Removal of gaseous elemental mercury by hydrogen chloride non-thermal plasma modified biochar

Research progress on elemental mercury (Hg0) removal in flue gas using non-thermal plasma technology

Elemental mercury (Hg0) removal is a crucial target for mercury pollution control in flue gas. This article focuses on Hg0 removal in flue gas using corona discharge (CD) and dielectric barrier discharge (DBD) technologies, and provides a mechanistic perspective on the development and influencing factors of non-thermal plasma (NTP) technology for Hg0 removal. The influence factors include reactor configurations, power supplies, energy density, residence time, oxidation methods, gas composition, and the synergy between NTP and catalysis/adsorption, etc. This study reveals that the use of a pulsating electrical power supply significantly increases electron densities in both CD and DBD systems, thereby ensuring high energy efficiency and economic viability. Cl2 proves to be more effective than HCl as a chlorine source for Hg0 removal. NO significantly reduces Hg0 oxidation efficiency, while the effects of SO2 and H2O remain unclear. Energy density distribution is closely related to plasma devices, power supplies, and overall reactor configurations. Direct oxidation proves to be more effective than indirect oxidation for Hg0 removal. The combination of NTP with adsorption/catalysis technologies shows significantly better Hg0 removal efficiency compared to using NTP alone. This study can provide theoretical support for enhancing Hg0 removal mechanisms and optimizing process control parameters in industrial applications of NTP technology.

Study on H2SO4-modified corn straw biochar as substrate material of constructed wetland

The high value resource utilization of corn straw is a long-term problem at present and in the future. Biochar preparation is an important utilization way of corn straw. The research on city tail water treated by constructed wetland (CW) with biochar was carried out to further increase the wastewater treatment capacity of the CW. Surface characterization, structural characteristics, and adsorption of straw biochar modified by different acids were measured. The study found that the ability of H2SO4 to remove ash from biochar was stronger than other acids and H2SO4-biochar was easy to be cleaned without H2SO4 residue. The performance of biochar modified by H2SO4 was obviously better than other acids, and the biochar adsorption was enhanced. The modification of biochar substrate modified by H2SO4 in CW reduced the change of electrical conductivity (EC) and promoted denitrification. H2SO4-modified biochar promoted the absorption of N and P by Iris pseudacorus L. The compound modification effect of straw biochar was obvious. The results revealed the acid modification characteristics of straw biochar, which were beneficial for increasing the wastewater treatment rate by CW. This study will promote the sustainable development of CW.

Adsorption and photodegradation of reactive red 120 with nickel-iron-layered double hydroxide/biochar composites

Layered double hydroxide (LDH) materials were widely applied for adsorption and photodegradation of pollutants for wastewater treatment. New efficient LDH materials with adsorption and photodegradation abilities will be promising candidates for pollutants removal. Hence, a series of NiFe-LDH/biochar (NiFe/BC) were fabricated by the coprecipitation method for synergistic adsorption and photodegradation anionic dyes of reactive red 120 (RR120). The removal experiment showed that the addition of an appropriate amount of biochar into NiFe-LDH enhanced the adsorption capacity and its photocatalytic ability. The optimized NiFe/BC2 composite can remove 88.5 % of RR120 under visible light by adsorption and photocatalysis, which was much better than NiFe-LDH (63.3 %) and biochar (2.6 %). The photodegradation kinetic constant of the NiFe/BC2 composite was 3.1 and 104.8 times that of NiFe-LDH and BC. In addition, active species capture experiments and electron spin resonance (ESR) tests revealed the removal mechanisms of NiFe/BC composites for RR120 removal. This work affords a feasible strategy for preparing LDH-based photocatalyst with excellent adsorption and photocatalytic performance for wastewater treatment.

Identification of mercury species in coal combustion by-products from power plants using thermal desorption-atomic fluorescence spectrometry on-line coupling system

Along with the environmental protection policies becoming strict in China, the air pollution control devices (especially selective catalytic reduction (SCR)) are widely equipped in coal-fired power plants. The installation and run of these devices will inevitably affect mercury (Hg) species distribution in coal fired by-products such like fly ash (FA) and gypsum. In this work, a new on-line coupling system based on atomic fluorescence spectrometry (AFS) with a home-made chromatographic workstation was successfully developed to identify Hg species through thermal programmed desorption (TPD). The influences of matrix, furnace temperature, and carrier gas flow on analytical performance were investigated and the parameters were optimized. The FA and gypsum samples from coal-fired power plants equipped with SCR were collected and the mercury species were analyzed by the developed coupling system. HgCl₂ and HgO were the main species in FA, while Hg₂Cl₂ and HgO were the main species in gypsum. All of Hg species in the studied FA and gypsum samples were released below 400 °C. A sequential extraction procedure was applied to further verify the operational Hg species including mobile and non-mobile fractions in FA and gypsum samples. This study demonstrated that AFS coupled with TPD procedure was an effective method to analyze Hg species in coal combustion by-products from power plants.

Emerging Modification Technologies of Lignin-based Activated Carbon toward Advanced Applications

Lignin-based activated carbon (LAC) is a promising high-quality functional material due to high surface area, abundant porous structure, and various functional groups. Modification is the most important step to functionalize LAC by altering its porous and chemical properties. This Review summarizes the state-of-the-art modification technologies of LAC toward advanced applications. Promising modification approaches are reviewed to display their effects on the preparation of LAC. The multiscale changes in the porosity and the surface chemistry of LAC are fully discussed. Advanced applications are then introduced to show the potential of LAC for supercapacitor electrode, catalyst support, hydrogen storage, and carbon dioxide capture. Finally, the mechanistic structure-function relationships of LAC are elaborated. These results highlight that modification technologies play a special role in altering the properties and defining the functionalities of LAC, which could be a promising porous carbon material toward industrial applications.

Removal of flue gas mercury by porous carbons derived from one-pot carbonization and activation of wood sawdust in a molten salt medium

Porous carbons derived from one-pot carbonization and activation of wood sawdust in a molten salt (LiCl-KCl) medium were employed for Hg⁰ removal. The carbons derived from molten salt carbonization (MSC) displayed much superior Hg⁰ removal performance comparing with the carbons derived from N₂ pyrolysis method (NC). The best molar ratio of LiCl-KCl was 59:41, the optimal molten salt temperature was 700 °C, and the best mass ratio of wood sawdust to molten salt was 1:10. The MSC displayed good applicability at 50-125 °C. The saturation Hg⁰ adsorption capacity of MSC was about 7828.39 μg·g^-1, far exceeding those for carbonaceous adsorbents reported in literatures. A Hg⁰ removal mechanism over MSC was proposed, i.e., the hierarchical porous structure accelerated mass transfer of Hg⁰, and the CO groups served as electron acceptors from Hg⁰ atoms to form organic matter bonded mercury (Hg-OM). The molten salt could be easily separated from the mixture of MSC for recycling multiple times. Thus, molten salt carbonization method appears to be promising in one-pot carbonization and activation of biomass as efficient adsorbents for gaseous Hg⁰.

Artificial intelligence (AI) applications in adsorption of heavy metals using modified biochar

The process of removal of heavy metals is important due to their toxic effects on living organisms and undesirable anthropogenic effects. Conventional methods possess many irreconcilable disadvantages pertaining to cost and efficiency. As a result, the usage of biochar, which is produced as a by-product of biomass pyrolysis, has gained sizable traction in recent times for the removal of heavy metals. This review elucidates some widely recognized harmful heavy metals and their removal using biochar. It also highlights and compares the variety of feedstock available for preparation of biochar, pyrolysis variables involved and efficiency of biochar. Various adsorption kinetics and isotherms are also discussed along with the process of desorption to recycle biochar for reuse as adsorbent. Furthermore, this review elucidates the advancements in remediation of heavy metals using biochar by emphasizing the importance and advantages in the usage of machine learning (ML) and artificial intelligence (AI) for the optimization of adsorption variables and biochar feedstock properties. The usage of AI and ML is cost and time-effective and allows an interdisciplinary approach to remove heavy metals by biochar.

Humulus scandens-Derived Biochars for the Effective Removal of Heavy Metal Ions: Isotherm/Kinetic Study, Column Adsorption and Mechanism Investigation

Humulus scandens was first adopted as a biomass precursor to prepare biochars by means of a facile molten salt method. The optimized biochar exhibits a high specific surface area of ~450 m²/g, a rich porous structure and abundant oxygen functional groups, which demonstrate excellent adsorption performance for heavy metal ions. The isotherm curves fit well with the Langmuir models, indicating that the process is governed by the chemical adsorption, and that the maximum adsorption capacity can reach 748 and 221 mg/g for Pb²⁺ and Cu²⁺, respectively. In addition, the optimized biochar demonstrates good anti-interference ability and outstanding removal efficiency for Cu²⁺ and Pb²⁺ in simulated wastewater. The mechanism investigation and DFT calculation suggest that the oxygen functional groups play dominant roles in the adsorption process by enhancing the binding energy towards the heavy metal ions. Meanwhile, ion exchange also serves as the main reason for the effective removal.

Tuning oxygenated functional groups on biochar for water pollution control: A critical review

Biochar has attracted increasing attention in water pollution control, attributed to its various merits, e.g., tunable physico-chemical properties. The oxygenated functional groups (OFGs) on biochar are key active sites for removing pollutants from water through interfacial adsorption/redox reaction. However, there is still a lack of comprehensive knowledge and perspective on tuning OFGs on biochar for enhanced performance in water pollution control. Here, this review highlighted the mechanisms of biochar OFGs in water pollution control, analyzed the strategies and mechanisms for tuning OFGs on biochar, and investigated the performances of biochars with tuned OFGs in removing inorganic/organic pollutants via adsorption/redox reactions. Specifically, strategies for tuning OFGs on biochar are far more than the well-recognized ex-situ oxidation of pristine biochar. These strategies include in-situ low temperature preservation of hydroxyl and carboxyl, in-/ex-situ oxidation of biochar, and in-/ex-situ grafting of carboxyl on biochar via cycloaddition/acylation reaction. The resultant biochars showed enhanced performances in adsorption (mainly mediated by hydroxyl, carboxyl and ketone through surface complexation, H-bonding, and electrostatic attraction) and redox reaction (mainly mediated by redox-active hydroxyl and ketone). Finally, this review presented future directions on developing biochar with specially tuned surface OFGs as a sustainable high-performance adsorbent/carbocatalyst for water pollution control.

Phosphate removal from river water using a highly efficient magnetically recyclable Fe3O4/La(OH)3 nanocomposite

Lanthanum based nanocomposites have attracted much attention for their efficiency and capacity in removing phosphate from water. This study developed a Fe₃O₄/La(OH)₃ nanocomposite through a precipitation route at room temperature and used the nanocomposite to remove phosphate from river water. Performance of the Fe₃O₄/La(OH)₃ nanocomposite was evaluated in terms of sorption kinetics, sorption isotherms, different solution pH values, competing ions, and regenerative ability. The Fe₃O₄/La(OH)₃ nanocomposite showed a nanosphere-like morphology with 97% magnetic separation efficiency, excellent phosphate removal capacity of 253.83 mg/g, 99% phosphate selectivity in the presence of chloride, nitrate, sulfate, fluoride, and calcium as competing ions and excellent reusability in ten cycles. Based on these findings, the Fe₃O₄/La(OH)₃ nanocomposite was used to remove phosphate from river water. It was found that, in 60 min, a 0.1 g/L dosage of the nanocomposite was able to reduce the phosphate in the water from 0.087 mg/L to 0.002 mg/L. Moreover, studying of the removal mechanism of the nanocomposite revealed that surface complexation and the electrostatic interaction between phosphate species and lanthanum hydroxide played a prominent role in the sorption of phosphate.

New applications of quinone redox mediators: Modifying nature-derived materials for anaerobic biotransformation process

Due to their wide-distribution, high-biocompatibility and low-cost, nature-derived quinone redox mediators (NDQRM) have shown great potential in bioremediation through mediating electron transfers between microorganisms and between microorganisms and contaminants in anaerobic biotransformation processes. It is obvious that their performance in bioremediation was limited by the availability of quinone-based groups in NDQRM. A sustainable solution is to enhance the electron transfer capacity and retention capacity by the modification of NDQRM. Therefore, this review comprehensively summarized the modification techniques of NDQRM according to their multiple roles in anaerobic biotransformation systems. In addition, their potential applications in greenhouse gas mitigation, contaminant degradation in anaerobic digestion, contaminant bioelectrochemical remediation and energy recovery were discussed. And the problems that need to be addressed in the future were pointed out. The obtained knowledge would promote the exploration of novel NDQRM, and provide suggestions for the design of anaerobic consortia in biotransformation systems.