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Acoustic Treatment of Coal Gasification Residue for Extraction of Selenium Baharak Sajjadi, Wei-Yin Chen, and Riya Chatterjee Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00683 • Publication Date (Web): 20 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019
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Energy & Fuels
Acoustic Treatment of Coal Gasification Residue for Extraction of Selenium
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Baharak Sajjadi*, Wei-Yin Chen, Riya Chatterjee
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Department of Chemical Engineering, School of Engineering, University of Mississippi, 134
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Anderson Hall, MS 38677-1848, U.S.A.
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*E-mail:
[email protected], Tel No: 662-915-1640, Fax No: 662-915-7023
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The present study demonstrates that acoustic treatment of coal gasification residue in water
8
with dissolved CO2 is an effective route for extracting selenium (Se). It was recently revealed that
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the acoustic treatment can enhance the extraction of a series of metals, Na, K, Ca, Si, from the
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carbonaceous structures, e.g., biochar, due to the cavitation-induced phenomena including
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generation of micro-jets, shock-waves and hot-spots. Ultrasound (US) irradiation of graphitic
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carbonaceous structures in water/CO2 partially exfoliates the graphitic clusters, creates new and
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opens the blocked pores, and increases the internal surface area, carbon content and hydrogen
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content through a group of sonolysis reactions of CO2 and H2O1,2. The present study utilized
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ultrasound treatment with different amplitudes and total US energy consumption or irradiation
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durations under CO2 blanket in the headspace of the solution to determine the extraction of a series
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of heavy metals, including selenium, from a coal gasification residue. The experimental results
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(not yet optimized) showed a significant reduction in selenium content (71%) in treated samples
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at 2000 J - 50% ultrasound (US) amplitude. A thorough study of this approach is certainly
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warranted.
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The coal combustion and gasification residues typically contain toxic heavy metals including
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arsenic, lead, mercury, cadmium, chromium, selenium etc. causing cancer, pulmonary disorder,
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cognitive defects, developmental delays, and behavioral problems 3. Coal gasification residue is a
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major waste streams from the power plants, which is often stored in landfills as a long-term
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disposal mean 3. A typical concern regarding the landfills involves the risk of catastrophic failure
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(such as impoundment of ponds containing coal ash slurry), groundwater contamination, and their
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hazardous nonreversible effects on the local environment, especially with selenium. The major
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environmental implication of selenium is its propensity to accumulate in aquatic food chains
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endangering human lives4. No satisfactory chemical, physical or biological treatment exists to
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remove selenium because the existing treatments can only work on small scale and are inefficient
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and expensive for large scale5.
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Acoustic treatment of biochar has been considered a new route for the production of advanced
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sorbents for CO2 capture6 and adsorption of heavy metals in wastewater7. The residue used in the
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current study was obtained from a coal gasifier in China. It was derived from lignite and has
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unusually high carbon content, 20.6%, representing a significant fraction of unused energy. The
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company is currently storing the residue in a landfill. In the current study, the acoustic energy was
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applied to the mixture of 7.5 g residual powders in 125 ml of water saturated with CO22. The US
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treatment was conducted with a QSonica - Q700 sonicator of 20 kHz and 700 W in presence of
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7% CO2 in head-space. In order to control CO2 concentration at 7%, a mixture of CO2 (50 ml/min)
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balanced with helium (664 ml/min) was blown into the head space of the solution. The following
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US amplitude and energy chosen to evaluate their effects on mineral leaching: 5% US amplitude
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and 2000 J energy (US-A5%-E2000J), 50% US amplitude and 2000 J energy (US-A50%-E2000J)
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and 50% US amplitude and 4000 J energy (US-A50%-E4000J). The change of electrical energy
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into mechanical energy in ultrasound transducer causes its tip to move up and down. The
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displacement of the ultrasound tip is called its amplitude, and this is adjustable. In this work, an
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ultrasound tip with the amplitude of 120 microns was used (amplitude of 100%). Therefore, at
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settings of 50% and 5% amplitude, the probe will achieve the amplitudes of approximately 60 and
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6 μm respectively. QSonica - Q700 sonicator is equipped with a monitor which shows the
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amplitude and the total amount of electrical energy. Furthermore, ultrasonication for 1800 s was
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employed at 50% amplitude as well to assess the effect of prolonged acoustic treatment. To
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illuminate the impact of US irradiation, the leaching of metal was studied with water washing for
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167 and 1800 s without irradiation. The durations were selected based on the experiments with 5%
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amplitude-2000 J energy, and 50% amplitude-1800 s, respectively. After each treatment, the
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mixture was filtered, and the residue was dried at 60 °C overnight under vacuum and analyzed for
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mineral and organics changes as well as elemental compositions and metal removal.
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Effects of Ultrasound Amplitude, Energy and Irradiation Duration: Table 1 depicts the role of
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acoustic treatment in the extraction of heavy metals. Leaching of selenium, sodium, and potassium
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was accelerated when the US amplitude was increased from 5% to 50% (at constant ultrasound
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energy of 2000 J). The calculations of this study are based on “weight change” not direct
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subtraction. In this method, the overall mass change of the carbonaceous structure during treatment
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is taken into account. For example, US-A50%-E2000J treatment showed a weight loss of 22.78%
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(or weight change of -22.78%) (Table 2). Now %removal can be calculated as:
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[(1 + WC) ∗ (C𝑎𝑠ℎ of TR) ∗ (C𝑆𝑒 of TR)] ― [( C𝑎𝑠ℎ of RR) ∗ ( C𝑆𝑒 of RR)] ( C𝑎𝑠ℎ of RR) ∗ ( C𝑆𝑒 of RR)
∗ 100
(1)
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Where, WC, TR, RR, Cash and CSe denote the weight change, Treated residual, Raw residual, ash
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content and Selenium content, respectively. A similar calculation was used for the other metals
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including arsenic.
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Se removal increased from 33% for US-A5%-E2000J to 71% for US-A50%-E2000J. This
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behavior can be explained as a function of acoustic cavitation intensity, which increased 3 ACS Paragon Plus Environment
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significantly at high amplitude accelerating the mass transfer. This facilitates the leaching of
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minerals which have strong bonding with the carbonaceous support. Meanwhile, the removal of
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cadmium and chromium was reduced with ultrasound amplitude. This may be due to the formation
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of agglomeration of metal due to fusion at high temperature as a result of intensified sonication8.
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Similar behavior was observed when greater acoustic energy (4000 J) was applied at the same
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amplitude (50%). The results showed 12% reduction in Se and 2% in Cr removals, when the energy
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increased from 2000 to 4000 J. On the contrary, Cd removal was almost doubled with increasing
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acoustic energy. These results suggest that optimizing the interaction of ultrasound energy and
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amplitude can further improve the removal of target heavy metals. On the contrary, Cd removal
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was almost doubled with increasing acoustic energy. This necessitates a thorough analysis of the
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interaction between the kinetics of leaching of different minerals with acoustic intensity. In the
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next step, the effect of ultrasonication duration was investigated by treating the residue sample for
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a longer duration (1800 sec) at 50% amplitude. Data in Table 1 suggest that longer duration does
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not enhance the leaching of heavy metals, which could be attributed to aggregation during the long
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treatment time9.
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Effect of Dissolved CO2: Dissolved CO2 is the main source of water acidity in the system
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of interest. CO2 keeps entering water through its interface with the atmosphere. Water reacts with
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aqueous CO2 (aq), forming carbonic acid; 𝐻2𝑂 + 𝐶𝑂2(𝑎𝑞)↔𝐻2 𝐶𝑂3(𝑎𝑞). Dissolved CO2 in the
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form of carbonic acid may lose protons to form bicarbonate (R1, K=2.00×10-4) and carbonate
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(K=4.69×10-11)2. O
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O C O + H
O
H
H+ +
O C(OH)2
HO
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O-
(2)
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The generated H+ ions interact with the metallic ions present in residue sample through ion
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exchange and remove them from the structure. This phenomenon contributes in hydrogen uptake
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as well. Moreover, sonolysis of CO2 in water yields acidic organics including formaldehyde and
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formic acid in the following way. It is known that ultrasound splits water to form hydrogen and
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hydroxyl radicals (R3). On the other hand, several studies showed the sonolysis of CO2 in aqueous
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solution 10, 11, 12. Henglein et al. 12 revealed that CO2 plays two major roles: scavenging hydrogen
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radicals and decomposition to CO. CO2 Scavenges •H from the sonolysis of water to form •COOH
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which is followed by formation of a small quantity of formic acid (R4-R5): Formaldehyde can
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then be formed as the byproduct of HCOOH degradation (R6-R7) 13. More details can be found in
10 11 12 13 14 15 16 17
our previous work 2. H
OH
H
O C O H + H O C O+H
OH
O C OH H
C
H
(4)
O C O H
O C OH + OH H
O
(3)
O C O H + H O H
H O
(5)
C
H
(6)
H O
C H
(7)
18 19 20
These compounds could react with graphitic carbonaceous structures containing Lewis base and
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the π electrons. The basic nature of residue in water increased alkalinity that helped to trap a higher
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amount of CO2 and organic acids during CO2 bubbling and treatment thus enhanced the removal
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of basic cations through ion exchange and acid/base reactions.
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Effect of Water Washing: The heavy metals are partially soluble in water. Hence, the water
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washing tests were conducted to illuminate the impact of ultrasound treatment. As observed, 29%
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of Se was leached out of the residue by simple washing. The value increased to 71% by using a
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short-term of ultrasound irradiation under CO2 blanket. Therefore, enhanced removal of selenium
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through acoustic cavitation serves as a viable treatment technique for overcoming the challenges
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of handling coal fired residues. The dissolved selenium (selenite or selenate) can be further
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removed from the wastewater through either biological or chemical/physical processes. In the
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former, the dissolved selenium can be biologically reduced by bacteria under anoxic conditions to
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elemental selenium 14. The latter technologies include: oxidation/reduction, iron co-precipitation
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15
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water washing. As observed, 80% of arsenic that was removed from the residue was water
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leachable as reported earlier17. Therefore, integration and optimization of water and ultrasonic
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washing can effectively remove a wide range of metals, particularly As and Se, which can induce
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plant toxicity and subsequent effects on animals and humans17.
, ion exchange, and adsorption 16. On the other hand, arsenic exhibited easy percolation during
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Implications of mineral and organics change during treatment: The various parameters
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involving in the acoustic treatment process and their changes along with the elemental
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compositions are reported in Tables 2-3. Weight change (in Table 2) represents a combination of
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mineral leaching, organics leaching as well as fixation of organics and minerals on the graphitic
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carbonaceous structure (herein residue) during the treatment. All weight changes show negative
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values, indicating the acoustic treatment induced a weight loss through the removal of inorganics
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or minerals and a small amount of change in organic compounds.
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The weight loss was more pronounced with increasing the ultrasonic amplitude and energy.
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This trend was also observed in our previous studies1-2. In addition, the observed trend of weight
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loss is consistent with the % ash reduction and mineral leaching (Table 1). As a result, an increase
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in sonication amplitude (from 5% to 50% at constant energy consumption 0.06 kcal/g) and energy
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(0.06 kcal/g to 0.13 kcal/g) enhanced the mineral loss from 23.75 to 25.03%.
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Organic content is a measure of the combustible (i.e., the gasified) fraction of the residue such
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as %C, %H, %O, %N and %S content. Change in organic composition during the treatments has
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been calculated and reported in Table 3. The highest hydrogen fixation was observed in water
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washed samples, while a slight H increase was obtained in ultrasonic-treated samples. This
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attributes to the different types of minerals removed and the kinetic of removal. The gain in
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hydrogen content of water-treated samples suggested the attachment of H+ ion through ion
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exchange, 𝑀 + + 𝐻 + ↔𝐻 + + 𝑀 + ; however, removal of mineral compounds under ultrasound
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irradiation is most likely due to cavitation and its implications.
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On the other hand, the reduction in O and C contents of samples treated with ultrasound is
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significantly lower compared with water washed residue. Since the weight loss is a combination
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of mineral leaching, organics leaching and fixation of organics, and given the fact that the values
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of ash loss in both ultrasound and water treated residues are very close, data in Table 3 imply
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significant losses of both organic content (C and O) during water washing. Nevertheless,
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ultrasound appears to reduce these losses and possible induces C and O fixation. Generally,
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nineteen complicated reactions occur in sonolysis of pure water alone, generating a series of highly
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reactive species and radicals including 𝐻, 𝑂𝐻, 𝐻2, 𝐻𝑂2, 𝑂2, 𝐻2𝑂2, 𝑂 18. On the other hand, CO2
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and H2O are the only sources for C, O and H fixation on the residue. Since an initial sonochemical
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reaction is the addition of H• to CO2 to form a carbon-centered •COOH radical, it is possible that
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the radical adds to a π bond, and the resulting radical combines with another radical, giving a
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carboxylic acid (R5). C
+ O=C O H
C
23
H O C
H
+ H—Ar
O C
C
C
+ •H
O H
HO CH27 Ar ACS Paragon Plus Environment
O H C (R15)
C
C
O H
(R14)
(8)
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Sonolysis of dissolved CO2 can also directly split CO2 to form highly reactive •O•. Diradical
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oxygen initiates a chain of reactions to form 𝑂2, carbon atom and CO. Formic acid and
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formaldehyde may also be fixed on the residue that cause increases in C. We remain interested in
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the possible fixation of carbon from CO2 to the graphitic carbonaceous structure2. Additionally,
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the IR spectrum shown in Fig 1 represents the spectrum of raw and activated residuals under
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different conditions. The overall shape of the spectrums remains same except the C-O peak at 1100
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cm-1 1. The figure shows that the intensity of C-O peak (which comes from ether linkage) increased
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for activated samples with enhancing the sonication amplitude and energy. The peak is ascribed to
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alkoxy group (ether) stretching and the increase in intensity can be explained as a result of
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interaction between C-O and CO2 19.
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This study introduced a fast and economically feasible method for removal of selenium from
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coal combustion and gasification residues in water/CO2 system under ultrasound irradiation. The
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process was applied at room temperature and effectively removed 71% Se within 41 seconds.
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Additionally, the study revealed that simple water washing dissolved with CO2 can remove
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significant amount of arsenic (80%) from the residue samples. The results suggested that
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integration and optimization of ultrasonic treatment of different acoustic amplitudes (5 to 50%)
17
with simple water washing can maximize the removal of a wide range of residual’s heavy metals.
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The results were obtained at early stage of our study thus it needs further through investigation to
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optimize the process aiming at maximum removal of all the heavy metals.
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Acknowledgment
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This project is supported by the National Science Foundation of the United States (NSF EPSCoR
22
RII Grant No. OIA-1632899), to whom the authors are grateful.
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Energy & Fuels
References 1. Chen, W. Y.; Mattern, D. L.; Okinedo, E.; Senter, J. C.; Mattei, A. A.; Redwine, C. W., Photochemical and acoustic interactions of biochar with CO2 and H2O: applications in power generation and CO2 capture. AIChE Journal 2014, 60 (3), 1054-1065. 2. Sajjadi, B.; Chen, W.-Y.; Adeniyi, A.; Mattern, D. L.; Mobley, J.; Huang, C.-P.; Fan, R., Variables governing the initial stages of the synergisms of ultrasonic treatment of biochar in water with dissolved CO2. Fuel 2019, 235, 1131-1145. 3. Kim, A. G., Soluble metals in coal gasification residues. Fuel 2009, 88 (8), 1444-1452. 4. Lemly, A. D., Environmental implications of excessive selenium: a review. Biomed Environ Sci 1997, 10 (4), 415-35. 5. Presser, T. S.; Luoma, S. N., Forecasting selenium discharges to the San Francisco Bay-Delta Estuary: ecological effects of a proposed San Luis Drain extension. Geological Survey (US): 2006. 6. Chatterjee, R.; Sajjadi, B.; Mattern, D. L.; Chen, W.-Y.; Zubatiuk, T.; Leszczynska, D.; Leszczynski, J.; Egiebor, N. O.; Hammer, N., Ultrasound cavitation intensified amine functionalization: A feasible strategy for enhancing CO2 capture capacity of biochar. Fuel 2018, 225, 287-298. 7. Sajjadi, B.; Broome, J. W.; Chen, W. Y.; Mattern, D. L.; Egiebor, N. O.; Hammer, N.; Smith, C. L., Urea functionalization of ultrasound-treated biochar: A feasible strategy for enhancing heavy metal adsorption capacity. Ultrasonics Sonochemistry 2019, 51, 20-30. 8. Vyas, S.; Ting, Y.-P., A review of the application of ultrasound in bioleaching and insights from sonication in (bio) chemical processes. Resources 2018, 7 (1), 3. 9. Ali, F.; Reinert, L.; Lévêque, J.-M.; Duclaux, L.; Muller, F.; Saeed, S.; Shah, S. S., Effect of sonication conditions: solvent, time, temperature and reactor type on the preparation of micron sized vermiculite particles. Ultrasonics sonochemistry 2014, 21 (3), 1002-1009. 10. Shah, Y. T.; Pandit, A.; Moholkar, V., Cavitation reaction engineering. Springer Science & Business Media: 2012. 11. Rooze, J., Cavitation in gas-saturated liquids. Eindhoven: Technische Universiteit Eindhoven– 2012.–120 pp. DOI 2012, 10. 12. Henglein, A., Sonolysis of carbon dioxide, nitrous oxide and methane in aqueous solution. Zeitschrift für Naturforschung B 1985, 40 (1), 100-107. 13. Navarro, N. M.; Chave, T.; Pochon, P.; Bisel, I.; Nikitenko, S. I., Effect of Ultrasonic Frequency on the Mechanism of Formic Acid Sonolysis. The Journal of Physical Chemistry B 2011, 115 (9), 2024-2029. 14. Hageman, S. P. W.; van der Weijden, R. D.; Stams, A. J. M.; van Cappellen, P.; Buisman, C. J. N., Microbial selenium sulfide reduction for selenium recovery from wastewater. Journal of Hazardous Materials 2017, 329, 110-119. 15. (a) Li, Y.; Guo, X.; Dong, H.; Luo, X.; Guan, X.; Zhang, X.; Xia, X., Selenite removal from groundwater by zero-valent iron (ZVI) in combination with oxidants. Chemical Engineering Journal 2018, 345, 432-440; (b) Liu, J.; Taylor, J. C.; Baldwin, S. A., Removal of selenate 9 ACS Paragon Plus Environment
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from brine using anaerobic bacteria and zero valent iron. Journal of Environmental Management 2018, 222, 348-358. 16. He, Y.; Xiang, Y.; Zhou, Y.; Yang, Y.; Zhang, J.; Huang, H.; Shang, C.; Luo, L.; Gao, J.; Tang, L., Selenium contamination, consequences and remediation techniques in water and soils: A review. Environmental Research 2018, 164, 288-301. 17. Dellantonio, A.; Fitz, W. J.; Repmann, F.; Wenzel, W. W., Disposal of coal combustion residues in terrestrial systems: contamination and risk management. Journal of environmental quality 2010, 39 (3), 761-775. 18. (a) Adewuyi, Y. G., Sonochemistry: Environmental Science and Engineering Applications. Industrial & Engineering Chemistry Research 2001, 40 (22), 4681-4715; (b) Gong, C.; Hart, D. P., Ultrasound induced cavitation and sonochemical yields. The Journal of the Acoustical Society of America 1998, 104 (5), 2675-2682. 19. Coates, J., Interpretation of infrared spectra, a practical approach. 2000.
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Energy & Fuels
Table 1. Changes in elemental metal concentration of the residue upon treatment (weight-based calculation*). Maximum possible selenium removal was achieved at 50% US amplitude and 2000 J US energy consumption within 41 s. 80% of arsenic was soluble in water for 1800 s implying the risk of leaching of metals in water. Sample Description
Na Cd Se K Ca As Hg Cr Pb (μg/g) (μg/g) (μg/g) (μg/g) (μg/g) (μg/g) (μg/g) (μg/g) (μg/g)
Raw 10,900 0.66 2.6 13,400 43,000 7.7
