Mediation of rhodamine B photodegradation by biochar

A state-of-art review on the redox activity of persistent free radicals in biochar

Biochar-bound persistent free radicals (biochar-PFRs) attract much attention because they can directly or indirectly mediate the transformation of contaminants in large-scale wastewater treatment processes. Despite this, a comprehensive top-down understanding of the redox activity of biochar-PFRs, particularly consumption and regeneration mechanisms, as well as challenges in redox activity assessment, is still lacking. To tackle this challenge, this review outlines the identification and determination methods of biochar-PFRs, which serve as a prerequisite for assessing the redox activity of biochar-PFRs. Recent developments concerning biochar-PFRs are discussed, with a main emphasis on the reaction mechanisms (both non-free radical and free radical pathways) and their effectiveness in removing contaminants. Importantly, the review delves into the mechanism of biochar-PFRs regeneration, triggered by metal cations, reactive oxygen species, and ultraviolet radiations. Furthermore, this review thoroughly explores the dilemma in appraising the redox activity of biochar-PFRs. Components with unpaired electrons (particular defects and metal ions) interfere with biochar-PFRs signals in electron paramagnetic resonance spectra. Scavengers and extractants of biochar-PFRs also inevitably modify the active ingredients of biochar. Based on these analyses, a practical strategy is proposed to precisely determine the redox activity of biochar-PFRs. Finally, the review concludes by presenting current gaps in knowledge and offering suggestions for future research. This comprehensive examination aims to provide new and significant insights into the redox activity of biochar-PFRs.

A hydrogel-based ratiometric fluorescent sensor relying on rhodamine B labelled AIE-featured hyperbranched poly(amido amine) for heparin detection

The fluorescent flexible sensor for point-of-care quantification of clinical anticoagulant drug, Heparin (Hep), is still an urgent need of breakthrough. In this research, a hyperbranched poly(amido amine) (HPA) was decorated with tetraphenylethene (TPE) and Rhodamine B (RhB), constructing a ratiometric fluorescent sensor (TR-HPA) for Hep. When the sensor was exposed to Hep, the TPE units within the probe skeleton would aggregate, resulting in an increasing fluorescent emission at 483 nm. The 580 nm of fluorescence came from RhB enhance, simultaneously, due to the fluorescence resonance energy transfer. As a result, there are two good linear correlation between the fluorescence emission ratio (E483/E580) of TR-HPA and the Hep concentration over a range of 0-1.0 μM, with a low limit of detection of 3.0 nM. Furthermore, we incorporate the TR-HPA probe into a polyvinyl alcohol (PVA) hydrogel matrix to create a flexible fluorescent sensing system platform, denoted as TR-HPA/PVA. This approach offers a straightforward visual detection method by causing a fluorescence color change from pink to blue when trace amounts of Hep are present. The hydrogel-based fluorescent sensor streamlines the detection procedures for Hep in biomedical applications. It shows great potential in rapid and point-of-care human blood clotting condition monitoring, making it suitable for next-generation wearable medical devices.

Amino-grafted Biochar as a Novel Photocatalyst for degradation of high concentration dye

Photocatalytic degradation of organic pollution by biochar was a sustainable strategy for waste water remediation, nevertheless, it still suffers drawbacks like low efficiency due to the poor photocatalytic properties of pristine biochar. Herein, amino groups were grafted on the edge sites/defects of biochar by Friedel-Crafts acylation to enhance the degradation of high concentration dye solutions. The results suggested that the amino groups played an important role in imparting photocatalytic properties to biochar. Owing to the strong Lewis basicity and electron-donating ability of amino groups, their interaction with oxygen-containing functional groups/aromatic structures in biochar was improved, which enhanced the electron exchange ability of biochar under visible light irradiation, resulting in excellent degradation performances of high concentration RhB (∼10 times faster than ungrafted biochar). In this work, amino-grafted garlic peel biochar delivered a new idea for the future direction of biochar-based photocatalysis in wastewater remediation.

Effects of carbon-silicon structure on photochemical activity of biochars

Biochar has raised increasing concerns because of its great environmental impacts. It is known that the photocatalytic property of biochar is related to its carbon component and dissolved black carbon, but the effect of silicon component is ignored, and the effect of silicon and carbon phases was far less studied. This study systematically explored the photochemistry of silicon-rich and silicon-deficient biochar under light irradiation by using hexavalent chromium (Cr(VI)) and sulfadiazine as representative pollutants for photoreduction and photooxidation, respectively. It was found that biochar had photoreduction activity under the enhancement of electron donors, and 80.1% Cr(VI) can be removed by biochar with crystalline silicon and carbon (i.e., RH900) after 12 h irradiation. Meanwhile after low temperature pyrolysis, biochar with amorphous silicon and carbon (i.e., RH600) had great photooxidation capacity, and 71.90% organic pollutant was degraded within 24 h. The reaction was illustrated by transient photocurrent response, and hydroxyl radical generation measurement, and other tests. A new photochemical mechanism of the synergy between silicon and carbon model was proposed to elucidate the redox reactions of pollutants under the light. Graphitic carbon or crystalline silicon formed under high temperature played a role of valence band which was excited under light irradiation and the effect of electron donors to benefit photoreduction, while amorphous silicon formed under low temperature facilitated photooxidation process by increasing reactive oxygen species concentration. This study provided a gist for biochar production and application in the field of photocatalysis, and contributed to the broader understanding of biochar geochemical behavior in natural sunlit system.

Degradation of organic pollutants from water by biochar-assisted advanced oxidation processes: Mechanisms and applications

Biochar has shown large potential in environmental remediation because of its low cost, large specific surface area, porosity, and high conductivity. Biochar-assisted advanced oxidation processes (BC-AOPs) have recently attracted increasing attention to the remediation of organic pollutants from water. However, the effects of biochar properties on catalytic performance need to be further explored. There are still controversial and knowledge gaps in the reaction mechanisms of BC-AOPs, and regeneration methods of biochar catalysts are lacking. Therefore, it is necessary to systematically review the latest research progress of BC-AOPs in the treatment of organic pollutants in water. In this review, first of all, the effects of biochar properties on catalytic activity are summarized. The biochar properties can be optimized by changing the feedstocks, preparation conditions, and modification methods. Secondly, the catalytic active sites and degradation mechanisms are explored in different BC-AOPs. Different influencing factors on the degradation process are analyzed. Then, the applications of BC-AOPs in environmental remediation and regeneration methods of different biochar catalysts are summarized. Finally, the development prospects and challenges of biochar catalysts in environmental remediation are put forward, and some suggestions for future development are proposed.

Visible-light-driven simultaneous decontamination of multi-antibiotics by facile synthesized BiOCl loaded food wastes biochar

Environmental dissemination caused by widespread use of antibiotics has been regarded as a possible hazard to aquatic ecosystem and human health. The increasing misgivings make it imperative to develop a novel catalyst with remarkable visible-light-driven activity to remove antibiotics, especially for their simultaneous decontamination. Herein, C/BiOCl composites were successfully prepared by decorating BiOCl nanosheets on food wastes biochar (C) by a simple hydrolysis strategy. Not only the binary system of tetracycline antibiotics, but also the ternary mixture could be simultaneously photodegraded over 25% C/BiOCl within 15 min irradiation. The improved photocatalytic activities could be ascribed to the introduction of biochar, endowing increased surface area, enhanced separation of photo-generated charge carriers, and better light absorption. The as-prepared 25% C/BiOCl also demonstrated satisfactory stability and positive removal effect in actual water samples. The present work provides new insights into the development of biochar-based photocatalysts for simultaneous degradation of multiple antibiotics.

Improving photocatalytic activity under visible light over a novel food wastes biochar-based BiOBr nanocomposite

Biochar (C) applied in synthesizing photocatalysts to eliminate water pollution has been intensively investigated. Herein we report the first use of biochar pyrolyzed from food wastes at 400 ^°C (400C) and 700 ^°C to construct C/BiOBr composites via a facile hydrolysis approach. Photocatalytic performances could be significantly improved by choosing the appropriate carbonization temperature and adjusting the content of C in C/BiOBr composites. The prepared 1%400C/BiOBr exhibited the best photodegradation capacity towards methylene orange (20 mg/L) and tetracycline (50 mg/L). A series of characterization results illustrated that smooth structure and surface properties (oxygen functional groups and persistent free radicals) of 400C played an important role in enhancing the photocatalytic activities. Mechanism exploration suggested that h⁺ and ˙O₂^- were the main active species thus contributing to photodegradation. This study provided a new insight into utilization of biochar derived from food wastes in photocatalysis and environmental remediation.

Heterogeneous compositions of oxygen-containing functional groups on biochars and their different roles in rhodamine B degradation

The reactivity of pyrogenic carbon has attracted a great deal of research attentions recently. The oxygen-containing structures are rich on the surface of biochars, and involved in accepting and donating electrons during the interactions between biochar and organic contaminants. In this work, the species and content of oxygen-containing functional groups on biochar surface were regulated through chemical modification, and batch sorption/degradation experiments were carried out for rhodamine B (RhB). Based on the comparison of surface functional groups, biochars produced below 200 °C mediated RhB degradation through phenol hydroxyl group, while semiquinone and carboxylic acid groups were the main reaction active sites for biochars produced at higher than 500 °C. Considering that various biochar properties play roles in mediating organics degradation, the strategies in manipulating biochar properties should be carefully considered.

The chemical structure characteristics of dissolved black carbon and their binding with phenanthrene

The elucidation of interactions between the dissolved black carbon (DBC) in biochar and hydrophobic organic contaminants (HOCs) is crucial for controlling the environmental behavior of HOCs. The complicated chemical structures of DBCs result in diverse interaction mechanisms between DBCs and HOCs, which were driven by different chemical structures in DBCs. In the present study, ten DBCs were extracted from rice straw and corncob biochars and their chemical structures were characterized and analyzed. The binding of phenanthrene (Phen) with DBC were studied through fluorescence quenching experiments. DBCs with low concentration (1 mg C/L) were found to complex with high amounts of Phen per unit mass. No significant difference was found in the amount of the bound Phen per unit amount of DBC when the concentration of DBC increased beyond >5 mg C/L. The dominant mechanisms involved in the binding of Phen by DBCs are speculated to be hydrophobic interactions, π-π electron donor-acceptor (EDA), and chemical partition, which was driven by the fatty carbon chain, aromatic rings, and quinone groups or ester groups, respectively. This study elucidates the interactions between DBC and Phen, which is of great significance for understanding the environmental behavior of HOCs.

A critical review of environmentally persistent free radical (EPFR) solvent extraction methodology and retrieval efficiency

Long-lived environmentally persistent free radical (EPFR) exposures have been shown in toxicology studies to lead to respiratory and cardiovascular effects, which were thought to be due to the persistence of EPFR and their ability to produce reactive oxygen species. To characterize EPFR exposure and resulting health impacts, it is necessary to identify and systematize analysis protocols. Both direct measurement and solvent extraction methods have been applied to analyze environmental samples containing EPFR. The use of different protocols and solvents in EPFR analyses makes it difficult to compare results among studies. In this work, we reviewed EPFR studies that involved solvent extraction and carefully reported the details of the extraction methodology and retrieval recovery. EPFR recovery depends on the structure of the radical species and the solvent. For the limited number of studies available for review, the polar solvents had superior recovery in more studies. Radicals appeared to be more oxygen-centered following extraction for fly ash and particulate matter (PM) samples. Different solvent extraction methods to retrieve EPFR may produce molecular products during the extraction, thus potentially changing the sample toxicity. The number of studies reporting detailed methodologies is limited, and data in these studies were not consistently reported. Thus, inference about the solvent and protocol that leads to the highest EPFR extraction efficiency for certain types of radicals is not currently possible. Based on our review, we proposed reporting criteria to be included for future EPFR studies.

Environmental persistent free radicals in diesel engine exhaust particles at different altitudes and engine speeds

The occurrence of environmental persistent free radicals (EPFRs) in the environment has attracted a great deal of research attention. Although the major sources of EPFRs in the environment is diesel engine exhaust, the study on the emission characteristics of EPFRs at different working conditions is still very limited. An integrated engine system was adopted to simulate different working conditions of various altitudes and engine speeds, and to examine the emission process of a diesel engine. The results suggested that low engine speed and high altitude are generally associated with high PM₁₀ emission with more stable and ordered structures. Based on the analysis of PAHs on solid and gas phases, PM₁₀ generated from diesel engine at altitude higher than 2000 m may contain substantial amounts of PAHs embedded inside particles, but not adsorbed on the surface. EPFRs signal up to 1.66 × 10²⁰ spins/g were detected in PM₁₀ of the diesel exhaust. Higher engine speed and lower altitude were associated with stronger EPR signals on PM₁₀. However, the accumulated EPR signal intensities after consuming 1 L of diesel were higher at lower engine speed and higher altitude, suggesting higher overall risks. A positive correlation between R value (signal strength ratio of D and G peaks on the Raman spectra) and EPFRs intensity indicated that the EPR signals were associated with the defects of carbon structure. EPFRs intensity in particles showed no significant change in dark, and over 70% of the EPR signals survived under UV light in a one-month aging simulation. The strong persistence of these EPFRs suggested their potential long lasting and widespread risks, which should be investigated extensively.

Reduction of silver ions to silver nanoparticles by biomass and biochar: Mechanisms and critical factors

The reduction of metal ions by biomasses (BMs) and biochars (BCs) is often neglected when evaluating the environmental behavior and risk of heavy metals. In this study, the formation mechanisms of silver nanoparticles (AgNPs) when Ag⁺ coexists with BMs/BCs were investigated. Four types of BMs (pine sawdust, bagasse, lignin, and cellulose) as well as their BCs were investigated for their roles in transforming Ag⁺ to AgNPs. The electron donating capacity (EDC) of all the BMs/BCs was larger than zero. The UV-Vis spectrometer and scanning electron microscopy-energy dispersive X-ray spectrometer (SEM-EDX) analysis confirmed the formation of AgNPs. The quantities of AgNPs formed by BM systems were higher than that by their corresponding BCs. The quantities of formed AgNPs by bagasse and its BCs were the highest when compared with other BMs/BCs, which may be due to their highest EDC values. We found that hydroxyl group (-OH) was the important redox-active functional group in BMs and BCs that contributed to Ag⁺ reduction according to the results from X-ray photoelectron spectrometric (XPS) and Fourier transform infrared spectroscopic (FTIR) analyses. AgNPs formation was enhanced at elevated pH, probably because of the deprotonated functional groups with high EDC values and electron density. The higher temperature could enhance the formation of AgNPs, suggesting that the reduction of Ag⁺ by BMs/BC was a thermodynamically favored process. This study illustrated that Ag⁺ was transformed to AgNPs by BMs and BCs through the redox reactive -OH of BMs/BCs, which further improved our understanding on the formation of AgNPs in the environment.

Two new layered metal chalcogenide frameworks as photocatalysts for highly efficient and selective dye degradation

Dye photodegradation is an important research topic, and great efforts have been made to target the photocatalysts with highly efficient and selective performance. Reported here are two layered anion chalcogenide frameworks with semiconducting properties combined with highly open interlayer spaces, which are used as efficient photocatalysts to show excellent size and charge selectivity towards organic dye molecules. In addition, the organic templates inside the chalcogenide frameworks are exchanged via an ion-exchange process, and the resulting host frameworks with much looser internal spaces play significant roles in improving the photocatalytic activity.

Surface characterization of maize-straw-derived biochar and their sorption mechanism for Pb2+ and methylene blue

Biochar derived from straw is a potential low-cost adsorbent for metal ions and organic pollutants, but its practical application is still limited by the adsorption capacity. In this study, the correlation between the biochar’s properties and pyrolysis temperature was explored. The adsorption mechanism was studied by monitoring the changes of biochar properties before and after adsorption using BET, SEM, XPS and FT-IR spectroscopy. The adsorption mechanism was revealed following the adsorption kinetics and the changes in biochar’s properties before and after adsorption. The methylene blue (MB) and Pb²⁺ adsorption removal efficiency reached 95% at the initial concentration of 125 and 500 mg/L, respectively. Physisorption, chemisorption, and pore filling mechanisms determined the adsorption process of MB and Pb²⁺ on biochar. The Pb²⁺ adsorption process was highly affected by chemical co-precipitation at higher pyrolysis temperatures. The appearance of tar particles increased the adsorption rate of Pb²⁺. The biochar obtained at the pyrolysis temperature at 500, 800 and 900°C proved to be applicable for Pb²⁺ removal. Chemisorption and porosity dominated the MB adsorption, and biochars produced at pyrolysis temperatures of 200, 800 and 900°C are potential materials for MB removal. This study provides optimal pyrolysis conditions for transforming maize straw into valuable, low-cost materials for the removal of different pollutants.