Environmental fate of tire-rubber related pollutants 6PPD and 6PPD-Q: A Review.

To enhance tire durability, the antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is used in rubber, but it converts into the toxic 6PPD quinone (6PPD-Q) when exposed to oxidants like ozone (O3), causing ecological concerns. This review synthesizes the existing data to assess the transformation, bioavailability, and potential hazards of two tire-derived pollutants 6PPD and 6PPD-Q. The comparative analysis of different thermal methods utilized in repurposing waste materials like tires and plastics into valuable products are analyzed. These methods shed light on the aspects of pyrolysis and catalytic conversion processes, providing valuable perspectives into optimizing the waste valorization and mitigating environmental impacts. Furthermore, we have examined the bioavailability and potential hazards of chemicals used in tire manufacturing, based on the literature included in this review. The bioavailability of these chemicals, particularly the transformation of 6PPD to 6PPD-Q, poses significant ecological risks. 6PPD-Q is highly bioavailable in aquatic environments, indicating its potential for widespread ecological harm. The persistence and mobility of 6PPD-Q in the environment, along with its toxicological effects, highlight the critical need for ongoing monitoring and the development of effective mitigation strategies to reduce its impact on both human health and ecosystem. Future research should focus on understanding the chronic effects of low-level exposure to these compounds on both terrestrial and aquatic ecosystems, as well as the potential for bioaccumulation in the food chain. Additionally, this review outlines the knowledge gaps, recommending further research into the toxicity of tire-derived pollutants in organisms and the health implications for humans and ecosystems.

Navigating the environmental dynamics, toxicity to aquatic organisms and human associated risks of an emerging tire wear contaminant 6PPD quinone.

N-1,3-Dimethylbutyl-N'-phenyl-p-quinone diamine (6PPDQ) is a derivative of 6PPD, a synthetic antioxidant used in tire manufacturing to control the degradation caused by oxidation and heat aging. Its discovery in 2020 has raised important environmental concern, particularly regarding its association with acute mortality in coho salmon, prompting surge in research on its occurrence, fate, and transport in aquatic ecosystems. Despite this attention, there remain notable gaps in grasping the knowledge, demanding an in depth overview. Thus, this review consolidates recent studies to offer a thorough investigation of 6PPDQ's environmental dynamics, pathways into aquatic ecosystems, toxicity to aquatic organisms, and human health implications. Various aquatic species exhibit differential susceptibility to 6PPDQ toxicity, manifesting in acute mortalities, disruption of metabolic pathways, oxidative stress, behavioral responses, and developmental abnormalities. Whereas, understanding the species-specific responses, molecular mechanisms, and broader ecological implications requires further investigation across disciplines such as ecotoxicology, molecular biology, and environmental chemistry. Integration of findings emphasizes the complexity of 6PPDQ toxicity and its potential risks to human health. However, urgent priorities should be given to the measures like long-term monitoring studies to evaluate the chronic effects on aquatic ecosystems and the establishment of standardized toxicity testing protocols to ensure the result comparability and reproducibility. This review serves as a vital resource for researchers, policymakers, and environmental professionals seeking appraisals into the impacts of 6PPDQ contamination on aquatic ecosystems and human health.

Decoding the molecular concerto: Toxicotranscriptomic evaluation of microplastic and nanoplastic impacts on aquatic organisms.

The pervasive and steadily increasing presence of microplastics/nanoplastics (MPs/NPs) in aquatic environments has raised significant concerns regarding their potential adverse effects on aquatic organisms and their integration into trophic dynamics. This emerging issue has garnered the attention of (eco)toxicologists, promoting the utilization of toxicotranscriptomics to unravel the responses of aquatic organisms not only to MPs/NPs but also to a wide spectrum of environmental pollutants. This review aims to systematically explore the broad repertoire of predicted molecular responses by aquatic organisms, providing valuable intuitions into complex interactions between plastic pollutants and aquatic biota. By synthesizing the latest literature, present analysis sheds light on transcriptomic signatures like gene expression, interconnected pathways and overall molecular mechanisms influenced by various plasticizers. Harmful effects of these contaminants on key genes/protein transcripts associated with crucial pathways lead to abnormal immune response, metabolic response, neural response, apoptosis and DNA damage, growth, development, reproductive abnormalities, detoxification, and oxidative stress in aquatic organisms. However, unique challenge lies in enhancing the fingerprint of MPs/NPs, presenting complicated enigma that requires decoding their specific impact at molecular levels. The exploration endeavors, not only to consolidate existing knowledge, but also to identify critical gaps in understanding, push forward the frontiers of knowledge about transcriptomic signatures of plastic contaminants. Moreover, this appraisal emphasizes the imperative to monitor and mitigate the contamination of commercially important aquatic species by MPs/NPs, highlighting the pivotal role that regulatory frameworks must play in protecting all aquatic ecosystems. This commitment aligns with the broader goal of ensuring the sustainability of aquatic resources and the resilience of ecosystems facing the growing threat of plastic pollutants.

Enhancing rice quality and productivity: Multifunctional biochar for arsenic, cadmium, and bacterial control in paddy soil.

The perilousness of arsenic and cadmium (As-Cd) toxicity in water and soil presents a substantial hazard to the ecosystem and human well-being. Additionally, this metal (loids) (MLs) can have a deleterious effect on rice quality and yield, owing to the existence of toxic stress. In response to the pressing concern of reducing the MLs accumulation in rice grain, this study has prepared magnesium-manganese-modified corn-stover biochar (MMCB), magnesium-manganese-modified eggshell char (MMEB), and a combination of both (MMCEB). To test the effectiveness of these amendments, several pot trials were conducted, utilizing 1% and 2% application rates. The research discovered that the MMEB followed by MMCEB treatment at a 2% rate yielded the most significant paddy and rice quality, compared to the untreated control (CON) and MMCB. MMEB and MMCEB also extensively decreased the MLs content in the grain than CON, thereby demonstrating the potential to enrich food security and human healthiness. In addition, MMEB and MMCEB augmented the microbial community configuration in the paddy soil, including As-Cd detoxifying bacteria, and decreased bioavailable form of the MLs in the soil compared to the CON. The amendments also augmented Fe/Mn-plaque which captured a considerable quantity of As-Cd in comparison to the CON. In conclusion, the utilization of multifunctional biochar, such as MMEB and MMCEB, is an encouraging approach to diminish MLs aggregation in rice grain and increase rice yield for the reparation of paddy soils via transforming microbiota especially enhancing As-Cd detoxifying taxa, thereby improving agroecology, food security, and human and animal health.

Arsenic and cadmium load in rice tissues cultivated in calcium enriched biochar amended paddy soil.

Arsenic (As) and cadmium (Cd) are unnecessary metal(loids) toxic at high concentration to plants and humans, hence lessening their rice grain accumulation is crucial for food security and human healthiness. Charred eggshell (EB), corncob biochar (CB), and eggshell-corncob biochar (ECB) were produced and amended to As and Cd co-polluted paddy soil at 1% and 2% application rates to alleviate the metal(loids) contents in rice grains using pot experiments. All the amendments increased paddy yields at 1%, while EB at 2% significantly reduced the yields compared to untreated control. The resulting yield loss in 2%EB was from the combined effects of its high CaCO3 supplementation, and the increment of rhizosphere soil pH which could insolubilize plant nutrients. The amendments were inefficient in decreasing rice grain As (AsGrain), but all the treatments significantly reduced the rice grain Cd (CdGrain) at both 1% (44.4-77.1%) and 2% (79.8-91.5%) application rates compared to that of control. Regression analysis for contribution weights of control factors revealed that rhizosphere soil Eh and pH were vital influential factors regulating the AsGrain, whereas porewater Cd was main factor controlling CdGrain accumulation. These investigations indicated that the Ca-enriched eggshell-corncob biochar even at high application rate (i.e., 2%ECB) could be a potential tactic for grain accumulation remediation of the cationic pollutant (i.e., Cd) from the paddy soil to rice grain scheme with concurrent increase in rice yields.

Mechanism of As(III) removal properties of biochar-supported molybdenum-disulfide/iron-oxide system.

Sulfate (SO4•-) and hydroxyl-based (HO•) radical are considered potential agents for As(III) removal from aquatic environments. We have reported the synergistic role of SO4•- and HO• radicals for As(III) removal via facile synthesis of biochar-supported SO4•- species. MoS2-modified biochar (MoS2/BC), iron oxide-biochar (FeOx@BC), and MoS2-modified iron oxide-biochar (MoS2/FeOx@BC) were prepared and systematically characterized to understand the underlying mechanism for arsenic removal. The MoS2/FeOx@BC displayed much higher As(III) adsorption (27 mg/g) compared to MoS2/BC (7 mg/g) and FeOx@BC (12 mg/g). Effects of kinetics, As(III) concentration, temperature, and pH were also investigated. The adsorption of As(III) by MoS2/FeOx@BC followed the Freundlich adsorption isotherm and pseudo-second-order, indicating multilayer adsorption and chemisorption, respectively. The FTIR and XPS analysis confirmed the presence of Fe-O bonds and SO4 groups in the MoS2/FeOx@BC. Electron paramagnetic resonance (EPR) and radical quenching experiments have shown the generation of SO4•- radicals as predominant species in the presence of MoS2 and FeOx in MoS2/FeOx@BC via radical transfer from HO• to SO42-. The HO• and SO4•- radicals synergistically contributed to enhanced As(III) removal. It is envisaged that As(III) initially adsorbed through electrostatic interactions and partially undergoes oxidation, which is finally adsorbed to MoS2/FeOx@BC after being oxidized to As(V). The MoS2/FeOx@BC system could be considered a novel material for effective removal of As(III) from aqueous environments owing to its cost-effective synthesis and easy scalability for actual applications.

Effect of calcium and iron-enriched biochar on arsenic and cadmium accumulation from soil to rice paddy tissues.

Arsenic (As) and cadmium (Cd) are nonessential toxic metal(loids) that are carcinogenic to humans. Hence, reducing the bioavailability of these metal(loids) in soils and decreasing their accumulation in rice grains is essential for agroecology, food safety, and human health. Iron (Fe)-enriched corncob biochar (FCB), Fe-enriched charred eggshell (FEB), and Fe-enriched corncob-eggshell biochar (FCEB) were prepared for soil amelioration. The amendment materials were applied at 1% and 2% application rates to observe their alleviation effects on As and Cd loads in rice paddy tissues and yield improvements using pot trials. The FCEB treatment increased paddy yields compared to those of FCB (9-12%) and FEB (3-36%); this could be because it contains more plant essential nutrients than FCB and a lower calcite content than that of FEB. In addition, FCEB significantly reduced brown rice As (AsBR, 29-60%) and Cd (CdBR, 57-81%) contents compared to those of the untreated control (CON). At a 2% application rate, FCEB reduced the average mobility of As (56%) and Cd (62%) in rhizosphere porewater and enhanced root Fe-plaque formation (76%) compared to those of CON. Moreover, the enhanced Fe-plaque sequestered a substantial amount of As (171.4%) and Cd (90.8%) in the 2% FCEB amendment compared to that of CON. Pearson correlation coefficients and regression analysis indicated that two key mechanisms likely control AsBR and CdBR accumulations. First, rhizosphere soil pH and Eh controlled As and Cd availabilities in porewaters and their speciation in the soil. Second, greater Fe-plaque formation in paddy roots grown in the amended soils provided a barrier for plant uptake of the metal(loids). These observations demonstrate that soil amendment with Fe-enriched corncob-eggshell biochar (e.g., 2% FCEB) is a prospective approach for the remediation of metal accumulation from the soil to grain system while simultaneously increasing paddy yield.

Mechanism of novel MoS2-modified biochar composites for removal of cadmium (II) from aqueous solutions.

The purpose of this study was to develop a MoS2-impregnated biochar (MoS2@BC) via hydrothermal reaction for adsorption of cadmium (Cd) from an aqueous solution. The prepared adsorbents were characterized, and their abilities to remove Cd(II) were evaluated. The Langmuir and pseudo-second-order models better described the removal of Cd(II) by MoS2@BC. The prepared MoS2@BC exhibited excellent monolayer adsorption capacity. The S-containing functional groups on MoS2@BC enhanced the adsorption of Cd(II). Multiple Cd(II) sorption mechanisms were identified; including Cd(II)-π interactions, ion exchange, electrostatic interaction, and complexation. The dominant mechanism involved Cd-O (38.3%) bonds and Cd-S complexation (61.7%) on MoS2@BC. The as-prepared MoS2@BC is both economical and efficient, making it an excellent material for environmental Cd(II) remediation.

Utilization of Citrullus lanatus L. seeds to synthesize a novel MnFe2O4-biochar adsorbent for the removal of U(VI) from wastewater: Insights and comparison between modified and raw biochar.

Uranium (U) is a radioactive and highly toxic metal. Its excessive concentrations in the aqueous environments may result in severe and irreversible damage. To fight this hazard, a raw biochar was prepared from Citrullus lanatus L. seeds, then characterized and compared with a MnFe2O4 modified biochar, both tested for U(VI) adsorption from wastewater, which was assayed for the first time in this study. The characterization of the adsorbent materials was performed by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) with elemental mapping, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) techniques. The effects of solution pH, concentration of sorbate and sorbents, temperature, time and ionic strength were assessed as regards their influence on U(VI) adsorption. The experimental adsorption data showed good fit to a pseudo-second-order kinetic model (reaching a value of qe = 15.12 mg g-1, R2 = 0.96 at equilibrium), and to the Langmuir isotherm (achieving a maximum score of qmax = 27.61 mg g-1, R2 = 0.96). The maximum adsorption capacity was found at 318 K. The results of the study indicate that the binding of negatively charged functional groups (carbonyls, hydroxyls, and some carboxylic groups) with MnFe2O4 significantly enhanced U(VI) adsorption. In view of the overall results, it can be concluded that the MnFe2O4 modification of the Citrullus lanatus L. seeds biochar could give an efficient alternative adsorbent for U(VI) removal in a variety of environmental conditions, simultaneously promoting resource utilization and good sustainable management of the materials studied, aiding to protect the environment and human health.

Watering techniques and zero-valent iron biochar pH effects on As and Cd concentrations in rice rhizosphere soils, tissues and yield.

Zero-valent iron amended biochar (ZVIB) has been proposed as a promising material in immobilizing heavy metals in paddy fields. In this study, the impacts of pH of ZVIB (pH 6.3 and pH 9.7) and watering management techniques (watering amount in the order of CON (control, 5/72)>3/72>1-3/72>3/100>1/72, with 5/72, for example, representing irrigation given to 5 cm above soil surface in 72 hr regular interval) on As and Cd bioavailability for rice and its grain yield (YieldBR) were investigated in a pot experiment. Brown rice As (AsBR) content was irrelative to the watering treatments, while significantly decreased (>50%) with the addition of both ZVIB materials. The diminutions of brown rice Cd (CdBR) content as well as the YieldBR were highly dependent on both the soil amendment materials' pH and watering amount. Among all the watering treatments, 3/72 treatment (15% less irrigation water than the CON) with ZVIB 6.3 amendment was the optimum fit for simultaneous reduction of AsBR (50%) and CdBR contents (19%) as well as for significant increment (12%) of the YieldBR. Although high pH (9.7) ZVIB application could also efficiently decrease As and Cd contents in brown rice, it might risk grain yield lost if appropriate (e.g. 3/72 in our study) watering management technique was not chosen. Therefore, ZVIB would be an environmentally friendly option as an amendment material with proper selection of watering management technique to utilize As and Cd co-contaminated arable soils safely for paddy cultivation.

Interaction of liming and long-term fertilization increased crop yield and phosphorus use efficiency (PUE) through mediating exchangeable cations in acidic soil under wheat-maize cropping system.

Low phosphorus use efficiency (PUE) is one of the main problems of acidic soil that limit the crop growth. Therefore, in the present study, we investigated the response of crop yield and PUE to the long-term application of fertilizers and quicklime (CaO) in the acidic soil under wheat-maize rotation system. Treatments included, CK (no fertilization), NP (inorganic nitrogen and P fertilization), NPK (inorganic N, P and potassium fertilization), NPKS (NPK + straw return), NPCa (NP + lime), NPKCa (NPK + lime) and NPKSCa (NPKS + lime). Results showed that, fertilizer without lime treatments, significantly (p ≤ 0.05) decreased soil pH and crop yield, compared to the fertilizer with lime treatments during the period of 2012-2018. Average among years, compared to the CK treatment, wheat grain yield increased by 138%, 213%, 198%, 547%, 688% and 626%, respectively and maize yield increased by 687%, 1887%, 1651%, 2605%, 5047% and 5077%, respectively, under the NP, NPK, NPKS, NPCa, NPKCa and NPKSCa treatments. Lime application significantly increased soil exchangeable base cations (Ca2+ and Mg2+) and decreased Al3+ cation. Compared to the NP treatment, phosphorus use efficiency (PUE) increased by 220%, 212%, 409%, 807% and 795%, respectively, under the NPK, NPKS, NPCa, NPKCa and NPKSCa treatments. Soil pH showed significant negative relationship with exchangeable Al3+ and soil total N. While, soil pH showed significant (p ≤ 0.05) positive relationship with exchangeable Ca2+, PUE and annual crop yield. PUE was highly negatively correlated with soil exchangeable Al3+. In addition, soil exchangeable Ca2+, pH, exchangeable Al3+ and available N were the most influencing factors of crop yield. Therefore, we concluded that lime application is an effective strategy to mitigate soil acidification and to increase PUE through increasing exchangeable base cations and reducing the acidic cations for high crop yield in acidic soil.

The sorbed mechanisms of engineering magnetic biochar composites on arsenic in aqueous solution.

The aim of this study was to produce magnetic biochar for the removal of As (III) from the aquatic environment. Magnetic biochar (MBC) was prepared from corn straw­derived biochar. Pristine biochar (BC) was impregnated with iron oxide and relative analyses were performed on the adsorption capacity of BC's and MBC's. After impregnation, the specific surface area of MBC800-0.6300 increased from 79.66 to 309.7 m2 g-1 and superparamagnetic magnetization was about 9.75 emu g-1 contributed by the contained Fe3O4. Results of MBC800-0.6300 showed maximum adsorption capacity (Qmax) 22.94 mg g-1 for As (III) based on Langmuir model which is 5.71 times higher than the adsorption capacity of BC800 (4.02 mg g-1). The adsorption of As (III) increased significantly due to the successful loading of iron oxide and the increased oxygen functional groups that were confirmed by XPS and FTIR results. The removal of As (III) followed Langmuir isotherm model and pseudo-second-order (R2 ≥ 0.99), indicated that the adsorption rate was monolayer and depended on the chemical adsorption process, respectively. Consequently, the simple preparation procedure and high adsorption performance suggest that MBC800-0.6300 could be used as an environment-friendly and extremely effective adsorbent for As (III) removal from aqueous environment.

Efficient As(III) Removal by Novel MoS2-Impregnated Fe-Oxide-Biochar Composites: Characterization and Mechanisms.

Sorbents that efficiently eliminate toxic metal(loid)s from industrial wastes are required for the protection of the environment and human health. Therefore, we demonstrated efficient As(III) removal by novel, eco-friendly, hydrothermally prepared MoS2-impregnated FeO x @BC800 (MSF@BC800). The properties and adsorption mechanism of the material were investigated by X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The synergistic effects of FeO x and MoS2 on MSF@BC800 considerably enhanced As(III)-removal efficiency to ≥99.73% and facilitated superior As(III) affinity in aqueous solutions (K d ≥ 105 mL g-1) compared to those of FeO x @BC800 and MS@BC800, which showed 37.07 and 17.86% As(III)-removal efficiencies and K d = 589 and 217 mL g-1, respectively, for an initial As(III) concentration of ∼10 mg L-1. The maximum Langmuir As(III) sorption capacity of MSF@BC800 was 28.4 mg g-1. Oxidation of As(III) to As(V) occurred on the MSF@BC800 composite surfaces. Adsorption results agreed with those obtained from the Freundlich and pseudo-second-order models, suggesting multilayer coverage and chemisorption, respectively. Additionally, MSF@BC800 characteristics were examined under different reaction conditions, with temperature, pH, ionic strength, and humic acid concentration being varied. The results indicated that MSF@BC800 has considerable potential as an eco-friendly environmental remediation and As(III)-decontamination material.

Properties and adsorption mechanism of magnetic biochar modified with molybdenum disulfide for cadmium in aqueous solution.

In this paper, we present the preparation of MoS2-modified magnetic biochar (MoS2@MBC) as a novel adsorbent by a simple hydrothermal method. MoS2@MBC contains abundant S-containing functional groups that facilitate efficient Cd(II) removal from aqueous systems. We employed various characterization techniques to explore the morphology, surface area, and chemical composition of MoS2@MBC; these included Brunauer-Emmett-Teller analysis scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction,. The results indicated the successful decoration of the surface of MoS2@MBC with iron and MoS2, and a higher surface area of MoS2@MBC than that of unmodified biochar. Moreover, adsorption properties including thermodynamics and kinetics were investigated along with the effects of pH, humic acid, and ionic strength on the Cd(II) adsorption onto MoS2@MBC. The O-, C-, S-, and Fe-containing functional groups on the surface of MoS2@MBC led to an electrostatic attraction of Cd(II) and strong Cd-S complexation. The Langmuir and pseudo second-order models fitted best for the batch adsorption experiments results. The adsorption capacity of MoS2@MBC (139 mg g-1 on the basis of the Langmuir model) was 7.81 times higher than that of pristine biochar. The adsorption process was found to be pH-dependent. The experimental results indicated that MoS2@MBC is an effective adsorbent for removing Cd(II) from water solutions. Further, the adsorption process involved the complexation of Cd(II) with oxygen-based functional groups, ion exchange, electrostatic attraction, Cd(II)-π interactions, metal-sulfur complexation, and inner-surface complexation. This work provides new insights into the Cd(II) ions removal from water via adsorption. It also demonstrates that MoS2@MBC is an efficient and economic adsorbent to treat Cd(II)-contaminated water.

Efficient oxidation and adsorption of As(III) and As(V) in water using a Fenton-like reagent, (ferrihydrite)-loaded biochar.

The by-product of the traditional Fenton reaction, colloidal arsenic-­iron oxide, is migratable and may cause secondary environmental pollution. This paper reported a new strategy involving oxidizing and immobilizing inorganic arsenic using the Fenton reaction, and avoiding the risk of secondary contamination. Lab synthesized ferrihydrite-loaded biochar (FhBC) was developed for oxidizing and binding As(III) and As(V) in aqueous solution. Batch experiments and a series of spectrum analysis (e.g., X-ray photoelectron spectroscopy [XPS], electron paramagnetic resonance [EPR], and Fourier transform infrared spectroscopy [FTIR]) were conducted to study the oxidizing or adsorption capacity and mechanism. The maximum adsorption capacity of FhBC for As(III) and As(V) is 1.315 and 1.325 mmol/g, respectively. In addition, FhBC has an efficient oxidizing capacity within a wide pH range, which is because biochar promotes the Fenton reaction by acting as an electron donator, electron shuttler, or by providing persistent free radicals. Moreover, the adsorption mechanism was studied by FTIR spectroscopy, XPS, and X-ray diffraction (XRD). The formation of internal spherical complexes and iron oxides with a higher degree of crystallization was observed, which indicate that the products of adsorption are stable and robust in a complex environment and can exist in a highly crystallized form after adsorbing arsenic ions. Therefore, the use of FhBC as an adsorbent for arsenic represents a new strategy of using the Fenton reaction while reducing secondary contamination. These results may contribute to further mechanistic studies or extensive practical applications of FhBC.

Mechanisms for cadmium adsorption by magnetic biochar composites in an aqueous solution.

There is a demand to develop techniques for the continuous removal/immobilization of heavy metals from contaminated soil and water bodies. In this study, a unique biochar preparation method was developed for the removal of cadmium. First, conventional biochars of corn straw were produced by pyrolysis at two temperatures and then treated using one-step synthesis at different ferric nitrate ratios and different calcination temperatures to produce magnetic biochars. Second, the prepared biochars were used as adsorbents for Cd(II) removal from a solution, and the best one was selected for further evaluation. Various techniques were used to characterize the adsorbents and determine the main adsorption mechanism. The results indicated that the biochars successfully carried iron particles within, which improved the specific surface area, formed inner-sphere complexes with oxygen-containing groups, and increased the number of oxygen-containing groups. The adsorption experiments revealed that MBC800-0.6300 had a higher affinity for Cd(II) than the other adsorbents. Batch adsorption experiments were performed to explore the influence of the kinetics, isotherm, pH, thermodynamics, ionic strength, and humic acid on Cd(II) adsorption. The results indicated that the Langmuir model fit the Cd(II) adsorption best with MBC800-0.6300 having the highest adsorption capacity (46.90 mg g-1). The sorption kinetics of Cd(II) on the adsorbent follows a pseudo-second-order kinetics model. Because MBC800-0.6300 is loaded with metal ions, it can be conveniently collected by a magnet. Thus, biochar modification methods with ferric nitrate impregnation provide an excellent approach to eliminating Cd(II) from aqueous solutions. The possible adsorption mechanisms include chemisorption, electrostatic interaction, and monolayer adsorption.

Enhanced As(III) removal from aqueous solution by Fe-Mn-La-impregnated biochar composites.

A novel Fe-Mn-La-impregnated biochar composite (FMLBC) was synthesized using an impregnation method for efficient As (III) adsorption. The pseudo-second-order model (R2 values are 0.996, 0.996, and 0.994 for different FMLBC rate used) better fitted the kinetic adsorption of arsenic (As) on the FMLBC than the pseudo-first order model (R2 values are 0.978, 0.971, and 0.991 respectively). The SEM-EDS, FTIR and XPS results confirmed the addition of Fe, Mn and La into the BC structure. Compared with pristine biochar (3.73 mg g-1) and Fe-Mn-impregnated biochar (9.48 mg g-1), the As (III) adsorption capacity of Fe-Mn-La impregnated biochar (14.9 mg g-1) was significantly improved. The presence of NO3- and SO42- did not influence As adsorption, whereas PO43- influenced As removal. The mechanism of As adsorption on the FMLBC involved oxidization, electrostatic attraction, ligand exchange, and formation of an inner-sphere R-O-As complex. Among them, the electrostatic attraction and inner-sphere complexation contributed the most. The simple preparation process and high adsorption performance suggest that the FMLBC could be served as a promising adsorbent for As (III) removal from aqueous solution.

Effects of biodegradable chelator combination on potentially toxic metals leaching efficiency in agricultural soils.

Soil washing with chelators, a viable method for treating soils contaminated with potentially toxic metals, has drawn increasing attentions. The objective of this study was to determine a new generation of mixed degradable chelating agents from N, N-bis (carboxymethyl) glutamic acid (GLDA), [S, S]-stereoisomer of ethyleneiaminedisucc--inic acid (EDDS), nitrilotriacetic acid (NTA), and citric acid (CA), and to evaluate its effectiveness and feasibility to reduce toxic metals contamination in two different agricultural soils. A comparative leaching test conducted on the four individual degradable chelating agents showed that the capacity of single chelator in mobilizing copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb) varied significantly. Using a combination of GLDA and NTA was more advantageous than using a single chelating agent in extracting potentially toxic metals. The removal efficiencies of Cu, Zn, Cd, and Pb reached 38.2, 9.8, 71.4, and 19.5% for soil 1, and 25.0, 5.2, 59.7, and 18.5% for soil 2, respectively, at mixed chelator (MC) concentrations of 3 mmol/L (GLDA) and 2 mmol/L (NTA), pH of 6.0, and a contact time of 4.0 h. The effects of washing conditions, chelator concentration, pH values, and contact time on the removal efficiencies of target toxic metals were investigated. The results showed that the combined chelating agent has a lower pH dependence, making it feasible for a wider range of applications. The effects of the chelating agents on the morphological distribution of potentially toxic metals and the soil enzyme activity before and after the treatments were also studied. After washing, the content of the water-soluble, acid-soluble, reducible, and oxidizable target metals showed a certain degree of decrease. Although the activities of catalase, urease, and invertase appeared to be inhibited during a short period of time, their activities were stimulated and later promoted with the degradation of the chelating agent. In general, the chelating agent combination has a great potential for toxic metals leaching.

Arsenic volatilization in flooded paddy soil by the addition of Fe-Mn-modified biochar composites.

We investigated the potential role of Fe-Mn-modified biochar composites (FMBCs) in the volatilization of toxic arsenic (As) in flooded paddy soil, by considering As fractionation, enzyme activities, and bacterial abundance. The results indicated that the addition of FMBCs reduced As volatilization from polluted soil, and this effect was more pronounced at higher dosages. Two types of FMBCs (i.e., FMBC1 and FMBC2) were analyzed, and FMBC2 exhibited a superior performance to FMBC1. Maximum volatilization was achieved in the fourth week and was followed by stabilization. In addition, the majority of As in the soil corresponded to crystalline and residual phases. Furthermore, the addition of FMBCs had little influence on the activities of various enzymes, although FMBC1 significantly affected catalase and peroxidase activities (P < 0.05). Moreover, FMBC application changed the relative abundances of different bacteria, where the abundances of Firmicutes were reduced, but a 2 g dose of FMBCs in highly polluted soil increased the bacterial abundance. In addition, the As volatilization, As fractionation, and enzyme activities displayed some correlation, in that As volatilization was negatively correlated to the presence of residual As phases but positively correlated to amorphous and poorly crystalline hydrous oxides of Fe and Al. As such, As fractionation and an improvement in soil properties are important mechanisms for reducing As volatilization.

Fe-Mn-Ce oxide-modified biochar composites as efficient adsorbents for removing As(III) from water: adsorption performance and mechanisms.

In this study, a novel Fe-Mn-Ce oxide-modified biochar composite (FMCBC) was synthesized via pyrolysis to enhance the adsorption capacity of biochar (BC). Scanning electron microscopy-energy-dispersive X-ray spectroscopy confirmed that Fe, Mn, and Ce were successfully loaded onto the surface of the BC. A series of adsorption experiments showed that the FMCBC exhibited improved adsorption of As(III) in an aqueous environment. The adsorption process was well expressed by the pseudo-second-order kinetic model. The adsorption capacity of FMCBC reached 8.74 mg L-1, which was 3.27 times greater than that of BC. The pH of the solution significantly influenced the adsorption of As(III), where the amount of As(III) adsorbed by FMCBC was maximized at pH 3. A high phosphate concentration inhibited adsorption, whereas nitrate and sulfate ions promoted As(III) adsorption and increased the FMCBC adsorption capacity. Similarly, with increasing humic acid concentration, the adsorption capacity of FMCBC for As(III) decreased; however, a low concentration of humic acid promoted adsorption. X-ray photoelectron spectroscopy analysis revealed that the adsorption of As(III) by FMCBC occurred through redox and surface complexation reactions. Therefore, FMCBC has excellent potential for purifying arsenic-contaminated water.