Mercury abatement in the environment: Insights from industrial emissions and fates in the environment

Mercury's neurotoxic effects have prompted the development of advanced control and remediation methods to meet stringent measures for industries with high-mercury feedstocks. Industries with significant Hg emissions, including artisanal and small-scale gold mining (ASGM)-789.2 Mg year-1, coal combustion-564.1 Mg year-1, waste combustion-316.1 Mg year-1, cement production-224.5 Mg year-1, and non-ferrous metals smelting-204.1 Mg year-1, use oxidants and adsorbents capture Hg from waste streams. Oxidizing agents such as O3, Cl2, HCl, CaBr2, CaCl2, and NH4Cl oxidize Hg0 to Hg2+ for easier adsorption. To functionalize adsorbents, carbonaceous ones use S, SO2, and Na2S, metal-based adsorbents use dimercaprol, and polymer-based adsorbents are grafted with acrylonitrile and hydroxylamine hydrochloride. Adsorption capacities span 0.2-85.6 mg g-1 for carbonaceous, 0.5-14.8 mg g-1 for metal-based, and 168.1-1216 mg g-1 for polymer-based adsorbents. Assessing Hg contamination in soils and sediments uses bioindicators and stable isotopes. Remediation approaches include heat treatment, chemical stabilization and immobilization, and phytoremediation techniques when contamination exceeds thresholds. Achieving a substantially Hg-free ecosystem remains a formidable challenge, chiefly due to the ASGM industry, policy gaps, and Hg persistence. Nevertheless, improvements in adsorbent technologies hold potential.

Co2+-adsorbed chitosan-grafted-poly(acrylic acid) hydrogel as peroxymonosulfate activator for effective dye degradation

In this work, chitosan-grafted-poly(acrylic acid) (CS-g-PAA) was synthesized for use as a Co2+ adsorbent and circularly utilized as a peroxymonosulfate (PMS) activator in the degradation of rhodamine B (RhB) dye. CS-g-PAA demonstrated 3.7 times higher adsorption capacity toward Co2+ than pristine chitosan. The impact of the adsorption conditions was evaluated. The pseudo-second-order kinetic model and the Langmuir isotherm model best described the adsorption process. Under optimum conditions, the adsorption capacity of CS-g-PAA for Co2+ was 212 mg/g. The Co2+-adsorbed CS-g-PAA hydrogel was further utilized in the RhB degradation process. The effects of catalyst dosage, initial RhB concentration, pH, and the coexistence of anions on the degradation of RhB were studied. The hydrogel catalyst could remove 98 % of RhB within 5 min, at a degradation rate of 0.624 per min. Electron paramagnetic resonance (EPR) analysis and the radical scavenger experiment suggested that SO4•-, HO•, 1O2, and O2•- were involved in the degradation. Furthermore, when tested in various water systems, high degradation efficiencies of 98 % were attained after 20 min. The hydrogel catalyst performed excellent degradation over ten cycles without any chemical recovery processes. Moreover, high degradation efficiencies were observed between 95 % and 98 % when tested with other dyes.

Chitosan-grafted hydrogels for heavy metal ion adsorption and catalytic reduction of nitroaromatic pollutants and dyes

Chitosan-grafted-poly(acrylic acid) (CS-g-PAA) and chitosan-grafted- poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (CS-g-P(AA-co-AMPS)) hydrogels were synthesized and then employed as adsorbents for the effective removal of Cu2+ and other heavy metal ions. The effect of hydrogel's composition on the Cu2+ adsorption was explored. The CS-g-PAA hydrogel demonstrated a superior adsorption capacity compared to pristine CS, PAA hydrogel, and CS-g-P(AA-co-AMPS) hydrogels. The adsorption followed the Langmuir isotherm model, and the pseudo-second order kinetic model. Additionally, the CS-g-PAA hydrogel exhibited relatively high adsorption performances toward Cr3+, Co2+, Ni2+, Pb2+, and Zn2+. Metal ions adsorbed within CS-g-PAA hydrogels underwent reduction to their corresponding metallic states and were reutilized as catalysts for the reduction of 4-nitrophenol. The comparative catalytic performances of the metal species in the hydrogel were in the order of Cu > Ni > Co > Zn. The reduction efficiency of Cu-CS-g-PAA increased with increased catalyst dosage, NaBH4 concentration, and temperature. A very low activation energy of 3.7 kJ/mol was observed. The catalyst maintained high catalytic performance even when subjected to real water samples and proved its reusability for up to three cycles. Moreover, the catalyst could effectively reduce 2-nitrophenol and methyl orange.

Study on the Dye Removal from Aqueous Solutions by Graphene-Based Adsorbents

In the current investigation, the removal efficiency regarding a cationic dye, methylene blue (MB), from three graphene-based materials was investigated. The materials' characterization process involved instrumental methods such as XRD, XPS, SEM, TEM, FTIR, and nitrogen adsorption at 77 K. The survey examined how various process factors influenced the ability of the studied materials to adsorb cationic dyes. These parameters encompassed contact time, initial dye concentrations, solution pH, and temperature. The adsorption procedure was effectively explained through the application of pseudo-second-order and Langmuir models. The maximum adsorption capacity for the best adsorbent at 293 K was found to be 49.4 mg g-1. In addition, the study also determined the entropy, enthalpy, and Gibbs free energy values associated with the removal of MB and showed that the adsorption of MB is endothermic, feasible, and spontaneous. The results also revealed that the studied materials are suitable adsorbents for the removal of cationic dyes.

Composite beads from chitosan and zeolitic imidazolate framework-8 for the adsorption and photocatalytic degradation of reactive red 141

This study describes the fabrication of composite beads comprising chitosan and zeolitic imidazolate framework-8 (ZIF-8) as a natural biodegradable dye adsorbent and support for ZnO photocatalyst. Chitosan beads were cross-linked with trisodium citrate dihydrate to enhance the adsorption capacity for the reactive red 141 dye (RR141). The ability was further improved by adding ZIF-8. The optimum loading was 2.5%, and the adsorption equilibrium was reached within 2 h. The maximum adsorption capacity of the composite beads was 6.51 mg g-1 at pH 4 when an initial concentration of 1000 mg L-1 was used. The pseudo-second-order kinetics model and the Langmuir isotherm model best described the adsorption process. The composite beads could also adsorb dyes like reactive black, Congo red, direct yellow, reactive orange, rhodamine B, crystal violet, and methylene blue (MB). Thermal stability was significantly improved after coating the surface of the 2.5% ZIF beads with a ZnO photocatalyst. After UV irradiation for 5 h, the photocatalytic beads containing 2.59 weight percent of ZnO could decolorize 99% of MB and 90% of RR141 dyes with a degradation rate of 0.6032 h-1 and 0.3198 h-1, respectively. Furthermore, the photocatalytic beads remained effective for at least ten consecutive cycles.

Synthesis, characterization, and application of pristine and clay-templated carbon xerogel microspheres for removing diclofenac and heavy metals from water solution

Organic xerogel microspheres (SX) were synthesized by inverse emulsion sol-gel polymerization and carbonized to obtain carbon xerogel spheres (SXCs). The catalyst was K₂CO₃ or Fe(C₂H₃O₂)₂, and the clay sodium sepiolite (SNa) or exfoliated vermiculite (V_exf) was added during the synthesis. Depending on the catalyst and clays, the SXCs were designated SXC-K, SXC-Fe, V_exf-K, V_exf-Fe, SNa-Fe, and SNa-K. At pH = 7 and T = 25 °C, the SXCs' adsorption capacities towards diclofenac (DCF) in water increased as follows: SXC-K < V_exf-Fe < SXC-Fe < SNa-Fe < SNa-K < V_exf-K and this order is associated with the SXCs' surface area and mesopore volume. The V_exf-K displayed the highest capacity for DCF due to its optimal textural and chemical properties, and the DCF maximum uptake was 560 mg/g at pH = 6 and T = 35 °C. The adsorption capacity towards Cd²⁺ and Pb²⁺ decreased as SX-K > SX-Fe > SXC-K > SXC-Fe, indicating that the non-carbonized materials (SX) presented higher adsorption capacity than the SXCs because the SXs had a higher acidic site content. Adding SNa or V_exf to SXs enhanced the adsorption capacity towards Cd(II), and SNa-SX-K presented an exceptionally high capacity of 182.7 mg/g. This synergistic effect revealed that the Cd²⁺ was adsorbed on the SX-K acidic sites and by cation exchange on the SNa.

Reduced Graphene Oxide-Zinc Sulfide Nanocomposite Decorated with Silver Nanoparticles for Wastewater Treatment by Adsorption, Photocatalysis and Antimicrobial Action

Reduced graphene oxide nanosheets decorated with ZnS and ZnS-Ag nanoparticles are successfully prepared via a facile one-step chemical approach consisting of reducing the metal precursors on a rGO surface. Prepared rGO-ZnS nanocomposite is employed as an adsorbent material against two model dyes: malachite green (MG) and ethyl violet (EV). The adsorptive behavior of the nanocomposite was tuned by monitoring some parameters, such as the time of contact between the dye and the adsorbent, and the adsorbent dose. Experimental data were also simulated with kinetic models to evaluate the adsorption behavior, and the results confirmed that the adsorption of both dyes followed a pseudo 2nd order kinetic mode. Moreover, the adsorbent was also regenerated in a suitable media for both dyes (HCl for MG and ethanol for EV), without any significant loss in removal efficiency. Ag doped rGO-ZnS nanocomposite was also utilized as a photocatalyst for the degradation of the selected organic contaminant, resorcinol. The complete degradation of the phenolic compound was achieved after 60 min with 200 mg of rGO-ZnS-Ag nanocomposite under natural sunlight irradiation. The photocatalytic activity was studied considering some parameters, such as the initial phenol concentration, the photocatalyst loading, and the pH of the solution. The degradation kinetics of resorcinol was carefully studied and found to follow a linear Langmuir-Hinshelwood model. An additional advantage of rGO-ZnS and rGO-ZnS-Ag nanocomposites was antibacterial activity against Gram-negative bacterium, E. coli, and the results confirmed the significant performance of the nanocomposites in destroying harmful pathogens.

A Review on Polyacrylonitrile as an Effective and Economic Constituent of Adsorbents for Wastewater Treatment

Water gets polluted due to the dumping of untreated industrial waste into bodies of water, particularly those containing heavy metals and dyes. Industrial water contains both inorganic and organic wastes. Numerous adsorbents that are inexpensive and easily available can be used to address the issue of water deterioration. This review report is focused on polyacrylonitrile as an efficient constituent of adsorbents to extract toxic ions and dyes. It discusses the various formulations of polyacrylonitrile, such as ion exchange resins, chelating resins, fibers, membranes, and hydrogels, synthesized through different polymerization methods, such as suspension polymerization, electrospinning, grafting, redox, and emulsion polymerization. Moreover, regeneration of adsorbent and heavy metal ions makes the adsorption process more cost-effective and efficient. The literature reporting successful regeneration of the adsorbent is included. The factors affecting the performance and outcomes of the adsorption process are also discussed.

Highly efficient poly(6-acryloylamino-N-hydroxyhexanamide) resin for adsorption of heavy metal ions

Heavy metal wastewater pollution has become an ecological challenge worldwide. This study reports the development of a novel poly (6-acryloylamino-N-hydroxyhexanamide) (PAHHA) resin for effective adsorption of heavy metal ions, including Cu²⁺, Pb²⁺ and Ni²⁺. The chelating resin was synthesized by the grafting reaction between 6-amino-N-hydroxyhexanamide and polyacrylic resin, thus containing the hydroxamate and acylamino groups. The batch adsorption experiments revealed that the PAHHA resin exhibited an excellent adsorption performance for Cu²⁺, Pb²⁺ and Ni²⁺. The maximum adsorption capacities of Cu²⁺, Pb²⁺ and Ni²⁺ were determined to be 238.59, 232.48 and 115.77 mg·g^-1, respectively. Based on the adsorption kinetics, the pseudo-second-order kinetic model was noted to fit well for all metal ions. The metal ion concentration as a function of the equilibrium adsorption capacity fitted well with the Langmuir isotherm, thus indicating the single layer adsorption process. The adsorption mechanism was investigated by using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS) and adsorption isotherms. It was revealed that the PAHHA resin possessed multiple active sites, including -CONHOH, -CONH- and -COOH, which could strongly adsorb the metal ions. Specifically, the -CONHOH group displayed a high affinity by forming a stable five-membered ring with heavy metal ions. Overall, the developed resin exhibits advantages such as simple synthesis, inexpensive raw material and good recyclability, along with high adsorption ability, thus providing a new approach for efficiently treating wastewater contaminated with heavy metal ions.

Facile synthesis and adsorption characteristics of a hybrid composite based on ethyl acetoacetate modified chitosan/calcium alginate/TiO2 for efficient recovery of Ni(II) from aqueous solution

In this study, chemical modification of chitosan has been carried out using epichlorohydrin as crosslinking agent and ethyl acetoacetate as a modifier to graft acetoacetyl moiety. The said organo-functionalization on chitosan and sodium alginate not only offered a novel support for TiO2 immobilization but also enhanced sorption performance for Ni(II) recovery from aqueous medium. So, a composite consisting of acetoacetyl moiety grafted chitosan, sodium alginate and titanium oxide (EAA-MCS/TiO2) was prepared and characterized by fourier transform-infra red (FT-IR) spectroscopy and scanning electron microscopy (SEM). The hybrid composite (3EAA-MCS/TiO2) which had TiO2 to EAA-MCS mass ratio of 20.0% by weight showed maximum sorption efficiency. The formulated sorbent was conditioned in the form of hydrogel beads for operation. Isothermal sorption and kinetics studies were performed at pH = 6.0 to configure the nature of sorption. Pseudo-2nd order rate expression better explained the sorption kinetics and chemisorption is the predominant mode of uptake. Langmuir adsorption model better explained the sorption process (R 2 ∼ 0.99) and maximum monolayer sorption capacity (q m ) at sorption/desorption dynamic equilibrium was computed as 403 mg/g at optimized pH.

Cr(VI) visualization via transmittance of electrorheological display medium with core/shell polystyrene/polyvinyltetrazole microspheres

In the course of time, significant amounts of heavy-ion pollutants have been dispersed into the environment. Rapid on-site detection of heavy metal ions is crucial to monitor their dispersion in the nascent stages. In this study, 2.2-μm-diameter polystyrene microspheres (PSM) were synthesized via emulsifier-free polymerization to coat polyacrylonitrile (PSM@PAN) and form core/shell-structured microspheres. Core/shell polystyrene/polyvinyltetrazole (PSM@PVT) microspheres were obtained after a cyano-to-tetrazole conversion reaction, loaded in an electrorheological device (ERD) display constructed using two indium tin oxide glasses with a spacer seal. The ERD loading dispersed the microsphere solution by scattering light through the ERD, resulting in a low transmittance in the absence of an alternating electric field (AEF). Particles in the fluid medium were polarized to induce negative and positive charges at each end of the particles under the AEF, and the resultant particle chains enhanced transmittance. The optimal frequency to generate the highest degree of particle chaining in the presence of an AEF is defined as its characteristic frequency (Fc), which also serves as an indicator to identify the shell materials. The Fc of PSM@PVT shifted from 350 kHz to 30 kHz after adsorbing Cr(VI) from the PVT coating. Transmittance of the ERD loading of PSM@PVT with Cr(VI) increased linearly with the concentration of Cr(VI). Approximately 40 ng mL^-1 of the limit of detection was calculated in the linear range of 10-540 ng mL^-1. The Fc of the PSM@PVT adsorbing the Cr(VI) was not influenced by Na(I), K(I), Ca(II), Mg(II), Fe(III), and Zn(II) coexisting in the ERD.

Removal of Cadmium (II) from Aqueous Medium Using Vigna radiata Leave Biomass: Equilibrium Isotherms, Kinetics and Thermodynamics

In the present study, a novel biosorbent Vigna radiata leaves biomass (L. biomass) was utilized for cadmium (II) extraction from aqueous medium. Cadmium (II) free and cadmium (II) loaded L. biomass was analyzed by Fourier transform infrared (FTIR) spectroscopy. Adsorption of cadmium (II) from aqueous medium was studied under various conditions such as adsorbent dose, agitation time, pH and temperature of the medium to optimize the process variables. Different models including Langmuir, Freundlich, Temkin and Dubinin–Radushkevich (DR) were used to elaborate the insight of adsorption process. Best interpretation of biosorption process was given by Langmuir model. Value of maximum adsorption capacity (qm) calculated from Langmuir isotherm model was found to be 13.44 mg/g. Results indicated the establishment of physical interaction between cadmium (II) ions and functional groups of L. biomass. Kinetic study for adsorption of cadmium (II) ions on L. biomass was done by applying pseudo first order, pseudo second order, elovich and intra-particles diffusion models. Biosorption process best followed the pseudo second order kinetics. Value of standard Gibbs energy (ΔG°) and standard enthalpy change (ΔH°) showed the feasibility, spontaneity and endothermic nature of adsorption process. Percentage removal efficiency of L. biomass for cadmium (II) was successfully maintained for four cycles. Biomass has a potential to be used as an efficient adsorbent for the removal of cadmium (II) from different polluted water samples.

Amidoxime-modified chitosan for pigment red 224 enrichment through reversible assembly

An amidoxime-modified chitosan, featuring favorable porosity and super-lipophilic properties, was successfully prepared for pigment red 224 enrichment.