Adsorption of mercury ions from wastewater by a hyperbranched and multi-functionalized dendrimer modified mixed-oxides nanoparticles

Synthesis and characterization of a lactose-based biosurfactant by a novel nanodendritic catalyst and evaluating its efficacy as an emulsifier in a food emulsion system

Nonionic lactose fatty acid esters are a class of synthetic biosurfactants with various uses in the food, pharmaceutical, personal care, and cosmetic industries. The objective of this research was the preparation and full characterization of a series of novel metallic encapsulated magnetic core/dendrimer shell composites as catalysts (CoII/MnII G2.0L1/2@SCMBNP) and their use in the chemo- and regioselective synthesis of a biosurfactant for the first time. Surface-active properties (such as contact angle (CA), surface tension (SFT), interfacial tension (IFT), critical micelle concentration (CMC), hydrophilic-lipophilic balance (HLB), foamability (FA) & foam stability (FS), emulsion ability (EmA) & emulsion stability (EmS), surface excess (Γ) and free energy of adsorption (ΔG) were also determined for all synthesized biosurfactants. In comparison to other works, these results suggested that the synthesized lactose fatty acid esters have potential application as synthetic emulsifiers featuring surface properties and are comparable with Ryoto sugar ester L-1695 (sucrose laurate) & Tween-20 (polysorbate 20) as industrial emulsifiers. The optimized conditions for biosurfactant syntheses are 8 days at 2 : 1 molar ratio of lactose sugar to lauric acid at 50 °C. Lactose ester as a biosurfactant exhibited a decrease of SFT & IFT and was able to stabilize a 20% soybean O/W emulsion. Furthermore, high conversion & yield, excellent chemo- and regioselectivity, and high operational stability over 5 runs were achieved for CoII/MnII-G2.0L1/2@SCMBNP, indicating the suitable efficiency of the catalytic process.

Heavy metal pollution and the role of inorganic nanomaterials in environmental remediation

Contamination of water and soil with toxic heavy metals is a major threat to human health. Although extensive work has been performed on reporting heavy metal pollutions globally, there are limited review articles on addressing this pernicious phenomenon. This paper reviews inorganic nanoparticles and provides a framework for their qualities required as good nanoadsorbents for efficient removal of heavy metals from water. Different inorganic nanoparticles including metals, metal oxides and metal sulfides nanoparticles have been applied as nanoadsorbents to successfully treat water with high contaminations of heavy metals at concentrations greater than 100 mg l^-1, achieving high adsorption capacities up to 3449 mg g^-1. It has been identified that the synthesis method, selectivity, stability, regeneration and reusability, and adsorbent separation from solution are critical parameters in deciding on the quality of inorganic nanoadsorbents. Surface functionalized nanoadsorbents were found to possess high selectivity and capacity for heavy metals removal from water even at a very low adsorbent dosage of less than 2 g l^-1, which makes them better than conventional adsorbents in environmental remediation.

Selective and efficient removal of Hg (II) from aqueous media by a low-cost dendrimer-grafted polyacrylonitrile fiber: Performance and mechanism

Polyacrylonitrile fiber was successfully modified with triazine-based dendrimer via grafting method as a promising adsorbent for removal of mercury species from aqueous media. The prepared adsorbent was characterized by elemental analysis, scanning electron microscope, Fourier transform infrared spectroscopy, porous structure analysis and X-ray photoelectron spectroscopy, providing the evidence of successful fabrication. The adsorption conditions were found via varying pH, dosage, coexisting substances, contact time, temperature and concentration. Adsorption performance, described better by the pseudo-second-order kinetics with intraparticle diffusion as rate controlling step and Langmuir isotherm model, indicated a chemisorption process with the maximum Langmuir adsorption amount of 227.64 mg g^-1 for mercury ions. Thermodynamically, adsorption of mercury ions was spontaneous and endothermic. Desorption and regeneration experiments demonstrated that it could be reused in five successive adsorption cycles without significant loss of its original performance. Experimental data and density functional theory calculation disclosed the coordination geometries and chelating mechanism between the adsorbent and mercury ions. The proposed study would provide a new prospect for the purification of mercury in aqueous system by functionalizing commercial polyacrylonitrile fiber with dendrimers.

Mercury Removal from Wastewater Using Cysteamine Functionalized Membranes

This study demonstrates a three-step process consisting of primary pre-filtration followed by ultrafiltration (UF) and adsorption with thiol-functionalized microfiltration membranes (thiol membranes) to effectively remove mercury sulfide nanoparticles (HgS NPs) and dissolved mercury (Hg2+) from wastewater. Thiol membranes were synthesized by incorporating either cysteine (Cys) or cysteamine (CysM) precursors onto polyacrylic acid (PAA)-functionalized polyvinylidene fluoride membranes. Carbodiimide chemistry was used to cross-link thiol (-SH) groups on membranes for metal adsorption. The thiol membranes and intermediates of the synthesis were tested for permeability and long-term mercury removal using synthetic waters and industrial wastewater spiked with HgS NPs and a Hg2+ salt. Results show that treatment of the spiked wastewater with a UF membrane removed HgS NPs to below the method detection level (<2 ppb) for up to 12.5 h of operation. Flux reductions that occurred during the experiment were reversible by washing with water, suggesting negligible permanent fouling. Dissolved Hg2+ species were removed to non-detection levels by passing the UF-treated wastewater through a CysM thiol membrane. The adsorption efficiency in this long-term study (>20 h) was approximately 97%. Addition of Ca2+ cations reduced the adsorption efficiencies to 82% for the CysM membrane and to 40% for the Cys membrane. The inferior performance of Cys membranes may be explained by the presence of a carboxyl (-COOH) functional group in Cys, which may interfere in the adsorption process in the presence of multiple cations because of multication absorption. CysM membranes may therefore be more effective for treatment of wastewater than Cys membranes. Focused ion beam characterization of a CysM membrane cross section demonstrates that the adsorption of heavy metals is not limited to the membrane surface but takes place across the entire pore length. Experimental results for adsorptions of selected heavy metals on thiol membranes over a wide range of operating conditions could be predicted with modeling. These results show promising potential industrial applications of thiol-functionalized membranes.

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.

Microfluidics combined with ionic gelation method for production of nanoparticles based on thiol-functionalized chitosan to adsorb Hg (II) from aqueous solutions

This work aimed at producing nanoparticles (NPs) based on thiol-functionalized chitosan (CS) using capillary microfluidic (MF) device combined with ionic gelation method to adsorb mercury ion [Hg (II)] from aqueous solutions. In this line, CS was functionalized with epichlorohydrin/cysteaminium chloride (2.73 M ratio) followed by fabricating NPs via MF and bulk mixing (BM) methods. To characterize the morphology, zeta potential, functionality, structure, and magnetic property of the samples, a series of tests such as SEM, TEM, DLS, FTIR, XRD, and VSM were carried out, respectively. The obtained results showed that MF technique was able to produce NPs with a diameter as small as 18 ± 3 nm, and a uniform shape compared to BM method. Thiol groups (-SH) functionalization on CS surface was confirmed by appearing a characteristic peak at 2579 cm^-1. Also, the XRD patterns indicated the appropriate synthesis of Fe₃O₄ (magnetite), and no change in the structure of CS NPs in the presence of magnetite. Moreover, adding the magnetite to thiol-functionalized CS NPs led to suitable saturation magnetization about 26 emu/g to facilitate their separation using a magnetic field. To evaluate the performance of the nanoadsorbent, it has been exposed to Hg (II) in an aqueous solution which in turn the parameters optimization for the adsorption was done via Box-Behnken design (BBD) method, exhibiting the effect of adsorbent dose and the initial concentration of Hg (II) was much more significant than that of pH. Different concentrations of total dissolved solids up to 1000 mg/L had no adverse impact on the adsorption process confirmed by EDAX spectra. The least value of RMSE (5.023) and χ² (0.3) were observed for Redlich-Peterson, Radke-Prausnitz, and UT isotherms. Maximum adsorption capacities calculated using Langmuir and UT models were 1192 mg/g and 1126 mg/g, respectively. Thermodynamic studies demonstrated that the nature of the adsorption process was spontaneous and endothermic. Recovery of nanoadsorbent was successfully carried out using HCl 0.5 mol/L. The adsorption studies revealed that the prepared nanoadsorbent is promising candidate used in mercury removal from a real wastewater potentially.

Carboxymethyl cellulose thiol-imprinted polymers: Synthesis, characterization and selective Hg(II) adsorption

Sulfur containing ion imprinted polymers (S-IIPs) were applied for the uptake of Hg(II) from aqueous solution. Cysteamine which was used as the ligand for Hg(II) complexation, was grafted along the epichlorohydrin crosslinked carboxylated carboxymethyl cellulose polymer chain through an amide reaction. The adsorption ability of S-IIPs towards Hg(II) was investigated by kinetic and isotherm models, which, corresponding, showed that the adsorption process followed a pseudo-second-order, fitted well with the Langmuir isotherm with a maximum adsorption capacity of 80 mg/g. Moreover, thermodynamic studies indicated an endothermic and spontaneous reaction with the tendency of an enhanced randomness at the surface of the S-IIPs with temperature increases. S-IIPs indicated a high degree of selectivity towards Hg(II) in the presence of Cu²⁺, Zn²⁺, Co²⁺, Pb²⁺ and Cd²⁺. Furthermore, the efficiency of S-IIPs was also evaluated against real samples showing 86.78%, 91.88%, and 99.10% recovery for Hg(II) wastewater, ground water and tap water, respectively. In this study, the adsorbent was successfully regenerated for five cycles, which allows for their reuse without significant loss of initial adsorption capability.

Colorimetric determination of Hg2+ in environmental water based on the Hg2+-stimulated peroxidase mimetic activity of MoS2-Au composites

A colorimetric assay is described for sensitive determination of Hg²⁺ ions based on the MoS₂-Au composites as peroxidase mimetics, which are synthesized by microwave-assisted solvothermal method. The addition of Hg²⁺ stimulates their peroxidase-like activity, along with lower Michaelis constant toward the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with H₂O₂, allowing the composites for direct determination of Hg²⁺. A broad linear response is obtained ranging from 20 nM to 20 μM with a detection limit (LOD) of 5 nM. The superior peroxidase-like activity is attributed to the large surface area of MoS₂ nanosheets and the synergistic catalytic effect of MoS₂ and Au. The Hg²⁺-stimulation effect implies the strong interaction between Hg²⁺ and MoS₂-Au, where the XPS results confirm the presence of metallic Hg⁰, indicative of an Au-Hg amalgam. This colorimetric assay is successfully applied for the determination of Hg²⁺ in environmental water (tap water and Yellow River water) with excellent selectivity over interfering cations.

Chitosan-Thiobarbituric Acid: A Superadsorbent for Mercury

In the present investigation, chitosan (CH) was supramolecularly cross-linked with thiobarbituric acid to form CT. CT was well characterized by UV, scanning electron microscopy-energy-dispersive X-ray analysis, Fourier transform infrared, NMR, differential scanning calorimetry, thermogravimetric analysis, and X-ray diffraction analyses, and its adsorption potential for elemental mercury (Hg⁰), inorganic mercury (Hg²⁺), and methyl mercury (CH₃Hg⁺) was investigated. Adsorption experiments were conducted to optimize the parameters for removal of the mercury species under study, and the data were analyzed using Langmuir, Freundlich, and Temkin adsorption isotherm models. CT was found to have high adsorption capacities of 1357.69, 2504.86, and 2475.38 mg/g for Hg⁰, Hg²⁺, and CH₃Hg⁺, respectively. The adsorbent CT could be reused up to three cycles by eluting elemental mercury using 0.01 N thiourea, inorganic mercury using 0.01 N perchloric acid, and methyl mercury with 0.2 N NaCl.