Carbon-based nanomaterials, especially carbon nanotubes and graphene, have drawn wide attention in recent years as novel materials for environmental applications. Notably, the functionalized ...derivatives of carbon nanotubes and graphene with high surface area and adsorption sites are proposed to remove heavy metals via adsorption, addressing the pressing pollution of heavy metal. This critical revies assesses the recent development of various functionalized carbon nanotubes and graphene that are used to remove heavy metals from contaminated water, including the preparation and characterization methods of functionalized carbon nanotubes and graphene, their applications for heavy metal adsorption, effects of water chemistry on the adsorption capacity, and decontamination mechanism. Future research directions have also been proposed with the goal of further improving their adsorption performance, the feasibility of industrial applications, and better simulating adsorption mechanisms.
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•Removal of heavy metals from water has drawn wide attention.•Various functionalized carbon nanotube and graphene nanomaterials were stated.•Effects of water environmental chemistry on heavy metals removal were discussed.•Adsorption isotherms, kinetics, thermodynamics, and pathways were discussed.•Some research prospects were proposed for future studies and application.
► Cr(VI) removal by Fe
0 loading on MWCNTs nanocomposites was first studied. ► Fe
0 on the surface or in the network of MWCNTs was observed by SEM, TEM, and XRD. ► Optimal mass ratio of MWCNTs to Fe
...0 was found and the efficiency was compared. ► Kinetics under different conditions were tested using pseudo first-order model. ► Effect of ionic strength, pH, and foreign ions on Cr(VI) removal was investigated.
For the first time, nanoscale zero-valent iron (nZVI)–multiwalled carbon nanotube (MWCNT) nanocomposites were adopted to remove Cr(VI) from wastewater. Such composites were prepared through depositing nZVI particles onto MWCNTs by in situ reduction of ferrous sulfate and then characterized by TEM, SEM and XRD. The results showed that nZVI particles could disperse on the surface or into the network of MWCNTs. Compared to bare nZVI or nZVI-activated carbon composites, the nZVI–MWCNT nanocomposites exhibited around 36% higher efficiency on Cr(VI) removal. The mass ratio of nZVI to MWCNTs was optimized at 1:2, at ionic strength of 0.05
M NaCl. The reaction followed a pseudo first-order model under different initial Cr(VI) concentrations and pHs. Low pH and initial Cr(VI) concentration could increase both removal efficiency and rate constants. Anions, such as
SO
4
2
-
,
NO
3
-
and
HCO
3
-
, exhibited negative effects on the removal of Cr(VI), while the effects of
PO
4
3
-
and
SiO
3
2
-
were insignificant. Overall, nZVI–MWCNT nanocomposites offer a promising alternative material for the removal of Cr(VI) ions from wastewater.
Sulfidation of nanoscale zerovalent iron (nZVI) has attracted increasing interest for improving the reactivity and selectivity of nZVI towards various contaminants, such as aqueous Cr(VI) removal. ...However, the benefits derived from sulfide modification that govern the removal of Cr(VI) remains unclear, which was studied in this work. S-nZVI with higher S/Fe molar ratio showed higher surface area, the discrepancy between the surface-area-normalized removal capacity of Cr(VI) by S-nZVI with different S/Fe indicated that the removal of Cr(VI) was also affected by other factors, such as electron transfer ability, surface-bounded Fe(II) species, and surface charges. High specific surface area would provide more active site for Cr(VI) removal, and as an efficient electron conductor, acicular-like FeSx phase would also favor electron transfer from Fe0 core to Cr(VI). Low initial pH also enhanced the Cr(VI) removal, and the Cr(VI) removal capacity by S-nZVI and nZVI was not affected by aging process, these results confirmed that the Fe(II) species also played an important role in the Cr(VI) removal. Other influence factors were also investigated for potential application, including temperature, initial Cr(VI) concentration, ionic strength, and co-existed ions. The removal mechanism of Cr(VI) by S-nZVI involved the sulfide modification to increase the specific surface area and provide more active sites, the corrosion of Fe0 to produce surface-bounded Fe(II) species to adsorb Cr(VI) species, followed by the favored reduction of Cr(VI) to Cr(III) due to the electron transfer ability of FeSx, then the formation of Cr(III)/Fe(III) hydroxides precipitates.
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•S-nZVI showed better reactivity than pristine nZVI towards Cr(VI).•S/Fe molar ratio had significant effects on the reactivity of S-nZVI.•Cr(VI) was removed through adsorption, reduction, and co-precipitation.•FeSx on S-nZVI facilitated the electron transfer and acted as reducing agent.•S-nZVI maintained high Cr(VI) removal efficiency after aged 12 days.
Herein, we report near‐infrared (NIR) light‐driven shape‐morphing of programmable MXene‐containing anisotropic hydrogel actuators that are fabricated through in situ free‐radical copolymerization of ...a judiciously designed MXene nanomonomer with thermosensitive hydrogel network. A low electric field (few V mm−1) was found to enable a spatial distribution of MXene nanosheets and hence introduce anisotropy into the hydrogel network. Programmable anisotropic hydrogel actuators were developed by controlling ITO electrode pattern, direct‐current (DC) electric field direction and mask‐assisted photopolymerization. As a proof‐of‐concept, we demonstrate NIR light‐driven shape morphing of the MXene‐containing anisotropic hydrogel into various shapes and devise a four‐arm soft gripper that can perform distinct photomechanical functions such as grasping, lifting/lowering down and releasing an object upon sequential NIR light exposure.
Programmable anisotropic hydrogel actuators with near‐infrared (NIR) light‐driven shape morphing properties were fabricated through in situ free‐radical copolymerization of a judiciously designed photopolymerizable MXene nanomonomer with thermosensitive PNIPAM‐based smart hydrogels. A shape‐programmed four‐arm soft gripper was demonstrated to perform distinct photomechanical functions under sequential exposure to spatiotemporal NIR light.
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► Fe0–Fe3O4 nanocomposites were synthesized and used for Cr(VI) removal. ► The synthesized nanocomposites were observed by SEM, HRTEM, XRD, and XPS. ► Effects of mass composition, pH, ...and initial Cr(VI) concentration were investigated. ► Kinetics and adsorption isotherms of such system were firstly described. ► Reaction mechanism has been deeply discussed and clearly presented in the figures.
For the first time, nanoscale zero-valent iron (nZVI)–Fe3O4 nanocomposites, prepared by an in situ reduction method, are employed for chromium(VI) removal in aqueous environment. 96.4% Cr(VI) could be removed by these novel materials within 2h under pH of 8.0 and initial Cr concentration of 20mgL−1, compared with 48.8% by bare nFe3O4 and 18.8% by bare nZVI. Effects of several factors, including mass composition of nZVI–Fe3O4 nanocomposites, initial pH and Cr(VI) concentration, were evaluated. The optimal ratio of nFe3O4 to nZVI mass lies at 12:1 with a fixed nZVI concentration of 0.05gL−1. Low pH and initial Cr(VI) concentration could increase both the Cr(VI) removal efficiency and reaction rate. Corresponding reaction kinetics fitted well with the pseudo second-order adsorption model. Free energy change (ΔG) of this reaction was calculated to be −4.6kJmol−1 by thermodynamic study, which confirmed its spontaneous and endothermic characteristic. The experimental data could be well described by the Langmuir and Freundlich model, and the maximum capacity (qmax) obtained from the Langmuir model was 100 and 29.43mgg−1 at pH 3.0 and 8.0, respectively. The reaction mechanism was discussed in terms of the mutual benefit brought by the electron transfer from Fe0 to Fe3O4.
Dewatering is important for activated sludge disposal. The dewaterability of activated sludge was first deteriorated and then ameliorated when the temperature was raised from 100 to 200 °C with a ...threshold temperature of 130 °C under hydrothermal treatment. Calcium chloride assisted hydrothermal treatment to improve the dewaterability of activated sludge, and eliminated the threshold temperature at as less as 20 mg/g dry solid (DS). An increase in temperature and dosage of CaCl2 till 60 mg/g DS allowed a continuous improvement of dewaterability. It is found that the charge neutralization resulted from biopolymers solubilization dominated the dewaterability evolution below 160 °C, while the decomposition of water-binding components played a more important role at higher temperatures. The variation of molecular weight of soluble protein and polysaccharides implies that CaCl2 interacted with the component of sludge and altered the constituent during the hydrothermal treatment. The integration of soluble biopolymers into the floc matrix by CaCl2 contributed to the compacted floc structure and thus improved the dewaterability. This work presents an insight into the floc variation in both the composition and structure associated with the dewaterability and offers a new understanding to the role of temperature and CaCl2 in hydrothermal treatment on activated sludge dewatering.
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•Hydrothermal treatment and CaCl2 play different roles in sludge dewatering.•Charge neutralization dominated the sludge dewatering below 160 °C.•Water-binding component decomposition prevailed in dewatering promotion above 160 °C.•Calcium ions altered the flocs constituent and consolidated the inner interaction.
Magnetic biochar is increasingly known as a multi-functional material and the appropriate synthesis method further increase its efficient applications. In this study, the effects of synthesis methods ...on the fabrication of Kans grass straw/biochar (KGS/KGB) with Fe super(3+)/Fe super(2+) by chemical co-precipitation and subsequently pyrolyzing at 500 degree C for 2 and 4 h were studied in details, and compared their As(III, V) adsorption potentials under different operating conditions. Magnetic biochars (MKGB3 and MKGB4) prepared from KGS revealed of superior Fe sub(3)O sub(4) loading, higher As(III, V) adsorption efficiency and saturation magnetization (45.7 Am super(2) kg super(-1)) than that of KGB (MKGB1 and MKGB2). Moreover, Thermogravimetric analysis (TGA) demonstrated three stages of decomposition and the MKGB3 and MKGB4 generated higher residual mass (>60%) at stage 3 (1000 degree C) due to greater Fe sub(3)O sub(4) composite in biochar matrix and turned to be thermally more stable. As(III) and As(V) adsorption equilibrium data well fitted in Langmuir model and followed the order: MKGB4 > MKGB3 > MKGB2 > MKGB1. The maximum As(III) and As(V) adsorption capacities were about 2.0 mg g super(-1) and 3.1 mg g super(-1), respectively. The data best fitted in pseudo-second-order (R super(2) > 0.99) rather than pseudo-first-order kinetics model indicating of more complex mechanism. The adsorption of As(III) and As(V) was found to decrease with increasing in ionic strength of competing ions and PO sub(4) super(3-) was found to strongly inhibit arsenic adsorption. Highest desorption was achieved at pH 13.5 using NaOH. This study suggests that selective adsorbent synthesis method could be useful to prepare effective adsorbent for toxic metals immobilization.
Activated sludge dewatering is of great importance in sludge treatment and disposal. To enhance the dewaterability, a novel method was performed by treating the sludge under mild temperature (50-90 ...°C) in CaCl(2) solution (3.7-1110.0 mg/g dry sludge). The capillary suction time, zeta potential, Fourier-transformed infrared spectra, concentration of soluble protein and carbohydrates were employed to characterize the dewaterability and influencing mechanism. The sludge dewaterability was deteriorated with single thermal treatment, but significantly promoted in CaCl(2) solution and advanced further together with thermal treatment. An increasing CaCl(2) dosage reduced the surface charge remarkably, and a higher temperature could strengthen this impact. The spectra indicate that Ca(2+) could interact with the protein, phenols and O-H functional group in the flocs. The thermal treatment could cause the solubilization of protein and carbohydrates, providing more binding sites for Ca(2+) to establish a strong bridging among the flocs. As CaCl(2) dosage elevated, the soluble carbohydrates showed a reduction trend, while the soluble protein lowered firstly and then bounced back except that remained unchanged at room temperature. A bridging equilibrium is presumed to exist between Ca(2+) and the soluble protein. And the bridging between Ca(2+) and the soluble carbohydrates plays a more important role in the dewatering. The sludge dewaterability was successfully and economically improved by thermal treatment in CaCl(2) solution.
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•nZVI assembled on magnetic Fe3O4/graphene nanocomposites is synthesized.•Better dispersion and stability of nZVI are achieved.•These nanocomposites exhibit high removal efficiency on ...Cr(VI).•Cr(VI) adsorption and reduction via nZVI-graphene/Fe3O4 battery work synergically.
Nanoscale Zero-Valent Iron (nZVI) assembled on magnetic Fe3O4/graphene (nZVI@MG) nanocomposites was synthesized for Cr(VI) removal from aqueous solution. nZVI particles were perfectly dispersed either among Fe3O4 nanoparticles (Fe3O4 NPs) or above the basal plane of graphene. This material shows Cr(VI) removal efficiency of 83.8%, much higher than those of individuals (18.0% for nZVI, 21.6% for Fe3O4 NPs and 23.7% for graphene) and even their sum of 63.3%. The removal process obeys pseudo-second-order adsorption model, suggesting that adsorption is rate-controlling step. Maximum Cr(VI) adsorption capacity varies from 66.2 to 101.0mgg−1 with decreasing pH from 8.0 to 3.0 at 30°C. Negative ΔG and ΔH indicate spontaneous tendency and exothermic nature. Robust performance of nZVI@MG arises from the formation of micro-nZVI-graphene/nZVI-Fe3O4 batteries and strong adsorption capability of broad graphene sheet/Fe3O4 surfaces. Electrons released by nZVI spread all over the surfaces of graphene and Fe3O4, and the adsorbed Cr(VI) ions on them capture these floating electrons and reduce to Cr(III). Fe3O4 NPs also served as protection shell to prevent nZVI from agglomeration and passivation.
