Adsorption and mechanistic study of the invasive plant-derived biochar functionalized with CaAl-LDH for Eu(III) in water.

Herein, we developed the invasive plant-derived biochar (IPB) functionalized with CaAl-LDH at five mass ratios using a physical mixture method, assessed their adsorption perform for Eu(III), and explored the relative mechanisms. Results show that the IPB successfully loaded CaAl-LDH in five composites and their Eu(III) sorption affinities were strongly affected by solution pH, contact time, temperature, and the mass ratio of LDH and IPB. All the sorpiton process for Eu(III) occurred on the heterogeneous surface of five composites and the boundary layer diffusion limited the chemical sorption rate. Interestingly, the CaAl-LDH/IPB composite with high ratio of IPB had higher sorption capacity than the one with high ratio of LDH due to larger porosity of the former. Three mechanisms containing ion exchange between Al and Eu ions, surface complexation with carboxyl- and oxygen-containing functional groups, and precipitation were involved in the Eu(III) sorption, but the dominant sorption mechanism for each CaAl-LDH/IPB composite differed with different mass ratio of CaAl-LDH and IPB. In composite with more IPB (e.g., CaAl-LDH/IPB-13), both ion exchange and surface complexes dominated the sorption process and the intensity of Eu3+ was identified with the one of Eu2O3. Whereas in composites with high LDH, ion exchange dominated the sorption and the intensity of Eu3+ was obviously higher than the one of Eu2O3. This research will provide a new perspective for the application of the LDH/biochar materials.

Enhanced Photocatalytic Simultaneous Removals of Cr(VI) and Bisphenol A over Co(II)-Modified TiO2.

To enhance the electron-hole separation and boost the practical performance of commercial titania (Degussa P25) under natural solar light, in this work, P25 was modified with Co(II) species (CoP25) through post-treatment with decomposition of Co-ethylenediaminetetraacetic acid precursors in a wet chemical anchoring process. With appropriate Co(II) loading amount as molecular cocatalyst, the resulted CoP25-4 showed significantly improved photocatalytic performance for Cr(VI) reduction and bisphenol A (BPA) oxidation under UV-light irradiation. The coexistence of Cr(VI) and BPA promoted mutually the degradation of both pollutants. Under simulated solar light (AM 1.5G) illumination, the Cr(VI) reduction rate over CoP25-4 was 8.5 times enhanced compared with that over P25, whereas the simultaneous degradation rate of BPA over CoP25-4 was 8 times higher than that over P25. Further investigations indicated that the covalent atomic Co(II) anchoring on P25 significantly promoted the photogenerated electron-hole separation and facilitated Cr(VI) reduction via the formation of a Co(I) intermediate and simultaneously boosted BPA oxidation. Our results demonstrated a facile strategy to modify P25 with remarkably improved performance for the practical application in environmental pollution management under natural light excitation.

Mechanistic insights into sequestration of U(VI) toward magnetic biochar: Batch, XPS and EXAFS techniques.

The magnetic iron oxide (Fe3O4) nanoparticles stabilized on the biochar were synthesized by fast pyrolysis of Fe(II)-loaded hydrophyte biomass under N2 conditions. The batch experiments showed that magnetic biochar presented a large removal capacity (54.35mg/g) at pH3.0 and 293K. The reductive co-precipitation of U(VI) to U(IV) by magnetic biochar was demonstrated according to X-ray diffraction, X-ray photoelectron spectroscopy and X-ray absorption near edge structure analysis. According to extended X-ray absorption fine structure analysis, the occurrence of U-Fe and U-U shells indicated that high effective removal of uranium was primarily inner-sphere coordination and then reductive co-precipitation at low pH. These observations provided the further understanding of uranium removal by magnetic materials in environmental remediation.

Controllable integration of nano zero-valent iron into MOFs with different structures for the purification of hexavalent chromium-contaminated water: Combined insights of scavenging performance and potential mechanism investigations.

This work combined the stability of the porous structure of metal-organic frameworks with the strong reducibility of nano zero-valent iron, for the controllable integration of NZVI into MOFs to utilize the advantages of each component with enhancing the rapid decontamination and scavenging of Cr(VI) from wastewater. Hence, four kinds of MOFs/NZVI composites namely ZIF67/NZVI, MOF74/NZVI, MIL101(Fe)/NZVI, CuBTC/NZVI, were prepared for Cr(VI) capture. The results indicated that the stable structure of ZIF67, MOF74, MIL101(Fe), CuBTC, was beneficial for the dispersion of NZVI that could help more close contact between MOFs/NZVI reactive sites and Cr(VI), subsequently, MOFs/NZVI was proved to be better scavengers for Cr(VI) scavenging than NZVI alone. The Cr(VI) capture achieved the maximum adsorption capacity at pH ~ 4.0, which might be due to the participation of more H+ in the reaction and better corrosion of NZVI at lower pH. Mechanism investigation demonstrated synergy of adsorption, reduction and surface precipitation resulted in enhanced Cr(VI) scavenging, and Fe(0), dissolved and surface-bound Fe(II) were the primary reducing species. The findings of this investigation indicated that the as-prepared composites of ZIF67/NZVI, MOF74/NZVI, MIL101(Fe)/NZVI, CuBTC/NZVI, with high oxidation resistance and excellent reactivity, could provide reference for the decontamination and purification of actual Cr(VI)-containing wastewater.

Hollow spherical nano-traps using pillararene-based polymer for efficient uranium extraction from seawater.

Herein, a hollow spherical pillar[5]arene-based polymer (P5-AO) adsorbent was synthesized. The P5-AO adsorbent was capable of effectively capturing uranium from simulated seawater (139.5 mg g-1) and real seawater (8.1 mg g-1). We also elucidated the uranium adsorption mechanism of P5-AOvia extended X-ray absorption fine structure (EXAFS). This study provides a novel direction for the development of uranium capture adsorbents.

Preparation and evaluation of celite decorated iron nanoparticles for the sequestration performance of hexavalent chromium from aqueous solution.

The increasing usage of an important heavy metal chromium for industrial purposes, such as metallurgy, electroplating, leather tanning, and other fields, has contributed to an augmented level of hexavalent chromium (Cr(VI)) in watercourses negatively impacting the ecosystems and significantly making Cr(VI) pollution a serious environmental issue. In this regard, iron nanoparticles exhibited great reactivity in remediation of Cr(VI)-polluted waters and soils, but, the persistence and dispersion of the raw iron should be improved. Herein, this article utilized an environment-friendly celite as a modifying reagent and described the preparation of a novel composites namaly celite decorated iron nanoparticles (C-Fe0) and evaluation of C-Fe0 for the sequestration performance of Cr(VI) from aqueous solution. The results indicated that initial Cr(VI) concentration, adsorbent dosage, and especially solution pH are all critical factors to control C-Fe0 performance in Cr(VI) sequestration. We demonstrated that C-Fe0 could achieve a high Cr(VI) sequestration efficiency with an optimized adsorbent dosage. Fitness of the pseudo-second-order kinetics model with data indicated that adsorption was the rate-controlling step and chemical interaction controlled Cr(VI) sequestration on C-Fe0. The adsorption isotherm of Cr(VI) could be the best depicted by Langmuir model with a monolayer adsorption. The underlying sequestration path of Cr(VI) by C-Fe0 was then put forward, and the combined effect of adsorption and reduction implied the potentials of C-Fe0 in Cr(VI) removal.

Highly efficient removal of rare earth elements by two-dimensional titanium carbide nanosheets as impacted via water chemistry.

The separation and recycling of rare earth elements (REEs) are very important owing to the high demand, limited resource, specific usages, and environmental issues. In this work, two-dimensional Ti3C2Tx MXene was introduced to remove REEs (Nd(III) and La(III)) from water, and its physicochemical properties were conducted by HRTEM, SEM-EDS, XRD, FTIR, and XPS. Various parameters, such as initial pH, REEs initial concentration, contact time, and temperature, were investigated by batch experiment, respectively. Furthermore, the adsorption kinetic and isotherm were examined to analyze the adsorption behavior and adsorption mechanism. Nd(III) and La(III) have a good affinity with Ti3C2Tx MXene surface functional groups (-F, -OH, and containing oxygen groups). The maximum adsorption capacities of Ti3C2Tx MXene for Nd(III) and La(III) were 229.85 mg/g and 175.83 mg/g at T = 333 K, respectively. The adsorption data of Nd(III) on Ti3C2Tx MXene fitted well with the Freundlich isotherms model and pseudo-second-order kinetic model. However, the best fitting for La(III) adsorption on Ti3C2Tx MXene was described by both pseudo-first-order and pseudo-second-order model. Thermodynamic study of Nd(III) and La(III) adsorption on Ti3C2Tx MXene showed that the reaction was a spontaneous and endothermic process. These results indicated Ti3C2Tx MXene had a great potential in extracting REEs from an aqueous solution.

Macroscopic and microscopic investigation of Ni(II) sequestration on diatomite by batch, XPS, and EXAFS techniques.

Sequestration of Ni(II) on diatomite as a function of time, pH, and temperature was investigated by batch, XPS, and EXAFS techniques. The ionic strength-dependent sorption at pH < 7.0 was consistent with outer-sphere surface complexation, while the ionic strength-independent sorption at pH = 7.0-8.6 was indicative of inner-sphere surface complexation. EXAFS results indicated that the adsorbed Ni(II) consisted of ∼6 O at R(Ni-O) ≈ 2.05 Å. EXAFS analysis from the second shell suggested that three phenomena occurred at the diatomite/water interface: (1) outer-sphere and/or inner-sphere complexation; (2) dissolution of Si which is the rate limiting step during Ni uptake; and (3) extensive growth of surface (co)precipitates. Under acidic conditions, outer-sphere complexation is the main mechanism controlling Ni uptake, which is in good agreement with the macroscopic results. At contact time of 1 h or 1 day or pH = 7.0-8.0, surface coprecipitates occur concurrently with inner-sphere complexes on diatomite surface, whereas at contact time of 1 month or pH = 10.0, surface (co)precipitates dominate Ni uptake. Furthermore, surface loading increases with temperature increasing, and surface coprecipitates become the dominant mechanism at elevated temperature. The results are important to understand Ni interaction with minerals at the solid-water interface, which is helpful to evaluate the mobility of Ni(II) in the natural environment.

Geochemical and U-Th isotopic insights on uranium enrichment in reservoir sediments.

Uranium (U) geochemistry and its isotopic compositions of reservoir sediments in U mine area were poorly understood. Herein, U and Th isotopic compositions were employed to investigate source apportionment and geochemical behavior of U in 41 reservoir sediments from a U mining area, Guangdong, China. The remarkably high contents of both total U (207.3-1117.7 mg/kg) and acid-leachable U (90.3-638.5 mg/kg) in the sediments exhibit a severe U contamination and mobilization-release risk. The U/Th activity ratios (ARs) indicate that all sediments have been contaminated apparently by U as a result of discharge of U containing wastewater, especially uranium mill tailings (UMT) leachate, while the variations of U/Th ARs are dominated by U geochemical behaviors (mainly redox process and adsorption). The U isotopic compositions (δ238U) showed a large variance through the sediment profile, varying from - 0.62 to - 0.04‰. The relation between δ238U and acid-leachable U fraction demonstrates that the U isotopic fractionation in sediments can be controlled by bedrock weathering (natural activity), UMT leachate (anthropogenic activity) and subsequent biogeochemical processes. The findings suggest that U-Th isotopes are a powerful tool to better understand U geochemical processes and enrichment mechanism in sediments that were affected by combined sources and driving forces.

Is the interaction between graphene oxide and minerals reversible?

The increased applications and production of graphene oxide (GO) make the necessity to study information on the interaction of GO with minerals. In this work, adsorption and desorption were used to study the reversibility of interaction between GO and goethite/kaolinite. Result showed that the pH value, ionic strength, and temperature had significant effects on the adsorption and desorption behavior of GO. Interaction force was stronger between GO and goethite than that of kaolinite. The interaction may be attributed to the electrostatic, hydrogen-bonding, and Lewis acid base interactions. The irreversible interaction between GO and minerals may be a main mechanism for the observed desorption hysteresis. These results are important for evaluating the fate and health risk of GO in the environment.

Thallium pollution in China and removal technologies for waters: A review.

Thallium (Tl) is a typical toxic metal, which poses a great threat to human health through drinking water and the food chain (biomagnification). China has rich Tl-bearing mineral resources, which have been extensively explored and utilized, leading to release of large amounts of Tl into the environment. However, research on Tl pollution and removal techniques is relatively limited, because Tl has not been listed within the scope of environmental monitoring in China for several decades. This paper reviewed Tl pollution in wastewater arising from various industries in China, as well as the latest available methods for treating Tl-containing industrial wastewater, in order to give an outlook on effective technologies for controlling Tl pollution. Conventional physical and chemical treatment technologies are efficient at removing trace amounts of Tl, but it proved to be difficult to achieve the stringent environmental standard (≤0.1-5 µg/L) cost-effectively. Adsorption by using newly developed nanomaterials, and metal oxide modified polymer materials and microbial fuel cells are highly promising and expected to become next-generation technologies for remediation of Tl pollution. With the potential for greater Tl contamination in the environment under accelerated growth of industrialization, researches based on lab-scale implementation of such promising treatment technologies should be further expanded to pilot and industrial scale, ensuring environmental protection and the safety of drinking water for sustainable development. Comprehensive insights into experiences of Tl pollution in China and in-depth perspectives on new frontier technologies of Tl removal from wastewaters will also benefit other nations/regions worldwide, which are susceptible to high exposure to Tl likewise.

Sorption properties of Th(IV) on the raw diatomite--effects of contact time, pH, ionic strength and temperature.

Diatomite has a number of unique physicochemical properties and has diversified industrial uses. Natural diatomite has been tested as a potential sorbent for the removal of Th(IV) from aqueous solutions. The results indicate that sorption of Th(IV) is strongly dependent on ionic strength at pH<3, and is independent of ionic strength at pH>3. Outer-sphere complexation or ion exchange may be the main sorption mechanism of Th(IV) to diatomite at low pH values, whereas the sorption of Th(IV) at pH>3 is mainly dominated by inner-sphere complexation or precipitation. The competition for Th(IV) between aqueous or surface adsorbed anions (e.g., herein ClO(4)(-), NO(3)(-) and Cl(-)) and surface functional groups of diatomite is important for Th(IV) sorption. The thermodynamic data (DeltaH(0), DeltaS(0), DeltaG(0)) are calculated from the temperature-dependent sorption isotherms. The results suggest that sorption process of Th(IV) on diatomite is spontaneous and endothermic.

Enhanced immobilization of U(VI) on Mucor circinelloides in presence of As(V): Batch and XAFS investigation.

The combined pollution of radionuclides and heavy metals has been given rise to widespread concern during uranium mining. The influence of As(V) on U(VI) immobilization by Mucor circinelloides (M. circinelloides) was investigated using batch experiments. The activity of antioxidative enzymes and concentrations of thiol compounds and organic acid in M. circinelloides increased to respond to different U(VI) and As(V) stress. The morphological structure of M. circinelloides changed obviously under U(VI) and As(V) stress by SEM and TEM analysis. The results of XANES and EXAFS analysis showed that U(VI) was mainly reduced to nano-uraninite (nano-UO2, 30.1%) in U400, while only 9.7% of nano-UO2 was observed in the presence of As(V) in U400-As400 due to the formation of uranyl arsenate precipitate (Trögerite, 48.6%). These observations will provide the fundamental data for fungal remediation of uranium and heavy metals in uranium-contaminated soils.

Adsorption and co-adsorption of graphene oxide and Ni(II) on iron oxides: A spectroscopic and microscopic investigation.

Graphene oxide (GO) may strongly interact with toxic metal ions and mineral particles upon release into the soil environment. We evaluated the mutual effects between GO and Ni (Ni(II)) with regard to their adsorption and co-adsorption on two minerals (goethite and hematite) in aqueous phase. Results indicated that GO and Ni could mutually facilitate the adsorption of each other on both goethite and hematite over a wide pH range. Addition of Ni promoted GO co-adsorption mainly due to the increased positive charge of minerals and cation-π interactions, while the presence of GO enhanced Ni co-adsorption predominantly due to neutralization of positive charge and strong interaction with oxygen-containing functional groups on adsorbed GO. Increasing adsorption of GO and Ni on minerals as they coexist may thus reduce their mobility in soil. Extended X-ray absorption fine structure (EXAFS) spectroscopy data revealed that GO altered the microstructure of Ni on minerals, i.e., Ni formed edge-sharing surface species (at RNi-Fe∼3.2 Å) without GO, while a GO-bridging ternary surface complexes (at RNi-C∼2.49 Å and RNi-Fe∼4.23 Å) was formed with GO. These findings improved the understanding of potential fate and toxicity of GO as well as the partitioning processes of Ni ions in aquatic and soil environments.

The enhancement roles of layered double hydroxide on the reductive immobilization of selenate by nanoscale zero valent iron: Macroscopic and microscopic approaches.

Herein, we utilized nanoscale zero-valent iron loaded on layered double hydroxide (NZVI/LDH) to immobilize Se(VI) and evaluated the enhancement role of LDH in the NZVI reaction system. The structural characterization indicated that LDH could stabilize and disperse NZVI as well as prevent NZVI from oxidation, thereby increasing iron reactivity. Batch experiments displayed that, compared with those by NZVI, both extent and rate of Se(VI) immobilized by NZVI/LDH significantly increased, owing to the prominent synergistic effect ascribing from adsorption and reduction. Kinetics studies under a series of conditions showed that Se(VI) reaction could be well described by pseudo first-order model. The performance of Se(VI) immobilization was inhibited to a considerable extent by most of co-existing ions, Nevertheless, the presence of Cu2+ improved performance of NZVI/LDH due to its role as a catalyst or medium of charge transfer during reduction. XANES revealed that LDH acted as a promoter for complete reduction of Se(VI) into Se(0)/Se(-II) over a wide pH range, whereas EXAFS suggested that LDH acted as a scavenger for insoluble products, making more reactive sites exposure to Se(VI) for reduction. These results suggested that NZVI/LDH as a promising candidate exhibited potential application in remediation of wastewaters containing Se(VI).

Macroscopic and spectroscopic studies of the enhanced scavenging of Cr(VI) and Se(VI) from water by titanate nanotube anchored nanoscale zero-valent iron.

Herein, a promising titanate nanotubes (TNT) anchored nanoscale zero-valent iron (NZVI) nanocomposite (NZVI/TNT) was synthesized, characterized and used for the enhanced scavenging of Cr(VI) and Se(VI) from water. The structural identification indicated that NZVI was uniformly loaded on TNT, thereby, the oxidation and aggregation of NZVI was significantly minimized. The macroscopic experimental results indicated that NZVI/TNT exhibited higher efficiency as well as rate on Cr(VI) and Se(VI) scavenging resulted from the good synergistic effect between adsorption and reduction. Besides, TNT can weaken the inhibitory effect of co-existing humic acid (HA) and fulvic acid (FA) on the scavenging of Cr(VI) and Se(VI) by NZVI, since TNT showed strong adsorption for HA and FA that inhibit potential reactivity. XPS analysis suggested that surface-bound Fe(II) played a critical role in Cr(VI) and Se(VI) scavenging. XANES analysis demonstrated that TNT acted as a promoter for the almost complete transformation of Cr(VI) into Cr(III), and Se(VI) into Se(0)/Se(-II) in NZVI system. EXAFS analysis indicated that TNT acted as a scavenger for insoluble products, and thus more reactive sites can be used for Cr(VI) and Se(VI) reduction. The excellent performance of NZVI/TNT provide a potential material for purification and detoxification of Cr(VI) and Se(VI) from wastewater.

Thallium contamination in arable soils and vegetables around a steel plant-A newly-found significant source of Tl pollution in South China.

Thallium (Tl) is a highly toxic rare element. Severe Tl poisoning can cause neurological brain damage or even death. The present study was designed to investigate contents of Tl and other associated heavy metals in arable soils and twelve common vegetables cultivated around a steel plant in South China, a newly-found initiator of Tl pollution. Potential health risks of these metals to exposed population via consumption of vegetables were examined by calculating hazard quotients (HQ). The soils showed a significant contamination with Tl at a mean concentration of 1.34 mg/kg. The Tl levels in most vegetables (such as leaf lettuce, chard and pak choy) surpassed the maximum permissible level (0.5 mg/kg) according to the environmental quality standards for food in Germany. Vegetables like leaf lettuce, chard, pak choy, romaine lettuce and Indian beans all exhibited bioconcentration factors (BCF) and transfer factors (TF) for Tl higher than 1, indicating a hyperaccumulation of Tl in these plants. Although the elevated Tl levels in the vegetables at present will not immediately pose significant non-carcinogenic health risks to residents, it highlights the necessity of a permanent monitoring of Tl contamination in the steel-making areas.

Enhanced sequestration of Cr(VI) by nanoscale zero-valent iron supported on layered double hydroxide by batch and XAFS study.

Herein, the reduction of nanoscale zero-valent iron (NZVI) and adsorption of layered double hydroxides (LDH) to sequester Cr(VI) were well combined by the immobilization of NZVI onto LDH surface (NZVI/LDH). The characterization results revealed that LDH decreased NZVI aggregation and thus increased Cr(VI) sequestration. The batch results indicated that Cr(VI) sequestration by NZVI/LDH was higher than that of NZVI, and superior to the sum of reduction and adsorption. The LDH with good anion exchange property allowed the adsorption of Cr(VI), facilitating interfacial reaction by increasing the local concentration of Cr(VI) in the NZVI vicinity. X-ray absorption near edge structure (XANES) results indicated that Cr(VI) was almost completely reduced to Cr(III) by NZVI/LDH, but Cr(VI) was partly reduced to Cr(III) by NZVI with a trace of Cr(VI) adsorbed on corrosion products. The coordination environment of Cr from extended X-ray absorption fine structure (EXAFS) analysis revealed that LDH could be a good scavenger for the insoluble products produced during reaction. So, the insoluble products on NZVI could be reduced, and its reactivity could be maintained. These results demonstrated that NZVI/LDH exhibits multiple functionalities relevant to the remediation of Cr(VI)-contaminated sites.

The roles of a pillared bentonite on enhancing Se(VI) removal by ZVI and the influence of co-existing solutes in groundwater.

The zero-valent iron permeable reactive barrier (ZVI-PRB) is a promising technology for in-situ groundwater remediation. However, its long-term performance often declined due to the blocked reactive sites by corrosion products and by interference of co-existing solutes. In order to address these issues, a pillared bentonite (Al-bent) was homogeneously mixed with ZVI for removing selenate (Se(VI)) from simulated groundwater in column experiments. The Se(VI) removal was enhanced because first Al-bent could facilitate the mass transfer of Se(VI) from solution to iron surface and accelerate Se(VI) reduction. XANES analysis indicated that Se(VI) was almost completely reduced to Se(0) and Se(-II) of less toxicity and solubility by the ZVI/Al-bent mixture, and the buffering effect of Al-bent could maintain the pH at a lower level that favored the Se(VI) removal. Besides, Al-bent could transfer the corrosion products away from iron surface, leading to the enhanced reactivity and longevity of ZVI. The inhibition on reactivity towards Se(VI) in both the single ZVI and the ZVI/Al-bent systems increased in the order of Cl(-)

Colloidal diatomite, radionickel, and humic substance interaction: a combined batch, XPS, and EXAFS investigation.

This work determined the influence of humic acid (HA) and fulvic acid (FA) on the interaction mechanism and microstructure of Ni(II) onto diatomite by using batch experiments, X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) methods. Macroscopic and spectroscopic experiments have been combined to see the evolution of the interaction mechanism and microstructure of Ni(II) in the presence of HA/FA as compared with that in the absence of HA/FA. The results indicated that the interaction of Ni(II) with diatomite presents the expected solution pH edge at 7.0, which is modified by addition of HA/FA. In the presence of HA/FA, the interaction of Ni(II) with diatomite increased below solution pH 7.0, while Ni(II) interaction decreased above solution pH 7.0. XPS analysis suggested that the enrichment of Ni(II) onto diatomite may be due to the formation of (≡SO)2Ni. EXAFS results showed that binary surface complexes and ternary surface complexes of Ni(II) can be simultaneously formed in the presence of HA/FA, whereas only binary surface complexes of Ni(II) are formed in the absence of HA/FA, which contribute to the enhanced Ni(II) uptake at low pH values. The results observed in this work are important for the evaluation of Ni(II) and related radionuclide physicochemical behavior in the natural soil and water environment.