Inner-sphere Surface Complexes Research Articles

Mechanisms of selenium removal by partially oxidized magnetite nanoparticles for wastewater remediation

Magnetite nanoparticles are a promising cost-effective material for the remediation of polluted wastewaters. Due to their magnetic properties and their high adsorption and reduction potential, they are particularly suitable for the decontamination of oxyanion-forming contaminants, including the highly mobile selenium oxyanions selenite and selenate. However, little is known how the remediation efficiency of magnetite nanoparticles in field applications is affected by partial oxidation and the formation of magnetite/maghemite phases. Here we characterize the retention mechanisms and capacity of partially oxidized nanoparticulate magnetite for selenite and selenate in an oxic system at different pH conditions and ionic strengths. Data from adsorption experiments showed that retention of selenate is extremely limited except for acidic conditions and strongly influenced by competing chloride anions, indicating outer-sphere adsorption. By contrast, although selenite adsorption capacity of oxidized magnetite is also adversely affected by increasing pH, considerable selenite quantities are retained even at alkaline conditions. Using spectroscopic analyses (XPS, XAFS), both mononuclear edge-sharing (2E) and binuclear corner-sharing (2C) inner-sphere selenite surface complexes were detected, while reduction to Se(0) or Se(–II) species could be excluded. Under favourable adsorption conditions, up to ~pH 8, the affinity of selenite to form 2C surface complexes is higher, whereas at alkaline pH values and less favourable adsorption conditions 2E complexes become more dominant. Our results demonstrate that magnetite can be used as a suitable reactant for the immobilization of selenite in remediation applications, even under (sub)oxic conditions and without the involvement of reduction processes.

Nanocellulose aerogel for highly efficient adsorption of uranium (VI) from aqueous solution

Cellulose nanofibers (CNFs) aerogel was prepared via simple covalent crosslinking and freeze-drying method. The porous cellulose aerogel possessed high specific surface area and high metal-chelating capacity, which showed fast adsorption kinetics and high adsorption capacity (440.60 mg g−1) in static uranium adsorption process. In the dynamic filtration system, the maximum adsorption capacity reached 194 mg g−1 with the initial concentration of 10 mg L−1. In addition, the CNFs aerogel possessed excellent selectivity and good regeneration ability for uranium adsorption. The integrated analyses of attenuated total reflection Fourier transform infrared (ATR-FTIR), X-Ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) suggested that the predominant UO22+ species formed inner-sphere surface complexes with two active carboxyl groups in the coordination model. This strategy may provide a sustainable route for development of efficient biomass-based adsorbents for selective uranium removal from aqueous solution.

Kinetics and equilibrium adsorption of phosphate on lanthanum oxide supported on activated carbon

Understanding the thermodynamics and kinetics of phosphate adsorption is crucial in the design of packed-bed columns for phosphate removal, particularly in clinical applications such as dialysis, in which limited contact time is available. Studies have shown that the kinetics of phosphate adsorption on metal oxides is a two-step process: rapid initial adsorption that occurs in a few minutes to a few hours is followed by slower adsorption that may last for days. In this work, the thermodynamics and kinetics of phosphate adsorption on lanthanum oxide impregnated on activated carbon (La-AC) are investigated to explain its phosphate adsorption behavior. The phosphate adsorption mechanism was investigated by studying (1) the effects of temperature on phosphate adsorption capacity, (2) the dynamics and enthalpy of adsorption using flow microcalorimetry (FMC), (3) the kinetics of phosphate adsorption, and (4) the relationship between the amount of phosphate adsorbed and the amount of hydroxide released. Phosphate adsorption on La-AC followed the Langmuir isotherm. FMC results indicate that phosphate adsorbed through several mechanisms, principally through the formation of monodentate and bidentate inner-sphere surface complexes. A pseudo-second-order kinetic model is typically used to model phosphate adsorption; however, it was found that a kinetic model developed based on the assumption of two parallel surface adsorption reactions fits the measured adsorption kinetics significantly better than the pseudo-second order kinetic model that is almost universally used to describe phosphate adsorption kinetics. • Lanthanum-impregnated activated carbon was investigated as a phosphate adsorbent. • Phosphate adsorption was well described by the Langmuir isotherm. • Phosphate anions adsorbed on lanthanum surface via a parallel path mechanism. • A kinetic model based on the parallel path mechanism was developed. • Parallel path model fit kinetics much better than the pseudo-2nd-order model.

Facile synthesis of novel tremella-like Mn0@Mn2O3 and its exceptional performance on removal of phosphate

Phosphorus is an essential nutrient for biological growth, but its excessive presence in water can lead to eutrophication. Reducing phosphorus concentrations is an important way to prevent eutrophication of surface water bodies. In this study, a novel zero-valent manganese (ZVM) Mn0@Mn2O3 sorbent was synthesized via a facile one-step reduction method at ambient temperature. The morphology and surface properties of the Mn0@Mn2O3 were analyzed by Scanning electron microscopy (SEM), X-ray diffraction (XRD), Zeta potential analyzer, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The obtained amorphous sample consists of many small tremella-like particles. The as-prepared sorbent shows exceptional phosphate removal performance and has a maximum adsorption capacity of 113.7 mg-P/g at pH 7.0, superior to many reported adsorbents. The removal of phosphate is obviously pH-dependent and is favorable under near-neutral and weak-alkaline conditions (pH 6.5–8.5). The kinetics data fit well to the pseudo-second-order model. The coexisting SO42-, CO32- and Cl- have no great influence on phosphate sorption, whereas, the SiO32- shows negative effect at high concentration. The sorbent is effective for phosphate removal from real municipal wastewater. The results of ionic strength effect experiment, SEM, XRD, FTIR and XPS analyses reveal that the phosphate was mainly removed through surface precipitation and formation of inner-sphere surface complexes. The Mn0@Mn2O3 is a promising sorbent for phosphate removal from surface water and wastewater, owning to its exceptional performance, high selectivity, environmental friendliness and easy preparation.

Revealing and elucidating chemical speciation mechanisms for lead and nickel adsorption on zeolite in aqueous solutions

The removal of heavy metals from water is crucial for human health and the environment, and finding an appropriate adsorption material to achieve this goal is extremely desirable. In this study we reported zeolite as the adsorption material to remove the Pb2+ and Ni2+ from the aqueous solution and elucidated the chemical speciation using batch technique. The adsorption of these metals by zeolite was investigated in relation to pH, ionic strength, contact time, co-existing ions, and the temperature. Zeolite exhibited better adsorption capacities than several other adsorption materials, due to its larger specific surface area. The adsorption of Pb2+ and Ni2+ is influenced by ionic strength at pH < 7.0 and pH < 8 and is independent of the ionic strength at pH > 7.0 and pH > 8, respectively. The chemical speciation of Pb2+ and Ni2+ is regulated by outer- and inner-sphere surface complexation, respectively at low and high pH values,. The maximum adsorption capacity of Pb2+ and Ni2+ predicted from the Langmuir model was found to be 45.00 and 28.57 mg g–1 respectively at 303 K. The thermodynamic studies revealed the spontaneous and endothermic nature of adsorption process whereby zeolite displayed an efficient adsorption for both metals with increasing temperature. The findings are very useful in evaluating zeolite interactions in aqueous solutions with the chemical speciation of Pb2+ and Ni2+. From above findings, the zeolite can be recommended as an appropriate adsorbent to adsorb metal ions from aqueous solution.

Synthesis of Mg–Al layered double hydroxides-functionalized hydrochar composite via an in situ one-pot hydrothermal method for arsenate and phosphate removal: Structural characterization and adsorption performance

• Mg–Al LDHs-FHC composite was synthesized via an in situ one-pot hydrothermal route. • The Mg:Al molar ratio and hydrothermal temperature were systematically optimized. • Synthetic conditions play significant roles in arsenate and phosphate removal. • Mechanism involves intercalation, inner-sphere, and outer-sphere surface complexes. • Competitive adsorption behavior mainly occurred at the binding sites on Mg(OH) 2 . In recent years, hydrochar has been recognized as an ideal and eco-friendly adsorbent for the removal of various pollutants, while it has a relatively low adsorption affinity toward negatively charged contaminants, such as arsenate and phosphate. Thus, to address serious worldwide water pollution caused by arsenate and phosphate, the Mg–Al layered double hydroxides-functionalized hydrochar (Mg–Al LDHs-FHC) composite was synthesized via an in situ one-pot hydrothermal method. The Mg:Al molar ratios (2:1–4:1) and hydrothermal temperatures (150–210 °C) played significant roles in influencing the structural properties of the Mg–Al LDHs-FHC composites. Specifically, the Mg–Al LDHs-FHC composite prepared at Mg:Al molar ratio of 2:1 and hydrothermal temperature of 150 °C showed the best arsenate (100%) and phosphate (99.7%) removal due to the highest charge density (4.132 e/nm 2 ) and the largest interlayer d -spacing (0.8371 nm), facilitating strong electrostatic adsorbent–adsorbate interaction and easy intercalation into the interlayer. Single-component isotherm data fit Langmuir and Freundlich models, while binary-component data used extended and ideal adsorbed solution theory combining these models. Overall, the adsorption affinity of the composite followed the order arsenate > phosphate (single-component system) and arsenate < phosphate (binary-component system). Spectroscopic analyses demonstrated that the different physicochemical characteristics of arsenate and phosphate, such as partial charge density, ionic radius, and the coordinate bonding characteristics were responsible for the competitive adsorption behavior. Stable reusability at least five times indicates that the Mg–Al LDHs-FHC composite could be a promising adsorbent for simultaneous arsenate and phosphate removal from water.

Investigation of phosphate removal mechanisms by a lanthanum hydroxide adsorbent using p-XRD, FTIR and XPS

Macroscopic batch experiments and microscopic spectroscopic characterization by powder X-ray diffraction (p-XRD), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were employed to accurately study the phosphate adsorption process onto lanthanum hydroxide adsorbent. The relatively low La release after phosphate adsorption suggested the formation of surface co-precipitation of lanthanum phosphate beyond the Lewis acid-base interaction under the acidic conditions. The maximum adsorption capacities calculated by the Langmuir isotherm at pH 4.0 and 8.0 were 122.0 mg/g and 109.9 mg/g, respectively. Cl−, SO42– and HCO3– on phosphate removal negligibly impacted on phosphate adsorption, while acetate slightly retarded the adsorption. The fitted FTIR spectra of P−O stretching vibration bands in 900–1200 cm−1 range indicated that diprotonated bidentate binuclear or monoprotonated bidentate mononuclear complexes were the dominant surface configurations at pH 4–9. Diprotonated bidentate binuclear complexes were dominant at pH 3 and monoprotonated bidentate binuclear complexes were the major configurations at pH > 9. The high resolution scans of La 3d, O 1s and P 2p spectra before and after phosphate adsorption and lanthanum phosphate indicated that inner-sphere surface complexes formed through the ligand exchange reaction, which were further determined to be rhabdophane (LaPO4·0.5H2O) by p-XRD analysis.

Insights into the adsorption mechanism of tannic acid by a green synthesized nano-hydroxyapatite and its effect on aqueous Cu(II) removal.

The polyphenolic tannic acid (TA) has been widely used in the stabilization and surface modification of nanomaterials. The interaction mechanism of TA with the biogenic nano-hydroxyapatite (nHAP) and its environmental importance, however, are poorly understood. This study explored the adsorption of TA using the green synthesized, eggshell-derived nHAP and implications of this process for the removal of aqueous Cu(II) via batch adsorption experiments, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) investigations. TA adsorption by nHAP was a complex pH-dependent process and significantly correlated with TA molecule speciation and amphoteric properties of nHAP via multiple adsorption modes including surface complexation, electrostatic attraction, and hydrogen bond. The maximum TA adsorption amount was found to be 94.8mg/g for less crystalline nHAP with lower calcination temperature. In the ternary Cu-TA-nHAP systems, TA promoted Cu(II) adsorption at pH<5 and reduced Cu(II) uptake at pH>5. Further studies of the effects of ionic strength and addition sequences, as well as Raman, FTIR, and XPS analyses revealed Cu(II) adsorption on nHAP was mainly dominated by inner-sphere surface complexation. These results can shed light on not only the utility of biogenic nHAP for TA and Cu(II) adsorption but also the evaluation of the effect of TA on the environmental behavior of heavy metals.

Highly efficient U(VI) capture by amidoxime/carbon nitride composites: Evidence of EXAFS and modeling

The recovery of uranium from wastewater and safe treatment of U(VI)-containing wastewater are of great important to ensure the sustainable development of nuclear-related energy. Although abundant studies of U(VI) sorption on various adsorbents have been widely achieved, U(VI) sorption at extreme pH and trace concentration is challenging issues due to limited sorption activity of natural adsorbents. The development of novel materials with highly efficient and excellent selectivity for capturing U(VI) from nuclear-related wastewater and seawater is highly desirable. In this study, amidoxime/carbon nitride (AO/g-C3N4) was fabricated and captured U(VI) under a variety of water chemistry. We demonstrated that AO/g-C3N4 exhibited the high adsorption capacities (312 mg/g at pH 6.8), fast removal equilibrium (>98% at 10 min) and superior selectivity for U(VI) compared with the other radionuclides (e.g., 19.76 mg/g of Cs(I)). In addition, AO/g-C3N4 exhibited the high uranium extraction capacity from natural seawater (9.55 mg/g at saturation time of 5.5 days) compared to vanadium (1.85 mg/g). U(VI) adsorption behavior at different pH can be excellently fitted by the surface complexation modeling with three inner sphere surface complexes (i.e., SOUO2(CO3)23−, SO(UO2)3(OH)50 and SOUO2+ species). According to XPS (X-ray Photoelectron Spectroscopy) analysis, the strong complexation of U(VI) with AO groups retained in C3N4 nanosheet. The split of U-Oeq2 subshell and the occurrence of U–C shell further demonstrated inner-sphere surface complexation by EXAFS (X-Ray Absorption Fine Structure) spectra analyses. These results revealed that the high potential of AO/g-C3N4 materials for selective U(VI) capture from wastewater and seawater.

Zirconium-modified natural clays for phosphate removal: Effect of clay minerals

Excessive amount of phosphate entering water bodies may cause eutrophication and have detrimental effects on ecosystems. Clay-based materials have been drawing attractive attention in mitigating phosphate release to aquatic environment. In this study, we prepared a series of zirconium (Zr)-modified clays to investigate the effect of clay structure and expansion property on phosphate adsorption. Kaolinite, montmorillonite, and vermiculite were selected as three representative natural clays for Zr modification, and the resulting Zr-modified clays were characterized using various techniques that included powder X-ray diffraction, scanning electron microscopy, and zeta potential measurement. Different Zr-modified clays exhibited substantially different phosphate adsorption behaviors, which may be related to the distinct structural and expansion properties of each clay substrate. Particularly, Zr-modified montmorillonite had fastest phosphate adsorption kinetics and highest phosphate adsorption capacity among all Zr-modified clays, which may be attributed to the good expansion property of montmorillonite that favored the uniform intercalation of Zr species, making the adsorption sites easily accessible by phosphate. Furthermore, all Zr-modified clays showed robust performance for phosphate adsorption under various water chemistry conditions. Combined aqueous sorption and solid characterization analyses suggested that formation of inner-sphere surface complexes may be the primary mechanism for phosphate adsorption by Zr-modified clays.

Facile synthesis of nanosheet-assembled γ-Fe2O3 magnetic microspheres and enhanced Sb(III) removal.

The development and utilization of magnetic nanoadsorption materials with large adsorption capacity and easy separation are the research hotspot nowadays. In this study, nanosheet-assembled maghemite (γ-Fe2O3) magnetic microspheres were successfully synthesized by an environmental friendly, quick, and simple method, for enhanced Sb(III) removal from aqueous solution. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), and Brunauer-Emmett-Teller (BET) were used to characterize the material. The results showed that the product contained flower-like γ-Fe2O3 microspheres composed of petal-shaped nanosheets interspersed with each other. The specific surface area and pore volume were 69.23 m2/g and 0.15 cm3/g, respectively. The material has a strong magnetic response, which allows rapid solid-liquid separation under the action of an external magnetic field. The effects of different dosages, solution pH, and contact time on the adsorption effect were studied by batch adsorption experiments, and the reusability of the materials was evaluated. Both Freundlich isothermal adsorption model and pseudo-second-order kinetic model were able to describe the uptake of Sb(III). The maximum adsorption capacity of the material was 47.48 mg/g under optimal conditions. The adsorption mechanism is mainly that Sb and lattice oxygen (OX2-) form Fe-O-Sb coordination bonds, which is incorporated into the crystal structure of γ-Fe2O3 as inner-sphere surface complexes. The synthetic material has the advantage of simple preparation process, good adsorption capacity, operation over a wide range of pH, and easy physical separation from treatment systems with good potential for future application to treat polluted wastewater.

Capture and Release of Orthophosphate by Fe-Modified Biochars: Mechanisms and Environmental Applications

Biochars have been suggested to have P capture potential from effluent streams and to recycle the captured P to agricultural soils. However, most biochars have low P sorption capacity. The objective of this study was to engineer biochar for enhanced P sorption affinity. Biochar was produced from corn stover biomass pre-treated with FeSO4 (ISIB) using autothermal (air-blown) pyrolysis at 500 °C. Point of zero charge (pHZPC) shifted from 8.48 to 4.31, indicating that Fe treatment increased the dominance of acid functional groups. Batch equilibration isotherm study showed that ISIB had 11–12 times more P sorption capacity (3763 versus 46,300 mg kg–1 and 6704 versus 48,821 mg kg–1 for non-oxidized and oxidized conditions, respectively), while P desorption rate was ∼1/3 relative to the control biochar. A column leaching study also shows that ISIB was effective for removing P from simulated agricultural effluent. XRD (X-ray diffraction) and SEM-EDS (scanning electron microscopy–energy-dispersive X-ray spectrometry) analyses showed the P sorption was predominately through inner-sphere surface complexation followed by surface precipitation and that P is preferentially sorbed by hematite (α-Fe2O3) relative to magnetite (FeIII2O3 + FeIIO) or maghemite (γ-Fe2O3). This study demonstrates that ISIB can be produced by pyrolyzing corn stover with FeSO4, and the resulting ISIB is effective for adsorption and recycling of P. When loaded with P, the ISIB can potentially be used as a slow-release P fertilizer.

Controlled preparation of a MCC-g-AM/EDA/PA loaded Fe(iii) adsorbent by the pre-radiation grafting method and its application for the adsorption removal of phosphate.

A new spherical cellulose-based adsorbent and high phosphate removal rate microcrystalline cellulose-g-acrylamide/ethylenediamine/phthalic anhydride (MCC-g-AM/EDA/PA) loaded Fe(iii) adsorbent was prepared by a pre-radiation grafting and chemical modification method. Fe(iii) was successfully introduced into the modified grafted chains of the MCC-g-AM/EDA/PA resin, and characterized by FTIR, TG, XRD, SEM and XPS. The optimized conditions for the grafting reaction of acrylamide (AM) onto MCC were 20% AM emulsion at an absorbed dose of 30 kGy, and a grafting rate as high as 247%. In addition, the adsorption performance of the adsorbent was tested by static adsorption experiments with phosphate. The adsorbent resin showed excellent adsorption performance under alkaline conditions, contributions to the synergetic effect of precipitation, and inner-sphere surface complex reactions. The adsorption efficiency can reach 89.7% at low concentration. In summary, the neotype spherical cellulose-based adsorbent has the advantages of being separation-free in bulk materials, avoiding secondary pollution, and being easy to recycle, and it could be employed as an environmentally friendly adsorbent for phosphate removal in eutrophic water.

Highly efficient adsorption and immobilization of U(VI) from aqueous solution by alkalized MXene-supported nanoscale zero-valent iron

A novel composite of zero-valent iron nanoparticles supported on alkalized Ti3C2Tx nanoflakes (nZVI/Alk-Ti3C2Tx) was constructed by an in-situ growth method for simultaneous adsorption and reduction U(VI) from aqueous solution in anoxic conditions. The effect of various factors such as adsorbent dose, pH, ionic strength, contact time, initial U(VI) concentration and environmental media were comprehensively investigated by batch experiments. Benefiting from the good dispersion uniformity of nZVI on MXene substrates, nZVI/Alk-Ti3C2Tx exhibited rapid removal kinetics, excellent selectivity, 100% removal efficiency and up to 1315 mg g−1 uptake capacity for U(VI) capture. In the presence of mimic groundwater, 1.0 mM NaHCO3 and 10 mg L−1 humic acid, the removal percentages of U(VI) by the composites could reach 95.1%, 88.9% and 69.5%, respectively. The reaction mechanism between U(VI) and nZVI/Alk-Ti3C2Tx has been clarified based on FTIR, XANES, XPS and XRD analysis. Depending on the consumption of reactive nZVI in the composites and the solution pH, the elimination of U(VI) could be realized by different pathways including reductive immobilization in the form of UO2, inner-sphere surface complexation and hydrolysis precipitation. The present study illustrates that the nZVI/Alk-Ti3C2Tx composite may be an efficient scavenger for radioactive wastewater purification in environmental remediation.

Application of FeMgMn layered double hydroxides for phosphate anions adsorptive removal from water

The development and application of new materials for phosphate removal are very important due to its increasing emissions from the activities of daily living and socially production which may lead to eutrophication. The FeMgMn layered double hydroxides (FeMgMn-LDH) have been synthesized via a co-precipitation method and their adsorption characteristics for phosphate were investigated in this study. Adsorption equilibrium could be fitted by the Dubinin-Kaganer-Radushkevich (DKR) and Langmuir models. FeMg2Mn-LDH has the largest phosphate removal capacity and the fastest adsorption kinetics among the three LDH with different Fe/Mg/Mn ratio. The maximum adsorption capacity of FeMg2Mn-LDH at 25 °C is 34.31 mg-P/g. The negative values of ΔG0 and the positive value of ΔH0 (6.725 kJ/mol) indicate that the phosphate adsorption process on FeMgMn-LDH is spontaneous and endothermic. The competition order of coexisting anions on phosphate adsorption with FeMgMn-LDH is CO32− > SO42− > NO3−. An ion exchange mechanism can be attested by the apparent adsorption energy (E) values and the change of Cl content. Moreover, the appearance of small particles on the SEM images of Phosphate-FeMgMn-LDH, and the results of FTIR and XPS spectra demonstrated that the formation of both outer- and inner-sphere surface complexes via electrostatic attraction and monodentate/bidentate complexations. Overall, the as-prepared FeMgMn-LDH is a promising adsorbent for dephosphorization.

Nanoanalytical Identification of Siderite Dissolution-Coupled Pb Removal Mechanisms from Oxic and Anoxic Aqueous Solutions

Lead(II) is a toxic pollutant often found in metal-contaminated soils and wastewaters. In acidic aqueous environments, Pb(II) is highly mobile. Chemical treatment strategies of such systems therefore often include neutralization agents and metal sorbents. Since metal solubility and the retention potential of sorbents depend on the redox state of the aqueous system, we tested the efficiency of the naturally occurring redox-sensitive ferrous iron carbonate mineral siderite to remove Pb(II) from acidic aqueous solutions in batch experiments under oxic and anoxic conditions over a total of 1008 h. Siderite dissolution led to an increase in reactive solution pH from 3 to 5.3 and 6.9, while 90 and 100% of the initial aqueous Pb(II) (0.48 × 10–3 mol kg–1) were removed from the oxic and anoxic systems, respectively. Scanning and transmission electron microscopy, combined with X-ray absorption and photoelectron spectroscopy, indicated that under oxic conditions, Pb(II) was consumed by cerussite precipitation and inner-sphere surface complexation to secondary goethite. Under anoxic conditions, Pb(II) was removed by the rapid precipitation of cerussite. This efficient siderite dissolution-coupled sequestration of Pb(II) into more stable solid phases demonstrates this potential method for contaminated water treatment regardless of the redox environment.

A constant composition technique for quantifying the effect of As(V) on struvite crystallization under various operational conditions

Struvite crystallization was one of the most promising techniques for nutrient recovery from nutrient-rich wastewater. However, As(V) derived from the wastewater has been proved to adversely affect struvite crystallization. Constant composition technique was employed to quantify the effect of As(V) on struvite crystallization in the presence of As(V) under various operational conditions such as supersaturation, pH and ionic strength. Subsequently, the interaction mechanisms between As(V) and struvite crystals were further elucidated by means of XPS and FTIR. Interestingly, the roles of adsorption and coprecipitation in the interaction under various operational conditions were primarily evaluated. At a supersaturation of 0.4, pH 8.5 and 0.1 mol/L NaCl, the crystal growth rate of struvite was substantially reduced from (14.2 ± 0.3) × 10−6 to (7.6 ± 0.1) × 10−6 mol/min with As(V) concentration increasing from 0 to 10 mg/L. Nevertheless, As(V) scarcely changed the surface diffusion growth mechanism of struvite. Moreover, the adsorption and coprecipitation of As(V) accounted for its interaction with struvite. Thereinto, the adsorption mechanism was triggered by the bidentate inner-sphere surface complexes and the electrostatic interaction between As(V) and struvite. Furthermore, the influence of supersaturation and ionic strength on the inhibitory effect was found to be highly dependent of the adsorption of As(V), whereas the coprecipitation of As(V) is mainly responsible for the impact of pH. The finding herein provided a novel approach to elucidate the inhibitory mechanisms of heavy metals on struvite crystallization, which facilitated to obtain pure struvite crystals from the wastewater contaminated by heavy metals.

In situ fabrication of ultrathin-g-C3N4/AgI heterojunctions with improved catalytic performance for photodegrading rhodamine B solution

Heterojunction photocatalysts with controllable compositions and textures have attracted extensive research interest in environmental pollutants degradation owing to the superior catalytic activity compared to their single component counterparts. Herein, a series of ultrathin-g-C3N4/AgI heterojunctions (UCNA) were prepared by ultrasonication-assisted liquid exfoliation of bulk g-C3N4 followed by characteristic inner-sphere surface complexation of Ag (I) and in-situ growth of AgI on ultrathin g-C3N4 nanosheets. The structural features and optical properties of all the prepared samples were investigated using X-ray powder diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffused reflectance spectroscopy (UV–vis DRS) and photoluminescence spectroscopy (PL) prior performing photocatalytic activity. The as-prepared heterojunctions showed significant improvement in photocatalytic activity in comparison with pure AgI and g-C3N4 nanosheets with the complete degradation of rhodamine B (RhB) in 60 min at UCNA-70% sample under visible light irradiation with good recycling characteristics. The superior photocatalytic performance of the heterojunctions can be attributed to the combined effects of tightly coupled high-quality interface, appropriate energy band structure and position, along with the increased charge separation and migration efficiency. Based on the analysis of interfacial charge-transfer process across the ultrathin-g-C3N4/AgI heterojunction, a plausible photocatalytic mechanism of ultrathin-g-C3N4/AgI composites was presented.

A nappies management by-product for the treatment of uranium-contaminated waters

The direct disposal of municipal solid waste such as nappies to the environment may create serious pollution problems. Based on the circular economy and waste management concepts, the conversion of nappies and/or their ingredients (such as super absorbent polymer (SAP)) to high added value products is of great importance. In this work, a modified SAP (MSAP) was examined as an adsorbent for treatment of contaminated waters and uranium recovery. Batch experiments and spectroscopic techniques were used to examine the effect of various parameters (pH, contact time, temperature, initial concentration, and ionic strength), and the mechanism of adsorption U(VI) and desorption process. The U(VI) concentration was determined by alpha spectroscopy after addition of 232U standard tracer solution to account for possible interferences during electrodeposition and alpha particle counting. The maximum adsorption monolayer capacity was found to be 217.4 mg/g at pH 4.0 and at 298 K. The adsorption of U(VI) on MSAP seems to occur mainly via the formation of inner-sphere surface complexes between U(VI) and the carboxylic surface moieties of MSAP. The MSAP could satisfactorily be regenerated with 0.1 M Na2CO3 (>90%) and it also shows a promising applicability to real wastewaters contaminated with U(VI).

Removal of uranium using MnO2/orange peel biochar composite prepared by activation and in-situ deposit in a single step

Orange peel is an important resource in view of its low-cost and universal nature. A novel material (MnO2/orange peel carbon, MnO2/OPC) was yielded via a single step involving activation and in situ deposit. The feasibility of uranium removal using MnO2/OPC was tested by varying MnO2 loading amount, pH, ionic strength, uranium initial concentration and temperature. 1.2MnO2/OPC yielded the most competitive uptake value as 246.3 mg g−1. The pH beneficial for adsorption was 5.5. The adsorption taken the way of inner-sphere surface complexation and the process was an endothermic, entropy increasing and spontaneous process in nature. The work gave enlightening results for reusing orange peel as raw materials to produce a kind of scavenger materials of economy and effectivity in the field of treating radioactive aqueous solution.