Effectiveness And Mechanism Studies Of Carbon-based Materials For The Mercury (?) Removal In Aqueous Phase

The heavy metal pollutants existed in the waterbody are with the typical characteristics of easy migration, persistence, bioaccumulation and biotoxicity etc., and are becoming a global issue nowadays. Most of the conventional and recent advancements in the heavy metal wastewater purification technologies are chemical precipitation, ion exchange, adsorption, membrane filtration, coagulation and flocculation, flotation and electrochemical treatment etc. Compared with other new technologies, the adsorption method still is widely applied owing to the low cost and easily available of raw materials, widely compatible with various pollutants, bio-friendly and nontoxic. And within various adsorbents, the carbonaceous materials showed significant status in the adsorption technology because of the intrinsic characteristics like a vast array of types, easily to conduct the chemical modifications and normally with high specific surface area. On the other hand, in viewing of the major compositions and the pollution conditions of industrial wastewater, mercury is the most serious one to take under control. Thus herein, the Hg²⁺ was adopted as the adsorbate, three types of widely used raw commercial activated carbons including peach stone activated carbon?PSAC?, coal activated carbon?CAC?, coconut husk activated carbon?CHAC?, and other three kinds of graphene carbon based materials, the reduced graphene oxide?RGO?, the chromium and cobalt doped graphene?RGO/Cr and RGO/Co? were used as adsorbent to investigate the adsorbateadsorbent interactions.The unmodified conventional commercial activated carbon and three types of graphene derivatives were used to investigate the removal efficiencies of Hg²⁺, and the possible related adsorption mechanisms and comparisons of these materials were studied as well in present study. The major research contents and results are shown as follows:?1? The characterization of three kinds of unmodified commercial raw activated carbonsThe characterization results were obtained via the scanning electron microscope?SEM?, the Fourier transform infrared spectroscopy?FTIR?, the measurement of the point of zero charge?p Hpzc?, the isoelectric point?IEP?, the X-ray diffraction spectra?XRD?, the X-ray photoelectron spectra?XPS?, the surface pore size distribution and surface specific analyzer?BET?, showing that the main composition of the three absorbents includes hydroxyl, sp2-bonded carbon skeleton and some alkoxy contained compounds, more negatively charged exterior surface than the interior part of all three types of activated carbons, which normally is favorable for the adsorption of metallic ions. The XRD spectra of three materials showed that the crystallinity degreeof CAC was the lowest one, which indicated that the nanographic structure of CAC was more amorphous. The results of NLDFT model indicated that the surface pore width of PSAC was the narrowest one with more proportion of micropores, while CAC with more proportion of mesopores. The XPS results showed that the CAC presented higher degree of oxidation and there was no apparent mercury reduction observed.?2? The removal efficiencies of three kinds of commercial activated carbons on Hg²⁺The results showed that the theoretical maximum monolayer adsorption capacities of three materials in Langmuir model at the initial Hg²⁺ concentration of 40 mg/L pH 4 were 59.47 mg/g, 48.92 mg/g and 44.98 mg/g, respectively. The nonlinear simulation results of Langmuir, Freundlich, Dubinin-Radushkevich?D-R?, Tempkin, Redlich-Peterson?R-P? and Toth model showed high correlations with the original data. The original data of kinetic experiments was well described by the pseudo-first-order and intra-particle diffusion model, which meant the physisorption process might be the predominant part in adsorption.?3? The chromium and cobalt doped reduced graphene oxide?RGO/Cr, RGO/Co? were synthesized by means of the reduction property of RGO and the oxidation property of transition metal salts. The results were obtained via the characterization of SEM, FTIR, the particle diameter in solution and the contact angle, showing that the RGO/Cr exhibited higher specific surface area, the utmost particle diameters of RGO/Cr and RGO/Co in solution were 4767.5 nm and 5308.7 nm, and the contact angles of RGO/Cr and RGO/Co were 131.8±0.65° and 143.6±1.29°, separately. The hydrophilic property was greatly enhanced.?4? The removal efficiency study of RGO, RGO/Cr and RGO/Co showed thatthe premium loading of Co²⁺ and Cr6+ on 50 mg of RGO were 0.01 mmol/L and 0.5 mmol/L, respectively, and the corresponding adsorption capacities were 0.98 mg/g and 3.345 mg/g, the corresponding removal efficiencies were 18.56% and 63.35%. Premium pH value was obtained at 7 with corresponding adsorption capacity of 5.228 and 5.205 mg/g for RGO/Cr and RGO/Co.The theoretical monolayer adsorption capacities of RGO, RGO/Cr and RGO/Co in Langmuir model were 128.98, 181.86 and 146.86 mg/g, and the adsorption processes of three materials were simulated well with the pseudo-first-order and pseudo-second-order kinetic model, the equilibrium adsorption was achieved within 2³ h.