Mercury Removal by Magnetic Biochar Derived from Simultaneous Activation and Magnetization of Sawdust

A novel synthesis of sulfurized magnetic biochar for aqueous Hg(II) capture as a potential method for environmental remediation in water

Due to public health threats resulting from mercury (Hg) and its distribution in the food chain, global restrictions have been placed on Hg use and emissions. Biochar is a porous, carbonaceous adsorbent typically derived from waste biomass or organic matter, making it an eco-friendly material for aqueous mercury (Hg(II)) control. Functionalization of biochar can improve performance in pollution control applications. In this work, carbonization, magnetization, and sulfurization of biochar were combined into a single heating step to prepare sulfurized magnetic biochar (SMBC) for Hg(II) removal from water. Results indicate that SMBC prepared at 600 °C adsorbed 8.93 mg/g Hg(II), more than materials prepared at 400, 500, 700, 800, and 900 °C. Additionally, Hg(II) adsorption onto SMBC was 53.0% and 11.5% greater than onto magnetic biochar (MBC) and biochar (BC), respectively. Hg(II) adsorption is shown to be favorable in acidic conditions (pH 3.5-5), thermodynamically spontaneous, and endothermic. Adsorption results fit the pseudo-second-order (R² = 0.990 and the sum of squared error (SSE) = 5.382) and external mass transfer (R² = 0.971 and SSE = 9.422) models. The partitioning coefficients were 4.964 mg/g/μM in freshwater, 0.176 mg/g/μM in estuary water, and 0.275 mg/g/μM in seawater, highlighting the importance of salinity in environmental remediation applications. In summary, SMBC can be readily prepared with minimal processing steps. The product is a robust adsorbent for Hg(II), and it can potentially be applied to remediate contaminated water/sediment/soil in the future.

Adsorption characteristics and the removal mechanism of two novel Fe-Zn composite modified biochar for Cd(II) in water

In this study, adsorbents (Fe/Zn-RBC and Fe/Zn-DBC) for the removal of Cd(II) in water were successfully prepared by iron/zinc composite modified biochar derived from the branches of Robinia pseudoacacia biochar (RBC) and durian shells biochar (DBC). The results revealed that the iron and zinc ions were successfully loaded onto the biochar. The adsorption data of Cd(II) on Fe/Zn-BC conformed to the models of pseudo-second-order kinetic, Langmuir isothermal, and Redlich-Paterson. According to the results of batch experiments, the maximum sorption capacities of Fe/Zn-RBC and Fe/Zn-DBC for Cd(II) were approximately five times and three times higher than RBC and DBC, respectively. As the most dominant adsorption mechanisms, Cd(II) and CO₃^2-, Fe-O, Zn-O, and oxygen-containing functional groups on the Fe/Zn-BC surfaces precipitated CdCO₃, Cd(OH)₂, and CdO. Therefore, Fe/Zn-BC is an excellent adsorbent that removes Cd(II) from aqueous solutions, and also can be used in waste resource utilization, which has potential applications prospects.

Persulfate Oxidation of Sulfamethoxazole by Magnetic Iron-Char Composites via Nonradical Pathways: Fe(IV) Versus Surface-Mediated Electron Transfer

Despite the vital roles of reactive radical species in the coupled iron-carbon composite/persulfate (PS) system for eliminating pollutants, nonradical contributions are typically overlooked. Herein, we developed two efficient magnetic iron-char composites via low-temperature (BCFe-400) and high-temperature (BCFe-700) pyrolysis. The two composites activated PS through nonradical pathways for sulfamethoxazole (SMX) degradation. In the BCFe-400/PS system, high-valent iron Fe(IV) was the dominant active species for the oxidation, evidenced by methyl phenyl sulfoxide-based probe tests, Mössbauer spectroscopy, and in situ Raman analyses with kinetic evaluation. In the BCFe-700/PS system, surface-mediated electron transfer dominated the oxidation, and the nonradical regime was probed by the electrochemical test and in situ Raman analysis. Furthermore, the BCFe-400/PS system maintained high efficiency in continuous degradation of SMX due to the feasible Fe²⁺generation toward Fe(IV) formation. In the BCFe-700/PS system, the stability of the system was limited due to the hindered electron transfer between the surface reactive complex (i.e., BCFe-700-PS*) and SMX, and thermal treatment would help recover the reactivity. Both BCFe-400/PS and BCFe-700/PS systems exhibited high performances for SMX removal in the presence of chloride and humic acid and in real water matrixes (e.g., seawater, piggery wastewater, and landfill leachate), exhibiting the great merits of the nonradical system. Overall, the study would provide new insights into PS activation by iron-loaded catalysts to efficiently degrade pollutants via nonradical pathways.

Magnetic biochar production alters the molecular characteristics and biological response of pyrolysis volatile-derived water-soluble organic matter

The formed Fe oxides (minerals) in the magnetic biochar production process can facilitate its recovery and carbon retention rate. However, the influence of Fe oxides on pyrolysis volatile-derived water-soluble organic matter (PVWSOM, also called wood vinegar) has been largely overlooked. Results demonstrated that in-situ formed Fe oxides (α-Fe₂O₃ and Fe₃O₄) could obviously inhibit biomass cracking and accordingly reduce PVWSOM emissions, as indicated by decreased PVWSOM concentrations from 28.7 to 6.8 mg C/g biomass. FT-ICR MS results further indicated that Fe oxides suppressed the formation of large-molecular-weight PVWSOM compounds with high degree of unsaturation (DBE value > 5) and oxygen content (oxygen number > 5), leading to lower polarity and aromaticity. Therefore, the changes in PVWSOM molecular structures caused by Fe oxides relieved its toxicity on wheat seed growth, and reduced negative impact on soil microbial diversity and promoted soil bacterial Proteobacteria and Acidobacteria. These results indicate that molecular structures of PVWSOM from biomass pyrolysis also can be changed by Fe oxides to affect its application.

Magnetic Fe3O4/attapulgite hybrids for Cd(II) adsorption: Performance, mechanism and recovery

Herein, a Fe₃O₄ decorated attapulgite adsorbent (FA) is fabricated for the removal of Cd(II) from wastewater, and subsequently a feasible strategy for converting the saturated waste adsorbent to CdS/FA photocatalyst is reported. Owing to the in situ growth of Fe₃O₄ on the attapulgite (ATP), the FA adsorbents exhibit enlarged surface area and increased adsorption sites. More importantly, the strong interaction between Fe₃O₄ and ATP leads to changes of coordination environment around the O‒Fe‒O bond with the ATP. Based on the results of density functional theory calculations, the electrons are more readily transferred from Fe to O, and the hanging O atoms with more electronegativity act as the efficient adsorption sites for Cd(II), efficiently improving the adsorption performance of the Fe₃O₄ phases. Furthermore, the waste FA adsorbent could be conveniently separated from the treated water by magnets and converted to CdS/FA photocatalyst, which exhibits satisfying degradation efficiency for tetracycline with low concentration. This work provides a potential strategy to optimize the ATP-based materials for heavy ions adsorption and reutilize the waste adsorbents.

Carboxymethylcellulose-chitosan film modified magnetic alkaline Ca-bentonite for the efficient removal of Pb(II) and Cd(II) from aqueous solution

In order to endow alkaline Ca-bentonite (ACB) with magnetic separation ability, simultaneously obtain better magnetic stability and stronger removal capacity of heavy metal cations; magnetic alkaline Ca-bentonite/carboxymethylcellulose-chitosan film (MACB/C-C) was prepared by organic modification of magnetic alkaline Ca-bentonite (MACB) using non-toxic carboxymethylcellulose and chitosan. Textural characterization results revealed that magnetic Fe3O4 nanoparticles were successfully immobilized on ACB and modified with C-C. The functionalized layer of C-C concurrently enhanced the stability of Fe3O4 and removal performances of heavy metal cations. Adsorption results indicated that MACB/C-C exhibited thorough separation from aqueous solution and greater uptake ability for Pb(II) and Cd(II) (483 mg·g-1 and 123 mg·g-1) than the nascent MACB (335 mg·g-1 and 76 mg·g-1), respectively, at pH 5 and 25 °C temperature. The adsorption of Pb(II) and Cd(II) on MACB/C-C mainly occurred via surface precipitation and complexation when pH > 2. MACB/C-C could be efficiently recycled with marginal decrease in adsorption capacity. The current approach credited to the convenient operation, simplified synthesis, and high efficiency of MACB/C-C could be deemed as a promising alternative for the removal of heavy metal cations from wastewater.

Adsorption and regeneration on iron-activated biochar for removal of microcystin-LR

Novel iron activated biochars (FA-BCs) were prepared via simultaneous pyrolysis and activation of FeCl₃-pretreated bermudagrass (BG) for removing microcystin-LR (MC-LR) in aqueous solution. Compared to the raw BC (without activation), the surface area and adsorption capacity of FA-BC at iron impregnation ratio of 2 (2 g FeCl₃/g BG) were enhanced from 86 m²/g and 0.76 mg/g to 835 m²/g and 9.00 mg/g. Moreover, FA-BC possessed various iron oxides at its surface which provided the catalytic capacity for regeneration of MC-LR spent FA-BC and magnetic separation after the MC-LR adsorption. Possible mechanisms for the MC-LR adsorption onto FA-BC would include electrostatic attraction, π⁺-π, hydrogen bond, and hydrophobic interactions. The detailed adsorption studies indicated mainly chemisorption and intra-particle diffusion limitation would participate in the adsorption process. The thermal regeneration at 300 °C kept high regeneration efficiency (99-100%) for the MC-LR spent FA-BC during four cycles of adsorption-regeneration. In addition, the high regeneration efficiency (close to 100%) was also achieved by persulfate oxidation-driven regeneration. FA-BC also exhibited high adsorption capacity for the MC-LR from the real lake water to meet the MC-LR concentration below 1 μg/L as a safe guideline suggested by WHO.

Preparation of magnetic hydrochar derived from iron-rich Phytolacca acinosa Roxb. for Cd removal

Considering that hyperaccumulators can accumulate high concentrations of iron salt, they can successfully obtain magnetic hydrochar from iron-rich hyperaccumulators. In this study, iron-rich biomass was obtained by irrigating Phytolacca acinosa Roxb. using iron salt. Magnetic nano-Fe₃O₄ hydrochar was prepared from iron-rich Phytolacca acinosa Roxb. via hydrothermal carbonization to remove Cd. The characterization results showed that the synthesized magnetic nanoparticles had an average size of 2.62 ± 0.56 nm and N elements were doped into magnetic nano-Fe₃O₄ hydrochar with abundant oxygenic groups. Cd adsorption on magnetic nano-Fe₃O₄ hydrochar was better fitted using the Langmuir isotherm and the pseudo-second-order kinetic model. The maximum adsorption capacity was 246.6 mg g^-1 of Cd. The research confirmed that Cd adsorption was controlled by multiple mechanisms from the jar test, transmission electron microscopy mapping, scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. CdCO₃ crystals can be formed after adsorption, indicating that surface precipitation played an important role in Cd adsorption. The abundance of O atoms and the doping of N atoms on the hydrochar surface were conducive to Cd adsorption, indicating that the mechanisms were related to surface complexation and electrostatic attraction. In addition, the significant decrease in Na⁺ content after Cd adsorption illustrated that ion exchange had a non-negligible effect on Cd adsorption. This study not only provides a strategy for preparing magnetic nano-Fe₃O₄ hydrochar derived from iron-rich plants but also verifies multiple Cd adsorption mechanisms using magnetic nano-Fe₃O₄ hydrochar.

One-step fabrication of artificial humic acid-functionalized colloid-like magnetic biochar for rapid heavy metal removal

A novel functional colloid-like magnetic biochar (Col-L-MBC) with high dispersibility is prepared by the one-step method with the prepared porous biochar as the skeleton. Notably, A-HA obtained from waste biomass through hydrothermal humification (HTH) technology has rich functional groups (i.e., phenolic-OH, -COOH, etc.), which is conducive to the uniform dispersion of magnetic nanoparticles on the porous biochar skeleton, providing rich active sites for heavy metal ion removal. Interestingly, the introduction of A-HA can also lead to the formation of new iron species. Besides, A-HA coated on the surface of the magnetic substance also improves the dispersion of the magnetic biochar (Col-L-MBC) in the solution, forming a colloid-like magnetic biochar adsorbent, bringing superior removal performance for Cd²⁺ (maximum removal capacity up to 169.68 mg/g). Various removal mechanisms, including Cd-π interaction, complexation, ion exchange, and precipitation are introduced, making a great contribution to rapid removal performance.

Cadmium removal by MgCl2 modified biochar derived from crayfish shell waste: Batch adsorption, response surface analysis and fixed bed filtration

Cadmium (Cd) pollution is regarded as a disturbing environmental problem due to its serious risks to the water body and human health. The removal of cadmium from wastewater is thus crucial to avoid its harmful effects on the ecosystem. This study comprehensively investigated Cd(II) adsorption onto MgCl₂ modified biochar (MgC600) and results showed that the adsorption capacity of MgC600 was more than twice of that of pristine biochar due to its enhanced ion exchange ability. Response surface analysis revealed that reaction time played a crucial role in the Cd(II) adsorption, followed by initial concentration and solution pH. Moreover, the optimal adsorption conditions and capacity were precisely given by the quadratic regression model and thus proved that the model can be applied to predict the operation conditions of Cd(II) adsorption. Finally, a new model defined as BJP model [Formula: see text] was proposed and proved to be more suitable for the fixed bed filtration process. Overall, our findings provide a promising material in treatment of Cd(II)-rich wastewater and give a clear picture of its application. More importantly, the newly developed BJP model can accurately describe the fixed bed filtration process and further promote its application in wastewater treatment.

Sorption mechanisms of antibiotic sulfamethazine (SMT) on magnetite-coated biochar: pH-dependence and redox transformation

Sorption of sulfonamides (SAs) on magnetite-coated biochar (MBC) is a promising approach for the remediation of antibiotic contaminants, due to its extended adsorption capacity and irreversibility. However, the actual sorption mechanisms of SAs on MBC remain unclear and the gap in knowledge hinders understanding of the fate of SAs in soils or sediments. In this study, various MBCs were prepared under different pyrolysis temperatures, with batch sorption experiments conducted using SMT as the model pollutant. Results of a two-compartment kinetic model demonstrated that aromatic components of MBCs dominated slow-sorption mechanisms, whereas the embedded magnetite further accelerated fast-sorption due to H-bonding. Modification of BC with magnetite improved the distribution coefficient (K_d) and isotherm linearity of SMT. Multi-parameter model results indicated that the pH-dependence of SMT sorption on BCs and MBCs occurred via a dominant mechanism of π-bond assisted H-bonding. Compared to pristine BCs, the change in pH-dependent sorption characteristics of SMT on MBC results from the regulation of π-bonding and proton configuration. Simultaneous transformation of SMT to sulfate ions on BCs or MBCs was also observed. The degradation of SMT occurred because of persistent free radicals (PFRs) on BCs or the inherent redox of iron minerals on MBCs. However, the small fraction of SMT transformed on BCs or MBCs was not found to result in overestimation of SMT sorption. This study presents the critical mechanisms of SMT sorption on pyrochars and provides novel understanding of the fate of SMT on carbonaceous materials during practical remediation applications.

Corn stover biochar increased edible safety of spinach by reducing the migration of mercury from soil to spinach

Mercury (Hg) is toxic and can affect human health through soil entering food chain. Spinach absorb easily heavy metals. Corn stover biochar can improve soil structure and physicochemical property. This study wanted to establish a Hg-corn stover biochar-soil-spinach model including 1 control group (without HgCl₂ and corn stover biochar) and 24 treatment groups (with HgCl₂ or/and corn stover biochar). Hg concentration was 0, 1, 2, 4, and 6 mg kg^-1, respectively. Corn stover biochar contents were 0%, 1%, 3%, 5%, and 7% w/w, respectively. The results showed that residual Hg concentrations was the largest and water soluble and exchangeable Hg as well as carbonate bound Hg concentrations were the lowest among five Hg forms. Hg concentrations in four Hg treatment groups were higher than the control group in dose-dependent manner. The deposition of 6 mg kg^-1 Hg was the highest. Corn stover biochar decreased Hg migration from soil to leaching solution and spinach, and passivation effect of 7% concentration of corn stover biochar was the best. Besides, corn stover biochar relieved the increase of methyl Hg caused by Hg in soil. Moreover, Hg concentration in roots was the highest and Hg concentration in stems was the lowest in spinach. Furthermore, Hg absorbed by roots was more than the sum of Hg absorbed by stems and leaves. In addition, we also found that the measured soil Hg concentrations were coincided with the predicted soil Hg concentrations under 1, 2, and 4 mg kg^-1 Hg concentrations, except 2 mg kg^-1 Hg at 7% C. Under 6 mg kg^-1 Hg concentration, measured soil Hg concentrations was lower than that of the predicted soil Hg concentrations. Taken together, our findings indicated that corn stover biochar can increase edible safety of spinach by immobilizing Hg in soil and be used as an organic amendment.

Facile synthesis of core-shell phase-transited lysozyme coated magnetic nanoparticle as a novel adsorbent for Hg(II) removal in aqueous solutions

Adsorption using nanomaterials is considered an effective method for controlling the levels of toxic heavy metal in wastewater. Herein, a novel adsorbent, core-shell phase-transited lysozyme film-coated magnetic nanoparticles (Fe₃O₄@SiO₂@PTL) for Hg(II) ions removal from aqueous solutions was explored via facile and fast phase transformation and self-assembly process of lysozyme. The physiochemical properties of Fe₃O₄@SiO₂@PTL were investigated using various characterization techniques. The adsorption performances such as kinetics, isotherms, selectivity, the effect of coexisting ions, and regeneration were evaluated. Fe₃O₄@SiO₂@PTL showed an extremely high Hg(II) uptake rate and achieved more than 90% equilibrium adsorption capacity in 5 min. Hg(II) adsorption was followed by a pseudo-second-order kinetic model and fitted the Langmuir model by achieving a maximum uptake of 701.51 mg/g. Furthermore, excellent Hg(II) selectivity was obtained in a mixed solution containing various heavy metal ions, along with good chemical stability owing to the high adsorption capacity maintained after five cycles. The adsorption analyses indicated that the amino, imino, amide, hydroxyl, carboxyl, and thiol groups exposed on the surface of Fe₃O₄@SiO₂@PTL were vital for Hg(II) removal. Consequently, this work will significantly assist in the development of an easily available, eco-friendly, and selective adsorbent material to remove heavy metal ions from wastewater.

Does soluble starch improve the removal of Cr(VI) by nZVI loaded on biochar?

A novel material that nano zero valent iron (nZVI) loaded on biochar with stable starch stabilization (nZVI/SS/BC) was synthesized and used for the removal of hexavalent chromium [Cr(VI)] in simulated wastewater. It was indicated that as the pyrolysis temperature of rice straw increased, the removal rate of Cr(VI) by nZVI/SS/BC first increased and then decreased. nZVI/SS/BC made from biochar pyrolyzed at 600 °C (nZVI/SS/BC600) had the highest removal efficiency and was suitable for a wide pH range (pH 2.1-10.0). The results showed that 99.67% of Cr(VI) was removed by nZVI/SS/BC600, an increase of 45.93% compared to the control group, which did not add soluble starch during synthesis. The pseudo-second-order model and the Langmuir model were more in line with reaction. The maximum adsorption capacity for Cr(VI) by nZVI/SS/BC600 was 122.86 mg·g^-1. The properties of the material were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) mapping, Brunauer-Emmett-Teller (BET), Fourier-transform infrared (FTIR), and X-ray diffraction (XRD). The results showed that the nZVI particles were uniformly supported on the biochar, and the BET surface areas of nZVI/SS/BC was 40.4837 m²·g^-1, an increase of 8.79 times compared with the control group. Mechanism studies showed that soluble starch reduced the formation of metal oxides, thereby improving the reducibility of the material, and co-precipitates were formed during the reaction. All results indicated that nZVI/SS/BC was a potential repair material that can effectively overcome the limitations of nZVI and achieve efficient and rapid repair of Cr(VI).

Iron-activated bermudagrass-derived biochar for adsorption of aqueous sulfamethoxazole: Effects of iron impregnation ratio on biochar properties, adsorption, and regeneration

This work focused on the impacts of FeCl₃ impregnation ratio on the properties of FeCl₃-activated bermudagrass (BG)-derived biochars (IA-BCs), adsorption of sulfamethoxazole (SMX) onto IA-BCs and regeneration of SMX-spent IA-BC. Compared with the control BC (85.82 m²/g), IA-BCs made via pyrolysis with FeCl₃ to BG mass ratio between 1 and 3 (1-3 g FeCl₃/g BG) resulted in significantly enhancing surface area (1014-1035 m²/g), hydrophobicity, Fe content in IA-BCs (3.87-7.27%), and graphitized carbon. The properties of IA-BCs supported magnetic separation and higher adsorption (32-265 mg SMX/g BC) than the control BC (6-14 mg SMX/g BC) at various pH. Adsorption experiments indicated various adsorption mechanisms between SMX and IA-BCs via π-π EDA, hydrophobic interactions, and hydrogen bond with intraparticle diffusion limitation. The adsorption was also found to be spontaneous and exothermic. The IA-BC made at FeCl₃ to BG mass ratio of 2 (IA-BC_2.0) showed the maximum adsorption capacity for SMX (253 mg SMX/g BC) calculated from Langmuir isotherm model. Additionally, both NaOH desorption and thermal oxidation showed effective regeneration of SMX-saturated IA-BC_2.0 over multiple cycles. After three cycles of adsorption-regeneration, 64% and 62% of regeneration efficiencies were still achieved under thermal treatment at 300 °C and desorption with 0.1 M NaOH solution, respectively, indicating a cost-efficient adsorbent for the elimination of SMX in water.

Surface modification of fly ash by non-thermal air plasma for elemental mercury removal from coal-fired flue gas

The fly ash from a coal-fired power plants was modified with non-thermal plasma in air to improve the elemental mercury (Hg⁰) removal performance. The Hg⁰ adsorption experiments were implemented via a bench-scale fixed-bed reactor system. Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), ultimate, X-ray diffraction (XRD) and XRF analysis were employed to characterize the fly ash. The effect of non-thermal plasma voltage and time on Hg⁰ removal efficiency was investigated. The results showed that fly ash had better Hg⁰ removal performance when treatment voltage was 3.0 kV and treatment time was 7 min. Furthermore, the results showed that non-thermal plasma treatment had little influence on the specific surface area. However, non-thermal plasma treatment increased the relative content of oxygen. XPS and temperature programmed desorption results indicated that Hg⁰ removal process included adsorption and oxidation of Hg⁰. Moreover, ester and carbonyl groups played extremely vital roles in the improvement of Hg⁰ removal performance, and their temperature programmed desorption peak occurred at around 250°C and 320°C, respectively.

Sustainable Chromium (VI) Removal from Contaminated Groundwater Using Nano-Magnetite-Modified Biochar via Rapid Microwave Synthesis

This study developed a nano-magnetite-modified biochar material (m-biochar) using a simple and rapid in situ synthesis method via microwave treatment, and systematically investigated the removal capability and mechanism of chromium (VI) by this m-biochar from contaminated groundwater. The m-biochar was fabricated from reed residues and magnetically modified by nano-Fe₃O₄. The results from scanning electron microscopy (SEM) and X-ray diffraction (XRD) characterisations confirmed the successful doping of nano-Fe₃O₄ on the biochar with an improved porous structure. The synthesised m-biochar exhibited significantly higher maximum adsorption capacity of 9.92 mg/g compared with that (8.03 mg/g) of the pristine biochar. The adsorption kinetics followed the pseudo-second-order model and the intraparticle diffusion model, which indicated that the overall adsorption rate of Cr(VI) was governed by the processes of chemical adsorption, liquid film diffusion and intramolecular diffusion. The increasing of the pH from 3 to 11 significantly affected the Cr(VI) adsorption, where the capabilities decreased from 9.92 mg/g to 0.435 mg/g and 8.03 mg/g to 0.095 mg/g for the m-biochar and pristine biochar, respectively. Moreover, the adsorption mechanisms of Cr(VI) by m-biochar were evaluated and confirmed to include the pathways of electrostatic adsorption, reduction and complexation. This study highlighted an effective synthesis method to prepare a superior Cr(VI) adsorbent, which could contribute to the effective remediation of heavy metal contaminations in the groundwater.

Influence of Synthesis Methods on the High-Efficiency Removal of Cr(VI) from Aqueous Solution by Fe-Modified Magnetic Biochars

Fe-modified biochars have been widely used in removal of Cr(VI) from water due to the resulting modified surface functional groups and magnetization property. However, few studies have synthetically investigated modification methods and synthesis parameters on the improvement of the removal efficiency of Cr(VI) by Fe-modified biochars. Herein, 10 types of corn straw-based magnetic biochars were produced using pre-modification and post-modification methods with various modifier ratios, and the highest heating temperature (HHT). Cr(VI) removal results suggest that the removal efficiency of pre-modified biochars ranged from 50.7 to 98.6%, which was much higher than that of post-modified (6.6-21.6%) and unmodified biochars (0.4-7.6%). The effect of synthesis methods on Cr(VI) adsorption was in the following order: Fe-modification method > modifier ratio > HHT. The adsorption kinetics and isotherm results of three types of pre-modified biochars were well fitted with the pseudo-second-order model (R ² > 0.99) and the Langmuir adsorption model (R ² > 0.99), respectively, indicating the surface homogeneity of the pre-modified biochars and unilayer chemisorptions of Cr(VI). Characterization results show that iron oxides or zerovalent iron particles were successfully deposited onto the surface of biochars and magnetism was introduced. A good Pearson correlation (r = -0.9694) between the removal efficiency and pH value in modified biochar suggests that the lower pH value may offer more positive charges and promote electrostatic attraction. Therefore, the dominant mechanism for enhanced Cr(VI) adsorption on pre-modified biochar was electrostatic attraction, resulting from its distinguished acidity nature. Our findings provide new insights into the high-efficiency removal of Cr(VI) onto Fe-modified magnetic biochars and will benefit future design of more efficient magnetic biochars.

Oil industry waste based non-magnetic and magnetic hydrochar to sequester potentially toxic post-transition metal ions from water

Solid waste conversion to value-added products is a stepping stone towards sustainable environment. Herein, sesame oil cake (SOC), an oil industry waste was utilized as a precursor to develop hydrochar (HC) samples by varying reaction temperature (150-250 °C) and time span (2-8 h), chemically treated with 10% H₂O₂ to optimize a sample with maximum yield and Pb(II) adsorption. Highest yield (29.2 %) and Pb(II) (24.57 mg/g at C_o: 15 mg/L) adsorption was observed on SOCHC@200 °C/6 h, magnetized (mSOCHC@200 °C/6 h) for comparative study. XRD displayed highly crystalline SOCHC@200 °C/6 h and amorphous mSOCHC@200 °C/6 h, both having a characteristic cellulose peak at 14.9°. mSOCHC@200 °C/6 h displayed superparamagnetic behavior with 11.2 emu/g saturation magnetization. IR spectra confirmed the development of samples rich in oxygen containing functionalities; an additional peak for iron oxides appeared at 586 cm^-1 in mSOCHC@200°C/6 h spectrum. Four major peaks at 531.9, 399.9, 348.2 and 284.7 eV, assigned to O 1s, N 1s, Ca 2p and C 1s, respectively were observed during XPS analyses. An additional peak at 710.3 eV, ascribed to Fe 2p was observed in mSOCHC@200C/6 h XPS spectrum, while a peak at 143.2 eV for Pb 4f appeared in spectra of both Pb(II) saturated samples. pH dependent (maximum at ∼6.7), exothermic Pb(II) adsorption was found. About 50-70% (at C_o: 25 mg/L) adsorption on both SOCHC@200 °C/6 h and mSOCHC@200 °C/6 h was accomplished in a minute, attaining equilibrium in 180 and 240 min, respectively. Error functions and superimposed q_{e, exp}. and q_{e, cal.} values supported Langmuir isotherm model applicability, with respective q_m values of 304.9 and 361.7 mg/g at 25 °C for SOCHC@200 °C/6 h and mSOCHC@200 °C/6 h. Kinetic data was fitted to PSO model. Highest (between 92.2 and 88.9 %) amount of Pb(II) from SOCHC@200 °C/6 h and mSOCHC@200 °C/6 h was eluted by 0.01 M HCl.

Preparation of mechanically robust Fe3O4/porous carbon/diatomite composite monolith for solar steam generation

Mechanically robust Fe₃O₄/porous carbon/diatomite composite monolith was prepared from waste corrugated cardboard box and diatomite via slurrying in FeCl₃ solution, dewatering, molding, and carbonization at 600 °C. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (SEM), N₂-adsorption/desorption, Raman spectroscopy, and ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy. The water wettability, photothermal conversion property, and solar steam generation performance of the products were also evaluated. Results showed that the presence of FeCl₃ led to the formation of more pores and magnetic Fe₃O₄ crystallites, while diatomite provided good hydrophilicity for the composite. The product exhibited light absorption above 65% within the wavelength ranging from 200 to1974 nm, and its surface temperature eventually increased by 30 °C under 0.25 sun irradiation due to photothermal effect. Moreover, solar steam yield under 0.25 sun irradiation for 3600 s was improved by 67% with the presence of the monolithic composite because of the occurrence of interfacial solar steam generation and heat transfer from the composite acted as a heat island.

Effective removal of carbamazepine and diclofenac by CuO/Cu2O/Cu-biochar composite with different adsorption mechanisms

In this study, the CuO/Cu₂O/Cu-biochar composite (CBC) was fabricated by calcining Cu²⁺-loaded cauliflower root at 500 °C. The CBC displayed the higher specific surface area and total pore volume than raw biochar, which attributed to Cu²⁺ acting as a pore-forming agent in the synthesis process. The adsorption experiments indicated that CBC could remove 88.96% diclofenac and 93.02% carbamazepine, which was nearly double higher than the raw biochar. The film diffusion mainly controlled the adsorption rate. Meanwhile, the common adsorption mechanisms for two pollutants were deemed to hydrogen-bonding interaction, π-π interaction and micropore filling effect, and copper oxide particles providing more adsorption sites. In addition, the adsorption of diclofenac involved electrostatic attraction. Lastly, the higher adsorption capacity of carbamazepine than diclofenac on CBC was mainly attributed to two mechanisms: Lewis acid-base interaction enhancing the adsorption of carbamazepine and size exclusion effect reducing the adsorption of diclofenac. Therefore, the study provided a possible method that Cu-contaminated biomass converted to CuO/Cu₂O/Cu-biochar, which could achieve win-win results.

Facile synthesis of multifunctional bone biochar composites decorated with Fe/Mn oxide micro-nanoparticles: Physicochemical properties, heavy metals sorption behavior and mechanism

The value-added utilization of waste resources to synthesize functional materials is important to achieve the environmentally sustainable development. In this work, novel micro-nano FeOx- and MnOx-modified bone biochars derived from waste bone meal were obtained at 300 °C, 450 °C and 600 °C, and applied to remove Cd(II), Cu(II) and Pb(II) from aqueous solutions. The results showed that the pyrolysis temperature greatly influenced the specific surface area (SSA), micropore creation, functional groups and heavy metal sorption capacities of FO-BCs and MO-BCs. The effects of solution pH, ionic strength, humic acid (HA), kinetics and thermodynamics on heavy metals adsorption were investigated. Langmuir and pseudo-second order kinetics models fit the adsorption data well, and the FO-BC-450 and MO-BC-600 displayed the highest sorption capacity for Cd(II) (151.3 mg/g and 163.4 mg/g), Cu(II) (219.8 mg/g and 259.0 mg/g) and Pb(II) (271.9 mg/g and 407.2 mg/g), respectively. Due to the dissolved partial hydroxyapatite (HAP), carbonate-bearing hydroxyapatite (CHAP) and the catalysis of Fe(NO₃)₃, the FO-BCs with higher SSA than the MO-BCs, whereas the sorption capacity displayed an opposite trend. The chemical complex, cation-π bonds, ion exchange and coprecipitation were the dominant mechanisms for metals adsorption. Overall, waste bone resource co-pyrolysis with Fe(NO₃)₃/KMnO₄ impregnation is a promising and high-efficient adsorbents for the remediation of heavy metals-contaminated waters.

Smart Modification of HZSM-5 with Manganese Species for the Removal of Mercury

In this study, a series of Mn-modified HZSM-5 samples were synthesized using the solid-state ion-exchange method, and the effects of the manganese loading amount, calcination temperature, reaction temperature, and gas components on mercury removal efficiency were systematically explored. Given that the mass ratio of HZSM-5 to KMnO₄ and the calcination and reaction temperatures were set to 10:2.6 and 400 and 150 °C, Hg⁰ removal efficiency could reach a peak value of 96.4% when exposed to the flue gas containing 5% O₂ and N₂ as the balance. Among the various gas components, O₂ and NO showed a positive impact on Hg⁰ removal; Hg⁰ removal efficiency could even reach ca. 100% when O₂ and NO were simultaneously introduced. In contrast, the introduction of SO₂ led to a decline of Hg⁰ removal efficiency by ca. 16%. In addition, Hg⁰ removal efficiency could still retain ca. 92% of that for the fresh sample after six regeneration and reuse cycles, which is indicative of a satisfactory stability and renewability. Finally, Mars-Maessen mechanisms dominated in the mercury chemical adsorption process.

Study on the Adsorption of CuFe2O4-Loaded Corncob Biochar for Pb(II)

A series of the magnetic CuFe2O4-loaded corncob biochar (CuFe2O4@CCBC) materials was obtained by combining the two-step impregnation of the corncob biochar with the pyrolysis of oxalate. CuFe2O4@CCBC and the pristine corncob biochar (CCBC) were characterized using XRD, SEM, VSM, BET, as well as pHZPC measurements. The results revealed that CuFe2O4 had a face-centered cubic crystalline phase and was homogeneously coated on the surface of CCBC. The as-prepared CuFe2O4@CCBC(5%) demonstrated a specific surface area of 74.98 m2·g-1, saturation magnetization of 5.75 emu·g-1 and pHZPC of 7.0. The adsorption dynamics and thermodynamic behavior of Pb(II) on CuFe2O4@CCBC and CCBC were investigated. The findings indicated that the pseudo-second kinetic and Langmuir equations suitably fitted the Pb(II) adsorption by CuFe2O4@CCBC or CCBC. At 30 °C and pH = 5.0, CuFe2O4@CCBC(5%) displayed an excellent performance in terms of the process rate and adsorption capacity towards Pb(II), for which the theoretical rate constant (k2) and maximum adsorption capacity (qm) were 7.68 × 10-3 g·mg-1··min-1 and 132.10 mg·g-1 separately, which were obviously higher than those of CCBC (4.38 × 10-3 g·mg-1·min-1 and 15.66 mg·g-1). The thermodynamic analyses exhibited that the adsorption reaction of the materials was endothermic and entropy-driven. The XPS and FTIR results revealed that the removal mechanism could be mainly attributed to the replacement of Pb2+ for H+ in Fe/Cu-OH and -COOH to form the inner surface complexes. Overall, the magnetic CuFe2O4-loaded biochar presents a high potential for use as an eco-friendly adsorbent to eliminate the heavy metals from the wastewater streams.

Emission control strategies of hazardous trace elements from coal-fired power plants in China

China's energy dependents on coal due to the abundance and low cost of coal. Coal provides a secure and stable energy source in China. Over-dependence on coal results in the emission of Hazardous Trace Elements (HTEs) including selenium (Se), mercury (Hg), lead (Pb), arsenic (As), etc., from Coal-Fired Power Plants (CFPPs), which are the major toxic air pollutants causing widespread concern. For this reason, it is essential to provide a succinct analysis of the main HTEs emission control techniques while concurrently identifying the research prospects framework and specifying future research directions. The study herein reviews various techniques applied in China for the selected HTEs emission control, including the technical, institutional, policy, and regulatory aspects. The specific areas covered in this study include health effects, future coal production and consumption, the current situation of HTEs in Chinese coal, the chemistry of selected HTEs, control techniques, policies, and action plans safeguarding the emission control. The review emphasizes the fact that China must establish and promote efficient and clean ways to utilize coal in order to realize sustainable development. The principal conclusion is that cleaning coal technologies and fuel substitution should be great potential HTEs control technologies in China. Future research should focus on the simultaneous removal of HTEs, PM, SOx, and NOx in the complex flue gas.

Thiol-functionalized magnetic covalent organic frameworks by a cutting strategy for efficient removal of Hg2+ from water

Covalent organic frameworks (COFs) have attracted tremendous attention due to their excellent performance in wastewater remediation, but their practical application still suffers from various challenges. The development of highly-efficient magnetic COFs along with fast adsorption kinetic and high adsorption capacity is very promising. To achieve the purpose, thiol-functionalized magnetic covalent organic frameworks (M-COF-SH) with abundant accessible chelating sites were designed and synthesized by utilizing disulfide derivative as building blocks and subsequently cutting off the disulfide linkage. After the cutting process, the crystallinity, porosity, superparamagnetism of pristine M-COF are well maintained, and the resultant M-COF-SH turned out to be an effective and selective platform for Hg²⁺ capture from water. Impressively, the resulting composite exhibited a maximum adsorption capacity of Hg²⁺ as high as 383 mg g^-1. In addition, it also displays a rapid kinetic, where the adsorption equilibrium can be achieved within 10 min. More importantly, there is no significant loss of its adsorption performance even after recycling 5 times. This work not only offers a reliable platform for wastewater remediation but also provides a conceptual guide to prepare functionalized M-COF composites which cannot be obtained through conventional approaches.

Construction of luffa sponge-based magnetic carbon nanocarriers for laccase immobilization and its application in the removal of bisphenol A

The raw material of resin, Bisphenol A (BPA), is an endocrine-disrupting compound that can be continuously released into the environment and directly harms health. In this study, luffa sponge was used as the raw material to prepare magnetic carbon chemicals for laccase immobilization and BPA degradation. The MLC-1 was synthesized by one-step carbonization-magnetization method, which showed good magnetic properties and a strong load capacity for laccase. Compared with free laccase, Laccase@MLC-1 showed stronger thermal stability, better acid-tolerate performance and reusability. Moreover, Laccase@MLC-1 showed higher BPA degradation efficiency than free laccase. 100 mg/L of BPA can be completely removed by Laccase@MLC-1 in 4 h, while only 62.70% of BPA was removed by the same amount of free laccase. By improving reuse strategies, a complete BPA degradation ratio was obtained in each reoperation process. All results proved that Laccase@MLC-1 might be a suitable biocatalyst candidate for BPA removal.

Mechanistic investigation of mercury removal by unmodified and Fe-modified biochars based on synchrotron-based methods

Improved surface characteristics and incorporated Fe, S, and Cl species are reported in Fe-modified biochar, which makes it a prospective material for Hg(II) removal. In this study, aqueous Hg(II) was removed from solution by unmodified, FeCl₃-modified, and FeSO₄-modified biochars pyrolyzed at 300, 600, or 900 °C. Higher pyrolytic temperature resulted in higher removal efficiency, with the biochars pyrolyzed at 900 °C removing >96% of Hg(II). Fe-modification enhanced Hg(II) removal for biochars pyrolyzed at 600 °C (from 88% to >95%) or 900 °C (from 96% to 99%). Based on synchronous extended X-ray absorption fine structure (EXAFS) analysis, Hg coordinated to S in modified and unmodified biochars pyrolyzed at 900 °C, where thiol was reported, and in FeSO₄-modified biochars pyrolyzed at 600 or 900 °C, where sulfide was recognized; in other biochars, Hg bound to O or Cl. Additionally, confocal micro-X-ray fluorescence imaging (CMXRFI) demonstrated Hg was distributed in agreement with S in biochars where HgS was formed; otherwise, Hg distribution was influenced by Hg species in solution and the pore characteristics of the biochar. This investigation provides information on the effectiveness and mechanisms of Hg removal that is critical for evaluating biochar applications and optimizing modification methods in groundwater remediation.

Mechanism study of enhanced interaction between gaseous elemental mercury and hydroxylated UIO-66

Novel hydroxylated UIO-66 for gaseous elemental mercury (Hg⁰) removal has been considered to be an emerging method because of its economical and reusable property. Density functional theory studies were investigated to reveal the enhanced heterogeneous interaction mechanisms between mercury and hydroxylated UIO-66 with and without the presence of H₂O₂ vapor. The adsorption and dissociation of H₂O₂ and the generation mechanism of surface hydroxyls on UIO-66 were investigated. Results indicated that H₂O₂ preferred to disconnect the O-O bond followed by the generation of two hydroxyls in the presence of H₂O₂. The hydroxyl adsorbed on UIO-66 and formed the UIO-66 hydroxylation product. The interaction performances between Hg⁰, H₂O₂, and UIO-66 as well as the interaction performances between Hg⁰ and hydroxylated UIO-66 systems were both evaluated through binding energy and the Mulliken charge analysis. Interacted energies indicated thermodynamically favorable processes of Hg-OH formation on hydroxylated UIO-66. The Mulliken charge changes revealed an oxidative process of mercury.

Activation of porous magnetized biochar by artificial humic acid for effective removal of lead ions

In this paper, we have successfully prepared porous magnetic biochar with excellent surface area and recovery rate using corn stalks (CS) and waste iron (WI) as precursors. Notably, in order to prevent the incorporated iron oxides from blocking the carbon pores, then resulting in a decrease in specific surface area and reducing the removal efficiency of the material, the optimum range of iron ions can be determined to be 0.04-0.06 mol/L according to the effect of the amount of iron on the magnetic biochar recovery rate and Pb²⁺ removal capacity. Furthermore, as-synthesized artificial humic acid (A-HA) obtained from waste biomass by hydrothermal humification (HTH) technology has abundant functional groups, which can complex with heavy metals and metal oxides. Therefore, A-HA is introduced as an activator to produce novel porous magnetic biochar materials (AHA/Fe₃O₄-γFe₂O₃@PBC) with abundant functional groups (i.e., phenolic-OH, -COOH, etc.), providing high dispersibility and stability, further leading to excellent removal performance (Langmuir removal capacity up to 99.82 mg/g) and recyclable performance (removal capacity after 5 removal cycles is 79.04 mg/g). Multiple removal mechanisms have been revealed, including reduction, complexation, and precipitation.

Biochar/iron (BC/Fe) composites for soil and groundwater remediation: Synthesis, applications, and mechanisms

Biochar/iron (BC/Fe) composites, such as nano zero-valent iron (nZVI)/BC, iron sulfide/BC, and iron oxide/BC, have been developed and applied to deal with various contaminants owing to their excellent physicochemical properties. This work summarizes the progress in the preparation of BC/Fe composites, the properties and applications of BC/Fe, and the mechanism of the synergistic effect between Fe and BC in the composites. Various methods, including pyrolysis, hydrothermal carbonization, fractional precipitation, and ball milling, have been used to synthesize BC/Fe composites. In addition, the introduction of stabilizers, such as carboxymethyl cellulose (CMC), in the fractional precipitation process further prevents the agglomeration of Fe particles, which enhances the stability and fluidity of the resultant composites to facilitate the application of the composites in soil and water remediation. The application of BC/Fe composites in water and soil remediation is discussed in three aspects based on the interaction mechanisms, namely adsorption, reduction, and oxidation. Overall, the composites showed the synergistic effect of BC and Fe owing to the combination of the specific properties of Fe, such as reduction, catalysis, and magnetism, which can enhance the properties of BC with a larger surface area, abundant functional groups, and increased electron transfer efficiency. This review systemically summarizes the recent developments in BC/Fe composites to maximize the efficiency of BC/Fe application in soil and groundwater remediation. Key challenges and further research needs are also suggested.

Review on Activated Carbons by Chemical Activation with FeCl3

This study reviews the most relevant results on the synthesis, characterization, and applications of activated carbons obtained by novel chemical activation with FeCl3. The text includes a description of the activation mechanism, which compromises three different stages: (1) intense de-polymerization of the carbon precursor (up to 300 °C), (2) devolatilization and formation of the inner porosity (between 300 and 700 °C), and (3) dehydrogenation of the fixed carbon structure (>700 °C). Among the different synthesis conditions, the activation temperature, and, to a lesser extent, the impregnation ratio (i.e., mass ratio of FeCl3 to carbon precursor), are the most relevant parameters controlling the final properties of the resulting activated carbons. The characteristics of the carbons in terms of porosity, surface chemistry, and magnetic properties are analyzed in detail. These carbons showed a well-developed porous texture mainly in the micropore size range, an acidic surface with an abundance of oxygen surface groups, and a superparamagnetic character due to the presence of well-distributed iron species. These properties convert these carbons into promising candidates for different applications. They are widely analyzed as adsorbents in aqueous phase applications due to their porosity, surface acidity, and ease of separation. The presence of stable and well-distributed iron species on the carbons’ surface makes them promising catalysts for different applications. Finally, the presence of iron compounds has been shown to improve the graphitization degree and conductivity of the carbons; these are consequently being analyzed in energy storage applications.

Preparation and application of magnetic biochar in water treatment: A critical review

In recent years, magnetic biochar has been widely used in removal of pollutants from water. In this paper, the preparation technologies of magnetic biochar are analyzed, and the performance and application of magnetic biochar in removal of inorganic pollutants such as heavy metals, and organic pollutants are investigated. Moreover, the adsorption behaviors, the key influencing factors and the adsorption mechanisms of magnetic biochars are summarized in this paper. Compared with common biochar, magnetic biochar is more effective in removal of water pollutants, including Cd(II), Pb(II), Zn(II), Cu(II), methylene blue, tetracycline, pesticide and phosphate. Langmuir and Freundlich models are adopted as the mainly adsorption isotherms, while pseudo-second-order model is employed as Kinetic model of heavy metal ions and organic contaminants in water. This study also investigates degradation of organic contaminants in water using magnetic biochar as catalyst. Results showed that encapsulated γ-Fe₂O₃ nanoparticles enhanced the catalytic ability of persulfate activator. Further researches on preparation and application of magnetic biochar in water treatment are prospected in this review.

Distribution and speciation of iron in Fe-modified biochars and its application in removal of As(V), As(III), Cr(VI), and Hg(II): An X-ray absorption study

Characterization of the spatial distribution and speciation of iron (Fe) in Fe-modified biochars is critical for understanding the mechanisms of contaminant removal. Here, synchrotron-based techniques were applied to characterize the spatial distribution and speciation of Fe in biochars modified by FeCl₃ or FeSO₄ and pyrolyzed at 300, 600, and 900 °C, respectively. Confocal micro-X-ray fluorescence imaging (CMXRFI) results indicated Fe, sulfur (S), and chlorine (Cl) diffused into the basic porous structure of the biochars and aggregated to the surface as pyrolysis temperature increased. Fe K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra revealed maghemite (γ-Fe₂O₃) as the primary Fe species in the modified biochars and Fe(0) was observed when pyrolyzed at 600 or 900 °C. Unmodified and FeCl₃-modified biochars pyrolyzed at 900 °C were evaluated in the removal of arsenate (As(V)), arsenite (As(III)), hexavalent chromium (Cr(VI)) and Hg(II) from aqueous solution and Fe-modification enhanced the removal efficiency from 42.0%, 62.5%, 19.6%, and 97.0%, respectively, to all 99.9%. X-ray absorption spectroscopy results indicate both adsorption and redox reaction contributed to the removal mechanisms. The present study provides a prospective and sustainable material and offers information relevant to tailoring Fe-modified biochars to specific environmental applications.

Significant Enhancement of Gaseous Elemental Mercury Recovery from Coal-Fired Flue Gas by Phosphomolybdic Acid Grafting on Sulfurated γ-Fe2O3: Performance and Mechanism

The existing technologies to control Hg emissions from coal-fired power plants can be improved to achieve the centralized control of Hg⁰ emissions, which continue to pose a risk of Hg exposure to human populations. In this work, MoS_x@γ-Fe₂O₃, formed by the sulfuration of phosphomolybdic acid (HPMo)-grafted γ-Fe₂O₃, was developed as a magnetic and regenerable sorbent to recover gaseous Hg⁰ from coal-fired flue gas as a cobenefit to the use of wet electrostatic precipitators. The thermal stability of γ-Fe₂O₃ was notably enhanced by HPMo grafting; thus, the magnetization of MoS_x@γ-Fe₂O₃ hardly decreased during the application. The kinetic analysis indicates that the chemical adsorption of gaseous Hg⁰ was mainly dependent on the amounts of surface S₂^2- and surface adsorption sites. Although the amount of S₂^2- on sulfurated γ-Fe₂O₃ decreased after HPMo grafting, the amount of surface adsorption sites significantly increased due to the formation of a layered MoS_x structure on the surface. Therefore, the ability of sulfurated γ-Fe₂O₃ to capture Hg⁰ was improved considerably after HPMo grafting. Furthermore, low concentrations of gaseous Hg⁰ in coal-fired flue gas can be gradually enriched by at least 1000 times by MoS_x@γ-Fe₂O₃, which facilitates the recovery and centralized control of gaseous Hg⁰ in flue gas.

Understanding reactions and pore-forming mechanisms between waste cotton woven and FeCl3 during the synthesis of magnetic activated carbon

FeCl₃ is a valuable iron salt used in the synthesis of magnetic waste cotton woven-based activated carbon. Although it has received extensive research attention, more information is required regarding its interactions with the carbon matrix. This systematic study describes the potential reactions of FeCl₃ and waste cotton woven. First, the textural properties of waste cotton woven-based activated carbon synthesized under various conditions were investigated via element analysis, N₂ sorption/desorption isotherms, and scanning electron microscopy. Then, the possible reaction mechanisms were deduced through various characterization methods. The results demonstrate that FeCl₃ can lower the initial decomposition temperature of WCW to 135 °C and catalyze decarboxylation and decarbonylation at 100-330 °C to elevate the formation of microporous structures. Moreover, FeCl₃ can also form Lewis acid sites at 330-700 °C and promote the cross-linking reaction to develop intricate microporous structures and carbonaceous materials with the synergistic effect of Fe³⁺ and Cl^-. FeCl₃ could be used as a template-like agent to form mesoporous structures. Meanwhile, it can also act as a magnetizer that Fe₃O₄ derived from the decomposition of FeCl₃ would insert into the carbon matrix and combine with C-Cl to tailor the magnetic controllable activated carbon. Finally, we confirmed that extending the activation time could convert the structure of waste cotton woven-based activated carbon and increase the number of active sites, thereby further improving the catalytic properties of FeCl₃ in pore formation.

Magnetic biochar for environmental remediation: A review

The difficulty of separating the powdered biochar from the environmental medium may lead to secondary pollution and hinder the large-scale application of biochar as an adsorbent. An effective strategy to solve this bottleneck is to introduce transition metals and their oxides into the biochar matrix, creating easily separable magnetic biochar. Magnetic biochar is also effective for the removal of pollutants from aqueous solution. This review comprises a systematic analysis of 109 papers published in recent years (From 2011 to June 2019), and summarises the synthetic methods and raw materials required for magnetic biochar preparation. The basic physicochemical properties of magnetic biochar are expounded, together with findings from relevant studies, and the application of magnetic biochar as an adsorbent or catalyst in environmental remediation are summarised. Other applications of magnetic biochar are also discussed. Finally, some constructive suggestions are given for the future direction of magnetic biochar research.

Critical review of magnetic biosorbents: Their preparation, application, and regeneration for wastewater treatment

The utilisation of magnetic biosorbents (metal or metal nanoparticles impregnated onto biosorbents) has attracted increasing research attention due to their manipulable active sites, specific surface area, pore volume, pore size distribution, easy separation, and reusability that are suitable for remediation of heavy metal(loid)s and organic contaminants. The properties of magnetic biosorbents (MB) depend on the raw biomass, properties of metal nanoparticles, modification/synthesis methods, and process parameters which influence the performance of removal efficiency of organic and inorganic contaminants. There is a lack of information regarding the development of tailored materials for particular contaminants and the influence of specific characteristics. This review focuses on the synthesis/modification methods, application, and recycling of magnetic biosorbents. In particular, the mechanisms and the effect of sorbents properties on the adsorption capacity. Ion exchanges, electrostatic interaction, precipitation, and complexation are the dominant sorption mechanisms for ionic contaminants whereas hydrophobic interaction, interparticle diffusion, partition, and hydrogen bonding are the dominant adsorption mechanisms for removal of organic contaminants by magnetic biosorbents. In generally, low pyrolysis temperatures are suitable for ionic contaminants separation, whereas high pyrolysis temperatures are suitable for organic contaminants removal. Additionally, magnetic properties of the biosorbents are positively correlated with the pyrolysis temperatures. Metal-based functional groups of MB can contribute to an ion exchange reaction which influences the adsorption capacity of ionic contaminants and catalytic degradation of non-persistent organic contaminants. Metal modified biosorbents can enhance adsorption capacity of anionic contaminants significantly as metal nanoparticles are not occupying positively charged active sites of the biosorbents. Magnetic biosorbents are promising adsorbents in comparison with other adsorbents including commercially available activated carbon, and thermally and chemically modified biochar in terms of their removal capacity, rapid and easy magnetic separation which allow multiple reuse to minimize remediation cost of organic and inorganic contaminants from wastewater.

Development of Recyclable Iron Sulfide/Selenide Microparticles with High Performance for Elemental Mercury Capture from Smelting Flue Gas over a Wide Temperature Range

Fast and effective removal of elemental mercury in a wide temperature range is critical for the smelting industry. In this work, a recyclable magnetic iron sulfide/selenide sorbent is developed to capture and recover Hg⁰ from smelting flue gas. Benefiting from Se doping, the Hg⁰ capture performance of prepared FeS_xSe_y is significantly enhanced compared with traditional iron sulfide, especially at high temperatures. Considering the recyclability and working temperature, FeS_1.32Se_0.11 exhibits the best Hg⁰ capture performance. The average capture rate of FeS_1.32Se_0.11 is 3.661 μg/g/min at 80 °C and its saturation adsorption capacity is 20.216 mg/g. The flue gas compositions have almost no effect on Hg⁰ capture. X-ray photoelectron spectroscopy and mercury thermal programmed desorption suggest that the stable active Se-S_n^2- adsorption site can combine with Hg⁰ to form HgSe, consequently improving Hg⁰ capture performance at high temperatures. After Hg⁰ capture, the spent FeS_xSe_y can be collected by magnetic separation and regenerated through selective extraction, which facilitates harmless treatment and resource reuse of mercury. With the advantages of excellent Hg⁰ capture performance, wide operating temperature range, and remarkable recycling property, FeS_xSe_y microparticles may be a promising sorbent for Hg⁰ capture in industrial applications, while opening a new avenue to realize the resource utilization toward toxic elements.

Preparation of magnetic Co-Fe modified porous carbon from agricultural wastes by microwave and steam activation for mercury removal

In this article, a magnetic cobalt-iron modified porous carbon derived from agricultural wastes by microwave and steam activation was developed to remove elemental mercury in coal-fired flue gas. The effects of operating parameters on Hg⁰ capture were discussed. Reaction mechanism and regeneration performance were also studied. Results show that the activation of microwave and steam significantly improves the pore structure of the porous carbon. The ultrasound-assisted impregnation promotes the dispersion of cobalt oxides and iron oxides on the samples. The Co_0.4Fe₁₂/RSWU(500) sorbent exhibits highest Hg⁰ removal efficiency at 130 °C. The characterization analysis shows that cobalt oxides and iron oxides are the main active components for Hg⁰ removal. The XPS analysis suggests that the chemisorption oxygen and the lattice oxygen (derived from Co³⁺/Co²⁺ and Fe³⁺/Fe²⁺) participate in the Hg⁰ capture process. Moreover, the cobalt-iron mixed oxide modified porous carbon has a good regeneration performance, which is conductive to reduce the costs in the future application.

Remediation of mercury contaminated soil, water, and air: A review of emerging materials and innovative technologies

Mercury contamination in soil, water and air is associated with potential toxicity to humans and ecosystems. Industrial activities such as coal combustion have led to increased mercury (Hg) concentrations in different environmental media. This review critically evaluates recent developments in technological approaches for the remediation of Hg contaminated soil, water and air, with a focus on emerging materials and innovative technologies. Extensive research on various nanomaterials, such as carbon nanotubes (CNTs), nanosheets and magnetic nanocomposites, for mercury removal are investigated. This paper also examines other emerging materials and their characteristics, including graphene, biochar, metal organic frameworks (MOFs), covalent organic frameworks (COFs), layered double hydroxides (LDHs) as well as other materials such as clay minerals and manganese oxides. Based on approaches including adsorption/desorption, oxidation/reduction and stabilization/containment, the performances of innovative technologies with the aid of these materials were examined. In addition, technologies involving organisms, such as phytoremediation, algae-based mercury removal, microbial reduction and constructed wetlands, were also reviewed, and the role of organisms, especially microorganisms, in these techniques are illustrated.

Efficient Removal of Diclofenac from Aqueous Solution by Potassium Ferrate-Activated Porous Graphitic Biochar: Ambient Condition Influences and Adsorption Mechanism

Porous graphitic biochar was synthesized by one-step treatment biomass using potassium ferrate (K₂FeO₄) as activator for both carbonization and graphitization processes. The modified biochar (Fe@BC) was applied for the removal of diclofenac sodium (DCF) in an aqueous solution. The as-prepared material possesses a well-developed micro/mesoporous and graphitic structure, which can strengthen its adsorption capacity towards DCF. The experimental results indicated that the maximum adsorption capacity (q_max) of Fe@BC for DCF obtained from Langmuir isotherm simulation was 123.45 mg·L⁻¹ and it was a remarkable value of DCF adsorption in comparison with that of other biomass-based adsorbents previously reported. Thermodynamic quality and effect of ionic strength studies demonstrated that the adsorption was a endothermic process, and higher environmental temperatures may be more favorable for the uptake of DCF onto Fe@BC surface; however, the presence of NaCl in the solution slightly obstructed DCF adsorption. Adsorption capacity was found to be decreased with the increase of solution pH. Additionally, the possible mechanism of the DCF adsorption process on Fe@BC may involve chemical adsorption with the presence of H-bonding and π–π interaction. With high adsorption capacity and reusability, Fe@BC was found to be a promising absorbent for DCF removal from water as well as for water purification applications.

Preparation of microwave-activated magnetic bio-char adsorbent and study on removal of elemental mercury from flue gas

In this study, novel activated magnetic bio-char adsorbents were proposed to remove the element mercury (Hg⁰) from flue gas. Microwave activation and Mn-Fe mixed oxides impregnation assisted by ultrasound treatment were applied on the modification of renewable cotton straw chars. The influence of different preparation methods, loading value of Mn-Fe, molar ratio of Mn/Fe, calcining temperature, reaction temperature and individual flue gas ingredients (O₂, NO, SO₂ and H₂O) on removal of Hg⁰ was investigated in a fixed bed system. The characterization results reveal that microwave activation is advantageous for the development of the pore structure, and ultrasound treatment can optimize the dispersion of Mn and Fe active ingredients. MnFe4%(3/10)/CSWU700 adsorbent exhibits the optimal Hg⁰ removing performance. O₂, NO, low concentration of SO₂ (<600 ppm) and low concentration of H₂O (<2%) are found to be favourable for the capture of Hg⁰, while high concentrations of SO₂ and H₂O inhibit the removal of Hg⁰. Chemical adsorption acts a pivotal part in the process of Hg⁰ removal. Mn and Fe active ingredients are consumed in large quantities during the Hg⁰ capture. In addition, chemisorbed oxygen (O_β) also plays an indispensable in the oxidation process of Hg⁰. Furthermore, the magnetic adsorbent MnFe4%(3/10)/CSWU700 presents a good regeneration performance and adsorption capacity.