Removal of trace mercury(II) from aqueous solution by in situ formed Mn-Fe (hydr)oxides

One-step synthesis of a core-shell structured biochar using algae (Chlorella) powder and ferric sulfate for immobilizing Hg(II)

Mercury (Hg) pollution poses a significant environmental challenge. One promising method for its removal is the sorption of mercuric ions using biochar. FeS-doped biochar (FBC) exhibits effective mercury adsorption, however may release excess iron into the surrounding water. To address this issue, a novel magnetic pyrrhotite/magnetite-doped biochar with a core-shell structure was synthesized for the adsorption of 2-valent mercury (Hg(II)). The proposed synthesis process involved the use of algae powder and ferric sulfate in a one-step method. By varying the ratio of ferric sulfate and alga powder (within the range of 0.18 - 2.5) had a notable impact on the composition of FBC. As the ferric sulfate content increased, the FBC exhibited a higher concentration of oxygen-containing groups. To assess the adsorption capacity, Langmuir and Freundlich adsorption models were applied to the experimental data. The most effective adsorption was achieved with FBC-4, reaching a maximum capacity (Qm) of 95.51 mg/g. In particular, at low Hg(II) concentrations, FBC-5 demonstrated the ability to reduce Hg(II) concentrations to less than 0.05 mg/L within 30 min. Additionally, the stability of FBC was confirmed within the pH range of 3.8 - 7.2. The study also introduced a model to analyze the adsorption preference for different Hg(II) species. Calomel was identified in the mercury saturated FBC, whereas the core-shell structure exhibited excellent conductivity, which most likely contributed to the minimal release of iron. In summary, this research presents a novel and promising method for synthesizing core-shell structured biochar and provides a novel approach to explore the adsorption contribution of different metal species.

Highly efficient capture of mercury from complex water matrices by AlZn alloy reduction-amalgamation and in situ layered double hydroxide

Mercury pollution is a critical, worldwide problem and the efficient, cost-effective removal of mercury from complex, contaminated water matrices in a wide pH range from strongly acidic to alkaline has been a challenge. Here, AlZn and AlFe alloys are investigated and a new process of synergistic reduction-amalgamation and in situ layered double hydroxide (SRA-iLDH) for highly efficient capture of aqueous Hg(II) is developed using AlZn alloys. The parameters include the pH values of 1-12, the Hg(II) concentrations of 10-1000 mg L-1, and the alloy's Zn concentrations of 20%, 50% and 70% and Fe concentrations of 10%, 20% and 50%. The initial rate of Hg(II) uptake by AlZn alloys decreases with increasing Zn concentration while the overall rate is not affected. Specifically, AlZn50 alloy removes >99.5% Hg(II) from 10 mg L-1 solutions at pH 1-12 in 5 min at a rate constant of 0.055 g mg-1 min-1 and achieves a capacity of 5000 mg g-1, being the highest value reported so far. The super-performance of AlZn alloy is attributed to multiple functions of chemical reduction, dual amalgamation, in situ LDH's surface complexation and adsorption, isomorphous substitution and intercalation. This study provides a simple and highly efficient approach for removing Hg(II) from complex water matrices.

Rational design of selective functionalized silica adsorbents for Hg(II) removal from FGD wastewater. A combined experimental and theoretical study

The binding of Hg2+ by mercaptan-functionalized silica possessing different physical characteristics was explored through experimental investigations and theoretical calculations. The impacts of the pore structure of silica adsorbents on functionalization and the kinetics of Hg2+ adsorption were also studied. The mechanism of Hg2+ adsorption was proposed based on the results of experimental and theoretical studies. The DFT calculations suggested a simultaneous monodentate and bidentate Hg2+ complex formation on sulfur-containing surface functional groups that strongly depends on the Hg2+ concentration. Theoretical Hg2+ adsorption isotherms predicted by DFT calculations have shown that even when the adsorbent surface had heterogeneous adsorption sites capable of simultaneous monodentate and bidentate complexation, the adsorption still followed the Langmuir model.

Effect of a Permanganate-Bearing Reactive Oxidant on Flocs in Electrocoagulation: Transformations and Interfacial Interactions

The electrocoagulation/ultrafiltration (ECUF) process is expected to address the issues of current wastewater increments and complex water reuse. However, the underlying mechanism associated with flocs remains unclear in the ECUF system, especially in the upgraded permanganate-bearing ECUF (PECUF) system. Herein, flocs and their formation, response to organic matter (OM), and interfacial features in the PECUF process were systematically explored. Results demonstrated that permanganate contributed to the rapid start-up of the coagulation process by forming MnO₂ and blocking the ligand-metal charge transfer process between adsorbed Fe(II) and solid-phase Fe(III). The response of flocs to natural OM (NOM) exhibited obvious time- and particle size-dependent characteristics. Based on this, the optimal NOM adsorption window was found to be in the interval of 5-20 min, whereas the optimal NOM removal window was located at the 20-30 min interval. Furthermore, the extended Derjaguin-Landau-Verwey-Overbeek theory revealed the underlying principle of the PECUF module for optimizing UF performance. On the one hand, it reduced the inherent resistance of the cake layer by modifying the colloidal solution, which guaranteed a small drop (15%) in initial flux. On the other hand, it enhanced the repulsive force among suspended particles to achieve a long-term antifouling effect. This study may provide insights into the selection and performance control of on-demand assembly modules in decentralized water treatment systems.

Ultra-high-efficiency capture of lead ions over acetylenic bond-rich graphdiyne adsorbent in aqueous solution

A satisfactory material with high adsorption capacity is urgently needed to solve the serious problem of environment and human health caused by lead pollution. Herein, hydrogen-substituted graphdiyne (HsGDY) was successfully fabricated and employed to remove lead ions from sewage and lead-containing blood. The as-prepared HsGDY exhibits the highest adsorption capacity of lead among the reported materials with a maximum adsorption capacity of 2,390 mg/g, i.e., ~five times larger than that of graphdiyne (GDY). The distinguished hexagonal hole and stack mode of HsGDY allows the adsorption of more lead via its inner side adsorption mode in one single unit space. In addition, the Pb 6s and H 1s hybridization promotes the strong bonding of lead atom adsorbed at the acetylenic bond of HsGDY, contributing to the high adsorption capacity. HsGDY can be easily regenerated by acid treatment and showed excellent regeneration ability and reliability after six adsorption-regeneration cycles. Langmuir isotherm model, pseudo second order, and density functional theory (DFT) demonstrated that the lead adsorption process in HsGDY is monolayer chemisorption. Furthermore, the HsGDY-based portable filter can handle 1,000 μg/L lead-containing aqueous solution up to 1,000 mL, which is nearly 6.67 times that of commercial activated carbon particles. And, the HsGDY shows good biocompatibility and excellent removal efficiency to 100 μg/L blood lead, which is 1.7 times higher than that of GDY. These findings suggest that HsGDY could be a promising adsorbent for practical lead and other heavy metal removal.

Application of response surface methodology and box-behnken design for the optimization of mercury removal by Ulva sp

Mercury (Hg) is a global and top priority contaminant, toxic at low concentrations. Although it has been progressively eliminated from processes, this metal continues to circulate in the atmosphere, soil, and water. In this work, the Response Surface Methodology (RSM) combined with a Box-Behnken Design (3 factors - 3 levels) was used to optimize key operational conditions that influence the removal and uptake of Hg by living macroalga Ulva sp. in a complex mixture containing several elements used in industry (potentially toxic elements, rare earth elements, and platinum-group elements) (initial concentration 10, 100 and 190 µg/L, salinity 15, 25 and 35, seaweed stock density 1.0, 3.0 and 5.0 g/L). Results evidenced the great capability of Ulva sp. to remove Hg, with removal efficiencies between 69 % and 97 %. 3-D surfaces showed that the most impactful variable was seaweed stock density, with higher densities leading to higher removal. Regarding the uptake, a positive correlation between initial concentration and q_t values was observed. The appliance of RSM made possible to obtain optimal operating conditions for removing virtually 100 % of Hg from waters with high ionic strength, which is a pivotal step in the direction of the application of this remediation biotechnology at large scale.

Selective mercury adsorption and enrichment enabled by phenylic carboxyl functionalized poly(pyrrole methane)s chelating polymers

Mercury decontamination from water requires highly effective and efficient methods for maintaining public health and environmental protection. Herein, based on the coordination theory between functional groups and metal ions, we proposed phenylic carboxyl group-based poly(pyrrole methane)s (PPDCBAs) as highly efficient mercury removal materials for environmental remediation applications. It was found that PPDCBAs can efficiently adsorb and remove mercury(II) from aqueous solutions by functionalizing the molecular structure with phenylic carboxyl groups. Among the as-prepared PPDCBAs, poly[pyrrole-2, 5-diyl (4-carboxybenzylidane)] (PPD4CBA) with the carboxyl group at the para position can not only adsorb mercury over 1400 mg⋅g^-1 but also achieve a 92.5 % mercury(II) uptake within 100 min by a very low dosage of 0.1 g⋅L^-1. In addition, PPDCBAs exhibited excellent adsorption selectivity for mercury(II) compared with copper(II), cadmium(II), zinc(II) and lead(II). Furthermore, as determined by Fourier transform infrared (FT-IR) spectra, X-ray photoelectron spectroscopy (XPS) and the density functional theory (DFT) calculation, the mercury removal was found to be mainly dependent on the high density of chelating sites, the phenylic carboxyl moieties, which helped us to realize an ultra-trace amount mercury removal (from 10.8 μg⋅L^-1 to 0.6-0.8 μg⋅L^-1) for meeting drinking water standard requirements (1.0 μg⋅L^-1).

Insights into the correlation between different adsorption/oxidation/catalytic performance and physiochemical characteristics of Fe-Mn oxide-based composites

Fe-Mn oxide-based composites have been widely used in the solidification of heavy metals or the removal of organic pollutants, which can not only show excellent adsorption/oxidation performance, but also show catalytic activity for common oxidants. At present, the correlation between adsorption/oxidation/catalytic performance and physicochemical characteristics of these composites, and the underlying mechanisms are still unclear. Therefore, the main purpose of this review is to disclose the internal relationship between the physicochemical properties of Fe-Mn oxide-based composites and the pollutant removal performance. From the perspective of crystal phase, the basic units of Fe-Mn oxide composites are divided into Fe-Mn binary oxide (FMBO) and spinel MnFe₂O₄, and the two species were discussed separately in most chapters. The selected physicochemical properties mainly include the type of Fe-Mn oxide composites, surface-to-volume ratio, pore volume, pH_pzc, crystal type, surface functional groups. Because the physicochemical properties that determine how effective Fe-Mn oxide material is at removing contaminants may differ as it performs different functions, we discussed the above problems under different application scenarios (adsorption, oxidation, and advanced oxidation process). Additionally, internal factor (Fe/Mn mole ratio) and external factors (pH_ini, co-ions and ionic strength) were analyzed, and several common synthetic strategies of these composites were presented.

Synthesis and characterization of poly (styrene-co-acrylamide)-graft-polyanilines as new sorbents for mercuric present in aqueous hydrocarbon liquids

Background

The unprocessing hydrocarbon oil often contains high concentrations of mercury, which damages the metallic processing components and have health risk on workers and environment. Mercuric removal unit associated with natural gas processing plant is failed to complete mercury removal and then mercury distributed in most places of removal unit. Most of unremoved mercury are found in polar solutions.

Results

Styrene-co-acrylamide-graft-polyanilines were synthesized and characterized. The copolymer formed by free radical emulsion copolymerization of styrene-acrylamide (14:1) using ammonium persulphate (APS) at 60 °C. In addition, the grafting process was also achieved by oxidation chemical polymerization of the above copolymer with both aniline and 2-chloroaniline using APS. The synthetic polymeric samples were characterized using infrared (IR), x-ray diffraction (XRD), scan electron microscope (SEM), transition electron microscope (TEM), thermogravimetric analysis (TGA) and Brunauer–Emmett–Teller (BET) to confirm the polymerization process and investigate the polymeric samples as new sorbents for Hg (II). Both adsorption kinetics and isotherm models were checked.

Conclusions

In most cases Hg (II) was adsorbed as multi-layer on the obtained mesopores materials. The grafting process enhances the copolymer activity towards Hg (II) removal. The complete removal of mercury from water solution portion of mercuric removal unit was achieved by introduction of synthetic polymeric mesopores material based on styrene-co-acrylamide-graft-polyanilines. The removal efficiency closed to 100% in case of grafting with poly (2-chloroaniline).

MXsorption of mercury: Exceptional reductive behavior of titanium carbide/carbonitride MXenes

Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have drawn considerable attention for application in the field of environmental remediation. In this study, we report the simultaneous reductive-adsorption behavior of Ti₃CNT_x for toxic metal ion Hg²⁺ ion in the aqueous phase. 2D Ti₃CNT_x and Ti₃C₂T_x MXene nanosheets were synthesized by exfoliation of Ti₃AlCN and Ti₃AlC₂ MAX phases, respectively. Various characteristics analysis confirmed the successful fabrication of MAX phases and their exfoliation into MXenes. The fabricated MXene nanosheets were used to investigate their Hg²⁺ removal, Hg²⁺ intercalation, and surface interaction mechanism efficiencies. Both MXenes were found to adsorb and reduce a large amount of Hg²⁺. Analytical techniques such as X-ray powder diffraction, field emission transmission electron microscopy, zeta-potential analyses, and X-ray photoelectron spectroscopy were used to investigate the material characteristics and structural changes after uptake of Hg²⁺. The quantitative investigation confirmed the interaction of bimetal and hydroxyl groups with Hg²⁺ using electrostatic interactions and adsorption-coupled reduction. In addition, both MXenes exhibited extraordinary Hg ion removal capabilities in terms of fast kinetics with an excellent distribution coefficient (K_d^Hg) up to 1.36 × 10⁺⁹. Based on batch adsorption results, Ti₃C₂T_x and Ti₃CNT_x exhibited removal capacities of 5473.13 and 4606.04 mg/g, respectively, for Hg²⁺, which are higher than those of previous Hg adsorbents.

Advances in the Synthesis and Application of Anti-Fouling Membranes Using Two-Dimensional Nanomaterials

This article provides a comprehensive review of the recent progress in the application of advanced two-dimensional nanomaterials (2DNMs) in membranes fabrication and application for water purification. The membranes fouling, its types, and anti-fouling mechanisms of different 2DNMs containing membrane systems are also discussed. The developments in membrane synthesis and modification using 2DNMs, especially graphene and graphene family materials, carbon nanotubes (CNTs), MXenes, and others are critically reviewed. Further, the application potential of next-generation 2DNMs-based membranes in water/wastewater treatment systems is surveyed. Finally, the current problems and future opportunities of applying 2DNMs for anti-fouling membranes are also debated.

Composite Zn(II) Ferrocyanide/Polyethylenimine Cryogels for Point-of-Use Selective Removal of Cs-137 Radionuclides

The feasibility of several approaches to the fabrication of monolith composite cryogels containing transition-metal ferrocyanides for Cs⁺ ion uptake has been evaluated. Although in the series of investigated metal ion precursors (Cu(II), Zn(II), Ni(II), and Co(II)), in situ formation of the sorption active phase in polyethyleneimine (PEI) cryogel was feasible only in the case of Zn(II) ferrocyanide, this approach has shown significant advantages over the immobilization of ex situ synthesized ferrocyanide nanoparticles. Nanoparticles of the mixed ferrocyanide Zn_1.85K_0.33[Fe(CN)₆] formed in situ had an average size of 516 ± 146 nm and were homogeneously distributed in the monolith located at the polymer surface rather than embedded in the matrix. The Young modulus of the PEI cryogel increased after modification from 25 to 57 kPa, but composites maintained high permeability to the flow. Sorption of Cs⁺ ions has been investigated at superficial velocity up to 8 m/h. Steep breakthrough profiles and uptake efficiency of >99.5% until breakthrough point confirmed that a supermacroporous structure of the monolith composite assured good mass transfer, so that intraparticle diffusion was not the limiting stage of sorption kinetics. Application of the rate-constant distribution model (RCD model) to analyze the breakthrough curves of Cs⁺ sorption allowed the identification of two types of sorption sites with a difference in sorption rate constants of ~1 log unit. Most likely, sorption on “fast” sorption sites was governed by ion exchange between Cs⁺ ions in solution and K⁺ ions in the ferrocyanide lattice. Cs-137 radionuclide removal was investigated using the monolith composite columns of various geometries at superficial velocity up to the 6.6 m/h; specific gamma activity was reduced from 265 kBq/L to the background level, showing high potential of these materials for POU application.

Migration characteristics of heavy metals in the weathering process of exposed argillaceous sandstone in a mercury-thallium mining area

Lanmuchang mercury-thallium mine, a typical polymetallic mine is located in southwestern Guizhou, China, is the most serious and typical area resulted from multi-metal contamination (Tl, Hg, As, and Sb). After the mercury-thallium mining, a large area of surrounding rocks such as argillaceous sandstone with high contents of Tl, Hg, As, and Sb is exposed to air. Weathering caused the argillaceous sandstone to form different weathering layers, including the grey-black external layer, the brown-yellow middle layer and the gray-white inner layer, and the external layer was enriched with higher heavy metals. However, the reason of heavy metal migration and transformation in argillaceous sandstone caused by weathering is unclear. The objective of this paper was to investigate the migration, transformation and release characteristics of Tl, Hg, As, and Sb in argillaceous sandstone during the weathering. The results indicated that weathering not only promoted an acidic oxidation environment in argillaceous sandstone, but also increased its specific surface area, pore volume and hydrophilicity, which are beneficial to the permeability of oxygen and etching liquids during the process of weathering and leaching. Meanwhile, weathering led to the transformation or decomposition of hydrophilic groups, such as -OH and -C˭O in the grey-black external layer of argillaceous sandstone, resulting in the further release of heavy metals bound to these groups. The concentration of Tl, Hg, As, and Sb in the leaching solution of argillaceous sandstone represented a positive correlation with that of Fe, Ca, Mg at different levels, indicating that Tl, Hg, As, and Sb were released with the dissolution of Fe, Ca and Mg during weathering and leaching. In summary, these results indicated that weathering caused the dissolution and migration of heavy metals in the argillaceous sandstone. Tl, Hg, As, and Sb migrated from the grey-white inner layer to the grey-black external layer and partially adsorbed by free alumina (Al_d), jarosite and Ca-bearing minerals, showing enrichment phenomena, partially released into the environment, causing environmental pollution.

Comparative Study of Mercury(II) Removal from Aqueous Solutions onto Natural and Iron-Modified Clinoptilolite Rich Zeolite

The contamination of soil and water bodies with mercury from anthropogenic sources such as mining and industry activities causes negative effect for living organisms due to the process of bioaccumulation and biomagnification through the food chain. Therefore, the need for remediation of contaminated areas is extremely necessary and very desirable when it is cost-effective by using low-cost sorbents. This paper compares the sorption abilities of natural and iron-modified zeolite towards Hg(II) ions from aqueous solutions. The influence of pH, solid/liquid ratio (S/L), contact time, and initial concentration on the sorption efficiency onto both zeolites was investigated. At the optimal pH = 2 and S/L = 10, the maximum amount of sorbed Hg(II) is 0.28 mmol/g on the natural zeolite and 0.54 mmol/g on the iron-modified zeolite. It was found that rate-controlling step in mass transfer is intraparticle diffusion accompanied by film diffusion. Ion exchange as a main mechanism, accompanied with surface complexation and co-precipitation were included in the Hg(II) sorption onto both zeolite samples. This is confirmed by the determination of the amount of sorbed Hg(II) and the amount of released exchangeable cations from the zeolite structure as well as by the scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS) of saturated zeolite samples. In a wide pH range, 4.01 ≤ pH ≤ 11.08, the leaching of Hg(II) was observed in the amount of only 0.28–0.78% from natural zeolite and 0.07–0.51% from iron-modified zeolite indicating that both zeolites could be used for remediation purposes while the results suggest that modification significantly improves the sorption properties of zeolite.

Adsorption of Hg(II) in an Aqueous Solution by Activated Carbon Prepared from Rice Husk Using KOH Activation

With the development of industry, the discharge of wastewater containing mercury ions posed a serious threat to human health. Using biomass waste as an adsorbent to treat wastewater containing mercury ions was a better way due to its positive impacts on the environment and resource saving. In this research, activated carbon (AC) was prepared from rice husk (RH) by the KOH chemical activation method. The characterization results of scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), Fourier transform infrared (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) showed that rice husk-activated carbon (RHAC) had good pore structure and oxygen-containing functional groups. The influences of contact time, initial concentration of Hg(II), adsorbent dosage, pH, and ionic strength on mercury ion removal were investigated. The Langmuir model was most suitable for the adsorption isotherm of RHAC, and its maximum adsorption capacity for Hg(II) was 55.87 mg/g. RHAC still had a high removal capacity for Hg(II) after five regeneration cycles. RHAC had excellent removal efficiency for mercury ion wastewater. At the same time, RH could be used as a nonpolluting and outstanding characteristic adsorbent material.

Binding of Hg to preformed ferrihydrite-humic acid composites synthesized via co-precipitation and adsorption with different morphologies

Iron (hydr)oxide-natural organic matter (NOM) colloids, the dominant components of soil, usually occur in varied circumstances and may affect Hg transport and fate in soil. This study aims to reveal the Hg binding to preformed composites rather than only focusing on Hg retention by iron (hydr)oxides in the presence of NOM. Ferrihydrite-humic acid (FH-HA) is chosen as a representative composite, and the effect of the complexation method and FH morphology on Hg binding to various composites is evaluated. Three types of composites are developed: a dense coprecipitated composite (p-d-f), a gel-like adsorbed composite (a-g-f) and a dense adsorbed (a-d-f) composite. Batch sorption and stirred-flow kinetic tests together with surface property analysis and modern spectral analyses are carried out to explore the binding behavior of Hg to the three composites and clarify the interactions in the ternary systems of FH-HA-Hg. The results show that the Hg sorption isotherms all fit well with the Langmuir model, and the maximum sorption capacities follow the order a-g-f> a-d-f > p-d-f, implying that the adsorbed composite is more favorable than the coprecipitated composite for Hg binding and a gel morphology is more beneficial than a dense morphology. The stirred-flow experiments show that the adsorbed composite has a small advantage in Hg sorption compared to the coprecipitated composite and that the gel-like composite can adsorb more Hg at a faster rate than the dense composite. Both FH and HA participate in Hg sorption, and FH-HA-Hg complexes are speculated to form. These findings are helpful to better understand the mobility and fate of Hg in soils, as well as the associated dynamic model for predicting Hg behavior in the environment where the iron (hydr) oxide-NOM composites are pre-existed.

Rational Design of Polyamine-Based Cryogels for Metal Ion Sorption

Here we report the method of fabrication of supermacroporous monolith sorbents (cryogels) via covalent cross-linking of polyallylamine (PAA) with diglycidyl ether of 1,4-butandiol. Using comparative analysis of the permeability and sorption performance of the obtained PAA cryogels and earlier developed polyethyleneimine (PEI) cryogels, we have demonstrated the advantages and disadvantages of these polymers as sorbents of heavy metal ions (Cu(II), Zn(II), Cd(II), and Ni(II)) in fixed-bed applications and as supermacroporous matrices for the fabrication of composite cryogels containing copper ferrocyanide (CuFCN) for cesium ion sorption. Applying the rate constant distribution (RCD) model to the kinetic curves of Cu(II) ion sorption on PAA and PEI cryogels, we have elucidated the difference in sorption/desorption rates and affinity constants of these materials and showed that physical sorption contributed to the Cu(II) uptake by PAA, but not to that by PEI cryogels. It was shown that PAA cryogels had significantly higher selectivity for Cu(II) sorption in the presence of Zn(II) and Cd(II) ions in comparison with that of PEI cryogels, while irreversible sorption of Co(II) ions by PEI can be used for the separation of Ni(II) and Co(II) ions. Using IR and Mössbauer spectroscopy, we have demonstrated that strong complexation of Cu(II) ions with PEI significantly affects the in situ formation of Cu(II) ferrocyanide nanosorbents leading to their inefficiency for Cs⁺ ions selective uptake, whereas PAA cryogel was applicable for the fabrication of efficient monolith composites via the in situ formation of CuFCN or loading of ex situ formed CuFCN colloids.

Analog synthesis of artificial humic substances for efficient removal of mercury

A cost-effective artificial humic substances (humic acid-modified biochar, HA-BCs) is fabricated by using conventional hydrothermal-assisted pyrolysis technique, and then is considered as a promising adsorbent material for removing mercury ions from aqueous solution. Artificial humic acid (A-HA), humic acid-modified biochar (HA-BCs) are analyzed by using SEM, EA, XRD, FTIR, XPS, and BET techniques. The removal efficiency of mercury ions was greater than 95% after reaching the adsorption equilibrium. Meanwhile, the adsorption kinetics coincided with the pseudo-second-order model and the isotherms for mercury ion sorption can be best interpreted using Freundlich isotherm model, with high regression coefficients (R² = 0.967-0.990). Furthermore, the surface properties of HA-BCs before and after mercury adsorption are compared and evaluated, realizing that the mechanisms of removal of mercury ions on HA-BCs mainly include surface complexation with oxygen/nitrogen functional groups (-OH, -COOH and -NH₂) and formation of precipitation with CO₃^2- and OH^-. Furthermore, the used HA-BCs can be regenerated via 0.05 mol/L KI solution and the adsorption capacity of mercury still reaches at 32.57 mg/g after four cyclic utilization.

Synthesis of a novel Fe-Mn binary oxide-modified lava adsorbent and its effect on ammonium removal from aqueous solutions

Ammonium is strongly related to eutrophication and a key control of eutrophication in aquatic systems, especially in agricultural runoff. In this study, a novel Fe-Mn binary oxide-modified lava (FMML) granular adsorbent was synthesized for ammonium removal from aqueous solutions by co-precipitation method. The kinetic data were described by pseudo-second-order kinetic model well and intraparticle diffusion had effects on ammonium adsorption. For pH between 4.0 and 10.0, the adsorption efficiency was >80%, and its optimum was recorded at pH 7.0. FMML exhibited strong ammonium adsorption selectivity under the single presence of cations like Na⁺ , K⁺ , Ca²⁺ , and Mg²⁺ . The optimum adsorbent dose and particle size were 4 g/L and 3-5 mm, respectively, for an aqueous solution containing 10 mg/L of ammonium under normal conditions (298 K and pH 7.0). Furthermore, the adsorption process was endothermic, following both the Langmuir (R²  > 0.98) and Freundlich (R²  > 0.96) models. Compared with other adsorbents, the FMML can be prepared following a simpler protocol. After 30 times of adsorption-regeneration cycle, the FMML also had a relatively high ammonium adsorption capacity; hence, we see it as a prospective adsorbent for ammonium adsorption from aqueous solutions. PRACTITIONER POINTS: Fe-Mn binary oxide-modified lava with Fe/Mn ratio 3:1 was prepared using co-precipitation method. Adsorption maximum of modified lava was 20.8 mg/g (298 K and pH 7.0). Adsorption was sensitive to changes in adsorbent dose, particle size, and pH. Inorganic cations decreased ammonium adsorption in order of Na⁺  > K⁺  > Ca²⁺  > Mg²⁺ . Mechanisms for ammonium removal by FMML include diffusion, electrostatic attraction, oxidation, and complexation reaction.

Application progress of enhanced coagulation in water treatment

Water industries worldwide consider coagulation/flocculation to be one of the major treatment methods for improving the overall efficiency and cost effectiveness of water and wastewater treatment. Enhancing the coagulation process is currently a popular research topic. In this review article, the latest developments in enhanced coagulation are summarized. In addition, the mechanisms of enhanced coagulation and the effect of process parameters on processing efficiency are discussed from the perspective of ballast-enhanced coagulation, preoxidation, ultrasound, and composite coagulants. Finally, improvements and new directions for enhanced coagulation are proposed.

Evaluating the adsorptivity of organo-functionalized silica nanoparticles towards heavy metals: Quantitative comparison and mechanistic insight

Organo-functionalized SiO₂ nanoparticles are regarded as promising adsorbents for capture of heavy metals. However, actual adsorptivity of a specific functional group onto SiO₂ surface is unclear, thus extending a debate on which type of organic group possesses a better affinity toward heavy metals. Herein, surface functionalization of SiO₂ with different groups (i.e., -EDTA (ethylenediamine triacetic acid), -COOH, -SO₃H, -SH and -NH₂) were achieved by a facile silylating reaction. Batch experiments indicated that adsorption capacity of SiO₂ was remarkably improved by surface functionalization. Quantitative analysis manifested that one mole of EDTA grafted onto SiO₂ surface can adsorb 1.51 mol of Pb(II) ions, which was 7.7, 17.1, 28.4 and 50.2-fold larger than those of COOH-, SO₃H-, SH- and NH₂-functionalized SiO₂, respectively. This is first time to evaluate adsorptivity of functionalized SiO₂ on the basis of per effective functional group, which may repair deficiency of conventional assessment method that calculated on the basis of per unit mass. Further, adsorption mechanism of these functionalized SiO₂ were identified and uncovered by experimental and theoretical studies. This work not only develops an efficient adsorbent for heavy metal remediation but also provides a valuable insight for evaluation and design of novel SiO₂-based materials.

Removal of mercury(II) from contaminated water by gold-functionalised Fe3O4 magnetic nanoparticles

Fe₃O₄ nanoparticles were prepared by co-precipitation of Fe²⁺ and Fe³⁺ and then modified with Au to produce an effective adsorbent (Fe₃O₄/Au) for aqueous Hg(II) in contaminated water. Rietveld refinement on the XRD pattern confirmed that the Fe₃O₄/Au was synthesised. Mössbauer spectra exhibited broad and asymmetric resonance lines with two sextets which can be assigned to tetrahedral Fe³⁺; and octahedral Fe³⁺/Fe²⁺. The quantitative analysis of magnetite confirms that the sample shows around 3 wt.% Au and 97 wt.% partially oxidised Fe₃O₄. High surface area: 121 m² g^-1, average pore sizes: 6.3 nm and pore volume: 1.64 cm³ g^-1. The kinetics data were better fitted with a pseudo-second-order and Dubinin-Radushkevich isotherm suggests the Hg(II) adsorption onto Fe₃O₄/Au nanoparticles was mainly by chemical adsorption forming complex with the Au metal immobilised on Fe₃O₄ surfaces. Adsorption capacity of 79.59 mg g^-1. Ionic strength and co-existing ions had a slight influence on the adsorption capacity.

Puffed Rice Carbon with Coupled Sulfur and Metal Iron for High-Efficiency Mercury Removal in Aqueous Solution

Development of low-cost, high-efficiency, and environmentally benign adsorbents for mercury removal is of significant importance for environmental remediation. Herein, we report a novel porous puffed rice carbon (PRC) with co-implanted metal iron and sulfur, forming a high-quality PRC/Fe@S composite as a high-efficiency adsorbent for mercury removal from aqueous solution. The in situ-formed Fe nanoparticles in PRC are strongly coupled with sulfur via a supercritical CO₂ fluid approach and dispersed homogeneously in the cross-linked hierarchical porous architecture. The pore formation mechanism of Fe on PRC is also proposed. The optimized PRC/Fe@S composite offers superior selective affinity, high removal efficiency, and ultrahigh adsorption capacity of up to 738.0 mg g^-1. It is demonstrated that the hierarchical porous carbon in the PRC/Fe@S composite not only acts as a framework to stabilize and disperse Fe nanoparticles but also provides abundant pores and voids for absorbing Hg(II) from aqueous solution. More importantly, the absorbed Hg(II) can be reduced to Hg(0) by Fe and further chemically immobilized by sulfur. The enhanced coupled effect is discussed and proposed. Therefore, an innovative adsorption mechanism of adsorption-reduction-immobilization is proposed, which offers a new prospect in developing high-efficiency carbon-based adsorbents in environmental remediation.

Effect of different rotation systems on mercury methylation in paddy fields

Rice (Oryza sativa) paddy is the hotspot of mercury (Hg) methylation. Given distinct influences of rotation systems on the physicochemical properties of paddy soils, we hypothesized different rotation systems in rice paddies inducing a large difference in Hg methylation. Here, we investigated Hg species distribution, dissolved organic matter (DOM) features, and Hg methylation in five rotation systems (Other farmland newly reclaim into paddy field, i.e., NR-R; Drain the water in winter, i.e., DW-R; Flooding in winter, i.e., FW-R; Rape-Rice rotation, i.e., Ra-R; Wheat-Rice rotation, i.e., Wh-R) of paddy fields to identify such hypothesis. Results shown that FW-R had the strongest Hg methylation, followed by Ra-R and Wh-R, then DW-R, and finally NR-R. We further found that much higher soil organic matter (SOM) and organo-chelated Hg (Hg-o) from straw residues and root exudates were the main cause for the greater Hg methylation in FW-R, Ra-R and Wh-R. This was because the protein-like fraction of SOM facilitated the net production of methyl Hg (MeHg), meanwhile the humin-like fraction had a strong affinity to MeHg in paddy soils. Therefore, it can be concluded that paddy soil under DW-R was the optimum pattern in order to reduce the occurrence of Hg methylation. However, paddy soils under Ra-R and Wh-R were the recommendable patterns if the productivity of paddy fields was considered.

Adsorption of low-concentration mercury in water by 3D cyclodextrin/graphene composites: Synergistic effect and enhancement mechanism

The efficient removal of mercury from aqueous media remains a severe challenge in ensuring environmental safety, especially for low-concentration mercury, which requires adsorbents with high mercury affinity. In this work, we reported a nanocomposite of β-cyclodextrin and three-dimensional graphene (3D CD@RGO) to enhance the adsorption affinity and capacity for mercury with low concentrations. Characterization of the nanocomposite revealed that cyclodextrin was well dispersed on the 3D graphene support structure to provide highly exposed hydroxyl groups. Adsorption experiments showed that CD@RGO exhibited different adsorption behaviors for mercury within different concentration ranges of 0.2-4.0 mg/L and 4.0-10.0 mg/L, and the adsorption affinity for the former range (K_L = 10.05 L/mg) was 1.5 times higher than that for the latter range (K_L = 6.69 L/mg). Moreover, CD@RGO had a high adsorption efficiency of 96.6% with a superb adsorption affinity (172.09 L/g) at C_e = 0.01 mg/L, which is 6.70 and 41.25 times higher than that of RGO and RCD (physical mixture of RGO and cyclodextrin), respectively, indicating a synergistic effect of CD@RGO for mercury adsorption. This enhancement can be attributed to the transformation of the adsorption mechanism from the outer-sphere force of electrostatic interaction in RGO to the inner-sphere surface complexation in CD@RGO.

Superior removal of Hg (II) ions from wastewater using hierarchically porous, functionalized carbon

The removal of heavy metal ions from industrial wastewater by adsorption has been central to the environment for decades, where common adsorbent materials are often limited by poor efficiency, complex fabrication and long processing time. Porous carbon derived from biospecies holds promise to address the limitations. In this study we converted bagasse into a carbon composite having hierarchically porous structure; the composite's dispersion phases - iron oxide and manganese oxide - were synthesized by a simple one-step liquid-phase reaction method. Featuring large specific surface area of 350.8 m² g^-1, the composite demonstrated exceptional Hg (II) removal efficiency of 96.8%, adsorption rate of up to 96.8% within 150 min and adsorption capacity of 9.8 mg g^-1. In comparison with other removal materials, our work is outstanding in terms of both removal efficiency and synthesis simplicity. The high efficiency is attributed to the synergy between physical adsorption referring to hierarchically porous structure and chemical adsorption relating to functional complexation processes. It provides a new avenue for the development of high-performance adsorbent materials for heavy metal removal from aqueous media.

Application of Iron-Based Materials for Remediation of Mercury in Water and Soil

Mercury contamination in soil and water has become a major concern to environmental quality and human health. Among the existing remediation technologies for mercury pollution control, sorption via iron-based materials has received wide attention as they are environmental friendly and economic, and their reactivity is high and controllable through modulating the morphology and surface properties of particulate materials. This paper aimed to provide a comprehensive overview on environmental application of a variety of iron-based sorbents, namely, zero valent iron, iron oxides, and iron sulfides, for mercury remediation. Techniques to improve the stability of these materials while enhancing mercury sequestration, such as nano-scale size control, surface functionalization, and mechanical support, were summarized. Mechanisms and factors affecting the interaction between mercury and iron-based materials were also discussed. Current knowledge gaps and future research needs are identified to facilitate a better understanding of molecular-level reaction mechanisms between iron-based materials and mercury and the long-term stability of the immobilized mercury.

Effective removal of inorganic mercury and methylmercury from aqueous solution using novel thiol-functionalized graphene oxide/Fe-Mn composite

A novel thiol-functionalized graphene oxide/Fe-Mn (SGO/Fe-Mn) was investigated for aqueous Hg²⁺ and CH₃Hg⁺ removal. Mercury were removed mainly through ligand exchange and surface complexation with surface active sites (i.e., -SH, OH, OCO, CC, SiO, and ππ bond). SH had the strongest binding ability with mercury, forming sulfur-containing organic matter or polymers with Hg²⁺, and sulfur-containing organometallic compounds or thiolate-like species with CH₃Hg⁺. The BET sorption isotherm model well simulated the sorption isotherm data of Hg²⁺ (R²=0.995, q_m=233.17 mg/g) and CH₃Hg⁺ (R²=0.997, q_m=36.69 mg/g), indicating a multilayer adsorption process. The mercury uptake was promoted with the increase of 3-MPTS content, adsorbent dosage, and pH (<5.5), whereas the uptake was inhibited by high pH (>5.5) and high concentrations of humic acid and electrolytes. SGO/Fe-Mn demonstrated high mercury uptake in simulated surface water/groundwater and in the presence of Pb, Cu, Ni, Sb, Cd and Zn. The mercury-laden SGO/Fe-Mn can be successfully regenerated and reused for three times with 98.1% and 67.0% of original Hg²⁺ and CH₃Hg⁺ sorption capacity when 5% thiourea + 2 M KI was used as the desorbing agent. This study demonstrates potential and viability of SGO/Fe-Mn for mercury remediation.

Sustainable treatment of municipal landfill leachate by combined association of air stripping, Fenton oxidation, and enhanced coagulation

The present world has been facing the problem of municipal solid waste disposal with the generation of highly complex and toxic landfill leachate. Thus, in this research work, treatability of landfill leachate had been investigated by the combined approach of air stripping, Fenton oxidation, and enhanced coagulation to comply with discharge standard. At the initial stage of treatment, air stripping of raw leachate was implemented which removes around 51.50% of COD, 74.60% of BOD₅, and 97.60% of NH₃-N within 36 h of optimum retention time. Following air stripping, Fenton oxidation was applied with an optimum molar ratio of 1.9 of H₂O₂/Fe⁺² which register a maximal removal of 67.70% of COD, 92.30% of BOD₅, and 14.90% of Hg. Finally, enhanced coagulation (EC) with in situ formed Mn-Fe hydr(oxides) was employed and optimized by central composite design (CCD) of response surface methodology (RSM). Response surface plots denote an optimum condition of 0.13 M ratio of Mn/Fe, 22.67 mM of coagulant dose, and 7.78 of pH which corresponds to a maximum removal of 55.98% of COD and 77.68% of Hg. FTIR analysis of the precipitates of EC explained that the hydroxyl groups are primarily involved in the process of Hg removal. Moreover, EDAX spectrum also assured the removal of Hg by its existence with Mn-Fe complexes. Thus, the present line of treatment record an overall removal of 90.80% of COD, 98.0% of BOD₅, 97.60% of NH₃-N, and 82.68% of Hg which proves to be effective for the removal of leachate pollutants.

Deposition Kinetics of Colloidal Manganese Dioxide onto Representative Surfaces in Aquatic Environments: The Role of Humic Acid and Biomacromolecules

The initial deposition kinetics of colloidal MnO₂ on three representative surfaces in aquatic systems (i.e., silica, magnetite, and alumina) in NaNO₃ solution were investigated in the presence of model constituents, including humic acid (HA), a polysaccharide (alginate), and a protein (bovine serum albumin (BSA), using laboratory quartz crystal microbalance with dissipation monitoring equipment (QCM-D). The results indicated that the deposition behaviors of MnO₂ colloids on three surfaces were in good agreement with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Critical deposition concentrations (CDC) were determined to be 15.5 mM NaNO₃ and 9.0 mM NaNO₃ when colloidal MnO₂ was deposited onto silica and magnetite, respectively. Both HA and alginate could largely retard the deposition of MnO₂ colloids onto three selected surfaces due to steric repulsion, and HA was more effective in decreasing the deposition rate relative to alginate. However, the presence of BSA can provide more attractive deposition site and thus lead to greater deposition behavior of MnO₂ colloids onto surfaces. The dissipative properties of the deposited layer were also influenced by surface type, electrolyte concentration, and organic matter characteristics. Overall, these results provide insights into the deposition behavior of MnO₂ colloids on environmental surfaces and have significant implications for predicting the transport potential of common MnO₂ colloids in natural environments and engineered systems.

Removal of Hg(ii) in aqueous solutions through physical and chemical adsorption principles

Adsorption has been the focus of research on the treatment of heavy metal mercury pollution since it is among the most toxic heavy metals in existence.

Simple synthesis and characterization of l-Cystine functionalized δ-FeOOH for highly efficient Hg(II) removal from contamined water and mining waste

l-Cystine functionalized δ-FeOOH nanoparticles (Cys-δ-FeOOH) were prepared by a cheap and straightforward method for using as an adsorbent of Hg(II) in aqueous solution. X-ray diffraction (XRD), attenuated total reflectance infrared spectroscopy (ATR-IR), and Raman spectroscopy confirmed that Cys-δ-FeOOH was successfully synthesized. Cys-δ-FeOOH with 14 nm crystal size, 34 m² g^-1 surface area, and 9 nm pore size were produced. The functionalization of the δ-FeOOH surface with cysteine decreases the point of zero charge of the iron oxyhydroxide from 8.4 in δ-FeOOH to 5.7 in Cys-δ-FeOOH, which is beneficial for the adsorption of Hg(II) near neutral pH. The maximum Hg(II) adsorption capacity of the δ-FeOOH and Cys-δ-FeOOH at pH 7 were found to be 35 mg g^-1 and 217 mg g^-1, respectively. The kinetics data were best fitted by a pseudo-second-order model, suggesting chemical adsorption on the surface and pores of Cys-δ-FeOOH nanoparticles. Finally, δ-FeOOH and Cys-δ-FeOOH filters were constructed for purifying mercury-contaminated water. The filters were highly efficient to treat mercury-contaminated water from a Brazilian river, reducing the concentration of mercury in water to values below the allowed limits by the current legislation.

Cd(ii) removal by Fe(ii) surface chemically modified layered double hydroxide–graphene oxide: performance and mechanism

Fe(ii)-decorated LDH–GO composites had a high capacity for Cd(ii) removal. The mechanisms were controlled by surface-induced precipitation and complex formation.

Removing mercury from aqueous solution using sulfurized biochar and associated mechanisms

Biochar has been used to remove heavy metals from aqueous solutions. In this study, a sulfurized wood biochar (SWB) by direct impregnation with elemental sulfur was produced and evaluated along with pristine wood biochar (WB) for adsorption characteristics and mechanism of mercury. Mercury adsorption by WB and SWB was well described by Langmuir model and pseudo second order model and the maximum adsorption capacities of WB and SWB were 57.8 and 107.5 mg g^-1, respectively. Intraparticle diffusion model showed that mercury adsorption was fast due to boundary layer and slow adsorption due to diffusion into biochar pores. Although, mercury adsorption by both WB and SWB was predominantly influenced by the pH, temperature, salt concentration, and biochar dosage, the SWB showed a relatively stable mercury adsorption compared to WB under different conditions, suggesting the strong affinity of SWB for mercury. The XPS analysis showed different adsorption mechanisms of mercury between WB and SWB. In particular, mercury adsorption in WB was due to Hg-Cπ bond formation and interaction with carboxyl and hydroxyl groups, whereas in SWB it is primarily due to mercury interaction with C-SO_x-C and thiophenic groups in addition to Hg-Cπ bond formation and interaction with carboxyl groups. The SEM-EDS mapping also demonstrated that mercury in SWB was related to carbon, oxygen and sulfur. Overall, the sulfurized biochar was effective for removing mercury from aqueous solution, and its direct production through pyrolysis with elemental sulfur impregnation of wood chips could make it an economic option as absorbent for treating mercury-rich wastewater.

Analysis of mercury adsorption at the gibbsite-water interface using the CD-MUSIC model

Mercury (Hg), one of the most toxic substances in nature, has long been released during the anthropogenic activity. A correct description of the adsorptive behavior of mercury is important to gain a better insight into its fate and transport in natural mineral surfaces, which will be a prerequisite for the development of surface complexation model for the adsorption processes. In the present study, simulation experiments on macroscopic Hg(II) sorption by gibbsite (α-Al(OH)₃), a representative aluminum (hydr)oxide mineral, were performed using the charge distribution and multi-site complexation (CD-MUSIC) approach with 1-pK triple plane model (TPM). For this purpose, several data sets which had already been reported in the literature were employed to analyze the effect of pH, ionic strength, and co-exisiting ions (NO₃^- and Cl^-) on the Hg(II) adsorption onto gibbsite. Sequential optimization approach was used to determine the acidity and asymmetric binding constants for electrolyte ions and the affinity constants of the surface species through the model simulation using FITEQLC (a modified code of FITEQL 4.0). The model successfully incorporated the presence of inorganic ligands at the dominant edge (100) face of gibbsite with consistent surface species, which was evidenced by molecular scale analysis. The model was verified with an independent set of Hg(II) adsorption data incorporating carbonate binding species in an open gibbsite-water system.

Mercuric ion capturing by recoverable titanium carbide magnetic nanocomposite

Two-dimensional metal carbides and nitrides (MXenes) have attracted increasing attention for application in water/wastewater treatment. The functionalization of MXenes to increase their stability while demonstrating high pollutant removal can facilitate sustainable water/wastewater treatment processes. In this study, the highly stable magnetic titanium carbide (Ti₃C₂T_x) MXene nanocomposite (MGMX nanocomposite) was successfully synthesized through a facile hydrothermal approach and was tested for aqueous-phase adsorptive removal of mercuric ions. The synthesized MGMX nanocomposite was studied using characteristic analyses, showing high stability as revealed by zeta-potential analysis and dynamic light-scattering technique. The MGMX nanocomposite presented excellent Hg(II) removal in a wide range of pH conditions, and an exceptional maximum experimental Hg(II) uptake capacity of 1128.41mgg^-1 was observed. The adsorption behavior was investigated using the Redlich-Peterson adsorption isotherm, pseudo second-order kinetics, and thermodynamics models. In the adsorption/desorption investigation, the MGMX nanocomposite was reusable for up to five cycles of adsorption/desorption. The stability, hydrophilic nature, available adsorptive surfaces, and easy separation after reaction make the MGMX nanocomposite an efficient sorbent for the removal of toxic Hg(II) for water purification.

Supercritical CO2 assisted synthesis of sulfur-modified zeolites as high-efficiency adsorbents for Hg2+ removal from water

A facile supercritical CO₂ (SC-CO₂) synthetic strategy has been successfully developed for fabricating a new kind of highly efficient sulfur-modified zeolite sorbent for the removal of Hg²⁺ from water.

Efficient removal of Cr(III)-organic complexes from water using UV/Fe(III) system: Negligible Cr(VI) accumulation and mechanism

Most available processes are incapable of removing Cr(III)-organic complexes from water due to their high solubility, extremely slow decomplexation rate, and possible formation of more toxic Cr(VI) during oxidation. Herein, we proposed a new combined process, i.e., UV/Fe(III) followed by alkaline precipitation (namely UV/Fe(III)+OH), to achieve highly efficient and environmentally benign removal of Cr(III)-organic complexes from water. The combined process could remove Cr(III)-citrate from 10.4 mg Cr/L to 0.36 mg Cr/L and ∼60% total organic carbon as well. More attractively, negligible Cr(VI) (<0.06 mg/L) was formed during the process. In the viewpoint of mechanism, the added Fe(III) generates ·OH radicals to transform Cr(III) into Cr(VI) and simultaneously released the citrate ligand to form Fe(III)-citrate simultaneously. Then, the photolysis of Fe(III)-citrate under UV irradiation involved the citrate degradation and the production of massive Fe(II) species, which in turn transformed the formed Cr(VI) back to Cr(III). The free metal ions, including Cr(III), Fe(II) and Fe(III) were removed by the subsequent alkaline precipitation. Also, the combined process is applicable to other Cr(III) complexes with EDTA, tartrate, oxalate, acetate. The applicability of the combined process was further demonstrated by treating two real tanning effluents, resulting in the residual Cr(III) below 1.5 mg/L (the discharge standard of China) and negligible formation of Cr(VI) (<0.004 mg/L) as well. In general, the combined process has a great potential for efficient removal of Cr(III) complexes from contaminated waters.

Removal of thallium from aqueous solutions using Fe-Mn binary oxides

In this study, Fe-Mn binary oxides, which harbor the strong oxidative power of manganese dioxide and the high adsorption capacity of iron oxides, were synthesized for Tl(I) removal using a concurrent chemical oxidation and precipitation method. The adsorption of Tl onto the Fe-Mn adsorbent was fast, effective, and selective, with equilibrium sorption reaching over 95% under a broad operating pH (3-12), and high ionic strength (0.1-0.5mol/L). The adsorption can be well fitted with both Langmuir and Freundlich isotherms, and the kinetics can be well described by the pseudo-second-order model. Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) spectra suggest that surface complexation, oxidation and precipitation were the main mechanisms for the removal of Tl. This study shows that the Fe-Mn binary oxides could be a promising adsorbent for Tl removal.

Surprisingly High Selectivity and High Affinity in Mercury Recognition by H-Bonded Cavity-Containing Aromatic Foldarands

In the absence of macrocyclic ring constraints, few synthetic systems, possessing a mostly solvent-independent well-folded conformation that is predisposed for highly selective and high affinity recognition of metal ions, have been demonstrated. We report here such a unique class of conformationally robust modularly tunable folding molecules termed foldarands that can recognize Hg²⁺ ions surprisingly well over 22 other metal ions. Despite the lack of sulfur atoms and having only oxygen-donor atoms in its structure, the best foldarand molecule, i.e., tetramer 4, exhibits a selectivity factor of at least 19 in differentiating the most tightly bound Hg²⁺ ion from all other metal ions, and a binding capacity that is ≥18 times that of thio-crown ethers. These two noteworthy binding characters make possible low level removal of Hg²⁺ ions. With a [4]:[Hg²⁺] molar ratio of 5:1 and a single biphasic solvent extraction, the concentration of Hg²⁺ ions could be reduced drastically by 98% (from 200 to 4 ppb) in pure water. 4 could also effect a highly efficient reduction in mercury content by 98% (from 500 to 10 ppb) in artificial groundwater via multiple successive extractions with an overall consumption of 4 being 9:1 in terms of [4]:[Hg²⁺] molar ratio.

Significantly improving trace thallium removal from surface waters during coagulation enhanced by nanosized manganese dioxide

Thallium (Tl) is an element of high toxicity and significant accumulation in human body. There is an urgent need for the development of appropriate strategies for trace Tl removal in drinking water treatment plants. In this study, the efficiency and mechanism of trace Tl (0.5 μg/L) removal by conventional coagulation enhanced by nanosized manganese dioxide (nMnO₂) were explored in simulated water and two representative surface waters (a river water and a reservoir water obtained from Northeast China). Experimental results showed that nMnO₂ significantly improve Tl(I) removal from selected waters. The removal efficiency was dramatically higher in the simulated water, demonstrating by less than 0.1 μg/L Tl residual. The enhancement of trace Tl removal in the surface waters decreased to a certain extent. Both adjusting water pH to alkaline condition and preoxidation of Tl(I) to Tl(III) benefit trace Tl removal from surface waters. Data also indicated that competitive cation of Ca²⁺ decreased the efficiency of trace Tl removal, resulting from the reduction of Tl adsorption on nMnO₂. Humic acid could largely low Tl removal efficiency during nMnO₂ enhanced coagulation processes. Trace elemental Tl firstly adsorbed on nMnO₂ and then removed accompanying with nMnO₂ settling. The information obtained in the present study may provide a potential strategy for drinking water treatment plants threatened by trace Tl.

Bioinspired carbide-derived carbons with hierarchical pore structure for the adsorptive removal of mercury from aqueous solution

Biosilica of the diatom species Thalassiosira pseudonana is used as hard template for the synthesis of silicon carbide-derived carbons.

Noncompetitive and Competitive Adsorption of Heavy Metals in Sulfur-Functionalized Ordered Mesoporous Carbon

In this work, sulfur-functionalized ordered mesoporous carbons were synthesized by activating the soft-templated mesoporous carbons with sulfur bearing salts that simultaneously enhanced the surface area and introduced sulfur functionalities onto the parent carbon surface. XPS analysis showed that sulfur content within the mesoporous carbons were between 8.2% and 12.9%. The sulfur functionalities include C-S, C═S, -COS, and SO_x. SEM images confirmed the ordered mesoporosity within the material. The BET surface areas of the sulfur-functionalized ordered mesoporous carbons range from 837 to 2865 m²/g with total pore volume of 0.71-2.3 cm³/g. The carbon with highest sulfur functionality was examined for aqueous phase adsorption of mercury (as HgCl₂), lead (as Pb(NO₃)₂), cadmium (as CdCl₂), and nickel (as NiCl₂) ions in both noncompetitive and competitive mode. Under noncompetitive mode and at a pH greater than 7.0 the affinity of sulfur-functionalized carbons toward heavy metals were in the order of Hg > Pb > Cd > Ni. At lower pH, the adsorbent switched its affinity between Pb and Cd. In the noncompetitive mode, Hg and Pb adsorption showed a strong pH dependency whereas Cd and Ni adsorption did not demonstrate a significant influence of pH. The distribution coefficient for noncompetitive adsorption was in the range of 2448-4000 mL/g for Hg, 290-1990 mL/g for Pb, 550-560 mL/g for Cd, and 115-147 for Ni. The kinetics of adsorption suggested a pseudo-second-order model fits better than other models for all the metals. XPS analysis of metal-adsorption carbons suggested that 7-8% of the adsorbed Hg was converted to HgSO₄, 14% and 2% of Pb was converted to PbSO₄ and PbS/PbO, respectively, and 5% Cd was converted to CdSO₄. Ni was below the detection limit for XPS. Overall results suggested these carbon materials might be useful for the separation of heavy metals.