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Hydrogen Ion (H+) in Waste Acid as a Driver for Environmentally Sustainable Processes: Opportunities and Challenges Michael German,† Arup K. SenGupta,†,* and John Greenleaf‡ †
Department of Civil and Environmental Engineering, Lehigh University, 1 West Packer Avenue, Bethlehem, Pennsylvania 18015, United States ‡ Department of Civil and Environmental Engineering, Lafayette College, Easton, Pennsylvania 18042, United States result, any energy recovery from waste acid has not been reported to date even with such favorable thermodynamics and kinetics. According to the data available in 2010, the global sulfuric acid production capacity was over 200 million tonnes annually and that of phosphoric acid was about 40 million tonnes.2,3 If one considers that only 1% of this acid appears in the waste stream, which must be neutralized, that is theoretically equivalent to a generation potential of 1100 × 106 kWh. The actual energy generation or energy substitution potential Acid−base neutralization reaction in the aqueous phase is derived from waste acid may go well beyond the above thermodynamically favorable and kinetically fast. Waste acid estimate because many industrial, food, and mining processes neutralization is also the most common waste management also generate a huge amount of acidic waste streams, such as practice globally. However, waste acid neutralization is yet to be sulfur dioxide in the flue gas of coal- and oil-fired electric used for any work/energy generation because of the low utilities, citric and other organic acids from the food industry, concentrations of the waste acid and the high heat capacity of naturally produced acid streams in abandoned mines, and spent aqueous solutions. In this paper, we address potential processes acid solutions after etching in the metals industry. that can effectively take advantage of the high energy inherent The general scientific premise of the present article is to in neutralization reactions, in accordance with the goal of present a diverse set of environmentally significant opporsustainable development. tunities that attempt to utilize the energy embedded in acid− base neutralization. The paper also highlights the challenges BACKGROUND AND SCIENTIFIC PREMISE that need to be overcome for viable and meaningful application Waste acid neutralization is an industrial pollution control of these opportunities. process practiced globally. In principle, acid−base neutralization involves association of hydrogen and hydroxyl ions to form PROCESSES DRIVEN BY H+ NEUTRALIZATION neutral water molecules. This is the most thermodynamically favorable aqueous-phase homogeneous reaction with an Brackish Water Desalination with Waste Acids as the equilibrium constant (K1/w) value of 1 × 1014 at 25 °C: Energy Source. Electrodialysis (ED), or electrodialysis with reversal (EDR), is a widely practiced brackish water H+ + OH− → H 2O, ΔGo = − 79.8kJ/mol, desalination process that uses electrical energy for separation of electrolytes from water by cation and anion exchange ΔHo = − 55.8kJ/mol (1) membranes.4,5 Figure 1A shows the schematic of a typical Carbonate and bicarbonate salts (e.g., limestone, dolomite, electrodialysis process for desalination of a brackish water feed soda ash and sodium bicarbonate) are often used for acid (say NaCl) with alternate cation and anion exchange neutralization with favorable thermodynamics where bicarbonmembranes and DC power supply. Continuous supply of ate (HCO−3 ) and carbonate (CO2− 3 ) act as proton acceptors: electricity is the primary energy requirement of the electrodialysis process. Here we demonstrate that the process of + 2− − o H + CO3 → HCO3 , ΔG = − 36.3kJ/mol, desalinationremoval of Na+ and Cl− from the feed o solutioncan be achieved by replacing the entire DC power ΔH = − 14.9kJ/mol (2) supply with waste acid and a neutralizing base; Figure 1B illustrates the same. From the middle of the feed chamber, Na+ H+ + HCO−3 → H 2CO3 , ΔGo = − 58.9kJ/mol, and Cl− are continuously dissipated through ion exchange ΔHo = − 7.66kJ/mol (3) membranes in exchange for H+ in the waste acid and anionic neutralizing base (e.g., OH−, HCO−3 , and CO2− 3 ), respectively. These aqueous-phase neutralization reactions are also The thermodynamically favorable and kinetically fast neutralkinetically very fast; reaction half-times are on the order of ization reaction taking place in the middle chamber drives the nanoseconds.1 Since waste acid solutions undergoing treatment desalination process to near completion. In principle, the are often dilute, the significant amount of thermal energy generated in the neutralization reaction causes miniscule increases in the temperature of the bulk aqueous phase. As a Published: January 30, 2013
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© 2013 American Chemical Society
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exchange membrane. From the equality in eq 4, the chloride concentration will also drop by an equivalent amount through its passage to the right chamber following exchange with OH− or HCO−3 across the anion exchange membrane. Overall, neutralization of the waste acid stream drives the desalination of NaCl and both occur concurrently. While Figure 2A shows the schematic of a three-chamber laboratory desalination setup separated by cation and anion
Figure 1. A. Traditional electrodialysis desalination system using cation- and anion-exchange membranes under an electric potential gradient ; B. Ion exchange membrane desalination by replacing electricity with waste acids and neutralizing bases.
neutralization reaction, in accordance with reactions 1−3, delivers the energy of separation and the desalination process proceeds spontaneously. The capacity of such a desalination system can be conveniently expanded or scaled up by repeating multiple stacks with a similar arrangement of cation and anion exchange membranes. Note that the waste acid and the neutralizing base do not come in direct contact with each other and are separated by the middle chamber carrying the feed solution to be desalinated. Considering NaCl to be the only electrolyte in the middle chamber being desalinated and the products of neutralization reactions being nonionized, the following relationship is valid at any time in the middle chamber from an electroneutrality viewpoint: M M + = C − C Na Cl
(4)
Figure 2. A. Schematic of a three-chamber laboratory desalination setup separated by cation and anion exchange membranes; B. Desalination of 0.001 M KCl with 0.01 M H2SO4 in the acid chamber and 0.01 M Ca(OH)2 in the base chamber (Data are adapted from ref 7); C. Desalination using 0.05 M citric acid (with and without 0.03 M NaCl) in the acid chamber and 0.05 M Na2CO3 in the base chamber (Data are adapted from ref 10).
Where “C” represents the molar concentration and superscript “M” represents the middle chamber. For exchange between sodium and hydrogen ions across the strong acid cation membrane, the following equilibrium relationship holds according to the Donnan membrane equilibrium under ideal conditions and the stepwise derivation for similar systems has been provided in a previous paper:6 M + C Na L + C Na
=
exchange membranes, Figure 2B provides the experimental results adapted from Igawa et al. 7 for desalination of very dilute 0.001 M KCl (75 mg/L) with 0.01 M H2SO4 in the acid cell and 0.01 M Ca(OH)2 in the base cell. Note that KCl in the middle chamber is near-completely desalinated. Deionization of tap water with low total dissolved solids has been achieved in the laboratory using pure acid and base8,9 but the process is economically unsustainable for brackish and seawater desalination due to the large requirement of pure mineral acids and neutralizing bases. Figure 2C presents the results obtained by German 10 for brackish water desalination using a biorenewable organic acid, namely, citric acid; again, high percentage
C HM+ C HL+
(5)
Thus, for a waste acid stream with pH ∼ 2 in the left chamber and a neutral NaCl feed in the middle chamber for desalination (i.e., pH ∼ 7.0) in a continuous system, L + C Na M + C Na
=
C HL+ C HM+
= 105 (6)
A tremendous potential thus exists to desalinate the NaCl feed by pulling Na+ to the left chamber across the cation 2146
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desalination of NaCl (i.e., reduction from 5000 mg/L NaCl to less than 1000 mg/L) is achieved in 70 h. In this experiment, sodium carbonate was used as a neutralizing base, implying magnesium carbonate, an abundant and inexpensive base, may also be quite suitable. A follow-up experiment was conducted under identical conditions excepting that 1500 mg/L NaCl was present along with 0.05 M citric acid in the acid chamber.10 The results are superimposed in Figure 2C and note that the desalination flux dropped significantly and the NaCl concentration in the middle chamber dropped to only 4000 mg/L after 60 h. Such a drastic reduction in the desalination rate demands an explanation and that can be presented as follows: As validated in previous studies, intramembrane diffusion is the rate-limiting step with ion exchange membrane processes in the absence of electric potential gradient.11,12 Sodium ion, Na+, has higher affinity than H+ for the cation exchange sites in the membrane. Due to the enhanced competition caused by the presence of Na+ in the acid chamber, exchanger-phase H+ concentration at the water-exchanger interface dropped, thus reducing the hydrogen ion flux into the middle desalination chamber. Consequently, the rate of desalination was severely impaired, as can be observed from the two sets of data in Figure 2C. It is obvious that most of the organic and mineral waste acids available are not pure and contain other electrolytes, be it hydrochloric acid after metal pickling, citric acid from fermentation broth, or phosphoric acid after ore digestion. The challenge lies in maintaining high hydrogen ion flux in the proposed desalination process while in the presence of other competing cations. Dual-functional ion exchange membranes, although not available commercially, have been synthesized experimentally and can overcome this shortcoming through incorporation of the weak-acid functional groups with high hydrogen ion selectivity.13 In particular, use of thinner homogeneous ion exchange membranes, as demonstrated in an earlier investigation,7−9 may significantly enhance the desalination flux. Additionally, earlier studies have demonstrated intelligent use of an ion-exclusion technique in efficiently separating pure citric acid from salts resulting from fermentation broth.6 The technique does not warrant addition of external chemicals, can be applied at ambient temperature, and is environmentally benign. Relatively pure citric acid may offer high desalination rates enhancing the viability of the proposed approach. Other weakly ionized waste acids, namely, phosphoric and tartaric, are also likely to be amenable to purification and subsequent usage to achieve desalination without requiring electricity. Mechanical Energy through Swelling/Shrinking of Functionalized Polymers. In weak-acid (or weak-base) ion exchange resins, the functional groups are covalently attached to the polymer phase and, hence, cannot diffuse out when in contact with the aqueous phase. Once immersed in basic solutions (e.g., NaOH or Na2CO3), the functional groups of the weak-acid ion exchanger (e.g., carboxylate) become deprotonated and the resin beads swell by imbibing water molecules inside the exchanger phase in accordance with the illustration in Figure 3A:
Figure 3. A. Schematic of a weak-acid cation exchange resin swelling from addition of base; B. Shrinking of weak-acid cation exchange resin from addition of acid. C. Photographs under an optical microscope of a gradually swelling weak-acid resin bead (Purolite C-104) from base addition. D. Conceptualized reciprocating engine to produce a compressed gas through swelling and shrinking of weak-acid cation exchange resin; E. Experimental evidence of cyclic expansion and contraction of a latex membrane caused by deprotonation and protonation of a weak-acid cation exchange resin.
Subsequently, when brought in contact with an acid solution, the ion exchange resins shrink due to protonation of carboxylate functional groups as depicted in Figure 3B:
R − COOH + NaOH⥂R − COO−Na + + H 2O + swelling
R − COO−Na + + HCl⥂R − COOH + Na + + Cl−
(7)
+ shrinking
Where the overbar indicates the resin phase. 2147
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Sorption/Hardness Removal
Combining reactions depicting illustrations in Figure 3A and B, one may write
2R − COOH + Ca(HCO3)2 ⥂(R − COO−)2 Ca 2 +
NaOH + HCl⥂Na + + Cl− + H 2O + mechanical work
+ 2H 2CO3
(9)
Desorption/Regeneration
Thus, weak-acid ion exchange resins mediate acid−base neutralization while generating mechanical work through successive cycles of swelling and shrinking. One single cationexchange resin bead (Purolite C-104, carboxylate acid-functional group, Purolite Co., Philadelphia) was placed in a Petri dish under an optical microscope (Testmaster). Following the addition of a drop of 4% NaOH solution to the resin, the swelling of the bead was observed over a period; it swelled over 200% of its original volume in less than 15 min (Figure 3C). Shrinking of the resin back to its original volume was observed upon addition of a few drops of 4% HCl onto the resin. In order to demonstrate the feasibility of transforming cyclic expansion and contraction into usable energy, an experiment was conducted using a latex-membrane covered container inside which weak-acid cation exchange resin beads were allowed to come in consecutive contact with solutions of acid (1% HCl) and base (1% NaOH) during the process of neutralization.14 Shrinking and swelling of the resin column caused a change in pressure in the air over the resin column, which resulted in deflation and inflation of the latex membrane. While Figure 3D depicts a conceptualized reciprocating engine to provide compressed air as an energy source from acid−base neutralization, Figure 3E provides the experimental evidence of inflation and deflation of the latex membrane for two consecutive cycles, validating the basic premise of the concept. The external work generated through the expansion and contraction of the resin phase in a constant-volume chamber can thus be transformed into an engine under isothermal conditions. Obviously, the efficiency of the process is directly dependent on the swelling/shrinking ratio of the pH-sensitive polymers, which, in turn, are intrinsically dependent on their degree of cross-linking and hydrophilicity. Many pH-sensitive polymers with a high swelling ratio (over 500%) are available and amenable to synthesis, including biorenewable chitosan materials.15 They are likely to be good candidates for efficient recovery of mechanical work from acid−base neutralization.
CO2 (g) + H 2O⥂H 2CO3 H 2CO3⥂H+ + HCO−3 (R − COO−)2 Ca 2 + + 2H+⥂2R − COOH + Ca 2 +
Overall Regeneration (R − COO−)2 Ca 2 + + 2CO2 (g) + 2H 2O⥂2R − COOH + Ca 2 + + 2HCO−3
From an equilibrium viewpoint, the regeneration is essentially an acid−base neutralization reaction leading to sequestration of CO2 in the aqueous phase as alkalinity with simultaneous removal of Ca2+ or hardness from ion exchanging materials. The kinetics of carbon dioxide regeneration, is, however, very slow for commercially available ion-exchange resin beads. Instead, use of ion exchange fibers with relatively short diffusion path lengths offers high efficiency of regeneration.21−23 Figure 4A−C illustrate weak-acid ion exchange fibers with carboxylate functional groups, virgin fiber materials at 10× magnification and an SEM photograph of a single fiber used for carrying out hardness removal and regeneration using carbon dioxide.22 The process was sustainable and the ion exchange fiber was found significantly more efficient than commercially available resin beads. Many high volume acidic streams, such as sulfur-dioxideladen flue gases, acid mine drainage or AMD, and spent etching solutions containing HF are routinely neutralized to comply with prevailing environmental regulations. There exist opportunities to extract useful energy from neutralization. From a thermodynamic viewpoint, removal of hydrogen ion (H+) from acid solutions of nearly any strength is favorable and accompanied by significant negative free energy change. For example, for weak-acid solutions at pH 3.0, ΔG R of neutralization is −45.6 kJ/mol of H+ and the value increases to −68.4 kJ/mol at pH 1.0. Additionally, all neutralization reactions are kinetically very fast. As energy becomes expensive, regulations stringent, and nonrenewable materials scarce, such environmentally benign processes are likely to be economically competitive. An indepth analysis would promptly reveal that many ongoing global research thrusts for innovative technologies essentially embrace and apply scientific fundamentals that have long existed. Many emerging separation technologies utilize fundamentals of ligand exchange, photocatalytic oxidation, and redox reactions as the foundation to address many persistent environmental problems and offer sustainable solutions.24−27
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OTHER OPPORTUNITIES Global efforts for carbon dioxide capture and sequestration (CCS) from large scale, point source emissions such as power plants have led to a diverse group of processes.16−19 In addition to burning large quantities of fossil fuels, these coal-, gas-, and oil-fired electric power generating facilities require high volumes of hardness-free water as the feed for steam generators and recirculating cooling water systems. While lime softening processes for hardness removal produce voluminous sludge to be disposed of, ion exchange softening processes generate concentrated brine as a waste regenerant stream.20 Carbon dioxide is a weakly acidic gas and upon dissolution in water produces a low concentration of hydrogen ions. In principle, neutralization reactions initiated by carbon dioxide may allow both regeneration and simultaneous CO2 sequestration. Calcium (or magnesium) hardness may be removed in a fixed-bed column and regenerated using carbon dioxide in accordance with the following reactions:
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SCIENTIFIC CHALLENGES AND POTENTIAL BARRIERS Examples were presented for energy efficient and environmentally significant processes that are aided by acid−base neutralization. Separately, the first example uses acid−base neutralization to replace electricity for brackish water desalination; the second demonstrates the possibility of 2148
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carbon. The dilution effect of the bulk aqueous phase in the acid−base neutralization reaction and high specific heat of water, however, do not allow significant temperature increase, thus rendering it impossible to harness any useful thermal energy. Although the examples presented here offer new opportunities, some barriers need to be overcome before they become viable. For every example, the development of more efficient new materials holds the key to its success. An ion exchange membrane allowing high hydrogen ion flux with a salt-containing waste acid stream, a functionalized hydrophilic polymer with a high swelling/shrinking ratio and ion exchange nanofibers with surface functional groups can greatly improve the viability of the proposed processes. In closing, as new and efficient materials emerge, effluent streams with excess H+ ions need not be viewed as wastes requiring aqueous-phase neutralization before discharge. There remain significant opportunities to extract useful energy from waste acid streams in accord with the goals of environmental sustainability.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biography Arup K. SenGupta is the P. C. Rossin Professor in the Department of Civil and Environmental Engineering at Lehigh University in Bethlehem, PA. Michael German is currently a doctoral student in Lehigh’s environmental engineering program. John Grenleaf is a visiting professor at Lafayette College and received his PhD in environmental engineering from Lehigh University.
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ACKNOWLEDGMENTS We are immensely thankful for partial financial support received through an exploratory National Science Foundation Grant (CBET-1065651) and Lehigh University Faculty Innovation Grant (FIG-607488).
Figure 4. A. Schematic of weak-acid ion exchange fibers with carboxylate groups. B. Weak acid ion-exchange fibers digitally photographed at 10× magnification. C. SEM photograph of a single ion exchange fiber.
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REFERENCES
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extracting mechanical energy from neutralization; and the third uses its chemical energy for hardness removal and CO2 sequestration. All three examples have one thing in common: H+ neutralization is mediated in the presence of another medium be it a membrane, swelling/shrinking polymer or ion exchange fiber. The direct aqueous-phase neutralization of H+ with a base is avoided altogether. Thermodynamically, each process is favorable; thus, other opportunities are equally possible. For an insightful understanding of the scientific opportunity embedded in waste acid neutralization, it is worthwhile to compare the standard state enthalpy change with that of carbon oxidation, a universal process for energy generation, including coal burning: C + O2 → CO2 (g), ΔHo = − 394kJ/mol H+ + OH− → H 2O, ΔHo = −55.8kJ/mol
Note that, although smaller on a molar basis, the negative enthalpy change per unit mass of hydrogen ion for the acid− base neutralization is greater than that for the combustion of 2149
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