Enhanced removal of Cr(VI) by nitrogen-doped hydrochar prepared from bamboo and ammonium chloride

Effect of oxygen-containing functional group contents on sorption of lead ions by acrylate-functionalized hydrochar

The surface functional groups of hydrochar are crucial to its surface properties, and their contents are strongly positively correlated with the adsorption performance. In this study, acrylate-functionalized hydrochar (AHC) with varying contents of O-containing functional groups (OFGs) was synthesized via hydrothermal carbonization (HTC) of bamboo, acrylic acid and an initiator, and then deprotonated with NaOH. The AHCs were analyzed by various characterization techniques. During HTC, the higher amount of acrylic acid added led to higher carbon, oxygen and carboxyl contents, and to the larger specific surface area and pore volume of AHC. The adsorption kinetics, isotherms, thermodynamic, ionic strength and pH effects of Pb(II) on AHC were studied. Adsorption isotherms and kinetics obeyed Langmuir and pseudo-second-order models, respectively, indicating adsorption is monolayer chemical process. The adsorptive ability was well linearly related to the OFG contents of AHC. When acrylic acid was added to 25 mL during HTC, the adsorbing ability of AHC over Pb(II) reached 193.90 mg g-1. Hence, direct HTC of acrylic acid, biomass and an initiator can prepare hydrochar with controllable OFG contents, which is a prospective adsorbent for treating metal cations.

Amino-Functionalized Lotus Stem Hydrochar for Rapid Adsorption and In Situ Detoxification of Cr(VI) from Water

The development of low-cost, efficient, and environmentally friendly adsorbents is the key to highly toxic hexavalent chromium [Cr(VI)] removal by adsorption. In this paper, amino-functionalized lotus stem hydrochar (ALSHC) was prepared from an agricultural waste lotus stem (LS) for the adsorption removal of Cr(VI) from water. The effects of the initial Cr(VI) concentration, contact time, temperature, coexisting anions, and reusability of ALSHC on Cr(VI) removal were examined in detail. The adsorption mechanism was further discussed by investigating the impact of the solution's initial pH, the relation between the pH change in solution and Cr(VI) removal during the process, the changes of chromium (Cr) species in solution and on ALSHC during adsorption, and the XPS characterization. The results demonstrated that ALSHC effectively removed Cr(VI) from water with rapid adsorption (the removal rate reached 80.90% in only 10 min) and in situ detoxification. Most importantly, ALSHC still had better adsorption performance (adsorption capacity of 30.95 mg g-1) than commercially activated carbon, even at pH = 9.00. The adsorption of Cr(VI) by ALSHC accorded with the pseudo-second-order kinetic model and Langmuir isotherm model, indicating a monolayer chemisorption process. The adsorption process was shown to be spontaneous and endothermic based on the thermodynamic characteristics (ΔG0 < 0, ΔH0 > 0, and ΔS0 > 0). The mechanism of Cr(VI) removal was mainly composed of three parts in sequence: Firstly, Cr(VI) in solution was quickly adsorbed onto ALSHC with protonated -NH2 through electrostatic attraction; subsequently, the adsorbed Cr(VI) on ALSHC was mostly detoxicated by in situ reduction; and finally, the reduced Cr(III) and the remaining Cr(VI) were fixed on the ALSHC surface by complexation. The prepared ALSHC displayed a certain superiority in Cr(VI) adsorption and had the prospect of further development.

Chromium immobilization from wastewater via iron-modified hydrochar: Different iron fabricants and practicality assessment

The recycling of industrial solid by-products such as red mud (RM) has become an urgent priority, due to their large quantities and lack of reutilization methods can lead to resource wastage. In this work, RM was employed to fabricate green hydrochar (HC) to prepare zero-valent iron (ZVI) modified carbonous materials, and conventional iron salts (IS, FeCl3) was applied as comparison, fabricated HC labeled as RM/HC and IS/HC, respectively. The physicochemical properties of these HC were comprehensively characterized. Further, hexavalent chromium (Cr(VI)) removal performance was assessed (375.66 and 337.19 mg/g for RM/HC and IS/HC, respectively). The influence of dosage and initial pH were evaluated, while isotherms, kinetics, and thermodynamics analysis were also conducted, to mimic the surface interactions. The stability and recyclability of adsorbents also verified, while the practical feasibility was assessed by bok choy-planting experiment. This work revealed that RM can be used as a high value and green fabricant for HC the effective removal of chromium contaminants from the wastewater.

A review: Hydrochar as potential adsorbents for wastewater treatment and CO2 adsorption

To valorize the biomass and organic waste, hydrothermal carbonization (HTC) stands out as a highly efficient and promising pathway given its intrinsic advantages over other thermochemical processes. Hydrochar, as the main product obtained from HTC, is widely applied as a fuel source and soil conditioner. Aside from these applications, hydrochar can be either directly used or modified as bio-adsorbents for environmental remediation. This potential arises from its tunable surface chemistry and its suitability to act as a precursor for activated or engineered carbon. In view of the importance of this topic, this review offers a thorough examination of the research progress for using hydrochar and its modified forms to remove organic dyes (cationic and anionic dyes), heavy metals, herbicides/pesticides, pharmaceuticals, and CO2. The review also sheds light on the fundamental chemistry involved in HTC of biomass and the major analytical techniques applied for understanding surface chemistry of hydrochar and modified hydrochar. The knowledge gaps and potential hurdles are identified to highlight the challenges and prospects of this research field with a summary of the key findings from this review. Overall, this article provides valuable insights and directives and pinpoints the areas meriting further investigation in the application potential of hydrochar in wastewater management and CO2 capture.

KOH-modified bamboo charcoal loaded with α-FeOOH for efficient adsorption of copper and fluoride ions from aqueous solution

In this work, bamboo charcoal (BC) is prepared by pyrolysis of bamboo. Then, KOH modification and surface deposition of Goethite (α-FeOOH) are performed to obtain a new KOH-modified BC loaded with α-FeOOH (FKBC) adsorbent for copper (Cu2+) and fluoride (F-) ion adsorption from aqueous solution. Surface morphology and physiochemical properties of the prepared adsorbent are characterized by scanning electron microscopy-energy dispersive spectrometer, X-ray diffraction, and N2 adsorption-desorption. The effect of pH, contact time, adsorbent dosage, and initial concentration on Cu2+ and F- adsorption is also investigated. In addition, adsorption kinetics and isotherms are fitted to pseudo-second-order kinetics and Langmuir model, respectively. Thermodynamic parameters suggest that the adsorption process is spontaneous and endothermic. The adsorption mechanism is further characterized by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The Cu2+ absorption mainly occurs through ion exchange, coordination reactions, and surface precipitation, while the F- adsorption mainly occurs via ion exchange and hydrogen bonding. The selective adsorption experiments reveal that FKBC has good selectivity for Cu2+ and F-. The adsorption-desorption experimental results indicate that FKBC can be reused for Cu2+ and F- adsorption after regeneration. Results indicate that FKBC can be a promising adsorbent for Cu2+ and F- removal from aqueous solutions.

Enhanced removal of Cr(VI) by polyethyleneimine-modified bamboo hydrochar

Hydrochar is an environmentally friendly and cheap adsorbent, but its adsorption amounts for anions is very limited. The functionalized hydrochar can overcome this shortcoming. Herein, polyethyleneimine-modified hydrochar (PEI-HC) was synthesized from hydrothermal carbonization (HTC) of methyl acrylate and bamboo after addition of initiator ammonium persulfate, and then modified by polyethyleneimine (PEI), which was used to treat Cr(VI). PEI-HC was tested by XANES, EXAFS, SEM-EDS, XPS, FTIR, N2 sorption isotherms, zeta potential, and elemental analyses. The characterizations showed that PEI was successfully grafted onto hydrochar, and the PEI-HC was rich in N and O functional groups, which presented high Cr(VI) sorption ability (528.41 mg·g-1 at pH 2). The bath experiments found the pseudo-second-order kinetic and Freundlich equations can well describe the adsorption kinetics and isotherm of the Cr(VI) adsorption onto PEI-HC, respectively. Electrostatic interaction, reduction, complexation, and H-bonding are the main removal mechanisms as supported by XANES, EXAFS, XPS, and FTIR. This study provides a strategy of combining HTC and free radical graft polymerization to convert agricultural and forestry wastes into functionalized hydrochar, showing highly efficient removal of Cr(VI).

Removal of methylene blue by porous biochar obtained by KOH activation from bamboo biochar

A series of activated biochar (KBBC-700, KBBC-800 and KBBC-900) which were modified by KOH and pyrolysis at various temperatures from ball-milling bamboo powder were obtained. The physicochemical properties and pore structures of activated biochar were investigated by scanning electron microscopy (SEM), fourier transform infrared spectoscopy (FT-IR), X-ray diffraction (XRD) and N2 adsorption/desorption. The adsorption performance for the removal of methylene blue (MB) was deeply studied. The results showed that KBBC-900 obtained at activation temperature of 900 °C exhibited a great surface area which reached 562 m2/g with 0.460 cm3/g of total pore volume. The enhancement of adsorption capacity could be ascribed to the increase of surface oxygen-containing functional groups, aromatization and mesoporous channels. The adsorption capacity was up to 67.46 mg/g under the optimum adsorption parameters with 2 g/L of adsorbent dose, 11 of initial solution pH and 298 K of the reactive temperature. The adsorption capacity was 70.63% of the first time after the material was recycled for three cycles. The kinetics indicated that the adsorption equilibrium time for MB on KBBC-900 was of about 20 min with the data fitted better to the pseudo-second-order kinetics model. The adsorption process was mainly dominated by chemical adsorption. Meanwhile, the adsorption isotherm showed that the Langmuir model fitted the best, and thermodynamic parameters revealed that the adsorption reaction was the endothermic nature and the spontaneous process. Adsorption of MB mainly attributed to electrostatic interactions, cation-π electron interaction and redox reaction. This study suggested that the activated biochar obtained by KOH activation from bamboo biochar has great potentials in the practical application to remove MB from wastewater.

Prediction of heavy metals adsorption by hydrochars and identification of critical factors using machine learning algorithms

Hydrochar has become a popular product for immobilizing heavy metals in water bodies. However, the relationships between the preparation conditions, hydrochar properties, adsorption conditions, heavy metal types, and the maximum adsorption capacity (Q_m) of hydrochar are not adequately explored. Four artificial intelligence models were used in this study to predict the Q_m of hydrochar and identify the key influencing factors. The gradient boosting decision tree (GBDT) showed excellent predictive capability for this study (R²=0.93, RMSE=25.65). Hydrochar properties (37%) controlled heavy metal adsorption. Meanwhile, the optimal hydrochar properties were revealed, including the C, H, N, and O contents of 57.28-78.31%, 3.56-5.61%, 2.01-6.42%, and 20.78-25.37%. Higher hydrothermal temperatures (>220 °C) and longer hydrothermal time (>10 h) lead to the optimal type and density of surface functional groups for heavy metal adsorption, which increased the Q_m values. This study has great potential for instructing industrial applications of hydrochar in treating heavy metal pollution.

Nitrogen-doped hydrochar prepared by biomass and nitrogen-containing wastewater for dye adsorption: Effect of nitrogen source in wastewater on the adsorption performance of hydrochar

Dye wastewater has become one of the main risk sources of environmental pollution due to its high toxicity and difficulty in degradation. Hydrochar prepared by hydrothermal carbonization (HTC) of biomass has abundant surface oxygen-containing functional groups, and therefore is used as an adsorbent to remove water pollutants. The adsorption performance of hydrochar can be enhanced after improving its surface characteristics through nitrogen-doping (N-doping). In this study, wastewater rich in nitrogen sources such as urea, melamine and ammonium chloride were selected as the water source for the preparation of HTC feedstock. The N atoms were doped in the hydrochar with a content of 3.87%-5.70%, and mainly in the form of pyridinic-N, pyrrolic-N and graphitic-N, which changed the acidity and basicity of the hydrochar surface. The N-doped hydrochar adsorbed methylene blue (MB) and congo red (CR) in wastewater through pore filling, Lewis acid-base interaction, hydrogen bond, and π-π interaction, and the maximum adsorption capacities of those were obtained with 57.52 mg/g and 62.19 mg/g, respectively. However, the adsorption performance of N-doped hydrochar was considerably affected by the acid-base property of the wastewater. In a basic environment, the surface carboxyl of the hydrochar exhibited a high negative charge and thus an enhanced electrostatic interaction with MB. Whereas, the hydrochar surface was positively charged in an acid environment by binding H⁺, resulting in an enhanced electrostatic interaction with CR. Therefore, the adsorption efficiency of MB and CR by N-doped hydrochar can be tuned by adjusting the nitrogen source and the pH of the wastewater.

Synergistic effect in simultaneous removal of cationic and anionic heavy metals by nitrogen heteroatom doped hydrochar from aqueous solutions

Industrial wastewater typically contains both cationic and anionic heavy metals; therefore, their simultaneous removal must be considered to ensure environmental sustainability. Herein, nitrogen heteroatom (N) doped hydrochar derived from corncob was prepared via facile NH₄Cl-aided hydrothermal carbonization and used for the simultaneous adsorption of divalent copper (Cu(II)) and hexavalent chromium (Cr(VI)) in aqueous solutions. During hydrothermal carbonization, NH₄Cl played a vital role as the porogen and N dopant, which contributed to the efficient adsorption affinity toward coexisting Cu(II) and Cr(VI). The theoretical maximum adsorption capacities of the N-doped hydrochar were determined to be 1.223 mmol/g for Cu(II) and 1.995 mmol/g for Cr(VI), which were much better than those of the pristine hydrochar. Furthermore, in the binary-component system, the synergistic effect between Cu(II) and Cr(VI) significantly promoted the adsorption affinity of N-doped hydrochar, resulting in adsorption capacities for Cu(II) and Cr(VI) 9.48 and 1.92 times higher than those of the single-component system, respectively. A series of adsorption experiments and spectroscopic analyses demonstrated that multiple mechanisms, including electrostatic shielding, cation bridging, and redox reactions, mutually contributed to the synergistic effect in the adsorption of coexisting Cu(II) and Cr(VI). Overall, the N-doped hydrochar proved to be effective in simultaneously removing both cationic and anionic heavy metal pollutants.

Capacitive deionization of high concentrations of hexavalent chromium using nickel-ferric-layered double hydroxide/molybdenum disulfide asymmetric electrode

To decontaminate wastewater affected by high concentrations of aqueous hexavalent chromium (Cr(VI)) and improve the capability of layered double hydroxide (LDH) as an electrode in the capacitive deionization (CDI) process, nickel-ferric-LDH (NiFe-LDH) and NiFe-LDH/molybdenum disulfide (NiFe/MoS₂) were synthesized using a hydrothermal method. Characterization results indicated that the flower-like cluster framework of MoS₂ was decorated with the NiFe-LDH. Addition of MoS₂ improved the conductivity, capacitance reversibility, charge efficiency, coulombic efficiency, and stability of NiFe/MoS₂. The CDI performance of aqueous Cr(VI) was evaluated using NiFe/MoS₂ and activated carbon as the anode and cathode, respectively. The process reached equilibrium within 240 min. The deionization capacity and removal ratio for Cr(VI) (100 mg/L, 100 mL) were 49.71 mg/g and 99.42 %, respectively, at 1.2 V and 20 mL/min. The isothermal data were accurately described using the Langmuir model, and the theoretical maximum deionization capacity of NiFe/MoS₂ for Cr(VI) was 106.2 mg/g. The interaction mechanisms included electrostatic attraction, surface complexation, and reduction. These findings indicate that NiFe/MoS₂ has feasible applications in practical wastewater treatment for Cr(VI) removal.

Interactions between Cr(VI) and the hydrochar: The electron transfer routes, adsorption mechanisms, and the accelerating effects of wood vinegar

Conversion of the low-valued invasive plant biomass into high-grade carbonaceous materials may provide a novel strategy to tackle the global issues of climate changes and exotic plant invasion. In this study, the hydrochar was fabricated from the biomass of Eupatorium adenophorum spreng. via hydrothermal carbonization (HTC) process to remove Cr(VI). The adsorption thermodynamics and kinetics were investigated via batch experiments, and the electron transfer routes and adsorption mechanisms were further revealed based on systematic characterization. The adsorption isotherms were well fitted by the Langmuir model with a maximum adsorption amount of 7.76 mg/g. The adsorption was spontaneous, and the surface adsorption and intraparticle diffusion may be the speed-limiting steps. Both -OH group and furan structures may donate the electrons to reduce Cr(VI), and the adsorption was governed by the surface complexation with the oxygen-containing functional groups including hydroxyl and carboxyl. Furthermore, the wood vinegar, as the by-product, can significantly accelerate the reduction rate of Cr(VI). Thus, this study provided a new strategy to fabricate carbonaceous materials which may facilitate to boost the carbon neutrality and control of invasive plants.

Synergistic reduction of Cr(VI) by graphite N and thiophene S of N, S-co-doped hydrochar derived from waste straw

An efficient, simple, and inexpensive N, S-co-doped hydrochar (SNHC) was synthesized from waste straw by a one-pot hydrothermal process without calcination for the removal of Cr(VI). SNHC demonstrated excellent adsorption performance for Cr(VI) and high stability, achieving a high capacity of 171.33 mg/g (293 K, pH 2) and a capacity retention of 82.73 % after five cycles. The adsorption behavior was determined as a multilayer adsorption process based on chemisorption according to the simulation the results of Freundlich adsorption isotherms and pseudo-second-order models. The characterization of SNHC revealed that graphite N and thiophene S formed by the material were the effective active sites, functioning as electron donors to contribute a significant amount of electrons to reduce Cr(VI) to Cr(III). Therefore, next to electrostatic adsorption and complexation, the synergistic reduction of Cr(VI) by graphite N and thiophene S was the main mechanism for Cr(VI) removal. Additionally, density functional theory calculations indicated a low adsorption energy of thiophene S, which increased the attractive interaction between SNHC and Cr(VI) and played the most important role in reducing Cr(VI). The mechanism of the effect of graphite N and thiophene S on Cr(VI) removal not only offered a comprehensive perspective on the role of N, S co-doped mediation in hydrochar but also provided the basic theory for its practical application.

Electrochemical Removal of Nitrogen Compounds from a Simulated Saline Wastewater

In the last few years, many industrial sectors have generated and discharged large volumes of saline wastewater into the environment. In the present work, the electrochemical removal of nitrogen compounds from synthetic saline wastewater was investigated through a lab-scale experimental reactor. Experiments were carried out to examine the impacts of the operational parameters, such as electrolyte composition and concentration, applied current intensity, and initial ammoniacal nitrogen concentration, on the total nitrogen removal efficiency. Using NaCl as an electrolyte, the N_TOT removal was higher than Na₂SO₄ and NaClO₄; however, increasing the initial NaCl concentration over 250 mg·L^-1 resulted in no benefits for the N_TOT removal efficiency. A rise in the current intensity from 0.05 A to 0.15 A resulted in an improvement in N_TOT removal. Nevertheless, a further increase to 0.25 A led to basically no enhancement of the efficiency. A lower initial ammoniacal nitrogen concentration resulted in higher removal efficiency. The highest N_TOT removal (about 75%) was achieved after 90 min of treatment operating with a NaCl concentration of 250 mg·L^-1 at an applied current intensity of 0.15 A and with an initial ammoniacal nitrogen concentration of 13 mg·L^-1. The nitrogen degradation mechanism proposed assumes a series-parallel reaction system, with a first step in which NH₄⁺ is in equilibrium with NH₃. Moreover, the nitrogen molar balance showed that the main product of nitrogen oxidation was N₂, but NO₃^- was also detected. Collectively, electrochemical treatment is a promising approach for the removal of nitrogen compounds from impacted saline wastewater.

Carbide-derived carbon as an extraordinary material for the removal of chromium from an aqueous solution

In the present work, the adsorptive removal of chromium (Cr) from water by carbide-derived carbon (CDC) was investigated. The morphology and structure of the CDC were characterized by using FTIR, SEM, TEM, XRD, and N₂ adsorption-desorption measurements. The effect of adsorption parameters including contact time, initial Cr concentration, temperature, initial solution pH, and CDC dosage was examined on the removal of Cr ions. The kinetic analysis revealed that the experimental data on the removal of Cr ions on CDC is well correlated with the pseudo-second order kinetic model (with R² > 0.999), while the equilibrium data were fitted by the Redlich-Peterson isotherm model (with R² > 0.992). The Langmuir and Sips models were also in good compliance with the equilibrium data, indicating a monolayer coverage of Cr ions onto the CDC surface with some heterogeneous active adsorption sites. The CDC revealed a notable Langmuir adsorption capacity of 159.1 mg/g for Cr ions at pH 6 and room temperature. The thermodynamic analysis illustrated that the Cr ions elimination by CDC is a feasible adsorption process and endothermic in nature. After five adsorption/desorption cycles, less than 18% reduction in the adsorption capacity was obtained indicating the stability and reusability of the CDC. Moreover, the CDC demonstrated an excellent potential in removing the Cr ions from real brackish water. According to the adsorption data, both physical and chemical adsorption processes occurred, and the adsorption was mainly controlled by electrostatic interactions with a possible reduction of hexavalent Cr to trivalent Cr at acidic conditions.

Efficient Removal of Pb2+ from Water by Bamboo-Derived Thin-Walled Hollow Ellipsoidal Carbon-Based Adsorbent

Lead ion (Pb²⁺) is one of the most common water pollutants. Herein, with bamboo as the raw material, we fabricate a thin-walled hollow ellipsoidal carbon-based adsorbent (CPCs900) containing abundant O-containing groups and carbon defects and having a specific surface area as large as 730.87 m² g^-1. CPCs900 shows a capacity of 37.26 mg g^-1 for adsorbing Pb²⁺ in water and an efficiency of 98.13% for removing Pb²⁺ from water. This is much better than the activated carbon commonly used for removing Pb²⁺ from water (12.19 mg g^-1, 30.48%). The bond interaction of Pb²⁺ with the O-containing groups on CPCs900 and the electrostatic interaction of Pb²⁺ with the electron-rich carbon defects on CPCs900 could be the main forces to drive Pb²⁺ adsorption on CPCs900. The outstanding adsorption performance of CPCs900 could be due to the abundant O-containing groups and carbon defects as well as the large specific surface area of CPCs900. Bamboo has a large reserve and a low price. The present work successfully converts bamboo into adsorbents with outstanding performances in removing Pb²⁺ from water. This is of great significance for meeting the huge industrial demand on highly efficient adsorbents for removing toxic metal ions from water.

Flexible and easy-handling pristine polypyrrole membranes with bayberry-like vesicle structure for enhanced Cr(VI) removal from aqueous solution

Polypyrrole has been extensively explored for Cr(VI) removal from wastewater towing to the advantages of superior performance, low cost, facile synthesis, and high environmental stability. However, the unsatisfactory adsorption capacity and complicated process of adsorbent separation from aqueous solutions remain a huge challenge, limiting its practical application. Herein, a flexible PPy membrane with bayberry-like vesicle structures (PPy-B) was prepared via template-assisted interfacial polymerization. It was found that sodium sulfosalicylate not only improved the flexibility and strength of the PPy-B membrane for easy-handling but also participated in the polymerization of PPy as a dopant to improve the specific surface area and doping level for increasing adsorption sites. Benefiting from these, the easy-handling PPy-B membrane exhibited a high adsorption capacity (586.90-682.50 mg/g at 298-318 K), a high reusability (five adsorption-desorption cycles), and a high ultimate adsorption capacity after adsorption-desorption cycles until membrane failure (1174.86 mg/g at 298 K). The proposed mechanisms of the enhanced Cr(VI) removal involve electrostatic adsorption, reduction, and ion exchange. This flexible PPy membrane therefore shows attractive advantages in wastewater treatment.

Efficient removal of aqueous Cr(VI) with ferrous sulfide/N-doped biochar composites: Facile, in-situ preparation and Cr(VI) uptake performance and mechanism

FeS nanoparticles loaded on nitrogen-doped biochar (FeS/BNC) were fabricated by pyrolyzing coffee husks pretreated with Mohr's salt. The nitrogen doping and FeS loading of biochar are simultaneously achieved in one-pot pyrolysis. The elemental analysis, SEM, TEM, XRD, XPS, Raman, FTIR and N₂ adsorption-desorption technologies were used to characterize the composition and structure of FeS/NBC. The appraisement for removing aqueous Cr(VI) testified that FeS/NBC offered a synergistic scavenging effect of Cr(VI) by FeS and NBC. The effect of crucial experimental conditions (FeS/NBC dosage, foreign ions, initial pH and concentration of Cr(VI) solution) were investigated. The Cr(VI) removal capacity was as high as 211.3 ± 26 mg g^-1 under the optimized condition. The practicability of FeS/NBC was examined by using simulated actual samples from tap water and lake water. The mechanism examination showed that surface adsorption/reduction and solution reduction were implicated in the removal of Cr(VI). The current work introduces a novel FeS/NBC composite prepared by an in situ pyrolysis method with excellent potential for chromium pollution remediation.

Enhanced removal of Cr(VI) from aqueous solutions by polymer-mediated nitrogen-rich reduced graphene oxide

The efficient removal of heavy metal by rationally designed carbon-based adsorbents is a key challenge in the field of water purification. Herein, we report a nitrogen-enriched lignosulfonate exfoliated graphene oxide (N-LEGO) for hexavalent chromium (Cr(VI)) removal from aqueous solution. The nitrogen content of N-LEGO reached 13.28%, and the ratio of N-bonding configurations (pyri-N:amine-N:pyrro-N:grap-N) was 2.3:1.6:1:2.3. For Cr(VI) with initial concentration of 70 mg L^-1 under pH= 2, the residuary concentration after treated by N-LEGO was close to 0.004 mg L^-1, which meets the industrial wastewater discharge standard. The Cr(VI) adsorption behavior on N-LEGO can be fitted with the pseudo-second-order kinetics and Freundlich isotherm model well. The adsorption mechanism of Cr(VI) on N-LEGO includes anions electrostatic attraction, reduction and surface chelation. Density functional theory (DFT) simulations showed that N atoms doping was feasible and thermodynamically stable, meanwhile the N-doped system was easier to adsorb Cr₂O₇^2- than HCrO₄^-. The findings of this work can provide a new idea for the development of N-doped carbon-based adsorbents for the removal of highly toxic Cr(VI) from aqueous solutions.

Ball-milled bismuth oxybromide/biochar composites with enhanced removal of reactive red owing to the synergy between adsorption and photodegradation

In this paper, bismuth oxybromide (BiOBr)/biochar composites were synthesized by a facile ball milling method for synergistic adsorption and photodegradation of Reactive red 120 (RR120). The characterizations show that ball milling changed the degree of crystallization, increased the surface area, and promoted the charge transfer ability of biochar. The 70% BiOBr/BC composite showed the best removal efficiency for RR120 removal with or without light illumination, which proves its enhanced removal ability by adsorption and photodegradation. The biochar is served as a support of BiOBr for preventing its aggregation and a transporter of charges for promoting the separation of photo-induced carriers in composites. BiOBr can release the adsorption sites on the surface of composites by degradation, which facilitated the RR120 removal and regenerated the photocatalyst for reusing. The strong interactions between BiOBr and biochar in composites resulted from ball milling were beneficial for the charge transfer and synergistic removal of adsorption and degradation. Findings of this work indicate that ball milling method is an effective method to prepare highly efficient biochar-based composites for RR120 removal through synergistic adsorption and photodegradation.

Enhanced removal of Cr(VI) by cation functionalized bamboo hydrochar

Chemical modification on hydrochars can significantly improve their ability of removing heavy metal ions from wastewater, but so far no research has focused on the chemical modification through free radical reaction. In this work, a cation functionalized hydrochar (CFHC) bearing - N⁺H₂R was synthesized by grafting-polymerization of glycidyl methacrylate (GMA) onto bamboo hydrochar under initiation by benzoyl peroxide, followed by the amination with the introduced epoxy group and diethylenetriamine and a subsequent hydrochloric acid treatment. The resulted CFHC exhibited a superior removal capacity of 424.09 mg·g^-1 for Cr(VI), and the highest sorption occurred at pH of 2. Combining a series of characterizations and tests, it was concluded that the sorption conformed to the pseudo-second-order and Freundlich equations, indicating a multilayer chemisorption process that mainly driven by electrostatic reaction, reduction, and surface complexation. This research proved that a free radical polymerization treatment could effectively transform hydrochars into super adsorbents for wastewater treatment.