Efficient reduction and adsorption of Cr(VI) using FeCl3-modified biochar: Synergistic roles of persistent free radicals and Fe(II)

Assessing the efficiency and reusability of zirconium-based MOF-biochar composite for the removal of Pb (II) and Cd (II) in single and multi-ionic systems

Recent studies have highlighted the promising properties of metal-organic frameworks (MOF) and biochar composites as cost effective adsorbents. Although MOF-biochar composites have shown significant potential for contaminant removal in aquatic environments, further research is needed for their scalable performance in removing a wide range of emerging contaminants from wastewater. In this paper, we introduce a novel UiO67-biochar composite (MBC) for the first time, synthesised via an in-situ solvothermal method, as an innovative solution for removing heavy metals from water. The composite was characterised by various analytical techniques (SEM, TEM, XRD, FTIR, XPS, BET, and TGA) and the results demonstrated that the specific surface area of the composite (≈540 m2/g) elevated 28 times compared to the unmodified biochar (≈20 m2/g). The adsorption tests indicate remarkable adsorption capacity and removal efficiency in the range of 121.1 mg/g and 90.8 % as well as 59.7 mg/g and 89.5 % for Pb (II) and Cd (II), respectively, which sustained under impacts of co-existing ions. Kinetic studies demonstrated that the experimental data for both heavy metal ions were best described by the Pseudo-second order kinetic model, inferring that chemical interactions mainly control adsorption. The formulated material showed promising stability (retained crystallinity confirmed by XRD analysis) over reusability tests with approximately 87 % removal efficiency. The ion exchange, surface complexation, and electrostatic interactions were the main adsorption mechanisms of the heavy metal ions on the MBC composite. The formulated composite proposed in this study offers scalable, sustainable, and affordable material to treat heavy metal-polluted water and wastewater.

Effective elimination of Pb (II) cations from waste water and polluted water using siderite magnetic biochar

Magnetic biochar composites were created by pyrolyzing siderite and sawdust in nitrogen gas (N2). adsorption was done in a variety of pH and temperature ranges on magnetic biochar. A magnet was used to extract the MB-liquid from each other following 24-hour shaking period. At Iran's Geological Survey, Pb(II) concentration was measured using an ICP (Inductively Coupled Plasma). The adsorption-desorption process was carried out five times in order to evaluate the magnetic biochar's reusability. The Pyrolysis of siderite in order to gain the MB changes its chemical composition and turns into a mixture of hematite, magnetite and maghemite, which imparts magnetism to the biochar and enriches its surface functional groups. The characterizations showed a higher specific surface area and porous structures in the magnetic biochar. An external magnetic field (magnet) was used to easily separate the magnetic biochar suspension because XRD investigation revealed that the primary component of the siderite magnetic biochar absorbent is magnetite, a ferrimagnetic mineral with substantial magnetic characteristics. The magnetic biochar composites' strong adsorption capabilities toward Pb (II) ions were demonstrated by the batch adsorption tests. At pH 5.0 and T = 45 °C, Pb had its highest adsorption capability on magnetic biochar 96.92%. The mesoporous structure of magnetic biochar was indicated by the type IV isotherm. It has been demonstrated that adsorption most closely matches Langmuir's model. Therefore, it can be said that monolayer adsorption has occurred. Biochar's active sites were probably responsible for the fast adsorption process. Kinetics of lead adsorption with MB have been harmonized with pseudo-second order, indicating that the predominant mechanism for Pb adsorption onto magnetic biochar is chemisorption/surface complexation. In summary, magnetic biochar serves as a dual-functional material, adsorbing Pb(II) species and reducing them to less harmful forms, with the added advantage of easy recovery and reuse due to its magnetic properties. This makes it a promising material for the remediation of lead-contaminated environments.

Removal of Cr6+ in water by superoxide anion-mediated redox reaction assisted by lignin-rich kiwifruit twig biochar: Application of DFT calculation

This research aims to investigate the role of reactive oxygen species (ROS) in the adsorption and reduction of Cr6+ on lignin-rich biochar under dark conditions and under various oxygen treatment conditions. The research found that under aerobic conditions, the reduction content of Cr6+ (0.38 mg) and the production content of ·O2- (20.36 × 10-6 mg·L-1) are the highest, followed by untreated conditions (0.32 mg, 15.03 × 10-6 mg·L-1), and the lowest under anaerobic conditions (0.21 mg, 5.14 × 10-6 mg·L-1). Compared with anaerobic conditions, the reduction content of Cr6+ increased by 1.52 times under untreated conditions. Meanwhile, under anaerobic conditions, ·O2- disappeared, indicating that ·O2- had played an important role in the reduction of Cr6+. Kinetic results showed that the role of ·O2- in the reduction of Cr6+ mainly occurred in liquid solution. DFT calculations confirmed that C-OH was the main electron supplier in the reduction process of Cr6+, and there was a positive correlation between the production content of ·O2- and the content of C-OH in liquid solution. The present research is expected to provide a scientific basis for the transformation of Cr6+ on lignin-rich biochar in liquid solution.

Differentiated effects and mechanisms of N-, P-, S-, and Fe-modified biochar materials for remediating Cd- and Pb-contaminated calcareous soil

To investigate the remediation effects of various modified biochar materials derived from different impregnation agents on Cd- and Pb-contaminated calcareous soil, nitrogen (N-), phosphorus (P-), sulfur (S-), and iron (Fe-) modified biochar materials (NBC, PBC, SBC, FBC) were fabricated through the impregnation-pyrolysis method and employed to immobilize Pb and Cd in the calcareous soil. The characterization results showed that NBC exhibited an uneven pore size distribution and increased aromaticity, while PBC and SBC had increased pH and ash content. Pot experiments demonstrated significantly different effects of various modified biochar materials on soil immobilization and plant uptake of Cd and Pb. With regard to soil pH, FBC caused a notable decrease in both rhizosphere and non-rhizosphere areas, while the other materials showed an increase. NBC, PBC, and SBC effectively immobilized Cd and Pb in the soil and significantly reduced their accumulation in Chinese cabbage by 34.4 %-58.9 % for Cd and 9.2 %-53.1 % for Pb, with PBC having the best effect, attributed to complexation, precipitation, and adsorption. However, FBC had strong acidity and poor immobilization ability, which increased the available concentrations of Cd and Pb in the soil. Additionally, PBC promoted the growth, enzyme activity, and tolerance to Cd- and Pb-contaminated soil of Chinese cabbage. Overall, NBC and PBC were identified as the most effective modified biochar materials for stabilizing Cd and Pb in the soil, reducing heavy metal uptake by Chinese cabbage, and boosting enzyme activity.

A novel advanced reduction process for the reduction of Cr(VI): Assistance of microbial metabolites

Advanced reduction processes (ARPs) have become hotspot because of their fast and efficient features in pollutant treatment. In this study, a novel ARP was raised through the assistance of biological wastewater degradation solutions (PDs), to completely reduce Cr(VI). Enterobacter cloacae YN-4, which could completely degrade 1500 mg/L phenol within 72 h, was isolated and identified. While, the content of organic acids and their derivatives in PDs was extremely high (74.76 %). After the combination of PDs with Fe(III) and UV, 10 mg/L Cr(VI) was completely reduced within 66 min, whose reduction rate of Cr(VI) was stable at various concentrations (10-100 mg/L), which was applicable on electroplating wastewater. In addition, Cr(VI) could be reduced stably (71.63 %) after 10 cycles. Compared with the reported ARPs, herein, the components was complex, which was firstly proposed that simultaneous action of polycarboxylic acids, monocarboxylic acids, amino acids and alcohols could promote and ensure the stable reduction of Cr(VI). Among them, the multispecies radicals·CO2- and·O2- generated in PDs were combined with Fe(II), to co-reduce Cr(VI). This strategy produces a wide variety of radicals, which can provide an alternative pathway for remediation of various heavy metals and organic pollutants.

Removal of Pb(II) and Cd(II) from a Monometallic Contaminated Solution by Modified Biochar-Immobilized Bacterial Microspheres

Lead (Pb) and cadmium (Cd) are toxic pollutants that are prevalent in wastewater and pose a serious threat to the natural environment. In this study, a new immobilized bacterial microsphere (CYB-SA) was prepared from corn stalk biochar and Klebsiella grimontii by sodium alginate encapsulation and vacuum freeze-drying technology. The removal effect of CYB-SA on Pb(II) and Cd(II) in a monometallic contaminated solution was studied. The results showed that the removal of Pb(II) and Cd(II) by CYB-SA was 99.14% and 83.35% at a dosage of 2.0 g/L and pH = 7, respectively, which was 10.77% and 18.58% higher than that of biochar alone. According to the Langmuir isotherm model, the maximum adsorption capacities of Pb(II) and Cd(II) by CYB-SA at 40 °C were 278.69 mg/g and 71.75 mg/g, respectively. A combination of the kinetic model, the isothermal adsorption model, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) analyses showed that the main adsorption mechanisms of CYB-SA encompass functional group complexation, ion exchange, electrostatic attraction and physical adsorption. The findings of this study offer practical and theoretical insights into the development of highly efficient adsorbents for heavy metals.

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.

The role of superoxide anion to Cr(VI) reduction by pine biochar

It has been reported that Cr(VI) can be reduced by biochar because of its redox activity. Considering the anionic form of Cr(VI), we hypothesize that the reduction in aqueous phase is significant. However, the contribution of different reactive oxygen species in the biochar-Cr(VI) reaction system has not been distinguished. Herein, we quantitatively identified Cr(VI) adsorption and reduction in biochar systems. The reduction content of Cr(VI) was 1.5 times higher in untreated conditions than in anaerobic conditions. The disappearance of·O2- under anaerobic conditions illustrated that·O2- may be involved in the reduction of Cr(VI). Quenching of·O2- resulted in a decrease of Cr(VI) reduction by 34%, while 1O2 was negligible, probably due to the stronger electron-donating capacity of·O2-. The degradation of nitrotetrazolium blue chloride (quenching agent of·O2-) confirmed that the reduction process of·O2- mainly occurred in the liquid-phase. Boehm titration and quantification of·O2- further elucidated the significant correlation (P < 0.05) between phenolic groups and the formation of·O2-, which implied that phenolic groups acted as the primary electron donors in generating·O2-. This study highlights the importance of the liquid-phase reduction process in removing Cr(VI), which provides theoretical support for biochar conversion of Cr(VI).

Efficient Adsorption of Nitrogen and Phosphorus in Wastewater by Biochar

Nitrogen and phosphorus play essential roles in ecosystems and organisms. However, with the development of industry and agriculture in recent years, excessive N and P have flowed into water bodies, leading to eutrophication, algal proliferation, and red tides, which are harmful to aquatic organisms. Biochar has a high specific surface area, abundant functional groups, and porous structure, which can effectively adsorb nitrogen and phosphorus in water, thus reducing environmental pollution, achieving the reusability of elements. This article provides an overview of the preparation of biochar, modification methods of biochar, advancements in the adsorption of nitrogen and phosphorus by biochar, factors influencing the adsorption of nitrogen and phosphorus in water by biochar, as well as reusability and adsorption mechanisms. Furthermore, the difficulties encountered and future research directions regarding the adsorption of nitrogen and phosphorus by biochar were proposed, providing references for the future application of biochar in nitrogen and phosphorus adsorption.

Persistent free radicals on biochar for its catalytic capability: A review

Biochar is an economical carbon material for water pollution control, which shows great promise to be applied in the up-scale wastewater remediation processes. Previous studies demonstrate that persistent free radicals (PFRs) on biochar are critical to its reactivity for wastewater remediation. A series of studies have revealed the important roles of PFRs when biochar was applied for organic pollutants degradation as well as the removal of Cr (VI) and As (III) from wastewater. Therefore, this review comprehensively concludes the significance of PFRs for the catalytic capabilities of biochar in advanced oxidation processes (AOPs)-driven organic pollutant removal, and applied in redox processes for Cr (VI) and As (III) remediation. In addition, the mechanisms for PFRs formation during biochar synthesis are discussed. The detection methods are reviewed for the quantification of PFRs on biochar. Future research directions were also proposed on underpinning the knowledge base to forward the applications of biochar in practical real wastewater treatment.

Sorptive removal of cadmium using the attapulgite modified by the combination of calcination and iron

Sorptive removal of cadmium (Cd) from the aqueous solutions using the easily available natural materials is an attractive method. However, the adsorption efficiencies of these materials, such as clays, are typically low. Besides, they are generally in relatively low stability and renewability, which restrict their application. Thus, modification of these materials to enhance their performance on Cd removal has gained growing attentions. Herein, the integration of calcination and ferric chloride (FeCl3) was used to modify a typical clay, i.e., attapulgite, to increase the adsorption sites, and thus to develop a robust adsorbent for Cd. Under the optimum conditions for attapulgite modification (i.e., the mass ratio of FeCl3 to attapulgite was 1:2, calcination temperature was 350 °C, and calcination time was 1.5 h) and Cd adsorption (i.e., initial pH of 6.0, adsorption temperature of 25 °C, and adsorbent dosage of 1.0 g/L), the maximum adsorption capacity of the modified attapulgite toward Cd was 149.9 mg/g. Mechanisms of surface complexation and electrostatic attraction were involved in the efficient removal of Cd. The adsorption of Cd increased with pH due to the increased electrostatic attraction. Metal cations inhibited the Cd adsorption through competing with the adsorption sites. The changes of Gibbs-free energy during the adsorption of Cd were lower than zero and decreased with temperature, suggesting the process was spontaneous and endothermic. The removal efficiency of Cd after 5 times of recycle maintained at 82% of that of the raw modified attapulgite demonstrated the stability of the adsorbent. These results suggested that the modified attapulgite is robust for Cd removal and is promising for land application.

Addressing the issue of surface mechanisms and competitive effects in Cr(VI) reductive-adsorption on tin-hydroxyapatite in the presence of co-ions

Our group recently proposed an innovative sustainable reductant-adsorbent material, tin(II)-hydroxyapatite (Sn/HAP, ca. 10 wt% Sn) for the interfacial Cr(VI) reductive adsorption process. In this study, Cr(VI) removal capacity was evaluated in multi-component solutions containing representative background ions (i.e., CaCl2, Ca(NO3)2, MgSO4, Na2SO4, Fe(NO3)3, AlCl3, Zn(NO3)2, or Mn(NO3)2). Sn/HAP was able to reduce Cr(VI) with complete Cr3+ adsorption on HAP surface, except in the presence of Fe3+ and Al3+ ions. Some metal ions co-existing in solution, such as Fe3+, Al3+, Zn2+, and Mn2+, were also adsorbed on HAP surface. Reuse experiments of the Sn/HAP sample, up to 7 runs, resulted in a total amount of reduced Cr(VI) of ca. 15-18 mg g-1. Fast kinetics of Cr(VI) reductive adsorption at 25 °C in a multi-metal component solution was observed. The pseudo-second order model was in excellent agreement with the experimental kinetic data, leading to a rate constant (k25°C) value of ca. 30 M-1 s-1. The collection of adsorption isotherms of Cr3+ and Fe3+, together with TEM-EDX analysis permitted the unveiling of competitive adsorption phenomena between metal ions. The obtained results demonstrate that Sn/HAP could be an efficient material for the removal of hexavalent chromium in aqueous solutions containing high concentrations of inorganic impurities.