Removal of Cr(VI) from aqueous solutions by Na2SO3/FeSO4 combined with peanut straw biochar

Synthesis of graphite/rGO-modified fungal hyphae for chromium (VI) bioremediation process

Bioremediation is a promising technology that can eliminate the drawbacks of conventional treatment methods in removing harmful toxic metals including chromium(VI). Therefore, in this study, fungal hyphae modified with graphite and reduced graphene oxide were synthesized and assessed for their potential to bioremediate heavy metals for the first time in the literature. The effects of the carbon-based materials on microbial structure were characterized using scanning electron microscopy analysis. Thermogravimetric, RAMAN, X-ray diffraction, and enzymatic analyzes were performed to determine the role of functional groups. In addition, batch adsorption experiments utilizing response surface methodology were conducted to optimize operating parameters such as time (1-11 h), chromium (10-50 mg/L), and graphite/reduced graphene oxide (0.1-1 g/L). The maximum adsorption capacity with the graphene fungal hyphae was determined to be 568 mg.g-1, which is 9.7 times that of the crude fungal hyphae. The Cr(VI) removal for fungal hyphae-graphite and fungal hyphae-reduced graphene oxide biocomposites was 98.25% and 98.49%, respectively. The isothermal and kinetic results perfectly matched the 2nd order pseudo-model and Langmuir model in terms of the nature of the adsorption process. The laboratory scale test results indicate that fungal hyphae modified with graphite and reduced graphene oxide have a high adsorption capacity, suitable for the removal of chromium (VI) from wastewater.

Removal of cadmium from aqueous solution by magnetic biochar: adsorption characteristics and mechanism

Experiments were conducted to investigate the potential of the efficient resource utilization of waste cow manure and corn straw in an agricultural ecosystem. In this study, a magnetic cow manure and straw biochar were synthesized by a co-precipitation method, and cadmium (Cd(II)) was removed by adsorption in aqueous solution. Several physicochemical characterization techniques were applied, including SEM, BET, Zeta, FTIR, Raman, XPS, and VSM. The effects of pH value, magnetic biochar content, adsorption kinetics, and isothermal adsorption on the adsorption of Cd(II) were investigated. The physicochemical characterizations revealed that the physical and chemical properties of the magnetic biochar were substantially changed compared to the unmodified biochar. The results showed that the surface of the biochar became rough, the number of oxygen (O)-containing functional groups increased, and the specific surface area increased. The results of the adsorption experiments showed that the adsorption capacity was affected by pH, magnetic biochar addition, Cd(II) concentration, and adsorption time. The adsorption kinetics and isothermal adsorption experiments showed that the Cd(II) adsorption processes of the cow manure and corn straw magnetic biochars were consistent with the Freundlich model and pseudo-second-order kinetic model. The results also showed that the Cd(II) adsorption effect of cow manure magnetic biochar was found to be more effective than that of corn straw magnetic biochar. The optimal conditions for Cd(II) adsorption were 800 ℃ for cow manure magnetic biochar, with a pH value of 5 and 0.14 g biochar addition, and 600 ℃ for straw magnetic biochar with a pH value of 8 and 0.12 g biochar addition. In conclusion, the cow manure magnetic biochar was an effective adsorbent for the absorption of Cd(II) in wastewater.

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.

Sulfite Poses a Risk of Hexavalent Chromium Rebound in Vadose Zone: A Challenge of the Stability of CrxFe1-x(OH)3

Cr(VI) rebound is the primary risk associated with the reduction remediation of Cr(VI)-contaminated soil. The potential impact of sulfites, which can be produced by microbial activities or originate from sulfur-containing remediation agents, on the Cr(VI) rebound in the vadose zone has been overlooked. When sulfites are present, the stability of CrxFe1-x(OH)3 is compromised and significantly inferior to that of Cr(OH)3, as demonstrated in this paper. First, Fe acts as a catalyst for the conversion of adsorbed sulfite to SO4·-, which subsequently triggers the oxidation of Cr(III) and results in the rebound of Cr(VI). The heterogeneous catalysis by Fe on the surface of CrxFe1-x(OH)3 plays a predominant role, contributing to 78% of the actual oxidation of Cr(III) among all employed catalytic processes. The presence of ambient Cl- can exacerbate the rebound effect of Cr(VI) by promoting the generation of HOCl. Furthermore, a portion of released Cr(VI) was reduced to Cr(III) by dissolved sulfite in the presence of dissolved Fe as a catalyst, thereby increasing the dissolution and migration risk associated with CrxFe1-x(OH)3. Hence, the presence of sulfites results in a significant increase in the Cr(VI) rebound and Cr(III) release from CrxFe1-x(OH)3. This challenges the conventional understanding of the stability of CrxFe1-x(OH)3.

Lanthanum nanoparticle (La2O3)-loaded adsorbents for removal of hexavalent chromium: a kinetics, isotherm, and thermodynamic study

Nanocomposite adsorbents for wastewater treatment gained popularity in recent times. In the present study, nanoparticles prepared from lanthanum have been loaded on the powdered form of aquatic plants Salvinia molesta (S. molesta) and Typha latifolia (T. latifolia). These new adsorbents were NPS (nanoparticle-loaded S. molesta) and NPT (nanoparticle-loaded T. latifolia). The batch study was carried out to assess the effect of several factors on the adsorption of Cr(VI) by novel adsorbents NPS and NPT. XRD, SEM, FTIR, EDX, and Zeta potential were used for the characterization of nanoparticles formed and novel adsorbents. The maximal adsorption was noticed by both adsorbents at pH 1, 20 ppm of initial metal concentration, and 1 h of contact period with 150 rpm at 25 ℃. The adsorbent dose of 60 mg and 80 mg was observed as the equilibrium dose for NPS and NPT, respectively. The maximum adsorption capacity observed was 27.18 mg/g for NPS and 19.85 mg/g for NPT. Freundlich isotherm was better fitted for both adsorbents. Pseudo-second-order kinetics depicts the better mechanism of adsorption with R2 = 0.9995 and 0.9982 for NPS and NPT, respectively. Thermodynamic parameters show that the adsorption process was exothermic and spontaneous.

Fast Detection of Different Water Contaminants by Raman Spectroscopy and Surface-Enhanced Raman Spectroscopy

Fast monitoring of water quality is a fundamental part of environmental management and protection, in particular, the possibility of qualitatively and quantitatively determining its contamination at levels that are dangerous for human health, fauna and flora. Among the techniques currently available, Raman spectroscopy and its variant, Surface-Enhanced Raman Spectroscopy (SERS), have several advantages, including no need for sample preparation, quick and easy operation and the ability to operate on the field. This article describes the application of the Raman and SERS technique to liquid samples contaminated with different classes of substances, including nitrates, phosphates, pesticides and their metabolites. The technique was also used for the detection of the air pollutant polycyclic aromatic hydrocarbons and, in particular, benzo(a)pyrene, considered as a reference for the carcinogenicity of the whole class of these compounds. To pre-concentrate the analytes, we applied a methodology based on the well-known coffee-ring effect, which ensures preconcentration of the analytes without any pretreatment of the sample, providing a versatile approach for fast and in-situ detection of water pollutants. The obtained results allowed us to reveal these analytes at low concentrations, close to or lower than their regulatory limits.

Functionalized biochars: Synthesis, characterization, and applications for removing trace elements from water

Biochar (BC) has been recognized as an effective adsorbent to remove trace elements (TEs) from water. However, low surface functionality and small pore size can limit the adsorption ability of pristine biochar. These limitations can be addressed by using functionalized biochars which are developed by physical, chemical, or biological activation of biochar to improve their physico-chemical properties and adsorption efficiency. Despite the large amount of research concerning functionalized biochars in recent decades, to our knowledge, no comprehensive review of this topic has been published. This review focuses solely on the synthesis, characterization, and applications of functionalized/engineered biochars for removing TEs from water. Firstly, we evaluate the synthesis of functionalized biochars by physical, chemical, and biological strategies that yield the desired properties in the final product. The following section describes the characterization of functionalized biochars using various techniques (SEM, TEM, EDS, XRD, XANES/NEXAFS, XPS, FTIR, and Raman spectroscopy). Afterward, the role of functionalized biochars in the adsorption of different TEs from water/wastewater is critically evaluated with an emphasis on the factors affecting sorption efficiency, sorption mechanisms, fate of sorbed TEs from contaminated environments and associated challenges. Finally, we specifically scrutinized the future recommendations and research directions for the application of functionalized biochar. This review serves as a comprehensive resource for the use of functionalized biochar as an emerging environmental material capable of removing TEs from contaminated water/wastewater.

A Novel Triazole Schiff Base Derivatives for Remediation of Chromium Contamination from Tannery Waste Water

Tannery industries are one of the extensive industrial activities which are the major source of chromium contamination in the environment. Chromium contamination has been an increasing threat to the environment and human health. Therefore, the removal of chromium ions is necessary to save human society. This study is oriented toward the preparation of a new triazole Schiff base derivatives for the remediation of chromium ions. 4,4′-((1E)-1,2-bis ((1H-1,2,4-triazol-3-yl) imino)ethane-1,2-diyl) diphenol was prepared by the interaction between 3-Amino-1H-1,2,4-triazole and 4,4′-Dihydroxybenzil. Then, the produced Schiff base underwent a phosphorylation reaction to produce the adsorbent (TIHP), which confirmed its structure via the different tools FTIR, TGA, ¹HNMR, ¹³CNMR, GC-MS, and Phosphorus-31 nuclear magnetic resonance (³¹P-NMR). The newly synthesized adsorbent (TIHP) was used to remove chromium oxyanions (Cr(VI)) from an aqueous solution. The batch technique was used to test many controlling factors, including the pH of the working aqueous solution, the amount of adsorbent dose, the initial concentration of Cr(VI), the interaction time, and the temperature. The desorption behaviour of Cr(VI) changes when it is exposed to the suggested foreign ions. The maximum adsorption capacity for Cr(VI) adsorption on the new adsorbent was 307.07 mg/g at room temperature. Freundlich’s isotherm model fits the adsorption isotherms perfectly. The kinetic results were well-constrained by the pseudo-second-order equation. The thermodynamic studies establish that the adsorption type was exothermic and naturally spontaneous.

Bisulfite activated permanganate for oxidative water decontamination

Recently, bisulfite-activated permanganate (MnO₄^-; Mn(VII)) process has attracted considerable attention as a novel class of advanced oxidation technology for destruction of organic contaminants in water. However, disputes over the underlying activation mechanism as well as reactive species generated in the Mn(VII)/bisulfite system remain for a long period due to the fairly complex chemistry involved in this system. This article aims to present a critical review on scientific development of the Mn(VII)/bisulfite system, with particular focus on the generation and contribution of various reactive intermediates. Both reactive manganese species (RMnS) (i.e., soluble Mn(III), Mn(V), and Mn(VI)) and radical species (primarily SO₄^•-) are identified as the oxidizing components responsible for enhanced degradation of organic contaminants by the Mn(VII)/bisulfite system. Bisulfite plays a dual role of being an activating agent for reactive intermediates generation and acting as a complexing agent to stabilize RMnS. Solution chemistry (e.g., the [Mn(VII)]/[bisulfite] molar ratio, solution pH, the type of contaminants, ligands, and water matrix components) greatly impacts the generation and consumption of RMnS and radicals, thus influencing the degradation kinetics and pathways of organics. Particularly, dissolved oxygen (DO) is a vital factor for driving the oxidation of organics since the absence of DO can block the generation of SO₄^•- and meantime causes the consumption of RMnS by excess SO₃^•- as a strong reductant. Interestingly, ferrate (FeO₄^2-, Fe(VI)) and hexavalent chromium (CrO₄^2-/HCrO₄^-, Cr(VI)) that are high-valent metal oxyanions analogous to Mn(VII) can be activated by bisulfite via a similar pathway (i.e. both high-valent metal-oxo intermediates and reactive radicals are involved). Furthermore, key knowledge gaps are identified and future research needs are proposed to address the potential challenges encountered in practical application of the Mn(VII)/bisulfite oxidation technology.

Treated activated carbon as a metal-free catalyst for effectively catalytic reduction of toxic hexavalent chromium

In this work, activated carbon treated in N₂ atmosphere, as a non-metallic catalyst, exhibits excellent catalytic performance in reduction of Cr (VI) to Cr (III) using HCOOH as the reducing agent at room temperature. A series of characterizations and control experiments were carried out to deduce the possible reaction mechanism. The results showed that the improved catalytic performance can be attributed to the enhanced graphitization degree and basic sites such as pyrone-like, which favor electron transferring and activation of reactant. The reaction rate constant observed herein for the C-800 was 22 and 6 times more than that for C-0 and Pd/C catalyst, respectively. In addition, C-800 showed good recycle performance, and no loss of activity was observed after 5 cycles. This study broadens the application of nonmetallic catalyst and provides an easy-available and cost-effective catalytic material for removing toxic Cr (VI).

The fast redox cycle of Cu(II)-Cu(I)-Cu(II) in the reduction of Cr(VI) by the Cu(II)-thiosulfate system

Thiosulfate (S₂O₃^2-) is an important ligand to complex metal cations, however, the reactivity of metal-thiosulfate complexes has barely been mentioned. In this study, the reactivity of the Cu(II)-S₂O₃^2- system in the reduction of Cr(VI) was investigated. Kinetic results show that the reduction rates of Cr(VI) decrease with increasing pH values from 3.0 to 5.0, and 94.3% and 97.5% of 10 mg L^-1 Cr(VI) was rapidly reduced within 1 min at pH 3.0 and within 30 min at pH 5.0, respectively at the molar ratio of Cu(II):S₂O₃^2- of 0.05. We rule out the contributions of S species of tetrathionate (S₄O₆^2-) and sulfite (SO₃^2-) to Cr(VI) reduction and point out that the produced Cu(I) in the Cu(II)-S₂O₃^2- system is the key reductant that mediates the reduction of Cr(VI). We suggest that complexation between Cu(II) and S₂O₃^2- with the formation of Cu^II(S₂O₃)₂^2- is the pre-requisite for the formation of Cu^I(S₂O₃)_n^1-2n, which plays an important role in Cr(VI) reduction, accompanied by the re-oxidation of Cu(I) to Cu(II) by Cr(VI), achieving the rapid redox cycling of Cu(II)-Cu(I)-Cu(II). Such a redox cycle also mediates the denitrification process of NO₂^- to NH₃/NH₄⁺ under weakly acidic conditions. This study enriches our understanding on the reducing reactivity of the Cu(II)-S₂O₃^2- system and the importance of the Cu(II)-Cu(I)-Cu(II) redox cycle towards environmental oxidizing contaminants.

Influence of pyrolysis temperature on characteristics and Cr(VI) adsorption performance of carbonaceous nanofibers derived from bacterial cellulose

The effects of pyrolysis temperature on properties and adsorption performance of carbonized bacterial cellulose (CBC) produced from bacterial cellulose at 300, 400, 600 and 800 °C were investigated. As pyrolysis temperature increased, the BET surface area, C and ash contents of CBC increased while its mass yield and the contents of H, N and O decreased. Higher pyrolysis temperature resulted in CBC having more aromatic structure and less hydrophilic. The impacts of pyrolysis temperature, solution pH, contact time and initial concentration on the absorption of Cr(VI) onto CBC were systematically studied as well. The results showed that CBC400 prepared at 400 °C exhibited the highest Cr(VI) adsorption capacity for Cr(VI) up to 250.0 mg/g. The equilibrium adsorption and adsorption kinetics fitted the Langmuir isotherm and pseudo-second-order kinetic models well. The mechanisms of adsorption of Cr(VI) on CBC included electrostatic interaction, π-π interaction and functional groups complexation.

Remediation of Cr(VI)-contaminated soil by combined chemical reduction and microbial stabilization: The role of biogas solid residue (BSR)

In this work, the use of chemical reduction combined with microbial stabilization to remediate Cr(VI) in contaminated soil was systematically investigated. The effectiveness, phytotoxicity and microbial diversity resulting from the combination of ferrous sulfate with microbial stabilization by biogas solid residue (BSR) were determined. The stabilization experiments showed that the optimum Cr(VI) conversion rate of 99.92% was achieved with an Fe (II)/Cr(VI) molar ratio of 3:1, a BSR dose of 5.2% (wt), and a water content of 40%. Under these conditions, the residual Cr(VI) content was 0.80 mg/kg, which satisfied the risk screening value (≤ 5.7 mg/kg) for soil contamination of land for general development in China. The remaining Cr(VI) level was stable for 90 days during the chemical reduction and biogenic stabilization process. Moreover, Zucconi test analysis suggested that the soil phytotoxicity to Brassica campestris L. disappeared. The results of microbial diversity analysis indicated that the bacterial community changed significantly during chemical reduction and microbial stabilization processes, and Bacillus, Pseudomonas and Psychrobacter may participate in the reduction of Cr(VI) into Cr(III).

Mechanism of Removal of Hexavalent Chromium from Aqueous Solution by Fe-Modified Biochar and Its Application

This study discussed the mechanism of Fe-modified biochar (FeBC) derived from rice straw biochar (BC) as an adsorbent for removing Cr(VI) from aqueous solution and assessed its applicability in actual industrial wastewater. The Cr(VI) removal percentage increased with the FeBC dose, which achieved a removal of 99.5% at 8.0 g/L FeBC. Increasing the solution pH from 2 to 10 slightly reduced Cr(VI) adsorption by 6.6%. Coexisting ions such as Ca2+, Na+ and Cl− inhibited the removal of Cr(VI); the removal rate decreased to 60% at their concentration of 0.25 mol/L. The adsorption isotherm and kinetics were better described by the Langmuir isotherm and pseudo-second-order kinetic models, respectively. Through scanning electron microscopy with energy dispersive X-ray, the Brunauer–Emmett–Teller method, Fourier transform infrared, X-ray diffraction and X-ray photoelectron spectroscopy, the analysis revealed that FeBC with iron oxides loaded onto its surface had more active sites than BC; the surface functional groups changed; the removal of Cr(VI) by FeBC was mainly attributed to electrostatic adsorption; the redox reaction of Cr, and Fe loaded onto BC enhanced Cr(VI) reduction process. FeBC showed a good removal performance on actual industrial wastewater with the concentration of both total Cr and Cr(VI) meeting the integrated wastewater discharge standard of China.

Nano-selenium alleviating the lipid metabolism disorder of LMH cells induced by potassium dichromate via down-regulating ACACA and FASN

Chromium (Cr) VI is a common environmental contaminant highly toxic to livers. To explore the protective effect of nano-selenium (NANO-Se) on broiler liver damage caused by Cr (VI), this experiment was conducted with chicken hepatocellular carcinoma cell line (LMH) as the research object, using potassium dichromate (PDC) and NANO-Se gel for culturing cells. The results indicated that: (1) in the PDC-exposure group, LMH cells being treated with 20 μmol/L PDC for 24 h, IC50 (median inhibition concentration) = 23.427 could significantly reduce cell activity (p < 0.01) which decreased over time. PDC markedly increased the concentration of triglycerides (TG) and low-density lipoprotein cholesterol (LDL-C) in LMH cells (p < 0.01), which increased over time. In addition, PDC could substantially augment the transcription and protein levels of acetyl-CoA carboxylases alpha (ACACA) and fatty acid synthase (FASN) in LMH cells (p < 0.01). (2) Compared with the PDC-exposure group, the addition of 8 μmol/L NANO-Se after 12 h of PDC treatment could significantly increase the cell viability (p < 0.01) but decreased over time; the levels of TG and LDL-C in LMH cells declined markedly (p < 0.01). In addition, the transcription and protein levels of ACACA and FASN in LMH cells were significantly reduced (p < 0.01). (3) The LMH cells were cultured in advance with 8 μmol/L NANO-Se for 12 h and then with PDC for 24 h. The obtained results were similar to the above. There were no obvious differences in TG and LDL-C levels (p > 0.05). However, significant differences were found in the activity of LMH cells and the expression of genes related to lipid metabolism (p < 0.05).All these results suggest that the exposure to PDC promotes the increase of lipid synthesis in LMH cells and causes disorders in the lipid metabolism. Moreover, NANO-Se can partially attenuate the damage caused by PDC through down-regulating of the lipid metabolism-related genes (ACACA and FASN) in LMH cells.

Enhancement of Cd(II) adsorption by rice straw biochar through oxidant and acid modifications

To develop high-efficient biochar adsorbents, the effects and mechanisms of oxidant modification and acid modification on Cd(II) adsorption by rice straw biochar were investigated. Three rice straws from Langxi in Anhui Province, Yingtan in Jiangxi Province, and Lianyungang in Jiangsu Province were collected to prepare biochars by anaerobic pyrolysis in a muffle furnace. Rice straw biochars were modified by 15% H₂O₂ and 1:1 HNO₃/H₂SO₄ mixed acid, respectively, to obtain modified biochars. The untreated rice straw biochar and HCl-treated rice straw biochar with carbonate removed were used as controls. The functional groups on the surfaces of the biochars were qualitatively and quantitatively determined by Fourier transform infrared spectra and Boehm titration, respectively. The adsorption and desorption of Cd(II) onto and from the biochars and modified biochars were measured under various pH conditions. The results showed that oxidant modification with 15% H₂O₂ and acid modification with 1:1 HNO₃/H₂SO₄ significantly increased the number of carboxyl functional groups on the surfaces of the biochars, and acid modification was more effective than oxidant modification in amplifying carboxyl functional groups on the surfaces of the biochars. The increase of surface functional groups effectively enhanced the specific adsorption of Cd(II) on the modified biochars. Therefore, both oxidant modification and acid modification enhanced the adsorption of Cd(II) on the biochars through increasing functional groups on the surfaces of the biochars.

Enhanced 4-FP removal with MnFe2O4 catalysts under dielectric barrier discharge plasma: Economical synthesis, catalytic performance and degradation mechanism

The dielectric barrier discharge plasma (DBDP) process has received extensive attention for the removal of organic contaminants from water. A novel microwave-assisted hydrothermal method was used to easily and rapidly synthesize MnFe₂O₄ catalysts. Based on the DBDP process, MnFe₂O₄ can enhance 4-fluorophenol (4-FP) abatement from 44.15% to 58.78% through the catalysis within 18 min. Then, the adjunction of O₃ generated by discharge can further boost 4-FP degradation to 94.94%. After the whole optimization process is complete, the associated pseudo-first-order reaction kinetic constant and energy efficiency were enhanced from 0.0327 to 0.1536 min^-1 and 2067.13 mg kW h^-1 to 4444.75 mg kW h^-1, respectively. With the help of the condition, blank and radical capture experiments, the catalytic performance caused by MnFe₂O₄ and O₃ was attributed to the joint action of Fenton-like reactions, photocatalysis (ultraviolet, UV), photoassisted Fenton reactions and O₃ catalysis. The overall downward trend of the possible intermediate toxicities indicated that the DBDP/MnFe₂O₄/O₃ process can effectively remove and mineralize 4-FP without the generation of more toxic intermediates. In addition, during the 5 cycles, MnFe₂O₄ can maintain excellent recovery, efficiency and durability. In summary, the coupling of discharge plasma and MnFe₂O₄ sheds new light on catalysis for wastewater treatment.

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.

Investigation on the adsorption of antibiotics from water by metal loaded sewage sludge biochar

Application of sewage sludge biochar as an adsorbent for antibiotics treatment has obtained special attention owning to its low cost and surface functionality. Three metal ions were selected to modify sewage sludge biochar through the pyrolysis with the metal loaded method. Fe loaded sewage sludge biochar (BC-Fe), Al loaded sewage sludge biochar (BC-Al) and Mn loaded sewage sludge biochar (BC-Mn) were characterized and used to explore the performance of adsorbing tetracycline (TC), sulfamethoxazole (SMZ) and amoxicillin (AMC). BC-Fe, BC-Al and BC-Mn possessed rougher surfaces, larger specific surface area and better pore structure. Intra-particle diffusion and Langmuir models were more suitable to describe the adsorption process. The maximum adsorption amount of TC, SMZ and AMC could reach 123.35, 99.01 and 109.89 mg/g by BC-Fe. Furthermore, the main mechanism of antibiotics adsorption by metal loaded sewage sludge biochars might be pores filling, Van der Waals forces and H-bonding. The study can not only solve the problems associated with the pollution of antibiotics from wastewater, but also reduced the treatment pressure of sewage sludge effectively.

Effects of Potassium Propyl Xanthate Collector and Sodium Sulfite Depressant on the Floatability of Chalcopyrite in Seawater and KCl Solutions

This study demonstrates the effects of a potassium propyl xanthate (PPX) collector and sodium sulfite (Na2SO3) depressant on pure chalcopyrite in synthetic seawater (SSW) and potassium chloride (KCl) solutions. SSW solutions with 35 g/L of salt and 0.01-M KCl were used for microflotation and zeta potential tests. Particles sized between 200# and 100# (75–150 µm) were used, and the pH was between 8.0 and 8.5. The surface of the mineral and its interaction with the collector were characterized using Raman spectrometry. The zeta potential of the chalcopyrite was measured in KCl solution at a pH range of 3–12 using the collector and depressant at a monodispersed particle size of 635# (20 µm). The results indicate that the floatability of chalcopyrite is not affected by the presence of PPX collectors in SSW solutions. SSW provides better recoveries than KCl solutions with values of 91.42% and 88.15%, respectively. The Na2SO3 depressant does not hinder the mineral floatability throughout the entire concentration range used; however, special care must be taken when adjusting the pH range to avoid increasing the zeta potential.

Preparation and application of magnetic biochar in water treatment: A critical review

In recent years, magnetic biochar has been widely used in removal of pollutants from water. In this paper, the preparation technologies of magnetic biochar are analyzed, and the performance and application of magnetic biochar in removal of inorganic pollutants such as heavy metals, and organic pollutants are investigated. Moreover, the adsorption behaviors, the key influencing factors and the adsorption mechanisms of magnetic biochars are summarized in this paper. Compared with common biochar, magnetic biochar is more effective in removal of water pollutants, including Cd(II), Pb(II), Zn(II), Cu(II), methylene blue, tetracycline, pesticide and phosphate. Langmuir and Freundlich models are adopted as the mainly adsorption isotherms, while pseudo-second-order model is employed as Kinetic model of heavy metal ions and organic contaminants in water. This study also investigates degradation of organic contaminants in water using magnetic biochar as catalyst. Results showed that encapsulated γ-Fe₂O₃ nanoparticles enhanced the catalytic ability of persulfate activator. Further researches on preparation and application of magnetic biochar in water treatment are prospected in this review.

High-Efficiency Removal of Cr(VI) from Wastewater by Mg-Loaded Biochars: Adsorption Process and Removal Mechanism

Biochars were produced with magnesium chloride as an additive for the sorption of hexavalent chromium dissolved in water using five types of straw (from taro, corn, cassava, Chinese fir, and banana) and one type of shell (Camellia oleifera) as the raw materials. The removal of hexavalent chromium by the six biochars mainly occurred within 60 min and then gradually stabilized. The kinetics of the adsorption process were second order, the Langmuir model was followed, and the adsorption of Cr(VI) by the six biochars was characterized by Langmuir monolayer chemisorption on a heterogeneous surface. Banana straw biochar (BSB) had the best performance, which perhaps benefitted from its special structure and best adsorption effect on Cr(VI), and the theoretical adsorption capacity was calculated as 125.00 mg/g. For the mechanism analysis, Mg-loaded biochars were characterized before and after adsorption by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry (XRD), and scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS). The adsorption mechanism differed from the adsorption process of conventional magnetic biochar, and biochar interactions with Cr(VI) were controlled mainly by electrostatic attraction, complexation, and functional group bonding. In summary, the six Mg-loaded biochars exhibit great potential advantages in removing Cr(VI) from wastewater and have promising potential for practical use, especially BSB, which shows super-high adsorption performance.

Removal performance and mechanisms of Cr(VI) by an in-situ self-improvement of mesoporous biochar derived from chicken bone

A high-performance mesoporous biochar (MBCX) was fabricated from chicken bone via a facile and low-energy consumption pyrolysis process without any additional activators and templates. The physicochemical properties of biochar were carried out by elemental compositions, N₂ adsorption-desorption isotherms, FTIR, and TG. The results illustrated that lower carbonization temperature leaded to a lower specific surface area and more polar functional groups. And the meso-structure of biochar was obtained at 350 °C. Combined with the result of batch experiment, Cr(VI) adsorption capacity was decreased with the increasing in pyrolysis temperature, which suggested that the removal performance was depended on the functional groups of mesoporous biochar rather than the surface area. Kinetic analysis showed that the Cr(VI) adsorption process on MBCX was suitable for Elovich kinetic. The experimental data was well explained by Langmuir isotherm models. And the maximum adsorption capacity was 58.195 mg/g, which was higher than that of most report pristine biochars. This work not only paved a way for subsequent mesoporous biochar preparation but also demonstrated the application potentials of MBCX as an environment benign Cr(VI) adsorbent.

Enhanced removal of hexavalent chromium by engineered biochar composite fabricated from phosphogypsum and distillers grains

Two kinds of industrial wastes (distillers grains and phosphogypsum) were used as raw materials to produce a new biochar composite for Cr(VI) removal in water. The influencing factors including pyrolysis temperature, dosage, initial solution pH as well as contacting time were explored. The adsorption kinetics, isotherms, and thermodynamics of two biochars were conducted. The results show that the adsorption of Cr(VI) by biochar is related to pH. The ideal pH was 3.0 and the adsorbed Cr(VI) decreases as the pH increases. The Cr(VI) adsorption process conformed to the pseudo-second-order equation. Phosphogypsum modified (PM)-biochar is well described by the Freundlich model. The maximum adsorption capacities of distillers grains (DG)-biochar and PM-biochar on Cr(VI) were 63.1 and 157.9 mg g^-1, respectively. The thermodynamic analysis indicates that the Cr(VI) adsorption occurs spontaneously which is an endothermic process. This study provided an alternative way for Cr(VI) removal from water.

Removal of chromium(VI) by MnFe2O4 and ferrous ion: synergetic effects and reaction mechanism

MnFe₂O₄ was a magnetic material that can be used to adsorb contaminants in the wastewater. Fe(II) could act as a reductant to transfer Cr(VI) into Cr(III). In this paper, mesoporous MnFe₂O₄ prepared by the coprecipitation method was incorporated with Fe(II) to remove Cr(VI). The samples before and after reaction were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. A total of 98~100% of Cr(VI) in solution was removed by MnFe₂O₄/Fe(II) hybrid system within a wide pH range (pH 3.0-9.0), which is due to the synergetic effects of adsorption from MnFe₂O₄ and reduction from Fe(II). Cr(VI) was reduced to Cr(III) by ≡Mn(II) on MnFe₂O₄ and Fe(II). Cr(III) and Fe(III) produced during reaction formed Cr(III)-Fe(III) hydroxides/oxyhydroxides and deposited on MnFe₂O₄. The inhibiting action of phosphate on the removal of Cr(VI) was greater than that of sulfate. Cr(VI) removal rate retained 94.5% at the fourth recycle test, which showed excellent re-usability of MnFe₂O₄.

Recent advances in hexavalent chromium removal from aqueous solutions by adsorptive methods

Chromium exists mainly in two forms in environmental matrices, namely, the hexavalent (Cr(vi)) and trivalent (Cr(iii)) chromium. While Cr(iii) is a micronutrient, Cr(vi) is a known carcinogen, and that warrants removal from environmental samples. Amongst the removal techniques reported in the literature, adsorption methods are viewed as superior to other methods because they use less chemicals; consequently, they are less toxic and easy to handle. Mitigation of chromium using adsorption methods has been achieved by exploiting the physical, chemical, and biological properties of Cr(vi) due to its dissolution tendencies in aqueous solutions. Many adsorbents, including synthetic polymers, activated carbons, biomass, graphene oxide, and nanoparticles as well as bioremediation, have been successfully applied in Cr(vi) remediation. Initially, adsorbents were used singly in their natural form, but recent literature shows that more composite materials are generated and applied. This review focused on the recent advances, insights, and project future directions for these adsorbents as well as compare and contrast the performances achieved by the mentioned adsorbents and their variants.

Mechanisms and reutilization of modified biochar used for removal of heavy metals from wastewater: A review

Heavy metals (HMs) pose serious threat to both human and environmental health and therefore, effective and low-cost techniques to remove HMs are urgently required. Because HMs are difficult to be biodegraded and transformed, adsorption is a most promising treatment method in recent times. Biochar (BC), a low-cost and sustainable adsorbent material, has recently attracted much research attention due to its broad application prospects. While BC has many merits, it has a lower HMs adsorption efficiency than traditional activated carbon, limiting its practical applications. Furthermore, the HMs retained by BC are difficult to be desorbed, making the used sorbent material hazardous wastes if not well disposed of under natural conditions. Therefore, it is critical to seek effective surface modifications for BC, to improve its ability to HMs removal ability and the recyclability of BC loaded with HMs. This review represents and evaluates the reported modification methods for BC, the corresponding HMs removal mechanisms and the potential for reutilization of BC loaded with HMs. This review provides a basis for the effective practical application of BC in the treatment of HMs containing wastewater.

Effects of extracellular polymeric substances of Pseudomonas fluorescens, citrate, and oxalate on Pb sorption by an acidic Ultisol

The continuous production of low molecular weight (LMW) organic acids by plants and microorganisms coupled with the continuous presence of extracellular polymeric substances (EPS) in soils is a guarantee that the mobility of heavy metals in soils will be controlled. The effects of citrate, oxalate, and EPS on the adsorption of Pb by an acidic Ultisol were studied both as a function of pH and ionic strength. Electrokinetic potential measurements were also employed to observe to what extent each ligand affected the surface charge property of the Ultisol. All the ligands shifted the zeta potential of the Ultisol to the negative direction, implying that the surface charge of the soil became more negative. The effect on the zeta potential of the soil was observed in the order of oxalate ˃ citrate ˃ EPS. The quantity of Pb adsorbed at each pH (3.0-7.0) reflected the corresponding change in the zeta potential as induced by each ligand. The presence of the ligands shifted the isoelectric point of the Ultisol from 4.8 to 3.2 for the EPS system and below 3.0 for the citrate and oxalate systems. More Pb was adsorbed in the presence of oxalate than in the presence of citrate and EPS. The two most outstanding mechanisms that governed the adsorption of Pb by the Ultisol were (1) electrostatic attraction which was supported by the increase in negative zeta potential of the Ultisol and, (2) complexation which was supported by the lesser proportion of Pb adsorbed in the citrate system at higher pH and also by the spectroscopic data for EPS. The combination EPS + citrate + oxalate was more effective in enhancing the adsorption of Pb than the combination EPS + oxalate and EPS + citrate.

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

The removal efficiency and mechanism of Cr(VI) removal from aqueous solution on semi-decomposed maize straw biochars pyrolyzed at 300 to 600 °C were investigated. The removal of Cr(VI) by the biochars decreased with pyrolysis temperature increasing from 300 to 600 °C, and the maximum removal capacity of Cr(VI) for maize straw biochar pyrolyzed at 300 °C was 91 mg/g at pH 2.0. The percentage removal of Cr(VI) rapidly decreased with pH increasing from 2.0 to 8.0, with the maximum (>99.9%) at pH 2.0. The variation of Cr(VI) and Cr(III) concentrations in the solution after reaction showed that Cr(VI) concentration decreased while Cr(III) increased and the equilibrium was reached after 48 h, while the redox potential after reaction decreased due to Cr(VI) reduction. X-ray photoelectron spectroscopy (XPS) semi-quantitative analysis showed that Cr(III) accounted for 75.7% of the total Cr bound to maize straw biochar, which indicated reductive adsorption was responsible for Cr(VI) removal by the biochars. Cr(VI) was firstly adsorbed onto the positively charged biochar surface and reduced to Cr(III) by electrons provided by oxygen-containing functional groups (e.g., C=O), and subsequently part of the converted Cr(III) remained on the biochar surface and the rest released into solution. Fourier transform infrared (FTIR) data indicated the participation of C=O, Si–O, –CH2 and –CH3 groups in Cr(VI) removal by the biochars. This study showed that maize straw biochar pyrolyzed at 300 °C for 2 h was one low-cost and efficient adsorbent for Cr(VI) removal from aqueous solution.

Enhanced removal of Cr(VI) by biochar with Fe as electron shuttles

Biochar is extensively used as an effective soil amendment for environmental remediation. In addition to its strong contaminant sorption capability, biochar also plays an important role in chemical transformation of contaminant due to its inherent redox-active moieties. However, the transformation efficiency of inorganic contaminants is generally very limited when the direct adsorption of contaminants on biochar is inefficient. The present study demonstrates the role of Fe ion as an electron shuttle to enhance Cr(VI) reduction by biochars. Batch experiments were conducted to examine the effects of Fe(III) levels, pyrolysis temperature of biochar, initial solution pH, and biochar dosage on the efficiency of Cr(VI) removal. Results showed a significant enhancement in Cr(VI) reduction with an increase in Fe(III) concentration and a decrease of initial pH. Biochar produced at higher pyrolysis temperatures (e.g., 700°C) favored Cr(VI) removal, especially in the presence of Fe(III), while a higher biochar dosage proved unfavorable likely due to the agglomeration or precipitation of biochar. Speciation analysis of Fe and Cr elements on the surface of biochar and in the solution further confirmed the role of Fe ion as an electron shuttle between biochar and Cr(VI). The present findings provide a potential strategy for the advanced treatment of Cr(VI) at low concentrations as well as an insight into the environmental fate of Cr(VI) and other micro-pollutants in soil or aqueous compartments containing Fe and natural or engineered carbonaceous materials.

Cr(VI) removal using different reducing agents combined with fly ash leachate: A comparative study of their efficiency and potential mechanisms

Remediation of high concentrations of Cr(VI) in wastewater involves its chemical reduction to Cr(III), a product with low toxicity that can be easily removed. To date, NaBH₄ has rarely been used to reduce Cr(VI). This article reports a comparative study of Cr(VI) removal by NaBH₄ and five sulfur-based reducing agents (FeSO₄, Na₂S₂O₅, NaHSO₃, Na₂S₂O₃, and Na₂SO₃). The potential mechanisms of Cr(VI) removal by these six reducing agents with and without fly ash leachate (FAL) are also discussed. The results revealed that the reduction and subsequent removal of Cr(VI) are influenced by the hydrolysis and ionization of the reducing agents in solution. Thus, the reduction reaction was significantly enhanced when Na₂S₂O₅ and NaHSO₃ were added in excess of 600 mg L^-1. Combined with FAL, smaller amounts of NaBH₄ were required to reduce Cr(VI) to Cr(III) at pH 3.0 compared to those with the other reducing agents. NaBH₄ combined with FAL at a dose of 100 mg L^-1 afforded a total Cr (Cr_T) removal of 96.32% within 20 min, a value much higher than that obtained with the other reducing agents. The catalytic mechanism of NaBH₄ for such a FAL-catalyzed Cr(VI) reduction system is similar to that of acid catalysis via the hydrolysis of the Fe(III) and Al(III) species in FAL. Improvement of the Cr_T removal was also observed via Cr(VI) entrapment in the structure of Fe(III) and Al(III) metal hydroxides. These results indicate that relatively low loadings of NaBH₄ combined with FAL show great promise for Cr(VI) pollution remediation.

Adsorption and Reduction of Cr(VI) Together with Cr(III) Sequestration by Polyaniline Confined in Pores of Polystyrene Beads

The simultaneous reduction and sequestration of Cr(VI) from wastewater is desirable as a cost-effective and environmentally friendly approach. In this study, we execute a one-step facile synthesis strategy on polyaniline (PANI) composites based on aniline adsorption and polymerization on pores of millimeter-scale polystyrene balls (PANI@PS). The well-defined PANI@PS increased the removal capacity of Cr(VI) by 5.4 times, going from 43.6 (bulky PANI) to 233.7 mg g^-1 near neutral pH (6.0) instead of the pH 1-3 documented in other reports, which was higher than that of many reported adsorbents due to its porous structure, numerous interaction sites, and confinement effects in the polymer. Most importantly, PANI@PS could efficiently sequester positive Cr(III) after reducing Cr(VI) to Cr(III) due to its negative surface created by confinement effects confined to the nanopores of PS. Conversely, positively charged bulky PANI repelled electrostatically positive Cr(III); thus, additional precipitation or adsorption treatments were needed in practical applications. Moreover, a coating of PANI can protect PS, as a substrate and a composite, from irreversible damage due to the strong oxidation capacity of Cr(VI), which is another major concern in adsorbing strong oxidants using polymers. A novel strategy to regenerate the exhausted PANI@PS was efficiently executed based on the electrochemical redox reversibility of PANI. Finally, the comprehensive adsorption/reduction/sequestration of Cr on PANI@PS was elucidated in detail.

Reinvestigating the role of reactive species in the oxidation of organic co-contaminants during Cr(VI) reactions with sulfite

Experimental work was undertaken in this study to re-investigate the mechanisms and active species responsible for oxidation of co-contaminants in the Cr(VI)/HSO₃^- reaction system. Batch experiments showed that the degradation rates of 4-chlorophenol (4-CP) correlated well with the rates of Cr(VI) reduction by sulfite in the same solutions, and that O_2(aq) was necessary for the oxidation of 4-CP. Multiple lines of evidences indicate that Cr(VI)/HSO₃^- reaction is a SO₄^--based oxidation process. SO₃^- was generated in Cr(VI)/HSO₃^- system based on the electron spin resonance spectra, which could be transformed to secondary radicals (SO₄^-, SO₅^-, and HO). The contribution of SO₅^- was ruled out through almost complete inhibition of methanol (MeOH) on 4-CP degradation. Considering the negligible inhibition of tert-butanol (TBA) on 4-CP degradation, SO₄^- was identified to be reactive species in Cr(VI)/HSO₃^- process. This result was further verified by almost no degradation of nitrobenzene and the inhibiting effect of Cl^- in Cr(VI)/HSO₃^- process. This mechanism is beneficial to application of Cr(VI)/HSO₃^- system in wastewater treatment.

Preparation and application synthesis of magnetic nanocomposite using waste toner for the removal of Cr(vi)

In this study, a novel magnetic nanocomposite was prepared using waste toner (WT) through high temperature decomposition, and calcination was conducted in different atmospheres (air, ammonia, and vacuum). WT calcined in ammonia (WT(NH₃)), and it was then utilized as an efficient absorbent for the reduction of Cr(vi) in aqueous solutions; a batch experiment with different conditions was performed to investigate its Cr(vi) removal ability. The effects of two pH-regulating acid (HCl and H₂SO₄) treatments were also studied. It was found that WT(NH₃) could remove about 99% Cr(vi) at pH 2 under H₂SO₄ treatment. The XRD and TEM results coupled with VSM results confirmed that WT(NH₃) is an Fe₃O₄/Fe₂N nanohybrid, which possesses excellent water-dispersibility and remarkable magnetic properties. XPS analysis showed the presence of Cr(vi) and Cr(iii) on the surface of WT(NH₃), which indicated that Cr(vi) was reduced to Cr(iii). Furthermore, H₂SO₄ regulation also promoted the reduction of Cr(vi) by WT(NH₃), and this reduction was higher than that obtained by HCl regulation.

A novel magnetic nanocomposite is prepared using waste toner via calcination in ammonia, which exhibits excellent magnetic properties and high efficiency for the removal of Cr(vi) via pH regulation using H₂SO₄.

Simultaneous removal of Cr(VI) and acid orange 7 from water solution by dielectric barrier discharge plasma

A feasibility study was conducted for simultaneous removal of hexavalent chromium (Cr(VI)) and azo dye acid orange 7 (AO7) by the dielectric barrier discharge (DBD) plasma. The results showed that there was a synergistic effect between Cr(VI) reduction and AO7 degradation. The presence of Cr(VI) enhanced the degradation efficiency of AO7. Meanwhile, the removal efficiency of Cr(VI) also increased in the presence of AO7. Under acidic conditions (pH = 3.0), the reduction efficiency of Cr(VI) was higher (approximately 94%). However, the presence of Cr(VI) diminished the effect of pH on the AO7 degradation efficiency. By increasing the input voltage from 80 to 120 V, the removal efficiencies of Cr(VI) and AO7 were observably increased from 54% to 88% and 62% to 89%, respectively. Adding organic matters inhibited the degradation of AO7 and promoted the reduction of Cr(VI). The addition of Cu(II), Co(II), Ni(II), Mn(II) and Fe(III) could inhibit the Cr(VI) reduction, but not significantly affect the degradation of AO7. The degradation intermediates of AO7 were identified by LC-MS/MS system and a possible degradation pathway was proposed. This study showed that the DBD plasma can simultaneously remove Cr(VI) and AO7, which provided a new idea for the actual wastewater treatment.

Response surface methodology investigation into optimization of the removal condition and mechanism of Cr(Ⅵ) by Na2SO3/CaO

The removal of Cr(Ⅵ) by chemical reduction-precipitation is widely applied in wastewater treatment plants. Nevertheless, the formation of Cr(OH)₃ with gel properties has weak settlement performance, making it necessary to add a coagulant aid to reduce the settling time and improve the settling effect. In this investigation, a high concentration of Cr(Ⅵ) was removed using Na₂SO₃ as a reducing agent and CaO as a coagulant. An improved reduction and precipitation experiment was modeled by applying a three-factor central composite experimental design (CCD). To reveal as many mechanisms as possible for Cr_T removal, other verification experiments were performed. The Cr_T removal efficiency decreased, which can be explained by the following three reasons: dissolution of Cr(Ⅲ), competition for adsorption between Ca²⁺ and Cr(Ⅲ) at different coagulation times, and formation of a solubility complex with Cr(Ⅲ) due to the surplus SO₃^2- in solution. The increasing Cr_T removal efficiency can be explained by the following two reasons: dissolved Ca²⁺ from CaO can neutralize CrO₂^- that is produced by the dissolution of Cr(OH)₃ in alkaline solution and can broaden the optimal final pH range of coagulation. Ca²⁺ could also strengthen the Cr_T removal through adsorption bridging and co-precipitation with CaO as the core of flocs.

Green synthesis of a magnetic hybrid adsorbent (CoFe2O4/NOM): Removal of chromium from industrial effluent and evaluation of the catalytic potential of recovered chromium ions

This work describes the removal of chromium ions from industrial effluent using a hybrid magnetic adsorbent, CoFe₂O₄/NOM, synthesized using water rich in natural organic matter. The hybrid obtained at ambient temperature (HbAmb) was calcined at 200, 400, and 800°C for 2h, and formation of the cobalt ferrite phase was confirmed by XRD, which indicated the presence of NOM in the structure of the material. Removal tests showed that HbAmb provided efficient removal of chromium at the natural pH of the effluent, while the other materials were effective at pH 6. Evaluation of the kinetics showed excellent performance of the process, with 70-87% removal in 20min, which provided a high degree of flexibility. The hybrid showed high removal during five reuse cycles, ranging from 96% in the first cycle to 82% in the final. The matrices containing the saturated adsorbent (HbAmb_Sat) and recovered chromium ions (CrD) showed high performance in the catalytic reduction of 4-nitrophenol, with conversion rates of 99.9% in short periods of time, as well as excellent potential for reuse in three cycles. The results demonstrated that the production of a technological material and its use for remediation could be achieved in an ecologically sustainable manner.

Mechanisms of metal sorption by biochars: Biochar characteristics and modifications

Biochar produced by thermal decomposition of biomass under oxygen-limited conditions has received increasing attention as a cost-effective sorbent to treat metal-contaminated waters. However, there is a lack of information on the roles of different sorption mechanisms for different metals and recent development of biochar modification to enhance metal sorption capacity, which is critical for biochar field application. This review summarizes the characteristics of biochar (e.g., surface area, porosity, pH, surface charge, functional groups, and mineral components) and main mechanisms governing sorption of As, Cr, Cd, Pb, and Hg by biochar. Biochar properties vary considerably with feedstock material and pyrolysis temperature, with high temperature producing biochars with higher surface area, porosity, pH, and mineral contents, but less functional groups. Different mechanisms dominate sorption of As (complexation and electrostatic interactions), Cr (electrostatic interactions, reduction, and complexation), Cd and Pb (complexation, cation exchange, and precipitation), and Hg (complexation and reduction). Besides sorption mechanisms, recent advance in modifying biochar by loading with minerals, reductants, organic functional groups, and nanoparticles, and activation with alkali solution to enhance metal sorption capacity is discussed. Future research needs for field application of biochar include competitive sorption mechanisms of co-existing metals, biochar reuse, and cost reduction of biochar production.

Remediation of hexavalent chromium spiked soil by using synthesized iron sulfide particles

Carboxymethyl cellulose (CMC) stabilized microscale iron sulfide (FeS) particles were synthesized and applied to remediate hexavalent chromium (Cr(VI)) spiked soil. The effects of parameters including dosage of FeS particles, soil moisture, and natural organic matter (NOM) in soil were investigated with comparison to iron sulfate (FeSO₄). The results show that the stabilized FeS particles can reduce Cr(VI) and immobilize Cr in soil quickly and efficiently. The soil moisture ranging from 40% to 70% and NOM in soil had no significant effects on Cr(VI) remediation by FeS particles. When molar ratio of FeS to Cr(VI) was 1.5:1, about 98% of Cr(VI) in soil was reduced by FeS particles in 3 d and Cr(VI) concentration decreased from 1407 mg kg^-1 to 16 mg kg^-1. The total Cr and Cr(VI) in Toxicity Characteristic Leaching Procedure (TCLP) leachate were reduced by 98.4% and 99.4%, respectively. In FeS particles-treated soil, the exchangeable Cr fraction was mainly converted to Fe-Mn oxides bound fraction because of the precipitation of Cr(III)-Fe(III) hydroxides. The physiologically based extraction test (PBET) bioaccessibility of Cr was decreased from 58.67% to 6.98%. Compared to FeSO₄, the high Cr(VI) removal and Cr immobilization efficiency makes prepared FeS particles a great potential in field application of Cr(VI) contaminated soil remediation.

Hydrogen inhibition in a wet aluminum dust collection system using dichromate solution

By using dichromate solution, hydrogen production reaction between aluminum dust and water is inhibited.

Adsorption of arsenic, phosphorus and chromium by bismuth impregnated biochar: Adsorption mechanism and depleted adsorbent utilization

Bismuth impregnated biochar were synthesized to deal with wastewater pollution. Nitrogen adsorption-desorption isotherms, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to determine the characteristics of adsorbents and explore the main adsorption mechanism. Results showed that bismuth particle was carried successfully within the biochar matrix, making contributions to creating micropore and boost specific surface area. The loaded bismuth, served as the adsorption site, rather than the specific surface area played an important role in arsenic and phosphorus adsorption. Batch adsorption experiments demonstrated a fit Langmuir model for arsenic (As) and phosphorus (P) and a suitable Freundlich model for chromium (Cr). Thermodynamic parameters depicted the endothermic nature and the spontaneous process for phosphate and arsenic adsorption. Besides, this contaminant-loaded carbon adsorbent was further applied for the removal of methylene blue from aqueous solution.

The oxidative degradation of polystyrene resins on the removal of Cr(VI) from wastewater by anion exchange

Cr(VI) is a powerful oxidant and is capable of oxidizing most of the organic materials. Therefore, it is possible for Cr(VI) to oxidize the polymeric resins and change the sorption properties of the resins on the removal of Cr(VI) from wastewater by anion exchange. In this study, three polystyrene resins (D201, D202, and D301) with different functional groups (-N(+)(CH3)3, -N(+)(CH3)2(C2H4OH), and N(CH3)2) were assessed on oxidation stability for Cr(VI) removal from wastewater in fixed-bed column experiments. After a 10-cycle operation, due to the oxidation of the resin, the sorption capacity of D201, D202, and D301 resins decreased by 23.5, 29.3, and 17.3%, when approximately 20-34%, 31-50%, and 18-30% of Cr(VI) was reduced to Cr(III) during each cycle respectively. The results of the Fourier transform infrared spectroscopy (FT-IR) showed that both the cleavage of CN and the formation of CO bonds occurred on the polystyrene resins during the Cr(VI) removal process. The resin simulation experiments further validated the oxidation of CC and CN bonds connected with phenethyl groups. Based upon the results from column operations and the resin simulated experiments, the oxidation mechanism of the polystyrene resin was proposed.

Kinetics and mechanism of hexavalent chromium removal by basic oxygen furnace slag

Basic oxygen furnace slag (BOFS) has the potential to remove hexavalent chromium (Cr(VI)) from wastewater by a redox process due to the presence of minerals containing Fe(2+). The effects of the solution pH, initial Cr(VI) concentration, BOFS dosage, BOFS particle size, and temperature on the removal of Cr(VI) was investigated in detail through batch tests. The chemical and mineral compositions of fresh and reacted BOFS were characterized using scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) system and X-ray diffractometer (XRD). The results show that Cr(VI) in wastewater can be efficiently removed by Fe(2+) released from BOFS under appropriate acidic conditions. The removal of Cr(VI) by BOFS significantly depended on the parameters mentioned above. The reaction of Cr(VI) with BOFS followed the pseudo-second-order kinetic model. Fe(2+) responsible for Cr(VI) removal was primarily derived from the dissolution of FeO and Fe3O4 in BOFS. When H2SO4 was used to adjust the solution acidity, gypsum (CaSO4·2H2O) could be formed and become an armoring precipitate layer on the BOFS surface, hindering the release of Fe(2+) and the removal of Cr(VI). Finally, the main mechanism of Cr(VI) removal by BOFS was described using several consecutive reaction steps.