Elemental mercury (Hg0) removal from coal syngas using magnetic tea-biochar: Experimental and theoretical insights

Mercury is ranked 3^rd as a global pollutant because of its long persistence in the environment. Approximately 65% of its anthropogenic emission (Hg⁰) to the atmosphere is from coal-thermal power plants. Thus, the Hg⁰ emission control from coal-thermal power plants is inevitable. Therefore, multiple sorbent materials were synthesized using a one-step pyrolysis method to capture the Hg⁰ from simulated coal syngas. Results showed, the Hg⁰ removal performance of the sorbents increased by the citric acid/ultrasonic application. T5CUF_0.3 demonstrated the highest Hg⁰ capturing performance with an adsorption capacity of 106.81 µg/g within 60 min at 200 °C under complex simulated syngas mixture (20% CO, 20% H₂, 10 ppmV HCl, 6% H₂O, and 400 ppmV H₂S). The Hg⁰ removal mechanism was proposed, revealing that the chemisorption governs the Hg⁰ removal process. Besides, the active Hg⁰ removal performance is attributed to the high dispersion of valence Fe₃O₄ and lattice oxygen (α) contents over the T5CUF_0.3 surface. In addition, the temperature programmed desorption (TPD) and XPS analysis confirmed that H₂S/HCl gases generate active sites over the sorbent surface, facilitating high Hg⁰ adsorption from syngas. This work represented a facile and practical pathway for utilizing cheap and eco-friendly tea waste to control the Hg⁰ emission.

Effect of Sonochemical Treatment on Thermal Stability, Elemental Mercury (Hg0) Removal, and Regenerable Performance of Magnetic Tea Biochar

Elemental mercury (Hg⁰) removal from a hot gas is still challenging since high temperature influences the Hg⁰ removal and regenerable performance of the sorbent. In this work, a facile yet innovative sonochemical method was developed to synthesize a thermally stable magnetic tea biochar to capture the Hg⁰ from syngas. A sonochemically synthesized magnetic sorbent (TUF_0.46) exhibited a more prodigious surface area with developed pore structures, ultra-paramagnetic properties, and high dispersion of Fe₃O₄/γ-Fe₂O₃ particles than a simply synthesized magnetic sorbent (TF_0.46). The results showed that TUF_0.46 demonstrated strong thermostability and attained a high Hg⁰ removal performance (∼98.6%) at 200 °C. After the 10th adsorption/regeneration cycle, the Hg⁰ removal efficiency of TUF_0.46 was 19% higher than that of TF_0.46. Besides, at 23.1% Hg⁰ breakthrough, TUF_0.46 achieved an average Hg⁰ adsorption capacity of 16.58 mg/g within 24 h under complex syngas (20% CO, 20% H₂, 5% H₂O, and 400 ppm H₂S). In addition, XPS results revealed that surface-active components (Fe⁺, O^2-, O*, C=O) were the key factor for high Hg⁰ removal performance over TUF_0.46 from syngas. Hence, sonochemistry is a promising practical tool for improving the surface morphology, thermal resistance, renewability, and Hg⁰ removal efficiency of a sorbent.

Recent Advances in Fly-Ash-Based Geopolymers: Potential on the Utilization for Sustainable Environmental Remediation

This Mini-Review provides the fundamentals and the state-of-the-art overview on geopolymers, novel inorganic polymeric materials (also known as alkali-bounded ceramics), synthesized from aluminosilicate sources and explores their current and potential sustainable environmental applications. It summarizes and examines concisely the recent scientific advances on geopolymers widely synthesized from abundantly available fly-ash-based aluminosilicate materials via alkaline activation at relatively low temperatures. Although geopolymerization is not a new concept and has offered valuable solutions to some environmental challenges as a low-cost and environmentally benign alternative to conventional energy-intensive Portland cement-based construction materials and has also been used as a barrier in immobilizing toxic and radioactive metals, the application of this technology to produce effective adsorptive materials for mitigation of liquid- and gas-phase contaminants is relatively recent. The valorization of the fly-ash waste in the sustainable and cost-effective development of geopolymeric adsorbents and catalysts for the treatment and control of environmental contaminants and energy production and storage could lead to many economic benefits due to the low cost and resource recycling of this globally abundant raw material. Perspectives on the synthesis and utilization of new geopolymer-based adsorbents for environmental and energy applications with insights into future research directions, prospects, and challenges for economic large-scale production are addressed.

Simultaneous NO and SO2 removal by aqueous persulfate activated by combined heat and Fe2+: experimental and kinetic mass transfer model studies

This study evaluates the chemistry, kinetics, and mass transfer aspects of the removal of NO and SO₂ simultaneously from flue gas induced by the combined heat and Fe²⁺ activation of aqueous persulfate. The work involves experimental studies and the development of a mathematical model utilizing a comprehensive reaction scheme for detailed process evaluation, and to validate the results of an experimental study at 30-70 °C, which demonstrated that both SO₂ and Fe²⁺ improved NO removal, while the SO₂ is almost completely removed. The model was used to correlate experimental data, predict reaction species and nitrogen-sulfur (N-S) product concentrations, to obtain new kinetic data, and to estimate mass transfer coefficient (K_La) for NO and SO₂ at different temperatures. The model percent conversion results appear to fit the data remarkably well for both NO and SO₂ in the temperature range of 30-70 °C. The conversions ranged from 43.2 to 76.5% and 98.9 to 98.1% for NO and SO₂, respectively, in the 30-70 °C range. The model predictions at the higher temperature of 90 °C were 90.0 and 97.4% for NO and SO₂, respectively. The model also predicted decrease in K_La for SO₂ of 1.097 × 10^-4 to 8.88 × 10^-5 s^-1 (30-90 °C) and decrease in K_La for NO of 4.79 × 10^-2 to 3.67 × 10^-2 s^-1 (30-50 °C) but increase of 4.36 × 10^-2 to 4.90 × 10^-2 s^-1 at higher temperatures (70-90 °C). This emerging sulfate-radical-based process could be applied to the treatment of flue gases from combustion sources. Graphical abstract.

Simultaneous removal of NO and SO2 from flue gas by combined heat and Fe2+ activated aqueous persulfate solutions

The use of advanced oxidation processes (AOPs) to integrate flue gas treatments for SO₂, NO_x and Hg⁰ into a single process unit is rapidly gaining research attention. AOPs are processes that rely on the generation of mainly the hydroxyl radical. This work evaluates the effectiveness of the simultaneous removal of NO and SO₂ from flue gas utilizing AOP induced by the combined heat and Fe²⁺ activation of aqueous persulfate, and elucidates the reaction pathways. The results indicated that both SO₂ in the flue gas and Fe²⁺ in solution improved NO removal, while the SO₂ is almost completely removed. Increased temperature led to increase in NO removal in the absence and presence of both Fe²⁺ and SO₂, and in the absence of either SO₂ or Fe²⁺, but the enhanced NO removal due to the presence of SO₂ alone dominated at all temperatures. The removal of NO increased from 77.5% at 30 °C to 80.5% and 82.3% at 50 °C and 70 °C in the presence of SO₂ alone, and from 35.3% to 62.7% and 81.2%, respectively, in the presence of Fe²⁺ alone. However, in the presence of both SO₂ and Fe²⁺, NO conversion is 46.2% at 30 °C, increased only slightly to 48.2% at 50 °C; but sharply increased to 78.7% at 70 °C compared to 63.9% for persulfate-only activation. Results suggest NO removal in the presence of SO₂ is equally effective by heat-only or heat-Fe²⁺ activation as the temperature increases. The results should be useful for future developments of advanced oxidation processes for flue gas treatments.

Advanced oxidation processes (AOPs) involving ultrasound for waste water treatment: a review with emphasis on cost estimation

Two things are needed for any technology to be suitable for use in the industry, viz. 1. Technical feasibility and 2. Economical feasibility. The use of ultrasound for waste water treatment has been shown to be technically feasible by numerous reports in the literature over the years. But there are hardly any exhaustive reports which address the issue of economical feasibility of the use of ultrasound for waste water treatment on industrial scale. Hence an attempt was made to estimate the cost for the waste water treatment using ultrasound. The costs have been calculated for 1000 L/min capacity treatment plant. The costs were calculated based upon the rate constants for pollutant degradation. The pollutants considered were phenol, trichloroethylene (TCE) and reactive azo dyes. Time required for ninety percent degradation of pollutant was taken as the residence time. The amount of energy required to achieve the target degradation was calculated from the energy density (watt/ml) used in the treatability study. The cost of treatment was calculated by considering capital cost and operating cost involved for the waste water treatment. Quotations were invited from vendors to ascertain the capital cost of equipments involved and operating costs were calculated based on annual energy usage. The cost was expressed in dollars per 1000 gallons of waste water treated. These treatment costs were compared with other established Advanced Oxidation Process (AOP) technologies. The cost of waste water treatment for phenol was in the range of $89 per 1000 gallons for UV/US/O(3) to $15,536 per 1000 gallons for US alone. These costs for TCE were in the range of $25 per 1000 gallons to $91 for US+UV treatment and US alone, respectively. The cost of waste water treatment for reactive azo dyes was in the range of $65 per 1000 gallon for US+UV+H(2)O(2) to $14,203 per 1000 gallon for US alone. This study should help in quantifying the economics of waste water treatment using ultrasound on industrial scale. We strongly believe that this study will immensely help the researchers working in the area of applications of ultrasound for waste water treatment in terms of where the technology stands today as compared to other available commercial AOP technologies. This will also help them think for different ways to improve the efficiency of using ultrasound or search for other ways of generating cavitation which may be more efficient and help reduce the cost of treatment in future.

Ultrasound-Induced Aqueous Removal of Nitric Oxide from Flue Gases: Effects of Sulfur Dioxide, Chloride, and Chemical Oxidant

The effects of sulfur dioxide (SO(2)), sodium chloride (NaCl), and peroxymonosulfate or oxone (2KHSO(5).KHSO(4).K(2)SO(4) with active ingredient, HSO(5)(-)) on the sonochemical removal of nitric oxide (NO) have been studied in a bubble column reactor. The initial concentration of NO studied ranged from about 500 to 1040 ppm. NaCl in the concentration range of 0.01-0.5 M was used as the electrolyte to study the effect of ionic strength. At the low NaCl concentration (0.01 M), the percent fractional removal of NO with initial concentration of 1040 ppm was enhanced significantly, while as the NaCl concentration increased, the positive effects were less pronounced. The presence of approximately 2520 ppm SO(2) in combination with 0.01 M NaCl further enhanced NO removal. However, with a NO initial concentration of 490 ppm, the addition of NaCl was detrimental to NO removal at all NaCl concentration levels. The combinative effect of sonication and chemical oxidation using 0.005-0.05 M oxone was also studied. While the lower concentrations of HSO(5)(-) enhanced NO removal efficiency, higher concentrations were detrimental depending on the initial concentration of NO. It was also demonstrated that in the presence of ultrasound, the smallest concentration of oxone was needed to obtain optimal fractional conversion of NO.

Sonochemistry in environmental remediation. 2. Heterogeneous sonophotocatalytic oxidation processes for the treatment of pollutants in water

Recent advances in advanced oxidation technologies for applications in environmental remediation involve the use of acoustic cavitation. Cavitation is the formation, growth, and implosive collapse of gas- or vapor-filled microbubbles formed from acoustical wave-induced compression/ rarefaction in a body of liquid. Cavitation is effective in treating most liquid-phase pollutants but it is highly energy intensive and not economical or practically feasible when used alone. One of the most interesting topics in the recent advances in environmental sonochemistry is the intensification of the ultrasonic degradation process by coupling ultrasound with other types of energy, chemical oxidants, or photocataysts. In Part II of this series, a critical review of the applications of ultrasound in environmental remediation focusing on the simultaneous or hybrid use of ultrasonic irradiation and photocatalysis in aqueous solutions, namely, sonophotocatalytic oxidation processes, is presented.

Sonochemistry in environmental remediation. 1. Combinative and hybrid sonophotochemical oxidation processes for the treatment of pollutants in water

Sonoprocessing is the utilization of sonic and ultrasonic waves in chemical synthesis and processes. It is a new and rapidly growing research field with broad applications in environmental engineering, green chemical synthesis, and processing. The application of this environmentally benign technique in environmental remediation is currently under active research and development. Sonochemical oxidation is effective in treating toxic effluents and reducing toxicity. However, the ultrasonic treatment is highly energy intensive since sonication is relatively inefficient with respect to total input energy and is therefore not economically attractive or feasible alone. Hence, sonochemistry has not yet received much attention as an alternative for industrial and large-scale chemical and environmental processes. One of the most interesting topics in the recent advances in sonochemistry is the possibility of double or more excitations with ultrasound and other types of energy. The coupling of ultrasound with other free energy sources (i.e., UV) or chemical oxidation utilizing H2O2, O3, or ferrous ion presents interesting and attractive approaches. Therefore, many recent efforts have been devoted to improving the efficiency of sonochemical reactions by exploiting the advantages of combinative or hybrid processes involving the simultaneous or sequential use of ultrasonic irradiation and other advanced oxidation processes, electrochemical processes, and biological treatment. This paper provides a critical review of the applications of ultrasound in environmental remediation, focusing on recent developments and unifying analysis of combinative or hybrid systems, namely, sonophotochemical oxidation processes.

Destruction of carbon disulfide in aqueous solutions by sonochemical oxidation

Carbon disulfide (CS(2)) is toxic to animals and aquatic organisms, and can also decompose to carbonyl sulfide (OCS) and hydrogen sulfide (H(2)S) in aqueous environment. The kinetics of the sonochemical degradation of aqueous CS(2) was studied in a batch reactor at 20kHz and 20 degrees C, and the effects of process parameters (e.g. concentration, ultrasonic intensity, irradiating gas) investigated. The concentrations of unbuffered CS(2) solutions used were (6.4-7.0) x 10(-4), 10.5 x 10(-4) and (13.2-13.6) x 10(-4)M and the intensities were varied from 14 to 50W. The reaction rate was found to be zero-order and the rate constant for the degradation at 20 degrees C and 14W in air was 21.1 microM/min using the largest initial concentration range studied. At the same initial concentration range but at 50W (39.47W/m(2)) the degradation rate of CS(2) was 46.7 microM/min, more than two times that at 14W (11.04W/m(2)). The rate of CS(2) sonochemical degradation in the presence of the different gases was in the order of He>air> or =N(2)O>Ar; the rate with helium was found to be about three times that of argon. The formation of sulfate (SO(4)(2-)) as reaction product with air as the irradiating gas was enhanced in the presence of hydrogen peroxide (H(2)O(2)) and inhibited in the presence of 1-butanol. The sonochemical oxidation of CS(2) may prove to be an efficient and environmentally benign way for the removal of this hazardous pollutant from natural water and wastewater.