process conditions. To improve the retention of Hg in the fly ashes a good knowledge of the capture mechanism and Hg species present in the fly ashes is essential. The temperature programmed decomposition technique was chosen to identify the Hg species present in fly ashes obtained from two Pulverized Coal Combustion (PCC) plants and a Fluidized Bed Combustion (FBC) plant. The fly ashes were then used as Hg sorbents in a simulated flue gas of coal combustion and gasification. The Hg compounds found in the fly ash from the FBC plant after elemental mercury retention were mainly HgCl₂ and HgSO₄. The Hg species present in the two fly ashes from the two PCC plants were HgCl₂ and Hg⁰. The Hg species formed in the coal gasification atmosphere was HgS for all three fly ashes. The only Hg compound identified in the fly ashes after the retention of mercury chloride was HgCl₂.     

Effects of NOx, α-Fe2O3, γ-Fe2O3, and HCl on mercury transformations in a 7-kW coal combustion system

Bench-scale investigations indicate that NO, NO₂, hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃), and HCl promote the conversion of gaseous elemental mercury (Hg⁰) to gaseous oxidized mercury (Hg²⁺) and/or particle-associated mercury (Hg[p]) in simulated coal combustion flue gases. In this investigation, the effects of NO_x, α-Fe₂O₃, γ-Fe₂O₃, and HCl on Hg transformations were evaluated by injecting them into actual coal combustion flue gases produced from burning subbituminous Absaloka and lignitic Falkirk coals in a 7-kW down-fired cylindrical furnace. A bituminous Blacksville coal known to produce an Hg²⁺-rich combustion flue gas was also burned in the system. The American Society for Testing and Materials Method D6784-02 (Ontario Hydro method) or an online Hg analyzer equipped to measure Hg⁰ and total gaseous mercury (Hg[tot]) was used to monitor Hg speciation at the baghouse inlet (160–195 °C) and outlet (110–140 °C) locations of the system. As expected, the baseline Blacksville flue gas was composed predominantly of Hg²⁺ (Hg²⁺/Hg[tot]=0.77), whereas Absaloka and Falkirk flue gases contained primarily Hg⁰ (Hg⁰/Hg[tot]=0.84 and 0.78, respectively). Injections of NO₂ (80–190 ppmv) at 440–880 °C and α-Fe₂O₃ (15 and 6 wt.%) at 450 °C into Absaloka and Falkirk coal combustion flue gases did not significantly affect Hg speciation. The lack of Hg⁰ to Hg²⁺ conversion suggests that components of Absaloka and Falkirk combustion flue gases and/or fly ashes inhibit heterogeneous Hg⁰–NO_x–α-Fe₂O₃ reactions or that the flue gas quench rate in the 7-kW system is much different in relation to bench-scale flue gas simulators.An abundance of Hg²⁺, HCl, and γ-Fe₂O₃ in Blacksville flue gas and the inertness of injected α-Fe₂O₃ with respect to heterogeneous Hg⁰ oxidation in Absaloka and Falkirk flue gases suggested that γ-Fe₂O₃ catalyzes Hg²⁺ formation and that HCl is an important Hg⁰ reactant. The filtration of Absaloka and Falkirk combustion flue gases at 150 °C through fabric filters with ≈60 g/m² γ-Fe₂O₃ indicated that about 30% of the Hg⁰ in Absaloka and Falkirk flue gases was converted to Hg²⁺ and/or Hg(p). HCl injection (100 ppmv) into the Absaloka combustion flue gas converted most of the Hg⁰ to Hg²⁺, whereas HCl injection into the Falkirk flue gas converted most of the Hg⁰ and Hg²⁺ to Hg(p). Additions of γ-Fe₂O₃ and HCl did not have a synergistic effect on Hg⁰ oxidation. The filtration of Absaloka and Falkirk flue gases through much greater fabric filter loadings of 475 g/m² γ-Fe₂O₃ essentially doubled the baghouse Hg[tot] removal efficiency to about 50%. Results from this investigation demonstrate the importance of evaluating potential Hg⁰ reactants and oxidation catalysts in actual coal combustion flue gases.     

Speciation of As, Cr, Se and Hg under coal fired power station conditions

Coal combustion from power stations is an important anthropogenic contributor of toxic trace elements to the environment. Some trace elements may be emitted in range of valencies, often with varying toxicity and bioavailability. Hence, determination of trace element speciation in coals and their combustion products is important for conducting comprehensive risk assessments of the emissions from coal-fired power stations. This study focuses on speciation of selected trace elements, As, Cr, and Se, in coal combustion products and Hg in flue gas, which were sampled at one Australian power station. Different analytical methods such as secondary ion mass spectrometry (SIMS), ion chromatography-inductively coupled plasma mass spectrometry (IC-ICPMS) and X-ray absorption near edge structure spectrometry (XANES) were used to determine trace element speciation in coal and ash samples. Results showed that As, Cr and Se are all present in a range of valency states in coal. Concentrations of As and Se in the bottom ash as well as the more toxic hexavalent chromium were less than the detection limits. The more toxic As³⁺ form in fly ash was at 10% of the total arsenic, while selenium was mainly found in Se⁴⁺ form. Hexavalent chromium (Cr⁶⁺) in fly ash was 2.7% of the total fly ash chromium. Mercury speciation in flue gas was determined using the Ontario Hydro sampling train and analysis technique. Approximately 58% of the total mercury in flue gas was released in the elemental form (Hg⁰), which, among all mercury species, has the highest residence time in the environment due to lower solubility. This work summarises the performance of the selected analytical techniques for speciation of trace elements.     

Experimental study on mercury transformation and removal in coal-fired boiler flue gases

This paper reported mercury speciation and emissions from five coal-fired power stations in China. The standard Ontario Hydro Method (OHM) was used into the flue gas mercury sampling before and after fabric filter (FF)/electrostatic precipitator (ESP) locations in these coal-fired power stations, and then various mercury speciation such as Hg⁰, Hg²⁺ and Hg^P in flue gas, was analyzed by using EPA method. The solid samples such as coal, bottom ash and ESP ash, were analyzed by DMA 80 based on EPA Method 7473. Through analysis the mercury speciation varied greatly when flue gas went through FF/ESP. Of the total mercury in flue gas, the concentration of Hg²⁺ is in the range of 0.11–14.76 μg/N m³ before FF/ESP and 0.02–21.20 μg/N m³ after FF/ESP; the concentration of Hg⁰ ranges in 1.18–33.63 μg/N m³ before FF/ESP and 0.77–13.57 μg/N m³ after FF/ESP, and that of Hg^P is in the scope of 0–12.11 μg/N m³ before FF/ESP and 0–0.54 μg/N m³ after FF/ESP. The proportion of Hg²⁺ ranges from 4.87%–50.93% before FF/ESP and 2.02%–75.55% after FF/ESP, while that of Hg⁰ is between 13.81% – 94.79% before FF/ESP and 15.69%–98% after FF/ESP, with that of Hg^P is in the range of 0%–45.13% before FF/ESP and 0%–11.03% after FF/ESP. The mercury in flue gas mainly existed in the forms of Hg⁰ and Hg²⁺. The concentrations of chlorine and sulfur in coal and flue gas influence the species of Hg that are formed in the flue gas entering air pollution control devices. The concentrations of chlorine, sulfur and mercury in coal and the compositions of fly ash had significant effects on mercury emissions.     

Abatement of Gas-Phase Mercury—Recent Developments

Among various pollutants, mercury has a significant impact on the environment, human beings, and wildlife with its different forms, namely, elemental mercury (Hg⁰), oxidized mercury (Hg²⁺), and particle-bound mercury (Hg_p). Mercury dispersions mainly occur from coal burning, which is the world's major energy source. Among the three forms, Hg²⁺ and Hg_p are relatively easy to remove from the flue gas by employing typical air pollution control devices; on the other hand, Hg⁰ is difficult to remove. Various methods are available to detain elemental mercury. Recent developments in mercury removal options, especially during the last years, are reviewed. Main concentration has been focused on the removal methods of elemental mercury by novel sorbents and catalytic systems. A current challenge is to develop novel nanomaterials meeting rigorous requirements (easy separation, recyclability, and cost-effectiveness) for eventual exploitation.     

半干法脱硫灰对Hg~0的催化氧化及吸附特性

采用新型一体化脱硫工艺和循环流化床烟气脱硫工艺的脱硫灰为吸附剂,使用固定床反应器,在模拟烟气的条件下研究了两种半干法脱硫灰对汞的吸附及催化氧化特性。研究结果表明,吸附于脱硫灰表面的汞主要以Hg2+的形态存在,多数情况下,更高的汞氧化率伴随有更高的汞吸附率,HCl、Cl2、NO2在脱硫灰的催化作用下能有效氧化Hg0,且不同组分对Hg0的氧化作用可以累积,而NO和SO2抑制了脱硫灰对汞的吸附。脱硫灰中未燃尽碳和Fe2O3对脱硫灰吸附和催化氧化气态汞具有显著促进作用。汞吸附率和氧化率在使用两种脱硫灰作为吸附剂时均随温度升高先增大后减小,这是传质过程和反应速率共同作用的结果。     

Effect of selective catalytic reactor on oxidation and enhanced removal of mercury in coal-fired power plants

The present study investigated the variation of mercury (Hg) speciation within the air pollution control devices (APCDs) in bituminous coal-fired power plants. The effect of selective catalytic reduction (SCR) system, which is mainly installed for NOx removal, on elemental Hg (Hg⁰) oxidation and enhancement of Hg removal within APCDs, was studied. Hg speciations in flue gas at the inlet and outlet of each APCDs, such as SCR, cold-side electrostatic precipitator (CS-ESP) and flue gas desulphurization (FGD), were analyzed. Sampling and analysis were carried out according to Ontario Hydro Method (OHM). Overall Hg removal efficiency of APCDs, on average, was about 61% and 47% with and without SCR system, respectively. In the flue gas, Hg was mainly distributed in gaseous (elemental and oxidized) form. The oxidized to elemental Hg partitioning coefficient increased due to oxidation of Hg⁰ across the SCR system and decreased due to the removal of oxidized Hg (Hg²⁺) across a wet FGD system. Hg⁰ oxidation across the SCR system varied from 74% to 7% in tested coal-fired power plants. The comparative study shows that the installation of an SCR system increased Hg removal efficiency and suppressed the reemission of captured Hg⁰ within a wet FGD system.     

Unconventional ion exchange resins and their retention properties for Hg2+ ions

Synthesis of two unconventional ion exchange resins and their behaviour on the mercury sorption experiments were investigated.The ion exchange resins were obtained by the quaternization reaction of 4-vinylpyridine:divinylbenzene copolymer, gel-type, by two ways namely, the nucleophilic substitution of the pyridine matrix with 2-chloroacetamide and the nucleophilic addition of protonated copolymer to acrylamide.Comparative sorptions of Hg²⁺ ions on the synthesized pyridine resins by batch experiments in mono- and binary system were analyzed. Mercury retention experiments aimed to study the influence of the solution concentration, contact time and solution pH. The removal of Hg²⁺ ions from aqueous solutions depends on the pH values, the amount of the retained mercury increased with the pH value.The studied strong base pyridine anion exchange resins presented a good selectivity for the Hg²⁺ ions during the competitive sorption of Hg²⁺/Cu²⁺, Hg²⁺/Zn²⁺ and Hg²⁺/Fe³⁺ at metal cations ratio of 1:1.     

Study of elemental mercury re-emission through a lab-scale simulated scrubber

This paper describes a lab-scale simulated scrubber that was designed and built in the laboratory at Western Kentucky University’s Institute for Combustion Science and Environmental Technology. A series of tests on slurries of CaO, CaSO₃, CaSO₄/CaSO₃ and Na₂SO₃ were carried out to simulate recirculating slurries in different oxidation modes. Elemental mercury (Hg⁰) re-emission was replicated through the simulated scrubber. The relationship between the oxidation-reduction potential (ORP) of the slurries and the Hg⁰ re-emissions was evaluated. Elemental mercury re-emission occurred when Hg²⁺ that was absorbed in the simulated scrubber was converted to Hg⁰; then, Hg⁰ was emitted from the slurry together with the carrier gas. The effects of both the reagents and the operational conditions (including the temperature, pH, and oxygen concentrations in the carrier gas) on the Hg⁰ re-emission rates in the simulated scrubber were investigated. The results indicated that as the operational temperature of the scrubber and the pH value of the slurry increased, the Hg⁰ concentrations that were emitted from the simulated scrubber increased. The Hg⁰ re-emission rates decreased as the O₂ concentration in the carrier gas increased. In addition, the effects of additives to suppress Hg⁰ re-emission were evaluated in this paper. Sodium tetrasulfide, TMT 15, NaHS and HI were added to the slurry, while Hg²⁺, which was absorbed in the slurry, was retained in the slurry as mercury precipitates. Therefore, there was a significant capacity for the additives to suppress Hg⁰ re-emission.     

Analysis of mercury species present during coal combustion by thermal desorption

Mercury in coal and its emissions from coal-fired boilers is a topic of primary environmental concern in the United States and Europe. The predominant forms of mercury in coal-fired flue gas are elemental (Hg⁰) and oxidized (Hg²⁺, primarily as HgCl₂). Because Hg²⁺ is more condensable and far more water soluble than Hg⁰, the wide variability in mercury speciation in coal-fired flue gases undermines the total mercury removal efficiency of most mercury emission control technologies. It is important therefore to have an understanding of the behaviour of mercury during coal combustion and the mechanisms of mercury oxidation along the flue gas path. In this study, a temperature programmed decomposition technique was applied in order to acquire an understanding of the mode of decomposition of mercury species during coal combustion. A series of mercury model compounds were used for qualitative calibration. The temperature appearance range of the main mercury species can be arranged in increasing order as HgCl₂ < HgS < HgO < HgSO₄. Different fly ashes with certified and reference values for mercury concentration were used to evaluate the method. This study has shown that the thermal decomposition test is a newly developed efficient method for identifying and quantifying mercury species from coal combustion products.     

Stability of mercury on three activated carbon sorbents

The stability of adsorbed mercury on activated carbon (AC) is very important for avoiding reemission. Based on research concerning the stability of mercury on the AC relative to leaching and thermal desorption, our conclusions are as follows. Leaching tests show that mercury appears to be very stable on the AC. The Hg concentration in the leachate is much lower than the TCLP safety limit of 0.025 mg/l. Leaching time and liquid to solid (L/S) have some influence on the leaching results, but the influence is far less than which leads the Hg concentrate to exceed the safety limit. Leaching tests for mercury at lower and higher pH are aggressive compared with the neutral pH test. There is much more mercury released from the AC at longer heating time for mercury. At the same time, it seems that the stability of adsorbed original Hg⁰ on the AC is stronger than that of adsorbed original Hg²⁺.     

Mercury oxidation and adsorption characteristics of chemically promoted activated carbon sorbents

Previous entrained-flow tests conducted under elemental mercury (Hg⁰)-laden air found that significant amounts of oxidized mercury (Hg²⁺) are not adsorbed onto cupric chloride-impregnated carbon (CuCl₂-AC) and brominated activated carbon (DARCO Hg-LH), but entrained to the gas phase. In this study, these sorbents were tested in a fixed-bed system and a filter-added entrained-flow system to further investigate Hg⁰ oxidation and adsorption characteristics of CuCl₂-AC and DARCO Hg-LH. These test results suggested that CuCl₂-AC has different sites available for Hg⁰ oxidation and Hg adsorption, and the resultant oxidized mercury generated from the reaction between Hg⁰ and CuCl₂ is re-adsorbed at the site of CuCl₂-AC available for adsorption. The resultant oxidized mercury was also found to be easily re-adsorbed onto CuCl₂-AC and DARCO Hg-LH in the filter connected to the entrained-flow reactor.     

Alanine containing porous beads for mercury removal from artificial solutions

N‐methacryloyl‐(L )‐alanine (MALA) was synthesized by using methacryloyl chloride and alanine as a metal‐complexing ligand or comonomer. Spherical beads with an average diameter of 150–200 μm were obtained by suspension polymerization of MALA and 2‐hydroxyethyl methacrylate (HEMA) conducted in an aqueous dispersion medium. Poly(HEMA–MALA) beads were characterized by SEM, swelling studies, surface area measurement, and elemental analysis. Poly(HEMA–MALA) beads have a specific surface area of 68.5 m²/g. Poly(HEMA–MALA) beads with a swelling ratio of 63%, and containing 247 μmol MALA/g were used in the removal of Hg²⁺ from aqueous solutions. Adsorption equilibrium was achieved in about 60 min. The adsorption of Hg²⁺ ions onto PHEMA beads was negligible (0.3 mg/g). The MALA incorporation into the polymer structure significantly increased the mercury adsorption capacity (168 mg/g). Adsorption capacity of MALA containing beads increased significantly with pH. The adsorption of Hg²⁺ ions increased with increasing pH and reached a plateau value at around pH 5.0. Competitive heavy metal adsorption from aqueous solutions containing Cd²⁺, Cu²⁺, Pb²⁺, and Hg²⁺ was also investigated. The adsorption capacities are 44.5 mg/g for Hg²⁺, 6.4 mg/g for Cd²⁺, 2.9 mg/g for Pb²⁺, and 2.0 mg/g for Cu²⁺ ions. These results may be considered as an indication of higher specificity of the poly(HEMA–MALA) beads for the Hg²⁺ comparing to other ions. Consecutive adsorption and elution operations showed the feasibility of repeated use for poly(HEMA–MALA) chelating beads. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1222–1228, 2006     

Radiometric studies of mercury loss from fungicidal paints. 1. Loss of phenyl mercuric acetate

Exposure tests have been made on paint films prepared from an emulsion paint containing phenyl mercuric acetate (PMA) labelled with ²⁰³Hg. Loss of mercury from the films has been followed radiometrically. Films exposed indoors in a confined space (no air circulation) showed little, if any, mercury loss for up to 8 months. Films exposed in the open laboratory lost from 20 to more than 60% of mercury after 250 days. This agrees qualitatively with the results of other workers on similar paints. At 50°, films lost mercury at rates about 10 times greater than those at ambient temperature. Films exposed out-of-doors lost the majority of their mercury after a few days of rain. Pre-treatment of films, e.g. by u.v. radiation, had no effect on their subsequent outdoor behaviour. Even well-dried films lost mercury at the same rate as newly applied films. Films with a low PMA content (0·02% Hg) lost mercury at a higher percentage rate than films with a high one (0·2% Hg). Thick films (0·002 in.) lost mercury at a lower rate than thin ones (0·001 in.). A mercury-free top coat also reduced loss rate. All loss rates decreased with increasing time. These trends applied to all the exposure conditions studied. Mercury loss appears to be due to both volatilisation and dissolution of PMA and not to decomposition. The results indicate a non-homogeneous distribution of PMA throughout the paint film, and a diffusion-controlled process.     

Preparation and structural characterization of mercury 21-thiaporphyrin complex: HgII(Stpp)Cl (Stpp=tetraphenyl-21-thiaporphyrin anion)

Treatment of tetraphenyl-21-thiaporphyrin (StppH) with Hg(OAc)₂ in CH₂Cl₂ yields diamagnetic Hg^II(Stpp)Cl complex. The coordination sphere around Hg²⁺ in the monomeric molecule is described as a five-coordinate distorted trigonal bipyramid with the bonding to the three pyrrole nitrogens [Hg(1)–N=2.104(4), 2.626(4), 2.640(4) Å], the thiophene sulfur [Hg(1)–S=2.801(1) Å], and one axial chloride ligand [Hg(1)–Cl(1)=2.318(1) Å]. The plane of the three pyrrole nitrogen atoms [i.e., N(1), N(2), N(3)] bonded to Hg²⁺ is adopted as a reference plane 3N. Because of its larger size, the Hg²⁺ is considerably out of the 3N plane; its displacement of 1.41 Å is in the same direction as that of the apical Cl⁻ ligand. The thiophene ring is slightly folded so that the dihedral angle between the C(13)–C(14)–C(15)–C(16) and C(13)–S(1)–C(16) planes is 7.3°.     

Understanding mercury binding on activated carbon

Understanding the mechanism by which mercury adsorbs on activated carbon is crucial to the design and fabrication of effective capture technologies. In this study, the possible binding mechanism of mercury (Hg) and its species, i.e., HgCl and HgCl₂ on activated carbon is investigated using ab initio-based energetic calculations. The activated carbon surface is modeled by a single graphene layer in which the edge atoms on the upper side are unsaturated in order to simulate the active sites. In some cases, chlorine atoms are placed at the edge sites to examine the effect of chlorine on the binding of Hg, HgCl and HgCl₂. It has been concluded that both HgCl and HgCl₂ can be adsorbed dissociatively or non-dissociatively. In the case of dissociative adsorption, it is energetically favorable for atomic Hg to desorb and energetically favorable for it to remain on the surface in the Hg¹⁺ state, HgCl. The Hg²⁺ oxidized compound, HgCl₂ was not found to be stable on the surface. The most probable mercury species on the surface was found to be HgCl.     

Characterization of mercury- and zinc-doped alkali-activated slag matrix: Part I. Mercury

The physical properties, pore structure, hydration process and hydration products of mercury-doped (Hg-doped) alkali-activated slag (AAS) matrixes have been evaluated by examination of physical properties, pore structure analysis and XRD, TG-DTG, FTIR and TCLP methods. Low concentrations of Hg²⁺ ions had little effect on the compressive strength, pore structure and degree of hydration of AAS matrixes. The addition of 2% Hg²⁺ ions into the AAS matrix brought out an evident retardation on early hydration and reduction of early compressive strength, but no negative effects were noticed after hydration for 28 days. The results also show that up to 2% of Hg²⁺ ions can be effectively immobilized in the AAS matrix, with the leaching meeting the TCLP mercury limit. Two mechanisms, physical encapsulation and chemical fixation, are assumed to be responsible for the immobilization of mercury in the AAS matrix.