Removal Efficiency and Mechanism of Cr(VI) from Aqueous Solution by Maize Straw Biochars Derived at Different Pyrolysis Temperatures

Synthesis of High-Entropy Alloys with a Tailored Composition and Phase Structure Using a Single Configurable Target

Previously, refractory high-entropy alloys (HEAs) with high crystallinity were synthesized using a configurable target without heat treatment. This study builds upon prior investigations to develop nonrefractory elemental HEAs with low crystallinity using a novel target system. Different targets with various elemental compositions, i.e., Co20Cr20Ni20Mn20Mo20 (target 1), Co30Cr15Ni25Mn15Mo15 (target 2), and Co15Cr25Cu20Mn20Ni20 (target 3), are designed to modify the phase structure. The elemental composition is varied to ensure face-centered cubic (FCC) or body-centered cubic (BCC) phase stabilization. In target 1, the FCC and BCC phases coexist, whereas targets 2 and 3 are characterized by a single FCC phase. Thin films based on targets 1 and 2 exhibit crystalline phases followed by annealing, as indicated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. In contrast, target 3 yields crystalline thin films without any heat treatment. The thin-film coatings are classified based on the atomic size difference (δ). The δ value for the target with the elemental composition CoCrMoMnNi is 9.7, i.e., ≥6.6, corresponding to an HEA with an amorphous phase. However, the annealed thin film is considered a multiprincipal elemental alloy. In contrast, δ for the CoCrCuMnNi HEA is 5, i.e., ≤6.6, upon the substitution of Mo with Cu, and a solid solution phase is formed without any heat treatment. Thus, the degree of crystallinity can be controlled through heat treatment and the manipulation of δ in the absence of heat treatment. The XRD results clarify the crystallinity and phase structure, indicating the presence of FCC or a combination of FCC and BCC phases. The outcomes are consistent with those obtained through the analysis of the valence electron concentration based on X-ray photoelectron spectroscopy. Furthermore, a selected area electron diffraction analysis confirms the presence of both amorphous and crystalline structures in the HEA thin films. Additionally, phase evolution and segregation are observed at 500 °C.

Experimental Investigation of Cobalt Deposition on 304 Stainless Steel in Borated and Lithiated High-Temperature Water

The radioactive corrosion products ⁵⁸Co and ⁶⁰Co in the primary loops of pressurized water reactors (PWRs) are the main sources of radiation doses to which workers in nuclear power plants are exposed. To understand cobalt deposition on 304 stainless steel (304SS), which is the main structural material used in the primary loop, the microstructural characteristics and chemical composition of a 304SS surface layer immersed for 240 h in borated and lithiated high-temperature water containing cobalt were investigated with scanning electron microscopy (SEM), X-ray diffraction (XRD), laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectrometry (GD-OES), and inductively coupled plasma emission mass spectrometry (ICP-MS). The results showed that two distinct cobalt deposition layers (an outer layer of CoFe₂O₄ and an inner layer of CoCr₂O₄) were formed on the 304SS after 240 h of immersion. Further research showed that CoFe₂O₄ was formed on the metal surface by coprecipitation of the iron preferentially dissolved from the 304SS surface with cobalt ions from the solution. The CoCr₂O₄ was formed by ion exchange between the cobalt ions entering the metal inner oxide layer and (Fe, Ni) Cr₂O₄. These results are useful in understanding cobalt deposition on 304SS and have a certain reference value for exploring the deposition behavior and mechanism of radionuclide cobalt on 304SS in the PWR primary loop water environment.

High Entropy Oxide (CrMnFeNiMg)3 O4 with Large Compositional Space Shows Long-Term Stability as Cathode in Lithium-Sulfur Batteries

The repeated formation and irreversible diffusion of liquid-state lithium polysulfides (LiPSs) are the primary challenges in the development of high-energy-density lithium-sulfur battery (LSB). An effective strategy to alleviate the resulting polysulfide loss is critical for the stability of LSBs. In this regard, high entropy oxides (HEOs) appear as a promising additive for the adsorption and conversion of LiPSs owing to the diverse active sites, offering unparalleled synergistic effects. Herein, we have developed a (CrMnFeNiMg)₃ O₄ HEO as a functional polysulfide trapper in LSB cathode. The adsorption of LiPSs by the metal species (i. e., Cr, Mn, Fe, Ni, and Mg) in the HEO takes place through two different paths and leads to enhanced electrochemical stability. We demonstrate that the optimal sulfur cathode with the (CrMnFeNiMg)₃ O₄ HEO attains a high peak and reversible discharge capacities of 857 mAh g^-1 and 552 mAh g^-1 , respectively, at a cycling rate of C/10, a long cycle life of 300 cycles, and a high rate performance at the cycling rates from C/10 to C/2.

Crosslinked modified chitosan biopolymer for enhanced removal of toxic Cr(VI) from aqueous solution

Two new crosslinked modified chitosan biopolymers, namely (CTS-VAN) and (Fe₃O₄@CTS-VAN) bioadsorbents were prepared starting from chitosan and 4-hydroxy-3-methoxybenzaldehyde (VAN) in presence of epichlorohydrin. The analytical techniques FT-IR, EDS, XRD, SEM and XPS besides BET surface analysis were utilized for full characterization of the bioadsorbents. Batch experiments were conducted to study the effect of various influencing parameters in Cr (VI) removal such as initial pH, contact time, adsorbent amount and initial Cr (VI) concentration. The adsorption of Cr (VI) was found out to be maximum at pH = 3 for both bioadsorbents. Langmuir isotherm fit well the adsorption process with a maximum adsorption capacity of 188.68 and 98.04 mg/g for CTS-VAN and Fe₃O₄@CTS-VAN, respectively. The adsorption process followed pseudo second-order kinetics with R² values of 1 and 0.9938 for CTS-VAN and Fe₃O₄@CTS-VAN, respectively. X-ray photoelectron spectroscopy (XPS) analysis showed that Cr(III) accounted for 83 % of the total Cr bound to bioadsorbents surface, which indicated reductive adsorption was responsible for Cr(VI) removal by the bioadsorbents. Cr(VI) was initially adsorbed on the positively charged surface of the bioadsorbents and reduced to Cr(III) by electrons provided by oxygen-comprising functional groups (e.g., CO), and consequently part of the converted Cr(III) stayed on the surface and the rest released into solution.

Construction of efficient Ni-FeLDH@MWCNT@Cellulose acetate floatable microbeads for Cr(VI) removal: Performance and mechanism

Water pollution is an aggravating dilemma that is extending around the world, threatening human survival. Strikingly, the notorious heavy metals like hexavalent chromium ions (Cr⁶⁺) cause environmental problems raising awareness of the essentials for finding feasible solutions. For this purpose, the self-floating Ni-FeLDH@MWCNT@CA microbeads were prepared for removing Cr⁶⁺. The morphological, thermal, and composition characteristics of Ni-FeLDH@MWCNT@CA microbeads were analyzed using XRD, FTIR, TGA, SEM, XPS, and zeta potential. Notably, the adsorption aptitude of Cr⁶⁺ was enhanced by raising the MWCNTs proportion to 5 wt% in microbeads. The Cr⁶⁺ adsorption onto Ni-FeLDH@MWCNT@CA fitted Langmuir and Freundlich isotherm models with q_m of 384.62 mg/g at pH 3 and 298 K. The adsorption process was described kinetically by the pseudo-2nd order model. More importantly, the adsorption of Cr⁶⁺ onto Ni-FeLDH@MWCNT@CA occurred via electrostatic interactions, inner/outer sphere complexations, ion exchange, and reduction mechanisms. Besides, the cycling test showed the remarkable reusability of Ni-FeLDH@MWCNT@CA floatable microbeads for five subsequent cycles. The self-floating Ni-FeLDH@MWCNT@CA microbeads in this work provide essential support for the potential applications for the remediation of heavy metals-containing wastewater.

Adsorption of Cr(VI) by NaOH-modified microporous activated carbons derived from the wastes of Amaranthus retroflexus, Magnolia soulangeana, and Tanacetum Vulgar L.: mechanism, isotherms, and kinetic studies

The study was designed to synthesize microporous activated carbons from the wastes of three medicinal herbs by NaOH as an activator followed by pyrolysis. The prepared microporous activated carbons R. Weed (Amaranthus retroflexus Redroot pigweed modified with NaOH), S. Bod Magnolia soulangeana Soul-Bod treated with NaOH) and S. TY (Tanacetum Vulgar L. (Tansy) treated with NaOH) were deployed for dangerous Cr(VI) ion remediation from the aquatic system. The synthesized modified biosorbents were described by FT-IR, SEM, EDAX, and BET. Furthermore, it was found that biosorbent made from R. Weed seems to have a surface area of 588.155 m²/g and micropore volume of 0.331Cm³/g whereas biosorbent made from S. Bod and S. TY does have a surface area of 489.613 and 445.615 m²/g respectively. The effects of several variables such as pH, temperature, and contact time were explored. The kinetic studies were accomplished and it was noticed that equilibrium was confirmed at 6 min for R. Weed while at 8 and 10 min for S. Bod and S. TY respectively. The pertinence of different adsorption isotherms like Langmuir, Freundlich, and Temkin was explored. The optimum adsorption capacity for each adsorbent material in respect of monolayer coverage (Q_max) was calculated. The Q_max of Redroot pigweed (R. Weed), Soul-Bod (S. Bod), and Tansy (S. TY) biosorbents were 326.62, 181.69, and 108.14 mg/g respectively. The adsorption kinetics was described using pseudo-first-order and pseudo-second-order equations. Furthermore, it was found that each adsorbent material followed the pseudo-second-order kinetics which affirmed the chemosorption nature of adsorption. The thermodynamic variables that include ΔG, ΔH, and ΔS were determined for each adsorbent material. Moreover, the manufactured R. Weed biosorbent seems to have high recyclability. It will remediate Cr(VI) ions with a maximum remediation efficiency of up to 81% during six cycles of regeneration.

Optimization of Cr (VI) removal from aqueous solution with activated carbon derived from Eichhornia crassipes under response surface methodology

Tannery industries' effluent contains a high concentration of Cr (VI) which has the potential to affect the environment and public health. Therefore, this study aimed to investigate the optimization of Cr (VI) adsorption by activated carbon (AC) derived from Eichhornia crassipes from an aqueous solution. The adsorbent was activated with dilute sulfuric acid followed by thermal activation. AC was characterized using proximate analysis, SEM, FTIR, X-ray diffraction, and the BET method. The Cr (VI) removal optimization process was performed using a central composite design under the response surface methodology. The proximate analysis showed that the moisture content, volatile matter, ash content, and fixed carbon of the activated carbon were 5.6%, 18.2%, 14.4%, and 61.8% respectively. The surface areas of the Eichhornia crassipes before activation, after activation, and after adsorption were 60.6 g/m², 794.2 g/m², and 412.6 g/m² respectively. A highly porous structure with heterogeneous and irregular shapes was observed in the SEM micrograph. In the FTIR analysis, different peaks are indicated with various functional groups. The intensity of XRD peaks decreased as 2 theta values increased, which indicates the presence of an amorphous carbon arrangement. The point of zero charge (pH_pzc) of the activated carbon was found to be 5.20. A maximum Cr (VI) removal of 98.4% was achieved at pH 5, contact time 90 min, adsorbent dose 2 g, and initial Cr (VI) concentration of 2.25 mg/L. Statistically significant interactions (P < 0.05) were observed between the initial Cr (VI) concentration and adsorbent dose as well as the initial Cr (VI) concentration and contact time. Langmuir adsorption isotherm fitted the experimental data best, with an R² value of 0.99. The separation constant (RL) indicates that the adsorption process is favorable. The kinetic experimental data were best fitted with the pseudo-second-order model with an R² value of 0.99 whereas the adsorption rate is controlled by intraparticle and extragranular diffusion processes. Generally, the AC has the potential to be a strong adsorbent candidate for wastewater treatment at the industrial level.

Immobilization on anionic metal(loid)s in soil by biochar: A meta-analysis assisted by machine learning

Metal pollution in soil has become one of the most serious environmental problems in China. Biochar is one of the most widely used remediation agents for soil metal pollution. However, the literature does not provide a consistent picture of the performance of biochar on the immobilization of anionic metal(loid)s, especially arsenic, in soil. To obtain a baseline understanding on the interactions of metals and biochar, 597 data records on four metal(loid)s (As, Cr, Sb and V) were collected from 70 publications for this meta-analysis, and the results are highlighted below. Biochar has a significant immobilization effect on anionic metal(loid)s in soil and reduces the bioavailability of these metals to plants. Subgroup analysis found that biochar could decrease the potential mobility of Cr, Sb and V, but the immobilization effect on As was not always consistent. Meanwhile, biochar pH and soil pH are the most key factors affecting the immobilization effect. To summarize, biochar can effectively immobilize Cr, Sb and V in soil, but more attention should be given to As immobilization in future applications. By regulating the properties of biochar and appropriate modification, anionic metal(loid)s in soil can be immobilized more effectively. Hence, both of the soil quality and crop quality can be improved.

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.

Photoreduction of Cr(VI) in wastewater by anodic nanoporous Nb2O5 formed at high anodizing voltage and electrolyte temperature

In this study, nanoporous anodic film was produced by anodization of niobium, Nb in a fluoride ethylene glycol electrolyte. The effect of anodization voltage and electrolyte temperature was studied to find an optimum condition for circular, ordered, and uniform pore formation. The diameter of the pores was found to be larger when the applied voltage was increased from 20 to 80 V. The as-anodized porous film was also observed to comprise of nanocrystallites which formed due to high field-induced crystallization. The nanocrystallites grew into orthorhombic Nb₂O₅ after post-annealing treatment. The Cr(VI) photoreduction property of both the as-anodized and annealed Nb₂O₅ samples obtained using an optimized condition (anodization voltage: 60 V, electrolyte temperature: 70 °C) was compared. Interestingly, the as-anodized Nb₂O₅ film was found to display better photoreduction of Cr(VI) than annealed Nb₂O_5. However, in terms of stability, the annealed Nb₂O₅ presented high photocatalytic efficiency for each cycle whereas the as-anodized Nb₂O₅ showed degradation in photocatalytic performance when used continually.

A Statistical Modeling and Optimization for Cr(VI) Adsorption from Aqueous Media via Teff Straw-Based Activated Carbon: Isotherm, Kinetics, and Thermodynamic Studies

Currently, the growth of tannery industries causes a significant volume of waste disposal to the environment due to harmful Cr(VI). Long-time exposure to Cr(VI) imposes serious hazards on all living organisms. Hence, the treatment of tannery waste to remove Cr(VI) is not a choice but mandatory. Therefore, this study focused on the removal of Cr(VI) from the aqueous solutions via a teff (Eragrostis tef) straw based-activated carbon (TSAC) which was derived from locally available agricultural solid waste, teff straw (TS). The prepared TSAC was characterized using BET, FTIR, SEM, and XRD. A central composite approach-based RSM analysis was undertaken for statistical modeling and optimization for maximized Cr(VI) removal with respect to four important factors, namely, initial concentration of Cr(VI), the dosage of TSAC, pH, and adsorption time. Optimized values for maximizing adsorption of Cr(VI) (95% of removal) were acquired to be initial Cr(VI) concentration: 87.57 mg/L, TSAC dosage: 2.742 g/100 mL, pH: 2.2, and contact time:109 min. The results from the design of the experiment were also analyzed for the significance of the interaction between the selected process parameters. In addition, the pseudo-second-order kinetic and Langmuir isotherm models were found suitable for describing the adsorption data. The adsorption capacity of Cr(VI) on TSAC was 19.48 mg/g. The observed thermodynamic characteristics reveal that Cr(VI) adsorption on TASC is endothermic in nature. From the results, TSAC had shown a potential Cr(VI) efficiency on optimized process conditions that can be exploited effectively as adsorbent for removal of Cr(VI)-contaminated wastes.

Tomato green waste biochars as sustainable trivalent chromium sorbents

Chromium removal from aqueous solutions has gained attention due to its hazardous impact on life organisms. In the present study, sorption processes were performed to examine the opportunity to apply biochar derived from waste tomato leaves and stems for Cr(III) ion removal. Biochars were produced through pyrolysis in a wide range of temperature (250-800 °C). The obtained biochars were investigated in detail by means of ultimate and proximate analyses, pH point of zero charge, FT-IR, scanning electron microscopy, and mercury porosimetry. Biochars are characterized by high amount of ash varying from 23 to 44% and as a result high pHpzc values of about 13. It was proven that increasing pyrolysis temperature positively affected sorption of Cr(III) ions. Mineral matter in the biochars plays a crucial role in the removal of Cr(III) ions from aqueous solution mainly due to their precipitation. The sorption capacity of biochar produced at 800 °C was 169.5 mg g^-1, whereas at 250 °C only 62.2 mg g^-1. It was found that biochar can be reused in sorption process after desorption using 0.1 M HCl, while the sorption capacity decreased 4-fold.

Facile fabrication of novel magnetic ZIF-67 MOF@aminated chitosan composite beads for the adsorptive removal of Cr(VI) from aqueous solutions

Metal organic frameworks (MOFs) have become premium candidates for the removal of hazardous contaminants from wastewater. However, MOFs have a vast obstacle which is their poor recyclability. In this study, ZIF-67 was decorated with magnetic Fe₃O₄ nanoparticles, and then embedded into aminated chitosan (AmCs) matrix to form core-dual shell Fe₃O₄/ZIF-67@AmCs composite beads. Diverse analysis tools were utilized to ensure the successful fabrication of the magnetic composite beads. The fabricated magnetic composite beads were examined their adsorptive removal aptitude towards toxic Cr(VI) ions. The gained results refereed that a maximum adsorption capacity of 119.05 mg/g was attained by magnetic Fe₃O₄/ZIF-67@AmCs composite beads at 25 °C. The process obeyed both of Langmuir and Freundlich isotherm models, and the pseudo 2nd order was more suitable kinetic model to represent the adsorption process. Besides, Fe₃O₄/ZIF-67@AmCs composite showed an excellent recyclability for the removal of Cr(VI) ions from their aqueous solutions for seven consecutive cycles.

Synthesis of calcite-based bio-composite biochar for enhanced biosorption and detoxification of chromium Cr (VI) by Zhihengliuella sp. ISTPL4

The current study presents a comprehensive analysis of the potential of actinobacterium Zhihengliuella sp. ISTPL4 and different composite materials for the removal of hexavalent chromium [Cr (VI)]. Genome analysis of strain indicated the presence of several oxidoreductases which includes chromate reductase, nitrate reductase, thioredoxin, superoxide dismutase and hydrogenase are other major candidate genes. Catalytic calcite-based bio-composite material was absorbed on biochar had highest Cr removal efficiency. The main mechanism involved in Cr biosorption by this strain was explained by the Langmuir isotherm model; under equilibrium conditions the maximum adsorption was observed 49 ± 0.3 mgg^-1. Kinetic studies showed that biosorption of Cr (VI) by this strain was a rate-limiting step and followed a pseudo-second-order kinetics (R2 = 0.99). SEM analysis is in line with EDX result indicating highest Cr removal by calcined biochar. MTT assay shown that the bacteria successfully convert toxic Cr (VI) to comparatively less toxic form such as Cr (III).

Removal of Hexavalent Chromium from Aqueous Solutions Using Sulfonated Peat

Peat, a loose and porous material, contains rich organic matter and can be used as an adsorbent. In this study, it is chemically modified by adding sulfuric acid under different conditions, with the aim of producing a modified peat with optimized Cr(VI) adsorption capability. The modified peat exhibited a higher adsorption efficiency than the natural peat throughout the adsorption experiments. The adsorption of Cr(VI) from aqueous solutions correlates with the pseudo-second order kinetic model. In addition, the Langmuir model indicated a maximum loading capacity approximately of 105.4 mg/g, which is a markedly high value compared to some other reported adsorbents. The present study performed single factor experiments and the results indicated that higher temperature conditions result in better adsorption capability, whilst an increase in the pH played a contrary role. According to the orthogonal tests, the pH had the greatest impact on adsorption. The obtained results indicated that sulfonated peat can be effectively applied in removing Cr (VI).

A novel clean production approach to utilize crop waste residues as co-diet for mealworm (Tenebrio molitor) biomass production with biochar as byproduct for heavy metal removal

Proper management of waste crop residues has been an environmental concern for years. Yellow mealworms (larvae of Tenebrio molitor Linnaeus, 1758) are major insect protein source. In comparison with normal feed wheat bran (WB), we tested five common lignocellulose-rich crop residues as feedstock to rear mealworms, including wheat straw (WS), rice straw (RS), rice bran (RB), rice husk (RH), and corn straw (CS). We then used egested frass for the production of biochar in order to achieve clean production. Except for WS and RH, the crop residues supported mealworms' life activity and growth with consumption of the residues by 90% or higher and degraded lignin, hemicellulose and cellulose over 32 day period. The sequence of degradability of the feedstocks is RS > RB > CS > WS > RH. Egested frass was converted to biochar which was tested for metal removal including Pb(II), Cd(II), Cu(II), Zn(II), and Cr(VI). Biochar via pyrolysis at 600 °C from RS fed frass (FRSBC) showed the best adsorption performance. The adsorption isotherm fits the Langmuir model, and kinetic analysis fits the Pseudo-Second Order Reaction. The heavy metal adsorption process was well-described using the Intra-Particle Diffusion model. Complexation, cation exchange, precipitation, reduction, deposition, and chelation dominated the adsorption of the metals onto FRSBC. The results indicated that crop residues (WS, RS, RB, and CS) can be utilized as supplementary feedstock along with biochar generated from egested frass to rear mealworms and achieve clean production while generating high-quality bioadsorbent for environment remediation and soil conditioning.

Biochar from A Freshwater Macroalga as A Potential Biosorbent for Wastewater Treatment

The multi-elemental composition, surface texture and morphology of biochar, produced by pyrolysis at 300, 350, 400 and 450 °C from freshwater macroalga Cladophora glomerata, as a biosorbent of toxic metals was examined with Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Scanning Electron Microscopy (SEM), and Fourier Transform Infrared Spectroscopy (FT-IR) techniques. It was found that the yield of pyrolysis was inversely proportional to temperature: for 300 °C it was 63%, whereas for 450 °C—47%. The proximate analysis revealed that also biochar’s moisture and volatile matter was inversely proportional to temperature. The content of ash increased with temperature. All biochars were characterized by a similar total pore area of about 20 m2 g−1. FT-IR analysis showed that all biochars peaked at 3500–3100 cm−1 which was attributed to O–H stretching of the hydroxyl groups, at 2850–2970 cm−1, stretching vibrations of C–H bonds in aliphatic CH2 and CH groups, at 1605 cm−1, stretching vibrations from C=C of aromatics, at 1420 cm−1, bending oscillations from CH2, at about 1111 cm−1, stretching vibrations of Si–O, at 618 cm−1, vibrations from Fe–O bonds, and at 475 cm−1—Si–O–Si deformation vibrations. The biosorption properties of biochar towards Cr(III) ions were examined in kinetic studies. The biosorption capacity of biochar increased with an increase of pyrolysis temperature: the highest was for biochar obtained at 450 °C—87.1 mg Cr(III) g−1 and the lowest at 300 °C—45.9 mg g−1. Cladophora biochar also demonstrated a good ability to simultaneously remove metal ions from a multi-metal system, e.g., wastewater. The removal efficiency for Cr(III) was 89.9%, for Cu(II) 97.1% and for Zn(II) 93.7%. The biochar derived from waste-freshwater macroalgae can be a potent and eco-friendly alternative adsorptive material.

Efficient Removal of Cr(VI) from Water by Biochar and Activated Carbon Prepared through Hydrothermal Carbonization and Pyrolysis: Adsorption-Coupled Reduction Mechanism

Three carbonaceous porous materials (biochar and activated carbon) were developed from the Tectona grandis tree sawdust. The applied process of two-stage preparation included pre-treatment through hydrothermal carbonization at 190 °C and subsequent pyrolysis at 800 °C. Two chemical activating agents (K2CO3 and ZnCl2) were used to prepared activated carbons (K2CO3-AC and ZnCl2-AC), respectively. They were characterized by textural property, morphology, and surface element components and applied to remove Cr(VI) from solution at various solution pH values and initial Cr(VI) concentrations. Results showed that the textural parameters (SBET and VTotal) of the prepared material were 1757 m2/g and 1.027 cm3/g for Zn-Cl2-AC, 1013 m2/g and 0.418 cm3/g for K2CO3-AC, and 792 m2/g and 0.345 cm3/g for biochar. The adsorption process reached the highest efficiency at pH 3.0. The Langmuir maximum adsorption capacity indicated the decreasing order: ZnCl2-AC (127 mg/g) > K2CO3-AC (103 mg/g) > biochar (83.5 mg/g). The removal mechanism of Cr(V) from solution was regarded as an adsorption-coupled reduction, namely (1) partial reduction of Cr(VI) into Cr(III) during the adsorption process and (2) adsorption of the Cr(VI) anions through electrostatic attraction and pore filling and the reduced Cr(III) cations through complexation, Cπ–cation interaction, cation exchange, and pore filing. Therefore, the prepared biochar and activated carbon can server as promising adsorbents to efficiently remove both Cr(VI) and Cr(III) from water.