Removal of mercury from water by carbonaceous sorbents derived from walnut shell

Green technology approach for heavy metal adsorption by agricultural and food industry solid wastes as bio-adsorbents: a review

Heavy metal discharge from various metallurgical industries has been of particular concern in India over the last few decades. Similarly, management and disposal of wastes that are generated out of agricultural commodities processing is a huge task for processors. The researchers have been focusing on a new process for remediation of heavy metals, among which biosorption is an emerging technology. Adsorption using agricultural and food industry wastes (AFW) has shown a greater absorption rate than the conventional system due to the presence of the functional groups. In addition, these reported AFW exhibited better adsorption efficiency when modified with acid, alkaline, and other chemical solvents. In this context, utilization of agricultural and food waste as bio-sorbent could simultaneously benefit both water treatment and waste management. This review seeking to address the possibilities of using biosorption as green technological approach for removal of heavy metals and also focuses on various parameters that are required to use AFW as an efficient system for biosorption. However, commercialization and implementation of this process in industrial scale is necessary for successfully utilizing AFW as low-cost adsorbents.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-022-05486-1.

Nanoporous carbon materials as a sustainable alternative for the remediation of toxic impurities and environmental contaminants: A review

Due to rapidly deteriorating water resources, the world is looking forward to a sustainable alternative for the remediation of noxious pollutants such as heavy metals and organic and gaseous contaminants. To address this global issue of environmental pollution, nanoporous carbon materials (NPCMs) can be used as a one-stop solution. They are widely applied as adsorbents for many toxic impurities and environmental contaminants. The present review provides a detailed overview of the role of different synthesis factors on the porous characteristics of carbon materials, activating agents, reagent-precursor ratio and their potential application in the remediation. Findings revealed that synthetic parameters result in the formation of microporous NPCMs (S_BET: >4000 m³/g; V_Total (cm³/g) ≥ 2; V_Micro (cm³/g) ≥ 1), micromesoporous (S_BET: >2500 m³/g; V_Total (cm³/g) ≥ 1.5; V_Micro (cm³/g) ≥ 0.7) and mesoporous (S_BET: >2500 m³/g; V_Total (cm³/g) ≥ 1.5; V_Micro (cm³/g) ≥ 0.5) NPCMs. Moreover, it was observed that a narrow pore size distribution (0.5-2.0 nm) yields excellent results in the remediation of noxious contaminants. Further, chemical activating agents such as NaOH, KOH, ZnCl₂, and H₃PO₄ were compared. It was observed that activating agents KОН, H₃PO₄, and ZnCl₂ were generally used and played a significant role in the possible large-scale production and commercialization of NPCMs. Thus, it can be interpreted that with a well-planned strategy for the synthesis, NPCMs with a "tuned" porosity for a specific application, in particular, microporosity for the accumulation and adsorption of energetically important gases (CO₂, CH₄, H₂), micro-mesoporosity and mesoporosity for high adsorption capacity for towards metal ions and a large number of dyes, respectively.

Impacts of Forest Fire Ash on Aquatic Mercury Cycling

Mercury (Hg) is a ubiquitous contaminant in the environment and its methylated form, methylmercury (MeHg), poses a worldwide health concern for humans and wildlife, primarily through fish consumption. Global production of forest fire ash, derived from wildfires and prescribed burns, is rapidly increasing due to a warming climate, but their interactions with aqueous and sedimentary Hg are poorly understood. Herein, we compared the differences of wildfire ash with activated carbon and biochar on the sorption of aqueous inorganic Hg and sedimentary Hg methylation. Sorption of aqueous inorganic Hg was greatest for wildfire ash materials (up to 0.21 μg g^-1 or 2.2 μg g^-1 C) among all of the solid sorbents evaluated. A similar Hg adsorption mechanism for activated carbon, biochar made of walnut, and wildfire ash was found that involves the formation of complexes between Hg and oxygen-containing functional groups, especially the -COO group. Notably, increasing dissolved organic matter from 2.4 to 70 mg C L^-1 remarkably reduced Hg sorption (up to 40% reduction) and increased the time required to reach Hg-sorbent pseudo-equilibrium. Surprisingly, biochar and wildfire ash, but not activated carbon, stimulated MeHg production during anoxic sediment incubation, possibly due to the release of labile organic matter. Overall, our study indicates that while wildfire ash can sequester aqueous Hg, the leaching of its labile organic matter may promote production of toxic MeHg in anoxic sediments, which has an important implication for potential MeHg contamination in downstream aquatic ecosystems after wildfires.

Removal of inorganic toxic contaminants from wastewater using sustainable biomass: A review

Lignocellulosic biomass is most abundant, ecofriendly and sustainable material on this green planet which has received great attention due to exhaustion of petroleum reserves and various environmental complications. Due to its abundance and sustainability, it has been opted in number of advanced applications i.e. synthesis of green chemicals, biofuels, paper, packaging, biocomposite and for discharge of toxic contaminants from wastewaters. Utilization of sustainable biomass for removal of toxic pollutants from wastewater is robust technique due to its low-cost and easy availability. In this review, we have summarized removal of inorganic pollutants by sustainable lignocellulosic biomass in their natural as well as in chemically functionalized form. Various techniques for modification of sustainable biomass have been discussed and it was found that modified biomass showed better biosorption ability as compared to natural biomass. We conclude that modified biomass biosorbents are useful for removal of toxic inorganic pollutants to deficient levels. Several modification strategies can improve the qualities of biosorbent, however grafting is the most successful among them, as demonstrated in this work. The numerous grafting methods using a free radical grafting process are also summarized in this review article. This review also gathers studies comparing sorption capabilities with and without modification using modified and unmodified biosorbents. Chemically modified cellulosic biomass is favoured over untreated biomass because it has a higher adsorption efficiency, which is favoured by a large number of reactive binding sites, improved ion-exchange characteristics, and more functional groups available after modification.

Remarkable adsorption of hydroquinone as an anion contaminant by using the magnetic supported bimetallic (NiCu-MOF@MAC) nanocomposites in aqueous solutions

The purposes of this work were to synthesize the core-shell magnetic and nonmagnetic supported bimetallic metal-organic frameworks (MOFs) on the walnut-based activated carbon by the facile preparation method to investigate the feasibility of the performance adsorption of hydroquinone in the aqueous solutions. Activated carbon as a substrate and nickel, copper, and trimesic acid were employed in the structure of the prepared MOFs. The adsorbents were characterized by XRD, FTIR, FESEM, EDX, TEM, BET, and VSM analysis. The goethite and magnetite phases were detected in the morphology of the magnetic adsorbent as confirmed by the XRD pattern. Increases in the pH value from 6 and the adsorption temperature led to a lower adsorption capacity of the samples. The maximum adsorption capacity for the well-dispersed nanoparticles of magnetic (NiCu-MOF@MAC and nonmagnetic (NiCu-MOF@AC) was calculated to be 303.03 and 454.54 mg/g by using linear Langmuir isotherm as an appropriate model, respectively. The achievements from the reusability evaluation illustrated that the magnetic bimetallic MOF nanocomposite could successfully be applied to remove hydroquinone from the wastewater on an industrial scale. The kinetic experimental data was in good agreement with the pseudo-second-order model.

Adsorption of Hexavalent Chromium and Divalent Lead Ions on the Nitrogen-Enriched Chitosan-Based Activated Carbon

Optimizing the physicochemical properties of the chitosan-based activated carbon (Ch-ACs) can greatly enhance its performance toward heavy metal removal from contaminated water. Herein, Ch was converted into a high surface area (1556 m2/g) and porous (0.69 cm3/g) ACs with large content of nitrogen (~16 wt%) using K2CO3 activator and urea as nitrogen-enrichment agents. The prepared Ch-ACs were tested for the removal of Cr(VI) and Pb(II) at different pH, initial metal ions concentration, time, activated carbon dosage, and temperature. For Cr(VI), the best removal was at pH = 2, while for Pb(II) the best pH for its removal was in the range of 4-6. At 25 °C, the Temkin model gives the best fit for the adsorption of Cr(VI), while the Langmuir model was found to be better for Pb(II) ions. The kinetics of adsorption of both heavy metal ions were found to be well-fitted by a pseudo-second-order model. The findings show that the efficiency and the green properties (availability, recyclability, and cost effectiveness) of the developed adsorbent made it a good candidate for wastewaters treatment. As preliminary work, the prepared sorbent was also tested regarding the removal of heavy metals and other contaminations from real wastewater and the obtained results were found to be promising.

Content and morphology of lead remediated by activated carbon and biochar: A spectral induced polarization study

Soil and groundwater contamination with lead (Pb) poses serious challenges for the environment. Activated carbon (AC) and biochar have huge potential application in the in-situ remediation processes through permeable reactive barriers (PRB). Spectral induced polarization (SIP) technique recently showed promises in nondestructively monitoring the spatio-temporal characteristics of physical, chemical and biological processes in porous media. In this study SIP technique was used for monitoring Pb remediation by AC and biochar in column scale. The calculated characteristic grain/pore size evolutions from SIP signals on AC, agreed well with the size of precipitates measured by SEM and mercury intrusion porosimetry (MIP) methods. The content increment process of the retained Pb on AC was also recorded via the magnitude increment of the imaginary conductivity. The mechanisms of Pb-AC and Pb-biochar interactions were investigated using SEM-EDS, TEM, FTIR, XRD, and XPS measurements. It showed that AC immobilizes through physical adsorption and precipitation, whereas complexation with functional groups is the remediation mechanism for biochar. Furthermore, the observed SIP responses of both AC and biochar are two orders of magnitude higher than those of typical natural soils or silica materials. This distinct difference is an additional advantage for the field application of SIP technique in PRB scenarios.

Synthesis of the magnetic core-shell bi-metallic and tri-metallic metal-organic framework nanocomposites for dye adsorption

Bi-metallic and tri-metallic metal-organic frameworks (MOFs) supported on the magnetic activated carbon (MAC) were synthesized for the reduction of methylene blue (MB) concentration in the aqueous solutions. The adsorbent nanocomposites were characterized by applying the general tests including XRD, FTIR, FESEM, TEM, BET, and VSM. The XRD achievements demonstrated that crystalline structure of MOFs was derived on the MAC by the presented method. The core-shell morphology with nano-scale size of the magnetic carbonaceous MOFs was detected in TEM and FESEM micro-images. The acceptable magnetic strength of the prepared adsorbents was proved by using the VSM analysis. The important operating conditions including pH and temperature were also evaluated, while the other parameters were kept constant. The pseudo-second-order kinetic model was matched with the experimental data to show the kinetic behavior of the multi-component MOFs. The isotherm studies showed that the good agreement between the experimental data with both Langmuir model and the maximum capacities was calculated to be about 66.51 and 71.43 mg/g for the bi-metallic and tri-metallic nanocomposites, respectively. Regeneration experiments indicated that the fabricated adsorbents have an excellent reusing adsorption capacity which can be a proper selection for the industrial applications. PRACTITIONER POINTS: Bi-metallic and tri-metallic MOFs supported on the magnetic activated carbon were synthesized by the facile preparation method. Adsorption of methylene blue by using MOFs were successfully done. Nanocomposites were evaluated by XRD, FTIR, BET, FESEM, TEM, and VSM techniques. Maximum of adsorption capacity was observed for tri-metallic MOF as 71.43 mg/g.

Adsorption Mechanisms and Characteristics of Hg2+ Removal by Different Fractions of Biochar

The adsorption mechanisms of mercury ion (Hg2+) by different fractions of biochar were studied, providing a theoretical basis and practical value for the use of biochar to remediate mercury contamination in water. Biochar (RC) was prepared using corn straw as the raw material. It was then fractionated, resulting in inorganic carbon (IC), organic carbon (OC), hydroxyl-blocked carbon (BHC), and carboxyl-blocked carbon (BCC). Before and after Hg2+ adsorption, the biochar fractions were characterized by several techniques, such as energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Obtained results indicate that the reaction mechanisms of RC for Hg2+ removal mainly include electrostatic adsorption, ion exchange, reduction, precipitation, and complexation. The equilibrium adsorption capacity of RC for Hg2+ is 75.56 mg/g, and the adsorption contribution rates of IC and OC are approximately 22.4% and 77.6%, respectively. Despite the lower rate, IC shows the largest adsorption capacity, of 92.63 mg/g. This is attributed to all the mechanisms involved in Hg2+ adsorption by IC, with ion exchange being the main reaction mechanism (accounting for 39.8%). The main adsorption mechanism of OC is the complexation of carboxyl and hydroxyl groups with Hg2+, accounting for 71.6% of the total OC contribution. BHC and BCC adsorb mercury mainly via the reduction–adsorption mechanism, accounting for 54.6% and 54.5%, respectively. Among all the adsorption mechanisms, the complexation reaction of carboxyl and hydroxyl groups with Hg2+ is the dominant effect.

Current advancement and future prospect of biosorbents for bioremediation

The increasing use of heavy metals, synthetic dyes and pesticides is a major environmental concern. Wastewaters containing heavy metals and dyes, extensively released from small and large scale industries enter excessively into food chains resulting in mutagenesis, carcinogenicity and serious health impairments in living systems. The arrays of technologies are implemented to date to remediate both inorganic and organic contaminants from wastewaters. Among which, adsorption is the most attractive method as it employs eco-friendly, sustainable and cost-effective biomaterials. Use of bioadsorbents is advantageous over the conventional adsorbents. Clay, chitin, peat, microbial biomass and agricultural wastes are commonly used bioadsorbants. These bioadsorbents are extensively used for elimination of dyes, heavy metals, adsorption of toxic industrial effluents, removal of fertilizers/pesticides, atmospheric pollutants and nuclear waste from the environment. The current review presents state of the art knowledge on various types of biosorbents, their uses, and mechanism of action. Various strategies to enhance the efficiency of bioadsorbents and physicochemical conditions to remediate dyes and heavy metals from waste streams are also incorporated in this review. Use of nano-bioadsorbents in industries to minimize the hazardous effect of solid and liquid waste has also been discussed.

Nanocellulose-based composites for the removal of contaminants from wastewater

Polymers derived from plant and animal sources are of great interest in wastewater remediation due to their cost-effectiveness and renewable adsorption capabilities, one such polymer is nanocellulose (NC). NC has gained a lot of attention in various research fields due to its abundance in nature, nano-dimension, high surface area, stability and bio-compatibility. As a result, NC has emerged as a great potential adsorbent for the removal of contaminants such as heavy metals, organic dyes, oils, pharmaceutical and etc. in the environmental remediation. This review focuses on the description of the building blocks, structure, properties, isolation and also discusses the potential of nanocellulose based composites materials with reinforcements such as activated carbon, carbon nanotube, graphene oxides, metals, non-metals and ceramics that were effectively used as an adsorbents for diverse organic and inorganic contaminants in water.

Recent advances in the use of walnut (Juglans regia L.) shell as a valuable plant-based bio-sorbent for the removal of hazardous materials

The effective use of agricultural by-products is definitely a major challenge in waste management. In the walnut fruit processing industry, large amounts of shells are produced as agricultural by-products and discarded or burned produced as fuel. Walnut (Juglans regia L.) is a valuable tree nut in the Juglandaceae family. The fruit is composed of four main parts: the kernel, the skin, the shell, and the husk. The importance of walnuts is mostly related to theirs valuable kernels. However, their shells are currently experiencing as much interest as their kernels due to the beneficial effects of the shells. In the past several years, walnut shell (WS) has been widely explored as a naturally inert plant-based biosorbent. In this review, we first highlight recent scientific literature regarding the development of adsorbents from WS in the form of carbon-based materials including unmodified/modified WS, and activated carbons (ACs). Next, we discuss the potential applications of WS-derived by-products as natural yet effective adsorbents for the removal of various hazardous materials including heavy metals (HMs), synthetic industrial dyes, and harmful chemicals.

The effective use of agricultural by-products is definitely a major challenge in waste management.

Fabrication of novel magnetic graphene oxide nanocomposites for selective adsorption of mercury from aqueous solutions

In this work, a novel functionalized graphene oxide (GO) was used as an effective and selective adsorbent for removal of mercury (Hg²⁺). The magnetic nanocomposite adsorbent (MNA) based on GO was prepared through surface reversible addition-fragmentation chain transfer copolymerization of acrylic monomers and then the formation of Fe₃O₄ nanoparticles. The structure of MNAs was characterized by using FTIR, SEM, TEM, VSM, XRD, and nitrogen adsorption/desorption isotherms. The results of ion adsorption of MNAs demonstrated high selectivity and adsorption efficiency for Hg²⁺ in the presence of competing ions. Furthermore, the removal of Hg²⁺ obeyed a pseudo-second-order model and fitted well to the Langmuir isotherm model with the maximum Hg²⁺ uptake of 389 mg g^-1. The MNA was also confirmed as good materials for re-use and maintained 86% of its initial adsorption capacity for mercury after the fifth regeneration cycles. Finally, the experimental results demonstrated that the solution pH, ion concentration, and temperature had a major impact on Hg(II) adsorption capacity. The results indicate that the MNAs with high adsorption abilities could be very promising adsorbents for the selective recovery of ions in wastewater treatment process. Graphical abstract.

Green Activated Magnetic Graphitic Carbon Oxide and Its Application for Hazardous Water Pollutants Removal

Graphitic carbon oxide (GCO) and magnetic graphitic carbon oxide (MGCO) were prepared from sugar via optimized green activation by employing ozone oxidation, and applied to wastewater treatment. The maximal oxidation and adsorption yield of pollutants were achieved at pH 2.0−4.0, which is the optimized pH for ozone oxidation of GC to generate GCO. As-prepared GCO and MGCO were characterized using X-ray, infrared, and microscopic techniques. The MGCO has enough saturation magnetization (MS) of 41.38 emu g−1 for separation of the sorbent from the reaction medium by applying an external magnetic field. Batch adsorption of radioactive and heavy metals (Th(IV), Pb(II)), and a dye (methylene blue (MB)) using GCO and MGCO was evaluated by varying the adsorbent dose, equilibrium pH, contact time, initial metal and dye concentrations, and kinetics and isotherms. Adsorption kinetics and isotherm studies indicated that Th(IV), Pb(II), and MB adsorption were best described by pseudo-second-order kinetics and Langmuir isotherm with R2 (correlation coefficient) > 0.99, respectively. The maximum adsorption capacities for Th(IV), Pb(II), and MB were 52.63, 47.39, and 111.12 mg g−1 on GCO and 76.02, 71.94, and 76.92 mg g−1 on MGCO. GCO and MGCO are prospectively effective and low-cost adsorbents for ion removal in wastewater treatment. As prepared MGCO can be reused up to three cycles for Th(IV), Pb(II), and MB. This work provides fundamental information about the equilibrium adsorption isotherms and mechanisms for Th(IV), Pb(II), and MB on GCO and MGCO.

Selective removal behavior and mechanism of trace Hg(II) using modified corn husk leaves

Removal of Hg(II) from wastewater was beneficial to satisfy the discharge standards of China's mercury-containing wastewater (50 ppb). An adsorbent was prepared via modifying corn husk leaves with bismuthiol I. The results revealed that the mercury removal rate was more than 98.5% at pH 1.0-7.0. Moreover, the removal rate reached 96% at 5 min and the residual concentration decreased from 10 ppm to approximately 30 ppb. In addition, the adsorbent owned a conspicuous selective absorbability for trace Hg(II) from wastewater. The adsorption process followed a Hill isotherm model. The actual saturated adsorption quantity of the adsorbent was 707 mg/g. The repeatability experiment indicated that the mercury removal efficiency was still beyond 99% after three cycles. The X-ray photoelectron spectroscopy suggested that the main adsorption mechanism was chelation between nitrogen/sulfur groups and Hg(II). The adsorbent was hopeful to remove mercury from wastewater in a sustainability perspective.

Nanomaterials: Solutions to Water-Concomitant Challenges

Plenty of fresh water resources are still inaccessible for human use. Calamities such as pollution, climate change, and global warming pose serious threats to the fresh water system. Although many naturally and synthetically grown materials have been taken up to resolve these issues, there is still plenty of room for enhancements in technology and material perspectives to maximize resources and to minimize harm. Considering the challenges related to the purification of water, materials in the form of nanofiber membranes and nanomaterials have made tremendous contributions to water purification and filtration. Nanofiber membranes made of synthetic polymer nanofibers, ceramic membranes etc., metal oxides in various morphologies, and carbonaceous materials were explored in relation to waste removal from water. In this review, we have discussed a few key materials that have shown effectiveness in removing pollutants from waste water, enabling solutions to existing problems in obtaining clean drinking water.

Adsorption of inorganic mercury from aqueous solutions onto dry biomass of Chlorella vulgaris: kinetic and isotherm study

This study focused on kinetics and equilibrium isotherms of mercury biosorption from water using dry biomass of Chlorella vulgaris as biosorbent at pH 5.0. Biosorption tests were performed at 2.0 g/L biomass dosage varying initial Hg concentration from 11.0 to 90.6 mg/L. The Lagergren equation was found to best describe the process, with R² of 0.984 and specific rate constant of 0.029 ± 0.004 min^-1. Although equilibrium data were well fitted by the Dubinin and Radushkevich isotherm (R² = 0.870; q_DR = 16.6 mg/g), important insights on phenomenological events occurring at equilibrium were concurrently provided by the Lamgmuir one (R² = 0.826; q₀ = 32.6 mg/g; K_L = 0.059 L/mg). FT-IR analysis confirmed that Hg biosorption took place via physisorption. Since C. vulgaris is a fresh-water microalga that can be easily cultivated anywhere, these promising results suggest its possible use as an effective, low-cost biosorbent to treat industrial effluents contaminated by this metal.

Synthesis and Application of Polypyrrole/Fe3O4 Nanosize Magnetic Adsorbent for Efficient Separation of Hg2+ from Aqueous Solution

In order to prepare the magnetic adsorbent, polymerization of pyrrole is performed in a mixture containing Fe3O4 and FeCl3. FTIR, XRD, SEM, EDAX, BET and VSM techniques are employed to characterize the synthesized adsorbent. The results indicate that a homogeneous film of polypyrrole is formed on the surface of magnetic material. The synthesized adsorbent uptakes 173.16 mg g-1 of Hg2+ from aqueous solution, which is superior to the previously reported results for a similar adsorbent. Magnetic performance of the adsorbent is sufficient to separate the used adsorbent from the solution by use of a magnetic bar placed outside of the vessel. Langmuir, Freundlich, Temkin, Redlich-Peterson, and Sips isotherm models are employed to evaluate the experimental adsorption data. The kinetic models are studied and the experimental data are described by the pseudo-second-order kinetic model. The calculated thermodynamic parameter shows that the sorption process is endothermic and spontaneous. Regeneration of the used adsorbent indicates that more than 90% of the initial capacity remains after regeneration.

Effective remediation of low-concentration cadmium in groundwater using nano-scale magnesia

Cadmium (Cd), one of the hazardous elements in groundwater, is a severe threat to human health and ecological systems even at low concentrations. This study explores the effectiveness of commercial and self-synthesized nano-scale magnesia (NMgO) for remediating low-concentration Cd in groundwater as well as their associated removal mechanisms. The sorption kinetic data for both NMgOs were well fitted to the pseudo-second-order model and the calculated q _e values matched the experimental q _e values for both commercial and self-synthesized NMgOs. The sorption equilibrium data for both NMgOs were well fitted to the Langmuir isotherm model, with the maximum Cd sorption capacity (q _e) of 19.25 and 16.54 mg/g at an initial concentration range of 5-200 μg/L and a temperature of 25 °C, for commercial and self-synthesized NMgOs, respectively. The combined sorption kinetics and equilibrium data suggest that the sorption onto both NMgOs follows a monolayer chemisorption. The scanning electron microscope-energy dispersive X-ray (SEM-EDX), Fourier transform infrared spectroscopy (FTIR), and X-ray diffractometer (XRD) analyses show that the chemisorption of Cd onto commercial NMgO is due to the formation of Cd(OH)₂, which precipitates on the sorbent surface. For the self-synthesized NMgO, it was demonstrated that the hydroxyl group plays a role in the chemisorption process and the amount of Cd sorbed on the sorbent was quantified. The results of batch experiments showed that both NMgOs removed Cd effectively, obtaining a removal efficiency of more than 99%, under different experimental conditions of pH, sorbent dosage, co-existing ions, and simulated groundwater. Results from both the sorption isotherm and desorption experiments indicated strong bonding between Cd and both NMgOs, suggesting that NMgOs are safe, effective, and practical sorbents to remediate Cd in groundwater.

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.

Adsorption of mercury(ii) with an Fe3O4 magnetic polypyrrole–graphene oxide nanocomposite

The Fe₃O₄ magnetic polypyrrole–graphene (PPy–GO) has a Langmuir adsorption capacities of 400.0 mg g⁻¹ for Hg(ii). And it has a favorable saturation magnetization of 19.0 emu g⁻¹, easily separated from solutions via additional exterior magnets.

Removal of mercury(II) ions in aqueous solution using the peel biomass of Pachira aquatica Aubl: kinetics and adsorption equilibrium studies

Mercury is a highly toxic substance that is a health hazard to humans. This study aims to investigate powders obtained from the peel of the fruit of Pachira aquatica Aubl, in its in natura and/or acidified form, as an adsorbent for the removal of mercury ions in aqueous solution. The materials were characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis. The infrared spectra showed bands corresponding to the axial deformation of carbonyls from carboxylic acids, the most important functional group responsible for fixing the metal species to the adsorbent material. The thermograms displayed mass losses related to the decomposition of three major components, i.e., hemicellulose, cellulose, and lignin. The adsorption process was evaluated using cold-vapor atomic fluorescence spectrometry (CV AFS) and cold-vapor atomic absorption spectrometry (CV AAS). Three isotherm models were employed. The adsorption isotherm model, Langmuir-Freundlich, best represented the adsorption process, and the maximum adsorption capacity was predicted to be 0.71 and 0.58 mg g(-1) at 25 °C in nature and acidified, respectively. Adsorption efficiencies were further tested on real aqueous wastewater samples, and removal of Hg(II) was recorded as 69.6 % for biomass acidified and 76.3 % for biomass in nature. Results obtained from sorption experiments on real aqueous wastewater samples revealed that recovery of the target metal ions was very satisfactory. The pseudo-second-order model showed the best correlation to the experimental data. The current findings showed that the investigated materials are potential adsorbents for mercury(II) ion removal in aqueous solution, with acidified P. aquatica Aubl being the most efficient adsorbent.

Study of novel mechano-chemical activation process of red mud to optimize nitrate removal from water

Red mud (RM) is the industrial waste of alumina production and causes serious environmental risks. In this paper, a novel activation procedure for RM (mechano-chemical processing) is proposed in order to improve the nitrate adsorption from water. High-energy milling and acidification were selected as mechanical and chemical activation methods, respectively. Synthesized samples of adsorbent were produced considering two parameters of activation: acid concentrations and acidification time in two selected milling times. Optimization of the activation process was based on nitrate removal from a stock solution. Experimental data were analyzed with two-way analysis of variance and Kruskal-Wallis methods to verify and discover the accuracy and probable errors. Best conditions (acceptable removal percentage > 75) were 17.6% w/w for acid concentrate and 19.9 minutes for acidification time in 8 hours for milling time. A direct relationship between increase in nitrate removal and increasing the acid concentration and acidification time was observed. The adsorption isotherms were studied and compared with other nitrate adsorbents. Characterization tests (X-ray fluorescence, X-ray diffraction, Fourier transform infrared spectrophotometry, dynamic light scattering, surface area analysis and scanning electron microscopy) were conducted for both raw and activated adsorbents. Results showed noticeable superiority in characteristics after activation: higher specific area and porosity, lower particle size and lower agglomeration in structure.

A review on progress of heavy metal removal using adsorbents of microbial and plant origin

Heavy metals released into the water bodies and on land surfaces by industries are highly toxic and carcinogenic in nature. These heavy metals create serious threats to all the flora and fauna due to their bioaccumulatory and biomagnifying nature at various levels of food chain. Existing conventional technologies for heavy metal removal are witnessing a downfall due to high operational cost and generation of huge quantity of chemical sludge. Adsorption by various adsorbents appears to be a potential alternative of conventional technologies. Its low cost, high efficiency, and possibility of adsorbent regeneration for reuse and recovery of metal ions for various purposes have allured the scientists to work on this technique. The present review compiles the exhaustive information available on the utilization of bacteria, algae, fungi, endophytes, aquatic plants, and agrowastes as source of adsorbent in adsorption process for removal of heavy metals from aquatic medium. During the last few years, a lot of work has been conducted on development of adsorbents after modification with various chemical and physical techniques. Adsorption of heavy metal ions is a complex process affected by operating conditions. As evident from the literature, Langmuir and Freundlich are the most widely used isotherm models, while pseudo first and second order are popularly studied kinetic models. Further, more researches are required in continuous column system and its practical application in wastewater treatment.

Aqueous mercury adsorption by activated carbons

Due to serious public health threats resulting from mercury pollution and its rapid distribution in our food chain through the contamination of water bodies, stringent regulations have been enacted on mercury-laden wastewater discharge. Activated carbons have been widely used in the removal of mercuric ions from aqueous effluents. The surface and textural characteristics of activated carbons are the two decisive factors in their efficiency in mercury removal from wastewater. Herein, the structural properties and binding affinity of mercuric ions from effluents have been presented. Also, specific attention has been directed to the effect of sulfur-containing functional moieties on enhancing the mercury adsorption. It has been demonstrated that surface area, pore size, pore size distribution and surface functional groups should collectively be taken into consideration in designing the optimal mercury removal process. Moreover, the mercury adsorption mechanism has been addressed using equilibrium adsorption isotherm, thermodynamic and kinetic studies. Further recommendations have been proposed with the aim of increasing the mercury removal efficiency using carbon activation processes with lower energy input, while achieving similar or even higher efficiencies.

Supported copper and cobalt oxides on activated carbon for simultaneous oxidation of toluene and cyclohexane in air

Dispersed nano mixed copper–cobalt oxides bimetallic catalysts were prepared by deposition–precipitation method for simultaneous and complete oxidation of toluene and cyclohexane.