Green-synthesized, biochar-supported nZVI from mango kernel residue for aqueous hexavalent chromium removal: Performance, mechanism and regeneration

A biochar-supported green nZVI(G-nZVI@MKB)composite was synthesized using mango kernel waste with“dual identity”as reductant and biomass of biochar.The G-nZVI@MKB with a Fe/C mass ratio of 2.0(G-nZVI@MKB2)was determined as the most favorable composite for hexavalent chromium(Cr(VI))removal.Distinct influencing parameters were discussed,and 99.0%of Cr(VI)removal occurred within 360 min under these optimized parameters.Pseudo-second order kinetic model and intra-particle diffusion model well depicted Cr(VI)removal process.The XRD,FTIR,SEM,and XPS analyses verified the key roles of G-nZVI and functional groups,as well as the primary removal mechanisms involving electrostatic attraction,reduction,and complexation.G-nZVI@MKB2 exhibited good stability and reusability with only a 16.4%decline in Cr(VI)removal after five cycles.This study offered evidence that mango kernel could be recycled as a beneficial resource to synthesize green nZVI-loaded biochar composite for efficient Cr(VI)elimination from water.

Efficient removal of Cr(VI) by polydopamine-modified lignin from aqueous solution: Batch and XAFS studies

Lignocellulose has the potential to become a bio-based adsorbent due to its biodegradability and renewability.In this study,a novel polydopamine functionalized-lignin(lignin@PDA),prepared via self-polymerization of dopamine(PDA)on lignin,was used as a bio-based adsorbent for rapid scavenging of hexavalent chromium(Cr(VI)).The morphology,functional groups,crystalline structure,and chemical composition of lignin@PDA were characterized with a scanning electron microscope-energy dispersive spectrometer,Fourier transform infrared spectroscopy,X-ray diffraction,and X-ray photoelectron spectroscopy.The Cr(VI)adsorption process of lignin@PDA was studied using batch experiments as a function of pH,ionic strength,adsorbent dose,and contact time at room temperature.The adsorption rate of lignin@PDA was five times greater than that of the unmodified lignin,with a maximum adsorption capacity of 102.6 mg/g in an acidic medium.The adsorption of Cr(VI)on lignin@PDA fit the pseudo-secondorder equation and the Freundlich model,indicating that the adsorption process was mainly dominated by chemisorption and surface complexation.The thermodynamic parameters showed that adsorption of Cr(VI)on lignin@PDA was an endothermic and spontaneous process.The X-ray absorption fine structure results showed that sorption and reduction of Cr(VI)into Cr(II)occurred simultaneously on lignin.Moreover,PDA coating not only improved the reactivity of lignin but also promoted the complete reduction of Cr(VI)by lignin.According to these results,polydopamine functionalized-lignin is a promising bio-based adsorbent for immobilization of Cr(VI)from wastewater.

Assessing the effectiveness of nanoscale zero-valent iron particles produced by green tea for Cr(VI)-contaminated groundwater remediation

Nanoscale zero-valent iron particles(NZVI) produced by using green tea(GT) extract as a reductant can remove Cr(Ⅵ) from water effectively,which can be utilized in groundwater remediation.In order to define the reaction mechanism and removal effect in the aquifer,in this study,GT-NZVI particles were prepared and measured by some characterization methods to define their surface performance,and then batch and one-dimensional experiments were carried out to reveal the reaction properties of GT-NZVI and Cr(Ⅵ) in groundwater.The results showed that the prepared GT-NZVI particles were regular spherical with a diameter of 10-20 nm,which could disperse in water stably.The main component of GT-NZVI wasα-Fe with superficial polyphenols as a stabilizer.GT-NZVI suspension had good ability to reduce the Cr(Ⅵ) to Cr(Ⅲ) in water.When the concentration of GT-NZVI was 1 g/L,the removal efficiency of Cr(Ⅵ)with an initial concentration of 100 mg/L reached 92.8% in 1 h reaction.In column tests,GT-NZVI passed through the natural sand column successfully with an average outflow percentage of 71.2%.The simulated in-situ reaction zone(IRZ) with GT-NZVI was used to remediate Cr(Ⅵ) contaminated groundwater.The oufflow concentration of Cr(Ⅵ) kept in 0.14-0.32 mg/L corresponding to the outflow rate below 0.32%within 15 days,and the removal efficiency of Cr(Ⅵ) by IRZ with GT-NZVI decreased with the increase of aquifer medium particle size,groundwater flow rate and ionic strength.Most of Cr(Ⅲ) as reduzate was adsorbed or immobilized on the surface or in the lattice of GT-NZVI,which indicated effective immobilization for chromium.

Reactions of Chromium during the Calcination of Cement Clinker Produced Using Steel Slag

We investigated the effects of calcination temperature(950-1450℃),steel slag content,and the total chromium content of steel slag on the Cr^(6+)contents of clinker samples produced using steel slags with different chromium contents.Additionally,the reactions of chromium in clinker(produced using steel slag)during calcination were studied.It is found that Cr^(6+)conversion increases with increasing calcination temperature to 1250℃,reaching a maximum of 43%-79%,before decreasing to 18%-42%at 1450℃.Cr^(6+)is mainly formed by the oxidation of trivalent chromium(Cr^(3+))during the solid-phase reaction stage of clinker calcination.Furthermore,the Cr^(6+)content of a clinker sample is proportional to the chromium content of its raw meal precursor and is mainly in the form of water-insoluble calcium chromate(CaCrO_(4)).The chromium in clinker is mainly distributed in tricalcium aluminate and tetracalcium aluminoferrite,however,some is present in silicate minerals.We expect to inform the monitoring and control of the Cr^(6+)content of clinker(produced using steel slag)and resulting cement.

TiO_(2)@NH_(2)-MIL-125(Ti) composite derived from a partial-etching strategy with enhanced carriers' transfer for the rapid photocatalytic Cr(Ⅵ) reduction

Metal-organic frameworks (MOFs)-based composites have been widely applied as photocatalysts because of their synergistic effect between the two individual component.Herein,TiO_(2)@NH_(2)-MIL-125(Ti) nanocomposites which possess unsaturated titanium–oxo clusters,mesoporous structure,and intimate interface were successfully constructed via an in-situ distilled water-etched route.The X-ray photoelectron spectroscopy (XPS) measurements indicated strong electronic interaction between TiO_(2)and NH_(2)-MIL-125(Ti),confirming the formation of TiO 2@NH_(2)-MIL-125(Ti) nanocomposite.Photoelectrochemical and thermodynamics measurements showed that TiO_(2)@NH_(2)-MIL-125(Ti)nanocomposites have improved charge separation efficient and decreased transfer resistance of the carriers within the heterojunction interfaces,which facilitates the photoexcited electrons transfer and reduction of the Cr(VI) species.Therefore,the optimal TiO_(2)@NH_(2)-MIL-125(Ti)nanocomposite demonstrated superior performance compared to NH_(2)-MIL-125(Ti) and NH_(2)-MIL-125(Ti) derived TiO_(2).Based on the free radical trapping experiment and electron paramagnetic resonance (EPR) measurements,a possible type-II scheme was proposed for the enhanced photocatalytic activity over the TiO_(2)@NH_(2)-MIL-125(Ti) nanocomposite.

Thermodynamics of Cr(Ⅵ) adsorption on strong alkaline anion exchange fiber

Removal of Cr（VI） from aqueous solution by strong alkaline anion exchange fiber （SAAEF） was achieved using batch adsorption experiments. The effect of contact time, initial Cr（VI） concentration and pH was investigated. The results showed that the maximum adsorption capacity of SAAEF was 187.7 mg/g at pH=1.0. The adsorption capacity increased with Cr（VI） concentration but decreased with pH value when pH〉1.0. Adsorption isotherms at various temperatures were obtained. Langmuir, Freundlich, Dubinin-Radushkevich and Temkin models were adopted and the equilibrium data fitted best with the Langmuir isotherm. The constants of these models indicated that the adsorption process involved both chemisorption and physisorption. The values of thermodynamic parameters, including DH, DG and DS, suggested that the adsorption of Cr（VI） on SAAEF was a spontaneous, entropy-driven and endothermic process. Q（iso） was not a constant value, which indicated an inhomogenous energy distribution on SAAEF.

Facile fabrication and enhanced photocatalytic performance of visible light responsive UiO-66-NH2/Ag2CO3 composite

A series of UiO-66-NH2/Ag2CO3 Z-scheme heterojunctions were prepared by a simple ion-exchange-solution method using UiO-66-NH2 and semiconductor Ag2CO3 as precursors.The photocatalytic activities of UAC-X(UAC-20,50,100,150,200)Z-scheme heterojunctions toward the hexavalent chromium(Cr(VI))reduction and UAC-100 toward oxidative degradation of four organic dyes like rhodamine B(RhB),methyl orange(MO),congo red(CR),and methylene blue(MB)under visible light irradiation were investigated.The effects of different pH(pH=2,3,4,6,8),small organic acids(citric acid,tartaric acid,and oxalic acid),and foreign ions(ions in tap water and surface water)on Cr(VI)reduction were explored.The results revealed that the UAC-100 heterojunctions displayed more remarkable Cr(VI)reduction performance than the pristine UiO-66-NH2 and Ag2CO3,resulting from the improved separation of photo-induced electrons and holes.The enhanced photocatalytic activity of UAC-100 was further confirmed by the photoluminescence measurement,electrochemical analysis,and active species trapping experiments.After four cycles’experiments,the photocatalytic Cr(VI)reduction efficiency over UAC-100 was still over 99%,which exhibited that UAC-100 had excellent reusability and stability.Finally,the corresponding photocatalytic reaction mechanism was proposed and tested.

Removal of hexavalent chromium in aquatic solutions by pomelo peel

This study investigated the removal of hexavalent chromium(Cr(VI)) in aqueous solutions using pomelo peel(PP) and FeCl3-modified pomelo peel(FPP) as novel biomass adsorbents.Batch adsorption experiments were performed to evaluate the effects of pH,time,temperature,initial concentration,and adsorbent dose on Cr(VI) removal by PP and FPP.The results show that the maximum adsorption capacity of Cr(VI)was 21.55 mg/g for FPP and 0.57 mg/g for PP at a pH of 2.0 and a temperature of 40℃.The surface shape,microstructure,and chemical composition of FPP were analyzed with scanning electron microscopy(SEM),Fourier transform infrared spectroscopy(FTIR),and energy dispersive spectroscopy(EDS),and compared with those of PP.The results show that the adsorption performance of FPP was much better than that of PP,indicating that FPP can be an alternative high-efficiency adsorbent for Cr(VI) removal.

Bioremediation of Hexavalent Chromium Pollution by Sporosarcina saromensis M52 Isolated from Offshore Sediments in Xiamen, China

Objective Cr（VI） removal from industrial effluents and sediments has attracted the attention of environmental researchers. In the present study, we aimed to isolate bacteria for Cr（VI） bioremediation from sediment samples and to optimize parameters of biodegradation. Methods Strains with the ability to tolerate Cr（VI） were obtained by serial dilution and spread plate methods and characterized by morphology, 16 S r DNA identification, and phylogenetic analysis. Cr（VI） was determined using the 1,5-diphenylcarbazide method, and the optimum p H and temperature for degradation were studied using a multiple-factor mixed experimental design. Statistical analysis methods were used to analyze the results. Results Fifty-five strains were obtained, and one strain（Sporosarcina saromensis M52; patent application number： 201410819443.3） having the ability to tolerate 500 mg Cr（VI）/L was selected to optimize the degradation conditions. M52 was found be able to efficiently remove 50-200 mg Cr（VI）/L in 24 h, achieving the highest removal efficiency at p H 7.0-8.5 and 35 °C. Moreover, M52 could completely degrade 100 mg Cr（VI）/L at p H 8.0 and 35 °C in 24 h. The mechanism involved in the reduction of Cr（VI） was considered to be bioreduction rather than absorption. Conclusion The strong degradation ability of S. saromensis M52 and its advantageous functional characteristics support the potential use of this organism for bioremediation of heavy metal pollution.

Heavy metal contamination, physico-chemical and microbial evaluation of water samples collected from chromite mine environment of Sukinda, India

Water samples from chromite mine quarry of Sukinda and its adjacent areas were analyzed for their heavy metal contamination along with physico-chemical and microbial contents. The chromite mine water samples possessed high concentrations of heavy metals in the order of Cr〉Fe〉Zn〉Ni〉Co〉Mn while ground water did not show any heavy metal contamination except Fe. Physico-chemical parameters of mine water samples showed deviation from those of normal water. Mine water harboured low microbial populations of bacteria, fungi and actinomycetes in comparison with mine adjacent water samples. The correlation of data between metals with physico-chemical parameters showed both positive and negative responses while that of metal and microbial population exhibited negative correlation. Bacterial strains isolated from chromite mine water exhibited high tolerance towards chromium and other heavy metals as well as antibiotics which could be used as an indicator of heavy metal pollution.

Leaching kinetics of acid-soluble Cr(Ⅵ) from chromite ore processing residue with hydrofluoric acid

Leaching kinetics of acid-soluble Cr（VI） in chromite ore processing residue （COPR） using hydrofluoric （HF） acid solution as a leaching agent was investigated for potential remediation of COPR with industrial waste water containing HF. The results show that HF can effectively destabilize the Cr（VI）-bearing minerals, resulting in the mobilization of Cr（VI） from COPR into the leachate. Particle size significantly influences the leaching of acid-soluble Cr（VI） from COPR, followed by leaching time, whereas the effects of HF concentration and leaching temperature are slight and the influence of stirring rate is negligible. The leaching process of acid-soluble Cr（VI） from COPR is controlled by the diffusion through the product layer. The apparent activation energy is 8.696 kJ/mol and the reaction orders with respect to HF concentration and particle size is 0.493 8 and -2.013 3, respectively.

Photocatalytic Cr(Ⅵ)elimination over BUC-21/N-K2Ti4O9 composites:Big differences in performance resulting from small differences in composition

A series of BUC-21/N-K2Ti4O9 composites(B1NX)were facilely fabricated from BUC-21 and N-K2Ti4O9 via ball-milling,and they were fully characterized using various techniques.The photocatalytic reduction of Cr(Ⅵ)over the B1NX composites was investigated systematically under various conditions,including different light sources,pH values,hole scavengers,and inorganic ions,in both real lake water and tap water.The BUC-21/N-K2Ti4O9 composites demonstrated remarkable photocatalytic Cr(Ⅵ)reduction performance,good reusability,and stability under both UV and white light irradiation.The introduction of N-K2Ti4O9 into BUC-21 not only broadened its light absorption region to white light,but also strongly inhibited the recombination of the photo-generated electrons and holes.Mechanisms of photocatalytic Cr(Ⅵ)reduction under both UV light and white light were proposed and verified by electrochemical measurements,active species capture experiments,and ESR measurements.

Modelling and Fixed Bed Column Adsorption of Cr(Ⅵ) onto Orthophosphoric Acid-activated Lignin

The advantage of using an available and abundant residual biomass, such as lignin, as a raw material for activated carbons is that it provides additional economical interest to the technical studies. In the current investigation, a more complete understanding of adsorption of Cr（VI） from aqueous systems onto HaPO4-acid activated lignin has been achieved via microcolumns, which were operated under various process conditions. The practice of using microcolumn is appropriate for defining the adsorption parameters and for screening a large number of poten- tial adsorbents. The effects of solution pH （2-8）, initial metal ion concentration （0.483-1.981 mmol.L-1）, flow rate （1.0-3.1 cm3-min-1）, ionic strength （0.01-0.30 mmol-L-1） and adsorbent mass （0.11 0.465 g） on Cr（VI） adsorption were studied by assessing the microcolmnn breakthrough curve. The microcolumn data were fitted by the Thomas model, the modified Dose model and the BDST model. As expected, the adsorption capacity increased with initial Cr（VI） concentration. High linear flow rates, pH values and ionic strength led to early breakthrough of Cr（VI）. The model constants obtained in this study can be used for the design of pilot scale adsorption process.

Using humic acid for remediation of sandy soils contaminated by heavy metal

This paper presents the development of a new remediation technology for contaminated sandy soil using humic acid (HA). Distribution of amount of Cr (VI) in the aqueous or solid system containing humic acid and sandy soil, was studied using batch experiments, es-pecially for effects of reaction time, pH, concentrations, temperature and irradiation on the reduction of Cr (VI), and the optimum reaction conditions. The results indicated a significant increase of the adsorption of Cr (VI) because of the complexion reaction between HA and Cr (VI) that occurred under acidic condition. The reaction mechanisms of HA with chromium on sand surfaces were certified. Thus it came to a conclusion that HA could be used effectively on remediation of Cr (VI)–contaminated soil and groundwater in a wide range of pH, with or without sunlight. These results suggest that the organic-inorganic complex—such as sandy soils coated with humic substances—is important as a metal reservoir in the environment.

Application of Iron Nanoparticles Synthesized by Green Tea for the Removal of Hexavalent Chromium in Column Tests

Nano zero valent iron particles (nZVI) are popular the last few years because of the numerous applications in remediation of a wide range of pollutants in contaminated soils and aquifers. The nZVI particles can be 10 - 1000 times more reactive than granular or micro-scale ZVI particles due to the small particle size, large specific surface area and high reactivity. An alternative green synthesis procedure was used for the production of nano zero valent iron particles (nZVI) using green tea (GT) extract, which is characterized by its high antioxidant content. Polyphenols in green tea extract possess double role in the synthesis of nZVI, because they not only reduce ferric cations, but also protect nZVI from oxidation and agglomeration as capping agents. The objective of current study was to simulate ata laboratory scale the attachment of GT-nZVI particles on soil material and study the effectiveness of attached nanoparticles for removing hexavalent chromium (Cr(VI)) from contaminated groundwater flowing through the porous soil bed. Column tests were carried out with various flowrates in order to examine the effect of contact time between the attached on porous medium nZVI and the flow-through solution on Cr(VI) reduction. After the completion of column tests the soil material in each column was split in 5 vertical sections, which were further subjected to chemical analyses and leaching tests. According to the results of the study increasing the contact time favors the reduction and removal of Cr(VI) from the aqueous phase. The reductive precipitation of Cr can be described as a reaction that follows a pseudo-first order kinetic law, with rate constant equal to k = 0.0243 ± 0.0011 min-1. Leaching tests indicated that precipitated chromium is not soluble. In the examined soil material, the total amount of precipitated Cr was found to range between 280 and 890 mg/(kg soil), while soluble Cr was less than 1.4 mg/kg and most probably it was due to the presence of residual Cr(VI) solution in the porosity of soil.

Removal of hexavalent chromium from aqueous solution by iron nanoparticles

Groundwater remediation by nanoparticles has received increasing interest in recent years. This report presents a thorough evaluation of hexavalent chromium removal in aqueous solutions using iron (Fe0) nanoparticles. Cr(VI) is a major pollutant of groundwater. Zero-valent iron, an important natural reductant of Cr(VI), is an option in the remediation of contami- nated sites, transforming Cr(VI) to essentially nontoxic Cr(III). At a dose of 0.4 g/L, 100% of Cr(VI) (20 mg/L) was degraded. The Cr(VI) removal efficiency decreased significantly with increasing initial pH. Different Fe0 type was compared in the same con- ditions. The reactivity was in the order starch-stabilized Fe0 nanoparticles>Fe0 nanoparticles>Fe0 powder>Fe0 filings. Electro- chemical analysis of the reaction process led to the conclusion that Cr(OH)3 should be the final product of Cr(VI). Iron nanopar- ticles are good choice for the remediation of heavy metals in groundwater.

Evaluation of the Adsorption of Hexavalent Chromium on Kaolinite and Illite

The adsorption of hexavalent chromium on Kaolinite and Illite was studied in order to evaluate their potential for the reduction of hexavalent chromium mobility and their possible application for the treatment of polluted sediment. The influence of various parameters affecting the adsorption of hexavalent chromium, such as the pH of aqueous solution, the ionic strength and the initial metal ion concentration were investigated. The optimal pH range corresponding to the hexavalent chromium adsorption maximum on the Kaolinite and Illite is 2 - 4 and 2 - 2.6, respectively. The results showed that hexavalent chromium sorption on Kaolinite and Illite was strongly influenced by the pH, the ionic strength and the initial metal ion concentration. Langmuir and Freundlich adsorption isotherms are employed to understand the nature of adsorption at room temperature. The characteristic parameters for each isotherm have been determined. This showed that the Freundlich isotherm model well described the equilibrium data. The data suggest that the charge of the clay mineral surface is one of the main factors controlling hexavalent chromium desorption at alkaline pHs.

Adsorption kinetics,isotherm,and thermodynamic studies of adsorption of pollutant from aqueous solutions onto humic acid

In the present study,humic acid was used as an adsorbent for the investigation of the adsorption kinetics,isotherms,and thermo-dynamic parameters of hexavalent chromium from aqueous solution at varying pH,temperatures,and concentrations.Adsorption isotherms and equilibrium adsorption capacities were determined by the fittings of the experimental data to three well-known iso-therm models:Langmuir,Freundlich,and Redlich-Peterson.The results showed that the Langmuir and Redlich-Peterson models appear to fit the adsorption better than did the Freundlich adsorption model for the adsorption of chromium onto humic acid.The equilibrium constants were used to calculate thermodynamic parameters such as the change of free energy,enthalpy,and entropy.The derived adsorption constants (logaL) and their temperature dependencies from Langmuir isotherm have been used to calculate the corresponding thermodynamic quantities such as the free energy of adsorption,heat,and entropy of adsorption.The thermo-dynamic data indicate that Cr (VI) adsorption onto humic acid is entropically driven and characterized by physical adsorption.

Rapid biosorption and reduction removal of Cr(Ⅵ) from aqueous solution by dried seaweeds

Four types of common seaweeds(Laminaria japonica,Undaria pinnatifida,Porphyra haitanensis,and Gracilaria lemaneiformis) were examined to remove Cr(Ⅵ) ions from aqueous solution.The experimental parameters that affected the biosorption process including pH,biomass dosage,contact time and temperature were investigated via batch experiments.The surface characteristics of seaweeds before and after Cr(Ⅵ) adsorption were studied with scanning electron microscopy and Fourier transform infrared spectroscopy.The results show that an initial solution with the pH of 1.0 is most favorable for Cr(Ⅵ) adsorption.Rapid adsorption is observed in the initial stage and adsorption equilibrium state is reached within 1 h.The adsorption efficiency by Porphyra haitanensis is the maximum among four types of seaweed powders,followed by Laminaria japonica and Undaria pinnatifida with biosorption efficiency up to 90%.The removal rate of Gracilaria lemaneiformis is less than 60%.The kinetic data obtained using the seaweeds are found to follow pseudo-second order kinetic model.Experimental sorption data adequately correlate with the Langmuir model.FTIR indicates that amino and carboxyl groups play an important role in the process of Cr(Ⅵ) adsorption and a large percentage of Cr(Ⅵ) ions are reduced by reductive groups on the surface of seaweeds.

Titanium dioxide-graphene composite electrochemical sensor for detection of hexavalent chromium

Hexavalent chromium(Cr(VI))compound is useful to various industries but is toxic and carcinogenic.In this research work,we fab-ricate an amperometric sensor for the determination of Cr(VI),using a titanium dioxide(TiO2)-reduced graphene oxide(rGO)composite as the sensing element.The composite was synthesized following sol−gel chemistry,yielding TiO2 nanoparticles of~50 nm in size,immobilized on chemically exfoliated rGO sheets.The composite was employed in a 3-electrode electrochemical cell and operated in an amperometric mode,exhibiting good responses to the 50 to 500 ppb Cr(VI).Our best result from pH 3 Mcilvane’s buffer medium reveals the sensitivity of 9.12×10−4 ppb−1 and a detection limit of 6 ppb with no signal interference from 200 ppm Ca(II),150 ppm Mg(II),and 50 ppb Pb(II).The excellent results of the TiO2-rGO sensor can be attributed to synergic effects between TiO2 and rGO,resulting from the presence of n-p heterojunctions and the formation of the TiO2 nanoparticles on rGO.