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.     

Determination of haloacetic acids in hospital effuent after chlorination by ion chromatography

The ion chromatography combined solid phase extraction (SPE) method was developed for the analysis of low concentration haloacetic acids (HAAs), a class of disinfection by-products formed from chlorination of hospital wastewater. The monitored HAAs included monochloroacetic acid, monobromoacetic acid, dichloroacetic acid, dibromoacetic acid and trichloroacetic acid. The method employed a sodium hydroxide eluent at a flow rate of 0.8 mlmin, electrolytically generated gradients, and suppressed conductivity detection. To analyze the HAAs in real hospital wastewater samples, C18 pretreatment cartridge was utilized to reduce samples' turbidity. Preconcentration with SPE and matrix elimination with treatment cartridges were investigated and found to be able to obtain acceptable detection limits. Linearity, repeatability and detection limits of the above method were evaluated. The detection limits of monobromoacetic acid and dibromoacetic acid were 2.61 μgL and 1.30 μgL, respectively, and the other three acids are ranging from 0.48 to 0.82 μgL under 25-fold preconcentration. When the above optimization procedure was applied to three hospital wastewater samples with different treatment processes in Tianjin, it was found that the dichloroacetic acid was the major compound, and the growth ratios of the HAAs after disinfection by sodium hypochlorite were 91.28%, 63.61% and 79.50%, respectively.     

A collaborative strategy for enhanced reduction of Cr(Ⅵ) by Fe(0) in the presence of oxalate under sunlight:Performance and mechanism

Nanometer-size zero-valent iron(NZVI) is an efficient reducing agent,but its surface is easily passivated with an oxide layer,leading to reaction inefficiency.In our study,oxalate(OA) was introduced into this heterogeneous system of NZVI,which could form ferrioxalate complexes with the NZVI surface-bound Fe~(3+) and dissolved Fe~(3+) in the solution.Photolysis of ferrioxalate complexes can facilitate the generation of Fe~(2+) from Fe~(3+)and CO_2~(·-) radical,both species have strong reduction capacity.Hence,a photo-oxalate-Fe(0)system through sunlight induction was established,which not only prohibited the formation of a surface passivation layer,but also displayed a synergetic mechanism of ferrioxalate photolysis to enhance reduction,exhibiting remarkably higher degradation activity(several times faster) toward the model pollutant Cr(Ⅵ) than the mechanism with NZVI alone.Factor tests suggested that both NZVI dosage and OA content markedly affected the reduction rate.Low pH was beneficial to the reduction efficiency.Moreover,recyclability experiment showed that the reduction rate decreased from 0.21706 to 0.03977 min~(-1) after three cycles of reuse due to the NZVI losing reaction activity generally,but the system still maintained considerable reduction capacity.Finally,a mechanism was revealed whereby NZVI would transform to Fe oxides after the exhaustion of its reductive power,and the photolysis of ferrioxalate to promote the cycling of iron species played the predominant role in providing extra reduction ability.These features confirm that introduction of OA into Cr(Ⅵ) reduction by NZVI through sunlight induction is advantageous and promising.     

Novel functional fiber loaded with carbon dots for the deep removal of Cr(Ⅵ) by adsorption and photocatalytic reduction

A novel functional fiber(PAN-CDs) loaded with carbon dots(CDs) with excellent photoreduction and adsorption properties for Cr(VI) was prepared via an amidization reaction between the CDs' carboxyl groups and amine groups on polyacrylonitrile(PAN)-based ion exchange fibers, which could completely preserve the fluorescence properties of the CDs. The photoluminescence(PL), photocatalysis and adsorption properties of PAN-CDs were characterized and analyzed. The PAN-CDs possess high adsorption capacity(297.6 mg/g) and excellent kinetic behavior(attaining adsorption equilibrium in 30 min)for Cr(VI) adsorption. Furthermore, the residual Cr(VI)(approximately 3 mg/L) after adsorption could be removed completely by subsequent photoreduction by the PAN-CDs.The Cr-saturated PAN-CDs could be easily separated by filtering and regenerated, with no observable decay of removal efficiency after five regeneration cycles. In addition, due to the PL quenching action of Cr(VI), the PAN-CDs can also be used as sensor for quantitative detection of trace Cr(VI) in aqueous solution.     

Performance and mechanism of Cr(Ⅵ) removal by zero-valent iron loaded onto expanded graphite

Zero-valent iron(ZⅥ) was loaded on expanded graphite(EG) to produce a composite material(EG-ZⅥ) for efficient removal of hexavalent chromium(Cr(Ⅵ)). EG and EG-ZⅥ were characterized by X-ray diffraction(XRD), scanning electron microscopy(SEM),Fourier-transform infrared(FTIR) spectroscopy and Brunauer–Emmett–Teller(BET) analysis. EG-ZⅥ had a high specific surface area and contained sub-micron sized particles of zero-valent iron. Batch experiments were employed to evaluate the Cr(Ⅵ) removal performance. The results showed that the Cr(Ⅵ) removal rate was 98.80% for EG-ZⅥ,which was higher than that for both EG(10.00%) and ZⅥ(29.80%). Furthermore, the removal rate of Cr(Ⅵ) by EG-ZⅥ showed little dependence on solution p H within a p H range of 1–9.Even at pH 11, a Cr(Ⅵ) removal rate of 62.44% was obtained after reaction for 1 hr. EG-ZⅥ could enhance the removal of Cr(Ⅵ) via chemical reduction and physical adsorption,respectively. X-ray photoelectron spectroscopy(XPS) was used to analyze the mechanisms of Cr(Ⅵ) removal, which indicated that the ZⅥ loaded on the surface was oxidized, and the removed Cr(Ⅵ) was immobilized via the formation of Cr(III) hydroxide and Cr(III)–Fe(III)hydroxide/oxyhydroxide on the surface of EG-ZⅥ.     

Stability of Fe–As composites formed with As(V) and aged ferrihydrite

During the aging process, ferrihydrite was transformed into mineral mixtures composed of different proportions of ferrihydrite, goethite, lepidocrocite and hematite. Such a transformation may affect the fixed ability of arsenic. In this study, the stability of Fe-As composites formed with As(V) and the minerals aged for 0, 1, 4, 10 and 30 days of ferrihydrite were systematically examined, and the effects of molar of ratios Fe/As were also clarified using kinetic methods combined with multiple spectroscopic techniques. The results indicated that As(V) was rapidly adsorbed on minerals during the initial polymerization process, which delayed both the ferrihydrite conversion and the hematite formation. When the Fe/As molar ratio was 1.875 and 5.66, the As(V) adsorbed by ferrihydrite began to release after 6 hr and 12 hr, respectively. The corresponding release amounts of As(V) were 0.55 g/L and 0.07 g/L, and the adsorption rates were 92.43% and 97.50% at 60 days, respectively. However, the As(V) adsorbed by the transformation products aged for 30 days of ferrihydrite began to release after adsorbed 30 days. The corresponding release amounts of As(V) were 0.25 g/L and 0.03 g/L, and the adsorption rates were 84.23% and 92.18% after adsorbed 60 days, for the Fe/As=1.875 and 5.66, respectively. Overall, the combination of As(V) with ferrihydrite and aged products transformed from a thermodynamically metastable phase to a dynamically stable state within a certain duration. Moreover, the aging process of ferrihydrite reduced the sorption ability of arsenate by iron (hydr)oxide but enhanced the stability of the Fe-As composites.     

Efficient reduction and adsorption of Cr(Ⅵ) using FeCl3-modified biochar:Synergistic roles of persistent free radicals and Fe(Ⅱ)

Transition metal iron and persistent free radicals(PFRs) both affect the redox properties of biochar, but the electron transfer relationship between them and the coupling reduction mechanism of Cr(Ⅵ) requires further investigation. To untangle the interplay between iron and PFRs in biochar and the infuences on redox properties, FeCl₃-modified rice husk biochar(FBCs) was prepared and its reduction mechanism for Cr(Ⅵ) without light was evaluated. The FBCs had higher surface positive cha...     

Simultaneous removal of Cu(Ⅱ) and Cr(Ⅵ) by Mg–Al–Cl layered double hydroxide and mechanism insight

Mg–Al–Cl layered double hydroxide(Cl-LDH) was prepared to simultaneously remove Cu(Ⅱ)and Cr(VI) from aqueous solution. The coexisting Cu(Ⅱ)(20 mg/L) and Cr(VI)(40 mg/L) were completely removed within 30 min by Cl-LDH in a dosage of 2.0 g/L; the removal rate of Cu(Ⅱ) was accelerated in the presence of Cr(VI). Moreover, compared with the adsorption of single Cu(Ⅱ) or Cr(VI), the adsorption capacities of Cl-LDH for Cu(Ⅱ) and Cr(VI) can be improved by 81.05% and 49.56%, respectively, in the case of coexisting Cu(Ⅱ)(200 mg/L) and Cr(VI)(400 mg/L). The affecting factors(such as solution initial p H, adsorbent dosage, and contact time) have been systematically investigated. Besides, the changes of p H values and the concentrations of Mg~(2+) and Al~(2+)in relevant solutions were monitored. To get the underlying mechanism, the Cl-LDH samples before and after adsorption were thoroughly characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. On the basis of these analyses, a possible mechanism was proposed. The coadsorption process involves anion exchange of Cr(VI) with Cl-in Cl-LDH interlayer, isomorphic substitution of Mg~(2+) with Cu~(2+), formation of Cu_2Cl(OH)_3precipitation, and the adsorption of Cr(VI) by Cu_2Cl(OH)_3. This work provides a new insight into simultaneous removal of heavy metal cations and anions from wastewater by Cl-LDH.     

Microbial reduction of uranium(Ⅵ) by Bacillus sp.dwc-2:A macroscopic and spectroscopic study

The microbial reduction of U(VI) by Bacillus sp. dwc-2, isolated from soil in Southwest China,was explored using transmission electron microscopy(TEM), X-ray photoelectron spectroscopy(XPS) and X-ray absorption near edge spectroscopy(XANES). Our studies indicated that approximately 16.0% of U(VI) at an initial concentration of 100 mg/L uranium nitrate could be reduced by Bacillus sp. dwc-2 at pH 8.2 under anaerobic conditions at room temperature.Additionally, natural organic matter(NOM) played an important role in enhancing the bioreduction of U(VI) by Bacillus sp. dwc-2. XPS results demonstrated that the uranium presented mixed valence states(U(VI) and U(IV)) after bioreduction, which was subsequently confirmed by XANES. Furthermore, the TEM and high resolution transmission electron microscopy(HRTEM) analysis suggested that the reduced uranium was bioaccumulated mainly within the cell and as a crystalline structure on the cell wall.These observations implied that the reduction of uranium may have a significant effect on its fate in the soil environment in which these bacterial strains occur.     

Removal of Cr(Ⅵ) by glutaraldehyde-crosslinked chitosan encapsulating microscale zero-valent iron: Synthesis,mechanism, and longevity

Microscale zero-valent iron(mZVI) has shown great potential for groundwater Cr(Ⅵ) remediation. However, low Cr(Ⅵ) removal capacity caused by passivation restricted the wide use of mZVI. We prepared mZVI/GCS by encapsulating mZVI in a porous glutaraldehydecrosslinked chitosan matrix, and the formation of the passivation layer was alleviated by reducing the contact between zero-valent iron particles. The average pore diameter of mZVI/GCS was 8.775 nm, which confirmed the mesoporous characteristic ...     

Enhanced U(Ⅵ) bioreduction by alginate-immobilized uranium-reducing bacteria in the presence of carbon nanotubes and anthraquinone-2,6-disulfonate

Uranium-reducing bacteria were immobilized with sodium alginate, anthraquinone-2,6-disulfonate(AQDS), and carbon nanotubes(CNTs). The effects of different AQDS-CNTs contents, U(Ⅳ) concentrations, and metal ions on U(Ⅳ) reduction by immobilized beads were examined. Over 97.5% U(Ⅵ)(20 mg/L) was removed in 8 hr when the beads were added to 0.7% AQDS-CNTs, which was higher than that without AQDS-CNTs. This result may be attributed to the enhanced electron transfer by AQDS and CNTs. The reduction of U(Ⅵ) occurred at initial U(Ⅵ) concentrations of 10 to 100 mg/L and increased with increasing AQDS-CNT content from 0.1% to 1%. The presence of Fe(Ⅲ), Cu(Ⅱ) and Mn(Ⅱ)slightly increased U(Ⅵ) reduction, whereas Cr(Ⅵ), Ni(Ⅱ), Pb(Ⅱ), and Zn(Ⅱ) significantly inhibited U(Ⅵ) reduction. After eight successive incubation-washing cycles or 8 hr of retention time(HRT) for 48 hr of continuous operation, the removal efficiency of uranium was above 90% and 92%, respectively. The results indicate that the AQDS-CNT/AL/cell beads are suitable for the treatment of uranium-containing wastewaters.     

Removal of perchlorate from aqueous solution by cross-linked Fe(Ⅲ)-chitosan complex

Cross-linked Fe（III）-chitosan composite （Fe-CB） was used as the adsorbent for removing perchlorate from the aqueous solution. The adsorption experiments were carried out by varying contact time, initial concentrations, temperatures, pH, and the presence of co-existing anions. The morphology of the adsorbent was discussed using FT-IR and SEM with X-EDS analysis. The pH ranging from 3.0-10.2 exhibited very little effect on the adsorption capability. The perchlorate uptake onto Fe-CB obeyed Langmuir isotherm model. The adsorption process was rapid and the kinetics data obeyed the pseudo second-order model well. The eluent of 2.5% （W/V） NaC1 could regenerate the exhausted adsorbent efficiently. The adsorption mechanism was also discussed.     

金属有机框架MIL-53(Fe)可见光催化还原水中U(Ⅵ)

U（Ⅵ）是放射性废液中铀的主要存在形式.将可溶的U（Ⅵ）还原为难溶的U（Ⅳ）是治理铀污染的有效方法.以溶剂热法合成了铁基金属有机框架材料MIL-53（Fe）.在表征了材料的结构及光响应特性基础上,开展了MIL-53（Fe）在可见光下催化还原水中U（Ⅵ）的研究.探究了空穴捕获剂种类、空穴捕获剂浓度、反应体系pH及催化剂用量等对U（Ⅵ）光催化还原的影响.结果表明,空穴捕获剂甲酸的加入可有效提高光催化反应的电荷分离效率.当甲酸浓度为1 mmol·L^-1时,400 mg·L^-1的MIL-53（Fe）在可见光下,2 h内对初始浓度为50 mg·L^-1的U（Ⅵ）去除率达到80%;光电子能谱检测结果显示反应中有U（Ⅳ）生成,推测其主要反应机制是甲酸与MIL-53（Fe）的光生空穴反应产生强还原性的·COO^-,将U（Ⅵ）还原为U（Ⅳ）,从而实现对水中U（Ⅵ）的光催化去除.     

Efficient reduction of Cr(Ⅵ) by guava(Psidium guajava) leaf extract and its mitigation effect on Cr toxicity in rice seedlings

Hexavalent chromium(Cr(Ⅵ)) is a toxic element that has negative impacts on crop growth and yield. Using plant extracts to convert toxic Cr(Ⅵ) into less toxic Cr(Ⅲ) may be a more favorable option compared to chemical reducing agents. In this study, the potential effects and mechanisms of using an aqueous extract of Psidium guajava L. leaves(AEP) in reducing Cr(Ⅵ)toxicity in rice were comprehensively studied. Firstly, the reducing power of AEP for Cr(Ⅵ)was confirmed by the cyclic voltammetry combi...     

Bacterial reduction and release of adsorbed arsenate on Fe(Ⅲ)-, Al- and coprecipitated Fe(Ⅲ)/Al-hydroxides

Mobilization of arsenic under anaerobic conditions is of great concern in arsenic contaminated soils and sediments. Bacterial reduction of As(V) and Fe(Ⅲ) influences the cycling and partitioning of arsenic between solid and aqueous phase. We investigated the impact of bacterially mediated reductions of Fe(Ⅲ)/Al hydroxides-bound arsenic(V) and iron(Ⅲ) oxides on arsenic release. Our results suggested that As(V) reduction occurred prior to Fe(Ⅲ) reduction, and Fe(Ⅲ) reduction did not enhance the release of arsenic. Instead, Fe(Ⅲ) hydroxides retained their dissolved concentrations during the experimental process, even though the new iron mineral-magnetite formed. In contrast, the release of reduced As(Ⅲ) was promoted greatly when aluminum hydroxides was incorporated. Thus, the substitution of aluminum hydroxides may be responsible for the release of arsenic in the contaminated soils and sediments, since aluminum substitution of Fe(Ⅲ) hydroxides universally occurs under natural conditions.     

Experimental study on Cr(Ⅵ) reduction by Pseudomonas aeruginosa

Investigation on Cr(Ⅵ) reduction was conducted using Pseudomonas aeruginosa. The study demonstrated that the Cr(Ⅵ) can be effectively reduced to Cr(Ⅲ) by Pseudomonas aeruginosa. The effects of the factors affecting Cr(Ⅵ) reduction rate including carbon source type, pH, initial Cr(Ⅵ) concentration and amount of cells inoculum were thoroughly studied. Malate was found to yield maximum biotransformation, followed by succinate and glucose, with the reduction rate of 60.86%, 43.76% and 28.86% respectively. The optimum pH for Cr(Ⅵ) reduction was 7.0, with reduction efficiency of 61.71% being achieved. With the increase of initial Cr(Ⅵ) concentration, the rate of Cr(Ⅵ) reduction decreased. The reduction was inhibited strongly when the initial Cr(Ⅵ) concentration increased to 157 mg/L. As the amount of cells inoculum increased, the rate of Cr(Ⅵ) reduction also increased. The mechanism of Cr(Ⅵ) reduction and final products were also analysed. The results suggested that the soluble enzymes appear to be responsible for Cr(Ⅵ) reduction by Pseudomonas aeruginosa, and the reduced Cr(Ⅲ) was not precipitated in the form of Cr(OH)3.     

Impact of environmental factors on the sampling rate of β-blockers and sulfonamides from water by a carbon nanotube-passive sampler

Passive techniques are a constantly evolving approach to the long-term monitoring of micropollutants, including pharmaceuticals, in the aquatic environment. This paper presents, for the first time, the calibration results of a new CNTs-PSDs (carbon nanotubes used as a sorbent in passive sampling devices) with an examination of the effect of donor phase salinity, water pH and the concentration of dissolved humic acids (DHAs), using both ultrapure and environmental waters. Sampling rates (R_s) were determined for the developed kinetic samplers. It has been observed that the impact of the examined environmental factors on the R_s values strictly depends on the type of the analytes. In the case of β-blockers, the only environmental parameter affecting their uptake rate was the salinity of water. A certain relationship was noted, namely the higher the salt concentration in water, the lower the R_s values of β-blockers. In the case of sulfonamides, water salinity, water pH 7–9 and DHAs concentration decreased the uptake rate of these compounds by CNTs-PSDs. The determined R_s values differed in particular when the values obtained from the experiments carried out using ultrapure water and environmental waters were compared. The general conclusion is that the calibration of novel CNTs-PSDs should be carried out under physicochemical conditions of the aquatic phase that are similar to the environmental matrix.     

Treatability studies of naphthalene in soil,water and air with persulfate activated by iron(Ⅱ)

Chemical oxidation was applied to an artificially contaminated soil with naphthalene (NAP). Evaluation of NAP distribution and mass reduction in soil, water and air phases was carried out through mass balance. Evaluation of NAP distribution and mass reduction in soil, water and air phases was carried out through mass balance. The importance of the air phase analysis was emphasized by demonstrating how NAP behaves in a sealed system over a 4 hr reaction period. Design of Experiments method was applied to the following variables: sodium persulfate concentration [SP], ferrous sulfate concentration [FeSO₄], and pH. The system operated with a prefixed solid to liquid ratio of 1:2. The following conditions resulted in optimum NAP removal [SP] = 18.37 g/L, [FeSO₄] = 4.25 g/L and pH = 3.00. At the end of the 4 hr reaction, 62% of NAP was degraded. In the soil phase, the chemical oxidation reduced the NAP concentration thus achieving levels which comply with Brazilian and USA environmental legislations. Besides the NAP partitioning view, the monitoring of each phase allowed the variabilities assessment over the process, refining the knowledge of mass reduction. Based on NAP distribution in the system, this study demonstrates the importance of evaluating the presence of semi-volatile and volatile organic compounds in the air phase during remediation, so that there is greater control of the system as to the distribution and presence of the contaminant in the environment. The results highlight the importance of treating the contaminant in all its phases at the contaminated site.     

Radiation-induced reduction of chromium(Ⅵ) in aqueous solution by γ-irradiation in a laboratory-scale

Radiation-induced reduction of chromium（Ⅵ）（Cr（Ⅵ） by ,γ-irradiation was studied with an initial concentration of 42 mg/L in aqueous solutions. Several factors which might affect the reduction of Cr（Ⅵ） to Cr（Ⅲ） were examined, pH of aqueous solution affects the reduction efficiency significantly. Acidic condition of aqueous solution accelerates the process. At pH 2, a reduction of 86.2% was achieved with the absorbed dose of 15 kGy, while, with the same dose, at pH 5 and 7, the reduction ofCr （Ⅵ） were only 36.3% and 22.2%, respectively. Ethanol （0.1% in V：V） and sodium carbonate （1 mmol/L） were added into the solution respectively as relatively non-toxic hydroxyl radical scavengers. Reduction rate increased greatly in the presence of ethanol at each pH. Reduction efficiency of Cr（Ⅵ） was enhanced in neutral condition with the addition of sodium carbonate, however, no enhancement was found in acidic condition. The reduction of Cr（Ⅵ） was restrained when the solution was saturated with oxygen; however, the restraint was not significant.     

Radiation-induced reduction of chromium(Ⅵ) in aqueous solution by γ-irradiation in a laboratory-scale

Radiation-induced reduction of chromium(Ⅵ)(Cr(Ⅵ) by γ-irradiation was studied with an initial concentration of 42 mg/L in aqueous solutions. Several factors which might affect the reduction of Cr(Ⅵ) to Cr(Ⅲ) were examined. pH of aqueous solution affects the reduction efficiency significantly. Acidic condition of aqueous solution accelerates the process. At pH 2, a reduction of 86.2%was achieved with the absorbed dose of 15 kGy, while, with the same dose, at pH 5 and 7, the reduction of Cr (Ⅵ) were only 36.3%and 22.2%, respectively. Ethanol (0.1% in V:V) and sodium carbonate (1 mmol/L) were added into the solution respectively as relatively non-toxic hydroxyl radical scavengers. Reduction rate increased greatly in the presence of ethanol at each pH. Reduction efficiency of Cr(Ⅵ) was enhanced in neutral condition with the addition of sodium carbonate, however, no enhancement was found in acidic condition. The reduction of Cr(Ⅵ) was restrained when the solution was saturated with oxygen; however, the restraint was not significant.