Investigations of Cr(VI) removal by millet bran biochar modified with inorganic compounds: Momentous role of additional lactate

Synergistic effects of adsorption and chemical reduction towards the effective Cr(VI) removal in the presence of the sulfur-doped biochar material

In this study, the anaerobic sludge withdrawn from thickener in a sewage treatment plant served as the precursor for sludge-based biochar fabrication, which was further modified via sulfur (S) heteroatom doping (i.e., S-BC). The S atom doping resulted in the adjustment of the physicochemical properties towards the carbon material, endowment of abundant functional groups on biochar surface, and increasing the binding sites between biochar and Cr(VI). Compared to the primary biochar (i.e., biochar without heteroatomic doping, named BC), S-BC exhibited a rough surface and possessed remarkable advantages in ash content, specific surface area, and pore volume. The existence of graphene carbon crystal structure for S-BC was confirmed through S-BC by XRD and FTIR analysis. The studies of adsorption kinetics and isotherms showed that pseudo-second-order kinetics and the Langmuir model more fitted the Cr(VI) removal behavior in the presence of S-BC. Therefore, the chemisorption and monolayer adsorption were the primary mechanisms involved in the Cr(VI) removal process. Additionally, XPS analysis results illustrated the aqueous Cr(VI) was efficiently eliminated through the synergistic effect of chemisorption and reduction to Cr(III) in the presence of S-BC. Moreover, S-BC could still achieve the Cr(VI) eliminating efficiency of 85.31% undergoing five cycles with unchanged functional group and crystal structure via FTIR and XRD analysis. Thus, the results of this study may shed light on a new approach for simultaneous economical sludge disposal and the sustainable remediation of the Cr(VI)-contaminated wastewater.

The dual effect of disodium anthraquinone-2,6-disulfonate (AQDS) on the Cr(VI) removal by biochar: The enhanced electron transfer and the inhibited adsorption

Due to large specific surface area, abundant surface functional groups, and stable chemical structure, biochar has been widely used in many environmental fields, including the remediation of Cr pollution. Alternatively, electrochemically active organic matter (e-OM), which is prevalent in both natural environments and industrial wastewater, exerts an inevitable influence on the mechanisms underlying Cr(VI) removal by biochar. The synergistic interplay between biochar and e-OM in the context of Cr(VI) remediation remains to be fully elucidated. In this study, disodium anthraquinone-2,6-disulfonate (AQDS) was used as a model for e-OM, characterized by its quinone group's ability to either donate or accept electrons. We found that AQDS sped up the Cr(VI) removal process, but the enhancement effect decreased with the increase in pyrolysis temperature. With the addition of AQDS, the removal amount of Cr(VI) by BC300 and BC600 increased by 160.0% and 49.5%, respectively. AQDS could release more electrons trapped in the lower temperature biochar samples (BC300 and BC600) for Cr(VI) reduction. However, AQDS inhibited the Cr(VI) removal by BC900 due to the adsorption of AQDS on biochar surface. In the presence of the small molecule carbon source lactate, more AQDS was adsorbed onto the biochar surface. This led to an inhibition of the electron transfer between biochar and Cr(VI), resulting in an inhibitory effect. This study has elucidated the electron transfer mechanism involved in the removal of Cr(VI) by biochar, particularly in conjunction with e-OM. Furthermore, it would augment the efficacy of biochar in applications targeting the removal of heavy metals.

Reduced and oxidized rice straw biochar for hexavalent chromium adsorption: Revisiting the mechanism of adsorption

Surface oxygen functional groups of biochar were tuned by oxidation and reduction of biochar for establishing Cr(VI) adsorption mechanism. Oxygen functional groups (OFGs) on the surface of leached rice straw biochar (LBC4-6) obtained from pyrolysis at 400, 500 and 600 °C, were oxidized to furnish OBC4-6 using modified Hummer's method. Reduced biochar RBC4-6 were obtained by esterification and NaBH4/I2 reduction of oxidized biochar (OBC4-6). The modified biochar were characterized by increase in O/C and H/C ratio, respectively, in case of OBC4-6 and RBC4-6. The Cr(VI) adsorption by modified biochar LBC4-6, OBC4-6, and RBC4-6 showed optimum conditions of pH 3 and dose 0.1 g/L with a good non-linear fit for Langmuir & Freundlich isotherm. The maximum adsorption (Qm) followed the trend: OBC4 (17.47 mg/g) > RBC4 (15.23) > OBC5 (13.23) > LBC4 (10.23) > RBC5 (9.83) > OBC6 (9.60) > RBC6 (7.24) > LBC5 (6.32) > LBC6 (5.98). The adsorption kinetics for adsorption of Cr(VI) on to modified biochar fits pseudo second order (PSO), Elovich and intraparticle diffusion kinetics, showing a chemisorptions in case of biochar L/O/RBC4-6. The lower temperature modified biochar O/RBC4 show better Cr(VI) adsorption. X-ray Photoelectron Spectroscopy (XPS) studies establish optimum OFGs for reduction of Cr(VI) and chelation of the reduced Cr(III). Adsorption and stripping cycles show the oxidized and reduced biochar as better adsorbents with excellent stripping of Cr up to >98 % upon desorption with 1 M NaOH.

Biochar Decreases Cr Toxicity and Accumulation in Sunflower Grown in Cr(VI)-Polluted Soil

Biochar is preferentially recommended for the remediation of heavy metal-polluted soils. Sunflower is an important high-biomass oil crop with a promising potential for phytoremediation of Cr(VI)-polluted soil. However, how biochar affects sunflower growth and Cr accumulation in Cr(VI)-polluted soil needs to be elucidated. Here, a pot culture experiment was conducted to study whether soil amendment with biochar (0, 0.1%, 1%, and 5%, w/w) can mitigate Cr toxicity and accumulation in sunflower seedlings grown in soils artificially polluted with different levels of Cr(VI) (0, 50, and 250 mg Cr(VI)/kg soil). The addition of Cr(VI) exhibited significant phytotoxicity, as evidenced by inhibited plant growth and even the death of seedlings at 250 mg/kg Cr(VI). Overall, biochar amendment showed positive effects on plant growth and Cr immobilization, dependent on both the biochar dose and Cr addition level. When 50 mg/kg Cr(VI) was added, 1% biochar showed positive effects similar to 5% biochar on improving plant growth and mineral nutrition (particularly K), reducing Cr content in shoots and roots, and decreasing Cr availability and Cr(VI) content in the soil. In comparison with non-amendment, 1% and 5% biochar caused 85% and 100% increase in shoot dry weights, and 75% and 86% reduction in shoot Cr concentrations, respectively. When 250 mg/kg Cr(VI) was added, a 5% dose produced much better benefits than 1%, while a 0.1% dose did not help plants to survive. Overall, an appropriate dose of biochar enhanced Cr(VI) immobilization and subsequently decreased its toxicity and accumulation in sunflower seedlings. Our findings confirm that biochar can be used as an efficient amendment for the remediation of Cr(VI)-polluted soils and cleaner production of sunflower oil and biomass.

Effective elimination of hexavalent chromium and lead from solution by the modified biochar with MgMn2O4 nanoparticles: adsorption performance and mechanism

Heavy metal pollution is one of the environmental problems that need to be solved urgently. The adsorption method is thought as the most effective and economical treatment technology. Nature biochar usually showed unsatisfactory adsorption capacity due to its relatively small adsorption capacity and slow adsorption rate. The metal of Mn has been widely applied in the modification of biochar, which could effectively improve the adsorption capacity of biochar. However, leaching of Mn2+ on the adsorbent materials would appear during the adsorption process. And it would increase the risk of secondary pollution. The multifunctional binary modified biochar could improve the adsorption capacity of environmental pollutant removal. In addition, it could also act as a metal support carrier, reducing the risk of secondary pollution. A novel effective biochar loaded by Mg-Mn binary oxide nanoparticles (MgMn2O4@Biochar) was prepared and applied for the Cr(VI) and Pb(II) removal in aqueous solution. The characteristic of MgMn2O4@Biochar was analyzed by SEM, TEM, FTIR, and XRD. The irregular and somewhat flaky shaped particles of different shape and sizes clustered on the surface of MgMn2O4@Biochar appeared. Abundant functional groups of O-H, -C-OH, C-O, and C-OOH could be observed on the surface of MgMn2O4@Biochar. The elements of Mg and Mn elements besides of C, O, and Si elements were presented on the surface of MgMn2O4@Biochar. The wt% of C, O, Mg, Mn, and Si were 42.82%, 48.99%, 2.83%, 4.44%, and 0.93%, respectively. The operational parameters had an important influence on adsorption capacity of Cr(VI) and Pb(II) removal. The results showed that the adsorption capacity of MgMn2O4@Biochar for Cr(VI) and Pb(II) would reach 33.5 mg/g and 536 mg/g, respectively, within 360 min. Additionally, the adsorption processes of Cr(VI) and Pb(II) in solution could be described with pseudo-second-order. For Cr(VI), the Langmuir model was suitable to the adsorption process. However, the adsorption process of Pb(II) in solution could be described with Freundlich model. Furthermore, it could be concluded that the possible mechanism of Cr(VI) and Pb(II) removal by MgMn2O4@Biochar was physical adsorption, surface complexation reaction, and electrostatic adsorption.

Enhanced trichloroethylene biodegradation: The mechanism and influencing factors of combining microorganism and carbon‑iron materials

Trichloroethylene (TCE) is one of the most prevalent contaminants with long-term persistence and a strong carcinogenic risk. Biological dechlorination has gradually become the mainstream method due to its advantages of low treatment cost and high environmental friendliness. However, microorganisms are easily restricted by environmental factors, such as an insufficient energy supply and a slow biological dechlorination process. This study focused on the coupled degradation of TCE with the combination of microorganisms and assistant materials (biochar, nZVI, nZVI modified biochar, HPO₃ modified biochar), and set up microorganisms (alone) and materials (alone) as separate controls. Biochar provided nutrients, increased contact with pollutants, and promoted electron transfer to improve TCE degradation, although it did not change the pathway of degradation. The coupled treatment with anaerobic microorganisms (Micro) and 1 g/L unmodified biochar (BC) had the strongest degradation capacity. Compared with microorganisms alone, the addition of biochar resulted in the complete removal of TCE within 4 days. The influence of ambient temperature was mainly related to microbial activity, and 35 °C showed better degradation than 20 °C. Under 20 °C, 1 g/L of nZVI significantly promoted microbial dechlorination. As the dosage increased to 2 g/L and 4 g/L, nZVI showed a strong toxic effect. After 16 days, TCE was completely converted to ethylene by Micro-BC with C₃H₅O₃Na, while 4.40 μmol dichloroethane (DCE) and 1.48 μmol vinyl chloride (VC) remained in the treatment with Micro-BC alone. As an electron acceptor, NaNO₃ directly competed with TCE in the reduction process, which decreased the reduction efficiency of TCE. These findings provide a better understanding of the mechanism of the chemical materials coupling microbial dechlorination process and an optimal treatment method for trichloroethylene degradation.

Ball milling enhanced Cr(VI) removal of zero-valent iron biochar composites: Functional groups response and dominant reduction species

Zero-valent iron biochar composites (ZVI/BC) have been widely used to remove Cr(VI) from water. However, the application of ZVI/BC prepared by the carbothermal reduction was limited by the non-uniform dispersion of ZVI on the biochar surface. In this work, ball milling technique was introduced to modify ZVI/BC. Results showed that after ball milling, the maximum Langmuir adsorption capacity for Cr(VI) was 117.7 mg g-1 (298 K) which was 2.08 times higher than ZVI/BC. The initial adsorption rate of the Elovich model increased from 4.57 × 102 mg g-1 min-1 to 3.74 × 109 mg g-1 min-1 after ball milling. Dispersibility of ZVI on biochar surface and contact between ZVI and biochar were improved by the ball milling, thus accelerating the electron transfer. Besides, ball milling increased the content of oxygen-containing functional groups in biochar, contributing to the chemisorption of Cr(VI). The response sequence of oxygen-containing functional groups was analyzed by two-dimensional correlation spectroscopy, indicating that Cr(VI) preferentially complexed with phenolic -OH. Shielding experiments showed that Fe (0) was the dominant reducing species with a contribution of 73.4%, followed by surface-bound Fe(II) (21.3%) and dissolved Fe2+ (5.24%). Density functional theory calculations demonstrated that ball milled ZVI/BC improved the adsorption affinity and electron transfer flux towards Cr(VI) by introducing phenolic -OH and Fe (0). Combining all the textural characterization, the Cr(VI) removal mechanism of the ball milled ZVI/BC could be proposed as adsorption, reduction, and precipitation. Eventually, stable Cr-Fe oxides (FeOCr2O3 and Cr1·3Fe0·7O3) were formed. This work not only provides a simple method to modify ZVI/BC to remove Cr(VI) in water efficiently and rapidly, but also improves the mechanistic insight into the Cr(VI) removal by iron-carbon composites via the response sequence of functional group analysis and the quantitative analysis of reducing species.

Biochar for the removal of contaminants from soil and water: a review

Biochar shows significant potential to serve as a globally applicable material to remediate water and soil owing to the extensive availability of feedstocks and conducive physio-chemical surface characteristics. This review aims to highlight biochar production technologies, characteristics of biochar, and the latest advancements in immobilizing and eliminating heavy metal ions and organic pollutants in soil and water. Pyrolysis temperature, heat transfer rate, residence time, and type of feedstock are critical influential parameters. Biochar’s efficacy in managing contaminants relies on the pore size distribution, surface groups, and ion-exchange capacity. The molecular composition and physical architecture of biochar may be crucial when practically applied to water and soil. In general, biochar produced at relatively high pyrolysis temperatures can effectively manage organic pollutants via increasing surface area, hydrophobicity and microporosity. Biochar generated at lower temperatures is deemed to be more suitable for removing polar organic and inorganic pollutants through oxygen-containing functional groups, precipitation and electrostatic attraction. This review also presents the existing obstacles and future research direction related to biochar-based materials in immobilizing organic contaminants and heavy metal ions in effluents and soil.

Graphical Abstract

Fe/C materials prepared by one-step calcination of acidified municipal sludge and their excellent adsorption of Cr(VI)

Biochar derived from municipal sludge can be applied to adsorption. But it usually requires activation and pickling due to the generation of impurities such as metal oxide particles, which is uneconomical. Here, a facile strategy, acidification-one-step calcination, was developed and sludge-based Fe-C materials with good Cr(VI) removal effect were obtained by regulating the amount of hydrochloric acid. The results show that the adsorption capacity of Fe/C-5 (the best sample) for Cr(VI) was 150.84 mg g^-1. According to the Langmuir isotherm and pseudo-second-order kinetic model, the removal of Cr(VI) by Fe/C-5 is spontaneous and endothermic chemisorption process. In addition, Fe/C-5 has good ability to remove Cr(VI) under the interference of coexisting ions, and has good cycle stability. The removal of Cr(VI) by Fe/C-5 is considered to be synergistic process of adsorption and reduction. The Fe atoms were highly dispersed in Fe/C-5 and tightly bonded with C atoms, which not only strengthened the Cr(VI) adsorption by electrostatic attraction, but also activated the C atoms in the biochar material, so that the C atoms can reduce Cr(VI) to Cr(III) under acidic conditions. This may be due to the fact that acid pretreatment converted the iron in municipal sludge in the form of Fe-O/OH to free Fe³⁺ and entered the C lattice during the calcination process. In this work, Fe-C materials with excellent Cr(VI) adsorption capacity were prepared by one-step calcination method, which has important reference significance for the resource utilization of municipal sludge.

Removal of hexavalent chromium via biochar-based adsorbents: State-of-the-art, challenges, and future perspectives

Chromium originates from geogenic and extensive anthropogenic activities and significantly impacts natural ecosystems and human health. Various methods have been applied to remove hexavalent chromium (Cr(VI)) from aquatic environmental matrices, including adsorption via different adsorbents, which is considered to be the most common and low-cost approach. Biochar materials have been recognized as renewable carbon sorbents, pyrolyzed from various biomass at different temperatures under limited/no oxygen conditions for heavy metals remediation. This review summarizes the sources, chemical speciation & toxicity of Cr(VI) ions, and raw and modified biochar applications for Cr(VI) remediation from various contaminated matrices. Mechanistic understanding of Cr(VI) adsorption using different biochar-based materials through batch and saturated column adsorption experiments is documented. Electrostatic interaction and ion exchange dominate the Cr(VI) adsorption onto the biochar materials in acidic pH media. Cr(VI) ions tend to break down as HCrO₄^-, CrO₄^2-, and Cr₂O₇^2- ions in aqueous solutions. At low pH (∼1-4), the availability of HCrO₄^- ions attributes the electrostatic forces of attraction due to the available functional groups such as -NH₄⁺, -COOH, and -OH₂⁺, which encourages higher adsorption of Cr(VI). Equilibrium isotherm, kinetic, and thermodynamic models help to understand Cr(VI)-biochar interactions and their adsorption mechanism. The adsorption studies of Cr(VI) are summarized through the fixed-bed saturated column experiments and Cr-contaminated real groundwater analysis using biochar-based sorbents for practical applicability. This review highlights the significant challenges in biochar-based material applications as green, renewable, and cost-effective adsorbents for the remediation of Cr(VI). Further recommendations and future scope for the implications of advanced novel biochar materials for Cr(VI) removal and other heavy metals are elegantly discussed.

Insights into Cr(VI) removal mechanism in water by facile one-step pyrolysis prepared coal gangue-biochar composite

The acceleration of industrialization has increased the discharge of chromium-containing wastewater, posing serious threat to the eco-environment and human health. To remove Cr(VI) in wastewater and improve resource utilization of solid waste, coal gangue and rape straw were initially used to prepare coal gangue-rape straw biochar (CG-RS) composite. The effects of pyrolysis temperatures, solution pH, coexisting ions of Cr(VI) adsorption were investigated. Different adsorption models combined with site energy analysis were used to explore the adsorption behaviors and mechanisms. The results showed higher pyrolysis temperature (600 °C) prepared CG-RS had a larger adsorption capacity (9.2 mg/g) for Cr(VI) (pH = 5.0). Analysis of XPS indicated that CG-RS successfully loaded with Fe-O and Al-O functional groups, which mainly participated in the reduction of Cr(VI). Site energy analysis further proved that reduction and surface complexation were the main adsorption mechanisms. This study shows an effective removal of Cr(VI) by CG-RS, providing a new way for resource utilization of solid waste.