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Value-Added Humic Acid Derived from Lignite Using Novel Solid-Phase Activation Process with Pd/CeO2 Nanocatalyst: A Physiochemical Study Yafu Tang, Yuechao Yang, Dongdong Cheng, Bin Gao, Yongshan Wan, and Yuncong C. Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02094 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 2, 2017
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Title Page
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Value-Added Humic Acid Derived from Lignite Using Novel Solid-Phase
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Activation Process with Pd/CeO2 Nanocatalyst: A Physiochemical Study
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Authors: Yafu Tang1, Yuechao Yang1,3*, Dongdong Cheng1*, Bin Gao2, Yongshan
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Wan3, Yuncong C. Li3
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Affiliations:
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1
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Resources; National Engineering & Technology Research Center for Slow and
National Engineering Laboratory for Efficient Utilization of Soil and Fertilizer
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Controlled Release Fertilizers, College of Resources and Environment, Shandong
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Agricultural University, Daizong Street No. 61, Taishan District, Taian, Shandong
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271018, China;
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2
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Sciences (IFAS), University of Florida, Gainesville, FL 32611, USA.
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3
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IFAS, University of Florida, Homestead, FL 33031, USA.
Agricultural and Biological Engineering, Institute of Food and Agricultural
Department of Soil and Water Science, Tropical Research and Education Center,
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*Corresponding author: Yuechao Yang, Dongdong Cheng
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Phone: 86-538-824 2900
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E-mail:
[email protected] 21
[email protected] 1 ACS Paragon Plus Environment
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ABSTRACT: Soil, air and water pollution caused by lignite is considered a serious
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environmental problem. Activation methods thus have been developed to extract
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humic acid from lignite to support the agricultural production as the soil amendment
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or fertilizer synergist. The traditional activation methods of humic acid from lignite,
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however, are not environmentally friendly. As the first study, this work developed a
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novel solid-phase activation method with a Pd/CeO2 nanocatalyst for lignite-derived
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humic acid. This study analyzed the morphology and structures of as-synthesized
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Pd/CeO2 nanocatalyst with various characterization tools. The mechanisms of
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Pd/CeO2 nanocatalyst for lignite activation were determined. The Pd/CeO2 catalyst
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effectively promoted the production of water soluble humic acids from lignite via
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KOH solid-phase activation at room temperature. It increased the amount of small
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molecular active groups and the corresponding small molecules of humic acid. The
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existence of a strong synergistic effect at the interface sites between Pd/CeO2
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nanoparticles and lignite was one of the key factors for the outstanding catalytic
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performance. In conclusion, this study has great application perspectives in reducing
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lignite pollution and increasing humic acid utilization by crops, which can improve
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the sustainability of environment and agricultural systems.
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KEYWORDS: Lignite, Catalyst, Activation, Humic acid
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INTRODUCTION As the lowest rank of coal, lignite is an abundant natural resource and is often
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piled up as wastes in coal mining area because of its low calorific value and high ash
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content.1, 2 It not only occupied and destroyed the arable lands, but also cause
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environmental pollution such as air pollution and surface and underground water
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contamination.2 Mineralization process of lignite often releases acidic leachates
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containing heavy metals and trace elements which result in serious contamination of
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soil and water aquifers.3, 4 Therefore, the environmental problems caused by lignite
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are of great attention. However, recent studies have suggested to use lignite for the
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development of value-added products including humic and fulvic acids.5 Humic acid
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contains various types of acidic functional groups that play an important role in
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regulating many crucial ecological and environmental processes. Previous studies
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have demonstrated that humic acid can be used as a remediation agent in many
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environmental applications as well as soil amendment to improve soil properties.6
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Several studies have indicated that humic substances may have stronger effects on
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plant growth and crop yield than some of the traditional inorganic fertilizers.7 In
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particular, humic substances can not only enhance nutrient uptake and utilization in
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plants through chelating minerals, but also be the main source of organic carbon to
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plants through their own degradation.8 However, the humic substances in lignite
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cannot be directly utilized by crops. Activation processes thus are often needed to
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convert these humic substances into water-soluble forms that further stimulated the 3 ACS Paragon Plus Environment
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growth of the plants.9 Physical and chemical activation methods have been
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developed recently to better utilize the humic substances in lignite.6, 10 The
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traditional activation of humic acid from lignite often uses oxidants such as HNO3,
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H2O2 and KMnO4 to pretreat the samples to increase yield. However, most of these
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methods use liquid phase reactions and require external heating with high demand of
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energy, relatively long reaction time, and high standard of equipment.6 Furthermore,
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the activation process may not be environmentally friendly and release undesired
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byproducts into the natural environment. These drawbacks have limited the
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development of activation technologies for utilizing humic substances of lignite. It is
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thus necessary to develop novel activation methods that are low-cost, highly
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effective, and environmentally friendly to utilize lignite.
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With the development of nanotechnology, nanosized catalysts have attracted much
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attention recently.11 Metal and metal oxide-based composites are recognized as
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promising catalysts.12 In particular, Pd/CeO2 composites have been used as low-cost
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and high-efficiency catalysts in various applications.13-15 Pd/CeO2 composites as a
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three-way catalytic convertor are currently used in vehicles to oxidizing
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methane.16-18 In this case, ceria is an ideal support for methane oxidation catalysis
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because of it high oxygen storage capacity and high oxygen mobility.19 Vayssilov
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studied model Pt/CeO2 catalysts on electron and oxygen transfer and showed
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favorable interactions on nanostructured ceria that enhances activity.20 It has been
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demonstrated that, among the combination of noble metals (e.g., Pt, Ru, Pd and Au) 4 ACS Paragon Plus Environment
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and rare earth oxides (e.g., ZrO2, TiO2, and CeO2) for a single nanostructure for
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catalytic reactions, the Pd/CeO2 catalyst is considered the most effective one for the
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water-gas-shift (WGS) reaction.21-23 Tan et al. used CeO2 of different morphologies
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to support Pd nanocatalyst and found that Pd/ CeO2 can fully convert indoor
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formaldehyde into CO2 at ambient temperature.24 This is partially due to the fact that
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Ce is a multivalent ion (III and IV) and thus CeO2 has excellent redox potential with
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superior oxygen storage and release capacities.12 It has also been demonstrated that
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the CeO2 (100) surface is highly defective and contains more oxygen vacancies than
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the (111) and (110) surfaces.25 Additionally, it has been reported that the energy
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required for the formation of reactive oxygen vacancies on (100) surface is lower
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compared with (111) surface.26 Therefore, the CeO2 (100) surface is favorable for
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catalytic applications.
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Findings from previous studies have all pointed out that the Pd/CeO2
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nanocomposite is an excellent catalyst that can be used to oxidize and convert
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methane, formaldehyde, water gas, and other hydrocarbons with complicated carbon
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structures.17, 22, 24 Because lignite has high volatile hydrocarbons and is rich in
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carboxyl, hydroxyl, and phenol functional groups,27 it is anticipated that Pd/CeO2
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nanocomposites can also oxidize and activate the humic substances from lignite to
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promote its applications. However, little research has been conducted on the use of
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Pd/CeO2 catalyst to active lignite via solid-phase reaction at room temperature.
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The objective of this work was to take advantage of the excellent oxidation 5 ACS Paragon Plus Environment
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catalytic ability of Pd/CeO2 to develop and optimize the solid-phase activation
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process to produce value-added water-soluble humic substances from lignite. The
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activation of lignite often involves the convention of the insoluble calcium and
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magnesium humic matters into soluble potassium (sodium) salts through KOH
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treatment. In this work, a novel ball milling process was applied in the solid-phase
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activation process of lignite to promote the KOH conversion in the presence of
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Pd/CeO2 catalysts of different morphologies. Various laboratory experiments were
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conducted to determine the solid-phase activation mechanisms. In particular, the
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catalytic mechanisms of the Pd/CeO2 nanocomposites in surface oxidation of humic
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substances in lignite were explored. Findings from this work can be used to inform
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the development of low cost, highly effective, and environmentally friendly
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technologies to reduce the environmental pollution of lignite and produce
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value-added humic acid from lignite.
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EXPRIMENTAL SECTION
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Materials. Lignite (Shanxi, China), Ce(NO)3·6H2O (analytical grade, Aladdin
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Chemistry Co, Ltd, Shanghai, China), NaBH4 (analytical grade, Aladdin Chemistry
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Co, Ltd, Tianjin, China), PdCl2 (Aladdin Chemistry Co, Ltd, Shanghai, China), and
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KOH (analytical grade; Tianjin Kaitong Chemical Industry Co, Ltd, Tianjin, China)
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were used in this study. The solutions were made with deionized water.
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Synthesis of the CeO2 Catalysts. The CeO2 catalyst was prepared according to
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the previous methods.12, 24 The ceria materials were prepared via the template-free
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alkaline hydrothermal method.24 In brief, 1 g of Ce(NO)3·6H2O was dissolved in 30
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mL deionized water under stirring conditions. 10 mL NaOH (800 g/L) solution was
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then added. The mixture was stirred for 30 min at room temperature and then rapidly
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sealed into a 50 mL autoclave. The hydrothermal treatment was conducted at 473 K
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for 24 h. The final product was collected by filtration, centrifuged and rinsed several
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times with deionized water to remove any possible ionic remnants, and then dried
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and calcined at 337 K for 4 h.
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Preparation of the Pd/CeO2 Nanoparticles. The Pd/CeO2 nanoparticles were
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synthesized using a wet impregnation method.24 0.3 g of CeO2 samples were mixed
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with 100 mL deionized water and sonicated for 10 min at room temperature, then
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0.005 g of PdCl2 was added to the above solution and then the reaction mixture
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stirred for an hour. Afterward, the pH of the mixture was buffered to neutral using
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5% NaOH solution. Then, 2 mL of 0.005 g/ml NaBH4 solution was added into the
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suspension under stirring conditions. The final precipitates were thoroughly washed
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4 times with deionized water and ethanol. The resulting Pd/CeO2 nanoparticles were
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dried at 337 K overnight.
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Catalyst Characterization. The size and morphology of the synthesized sample
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were obtained by followed methods. Filed-emission scanning electron microscopy
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(FESEM, S-4800, Japan) was conducted at an accelerating voltage of 15 K.
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Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM)
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images were obtained with field emission transmission electron microscope (Tecnai 7 ACS Paragon Plus Environment
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G2 F20, USA) at an a working voltage of 200 kV.
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A JSM-6360LV scanning electron microscope (SEM) (JEOL) equipped with an
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X-act energy-dispersive X-ray spectrometer (EDX) (Oxford) was used to analyze the
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morphology and surface elemental composition of the synthesized sample.
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The powder X-ray diffraction (XRD) analysis of the synthesized sample was
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performed using an X-ray diffractometer (D8 ADVANCE, Germany) with
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Ni-filtered Cu Kα radiation (λ = 0.1541 nm), The 2θ angular region between 5 and
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90o was operated at a scan rate of 4 min−1.
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Raman spectra of the synthesized sample was conducted by spectrometer
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equipped with a liquid N2 cooled charge-coupled device detector and a confocal
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microscope (Renishaw inVia, Britain). A 350 mW near-infrared 785 nm laser was
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used for analysis under ambient conditions. The wavenumber values reported from
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the spectra are accurate to within 2 cm−1.
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X-ray photoelectron spectroscopy (XPS) was recorded using a Thermo escalab
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250Xi photoelectron spectrometer (USA) with a monochromatic Al Kα (hν = 1486.6
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eV) radiation source. The charging shift was calibrated with C 1 s value of
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adventitious carbon at binding energy of 284.8 eV. Smart background correction was
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used for peak fits with Avantage program.
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H2 temperature-programmed reduction (H2-TPR) was performed using a
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ChemiSorb 2720 (USA) apparatus equipped with a TCD detector. TPR was carried
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out at a heating rate of 5 oC min−1 using 10 vol % H2-Ar mixture and at a flow rate of 8 ACS Paragon Plus Environment
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15mL·min−1 to examine the redox behaviors of the samples. The Brunauer-Emmett-Teller (BET) surface area and pore volume of the sample
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were determined with N2 adsorption−desorption isotherms at −196 °C using
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Micrometrics ASAP 2020 (USA).
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Activation of Lignite. Lignite was milled and dried and then sieved to pass an 80
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mesh. Solid KOH was used as the activation agent. The experiment included four
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treatments: 1) raw lignite (RL) as the control; 2) 10% activation agent with lignite
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(AL); 3) 10% activation agent and 1% CeO2 nanocatalyst with lignite (ACL), and 4)
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10% activation agent and 1% Pd/CeO2 nanocatalyst with lignite (APL). The same
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amount of lignite was used in the above treatments. Each treatment was placed in a
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ball mill (QM-10-15, China) and was ground for 60 min at a speed 80 r/min. The
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experiments were repeated 3 times.
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Catalytic Activity Evaluations. After each of the treatments (RL, AL, ACL,
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APL), elemental compositions of the samples were determined using an elemental
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analysis instrument (Model 1106, Germany).Total humic acid and water soluble
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humic acid in the four treatments were determined with previously reported
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methods.28, 29 To observe the static grading phenomena of the four treatments, 1 g of
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each sample was added in 1,000 mL of water and place for 1 year. To further explore
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the catalytic effect, the humic acid was fractionated into 3 size ranges: below10,000
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Da; 10,000-50,000 Da and over 50,000 Da by continuous flow analytical system
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(DMJ60, China). Each molecular size fraction was determined following the 9 ACS Paragon Plus Environment
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methods of previous studies.30, 31 The light absorbance at 465 nm and 665 nm of the
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four treated humic acid was obtained using a Spectronic 20 Genesys
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Spectrophotometer on solution of 3.0 mg of each HA in 10 mL of 0.05 M NaHCO3
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and then the E4/E6 ratios were calculated.32, 33
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To further understand the activation and catalytic mechanisms, water-soluble
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calcium and magnesium in lignite was determined with atomic absorption
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spectrometer (AA-700, Japan). A JSM-6360LV scanning electron microscope (SEM,
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S-4800, Japan) equipped with an X-act energy-dispersive X-ray spectrometer (EDX)
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(Oxford) was also used for the morphological survey and elemental identification of
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the surface of the four treatments. Solid-state NMR spectroscopy
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(13C-CPMAS-NMR) was performed on a Bruker AV-300 (Germany) equipped with
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a 4 mm wide-bore MAS probe and NMR spectra were obtained by applying the
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following parameters: 13,000 Hz of rotor spin rate; 1 s of recycle time; 1 ms of
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contact time; 20 ms of acquisition time; 5,000 scans. Samples were packed in 4 mm
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zirconia rotors with Kel-F caps. The four treatment samples were re-dissolved in 1
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mL of hexane and transferred in a glass vial for gas chromatography-mass
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spectrometry (GC-MS, MSQ8100 GC/MS, China) analysis.
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Statistical Analysis. Tukey’s multiple range testing was performed to compare the
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average values among the parameters. The statistical significance was at a
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probability level of p
