Maximum Cr Research Articles

Structural and magnetic characterization of (FeCo)1−xCrx nanowires array prepared by pulsed electrodeposition

(FeCo)1−xCrx nanowires (NWs) were grown by a pulsed AC electrodeposition technique into aluminum oxide nanoporous templates with good consistency. The structural features and morphologies of the NWs array were examined by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). A vibrating sample magnetometer equipped with the first-order reversal curve (FORC) diagram was then used to find the magnetic properties. The formation of NW with the (110) texture was also confirmed for the whole series samples. With increasing the Cr addition on the FeCo NW arrays, the corresponding diffraction peak in the XRD patterns was shifted to the lower angles and the lattice constant was increased. FE-SEM analysis also revealed that (FeCo)1−xCrx NWs with the length of 10 µm were grown above the Cu pre-plating layer into aluminum oxide with the pore diameter of 30 nm. Further, the hysteresis loops showed that the coercivity were decreased from 2265 to 1388 Oe, respectively, with an increase in the Cr content from the concentration of 0 to 0.03 M. FORC diagrams proved a decrease of the magnetostatic interaction with Cr content and a non-interacting single domain structure for the maximum Cr addition. Local magnetic interactions were also found to prevail in the presence of the Cr element beside the magnetic phase.

Optimization of Hexavalent Chromium Biosorption by Shewanella putrefaciens Using the Box-Behnken Design

Cr(VI) is a ubiquitous pollutant that poses a serious threat to human health. Recently, the use of microorganisms to adsorb heavy metals has attracted research attention. However, there are few studies on the biosorption of Cr(VI) by Shewanella putrefaciens, which is a metal-reducing bacterium. In this paper, single-factor experiments were designed to investigate the effect of hexavalent chromium by Shewanella putrefaciens, and response surface methodology (RSM) based on the Box-Behnken design (BBD) was performed to study the Cr(VI) biosorption behavior of Shewanella putrefaciens. The coefficient of determination (R2 = 0.811 for Cr(VI)) and probability value (P < 0.05) demonstrated significance for the obtained regression model. The results showed that the model was suitable for experimental data, and the maximum Cr(VI) removal efficiency by Shewanella putrefaciens was 85.68% under the optimum conditions of a contact time of 16.57 h, pH value of 8, and biomass dosage of 0.42 g/L, which were verified by additional experiments. ANOVA and 3D response graph analysis showed that the variables with significant influences were pH and temperature. In addition, scanning electron microscopy (SEM) results demonstrated that after biosorption of Cr(VI) by Shewanella putrefaciens, granular complexes attached to rough cell surfaces were observed. Furthermore, Fourier transform infrared spectroscopy (FT-IR) analysis showed that the distribution of Cr(VI) on the cell surface was related to the carboxyl, ether, amide, hydroxyl, and phosphoric acid groups of Shewanella putrefaciens. This study is useful to explore the process and mechanism of heavy metal adsorption by Shewanella putrefaciens and provide new ideas for the microbial remediation of metal pollution in water.

A high Cr (VI) absorption efficiency and easy recovery adsorbent: Electrospun polyethersulfone/polydopamine nanofibers

AbstractCr (VI) is a highly toxic pollutant to humans, to achieve high adsorption capacity, easy recovery, and good reusability, polyethersulfone/polydopamine (PES/PDA) ultrafine fibers were prepared successfully. A series of preparing effect factors were investigated systematically and the optimum one is 8.5 pH value at room temperature and 2 g/L dopamine concentration. And then they were used as an adsorbent for the removal of Cr (VI) ions from wastewater. The effect factors pH, the adsorbent dosage, and time were discussed on Cr (VI) adsorption process and the Cr (VI) adsorption behavior was investigated. It is found that the maximum Cr (VI) adsorption capacity is 115.2 ± 4.8 mg/g at pH = 3 using 0.06 g PES/PDA with 80 mins. The Cr (VI) adsorption process followed the pseudo‐second‐order model (r2 ≥ 0.99) and adsorption isotherms were fitted to the Langmuir model (R2 ≥ 0.999). Furthermore, the Cr (VI) adsorption mechanism was supposed according to the X‐ray photoelectron spectroscopic results. Finally, PES/PDA ultrafine fibers were considered to be a promising adsorbent with good stability (decomposing temperature, 356°C), high adsorption efficiency (112.1 ± 2.5 mg/g), and good reusability (three times) on the coexistence of anions and the actual industry wastewater environment.

Adsorption of Cr(VI) onto a low‐cost ceramsite: Preparation, kinetics, and thermodynamics

AbstractA novel composite ceramsite derived from bamboo powders and bentonite was developed as adsorbents for waste water treatment. The microstructure, morphology, and thermal properties of ceramsite were characterized by X‐ray powder diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, energy dispersive X‐ray fluorescence, and differential thermal analysis. The effect of compositions, sintering temperatures, sintering time, pH, and adsorption time on the adsorption performance of the ceramsites were studied. The results revealed that the maximum Cr(VI) removal rate of 99% can be obtained in the ceramsite with weight ratio bamboo to bentonite of 6:4, the annealing temperature of 1000°C, the solution pH 1 and contact time of 12 h. The adsorption kinetics and adsorption isotherm analysis revealed that the adsorption process best followed pseudo‐second‐order reaction kinetics and the Freundlich adsorption isotherm. The adsorption mechanism was discussed and both chemisorption and physiosorption were proposed as adsorption mechanism of Cr(VI). Moreover, the thermodynamic parameters, including changes in Gibbs free energy, entropy, and enthalpy, were studied in detail. The bamboo powder/bentonite composite ceramsite developed in this work is a promising candidate as an effective and economical adsorbent for Cr(VI) ions removal.

Hydroxyl modification of silica aerogel: An effective adsorbent for cationic and anionic dyes

Hydrophobic silica aerogels are attractive and highly efficient adsorbent for oil wastewater. However, the hydrophobicity seriously affects their dispersibility in water and limits the application for cationic and anionic dyes adsorption. Herein, a green sol-gel method was developed to prepare silica aerogels by using rice husk ash as silica source. The results demonstrate that heat treatment significantly impacts surface functional groups and pores of silica aerogels, which can be easily adjusted by changing heat treatment temperature. Cationic (Methyl blue, MB) and anionic (Congo red, CR) dyes were used to evaluate the adsorption ability of modified silica aerogels. The maximum MB and CR adsorption capacities of silica aerogel treat at 500℃ (SA-500) reached to 428.9 and 437.2 mg/g, respectively, which was achieved by simple control of pH. Besides, the adsorption of MB and CR by the SA-500 obeys the pseudo-second-order adsorption model. Electrostatic attraction and hydrogen bonds are critical for the adsorption of MB and CR. Furthermore, the adsorption efficiency of SA-500 for MB and CR was still as high as 95.5 % and 89.5 %, respectively, after 10 cycles of adsorption/desorption. Thus, SA-500 shows a promising application in removing both cationic and anionic dyes from wastewaters.

Biosorption of chromium ions through modified Eichhornia crassipes biomass form the aqueous medium

Aim: To explore the potential of water hyacinth biomass as a low cost adsorbent for sequestrating chromium ions from aqueous solution. Methodology: The biosorption behavior of chromium ions (Cr(III) and Cr(VI)) from aqueous solution by powdered water hyacinth biomass was assessed through series of batch experiments using advanced instrumentation techniques like Scanning Electron Microscopy with Energy Dispersive X-ray analysis and Fourier Transform Infra-Red spectroscopy, pH zero point charge. The equilibrium relationship between sorbent and sorbate was determined in solution by using the isotherms Langmuir, Freundlich and Temkin models. Results: The optimum pH for Cr(III) and Cr(VI) adsorption was 5.0 and 2.0 whereas the maximum Cr(III) and Cr(VI) adsorption was 99.80 and 89.15%, respectively. Langmuir isotherm fitted the data well with R2 value of 0.999 for both Cr(III) and Cr(VI) by the biosorbent. Adsorption kinetics showed that the experimental data fitted the pseudo second order kinetic model with R2 value of 0.999 for both Cr(III) and Cr(VI). The EDX spectra peaks of Ca, O, C indicated the presence of functional groups such as –OH and –COOH in the biosorbent. The zeta potential analysis depicted that water hyacinth biomass had negative surface charge of (-) 23.5mV. Interpretation: Water hyacinth based biosorbent can be utilized for the removal of Cr ions from industrial waste water. Further, water hyacinth has additional advantage that it reduces or even eliminates the diverse impact of weeds on the environment. Key words: Biosorption, Chromium removal, Water hyacinth

Selective Removal of Hexavalent Chromium from Wastewater by Rice Husk: Kinetic, Isotherm and Spectroscopic Investigation

Chromium (Cr) in water bodies is considered as a major environmental issue around the world. In the present study, aqueous Cr(VI) adsorption onto rice husk was studied as a function of various environmental parameters. Equilibrium time was achieved in 2 h and maximum Cr(VI) adsorption was 78.6% at pH 5.2 and 120 mg L−1 initial Cr(VI) concentration. In isotherm experiments, the maximum sorption was observed as 379.63 mg g−1. Among four isotherm models, Dubinin–Radushkevich and Langmuir models showed the best fitting to the adsorption data, suggesting physical and monolayer adsorption to be the dominant mechanism. The kinetic modeling showed that a pseudo-second order model was suitable to describe kinetic equilibrium data, suggesting a fast adsorption rate of Cr(VI). The results of FTIR spectroscopy indicated that mainly –OH and C–H contributed to Cr(VI) adsorption onto rice husk. This paper provided evidence that rice husk could be a cost-effective, environment-friendly and efficient adsorptive material for Cr(VI) removal from wastewater due to its high adsorption capacity.

Selective separation of Chromium (VI) from aqueous solutions by adsorption onto truffle peels as novel biomass

ABSTRACT The current quest develops a simple and highly selective spectrophotometric method for trace determination of Cr(VI) in water using truffle peels powder (TPPs) as solid-phase extractor. Adsorption of Cr(VI) onto TPPs from aqueous media reached equilibrium at pH ≈1.7 in 20 min and was independent of the initial concentration of Cr(VI). The results showed that the maximum Cr(VI) adsorption capacity was found as 0.2776 mg g−1 from the Langmuir model at 25°C. The sorption of Cr(VI) followed pseudo-second-order kinetic with rate constant k2 = 0.013 g mg−1min−1 (R2 = 0.9982). Furthermore, thermodynamic parameters (ΔH, ΔS and ΔG) of Cr(VI) retention by the sorbent were calculated and assigned. The positive values of ΔH (45.61 J mol−1) proved the endothermic nature of Cr(VI) retention on TPPs and ΔS (162.45 Jmol−1K−1) reflected good absorption and/or adsorption of the chromate [(CrO3Cl)]− as an ion associate with the bulky cations and the surface area available on TPPs. The negative value of ΔG (−48.6 kJ mol−1 at 298 K) indicates the spontaneous and chemical sorption nature of Cr(VI) uptake onto TPPs.

ADSORPTIVE CAPACITY OF COFFEE HUSK IN THE REMOVAL OF CHROMIUM (VI) AND ZINK (II) FROM TANNERY EFFLUENT: KINETICS AND EQUILIBRIUM STUDIES

Tannery wastewater is the most challenging due to the complex toxic pollutants it releases into the environment. Similarly, coffee husk from coffee processing operations pollutes the environment. The use of coffee husk to remove hexavalent chromium (VI) and Zink (II)from tannery wastewater solves pollution problems due to untreated tannery wastewater and coffee husk. A batch adsorption process was conducted to determine chromium (VI) and Zink (II)adsorptive capacity of waste coffee husk to treat tannery wastewater. The effects of adsorbent dose (0.2-2.4 g/L), contact time (20- 60 min), agitation speed (50-250 rpm), and pH value (2-8) on the adsorptive capacity of coffee husk on the removal of hexavalent chromium (VI) &amp; zinc (II) from tannery wastewater were studied. Using 2.4 g/L adsorbent dose, 250 rpm agitation speed, 60 min contact time, and pH value of 2, a maximum Cr(VI) removal capacity of coffee husk was 83%. The maximum Zn(II) removal capacity of coffee husk was found to be 79% at an adsorbent dose of 2.4 g/L, 60 min contact time, pH 7, and agitation speed of 250 rpm. Therefore, it can be concluded that coffee husk achieved significant removal of hexavalent chromium (VI) and zinc (II) from tannery wastewater.

Crustal anorthosite formation by deep‐seated hydrothermal circulation beneath fast‐spreading axis: Constraints from chronological approach, Sr isotope, and fluid–chromite inclusion investigation

AbstractHydrothermal circulation beneath the spreading axis plays a significant role in the exchange of energy and mass between the solid Earth and the oceans. Deep‐seated hydrothermal circulation down to the crust/mantle boundary in the fast‐spreading axis has been introduced by a number of studies regarding geological investigations and numerical models. In order to assess a reaction between hydrothermal fluid and host rock around the crust/mantle boundary, we conducted bulk trace element and Sr isotope analyses with a series of in situ investigations for crustal anorthosite, a reaction product between hydrothermal fluid and gabbro in the lowermost crustal section along Wadi Fizh, northern Oman ophiolite. In addition, we conducted titanite U–Pb isotope analyses to evaluate timing of the crustal anorthosite formation in the framework of the evolutional process of the Oman ophiolite. We estimated the formation age of the crustal anorthosite at 97.5 Ma ± 5.0 Ma, overlapping with the timing of the crust formation in the paleo spreading axis. The crustal anorthosite shows high‐Th/U ratio (~2.5) and high‐initial 87Sr/86Sr ratio (0.7050) due to seawater‐derived hydrothermal fluid ingress into the precursor gabbro. With using analytical technique of micro‐excavation at cryo‐temperature, we detected Cl from a few micrometer‐sized inclusion of aqueous fluid and chromite grains. The solubility of Cr was enhanced by complexation reactions with Cl in the hydrothermal fluid. Regarding reconstructed three‐dimensional mass distribution of the inclusion and chromite composition, maximum Cr content of parental fluid was estimated at ~69 000 μg/g. The exceptionally high‐Cr content was achieved locally by leaking of fluid and synchronous chromite crystallization during fluid entrapment. Presence of the deep‐seated hydrothermal circulation could be assigned to the segment end, where cold seawater penetrates into the lowermost crust and extract heat along widely spaced network‐like fluid channel.

Using X-ray Fluorescence Technique Propagation of Chromium (Cr) from Poultry Feeds to Different Parts of Chicken including her Eggs†

This paper presents the detection and propagation of heavy metals, particularly Chromium (Cr), in poultry in the Rajshahi area using the X-ray fluorescence (XRF) technique. The investigation was done to assess the possible transfer of heavy metals from poultry feeds to chicken meat. The study was carried out in the Rajshahi including collection feeds to chicken. It was found that a considerable amount of Cr was confirmed in all four locally produced poultry feeds and also assured that the Cr contained in chicken propagates to the chicken meat and egg. Among the four most widely used feeds in the Rajshahi region, “Adorsho Feed (Pabna)” shows a maximum Cr concentration of 17.3 ppm. Six different parts of the chicken for different ages which were grown by “Adorsho Feed” were considered for further study. About 4.3 ppm of Cr was found in Yolk whereas a little small about 2.7 ppm was found in the albumin. The average value of Cr in six parts of chicken remains almost the same, which is 3.74 ppm, irrespective of age. It is found that about 21.6% of Cr was propagated from the poultry feed to chicken flesh and also about 90% of Cr was propagated from the chicken to egg. The experimental results indicate that the investigation of the transmission of heavy metals like Cr from feed to egg and poultry is efficiently evaluated by using X-ray Fluorescence Technique.

Synthesis and characterization of activated carbon from sugar beet residue for the adsorption of hexavalent chromium in aqueous solutions.

A series of micro–mesoporous activated carbons (ACs) were prepared from sugar beet residue by a two-step method including KOH chemical activation and were used for Cr(vi) removal from aqueous solutions. Several characterization techniques, including SEM, TEM, N2 adsorption, XRD, FTIR, and Raman spectroscopy, were used to determine the chemical and physical characteristics of the ACs, and the adsorption properties of the ACs were tested. The results indicated that the high specific surface area of the ACs reached 2002.9 m2 g−1, and the micropore surface area accounts for 85% of the total area. The optimal conditions for achieving the maximum Cr(vi) adsorption capacity of 163.7 mg g−1 by the ACs were activation with a KOH/carbon ratio of 3.0, an initial Cr(vi) concentration of 400 mg L−1, an adsorbent dose of 2.0 g L−1 and pH of 4.5. Therefore, the ACs exhibit excellent adsorption performance for removing Cr(vi) from aqueous solutions. According to an investigation of the adsorption process, the adsorption isotherm is most consistent with the Langmuir isotherm model, and the adsorption kinetics were well described by the pseudo-second-order model.

ADSORPTIVE CAPACITY OF COFFEE HUSK IN THE REMOVAL OF CHROMIUM (VI) AND ZINK (II) FROM TANNERY EFFLUENT: KINETICS AND EQUILIBRIUM STUDIES

Tannery wastewater is the most challenging due to the complex toxic pollutants it releases into the environment. Similarly, coffee husk from coffee processing operations pollutes the environment. The use of coffee husk to remove hexavalent chromium (VI) and Zink (II)from tannery wastewater solves pollution problems due to untreated tannery wastewater and coffee husk. A batch adsorption process was conducted to determine chromium (VI) and Zink (II)adsorptive capacity of waste coffee husk to treat tannery wastewater. The effects of adsorbent dose (0.2-2.4 g/L), contact time (20- 60 min), agitation speed (50-250 rpm), and pH value (2-8) on the adsorptive capacity of coffee husk on the removal of hexavalent chromium (VI) &amp; zinc (II) from tannery wastewater were studied. Using 2.4 g/L adsorbent dose, 250 rpm agitation speed, 60 min contact time, and pH value of 2, a maximum Cr(VI) removal capacity of coffee husk was 83%. The maximum Zn(II) removal capacity of coffee husk was found to be 79% at an adsorbent dose of 2.4 g/L, 60 min contact time, pH 7, and agitation speed of 250 rpm. Therefore, it can be concluded that coffee husk achieved significant removal of hexavalent chromium (VI) and zinc (II) from tannery wastewater.

Evaluation of Cr(VI) Reducing Capability of Shewanella putrefaciens (MTTC8410) and Optimization of Operational Parameters

Bioremediation is an important technology to remediate the chromium (Cr) contaminated soil and water. In this study, Shewanella putrefaciens (MTTC8410) was used to investigate the influence of carbon concentration, pH, and temperature on reduction of hexavalent chromium [Cr(VI)] into trivalent chromium [Cr(III)]. The increased bacterial growth rate was significantly reduced the Cr(VI) concentration. In batch mode experiments, 1% starch recorded the highest reduction of Cr(VI) (90%) followed by 1% glucose (88% reduction) and a reduction of 77% was by 1% cellulose. By using various pH conditions the maximum Cr(VI) reduction was achieved at pH 7.0. In this experiment the maximum Cr(VI) reduction (75%) was observed at 35°C, followed by 30°C with 62% of Cr(VI) reduction. Bioreactor analysis revealed the highest reduction of Cr(VI) (88%) in unsterile tannery effluent. The significant levels of physico- chemical parameters were reduced in unsterile tannery effluent, as compared to the sterile tannery effluent. The experimental results revealed that the S. putrefaciens (MTTC8410) could be used as a potential bacterial strain for reduction of Cr(VI) from contaminated groundwater.

Removal of aqueous Cr(VI) by magnetic biochar derived from bagasse

We prepared a novel adsorbent functionalized by bagasse magnetic biochar (BMBC). To study the removal behaviors and mechanisms of Cr(VI) by BMBC, batch adsorption experiments were conducted by modifying variables, such as pH, adsorption time, BMBC dosages, initial Cr concentration, co-existing ions, and ionic strength, and characterizing BMBC before and after Cr(VI) adsorption. BMBC was primarily composed of Fe2O3 and Fe3O4 on bagasse boichar with an amorphous structure. The specific surface area of BMBC was 81.94 m2 g−1, and the pHpzc of BMBC was 6.2. The fabricated BMBC showed high adsorption performance of Cr(VI) in aqueous solution. The maximum Cr(VI) adsorption capacity of BMBC was 29.08 mg g−1 at 25 ºC, which was much higher than that of conventional biochar sorbents. The adsorption process followed pseudo-second-order kinetics and could be explained by the involvement of the Langmuir isotherm in monolayer adsorption. The crystalline structure of Fe3O4 in the BMBC changed slightly during the adsorption process; Fe3O4 improved the adsorption of Cr(VI) on BMB. The desorption capacity of Cr(VI) was 8.21 mg g−1 when 0.2 mol L−1 NaOH was used as the desorption solution. After being reused three times, the removal efficiency is still as high as 80.36%.

A novel modified cellulose nanomaterials (CNMs) for remediation of chromium (VI) ions from wastewater

Wastewater (WW) remediation technologies were the most crucial issues all over the world at present time. Thus, the remediation of Cr (VI) ions from real WW was conducted using green biocompatible and biodegradable pristine (CNM) and succinic anhydride functionalized cellulose nanomaterial (S-CNM) adsorbents. Both CNM and S-CNM adsorbents were prepared by using sulfuric acid hydrolysis method and characterized for particle sizes, functional groups, and surface morphologies by using XRD, FT-IR, and SEM instruments, respectively. The physicochemical properties of the collected WW were investigated. Next, both the prepared adsorbents were applied for the remediation of Cr (VI) ions from WW. The remediation processes is spontaneous and have higher remediation efficiencies of Cr (VI) ions from WW. The Cr (VI) ions remediation mechanism was evaluated from both the Cr (VI) ions adsorption isotherms and kinetic concepts. Both Langmuir and Freundlich Cr (VI) ions adsorption isotherm models were certainly fixed to a maximum Cr (VI) ions uptake capability (qmax) of 60.24 and 156.25 mg g−1 by CNM and S-CNM sorbents, respectively, and it follows pseudo-second-order (PSO) kinetics model through chemisorption processes. The Cr (VI) ions uptake capabilities were hindered by the presence of organic matter and any other competing pollutants in the WW. The S-CNM sorbent was selected for the regeneration study due to its higher efficiencies of remediation relative to CNM sorbent and the study was conducted through desorption of Cr (VI) ions by using HCl. Findings have shown that the sorbent was easily recyclable and applicable for the remediation of pollutants from real WW after consecutive 13th cycles.

Comparing chromium phyto-assessment in Brachiaria mutica and Leptochloa fusca growing on chromium polluted soil

Industrial discharge of chromium (Cr) into environment puts serious threat on living beings due to its potent toxicity. Phytostabilization, a type of in-situ phytoremediation is aimed to immobilize and stabilize the toxic elements in soil using root system of metal resistant potential plants. To evaluate the phytostabilization potential of two grass species Brachiaria mutica and Leptochloa fusca, a pot study was conducted using soil spiked with different concentrations of Cr (control, 25, 50 and 100 mg kg−1). Three plants were sown in each pot with three replications and arranged following completely randomized design. After three months of growth, the plants were harvested and above and below ground plant’s parts were analyzed for various growth and physiological parameters. Data revealed that plant biomass, chlorophylls and carotenoids reduced substantially with increasing Cr concentration. Antioxidant enzymatic activity increased significantly in L. fusca as compared to B. mutica with increasing Cr levels (up to 50 mg kg−1), then reduced at maximum Cr level (100 mg kg−1) in both grasses. Leptochloa fusca performed better with maximum root Cr accumulation 93.7 μg plant−1, shoot Cr accumulation 24.7 μg plant−1, root bioconcentration factor (BCF) 2.0, shoot BCF 0.08, shoot TF 0.06 and MTI 87%. While B. mutica showed maximum root Cr accumulation 18.4 μg plant−1, shoot Cr accumulation 7.6 μg plant−1, shoot BCF 0.03, root BCF 1.28, shoot TF 0.04, and MTI 56%. These results showed that L. fusca possessed good potential with better Cr bioaccumulation, MTI, BCF and antioxidant activities compared to B. mutica. Hence L. fusca can be used as good phytostabilizing agent for the soils contaminated with lower to moderate levels of Cr.

Polypyrrole/sewage sludge carbon as low-cost and high-effective catalyst for enhancing hexavalent chromium reduction and bio-power generation in dual chamber microbial fuel cells

Microbial fuel cell, a potential bio-electrochemical energy transducer can extract the bio-energy from wastewater by the consumption of the electro-active microorganism adhered the surface of anode, and the MFC performance is greatly inhibited by the cathode catalysts. Herein, polypyrrole/sewage sludge carbon electrode is fabricated by an electro-polymerization loading method and the electrode is employed as the cathode electrode in MFC for the removal of Cr(VI) and bio-energy release at the same time. The electrochemical analysis finds the polypyrrole/sewage sludge carbon electrode exhibits the low charge transfer resistance and Tafel slope, which can demonstrate that the polypyrrole/sewage sludge carbon electrode possesses strong conductivity and excellent electrochemical activity. In addition, Cr(VI) in the cathode chamber of MFC with the polypyrrole/sewage sludge carbon electrode can be totally removed at 30 h. And the maximum output power and Cr(VI) reduction rate of MFCs are 760.7 mW m−2 and 2.16 g·m−3·h−3, which are 2.79 and 1.51 times higher than those of carbon cloth electrode (272.5 mW·m−2 and 1.43 g·m−3·h−3). Moreover, Cr(VI) can be removed and deposited on the electrode in the form of Cr(OH)3. And polypyrrole plays an electron shuttling role in reducing the internal resistance of MFC and enhancing electrochemical activity of the electrode for Cr(VI) reduction. Consequently, the polypyrrole/sewage sludge composite can achieve more stable Cr(VI) removal and power than those of sewage sludge carbon, polypyrrole, and carbon cloth electrodes, demonstrating an effective and low-cost catalyst for accelerating Cr(VI) reduction and strengthening bio-power production.

Response of microalgae Chlorella vulgaris to Cr stress and continuous Cr removal in a membrane photobioreactor

This study evaluated the toxicity of Cr(VI) to microalgae Chlorella vulgaris, and its removal by continuous microalgae cultivation in membrane photobioreactor (MPBR). Batch cultivation in photobioreactors showed that low concentration of Cr(VI) (0.5 and 1.0 mg L−1) stimulated the growth of C. vulgaris, while 2.0 and 5.0 mg L−1 Cr(VI) in the wastewater significantly inhibited the growth of C. vulgaris. Superoxide dismutase and catalase activities that represented cellular antioxidant capacity significantly increased at 0.5 and 1.0 mg L−1 Cr(VI), and then gradually decreased with the continuous increase of Cr(VI) concentration. The content of malondialdehyde, which represents the degree of cellular oxidative damage, increased with the increase of Cr(VI) concentration and reached the peak value at 2.0 mg L-1 Cr(VI). C. vulgaris was then cultured in MPBR equipped with hollow-fiber ultrafiltration membrane module to achieve continuous removal of Cr from wastewater. With the in-situ solid-liquid separation function of the membrane module, solid retention time (SRT) and hydraulic retention time (HRT) of the reactor could be controlled separately. Experimental results showed that both SRT and HRT had significant effects on the algal biomass production and pollutants removal. During the continuous operation, MPBR achieved a maximum total Cr reduction of 50.0% at HRT of 3-day and SRT of 40-day, and a maximum volumetric removal rate of total Cr of 0.21 mg L−1 d−1 at HRT of 2-day and SRT of 40-day.

Hazardous Waste Disposal in Stromatolitic-Limestone Terrain and Hexavalent Chromium Contamination in Chhattisgarh State, India.

Background.Hexavalent chromium-containing waste from chromite ore processing is a major environmental health hazard due to its high toxicity. There have been instances of improper and unsafe disposal of this waste, leading to environmental health hazards.Objectives.The objective of the present study was to identify the cause of yellow colored water discharge and reported health issues in nearby residents and cattle. In addition, it investigated the improper disposal of chromite ore processing residue (COPR), a hazardous waste, in an abandoned quarry in stromatolitic-limestone terrain in central-east India.Methods.Standard methods of analysis of water and wastewater were used for the analyses of variables, including hexavalent chromium (Cr(VI)), pH, sulfate (SO42−), chlorine (Cl−), total hardness, calcium (Ca(II)), magnesium (Mg(II)), alkalinity and sodium (Na(I)) with proper sampling, quality assurance, and quality control protocols. Onsite Cr(VI) was analyzed using a chromium testing kit, and in the laboratory by atomic absorption spectrophotometry.Results.Large-scale contamination of surface and groundwater was noted due to the migration of hexavalent chromium-contaminated yellow colored leachate. High levels of hexavalent chromium were noted in the samples. The maximum Cr(VI) concentration observed was 1050 mg/L in leachate, 22 mg/L in surface water and 0.26 mg/L in the groundwater sample. Acute health effects were noted in cattle and by residents who consumed the highly contaminated water.Conclusions.A large volume of discharge of hexavalent chromium contamination from the COPR landfill was found, indicating the absence of containment features in the design (double high-density polyethylene liners, clay, leachate collection). Disposal of COPR in an abandoned limestone mine is inadvisable. The highly fractured stromatolitic-limestone environment at the study site was found to offer almost no resistance to the mobilization of Cr(VI) due to the absence of organic or eukaryotic deposition in the stromatolitic environment. It was also noted that the drainage pattern of the area facilitates a possible translocation of contaminated discharge to the nearby river system. Nearby residents were unaware of the adverse impacts of the contaminated leachates and were using the contaminated water for bathing, washing, etc. Applicable Indian governmental regulations regarding the construction of hazardous waste landfills were found to be insufficient with respect to the use of inactive limestone mines as landfill sites.Competing Interests.The authors declare no competing financial interests.