Mercury (II) adsorption from aqueous solution using nitrogen and sulfur co-doped activated carbon

Study of Mercury [Hg(II)] Adsorption from Aqueous Solution on Functionalized Activated Carbon

Mercury and its compounds are toxic substances, whose uncontrolled presence in the environment represents a danger to ecosystems and the organisms that inhabit in it. For this reason, in this work, we carried out a study of mercury [Hg(II)] adsorption from aqueous solution on functionalized activated carbon. The activated carbons were prepared by chemical activation of a mango seed with solutions of CaCl₂ and H₂SO₄ at different concentrations, later, the carbonaceous materials were functionalized with Na₂S, with the aim of increasing the sulfur content in the carbonaceous matrix and its affinity to mercury. The materials were characterized using: proximal analysis, scanning electron microscopy, Boehm titrations, point zero charge (pH_PZC), and infrared spectroscopy. Additionally, immersion calorimetries were performed in the mercury solution. The results of textural and chemical characterization show materials with low Brunauer-Emmett-Teller (BET) surface areas between 2 and 33 m²·g^-1 and low pore volumes. However, they had a rich surface chemistry of oxygenated groups. The enthalpies of immersion in the mercury solutions are between -31.71 and -77.31 J·g^-1, showing a correlation between the magnitude of the enthalpic data and the adsorption capacity of the materials. It was evidenced that the functionalization process produces a decrease in the surface area and pore volume of the activated carbons, and an increase in the sulfur content of the carbonaceous matrix. It was evidenced that the functionalization process generated an increase in the mercury [Hg(II)] adsorption capacity between 21 and 49% compared to those of the nonfunctionalized materials, reaching a maximum adsorption capacity of 85.6 mgHg²⁺g^-1.

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.

Adsorption of aqueous Hg2+ and inhibition of Hg0 re-emission from actual seawater flue gas desulfurization wastewater by using sulfurized activated carbon and NaClO

The potential impacts of seawater flue gas desulfurization (SFGD) process used in coal-fired power plants have been greatly concerned because the wastewater containing Hg is directly discharged into the ocean environment without proper treatment. Furthermore, the re-emission of Hg as Hg⁰ to the atmosphere from SFGD wastewater caused by the reduction of aqueous Hg²⁺ has also been observed. This study investigated the dependence of Hg²⁺ adsorption behavior for sulfurized activated carbon (SAC) in actual SFGD wastewater on various influencing factors, including initial Hg²⁺ concentration, solution pH, contact time, temperature, and the addition of oxidant (sodium hypochlorite, NaClO). SAC exhibited greater Hg²⁺ adsorption than raw activated carbon at an initial Hg²⁺ concentration of more than 4,723 ng L^-1. The Hg²⁺ removal efficiency of SAC was slightly larger at pH 7.0 and 8.0 than that at pH within 2.0-6.0. Hg²⁺ adsorption on SAC was well correlated with the linear adsorption model. Kinetic analysis results indicate that pseudo-second-order adsorption may serve as the rate-limiting reaction of Hg²⁺ adsorption on SAC. Thermodynamic analyses confirmed the endothermic and spontaneous adsorption behavior of Hg²⁺ on SAC in the seawater environment. Notably, the addition of NaClO significantly reduced the Hg²⁺ removal efficiency when SAC was used as the adsorbent. Nevertheless, NaClO addition also inhibited the reduction reaction of Hg²⁺ to Hg⁰ by forming strong HgCl complexes, which decreased the risk of Hg⁰ reemitted into the atmosphere via a SFGD system.

Efficient adsorption of Hg(II) from aqueous solution by N, S co-doped MnFe2O4@C magnetic nanoparticles

N, S co-doped MnFe₂O₄@C magnetic nanoparticles were successfully synthesized by a simple method involving the preparation of MnFe₂O₄ nanoparticles and subsequent pyrolysis treatment. The physical and chemical properties of MnFe₂O₄, MnFe₂O₄@C and MnFe₂O₄@C-NS nanoparticles were characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), transmission electron microscopy (TEM), N₂ adsorption-desorption and the pH at the point of zero charge. Their performances in the adsorption of Hg(II) from water were investigated. The adsorption process followed pseudo-second-order kinetics and the experimental data of equilibrium isotherms fitted well with the Langmuir model. MnFe₂O₄@C-NS showed the highest adsorption capacity of 108.56 mg/g, increasing more than 1.7 times compared to MnFe₂O₄. The enhanced adsorption performance was attributed to the larger specific surface area as well as the complexation of N and S ligands on the surface. The thermodynamic parameters of ΔH°, ΔS° and ΔG° at 30 °C were -24.39 kJ/mol, -0.046 kJ/mol K and -10.45 kJ/mol, respectively, which indicated that the adsorption of Hg(II) on MnFe₂O₄@C-NS was exothermic and spontaneous in nature. Moreover, MnFe₂O₄@C-NS showed superior selectivity towards Hg(II) compared with other metal ions generally present in mercury-containing industrial wastewater.

Application of bimetallic Al-doped ZnO nano-assembly for heavy metal removal and decontamination of wastewater

In the present study, bimetallic aluminium doped zinc oxide (AZO) nano-assemblies were synthesized for heavy metal removal and disinfection of wastewater. These bimetallic nanoparticles (NPs) were prepared by a simple co-precipitation method and characterized using field emission transmission electron microscopy (FETEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET), a Litesizer, and energy dispersive X-ray spectroscopy (EDS). The AZO NPs was tested for lead removal at various environmental conditions and optimized at pH 4 and 25 °C. The kinetic data were well fitted to the pseudo-second-order model and the process consisted of both surface adsorption and intraparticle diffusion. Al doping enhanced the surface charge of AZO NPs four fold as compared to ZnO, which improved colloidal stability and contributed towards its reusability. AZO NPs exhibited excellent removal efficiency of 86% over three adsorption-desorption cycles. The adsorption was found to be an exothermic and physicochemical process. The prepared AZO NPs were also used to treat a real wastewater sample and found to effectively remove Pb(II) and kill all the bacteria present.

Mercury adsorption and re-emission inhibition from actual WFGD wastewater using sulfur-containing activated carbon

A series of batch experiments were conducted to obtain the optimal adsorption condition for removing aqueous Hg from actual lime-based wet flue gas desulfurization (WFGD) wastewater with sulfur-containing activated carbon (SAC). The experimental results showed that SAC1 had an average 0.32 μg mg^-1 larger aqueous Hg adsorption capacity and 21% larger Hg removal than the CS₂-treated SAC1 (i.e., SAC2) in all tested pH values, confirming that greater sulfur content associated with effective sulfur functional group (i.e., elemental S) caused the larger Hg adsorption capacity. Furthermore, as increasing pH from 4 to 7, the Hg adsorption capacity of SAC1 decreased by 22% (i.e., 0.27 μg mg^-1). The equilibrium Hg adsorption capacity was well fitted with linear and Freundlich adsorption isotherms. Kinetic simulations showed that both pseudo-second order and Elovich equations could well describe the chemisorption behavior of Hg to SAC1. Thermodynamic parameter calculation confirmed that Hg adsorption by SAC1 was thermodynamically spontaneous and exothermic. Re-emission of gaseous Hg markedly decreased by 88% as SO₃^2- addition increased from 0 to 0.01 mM. Notably, by the addition of SAC1, zero re-emission of gaseous Hg was achieved. These experimental results confirm that the capture of aqueous Hg²⁺ and the inhibition of gaseous Hg⁰ re-emission can be successfully and simultaneously achieved in actual WFGD wastewater via the addition of SAC.