Study of the involved sorption mechanisms of Cr(VI) and Cr(III) species onto dried Salvinia auriculata biomass

Biosorption of hexavalent chromium from aqueous solution by pristine and CaCl2-modified erythromycin production residues

In this study, the adsorptive removal of hexavalent chromium [Cr(VI)] from aqueous solutions by the pristine and salt-treated (CaCl₂) erythromycin production residue (EPRs and SEPRs) were investigated. Batch experiments were carried out to determine the effect of contact time, sorbent dosage, pH, initial Cr concentration, and temperature on Cr(VI) sorption by EPRs and SEPRs. The highest adsorptive removal capacities were achieved at the pH equal to 1.0, and the maximum adsorption capacities for EPRs and SEPRs at optimized conditions were 21.74 and 35.24 mg g^-1, respectively. The FTIR spectra and SEM studies were examined for the pristine adsorbent and after the adsorption of Cr(VI). Moreover, thermodynamic results indicated that Cr sorption by EPR/SERPs was feasible, spontaneous, and endothermic under the optimum conditions. Langmuir model fitted well with the experimental data. Kinetic modeling revealed that the biosorption of Cr(VI) by EPRs and SEPRs obeyed the second-order model than the first-order model. The process involving rate-controlling step is much complex involving both boundary layer and intra-particle diffusion processes. Furthermore, the adsorption-coupled-reduction process was believed as the main mechanism of Cr(VI) removal by EPRs and SEPRs. In summary, both adsorbents could be considered as promising low-cost biosorbent for the removal of Cr(VI) from aqueous systems.

Scavenging of hexavalent chromium from aqueous solution by Macadamia nutshell biomass modified with diethylenetriamine and maleic anhydride

Based on the premise that aqueous anions of hexavalent chromium (Cr(VI)) are capable of electrostatic interaction with cationic and polar active sites, acid-washed Madacamia nutshell biomass was sequentially treated with diethylenetriamine (DETA) and maleic anhydride (MA) to graft poly(diethylenetriamine-co-maleic anhydride). By displaying a new peak at 1685 cm^-1 ascribed to amide CO stretching vibrations, Fourier transform infrared spectroscopy highlighted the formation of amide groups through reaction of DETA with carboxyl groups on the biomass surface. Scanning electron microscopic images of the MA-modified biomass displayed polymeric growths attributed to copolymerization of DETA with MA. The polar and ionizable amide and amine groups of the grafted copolymer endowed the adsorbent with Cr(VI) removal capabilities over a wide pH range demonstrated by removal efficiencies between 70.9% and 81.7% in the pH 1.6 to pH 10.0 range for the treatment of 20 mL solutions containing 100 mg L^-1 Cr(VI) with 200 mg of adsorbent. Conformity of the adsorption isotherm data to the Freundlich model revealed the heterogeneous nature of the adsorbent surface, which comprised a variety of functional groups capable of interaction with Cr(VI) species in solution. The Sips isotherm model provided the best fit to the equilibrium experimental data, and the adsorption capacity was 779.1 mg g^-1 at pH 1.6, room temperature and an adsorbent dosage of 5.0 g L^-1. The findings indicate that Cr(VI) adsorption onto diethylenetriamine and maleic anhydride modified Madacamia nutshell biomass is a promising option for Cr(VI) removal from aqueous solutions.

A critical review with emphasis on recent pieces of evidence of Moringa oleifera biosorption in water and wastewater treatment

The increasing demand for using competent and inexpensive methods based on biomaterials, like adsorption and biosorption, has given rise to the low-priced alternative biosorbents. In the past few years, Moringa oleifera (MO) has emerged as a green and low-priced biosorbent for the treatment of contaminated waters with heavy metals and dyes, and given its availability, we can create another generation of effective biosorbents based on different parts of this plant. In this review paper, we have briefed on the application of MO as a miraculous biosorbent for water purification. Moreover, the primary and cutting-edge methods for the purification and modification of MO to improve its adsorption are discussed. It was found that MO has abundant availability in the regions where it is grown, and simple chemical treatments increase the effectiveness of this plant in the treatment of some toxic contaminants. The different parts of this miraculous plant's "seeds, leaves, or even husks" in their natural form also possess appreciable sorption capacities, high efficiency for treating low metal concentrations, and rapid adsorption kinetics. Thus, the advantages and disadvantages of different parts of MO as biosorbent, the conditions favorable to this biosorption, also, the proposal of a logical mechanism, which can justify the high efficiency of this plant, are discussed in this review. Finally, several conclusions have been drawn from some important works and which are examined in this review, and future suggestions are proposed.

Evaluation of Kinetic, Equilibrium and Thermodynamics of Cationic Ion Using Agro-Industrial Residues of Plantain (Musa paradisiaca)

This study aimed to evaluate the adsorptive capacity of Cr (VI) on the residues of the plantain starch extraction process in a batch system, determining the effect of temperature, initial concentration and adsorbent dose. The adsorbent was characterized by FTIR and SEM. The Cr (VI) solution was placed in contact with the adsorbent at pH 2 and 200 rpm. The results revealed the presence of COO−, OH− and CHx+ functional groups in the adsorbent. In addition, the adsorption process is controlled by chemisorption and electrostatic interactions. We also found that temperature and adsorbent dose are the variables with significant influence. The highest adsorption capacity was 64.46 mg/g at 55 °C, 200 mg/L and 0.14 g of biomaterial. Based on the kinetic behavior, it was found that the data are adjusted by the pseudo-second order, Elovich and intraparticle diffusion models. The fit of the isotherms to the Freundlich and Dubinin–Radushkevich models establishes that the limiting step of the process is the chemical reaction. The thermodynamic parameters determine that the process is endothermic, with strong biomass–metal bonds that are favorable and spontaneous as the temperature increases. The results indicate that the residual plantain pulp is a residue that can be used in the removal of Cr (VI) ions, and it contributes to the state of the art in terms of the use of new agro-industrial waste.

Synthesis of Calcium Silicate Hydrate from Coal Gangue for Cr(VI) and Cu(II) Removal from Aqueous Solution

Both the accumulation of coal gangue and potentially toxic elements in aqueous solution have caused biological damage to the surrounding ecosystem of the Huainan coal mining field. In this study, coal gangue was used to synthesize calcium silicate hydrate (C-S-H) to remove Cr(VI) and Cu(II)from aqueous solutions and aqueous solution. The optimum parameters for C-S-H synthesis were 700 °C for 1 h and a Ca/Si molar ratio of 1.0. Quantitative sorption analysis was done at variable temperature, C-S-H dosages, solution pH, initial concentrations of metals, and reaction time. The solution pH was precisely controlled by a pH meter. The adsorption temperature was controlled by a thermostatic gas bath oscillator. The error of solution temperature was controlled at ± 0.3, compared with the adsorption temperature. For Cr(VI) and Cu(II), the optimum initial concentration, temperature, and reaction time were 200 mg/L, 40 °C and 90 min, pH 2 and 0.1 g C-S-H for Cr(VI), pH 6 and 0.07 g C-S-H for Cu(II), respectively. The maximum adsorption capacities of Cr(VI) and Cu(II) were 68.03 and 70.42 mg·g⁻¹, respectively. Furthermore, the concentrations of Cu(II) and Cr(VI) in aqueous solution could meet the surface water quality standards in China. The adsorption mechanism of Cu(II) and Cr(VI) onto C-S-H were reduction, electrostatic interaction, chelation interaction, and surface complexation. It was found that C-S-H is an environmentally friendly adsorbent for effective removal of metals from aqueous solution through different mechanisms.

Isolated and combined effects of glyphosate and its by-product aminomethylphosphonic acid on the physiology and water remediation capacity of Salvinia molesta

We evaluate the isolated and combined effects of glyphosate and its by-product aminomethylphosphonic acid (AMPA) and the potential of the aquatic macrophyte Salvinia molesta to remove these chemicals from contaminated water. Plants were exposed to environmentally relevant concentrations of glyphosate (0, 20, 40, 60, 80 and 100 µg l-1) or AMPA (0, 10, 20, 30, 40 and 50 µg l-1) for seven days. Then, based on the effective concentrations of glyphosate found to reduce photosynthetic rates by 10% (EC10) and 50% (EC50), the plants were exposed to combinations of 0, 16 and 63.5 µg glyphosate l-1 and 0, 5, 15, 25 µg AMPA l-1. The EC(10) and EC(50) were lower for AMPA (6.1 µg l-1 and 28.4 µg l-1 respectively) than for glyphosate (16 and 63.5 µg glyphosate l-1 respectively). When occurring together, the deleterious effects of those chemicals to plants increased. S. molesta plants removed up to 74.15% of glyphosate and 71.34% of AMPA from culture water. Due to its high removal efficiency, S. molesta can be used in phytoremediation programs. It will be important to evaluate the combined effects of glyphosate and AMPA in any toxicological studies of the herbicide.

Performance of the graphite felt flow-through electrode in hexavalent chromium reduction using a single-pass mode

Flow-through electrodes generally outcompete traditional parallel-plate electrodes in current efficiency and mass transfer. High-performance electrode materials can be costly and complicated to fabricate, hindering their wide application. In this study, we used commercial graphite felt (GF) as the cathode of a flow-through electrochemical cell to investigate its potential in treating Cr(VI) solution through electroreduction. The flow-through design with the porous GF electrode allowed sufficient contact surface with Cr(VI) and single-pass tests demonstrated a high reduction efficiency (95~100%) [117 mg/L~3 mg/L Cr(VI)] under acidic conditions. Slow flow rate and high current promoted electroreduction of Cr(VI). The presence of other metal ions could further improve Cr(VI) reduction at low flow rates due to enhanced conductivity in dilute solutions and generation of low valent ions as reducing agents. At fast flow rates, competition of these ions for reduction decreased Cr(VI) reduction efficiency. Moreover, an acidic environment prevented the coating of an insoluble layer on the GF surface and promoted durable performance, with a lower energy consumption [0.46 kWh for treating 100 L 117 mg/L Cr(VI) solution per unit area of GF]. This work demonstrated the potential of Cr(VI) detoxification using GF cathodes in flow-through electrochemical cell.

Unraveling the Underlying Heavy Metal Detoxification Mechanisms of Bacillus Species

The rise of anthropogenic activities has resulted in the increasing release of various contaminants into the environment, jeopardizing fragile ecosystems in the process. Heavy metals are one of the major pollutants that contribute to the escalating problem of environmental pollution, being primarily introduced in sensitive ecological habitats through industrial effluents, wastewater, as well as sewage of various industries. Where heavy metals like zinc, copper, manganese, and nickel serve key roles in regulating different biological processes in living systems, many heavy metals can be toxic even at low concentrations, such as mercury, arsenic, cadmium, chromium, and lead, and can accumulate in intricate food chains resulting in health concerns. Over the years, many physical and chemical methods of heavy metal removal have essentially been investigated, but their disadvantages like the generation of chemical waste, complex downstream processing, and the uneconomical cost of both methods, have rendered them inefficient,. Since then, microbial bioremediation, particularly the use of bacteria, has gained attention due to the feasibility and efficiency of using them in removing heavy metals from contaminated environments. Bacteria have several methods of processing heavy metals through general resistance mechanisms, biosorption, adsorption, and efflux mechanisms. Bacillus spp. are model Gram-positive bacteria that have been studied extensively for their biosorption abilities and molecular mechanisms that enable their survival as well as their ability to remove and detoxify heavy metals. This review aims to highlight the molecular methods of Bacillus spp. in removing various heavy metals ions from contaminated environments.

Functionalized MWCNTs-quartzite nanocomposite coated with Dacryodes edulis stem bark extract for the attenuation of hexavalent chromium

Multiwalled carbon nanotubes/quartzite nanocomposite modified with the extract of Dacryodes edulis leaves was synthesized and designated as Q, which was applied for the removal of Cr(VI) from water. The adsorbents (PQ and Q) were characterized using the SEM, EDX, FTIR, TGA, XRD, and BET analyses. The XRD revealed the crystalline composition of the nanocomposite while the TGA indicated the incorporated extract as the primary component that degraded with an increase in temperature. The implication of the modifier was noticed to enhance the adsorption capacity of Q for Cr(VI) by the introduction of chemical functional groups. Optimum Cr(VI) removal was noticed at a pH of 2.0, adsorbent dose (50 mg), initial concentration (100 mg dm^-3), and contact time (180 min). The kinetic adsorption data for both adsorbents was noticed to fit well to the pseudo-second-order model. The adsorption equilibrium data were best described by the Langmuir model. The uptake of Cr(VI) onto PQ and Q was feasible, endothermic (ΔH: PQ = 1.194 kJ mol^-1 and Q = 34.64 kJ mol^-1) and entropy-driven (ΔS : PQ = 64.89 J K^-1 mol^-1 and q = 189.7 J K^-1 mol^-1). Hence, the nanocomposite demonstrated potential for robust capacity to trap Cr(VI) from aqueous solution.

Closing the loop in a constructed wetland for the improvement of metal removal: the use of Phragmites australis biomass harvested from the system as biosorbent

Among the numerous clean-up techniques for water treatment, sorption methods are widely used for the removal of trace metals. Phragmites australis is a macrophyte commonly used in constructed wetlands for water purification, and in the last decades, its use as biosorbent has attracted increasing attention. In view of a circularly economy approach, this study investigated improvement of trace metal removal by recycling the biomass of P. australis colonizing a constructed wetland, which operates as post-treatment of effluent wastewater from an activated sludge plant serving the textile industrial district of Prato (Italy). After the annual mowing of the reed plants, the biomass was dried and blended to derive a sustainable and eco-friendly biosorbent and its sorption capacity for Fe, Cu, and Zn was investigated comparing the batch system with the easier-to-handle column technique. The possibility of regeneration and reuse of the biosorbent was also evaluated. The biomaterial showed an interesting sorption capacity for Cu, Fe, and Zn, both in batch and in column experiments, especially for Fe ions. The immobilization of the biosorbent in column filters induced some improvement in the removal efficiency, and, in addition, this operation mode has the advantage of being much more suitable for practical applications than the batch process.

Insight into efficient removal of Cr(VI) by magnetite immobilized with Lysinibacillus sp. JLT12: Mechanism and performance

In this work, Lysinibacillus sp. JLT12 was used to remove the Cr(VI)-induced passive layer on the magnetite. Mechanism study via dynamic kinetics, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy analyses revealed that Lysinibacillus sp. JLT12 could remove the passive layer (lepidocrocite and goethite) to facilitate the further Cr(VI) reduction by magnetite. For large-scale applications, porous ceramsite (PC) was prepared with magnetite, kaolin, and fallen leaves. Lysinibacillus sp. was then immobilized on the holes in PC. Slow-released nutrients were added to immobilized porous ceramsite (IM-PC) at a ratio of 1.5:10 (g/g) to supply carbon, nitrogen, and phosphorus to Lysinibacillus sp. JLT12 with low secondary pollution. The performance of IM-PC was evaluated via a column experiment. The results indicate that, in the presence of Lysinibacillus, the break-through time and maximum adsorption ability of IM-PC were 11.67 h and 121.47 mg/g, respectively. These values are higher than those of PC. Additionally, break-through curves detected at 5, 10, and 15 days demonstrated that the usage life of IM-PC was significantly longer than that of PC.

Isolation and Identification of Chromium Reducing Bacillus Cereus Species from Chromium-Contaminated Soil for the Biological Detoxification of Chromium

Chromium contamination has been an increasing threat to the environment and to human health. Cr(VI) and Cr(III) are the most common states of chromium. However, compared with Cr(III), Cr(VI) is more toxic and more easily absorbed, therefore, it is more harmful to human beings. Thus, the conversion of toxic Cr(VI) into Cr(III) is an accepted strategy for chromium detoxification. Here, we isolated two Bacillus cereus strains with a high chromium tolerance and reduction ability, named B. cereus D and 332, respectively. Both strains demonstrated a strong pH and temperature adaptability and survival under 8 mM Cr(VI). B. cereus D achieved 87.8% Cr(VI) removal in 24 h with an initial 2 mM Cr(VI). Cu(II) was found to increase the removal rate of Cr(VI) significantly. With the addition of 0.4 mM Cu(II), 99.9% of Cr(VI) in the culture was removed by B. cereus 332 in 24 h. This is the highest removal efficiency in the literature that we have seen to date. The immobilization experiments found that sodium alginate with diatomite was the better method for immobilization and B. cereus 332 was more efficient in immobilized cells. Our research provided valuable information and new, highly effective strains for the bioremediation of chromium pollution.

CR(VI) phytoremediation by hairy roots of Brassica napus: assessing efficiency, mechanisms involved, and post-removal toxicity

Industrial activities such as leather tanning involve the use of highly toxic inorganic pollutants, like Chromium (Cr). This work evaluated Cr(VI) remediation by hairy roots (HR) of Brassica napus, paying close attention to the mechanisms involved and the toxicity of post-removal solutions. Results showed that these roots were capable of tolerating concentrations of up to 10 mg L^-1 Cr(VI), while higher concentrations were toxic for HR development. Removal efficiency was assessed through the use of synthetic solutions containing different initial Cr(VI) concentrations (2, 5, or 10 mg L^-1). Regardless of these initial concentrations, the highest removal efficiency values were between 80 and 90% after 24 and 48 h of treatment, using a 2.0 g inoculum. The mechanisms involved were Cr accumulation (60%) and to a lesser extent, adsorption to the root biomass (30%). A fraction of Cr(VI) was intracellularly reduced to Cr(III), which suggests reductases may have played a role. Additionally, post-removal toxicity was evaluated through two bioassays (Lactuca sativa L. and AMPHITOX test) after the removal of 10 mg L^-1 Cr(VI). The treated solutions showed moderate phytotoxicity for L. sativa L. and no toxicity for R arenarum. The ability of HR to remove 10 mg L^-1 Cr(VI) from real tannery effluents collected from a regional industry (Córdoba province, Argentina) was also determined. The high removal efficiency observed (98%) demonstrates this system can be successful in treating complex wastewaters.

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.

Assessment of the use of Moringa oleifera seed husks for removal of pesticide diuron from contaminated water

Considering the need for new technologies for the removal of pesticides from the aqueous environment, the Moringa oleifera seed husks (h-MO) were investigated for the diuron adsorption from contaminated water at different temperatures and solute concentrations. The biosorbent used in this study was characterized by chemical, structural and textural analyses. The best experimental condition for the biosorption was determined by evaluating the mass of the biosorbent and the pH solution. We found a good adsorption capacity for the herbicide where the maximum adsorption capacity was 14.74 mg/g at pH 5 and 45°C. In addition, the adsorption process of diuron by the h-MO occurred spontaneously, in which, ΔG° values increased as the temperature increased, meaning that the process tends to a more energetically favourable process at higher temperatures. Both Langmuir and Sips isotherm models presented satisfactory adjustment at all temperatures and the pseudo-second-order model presented the best fit for the experimental results. The application of the intra-particle diffusion model showed that the adsorption process started instantaneously through the boundary layer of the adsorbent and that the pore diffusion step was a limiting step in the process. Finally, the capacity of the h-MO was compared with other adsorbents that had been used for diuron removal from contaminated where it was found that the adsorption capacity of the h-MO is much higher than other natural adsorbents.

Biomaterials cross-linked graphene oxide composite aerogel with a macro-nanoporous network structure for efficient Cr (VI) removal

Chitosan (CS) is an attractive bio-adsorbent in pollutant removal due to its environment-friendly properties and abundant adsorption sites. However, the weak mechanical properties and strong dissolubility in acidic conditions of CS hinder its wide application. Herein, a facile method was proposed to fabricate polydopamine (PDA) and CS cross-linked graphene oxide (GO) (GO/CS/PDA) composite aerogel for Cr (VI) removal. GO was cross-linked with CS, forming a reinforced and three-dimensional macroporous structure; the introduced PDA was simultaneously cross-linked with CS and GO, providing more abundant nanopores and active sites for Cr(VI) removal. Based on the batch experiment results, GO/CS/PDA exhibited an optimized mass ratio (1:20:2) of GO, CS, and PDA for the most effective Cr(VI) adsorption; the adsorption removal rate of Cr(VI) was pH dependent, with the highest removal rate at pH = 3.0. The pseudo-second-order kinetic and Freundlich models were more suitable for fitting the adsorption kinetics and adsorption isotherms, respectively, and the maximum adsorption capacity for GO/CS/PDA was 312.05 mg/g at 298 K. Thermodynamics parameters indicated that the adsorption was a spontaneous and exothermic process. The excellent mechanical integrity and reusable adsorption performance of GO/CS/PDA promise the adsorbent with satisfactory reusability.

Simple and Competitive Adsorption Study of Nickel(II) and Chromium(III) on the Surface of the Brown Algae Durvillaea antarctica Biomass

In this work Ni(II) and Cr(III) adsorption on Durvillaea antarctica surface were studied, optimal condition of pH, adsorption time is achieved at pH 5.0, with contact times of 240 and 420 minutes for a maximum adsorption capacity of 32.85 and 102.72 mg g^-1 for Ni(II) and Cr(III), respectively. The changes in the vibration intensity of the functional groups detected in the starting material by Fourier transform infrared spectroscopy and the opening of the cavities after the biosorption process detected by scanning electron microscopy images suggested the interaction of the metal ions with the surface and the changes in the chemical behavior of the solid. The heavy metal adsorption equilibrium data fitted well to the Sips model. The effect of competitive ions on adsorption equilibrium was also evaluated, and the results showed that the two metals compete for the same active sites of the biosorbent; the increase of the Ni(II) initial concentration increases its adsorption capacity but decreases the adsorption capacity of Cr(III).

Study of the properties and mechanism of deep reduction and efficient adsorption of Cr(VI) by low-cost Fe3O4-modified ceramsite

To efficiently treat low-concentration chromium wastewater at low cost, adsorbent iron-modified ceramsite (FCM) was successfully prepared. The adsorbent was characterized by SEM, XRD, FT-IR, XPS, ζ potential and VSM. From the experimental results, the optimum adsorption pH, adsorbent dosage and adsorption time were 4, 1g/L and 120min, respectively. The percentage removal of Cr(VI) was 93% and the reduction efficiency of Cr(VI) was 74.3% when the concentration was 2mg/L. After 5cycles, using 0.1mol/L NaOH as the regeneration agent, the removal rate of Cr(VI) did not decrease significantly. Furthermore, the mechanism of deep reduction and adsorption, the process of regeneration, the kinetics and adsorption isotherms were also explored. This paper can provide theoretical and technical support for reducing the toxicity and the advanced treatment of hexavalent chromium.

Use of Salvinia sp on the adsorption of hexavalent chromium

The remotion of hexavalent chromium in the form of chromate in aqueous solution was done using the aquatic plant Salvinia sp as biosorbent. The chemical modification of the Salvinia surface was performed by organosolv adapted method. The untreated Salvinia and the modified were characterized by infrared spectroscopy, Boehm titration, scanning electron microscopy, energy dispersive system, point of zero charge, surface area analysis, and porosity. Batch adsorption experiments were performed to observe the effects of pH, contact time, initial concentration, and temperature on the metal removal process. The characterization results show the chemistry modifically changed the modified Salvinia structure compared with untreated Salvinia. The adsorption test results showed the maximum adsorption capacity of 26.03 mg g^-1. The kinetic equilibrium was reached in about 3 h, and the better temperature and pH were 298 K and 7, respectively. The adsorption and kinetic models were Freundlich and pseud-second order, respectively. This study showed the Salvinia sp after the chemical treatment can be used with biosorbent for hexavalent chromate in the form of chromate, being a natural material with low cost and plentiful in the environment.

Efficacy of Cu(II) as an electron-shuttle mediator for improved bioelectricity generation and Cr(VI) reduction in microbial fuel cells

Cu(II) ion was employed as an electron-shuttle mediator to enhance bioelectricity output and accelerate reduction rate of Cr(VI) in a dual-chamber microbial fuel cell (MFC). In the presence of Cu(II), power density and the Cr(VI) reduction rate were 1235.53 mW m^-2 and 1.191 g m^-3 h^-1, respectively, which were 1.44 times and 1.17 times than that of MFC in the absence of Cu(II). A series of electrochemical analysis confirmed the presence of Cu(II) can diminish overpotential and diffusional resistance of MFC, further accelerating electrochemical reduction process of Cr(VI) via an indirect mechanism. After reduction, Cr(VI) and Cu(II) in this work were mainly deposited on cathode electrodes in the form of Cr(OH)₃ and little Cu, thus wastewater containing Cr(VI) was successfully treated by bio-electrochemical technology. The aim of this work was to study the efficacy of Cu(II) as an electron-shuttle mediator for improved bioelectricity generation and Cr(VI) reduction in MFCs.

Unravelled keratin-derived biopolymers as novel biosorbents for the simultaneous removal of multiple trace metals from industrial wastewater

Biopolymers derived from modified poultry feathers (KB) were developed to target a broad range of potentially toxic trace elements for their removal from synthetic wastewater and industrial process affected water. The chemical modifications increased surface functionality of KBs for enhanced metal adsorption. Unmodified KB (SM-03) added to synthetic wastewater spiked with nine transition and redox sensitive elements (30-50 μg L^-1 each) removed >82% of Pb, Ni, Co and Zn, whereas modified KBs (SM-01 and SM-06) removed 68-100% of Se^IV, V^V and Cr^VI. Similar results were observed when spiked process water was used. Experimental observation suggested chemical reduction of redox sensitive elements on the modified KB surfaces to their non-toxic/non-mobile redox states. Biopolymer SM-06 showed a maximum adsorption capacity of 17 mg g^-1 for V^V and 15 mg g^-1 for Cr^VI at ~20 °C. Due to the abundance of raw material and simplicity of the modifications presented here, modified KBs may serve as a useful option for large-scale water treatment.

Using Myriophyllum aquaticum (Vell.) Verdc. to remove heavy metals from contaminated water: Better dead or alive?

This study aimed to investigate the potential of the invasive macrophyte Myriophyllum aquaticum to remove heavy metals. The elements tested were Cd, Cr, Ni, and Zn, in single-metal trials, and experiments were performed with both the living and dead biomass of the plant. In respect of metal removal by living plants, the element that was removed the most was Zn, though Cd showed the highest concentration in plant shoots. The metal negative effect on plant growth was, therefore, more important than the level of metal concentration in plant tissue in determining the removal percentages. All the metals were mostly accumulated in the roots, where a considerable fraction of the element was simply adsorbed to root cell wall, except in the case of Cr. In shoots, the fraction of the adsorbed metal was extremely low in respect to roots, thereby implying a lower apoplastic binding capacity. As regards a possible use of the dead biomass for metal removal, we proposed the generation of a hybrid biosorbent enclosing the dried and grounded plant biomass in cotton bags to improve its handling and its adsorption capacity, in view of a valid alternative to reduce the problems of packed beds. Cadmium-and especially Zn-were the elements removed most efficiently with respect to the other metals. On comparing the removal percentages of the living biomass and the hybrid biosorbent, our data deposed in favour of the use of M. aquaticum as dead biomass for a possible application of this invasive macrophyte in the biological treatment of metal-contaminated water. Our findings may be beneficial to metal removal application accompanying wetland management, devising a possible use of M. aquaticum waste material after its removal from the invaded habitats.

An efficient and reusable quaternary ammonium fabric adsorbent prepared by radiation grafting for removal of Cr(VI) from wastewater

A novel quaternary ammonium polyethylene nonwoven fabric for removing chromium ions from water was prepared via radiation-induced grafting of glycidyl methacrylate and further modification with N,N'-dimethylethylenediamine. The structural and morphological characteristics of the adsorbent were analyzed using Fourier transform infrared spectroscopy (FTIR), thermogravimetry and differential thermogravimetry (TG/DTG), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The influences of several principal factors, including pH value, initial Cr(VI) concentration, contact time, and coexisting anions (including SO₄^2-, CO₃^2-, NO₃^-, PO₄^3-, and Cl^-), on adsorption performance were investigated via batch tests. The results showed that the optimum removal efficiency was 99.2% at pH 3 and the maximum adsorption quantity for Cr(VI) at 25 °C was 336 mg/g. The adsorption kinetic parameters were better fitted with the pseudo-second-order kinetic model, and the equilibrium data were described very well by the Freundlich isotherm model. Furthermore, the as-synthesized adsorbent exhibited excellent regeneration and recyclability while maintaining high adsorption performance after five adsorption/desorption cycles.

Synthesis of graphene/SiO2@polypyrrole nanocomposites and their application for Cr(VI) removal in aqueous solution

A novel hybrid nanocomposite, polypyrrole nanoparticles (PPy) anchored on the graphene/silica nanosheets with the high specific surface area (polypyrrole-graphene/silica, GS-PPy), was synthesized by a facile in situ polymerization and shows great potential to remove hexavalent chromium [Cr(VI)] in aqueous solutions. Characterizations by XRD, TEM, SEM, BET, FT-IR and XPS, have confirmed that the PPy nanoparticles were well-distributed on the surface of GS nanosheets. The effects of pH, contact time, the concentration of Cr(VI), temperature, coexisting ions and the number of adsorption-desorption cycles were studied. The maximum adsorption capacity of the GS-PPy for Cr(VI) was 429.2 mg g^-1 at 298 K at pH 2, which was much higher than PPy nanoparticles and other related materials. The adsorption data fitted to the pseudo-second-order model and Langmuir isotherm model. The removal mechanism involved in electrostatic attraction, ion exchange and reduction process that partial adsorbed Cr(VI) was reduced to Cr(III). And Cr(III) was still retained on the surface of GS-PPy. The GS-PPy nanocomposite will be a potential candidate for the removal of Cr(VI) from the industrial waste water.

Enhanced removal of Cr(VI) from aqueous solution by supported ZnO nanoparticles on biochar derived from waste water hyacinth

Biochar derived from waste water hyacinth was prepared and modified by ZnO nanoparticles for Cr(VI) removal from aqueous solution with the aim of Cr(VI) removal and management of waste biomass. The effect of carbonization temperature (500-800 °C), ZnO content (10-50 wt%) loaded on biochar and contact time (0.17-14 h) on the Cr(VI) removal were investigated. It was found that higher than 95% removal efficiency of Cr(VI) can be achieved with the biochar loaded 30 wt% ZnO. The adsorption kinetics of the sorbent is consistent with the pseudo-second-order kinetic model and adsorption isotherm follows the Langmuir model with maximum adsorption capacity of 43.48 mg g^-1 for Cr(VI). Multiple techniques such as XRD, XPS, SEM, EDX and FT-IR were performed to investigate the possible mechanisms involved in the Cr (VI) adsorption. The results show that there is precipitation between chromium ions and Zn oxide. Furthermore, the ZnO nanoparticles acts as photo-catalyst to generate photo-generated electrons to enhance the reduction of Cr(VI) to Cr(III). The as-prepared ZnO/BC possess good recyclability and the removal ratio remained at about 70% in the fifth cycle, which suggests that both contaminants removal and effective management of water hyacinth can be achieved by the approach.