Enhanced sorption of mercury from compact fluorescent bulbs and contaminated water streams using functionalized multiwalled carbon nanotubes

A review on adsorption of heavy metals from wastewater using carbon nanotube and graphene-based nanomaterials

The rampant rise in world population, industrialization, and urbanization expedite the contamination of water sources. The presence of the non-biodegradable character of heavy metals in waterways badly affects the ecological balance. In this modern era, the unavailability of getting clear water as well as the downturn in water quality is a major concern. Therefore, the effective removal of heavy metals has become much more important than before. In recent years, the attention to better wastewater remediation was directed towards adsorption techniques with novel adsorbents such as carbon nanomaterials. This review paper primarily emphasizes the fundamental concepts, structures, and unique surface properties of novel adsorbents, the harmful effects of various heavy metals, and the adsorption mechanism. This review will give an insight into the current status of research in the realm of sustainable wastewater treatment, applications of carbon nanomaterials, different types of functionalized carbon nanotubes, graphene, graphene oxide, and their adsorption capacity. The importance of MD simulations and density functional theory (DFT) in the elimination of heavy metals from aqueous media is also discussed. In addition to that, the effect of factors on heavy metal adsorption such as electric field and pressure is addressed.

Optimization Study of the Capacity of Chlorella vulgaris as a Potential Bio-Remediator for the Bio-Adsorption of Arsenic (III) from Aquatic Environments

This study examined the ability of the green microalgae Chlorella vulgaris to remove arsenic from aqueous solutions. A series of studies was conducted to determine the optimal conditions for biological arsenic elimination, including biomass amount, incubation time, initial arsenic level, and pH values. At 76 min, pH 6, 50 mgL^-1 metal concentration, and 1 gL^-1 bio-adsorbent dosage, the maximum removal of arsenic from an aqueous solution was 93%. The uptake of As (III) ions by C. vulgaris reached an equilibrium at 76 min of bio-adsorption. The maximum adsorptive rate of arsenic (III) by C. vulgaris was 55 mg/gm. The Langmuir, Freundlich, and Dubinin-Radushkevich equations were used to fit the experimental data. The best theoretical isotherm of Langmuir, Freundlich, or/and Dubinin-Radushkevich for arsenic bio-adsorption by Chlorella vulgaris was determined. To choose the best theoretical isotherm, the coefficient of correlation was used. The data on absorption appeared to be linearly consistent with the Langmuir (q_max = 45 mgg^-1; R² = 0.9894), Freundlich (k_f = 1.44; R² = 0.7227), and Dubinin-Radushkevich (q_D-R = 8.7 mg/g; R² = 0.951) isotherms. The Langmuir and Dubinin-Radushkevich isotherms were both good two-parameter isotherms. In general, Langmuir was demonstrated to be the most accurate model for As (III) bio-adsorption on the bio-adsorbent. Maximum bio-adsorption values and a good correlation coefficient were observed for the first-order kinetic model, indicating that it was the best fitting model and significant in describing the arsenic (III) adsorption process. SEM micrographs of treated and untreated algal cells revealed that ions adsorbed on the algal cell's surface. A Fourier-transform infrared spectrophotometer (FTIR) was used to analyze the functional groups in algal cells, such as the carboxyl group, hydroxyl, amines, and amides, which aided in the bio-adsorption process. Thus, C. vulgaris has great potential and can be found in eco-friendly biomaterials capable of adsorbing arsenic contaminants from water sources.

Adsorption and Removal of Mercury(II) by a Crosslinked Hyperbranched Polymer Modified via Sulfhydryl

In this study, the highly crosslinked hyperbranched polyamide-amines (H-PAMAMs) were first prepared via one-pot methods and then modified with thiourea to synthesize a novel adsorbent containing sulfhydryl groups (CHAP-SH), which was used to adsorb Hg(II) ions from aqueous solutions. The adsorption characteristics and mechanism of CHAP-SH for Hg(II) ions were systematically studied. As expected, CHAP-SH exhibited a rapid removal performance toward Hg(II), and the maximum adsorption capacity was 282.74 mg/g at 318 K and pH = 4.5. The whole adsorption behavior could be well described by the pseudo-second-order kinetic model and Langmuir and Redlich-Peterson adsorption isotherm models, which reflected that the adsorption process was mainly monolayer chemisorption. Meanwhile, CHAP-SH had strong selectivity for Hg(II) in the presence of multimetal ions, and it had excellent recoverability after five cycles. In order to further elucidate the adsorption mechanism, the adsorbents before and after adsorption were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and energy-dispersive X-ray spectroscopy, and the results showed that the nitrogen-containing, oxygen-containing, and sulfur-containing groups in the adsorbent molecule had synergistic complexation with Hg(II). These results indicated that the adsorbents had great potential in the future treatment of aqueous solutions containing Hg(II).

A unique, inexpensive, and abundantly available adsorbent: composite of synthesized silver nanoparticles (AgNPs) and banana leaves powder (BLP)

The purpose of this study is to investigate the development of a new and inexpensive adsorbent by immobilization synthesized silver nanoparticles (AgNPs) onto banana leaves powder (BLP), and the prepared composite (BLP)/(AgNPs) was used as an adsorbent for Zn(II), Pb(II), and Fe(III) ion removal from aqueous solutions under the influence of various reaction conditions. (BLP)/(AgNPs) demonstrated remarkable sensitivity toward Zn (II), Pb (II), and Fe (III) ions; metal ions eliminations increased with increasing contact time, agitation speed, adsorbent dose, and temperature, yielding adequate selectivity and ideal removal efficiency of 79%, 88%, and 91% for Zn (II), Pb (II), and Fe (III) ions, respectively, at pH = 5 for Zn(II) and pH = 6 for Pb(II), and Fe(III). The equilibrium contact time for elimination of Zn (II), Pb (II), and Fe (III) ions was reaches at 40 min. Langmuir, Freundlich, and Temkin equations were used to test the obtained experimental data. Langmuir isotherm model was found to be more accurate in representing the data of Zn(II), Pb(II), and Fe(III) ions adsorption onto (BLP)/(AgNPs), with a regression coefficient (R² = 0.999) and maximum adsorption capacities of 190, 244, and 228 mg/g for Zn(II), Pb(II), and Fe(III) ions, respectively. The thermodynamic parameters proved that adsorption of metal ions is spontaneous, feasible, and endothermic, whereas Kinetic studies revealed that the process was best described by a pseudo second order kinetics.

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By reduction reaction, silver nanoparticles were impregnated in banana leaves homogeneous powder and used as an adsorbent.

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The fabricated composites are used as adsorbent for the removal of Zn (II), Pb (II), and Fe (III) ions from aqueous solutions.

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The new adsorbent exhibited high adsorption capacity with three metal ions and followed the order Pb (II)> Fe (III) >Zn (II) ions.

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The metal ions vanished from the solution within approximately 40 min.

Application of MXenes for water treatment and energy-efficient desalination: A review

MXenes are a new type of two-dimensional (2D) material which are rapidly gaining traction for a range of environmental, chemical and medical applications. MXenes and MXene-composites exhibit high surface area, superlative chemical stability, thermal conductivity, hydrophilicity and are environmentally compatible. Consequently, MXenes have been successfully employed for hydrogen storage, semiconductor manufacture and lithium ion batteries. In recent years, MXenes have been utilized in numerous environmental applications for treating contaminated surface waters, ground and industrial/ municipal wastewaters and for desalination, often outperforming conventional materials in each field. MXene-composites can adsorb multiple organic and inorganic contaminants, and undergo Faradaic capacitive deionization (CDI) when utilized for electrochemical applications. This approach allows for a significant decrease in the energy demand by overcoming the concentration polarization limitation of conventional CDI electrodes, offering a solution for low-energy desalination of brackish waters. This article presents a state-of-the-art review on water treatment and desalination applications of MXenes and MXene-composites. An investigation into the kinetics and isotherms is presented, as well as the impact of water constituents and operating conditions are also discussed. The applications of MXenes for CDI, pervaporation desalination and solar thermal desalination are also examined based on the reviewed literature. The effects of the water composition and operational protocols on the regeneration efficacy and long-term usage are also highlighted.

Heavy metal removal by biomass-derived carbon nanotubes as a greener environmental remediation: A comprehensive review

The concentrations of heavy metal ions found in waterways near industrial zones are often exceed the prescribed limits, posing a continued danger to the environment and public health. Therefore, greater attention has been devoted into finding the efficient solutions for adsorbing heavy metal ions. This review paper focuses on the synthesis of carbon nanotubes (CNTs) from biomass and their application in the removal of heavy metals from aqueous solutions. Techniques to produce CNTs, benefits of modification with various functional groups to enhance sorption uptake, effects of operating parameters, and adsorption mechanisms are reviewed. Adsorption occurs via physical adsorption, electrostatic interaction, surface complexation, and interaction between functional groups and heavy metal ions. Moreover, factors such as pH level, CNTs dosage, duration, temperature, ionic strength, and surface property of adsorbents have been identified as the common factors influencing the adsorption of heavy metals. The oxygenated functional groups initially present on the surface of the modified CNTs are responsible towards the adsorption enhancement of commonly-encountered heavy metals such as Pb²⁺, Cu²⁺, Cd²⁺, Co²⁺, Zn²⁺, Ni²⁺, Hg²⁺, and Cr⁶⁺. Despite the recent advances in the application of CNTs in environmental clean-up and pollution treatment have been demonstrated, major obstacles of CNTs such as high synthesis cost, the agglomeration in the post-treated solutions and the secondary pollution from chemicals in the surface modification, should be critically addressed in the future studies for successful large-scale applications of CNTs.

Green synthesized nanosilver-biochar photocatalyst for persulfate activation under visible-light illumination

This paper presents a novel eco-sustainable wastewater treatment using silver nanoparticles and biochar (AgNPs@BC) synthesized from an Eriobotryra japonica leaf. A simple green pyrolysis method was chosen to activate peroxydisulfate (PDS) for organic pollutant degradation under the action of visible light. Rhodamine B (RhB) was chosen as a typical containment to assess the photocatalytic performance of the AgNPs@BC/PDS system with visible-light illumination (AgNPs@BC/PDS-vis). When Ag@BC750 (referring to the AgNPs@BC pyrolyzed at 750 °C) was added, the photocatalytic activity of PDS for RhB degradation was enhanced (from k_app = 0.165-2.984 min^-1). The result confirmed that 99.9% of the RhB was degraded in the AgNPs@BC/PDS-vis system within 10 min and with a 2.0 g/L photocatalyst, 10 mM PDS and under pH 9.03 (k_app = 6.723 min^-1). Over 100 mg/L of RhB could be degraded completely in the AgNPs@BC/PDS-vis system within 30 min mainly attributed to various free radicals. The possible degradation pathways of RhB were illustrated and the recyclability of the AgNPs@BC photocatalyst was confirmed. The findings in this study reveal the virtue of using the AgNPs@BC/PDS-vis process for advanced water disinfection and its provision as an alternative solution in addressing the green way for further water treatment.

Nanoadsorbants for the Removal of Heavy Metals from Contaminated Water: Current Scenario and Future Directions

Heavy metal pollution of aquatic media has grown significantly over the past few decades. Therefore, a number of physical, chemical, biological, and electrochemical technologies are being employed to tackle this problem. However, they possess various inescapable shortcomings curbing their utilization at a commercial scale. In this regard, nanotechnology has provided efficient and cost-effective solutions for the extraction of heavy metals from water. This review will provide a detailed overview on the efficiency and applicability of various adsorbents, i.e., carbon nanotubes, graphene, silica, zero-valent iron, and magnetic nanoparticles for scavenging metallic ions. These nanoparticles exhibit potential to be used in extracting a variety of toxic metals. Recently, nanomaterial-assisted bioelectrochemical removal of heavy metals has also emerged. To that end, various nanoparticle-based electrodes are being developed, offering more efficient, cost-effective, ecofriendly, and sustainable options. In addition, the promising perspectives of nanomaterials in environmental applications are also discussed in this paper and potential directions for future works are suggested.

Functionalized Carbon Nanotubes (CNTs) for Water and Wastewater Treatment: Preparation to Application

As the world human population and industrialization keep growing, the water availability issue has forced scientists, engineers, and legislators of water supply industries to better manage water resources. Pollutant removals from wastewaters are crucial to ensure qualities of available water resources (including natural water bodies or reclaimed waters). Diverse techniques have been developed to deal with water quality concerns. Carbon based nanomaterials, especially carbon nanotubes (CNTs) with their high specific surface area and associated adsorption sites, have drawn a special focus in environmental applications, especially water and wastewater treatment. This critical review summarizes recent developments and adsorption behaviors of CNTs used to remove organics or heavy metal ions from contaminated waters via adsorption and inactivation of biological species associated with CNTs. Foci include CNTs synthesis, purification, and surface modifications or functionalization, followed by their characterization methods and the effect of water chemistry on adsorption capacities and removal mechanisms. Functionalized CNTs have been proven to be promising nanomaterials for the decontamination of waters due to their high adsorption capacity. However, most of the functional CNT applications are limited to lab-scale experiments only. Feasibility of their large-scale/industrial applications with cost-effective ways of synthesis and assessments of their toxicity with better simulating adsorption mechanisms still need to be studied.

Preparation and Characterization of the Sulfur-Impregnated Natural Zeolite Clinoptilolite for Hg(II) Removal from Aqueous Solutions

Sulfur-impregnated zeolite has been obtained from the natural zeolite clinoptilolite by chemical modification with Na2S at 150 °C. The purpose of zeolite impregnation was to enhance the sorption of Hg(II) from aqueous solutions. Chemical analysis, acid and basic properties determined by Bohem’s method, chemical behavior at different pHo values, zeta potential, cation-exchange capacity (CEC), specific surface area, X-ray powder diffraction (XRPD), scanning electron microscopy with energy-dispersive X-ray analysis (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), as well as thermogravimetry with derivative thermogravimetry (TG-DTG) were used for detailed comparative mineralogical and physico-chemical characterization of natural and sulfur-impregnated zeolites. Results revealed that the surface of the natural zeolite was successfully impregnated with sulfur species in the form of FeS and CaS. Chemical modification caused an increase in basicity and the net negative surface charge due to an increase in oxygen-containing functional groups as well as a decrease in specific surface area and crystallinity due to the formation of sulfur-containing clusters at the zeolite surface. The sorption of Hg(II) species onto the sulfur-impregnated zeolite was affected by the pH, solid/liquid ratio, initial Hg(II) concentration, and contact time. The optimal sorption conditions were determined as pH 2, a solid/liquid ratio of 10 g/L, and a contact time of 800 min. The maximum obtained sorption capacity of the sulfur-impregnated zeolite toward Hg(II) was 1.02 mmol/g. The sorption mechanism of Hg(II) onto the sulfur-impregnated zeolite involves electrostatic attraction, ion exchange, and surface complexation, accompanied by co-precipitation of Hg(II) in the form of HgS. It was found that sulfur-impregnation enhanced the sorption of Hg(II) by 3.6 times compared to the natural zeolite. The leaching test indicated the retention of Hg(II) in the zeolite structure over a wide pH range, making this sulfur-impregnated sorbent a promising material for the remediation of a mercury-polluted environment.

Removal of mercury(II) from wastewater using a new and effective composite: sulfur-coated magnetic carbon nanotubes

A sulfur-coated magnetic multi-walled carbon nanotube (S-M-MWCNT) composite was synthesized via coating a thin S layer on M-MWCNTs via a facile heating process. The prepared superparamagnetic adsorbent was employed for the uptake of mercury(II) (Hg(II)) from aqueous solutions and then magnetically separated without filtration or centrifugation steps. The adsorption of Hg(II) increased with increasing pH and reached a plateau value in the pH range 4.5-8.0. The adsorption kinetics followed the pseudo-second-order (PSO) model and equilibrium was reached within 3 h. The isotherm data obeyed the Langmuir isotherm model, and the maximum adsorption capacity of S-M-MWCNT adsorbent was acquired as 62.11 mg g^-1. The adsorption of Hg(II) by the prepared composite is possibly controlled by the interaction between Hg(II) as a soft acid and elemental coated sulfur as a soft base. In addition, the coexist metal ions including copper(II) (Cu(II)), cadmium(II) (Cd(II)), cobalt(II) (Co(II)), lead(II) (Pb(II)), manganese(II) (Mn(II)), zinc(II) (Zn(II)), and chromium(III) (Cr(III)) had no significant effects on Hg(II) removal performance. It was found that the S-M-MWCNT composite could be reused after successive Hg(II) removal without any loss of adsorption capacity. Furthermore, the magnetic adsorbent holds high potential in the treatment of Hg-contaminated wastewater samples.

Biochar-supported nanoscale zero-valent iron as an efficient catalyst for organic degradation in groundwater

High-efficiency and cost-effective catalysts are critical to completely mineralization of organic contaminants for in-situ groundwater remediation via advanced oxidation processes (AOPs). The engineered biochar is a promising method for waste biomass utilization and sustainable remediation. This study engineers maize stalk (S)- and maize cob (C)-derived biochars (i.e., SB300, SB600, CB300, and CB600, respectively) with oxygen-containing functional groups as a carbon-based support for nanoscale zero-valent iron (nZVI). Morphological and physiochemical characterization showed that nZVI could be impregnated within the framework of the synthesized Fe-CB600 composite, which exhibited the largest surface area, pore volume, iron loading capacity, and Fe⁰ proportion. Superior degradation efficiency (100% removal in 20 min) of trichloroethylene (TCE, 0.1 mM) and fast pseudo-first-order kinetics (k_obs =22.0 h^-1) were achieved via peroxymonosulfate (PMS, 5 mM) activation by the Fe-CB600 (1 g L^-1) under groundwater condition (bicarbonate buffer solution at pH = 8.2). Superoxide radical and singlet oxygen mediated by Fe⁰ and oxygen-containing group (i.e., CO) were demonstrated as the major reactive oxygen species (ROSs) responsible for TCE dechlorination. The effectiveness and mechanism of the Fe/C composites for rectifying organic-contaminated groundwater were depicted in this study.

Biosorption of Hg (II) from aqueous solution using algal biomass: kinetics and isotherm studies

The present work investigated the ability of algal biomass Chlorella vulgaris to remove mercury from aqueous solutions. The mercury biosorption process was studied through batch experiments 35 °C temperature with regard to the influence of contact time, initial mercury concentration, pH and desorption. The maximum adsorption capacity was registered at pH 6. The adsorption conduct of Hg(II) was defined by pseudo second order well rather pseudo first order as the experimental data (q_e) come to an agreement with the calculated value. The kinetics of adsorption was fast and a high capacity of adsorption occurred within only 90 min. The adsorption data were signified by many models but Langmuir (q_max = 42. mg g⁻¹) & Freundlich fitted well having regression coefficients near to unity. The thermodynamic parameters were also suited well as negative value of free energy cope up to spontaneity, positive value of the randomness described by ΔS attributed to affinity of Hg⁺² towards algal bioadsorbant and high positive value of heat of enthalpy designates that the adsorption process is expected due to robust interactions between the Hg(II) ions and various functional groups on surface of algal bioadsorbant. Field emission scanning electron microscopy integrated with energy dispersive X-ray spectroscopy analysis before and after adsorption of Hg(II) reveals the adsorption of metallic ions over the surface. FTIR study supported the existence of various functional groups (carboxylix, amines, hydroxyls, amides etc.) helped in adsorption. Continuous adsorption desorption experiments proved that algal cells was excellent biosorbents with potential for further development.

Silver/quartz nanocomposite as an adsorbent for removal of mercury (II) ions from aqueous solutions

Silver nanoparticles (AgNPs) and silver/quartz nanocomposite (Ag/Q)NPs)) were synthesized by sol-gel method using table sugar as chelating agent. The synthesized nanosized materials were used for mercury ions adsorption from aqueous solutions. The materials were characterized by X-ray diffraction (XRD), Transmission Electron microscope (TEM), and surface area (BET). Adsorption of Hg²⁺ (10 mg/l) is strongly dependent on time, initial metal concentration, dose of adsorbent and pH value. Silver/quartz nanocomposite ((Ag/Q)NPs)) shows better efficiency than individual silver nanoparticles (AgNPs). This composite removed mercury ions from the aqueous solution with efficiency of 96% at 60 min with 0.5g adsorbent dosage at pH 6. The adsorption process explained well by the pseudo-second-order kinetic model. In conclusion silver/quartz nanocomposite (Ag/Q)NPs)) shows higher removal efficiency for mercury ions from aqueous solutions than individual silver naoparticles (AgNPs) or quartz (Q).

Performance of novel MgS doped cellulose nanofibres for Cd(II) removal from industrial effluent - mechanism and optimization

Green environment friendly and novel nano MgS decorated cellulose nanofibres (MgS@CNF) were prepared, characterized and evaluated towards the removal of heavy metal namely, cadmium from aqueous solutions. Cellulose nanofibres acted as a template for effective dispersion of MgS nanoparticles and also aid in the complexation of cadmium ions. In depth X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Fourier transform infra red spectroscopy (FTIR) studies revealed that doped MgS on mild acidification yields insitu production of H₂S which effectively complexes cadmium ion to form cadmium sulfide. The reaction followed pseudo first order kinetics with regression coefficient in the order of 0.98. A very high Langmuir adsorption capacity in the order of 333.33 mg/g was obtained for MgS@CNF. Finally, MgS@CNF was applied towards the removal of cadmium from organic and TDS rich tannery waste water. MgS@CNF was effective in bringing down the concentration from ppm to ppb levels.

Enhanced activation of persulfate by nitric acid/annealing modified multi-walled carbon nanotubes via non-radical process

This study aim to explore an effective approach of persulfate (PS) activation based on multi-walled carbon nanotubes (CNTs) for the phenol degradation. The nitric acid/annealing modified CNTs were obtained by oxidation with concentrated nitric acid followed by annealing at various high temperatures (400-1000 °C). The modified CNTs catalysts were characterized and their catalytic degradation performances of phenol with PS were investigated. The results reveal that the modified CNTs can obviously enhance the phenol removal. The catalytic activity is improved by nitric acid modification and obviously influenced by the annealing temperature. PS concentration and solution pH could also affect the catalytic degradation of phenol. The mechanism of enhanced PS activation by the nitric acid/annealing modified CNTs was proposed that the nitric acid modification resulted in more defective edges and oxygen-containing groups, and subsequent annealing at high temperate facilitated the conversion of sp³ to sp² carbon, then the catalytic activity for PS activation was enhanced by the active sites of CO group and sp²-hybridized carbon at the defective edges. In addition, a non-radical process of phenol degradation was demonstrated in terms of the radical quenching tests and electron paramagnetic resonance (EPR) spectra analyses. It is suggested that ¹O₂ is likely the main reactive specie generated in PS activation by the modified CNTs for the phenol removal.

Clay based nanocomposites for removal of heavy metals from water: A review

The exponential increment in world population, recent industrialization, civilization, agricultural and household activities leads to greater levels of water pollution in terms of organic and inorganic contaminants. However, numerous workers have done research for the removal of these pollutants and various types of clays and/or modified clays have been extensively used for this purpose. But all identified adsorbent materials are not able to remove pollutants after certain concentration and sometimes these contaminants are left as such in environment which may create other environmental issues. This paper presents comprehensive information for the adsorption of heavy metal ions from water and waste water using various nanostructured adsorbents such as different clay minerals (kaolinite, montmorillonite) and clay (bentonite), carbon nanotube and nanocomposites. In addition to this, the efficiency of developed materials for the removal of heavy metals is also discussed in details along with comparison of their adsorption efficiencies, pH and change in specific surface area, initial metal ion concentration and contact time. This paper also states the future directions which could be followed to challenge the situation of removal of traces of heavy metals from water, hence protecting water bodies from high pollution load.

Phosphonium-based Ionic Liquid Modified Activated Carbon from Mixed Recyclable Waste for Mercury(II) Uptake

The contamination of water surfaces by mercury is a dangerous environmental problem due to its toxicity, which leads kidney damage. Activated carbon from mixed recyclable waste modified by phosphonium-based ionic liquid (IL-ACMRW) was therefore prepared and evaluated for Hg(II) remediation. The activated carbon used in this study was prepared from mixed waste, including cardboard, papers and palm wastes as cheap raw materials. The mixed Recyclable Waste Activated Carbon was combined with trihexyl(tetradecyl)phosphonium Bis2,4,4-(trimethylpentyl)phosphinate (Cyphos^® IL 104) ionic liquid to form an adsorbent with organic-inorganic content, in order to improve the Hg(II) uptake from aqueous solutions. FTIR confirms the presence of P, C=O and OH after this modification. The adsorption process was investigated and the evaluated results showed that the capacity was 124 mg/g at pH 4, with a contact time of 90 min, an adsorbent dose of 0.4 g/L, and a Hg(II) concentration of 50 mg/L. This Hg(II) adsorption capacity is superior than that reported in the literature for modified multiwall carbon nanotubes. The adsorption of Hg(II) on the modified activated carbon from mixed recyclable waste was found to follow the pseudo second-order kinetics model. Isotherms of adsorption were analyzed via Freundlich and Langmuir models. The results indicated that Freundlich is the best model to describe the process, suggesting multilayer adsorption.

A green l-cysteine modified cellulose nanocrystals biosorbent for adsorption of mercury ions from aqueous solutions

Using a green biosorbent to remove toxic mercury ions from aqueous solutions is a significant undertaking. In the present study, a novel biosorbent, l-cysteine modified cellulose nanocrystals (Lcys-CNCs), was prepared by functionalizing high surface area cellulose nanocrystals with l-cysteine through periodate oxidation and reductive amination reaction. Lcys-CNCs were characterized by FT-IR, ¹³C CP-MAS NMR, elemental analysis, XPS, zeta potential and SEM. As cellulose nanocrystals are the natural nanomaterial, and l-cysteine contains strong mercury chelating groups, Lcys-CNCs show excellent adsorption capacity for mercury ions. The experimental conditions such as pH, contact time, and initial mercury ion concentration are discussed. The pseudo-second order model can describe the removal kinetics of Hg(ii) more accurately than the pseudo-first order model. The adsorption isotherm study of Hg(ii) followed the Langmuir model of monolayer adsorption. The maximum uptake capacity of Lcys-CNCs was determined to be 923 mg g⁻¹. Lcys-CNCs can remove mercury ions with 93% removal efficiency within 5 min from a 71 mg L⁻¹ solution. For Cd(ii), Pb(ii), Cu(ii) and Zn(ii) ions, Lcsy-CNCs can selectively adsorb Hg(ii) ions and the removal efficiency is 87.4% for Hg(ii). This study suggests Lcsy-CNCs are a green and highly efficient biosorbent for adsorption of mercury ions from aqueous solutions.

A green biosorbent, l-cysteine modified cellulose nanocrystals, was successfully synthesized and applied to adsorb mercury ions from aqueous solutions.

Degradation of aqueous 2,4,4'-Trihydroxybenzophenone by persulfate activated with nitrogen doped carbonaceous materials and the formation of dimer products

In this work, we systematically investigated the persulfate (PS) activation potential of a series of nitrogen doped carbonaceous materials for the degradation of 2,4,4'-trihydroxybenzophenone (2,4,4'-HBP), an additive in polyvinyl acetate films and personal care products. Nitrogen originating from urea, NH₄NO₃, indole and polyaniline was doped into carbonaceous materials, including hydroxylated multi-walled carbon nanotubes (CNT-OH), large-inner thin-walled carboxylated carbon nanotubes (CNT-COOH) and graphite oxide (GO), to examine the catalytic effect. The NH₄NO₃-CNT-OH catalyst, which showed the best catalytic performance in 2,4,4'-HBP removal, was characterized by SEM, TEM, FT-IR, Raman, BET surface area, XRD and XPS, and pyrrolic nitrogen was found to play a highly important role in the activation of PS. Under the conditions of [2,4,4'-HBP]₀: [PS]₀ = 1: 500, T = 25 °C, pH₀ = 7.0, concentration of catalyst = 100 mg L^-1, 43.48 μM 2,4,4'-HBP was completely removed in 2 h. According to electron paramagnetic resonance (EPR) spectra and radical quenching experiments, hydroxyl and sulfate radicals on the surface of the catalyst contributed to the substrate oxidation. Cleavage of C-C bridge bond, hydroxylation and polymerization were mainly involved in the oxidation process, leading to the formation of 10 intermediates (e.g., dimers), as detected by the MS/MS spectra. To the best of our knowledge, this report is the first to describe the transformation mechanism of 2,4,4'-HBP in nitrogen doped carbonaceous materials catalyzed PS system.

Magnetic solid-phase extraction for the removal of mercury from water with ternary hydrosulphonyl-based deep eutectic solvent modified magnetic graphene oxide

A novel ternary hydrosulphonyl-based deep eutectic solvent (THS-DES) comprised of choline chloride/itaconic acid/3-mercaptopropionic acid (molar ratio 2:1:1) was firstly synthesized. The composition, property and microscopic structure of the new magnetic adsorbent (THS-DES@M-GO) based on the THS-DES modified the magnetic graphene oxide (M-GO) was characterized by the system. Magnetic solid-phase extraction (MSPE) based THS-DES@M-GO was firstly researched for the removal of mercury (Hg²⁺) from water. Various influencing factors such as the mass of adsorbent, solution pH, initial Hg²⁺ concentration, the removal time and temperature had been systematically tested. Under optimized conditions the removal efficiency (R%) could achieved 99.91%. The precision, repeatability and stability experiments were investigated in detail to evaluate the presented method. The relative standard deviations (RSD) of the removal efficiency were 0.053%, 1.49% and 1.55%, respectively. The maximum adsorption capacity (Q_max) was 215.1 mg g^-1 and the data of the experiment fitted well with Langmuir model. Elution experimental studies shown that 94.94% of Hg²⁺ could be eluted by ethylenediaminetetraacetic acid (EDTA). After seven cycles of adsorption-desorption processes, the THS-DES@M-GO still retained a high removal efficiency of 90.23%. Compared with other adsorbents prepared in this work, THS-DES@M-GO displayed higher removal efficiency for Hg²⁺. Interference study proved the composites was tolerated and stabled under the complex matrix. What's more, the analysis of mercury contaminated water (from Guizhou, P.R., China) proved that the proposed method could be used to remove Hg²⁺ in practical application.

Towards a better understanding on mercury adsorption by magnetic bio-adsorbents with γ-Fe2O3 from pinewood sawdust derived hydrochar: Influence of atmosphere in heat treatment

Pyrolysis under protective atmosphere was regarded as an indispensable process for the preparation of biomass-based adsorbents to achieve higher surface areas. In this paper, magnetic carbon composites (MCC) that fabricated under air atmosphere showed an adsorption capacity of 167.22 mg/g in 200 ppm Hg(II), which was significantly higher than magnetic biochar (MBC, 31.80 mg/g) that fabricated under traditional nitrogen protection, and this remarkable performance of MCC was consistent in a wide range of pHs. Based on BET, XRD, FTIR, SEM and Boehm titration, MCC was demonstrated with limited surface area (43.29 m²/g) but large amount of surface functional groups comparing with MBC. Additionally, γ-Fe₂O₃ with a high degree of crystallization was generated in MCC, which led to a better magnetic property and recyclability. Moreover, characterizations, Langmuir isotherm and pseudo-second-order kinetics demonstrated the chemisorption was dominant for MCC in mercury capture, and surface complexation co-precipitate of Hg₄Fe₈O₁₆C₅₆H₄₀ were also formed.

Characterization, evaluation, and mechanistic insights on the adsorption of antimonite using functionalized carbon nanotubes

Floating catalytic chemical vapor deposition technique was used for synthesizing carbon nanotubes (CNTs) using ferrocene in benzene as the hydrocarbon source. The functionalization of CNTs was carried out by oxidation followed by grafting of potassium iodide (KI) and mercaptoethanol (HS(CH₂)₂OH) ligands to produce iodide-grafted CNTs (CNT-I) and thiol-functionalized CNTs (CNT-SH), respectively. The resulting adsorbents have been thoroughly characterized by various techniques. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) studies revealed the efficient grafting of the ligands. Further, their adsorption capacities towards antimonite have been assessed. The adsorption kinetics fitted the pseudo-second-order model for both the adsorbents. Moreover, the adsorption of Sb(III) followed Langmuir and Freundlich's model. The maximum adsorption capacity of CNT-I and CNT-SH for Sb(III) at pH 7 was found to be 200 and 140.85 mg/g, respectively. The interference effect of various ions on the adsorption of antimonite was studied. A suitable mechanism for Sb(III) adsorption has been postulated using TEM, XRD, XPS, and FTIR. The adaptability of the adsorbents was demonstrated by the removal capacity of Sb(III) at parts per billion levels from nuclear decontamination formulation (NAC) and tap water matrix as well.

A review of functionalized carbon nanotubes and graphene for heavy metal adsorption from water: Preparation, application, and mechanism

Carbon-based nanomaterials, especially carbon nanotubes and graphene, have drawn wide attention in recent years as novel materials for environmental applications. Notably, the functionalized derivatives of carbon nanotubes and graphene with high surface area and adsorption sites are proposed to remove heavy metals via adsorption, addressing the pressing pollution of heavy metal. This critical revies assesses the recent development of various functionalized carbon nanotubes and graphene that are used to remove heavy metals from contaminated water, including the preparation and characterization methods of functionalized carbon nanotubes and graphene, their applications for heavy metal adsorption, effects of water chemistry on the adsorption capacity, and decontamination mechanism. Future research directions have also been proposed with the goal of further improving their adsorption performance, the feasibility of industrial applications, and better simulating adsorption mechanisms.

Designer carbon nanotubes for contaminant removal in water and wastewater: A critical review

The search for effective materials for environmental cleanup is a scientific and technological issue of paramount importance. Among various materials, carbon nanotubes (CNTs) possess unique physicochemical, electrical, and mechanical properties that make them suitable for potential applications as environmental adsorbents, sensors, membranes, and catalysts. Depending on the intended application and the chemical nature of the target contaminants, CNTs can be designed through specific functionalization or modification processes. Designer CNTs can remarkably enhance contaminant removal efficiency and facilitate nanomaterial recovery and regeneration. An increasing number of CNT-based materials have been used to treat diverse organic, inorganic, and biological contaminants. These success stories demonstrate their strong potential in practical applications, including wastewater purification and desalination. However, CNT-based technologies have not been broadly accepted for commercial use due to their prohibitive cost and the complex interactions of CNTs with other abiotic and biotic environmental components. This paper presents a critical review of the existing literature on the interaction of various contaminants with CNTs in water and soil environments. The preparation methods of various designer CNTs (surface functionalized and/or modified) and the functional relationships between their physicochemical characteristics and environmental uses are discussed. This review will also help to identify the research gaps that must be addressed for enhancing the commercial acceptance of CNTs in the environmental remediation industry.

Lead removal from water – dependence on the form of carbon and surface functionalization

Functionalization with same functional group results in varied lead adsorption performance for different nanostructured carbon materials.

Inhibiting Mercury Re-emission and Enhancing Magnesia Recovery by Cobalt-Loaded Carbon Nanotubes in a Novel Magnesia Desulfurization Process

Mercury re-emission, because of the reduction of Hg²⁺ to form Hg⁰ by sulfite, has become a great concern in the desulfurization process. Lowering the concentrations of Hg²⁺ and sulfite in the desulfurization slurry can retard the Hg⁰ formation and, thus, mitigate mercury re-emission. To that end, cobalt-based carbon nanotubes (Co-CNTs) were developed for the simultaneous Hg²⁺ removal and sulfite oxidation in this work. Furthermore, the thermodynamics and kinetics of the Hg²⁺ adsorption and effect of Hg²⁺ adsorption on catalytic activity of Co-CNTs were investigated. Experimental results revealed that the Co-CNTs not only accelerated sulfite oxidation to enable the recovery of desulfurization by-products but also acted as an effective adsorbent of Hg²⁺ removal. The Hg²⁺ adsorption rate mainly depended on the structure of the adsorption material regardless of the cobalt loading and morphological distribution. The catalytic activity of the Co-CNTs for sulfite oxidation was not significantly affected due to the Hg²⁺ adsorption. Additionally, the isothermal adsorption behavior was well-fitted to the Langmuir model with an adsorption capacity of 166.7 mg/g. The mercury mass balance analysis revealed that the Hg⁰ re-emission was decreased by 156% by adding 2.0 g/L of Co-CNTs. These results can be used as a reference for the simultaneous removal of multiple pollutants in the wet-desulfurization process.

Application of a novel magnetic carbon nanotube adsorbent for removal of mercury from aqueous solutions

In this research, multiwall carbon nanotube was magnetized and subsequently functionalized by thiosemicarbazide. After characterization by FTIR, BET, SEM, EDAX, and VSM techniques, the magnetized adsorbent (multi-walled carbon nanotubes (MWCNTs)/Fe₃O₄) was used for removal of Hg²⁺ from aqueous solutions and the experimental conditions were optimized. The adsorption capacity of 172.83 mg g^-1 was obtained at 25 °C and pH = 3 which was superior to the value obtained for initial multiwall carbon nanotube, magnetized sample, and many previously reported values. In the presence of Pb⁺² and Cd⁺², the adsorbent was selective towards mercury when their concentration was respectively below 50 and 100 mg L^-1. The adsorption process was kinetically fast and the equilibration was attained within 60 min with 69.5% of the capacity obtained within 10 min. The used adsorbent was regenerated by HNO₃ solution, and the regenerated adsorbent retained 92% of its initial capacity. The magnetic sensitivity of the adsorbent allowed the simple separation of the used adsorbent from the solution by implying an appropriate external magnetic field. The adsorption data was well fitted to the Langmuir isotherm model, indicating homogeneous and monolayer adsorption of mercury by the adsorbent.

Novel deep eutectic solvent-functionalized carbon nanotubes adsorbent for mercury removal from water

Due to the interestingly tolerated physicochemical properties of deep eutectic solvents (DESs), they are currently in the process of becoming widely used in many fields of science. Herein, we present a novel Hg²⁺ adsorbent that is based on carbon nanotubes (CNTs) functionalized by DESs. A DES formed from tetra-n-butyl ammonium bromide (TBAB) and glycerol (Gly) was used as a functionalization agent for CNTs. This novel adsorbent was characterized using Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, XRD, FESEM, EDX, BET surface area, and Zeta potential. Later, Hg²⁺ adsorption conditions were optimized using response surface methodology (RSM). A pseudo-second order model accurately described the adsorption of Hg²⁺. The Langmuir and Freundlich isotherm models described the absorption of Hg²⁺ on the novel adsorbent with acceptable accuracy. The maximum adsorption capacity was found to be 177.76mg/g.

Highly Efficient Magnetic Nitrogen-Doped Porous Carbon Prepared by One-Step Carbonization Strategy for Hg2+ Removal from Water

Hydrophilic magnetic N-doped porous carbon composites (MNPCs) with high special surface areas and rich nitrogen content was prepared via simple one-step carbonization of zinc oxide nanoparticles (ZnO NPs), 2-methylimidazole (HmIm), and Fe₃O₄@SiO₂ magnetic nanoparticles (MNPs) mixture directly. During the carbonization process, ZnO NPs directly reacts with HmIm to yield porous ZIF-8 while the MNPs are incorporated into the frameworks to generate magnetic metal-organic frameworks (MFCs), and the MFCs acts as a self-sacrificing template to prepare MNPC. The obtained MNPCs via simple one-step carbonization strategy display higher adsorption capacity (429 mg g^-1) for Hg²⁺ ions than MNPC-T700-M3-T (382 mg g^-1) which was obtained by two-step synthesis strategy for comparison. It also exhibits very fast adsorption dynamics (adsorption rate constant (K₂) = 2.45 g mg^-1 min^-1) for Hg²⁺ and could efficiently remove 95% Hg²⁺ in 2 min for 20 mg L^-1 Hg²⁺ solution. Furthermore, the prepared MNPC exhibits good chemical stability and the adsorption capacity is still more than 95% even after 10 adsorption-elution cycles. The proposed method is easy-processing and economic, which not only provides highly efficient MNPCs for metal ions capture but also paves the ways toward various MFCs with different ligands through solvent/additive-free synthesis approaches.

Design and construction of a new optical solid-state mercury(ii) sensor based on PVC membrane sensitized with colloidal carbon dots

A new Hg²⁺ ion solid-state double layer sensor impregnated with colloidal carbon dots and N′-(3-(4-(dimethylamino)phenyl)allylidene)isonicotinohydrazide (NDPAI, as a second layer) was inserted in plasticized polyvinyl chloride (PVC) as the first layer.

Highly efficient simultaneous adsorption of Cd(ii), Hg(ii) and As(iii) ions from aqueous solutions by modification of graphene oxide with 3-aminopyrazole: central composite design optimization

Efficient simultaneous adsorption of heavy metal ions from solutions by modified graphene oxide with 3-aminopyrazole using central composite design modeling.

Allyl triphenyl phosphonium bromide based DES-functionalized carbon nanotubes for the removal of mercury from water

Recently, deep eutectic solvents (DESs) have shown their new and interesting ability for chemistry through their involvement in variety of applications. This study introduces carbon nanotubes (CNTs) functionalized with DES as a novel adsorbent for Hg²⁺ from water. Allyl triphenyl phosphonium bromide (ATPB) was combined with glycerol as the hydrogen bond donor (HBD) to form DES, which can act as a novel CNTs functionalization agent. The novel adsorbent was characterized using Raman, FTIR, XRD, FESEM, EDX, BET surface area, TGA, TEM and Zeta potential. Response surface methodology was used to optimize the removal conditions for Hg²⁺. The optimum removal conditions were found to be pH 5.5, contact time 28 min, and an adsorbent dosage of 5 mg. Freundlich isotherm model described the adsorption isotherm of the novel adsorbent, and the maximum adsorption capacity obtained from the experimental data was 186.97 mg g^-1. Pseudo-second order kinetics describes the adsorption rate order.

Decolorization of azo dye by peroxymonosulfate activated by carbon nanotube: Radical versus non-radical mechanism

Carbon nanotube (CNT) has been shown to effectively activate peroxymonosulfate (PMS) to remove contaminants, whereas controversial activation mechanisms (radical vs non-radical mechanism) were previously proposed. Here we report that radical-induced decolorization of acid orange 7 (AO7) dominated in the CNT activated PMS system, but non-radical mechanism was also involved at high Cl^- concentration. CNT exhibited high activity in activating PMS to decolorize AO7. The decolorization rate of AO7 increased with increasing PMS dosages and CNT loadings, rising temperature and higher pH. Radical quenching and photoluminescence techniques confirmed the decolorization of AO7 in the CNT/PMS system was caused by the radical oxidation, which dominantly took place on the surface of CNT, rather than the bulk solution. The presence of Cl^- exhibited a dual effect on AO7 decolorization. Low concentration of Cl^- slightly inhibited AO7 decolorization, but further raising the concentration to above 0.1M significantly accelerated its decolorizaition. Cl^- was confirmed to react with PMS to generate HClO, which effectively bleached AO7 through non-radical process rather than radical process. The decolorization of AO7 induced from the non-radical process exhibited different degradation products and less mineralization in comparison to that derived from radical process.

Removal of antimonite (Sb(iii)) and antimonate (Sb(v)) using zerovalent iron decorated functionalized carbon nanotubes

Zerovalent iron decorated CNTs exhibited excellent adsorption capacity towards both Sb(iii) and Sb(v). Using XPS, FTIR and Raman mechanism of interaction is postulated.