Review pubs.acs.org/EF
Treatment of Waste Gases by Humic Acid Zhiguo Sun,*,† Bo Tang,‡ and Hongyong Xie† †
School of Urban Development and Environmental Engineering, Shanghai Second Polytechnic University, Shanghai 201209, P. R. China ‡ School of Petroleum Engineering, Changzhou University, Changzhou, Jiangsu 213016, P. R. China ABSTRACT: Humic acid (HA) is a natural adsorbent and has special physical and chemical characteristics that form the foundation for disposal of pollutants from a combustor flue gas. HA, which occurs widely in soil, water, and low-rank coals, has already been proposed as a candidate that can be used as a sorbent for air pollution control. A flue gas desulfurization and denitrification (FGDD) process employing HA seems like a promising approach. It is a better choice using HA−Na as a desulfurization additive to improve wet limestone scrubbers or other FGDD processes. This paper reviews the recent development of waste gas treatment by HA with special reference to HA for removal of SO2, NOx, CO2, H2S, and heavy metals.
1. INTRODUCTION Humic substances (HS) originate from the decay of animals, plants, and other biological activities of microorganisms in the environment, which are widely distributed in water, soil, and low-rank coals. As natural polyelectrolytes, the presence of HS is crucial to preserve the production and quality of soil, remove inorganic pollutants, improve industrial agents, and treat some diseases. Therefore, HS act as a important role in the fields of agriculture, environment, industry, human health, and medicine.1 The prominent characteristics of HS have caught the attention of more investigators. Research progress in the exploitation of their physicochemical characteristics and structure has been achieved in the recent decades, and HS have been applied to many practical applications under the guidance of that new knowledge.2 HS are referred to as a black or brown, amorphous, complex heterogeneous mixture of organic substances with similar properties. HS are composed of C, H, O, N, and S atoms; however, the structure is complicated. Some functional groups including −COOH, −OH, and −O− are contained, which endows HS with a heterogeneous mixture. Depending on their source, extraction, and analysis method, the typical molecular mass of HS is from a few hundred to several thousands, and the size range is from 1 nm to several hundred nanometers.3−5 The chemical and physical properties of HS have been studied by many researchers. On the basis of employing new analysis techniques (e.g., XPS, NMR, SEM, TEM, ESR), the physicochemical characteristics of HS have been revealed, such as chelation, compexation, adsorption, and ion-exchange capacity.6,7 It is worth noting that the chemical structure of HS is close to the source of the origin. The latest research indicates that HS are formed from relatively small molecules, which have similar characteristics, and are held together via supramolecular interactions.8 HS can be further subdivided depending on their solubility in acids and bases. The groupings are termed humic acid (HA), fulvic acid (FA), and humin. (1) Humic acid (the part of HS that is soluble in dilute alkali but insoluble in acidic solution); this characteristic gives a © XXXX American Chemical Society
theoretical basis for precipitation and separation of HA fertilizer after desulfurization and denitrification. (2) Fulvic acid (the part of HS soluble in all pH conditions). (3) Humin (the part of HS insolube in alkali or acid). HA can also be divided into “natural HA” and “artificial HA”. Natural HA is further classified as soil HA, water HA, and coal HA. Artificial HA includes fermentation HA (FHA), chemical synthesis HA, and oxidized regenerated HA. Although the total amount of soil HA and water HA is very large, the percentage is very low. The main material of HA in industry is low-grade coal, such as peat, lignite, and weathered coal. The total amount of HA runs up to one trillion tons. HA, partly bonded with potassium, calcium, and sodium, into potassium humate (HA−K), calcium humate (HA−Ca), and sodium humate (HA−Na) by their oxygen functional groups respectively, is the major component of low-rank coals (e.g., peat and lignite).9−11 HA and HA−Na possess potential application in industry because of their thermostability, and no obvious destruction takes place even after exposing them under 250 °C for 60 min.12 HA can be extracted from low quality coal with either sodium hydroxide (NaOH) or sodium carbonate (Na2CO3), and we can also get HA by the fermentation of waste biomass.13 To sum up, as a kind of natural adsorbent, HA has a wide range of origins, lower price, and special physical and chemical characteristics, which gives it the potential to control SO2, NOx, H2S, CO2, and heavy metals in exhaust gas.
2. PHYSICAL AND CHEMICAL BASIS OF HA IN WASTE GAS TREATMENT 2.1. Structural Characterization. Generally, the structural characterization of HA includes elemental analysis, measurement of oxygen-containing functional groups, measurement of molecular weight or distribution of molecular weight, and some other important features such as C/H, E4/E6, etc. HA is Received: October 13, 2014 Revised: February 3, 2015
A
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Figure 1. HA model suggested by F. J. Stevenson.4
2.3.2. Hydrophilicity. The hydrophilic degree of HA depends on the ratio between the aromatic nucleus and the side chain; that is, it depends on the condensation degree. 2.3.3. Surfactant. HA is a kind of amphiphilic molecule, and hence it has surface activity. 2.3.4. Ion Exchange. HA can combine to generate the corresponding humates with potassium ion, sodium ion, calcium ion, magnesium ion, etc., which gives HA a strong ion-exchange property. Because of its ion-exchange properties, HA can be utilized in industrial wastewater treatment, nutrient preserving capability of soil, and many other fields. 2.3.5. Complexation. HA contains carboxyl, phenolic hydroxyl, alcoholic hydroxyl, and many other active functional groups, so that its complexation/chelation and ion-exchange with metal ions could be carried out. HA possesses great adsorption capability due to its surface energy and large surface area resulting from its amorphous structure. Moreover, the swelling property also contributes to the adsorption capability of HA. Compared to HA, humates (e.g., HA−Na) possess a better swelling property, which allows the active radicals (e.g., OH− and COO−) to easily contact the adsorbed ions in water.15 This property offers a sound theoretical basis for treating trace amounts of heavy metals in waste gas using HA. Moreover, HA also has redox, catalytic activity, and modification.16 After reaction with sulphonation, oximation, nitration, and methylation, the structure of HA is modified, which is suitable for industrial and agricultural applications.17−19
composed of C, H, O, N, S, and other elements. Among them, C and O are the main elements, while H, N, and S only occupy a small amount.14 Functional groups of HA generally refer to oxygen-containing functional groups, which include total acid group, carboxyl, phenolic hydroxyl, total hydroxy, alcoholic hydroxyl, methoxy, total carbonyl, and quinone groups.2 Although many scientists have conducted a thorough study in the chemical structure of HA, there are disputes about it. However, it is accepted tendentiously that HA has the approximate molecular structure as Figure 1 shows. The adsorption behavior of HA is difficult to describe due to the lack of a precise molecular structure. 2.2. Physical Properties. HA is a kind of amorphous macromolecular compound, and it is usually black or brown in amorphous form. The relative density of HA usually ranges from 1.330 to 1.448. Its main physical properties include the following. 2.2.1. Adsorption. Since HA has a loose “sponge-like” structure, it has a large specific surface area (330−340 m2/g) and surface energy. HA has a heterogeneous porous structure, and the radius range of the porous from 1 to 7 nm, in which micropores (1−1.5 nm) are in the majority. Therefore, the pores in HA are almost a consistent size, the order of magnitude of which is same as the pore in activated carbon. Good adsorption properties and pore structure of HA indicate that HA can be adopted as an adsorbent in theory. 2.2.2. Colloid. According to the observation by the electronic microscopy, the approximate diameter of dispersed particles of HA in water ranges from 6 to 10 nm. Every particle, as long as its diameter is between 1 nm to 0.1 μm and it disperses stably in a medium, can be regarded as colloid. Hydrogen ions contained in HA may ionized, so that the surface of HA in solution is negatively charged. Monovalent salt of HA is soluble in water and weak alkaline, such as HA−Na, HA−K, ammonium humate (HA−NH4), etc. Thus, the weak alkalinity of HA−Na solution makes it possible to absorb acidic waste gases, such as SO2, NOx, and CO2. 2.2.3. Solubility. FA can dissolve in water, any alkaline and acidic solution as well as ethanol, acetone, and other organic solvents. While HA is soluble in strong alkaline solution, it also has a strong solubility in polar organic solvents containing nitrogen. In both of these solvents, the chemical reaction or irreversible adsorption will happen with HA. HA also has a certain solubility in some inorganic salts solution and basic salts solution of organic acids. However, high ionic strength will depress the dissolution of HA in any solution. 2.3. Chemical Properties. 2.3.1. Weak Acid. Owing to carboxyl and phenolic hydroxyl, HA is a weak acid.
3. APPLICATION OF HA IN WASTE GAS TREATMENT Current research on HA is mainly in terms of soil, fertilizer, industrial additives, sewage treatment, and some other fields, while previous research concerning the removal of exhaust gas by HA is insufficient. According to reports, HA and humates have been used in the treatment of NOx, SO2, H2S, CO2, and heavy metals in exhaust gas. 3.1. SO2. SO2 from the combustion of fossil fuels is considered a major pollutant and the source of acid rain. However, SO2 can act as a precursor of sulfur fertilizer after it is reasonably adsorbed and transformed. FGD is widely employed to control the emission of SO2 at present.20 Depending on the classification FGD processes, application of HA in waste treatment will be introduced by a dry process and wet process. 3.1.1. Dry Process. Materials containing HA (e.g., peat) can be used as an adsorbent to remove SO2, NOx, H2S, etc. Although HA has certain capability in adsorbing waste gas, its adsorption capability is lower than that of active carbons (AC), because of its lower specific surface area, which hinders its application in waste gas adsorption compared with AC. So HA B
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Figure 2. Mechanism of reaction of mass-transport for SO2 and HA−Na.
3.1.2.1. Natural HA−Na. Green et al.24,25 used coal HA mixed fly ash to absorb SO2. HA may enhance the dissolution of alkali metals from fly ash into HA solution. The alkali metals combine with HA into corresponding humates, which absorb SO2 effectively by the acid−base mechanism. Both the alkalinity and solubility of fly ash depend on the HA/fly ash ratios. In a scrubbing operation, the humates from the fly ash dissolved into HA solution can remove most of SO2. In those alkali metals, Ca2+ plays a key role, and much of SO2 reacts with Ca2+ to form CaSO3 and CaSO4. The reaction process is summarized as follows:
has to be considered as an additive to manage waste gas by combining it with other agents, such as ammonia (NH4OH) and calcium carbonate (CaCO3). Recently, Sun et al.15 made a new composite adsorbent of HA−Na/α-Al2O3 for FGD by the impregnation method. The desulfurization property of the adsorbent (HA−Na/α-Al2O3) impregnated with NH3·H2O in a fixed−bed reactor was studied. The results state that the HA−Na-coating on the α-Al2O3 fibers can enhance the desulfurization ability of the α-Al2O3 support. More ammonia is adsorbed by Ha−Na/α-Al2O3, and the longer a high SO2 conversation rate is maintained because of the excellent adsorption capability of HA−Na. Moreover, the byproduct of FGD is a compound fertilizer consisting of HA− NH4, (NH4)2SO4, and HA−Na. The regenerable α-Al2O3 can be achieved easily. On the basis of this study, Zhao et al.21 investigated the removal of NO2 and SO2 simultaneously using the absorbents of HA−Na/α-Al 2 O 3 impregnated with ammonia. The results show that 100% of SO2 and 80% of NO2 could be removed. The preferential absorption of SO2 rather than NO2 is obvious when SO2 and NO2 occur simultaneously. The key to this method is that HA−Na plays an additive role to improve the desulfurization capacity of ammonia. A cost-effective approach is provided to strengthen the ammonia scrubber through lowering ammonia loss due to the strong adsorption ability of HA−Na. Since the FGD process based on Ca has been the most widely used in coal-fired power plants nowadays, investigations on how to improve the SO2 removal efficiency and lower utility consumption of limestone have never been stopped. Zhao et al.22 used HA to modify Ca-based adsorbents for FGD. HA was used as an additive to control the crystallite morphology and specific surface area of CaCO3. Results show that with the increased amount of HA additive, the specific surface area can run from 28 m2/g to 50 m2/g, and the pore size becomes larger, while desulfurization efficiency is increased from 38.4% of normal CaCO3 to above 70%. 3.1.2. Wet Process. At present, FGD is adopted widely to remove SO2 from the combustion of fossil fuels. Despite the fact that the wet FGD processes based mainly on the limestone−gypsum method are used widely, they have many disadvantages such as a larger water requirement, high capital, poor quality of by-products, and even leading to secondary pollution. Thus, cost-effective technologies for removing SO2 is one of the most important issues.23 Among this research, removal of air pollutants by wet processes with HA is a good prospect. In addition to coal HA−Na, sluge HA−Na (SHA− Na), biochemical fulvic acid (BFA) can be used as a desulfurization agent. On the other hand, HA−Na is also a better desulfurization additive.
MnOx + 2x HA ↔ n[M(2x / n) + − (HA)−2x / n ] + x H 2O (1)
n[M(2x / n) + − (HA)−2x / n ] + 2xSO2 (aq) + 2x H 2O ↔ n M(2x / n)(aq) + 2x HSO−3 (aq) + 2x HA
(2)
The basic reaction: MnOx + 2xSO2 (aq) + x H 2O ↔ 2x HSO−3 (aq) + n M(2x / n) +(aq)
(3)
The study of SO2 absorption with HA−Na is adopted to estimate the absorption efficiencies and capacities of the fly ash mixtures. The major reaction mechanism is shown as follows: HA − Na(aq) + SO2 (g) + H 2O ↔ HA(s) + HSO−3 (aq) + Na +(aq)
(4)
“Complexation” resulting in the additional SO2 absorption. HA + SO2 (aq) ↔ HA‐‐‐SO2
(5)
The results show that the SO2 removal rate with HA−Na was above 98%. However, the probable methods for removal of build-up Na+ and the overall reactivity and kinetics of HA with fly ash in spent scrubber solution are still unclear. Despite the fact that it is impossible to predict the SO2−HA binding mechanism, due to the limited knowledge of the HA molecule structure, the practical effect of removal of SO2 by HA/fly ash mixtures would be negligible. Sun et al.23 carried out research on SO2 absorption using HA−Na as the raw material. They analyzed the mechanism and kinetics of SO2 absorption with HA−Na solution in a bubbling reactor. The results show that the concentration of HA−Na has a significant effect on the desulfurization effects. SO2 absorption is improved by low temperature and low gas flow rate. The mass transfer of SO2 and the SO2 consumption rate increase with increased SO2 inlet concentration. SO2 absorption ability is promoted by the presence of NO2 coexisting with SO2 since C
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Energy & Fuels Table 1. Cost and Efficiency of FGD by HA-Na (Per Ton of SO2)23 by-product FGD process HA−Na solution lime-gypsum
sorbent consumption (ton)
water consumption (ton)
neutralizer consumption (ton)
gypsum (ton)
fertilizer (ton)
desulfurization efficiency (%)
capital cost (US$)
5.5
circulating
0.3 (lime)
0.9
5.5
98
15
1.2
12
0
3.7
0
95
63
Figure 3. Simplified scheme of application of the SHA−Na for FGD.28
Figure 4. Representative structure of fulvic acid.30 2− it may accelerate oxidation of SO2− 3 to SO4 . HA−Na solution shows a strong ability of SO2 absorption, and above 98% of the SO2 absorption efficiency is maintained for 1.5 h at the optimum conditions. In addition to acid−base reaction eq 4, the additional reactions should be considered during the desulfurization process: SO2 dissolution:
SO2 + H 2O ↔ H+ + HSO−3
(6)
HSO−3 (aq) ↔ H+(aq) + SO32 −(aq)
(7)
R‐COO‐ + H+ ↔ R‐COOH
Mechanism of liquid HA sodium desulfurization is mainly the theory of acid−base neutralization. On the basis of the two-film theory, the process of SO2 absorption by HA−Na solution (shown in Figure 2) can be assumed as the following steps:26 (a) SO2 in the gas-phase diffuses into the gas−liquid interface. (b) SO2 dissolves in the gas−liquid interface and establishes dissolution equilibrium, which obeys Henry’s Law. (c) The hydrated SO2 partly ionizes into HSO32− and SO32−. (d) HSO32−, SO32−, and H+ migrate and diffuse in the liquid phase. (e) Some SO32− are oxidized into SO42− by dissolved O2. (f) The humate radicals (R−COO−) from ionized HA, mainly containing carboxyl (COO−), transfer to the gas−liquid interface.
Oxidization of sulfite ion: 2SO32 −(aq) + O2 (aq) → SO24 −(aq)
(8)
Ionization and hydrolyzation: R‐COONa ↔ R‐COO− + Na +
(10)
(9) D
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Energy & Fuels (g) H+ in the liquid phase combines with COO− to form in insoluble HA, according to eqs 6, 7, 9, and 10, which promotes more SO2 to dissolve into water for HA ionization to produce a large number of H+ ions. Hu et al.27 put forward a method of FGD and by-product hydrogen with HA−Na solution. After HA−Na solution was used to remove SO2 from flue gas, solar light photocatalytic hydrogen production from the desulfurization liquid, mainly containing HA and Na2SO3, is achieved over graphene and TiO2 photocatalysts. In this process, Na2SO3 plays a sacrificial agent role. It is recognized as an energy-saving regeneration technology, due to its achievement of both FGD and byproduct hydrogen. It is important to evaluate the capital cost of this desulfurization process. Compared to the lime−gypsum process, a lower capital cost and higher desulfurization efficiency can be achieved using HA−Na (Table 1). Thereby, HA−Na solution possesses potential commercial application in FGD. 3.1.2.2. Artificial HA−Na. (a) Sluge HA−Na. In addition to coal HA−Na derived from low quality coal, SHA−Na solution was also designed to remove SO2 from flue gas. Zhao et al.28 proposed a scheme of application of the SHA−Na for FGD (Figure 3) and studied the ability of HA−Na from alkaline treatment sludge on removing SO2. The SHA−Na shows great performance in SO2 absorption, and 98% desulfurization efficiency can be achieved under optimum operating conditions, while the highest sulfur capability is 0.037 g (SO2)/g (SHA−Na) in the process. The desulfurization products mainly contain sludge HA sediment, which can be used as fertilizer components. (b) Biochemical Fulvic Acid. FA, a fraction of HS, can be dissolved under all pH values. Because of the dissociation of acidic functional groups (e.g., COO− and OH−, Figure 4), FA can be used as buffer material in a wide pH range. The sources of FA include water, soil, lignite, peat, agricultural and forest biomass residues, etc. At present, due to low cost and abundant raw materials, extracting FA in large-scale from biomass residues has been adopted widely. Yang et al.29 put forward a new regenerable FGD process with BFA, and it is dependent on the acid−base buffering ability of BFA. The results show the maximum desulfurization efficiency is 97.5% under optimum operating conditions. It is different from other HS that the SO2loaded BFA solution can be desorbed and regenerated at 70 °C under ambient pressure, and the excellent absorption performance of resulting BFA is maintained after several absorption and desorption cycles. During this SO2 absorption process, trace metal ions combined with FA play a crucial role. No chemical change of BFA samples before and after absorbing SO2 is found, except that carboxylates are changed into carboxylic groups, which demonstrates that BFA is stable absorption and desorption cycles. Because of the low cost and regeneration, BFA can be used as a preferable agent for trace SO2 separation and purification. 3.1.2.3. Desulfurization Additives. Using HA−Na as a desulfurization additive is also a better choice. Sun et al.31 used HA−Na as an additive to strengthen the effect of limestone wet FGD. In addition to the SO2 absorption reactions eqs 6 and 7, ionization eq 9, and hydrolyzation eq 10, after SO2 dissolved in water these equilibriums exist in absorption solution: Limestone dissolution: CaCO3 ↔ CO32 − + Ca 2 +
Neutralization: CO32 − + H+ ↔ HCO−3
(12)
H+ + HCO−3 ↔ H 2CO3
(13)
H 2CO3 ↔ CO2 + H 2O
(14)
The role of HA−Na in improving SO2 absorption in wet limestone scrubbers was investigated. The result shows that the pH value of the absorption solution was increased after adding HA−Na. HA−Na is a strong base weak acid salt and contains a large number of carboxylates, which will combine with much H+ to generate carboxylic acid after hydrolysis. This reaction may move eqs 6 and 7 to the right, giving rise to a higher solubility of SO2 into solution. On the other hand, H+ ionized from the carboxylic acid may move eqs 12 and 13 to the right, which promotes the dissolution of limestone. It is concluded that, in the process of strengthening wet limestone scrubbers, the ionization and hydrolyzation reactions of HA−Na promote SO2 absorption and CaCO3 dissolution, which improves the removal rate of SO2 and limestone utilization. The SO2 absorption efficiency is increased 10% by adding HA−Na of 1.5 g/L to limestone slurry. On the basis of previous reports, SO2 can be absorbed effectively with HA−Na scrubbing. However, HA, a main byproduct after desulfurization, is difficult to be sulfonated in the same process owing to a low concentration of H2SO4 and a low reaction temperature. In order to prepare sulfonated desulfurization products, Zhao et al.32 put forward a different FGD process, including adopting HA−Na solution and caprolactam tetrabutyl ammonium bromide ionic liquid ([CPL][TBAB] IL). This investigation reveals both the SO2 absorption efficiency and resulting by-product of desulfurization. Moreover, the influence of the recycling number on the desulfurization effect was also studied. The experimental results show that the SO2 absorption efficiency can reach 95%. This mixed solution can be used for five cycles. In this process HA can be partly sulfonated. The results demonstrate that HA would react with NaHSO3 in a Michael addition reaction, and the ionic liquid acts as a catalyst in this reaction. During this process, eq 4 shows the primary mechanism of SO2 absorption by HA−Na solution, which is an acid−base reaction. Meanwhile, HA reacts with NaHSO3 as eq 15, and Figure 5 shows a plausible mechanism. HA(s) + NaHSO3(aq) → SHA(s)
(11)
(15)
Figure 5. Sulfonation reaction of HA.32 E
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Figure 6. Treatment process of exhaust gas with NOx by amidated HA.36
3.2. NOx. Acid rain is formed by SO2 and NOx from fossil fuel combustors, which leads to serious environmental problems. Different from the advanced technologies of SO2 gas removal, the elimination of NOx gas is far from mature. NO is insoluble in water and inactive, but about 90−95% of NOx emitted during combustion processes forms NO. In order to depress the generation and NOx emission, some processes for NOx control have been proposed, including adsorption, selective noncatalytic reduction (SNCR), selective catalytic reduction (SCR), and nonselective catalytic reduction (NSCR). Although wet FGD scrubbers can remove SO2 effectively, the method is not work for NOx. Thus, development of a new method of NOx control has been a research focus.33,34 Zhang et al.35 used peat (containing about 20% of HA) to absorb NOx in a fixed-bed quartz reactor (80 mm in diameter) with the following conditions: gas flow rate of 0.7 m3/h, contact time of 5 s, NOx inlet concentration of 3674 mg/m3, room temperature and ambient pressure. They found that untreated peat has a low adsorption efficiency of NOx (average at about 40%) and a weaker adsorption capacity (it can only maintain 6−7 h), while NO2 is much easier to be adsorbed by peat than NO. There is no obvious improvement after adding ammonia, but some improvements exist after adding ammonium bicarbonate, urea, and phosphate rock. The removal efficiency of NOx could reach 60−70%, 50−60%, >40% respectively than only using peat (30−40%). However, this method only applies to the exhaust gas, which contains a large amount of NOx, for example, the exhaust gas from a nitric acid plant. In addition, both its removal efficiency and sorbent utilization are low. Hu et al.36 put forward a treatment process of exhaust gas with NOx by amidated HA (shown in Figure 6). Processing exhaust gas NOx comprises: (a) choosing city life sewage mud or straw powder as raw material to do micro-organism fermentation, stirring material after fermentation, and aging, filtering grain and impurities to obtain HA solution; (b) taking the HA mixed solution into an aminated device, and adding strong aqua ammonia at the same time; the mass ratio of added ammonia and HA is 1:5−1:10, obtaining HA−NH4 after aminating HA; (c) taking HA−NH4 into an amidated device, heating, producing amidated HA (HA−NH2); (d) taking HA− NH2 into a prepared pool, and adding water in the prepared pool; (e) transporting the solution in the prepared pool into the exhaust gas process device, removing NOx from the exhaust
gas in the amidated device, in the exhaust gas process device; the mole ratio of NO importing into smoke at unit time and amide radical in the imported HA−NH2 is 1:1−1:1.5, and HA− NH2 reacts with NO and nitrous acid, then produces HA, discharging nitrogen, and HA dissolves in the reaction mixture; and (f) discharging the reaction mixture with HA from the denitrification device, taking it into the circulating pool, then taking it back to the aminated device, complementing ammonia for circular use. This method is the Hofmann rearrangement and addition reaction, which are useful reactions for NO control. It is important to get regenerative HA and change NOx to innoxious N2 during this process. The relative reactions are as follows: 2NO + O2 → 2NO2
(16)
2NO2 + H 2O → HNO3 + HNO2
(17)
R − COOH + NH3 → R − COONH4
(18)
dehydration
R‐COONH4 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ R‐CONH 2
(19)
R‐CONH 2 + NO → R‐COOH + N2 + H 2O
(20)
R‐CONH 2 + HNO2 → R‐COOH + N2 + H 2O
(21)
where R-COOH represents the structural formula of HA; RCOONH4 stands for the structural of ammonium humate; RCONH2 is the structural of annotated HA. This process gave a new method to remove NOx by HA from city life sewage mud or straw powder. While promising in theory, it is necessary to make investigations on both the conversion efficiency of HA and the removal rate of NOx. On the other hand, the energy consumption has to be estimated. 3.3. Simultaneous Removal of SO2 and NOx. In recent years, simultaneous flue gas desulfurization and denitrification (FGDD) have attracted intense interest. A number of technologies were developed, among which the stage treatment technology is deemed to be a mature one. According to traditional technology, a separate NOx control system (e.g., SNCR or SCR) is installed at the back of the equipment of removing SO2. Although it would remove both SO2 and NO, it is hard to widely apply in industry due to the high running cost and large occupying area. In order to decrease the cost, developing new technologies and equipment to remove SO2 and NOx simultaneously becomes a hot issue in the F
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(CCS). CO2 can be captured precombustion, postcombustion, from oxy-fuel combustion or from industrial process streams. Some approaches include the separation of CO 2 with membranes, sorbents, chemical-looping, and cryogenic distillation, among which chemical absorption becomes a widely adopted method, for example, capturing CO2 by ammonia scrubbing.47−49 Sun et al.50 put forward a method to absorb CO2 by humates and ammonia. This method involves CO2 absorption by a mixture of humates and ammonia−water. The humates and ammonia−water are stirred to obtain a mixed aqueous solution into an absorbing tower, where insetting gas contains CO2 to generate a solution of ammonium hydrogen carbonate (NH4HCO3) and HA−NH4. The obtained solution is poured into a mechanical dewatering device. The obtained mixture of (NH4HCO3) and HA−NH4 is a kind of compound fertilizer. This method provides a new route to reduce emissions of CO2 from power plants, which contains a separation improvement in ammonia scrubbing by adding HA−Na. Therefore, it seems that employing HA as an additive to enhance on CCS of other absorbents is a good choice, before CO2 sequestration mechanism by only HA is not reported. 3.6. Heavy Metals. Coal burning generates many air pollutants including not only SO2 and NOx, but also trace elements. Mercury (Hg) is considered as one of the most toxic trace elements, and the main influence of Hg is considered in an aqueous environment. If mercury enters water bodies, under anaerobic conditions, inorganic mercury will be methylated biotically to dimethyl mercury, which is the most toxic form of Hg, and biomagnifies readily in the food chain, threatening public health and ecosystems severely.51,52 In order to protect the environment and human health, it is necessary to reduce mercury emissions in an economical manner. The current technologies for capture Hg from flue gas mainly focuses on following aspects: activated carbon injection (ACI), electrostatic precipitator (ESP), and FGD, and the cobenefit of SCR. The combination of FGD, SCR, and ESP is a complicated process. First of all, the elemental Hg in the flue gas is oxidized over a SCR catalyst. Then the oxidized Hg is captured by FGD or ESP. However, catalysts with high active are still lacking, and the gaseous Hg just transfers into desulfurization slurry and fly ash after this process, which are the sources of secondary pollution. Although ACI processes Hg removal from flue gas with high efficiency, the recycling of fly ash is difficult because of the materials mixing with contaminated activated carbon powder.53−56 Thereby, proposing a suitable process to remove gas Hg is meaningful. An HA-based sorbent may play a role in control the Hg with a facile operation and low cost. HA can also treat trace amounts of heavy metals in the exhaust. Hu and Sun proposed a method for HA to manage SO2 bearing and heavy metallic waste gas and by-product compound fertilizer.57 The treatment mechanism of heavy metals is based on ion exchange, chelation, and physical adsorption. HA can carry out ion exchange with metal ions due to carboxyl (−COOH). For example,
environment area. Although some investigations on FGDD have been made, the related methods are difficult to apply widely due to the economic and technical defects.37−39 Hu et al.40,41 proposed a novel simultaneous FGDD process using HA−Na as raw material (shown in Figure 7).
Figure 7. Process chart of the FGDD by HA−Na solution.41
On the basis of the conclusion, simultaneous FGDD by HA− Na solution possesses some merits (a) almost no waste sludge, (b) lower costs and energy requirements, and (c) by-product sulfur-containing nitrogen fertilizer. However, in this FGDD process, HA−Na solution can only deal with NO2, but not NO, which accounts for most of the proportion of NOx in fact. Thereby, the potential method can be employed in a large scale after the new method of oxidation NO into NO2 effectively is developed. 3.4. H2S. H2S, with the characteristic odor of rotten eggs, is a toxic and corrosive gas. H2S emission from industry into the atmosphere would lead to serious problems; therefore how to collection and treat H2S is a important issue. Some technologies including chemical, physical, and biological methods have been provided to treat emitted H2S. Among those methods, biotechnology has grown remarkably and is accepted increasingly due its ability to decompose pollutants rather than simply transfer them from the gas to the liquid phase. Adsorption of the pollutants is the most common method to treat the waste gas.42−45 Natural peat can be used as an adsorbent to remove the exhaust pollutant of H2S. Natural peat reacts with H2S to form insoluble metal sulfide. Therefore, natural heat needs to be changed to cationic peat (e.g., Zn-peat, Co-peat, and Ca-peat) soaking in metal salt solutions, and then the functional groups of cationic peat can release hydrogen ions or metal cations. For example, the amount of H2S adsorption by Zn-peat is 101 mg of 28% Zn in 60 min. Cu-peat may adsorb 101 mg of H2S in 30 min. Almost 101 mg of H2S is removed by Cu-peat in 80 min. Products of metal sulfide are innocuous substances since they are insoluble.46 3.5. CO2. At present, fossil fuels provide over 85% energy demand all over the world. About 40% of total CO2 emissions come from fossil-fueled power plants, which are the main contributor. Emission of greenhouse gases (GHG) from combustion of fossil fuels is a global problem. Many technologies exist for each phase of carbon capture and storage
2R‐COOH + Ca 2 + ↔ (R‐COO)2 Ca + 2H+
(22)
It is obvious that the amount of ion-exchange depends on the amount of carboxyl. The rate and amount of ion-exchange of HA−Na is better than that of HA because of the acidity, hydrogen bond, and insufficient swelling property in HA solution. In addition to ion-exchange, HA can chelate divalent G
DOI: 10.1021/ef502299k Energy Fuels XXXX, XXX, XXX−XXX
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4.3. Medicine Applications. In the human medicine area, HA has been manufactured in large scale. In recent decades, there has been more and more interest in the application of HA in medicine. Researchers have studied the possibility of HA as a candidate as a base of products in medicine. The increasing interesting in HA results from its antiviral, antitumor, constringency, hemostasia, antibacterial, profibrinolytic, antiinflammatory and estrogenic activities. So HA has been applied to treat the following diseases: (a) dermatosis, burns and scalds, and ulcers, (b) gastrointestinal hemorrhage, diarrhea and enteritis, and dyspepsia, (d) rheumatoid arthritis, periarthritis, and hepatitis, (e) gynecopathy, and (f) cancer and tumors, etc.62
metal ions with its chelating groups (e.g., carboxyl, phenolic hydroxyl). The chelation reactions are shown by the following:
5. CONCLUSIONS The removal of waste gas from combustion flue gas by HA has been spotlighted in recent years as a potential way to reduce gas pollutants. The special physical and chemical characteristics of HA is a basis for waste gases treatment employing HA. Because of the great adsorption and weak alkalinity of HA−Na, it is used to remove the acidic waste gas SO2, NOx, H2S, and CO2. Disposal of heavy metals by HA is based on its ion-exchange and complexation. The different solubilities between HA and HA−Na make it possible to separate byproduct HA-fertilizers from the acidic desulfurization liquid. Among a range of technologies using HA to remove gas pollutants including SO2, NOx, H2S, CO2, heavy metals, the removal of SO2 or NO2 by HA−Na solution appears most promising, owing to low-cost, and high removal rate, especially by-product fertilizer. However, if it is used for large-scale implementation of FGDD, the point of this process is getting the qualified by-product of HA-fertilizer, which is permitted for use in soil or plants. FA is employed as a preferable agent for SO2 separation and purification owing to its high pH buffering capacity. For this process of removing SO2, FA is a regenerable absorbent, which allows this method to be prospectively applied in engineering with the merit of being cost-effective. Hence the regenerable HA is important to its application in a large scale on disposal of gas pollutants. It is a better choice using HA−Na as a desulfurization additive to improve wet limestone scrubbers or other FGD processes because the consumption of HA−Na (per ton SO2) is high, only requiring HA−Na as raw material to remove SO2. Although HA−Na may increase the SO2 removal rate in wet limestone FGD, it demands consideration of the effects of HA− Na additive on the quality of by-product gypsum. HA−Na can be also served as additives to enhance the CO2 capture by ammonia scrubbing, which is thought to be the most prospective in application among those methods of removing waste gases by HA. This work does not seek to suggest that waste gases adsorption by HA is the ideal way to achieve efficiency in FGDD and CCS. Rather, the composite adsorption materials composed by HA and other substances, such as Al2O3 and TiO2, appear to have good promise for waste gas treatment. Furthermore, using HA as an additive to modify other adsorbents is feasible. Although mechanisms of removing NO and Hg from flue gas by HA have been proposed, their feasibility has not been confirmed by investigations. If HA is to be used for large-scale implementation for treating waste gas, the physical and chemical properties of HA with the different origin, source, and extraction method must be well understood. To this end, it is necessary to attach importance to the analysis technologies for HA, which hinders
The capability of humate salts after sulfonation or nitration on adsorption of heavy metal ions will increase up to 180 mg/g to 420 mg/g. By chelation reaction, HA can passivate heavy metals such as Hg2+, Cu2+, Zn2+, Pb2+, Cd2+, Ni2+, and so on, which will prevent the plant system from being invaded by heavy metals.
4. APPLICATIONS IN AGRICULTURE, INDUSTRY, AND MEDICINE 4.1. Agriculture Applications. Most of investigations about HA focus on agriculture applications, for HA plays a key role in an agriculture area. They significantly influence both the productivity and quality of the soil. HA has a high base exchange capacity, and it is significant for soil fertility. Moreover, HA can improve the moisture conditions and physical properties of soil. Currently, humates (e.g., HA−Na, HA−NH4, and HA−Ca) are used as additives in fertilizers. HA combines with N, P, and K of essential elements for plant into HA compound fertilizer, functions of which include to (a) regulate the pH value of soils; (b) improve plant growth; (c) promote the efficiency of nitrogen fertilizers;58 (d) enhance the buffering powder of soil, improve the soil’s structure, optimize N, P, K absorption by plants; (e) enhance the resilience of crops;59 (f) adjust plant metabolism process. The last reports indicate that HA can be employed as farm animal feed due to its growth-promoting effect.60 4.2. Industry Applications. HA has widespread use in industry. HA has been used not only in removal of waste gas but also in industrial wastewater treatment. HA in the neutral pH range shows the best adsorption and stability performance in adsorbing heavy metal ions. The adsorption rate of Cu2+, Pb2+, and Cd2+ with HA can achieve 99.25%, 98.73%, and 97.86% respectively.61 HA has been used as an additive to adjust concrete’s setting rate. HA does not enhance the physicomechanical properties of concrete but also just reduces its consumption. The oil industry is another field where HA has been applied. It can be adopted in water base and oil base drilling fluids as a fluid loss additive. In the battery industry, HA is used as an expander for lead accumulators. They may prevent plates from cracking and hardening, and improve the capacity and starting discharge performance of battery. Furthermore, HA can be employed in the paper industry. In different production processes, HA acts respectively as a deinking agent, defoamer, antistatic agent, and scale inhibitor, etc.2 In addition, HA has been applied to ceramic mold release agents, binders, boiler descaling agent, vulcanized rubber reinforcing agent, etc. H
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the application development of HA. Progress in understanding the structure of HA will help to explore the mechanisms between HA and waste gas, HA and hydrates of waste gas.
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AUTHOR INFORMATION
Corresponding Author
*Tel/fax: +86-21-50217725. E-mail: sunzhiguo1998@gmail. com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the Natural Science Foundation of Shanghai (No. 15ZR1416900), the Innovation Program of Shanghai Municipal Education Commission (No. 13YZ130), and Cultivate Discipline Fund of Shanghai Second Polytechnic University (No. XXKPY1303).
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