Mercury removal by porous sulfur copolymers: Adsorption isotherm and kinetics studies

Biogeochemical dynamics and sustainable remediation of mercury in West African water systems

Pollution of environmental drinking water sources by mercury (Hg) in West Africa is challenging, with the need to develop strategies to understand its biogeochemical transformation and mitigation to provide clean drinking water void of Hg. This review evaluated the biogeochemical cycle of Hg in West African ecology and the mitigation of Hg contamination of drinking water sources in the West African region. The study revealed Hg-bearing mineral deposits and artisanal and small-scale gold mining as major sources of Hg in West African environment. West African countries must develop sustainable methods for removing Hg from water. However, bioremediation (including microbial and phytoremediation) and adsorption are promising methods for purifying Hg-contaminated environmental drinking water sources in West Africa. Microorganisms such as Arbuscular mycorrhizal, E. coli, Fusobacterium sp, Trichoderma viride, Gliocladium arborescens, Bascillus sp. and Brevibacterium cysticus have demonstrated the capacity to remediate Hg from the water system. Furthermore, plant species like Paspalum conjugatum, Cyperus kyllingia, and Lindernia crustacea revealed exciting capacity as phytoremediators of Hg. Activated carbon, clay and mineral clays are abundant resources in West Africa that can function as adsorbents for removing Hg during water treatment. However, future studies should focus on optimizing the field-scale application of bioremediation and adsorption methods as mitigation strategies and their long-term benefits in West Africa. It is essential that the government in West Africa fund initiatives and programmes that support the accomplishment of the Minamata Convention agreement, which favours the attainment of the sustainable development goal (SDG-6).

Inversely Vulcanized Porous Copolymer Monolith for Efficient Removal of Metal Ions from Water

Heavy metal pollution poses a significant threat to the environment and human health. Developing efficient adsorbent materials for removing these contaminants from water is crucial. Sulfur-based polymers derived from inverse vulcanization were found to be promising candidates for the adsorption of several heavy metals from water. In this work, polysulfides bearing β-diketone functional groups are synthesized for the first time through the inverse vulcanization between acetoacetoxyethyl methacrylate (AAEMA) and sulfur. Successful copolymerization and the incorporation of the acetoacetate moiety into the polymer chain are confirmed by NMR, FT-IR, Raman, XPS, XRD, DSC, and TGA analyses. A porous variant of the adsorbent is made using a facile template-assisted method exploring NaCl as a porogen. The porous polymer is analyzed using X-ray micro-CT and SEM to get insights into the internal porous architecture as well as the surface porosity of the adsorbent. Energy-dispersive X-ray (EDAX) and CHNS(O) analyses of the adsorbent reveal a sulfur-rich surface with uniformly distributed oxygen and carbon. Heavy metal adsorption studies of the synthesized polymer are conducted in a multielement solution containing different types of metal ions. The porous adsorbent exhibited remarkable heavy metal chelation properties, with a removal efficiency of 100% for Hg2+ and 72-96% removal for Cr3+, Pb2+, Co2+, Fe3+, Ni2+, Ag+, and Cu2+. The synergistic interplay between the thiol functional groups, polysulfide loops, and the strong binding affinity of the β-diketone moiety and hydroxyl group coupled with the polymer's well-defined porous structure collectively enhances its adsorption capacity toward heavy metal ions. A prototype of an identical porous adsorbent in monolithic form has been constructed. The porous monolith retains all the benefits of porous adsorbent in particulate form while allowing the separation of the adsorbent from the treated water by simply lifting it as a single unit, eliminating the necessity for filtration as is necessary with particulate adsorbents.

Novel synthesis and application of biochar for controlling release and exposure of mercury in the farmland: From human health risk perspective

Mercury (Hg)-contaminated farmlands have received wide attention because of the adverse risks posed to food security and human health. In addition, climate change altered the mobility of Hg in the soil, limiting soil productivity and nutrient bioavailability, hence elevating health risks. To adapt to these risks, pot experiments were employed to showcase the impacts of single-pyrolytic synthesized biochar with nitrogen and phosphorus impregnation (NPBC) on the nutrient accessibility, Hg immobilization, and human health risks, compared to pristine and control groups. Results revealed that, with increased surface area and abundant function groups, impregnation amplified bulk nitrogen, phosphorus, and oxygen content from 0.47, 0.25, and 9.47 % to 3.01, 4.50, and 21.4 %, respectively. The pot experiments indicated the effectiveness of NPBC900 in immobilizing soil Hg, hence reducing Brassica rapa chinensis' Hg uptake by 88 %. Notably, NPBC transformed ∼93 % of water soluble and exchangeable Hg species to stable fractions, enhancing the residue concentration three-fold higher than the control. Additionally, NPBC700-900 showcased characteristic phosphorus and nitrogen slow-release (best at NPBC900 and NPBC500, respectively; 5 %) contributing to controlled soil available nutrients. Hg bioaccessible fraction exhibited a notably higher level (1.7 mg kg-1) in the control group measured against BC (0.8 mg kg-1) and NPBC treatments (∼0.1 mg kg-1). Through dietary and soil ingestion pathways, NPBC900 treatment demonstrated the best health risk reduction for farmers and the public by ∼93 and 69 %, respectively. With versatile capabilities, NPBC emerges as a practical, green, and sustainable alternative in Hg remedy technologies, a breakthrough for climate change adaptation.

Complete Valorization of Cashew Nutshell Waste Enriched with Sulfur Copolymer for Efficient Mercury Removal

Integrating sustainable raw materials with efficient synthesis is key to advancing eco-friendly solutions. Renewable feedstocks like cashew nutshells (CNS) and elemental sulfur, an industrial byproduct, are underutilized resources. This study presents a simple method to valorize CNS and sulfur, creating a copolymer composite designed for efficient mercury removal from contaminated water.

Active Polymers Decorated with Major Acid Groups for Water Treatment: Potentials and Challenges

Polymers exhibiting ion-conduction capabilities are essential components of water-purifying devices. These polymers not only transport selective ions but are also mechanically robust; thus, they can be processed as membranes. In this review, we highlight major acidic polymers and their engineered morphologies and optimized properties, including metal selectivity and water permeation or retention. Crucial phenomena, such as self-assembly in acid-group-functionalized polymers for driving water transportation, are discussed. It was observed that the phosphonic acid groups containing polymers are rather suitable for the selective adsorption of toxic metals, and thus, are superior to their sulfonated counterparts. Additionally, due to their amphoteric nature, phosphonated polymers displayed several modes of metal complexations, which makes them appropriate for eliminating a wide range of metals. Further observation indicates that aromatic-acid-functionalized polymers are more durable. Temperature- and pH-responsive polymers were also found to be promising candidates for a controlled water-treatment process. Nevertheless, considering the morphology, water retention, and metal adsorption, acid-functionalized polymers, especially phosphonated ones, have the potential to remain as the materials of choice after additional advancements. Further perspectives regarding improvements in acidic polymers and their fabricated membranes for water treatment are presented.

Mercury adsorption study and DFT calculations of sulfur-containing biomass composites prepared by inverse vulcanization

This study introduces an innovative and cost-effective biomass adsorbent, the sulfur/cardanol/potato starch composite (SCP), synthesized through inverse vulcanization for the remediation of mercury-contaminated waters. The SCP was characterized using Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS), Fourier Transform Infrared Spectroscopy (FT-IR), Thermogravimetric Analysis (TGA) and X-ray Diffraction (XRD) to confirm its composition, morphology, and properties. The adsorption capacity of SCP for Hg(II) was 246.88 mg/g with a removal rate of 98.75 %. Adsorption kinetics followed a pseudo-second-order model, indicating chemisorption as the dominant mechanism, while adsorption isotherms were best described by the Langmuir model. Thermodynamic studies confirmed the adsorption process as spontaneous and endothermic. Density Functional Theory (DFT) calculations further elucidate the interaction mechanisms between mercury and the adsorbent, revealing that the thiyl radicals play a crucial role in the adsorption process. The SCP also exhibited a high selectivity for Hg(II) over other co-existing ions and maintains an adsorption capacity over 223.93 mg/g after five regeneration cycles, thus promising practical applications in environmental mercury management.

Mechanistic Evidence for Hg Removal from Wastewater by Biologically Produced Sulfur

A significant quantity of biologically produced sulfur (BPS) is generated as a by-product of chemical and biological desulfurization processes applied to landfill gas treatment. The beneficial upcycling of BPS has seen limited use in the environmental context. The effectiveness and underlying mechanism of BPS as an adsorbent for removing Hg2+ from both solution and wastewater were elucidated based on experiments encompassing surface characterization, adsorption isotherms, kinetics, and thermodynamics. The BPS exhibited remarkable efficacy in removing Hg2+ from solution, with the Langmuir model accurately describing the adsorption process and showing a maximum adsorption capacity of 244 mg g-1. Surface analysis through X-ray photoelectron spectroscopy and scanning electron microscopy revealed that Hg2+ complexed with sulfide on BPS surfaces, forming stable HgS. The adsorbed Hg was strongly retained in BPS, with less than 0.2% of the adsorbed Hg desorbed by strong acids. Adsorption kinetics followed the double-exponential first-order model, showing an initial rapid adsorption phase wherein 75% of the initial Hg2+ was removed within 5 min, followed by a slower adsorption rate. The thermodynamic parameters suggested that adsorption of Hg2+ by BPS was a spontaneous and endothermic process. Additionally, BPS effectively removed Hg2+ from wastewater, showing preference for Hg over other co-existing metals. These findings underscore the potential of BPS as an effective adsorbent for Hg2+ removal from wastewater.

Enhanced wastewater bioremediation by a sulfur-based copolymer as scaffold for microalgae immobilization (AlgaPol)

In recent years, there has been an increasing concern related to the contamination of aqueous ecosystems by heavy metals, highlighting the need to improve the current techniques for remediation. This work intends to address the problem of removing heavy metals from waterbodies by combining two complementary methodologies: adsorption to a copolymer synthesized by inverse vulcanization of sulfur and vegetable oils and phytoremediation by the microalga Chlorella sorokiniana to enhance the metal adsorption. After studying the tolerance and growth of Chlorella sorokiniana in the presence of the copolymer, the adsorption of highly concentrated Cd²⁺ (50 mg L^-1) by the copolymer and microalgae on their own and the combined immobilized system (AlgaPol) was compared. Additionally, adsorption studies have been performed on mixtures of the heavy metals Cd²⁺ and Cu²⁺ at a concentration of 8 mg L^-1 each. AlgaPol biofilm is able to remove these metals from the growth medium by more than 90%. The excellent metal adsorption capacity of this biofilm can be kinetically described by a pseudo-second-order model.

The recent development of inverse vulcanized polysulfide as an alternative adsorbent for heavy metal removal in wastewater

Inverse vulcanized polysulfides have been used as low-cost and effective adsorbents to remediate heavy metals in wastewater. Inverse vulcanization introduces sustainable polysulfide synthesis by solving the rapid desulfurization problem of unstable polysulfides, and provides superior performance compared to conventional commercial adsorbents. The review discussed the brief applications of the inverse vulcanized polysulfides to remove heavy metal wastewater and emphasized the modified synthesis processes for enhanced uptake ratios. The characteristics of polysulfide adsorbents, which play a vital role during the removal process are highlighted with a proper discussion of the interaction between metal ions and polysulfides. The review paper concludes with remarks on the future outlook of these low-cost adsorbents with high selectivity to heavy metals. These polysulfide adsorbents can be prepared using a wide variety of crosslinker monomers including organic hydrocarbons, cooking oils, and agro-based waste materials. They have shown good surface area and excellent metal-binding capabilities compared to the commercially available adsorbents. Proper postmodification processes have enabled the benefits of repetitive uses of the polysulfide adsorbents. The improved surface area obtained by appropriate choice of crosslinkers, modified synthesis techniques, and regeneration through post-modification has made inverse vulcanized polysulfides capable of removing.

Modelling mercury sorption of a polysulfide coating made from sulfur and limonene

A polymer made from sulfur and limonene was used to coat silica gel and then evaluated as a mercury sorbent. A kinetic model of mercury uptake was established for a range of pH values and concentrations of sodium chloride. Mercury uptake was generally rapid from pH = 3 to pH = 11. At neutral pH, the sorbent (500 mg with a 10 : 1 ratio of silica to polymer) could remove 90% of mercury within one minute from a 100 mL solution containing 5 ppm HgCl₂ and 99% over 5 minutes. It was found that sodium chloride, at concentrations comparable to seawater, dramatically reduced mercury uptake rates and capacity. It was also found that the spent sorbent was stable in acidic and neutral media, but degraded at pH 11 which led to mercury leaching. These results help define the conditions under which the sorbent could be used, which is an important advance for using this material in remediation processes.

Microwave-assisted functionalization of PAN fiber by 2-amino-5-mercapto-1,3,4-thiadiazol with high efficacy for improved and selective removal of Hg2+ from water

Mercury (Hg²⁺) contamination in water is associated with potential toxicity to human health and ecosystems. Many research studies have been ongoing to develop new materials for the remediation of Hg²⁺ pollution in water. In this study, a novel thiol- and amino-containing fibrous adsorbent was prepared by grafting 2-amino-5-mercapto-1,3,4-thiadiazol (AMTD) onto PAN fiber through a microwave-assisted method. The synthesized functional fiber was characterized by FTIR, SEM, and elemental analysis. Adsorption tests depicted that for mercury uptake, PAN_MW-AMTD fiber exhibited enhanced adsorption capacity compared with other fibrous adsorbents and selective adsorption feature under the interference of other metal ions, including Pb²⁺, Cu²⁺, Cd²⁺, and Zn²⁺. The influence of pH on the adsorption process was investigated and the effect of temperature revealed that the adsorption sorption process was endothermic and the adsorption performance of PAN_MW-AMTD was elevated with the increase of temperature. Kinetic studies of PAN_MW-AMTD fiber followed the pseudo-second-order and the adsorption isotherm of Hg²⁺ was well fitted by Sips and Langmuir equations, given the maximum adsorption amount of 332.9 mg/g. XPS results suggested that a synergetic coordination effect of sulfur and nitrogen in functional fiber with mercury took responsibility for the adsorption mechanism in the uptake process. In addition, the prepared PAN_MW-AMTD fiber could easily be regenerated with 0.1 M HCl for five times without significant reduction of mercury removal efficiency. Thus, this study will facilitate the research on novel functional material for the removal of mercury from water.

Adsorption Evaluation for the Removal of Nickel, Mercury, and Barium Ions from Single-Component and Mixtures of Aqueous Solutions by Using an Optimized Biobased Chitosan Derivative

In this experimental study, the use of 5-hydroxymethyl-furfural (HMF) organic compound as a grafting agent to chitosan natural polymer (CS) was examined. One optimized chitosan derivative was synthesized, and then tested (CS-HMF), in order to uptake nickel, mercury, and barium metal ions from single- and triple-component (multi-component) aqueous solutions. The characterization of the material before and after the metal uptake was achieved by scanning electron microscopy (SEM). The ability of the adsorption of CS-HMF was tested at pH = 6. The adjusting of temperature from 25 to 65 °C caused the increase in the adsorption capacity. The equilibrium data were fitted to the models of Langmuir and Freundlich, while the data from kinetic experiments were fitted to pseudo-1st and pseudo-2nd order models. The best fitting was achieved for the Langmuir model (higher R²). The adsorption capacity for nickel, mercury, and barium removal at 25 °C (single component) was 147, 107, and 64 (mg/g), respectively. However, the total adsorption capacity for the multi-component was 204 mg/g. A thermodynamic study was also done, and the values of ΔG⁰, ΔH⁰, and ΔS⁰ were evaluated.