Study On The Directional Construction Of Novel Nanoadsorbents And The Performance And Mechanism For Mercury Removal

Mercury（Hg）with long-range transport,high biological toxicity and accumulation capability has greatly threatened the ecosystems and human health.The increase of Hg（II）concentration in surface and underground water has caused severe safety problems to drinking water.Therefore,mercury pollution in water matrices have has drawn great concerns in many regions of the world,especially in the rural areas lacking available water treatment facilities.Among the treatment technologies（e.g.,flocculation,electrodeposition,membrane separation）,adsorption holds great promise in Hg（II）removal due to the merits of cost-effectiveness,and ease of operation;however,traditional adsorbents with generally weak adsorption forces do not have sufficient adsorption capacities and selectivity to removal Hg（II）from water to satisfy the stringent drinking water standard.To date,nanoadsorbents have been extensively investigated in Hg（II）removal,whereas their practical applications in water treatment are still restricted by some issues:（1）difficulty in rapid and deep Hg（II）removal,（2）insufficient adsorption capacity and weak selectivity,（3）inferior working performance in complex water matrixes,and（4）poor reusability and bottleneck in practical engineering applications.Moreover,the applications of nanoadsorbents in mercury removal are also hindered by the hydrolabil nature,low utilization rate of active sites,and unclear adsorption mechanisms.To develop the adsorbents with excellent performance in the treatment of Hg（II）-contaminated water,we designed and constructed a series of novel nanoadsorbents for their environmental functions.Based on the current researches on the nanomaterial development,a variety of preparation approaches were employed to synthesize several emerging nanosorbents,and their performance in actual water treatment,adsorption mechanisms,and the applicability in continuous flow mode were comprehensively investigated.Main findings are shown below:1.In order for efficient removal and emergency on-site remediation of Hg（II）in water,multilayered Ti₃C₂O_X nanosheets（M-Ti₃C₂）were designed and successfully synthesized by hydrofluoric acid etching method.The prepared M-Ti₃C₂ confirmed rapid adsorption rate,ultrahigh adsorption capacity(4806 mg g^-1),satisfactory anti-interfering ability to coexisting cations and dissolved organic matters,and an extended working p H range（3.0-12.0）.Density functional theory calculations with various characterizations revealed that this excellent Hg（II）removal performance is ascribed to the adsorption coupled catalytic reduction mechanism.Ti atoms on the（001）facets tend to capture Hg Cl OH,which then suffers homolytic cleavage to generate radical species（e.g.,·OH and·Hg Cl）,and eventually forms crystalline Hg₂Cl₂.Over～95%of high-purity Hg₂Cl₂ can be recycled by a facile thermal treatment.Ascribing to the generated·OH and heat released during the catalysis,M-Ti₃C₂ could be oxidized to Ti O₂/C nanocomposites,which showed better performance on the photo-degradation of organic pollutants than commercial Degussa P25.2.According to hard-soft acid-base theory,Zr-based MOF nanocrystals（i.e.,Zr-MSA）rich in alkyl mercaptan with high affinity for mercury were synthesized via one-step hydrothermal strategy.X-ray diffraction,inductively coupled plasma spectroscopy and hydrogen nuclear magnetic resonance spectroscopy indicated that the Zr-MSA had high crystallinity and obvious nanoparticle agglomeration,and its molecule was determined Zr₆O₄（OH）₄（HOOCCH₂CHSHCOOH）_5.7（HCOOH）_0.6.As evidenced by the adsorption experiment results,the Zr-MSA exhibited rapid removal rate,high adsorption capacity(425.5 mg g^-1),excellent acid resistance,superb mercuric selectivity,and regeneration capability.The above studies confirm that Zr-MSA can be used as the ideal foundation nanomaterial for the construction of novel high-performance nanocomposite adsorbent.3.To promote the practical water treatment applications of powdered MOF,a facile and environmentally benign dropping method was proposed to construct Zr-MSA into the cross-linking networks formed by calcium alginate and polyvinyl alcohol.The granular MOF beads（GMBs）were uniform in size（～1.76 mm）,and has high adsorption capacity(431.1 mg g^-1),fast adsorption kinetics(D_s～5.58×10^-11 m²s^-1),and broad working p H range（1.0-11.0）.Moreover,the adsorbent exhibited great anti-DOMs interference and excellent adsorption selectivity for Hg（II）(K_d～2.5×10⁷m L g^-1).Benefiting from the mechanical strength and Water researchistance,the GMBs maintained great mechanical integrity and 99.0%removal efficiency after 20cycles.Combined characterizations and experiments suggested that the proton exchange accompanied by Hg-S coordination mechanisms should account for the exceptional selectivity toward Hg（II）.Furthermore,the full-scale performance of GMBs under different operation conditions was systematically predicted and evaluated using the validated pore diffusion model.A short bed adsorber experiment was designed to verify the reliability of the pore diffusion model in predicting continuous flow breakthrough curves,and the applicability of GMBs in full-scale systems under different working conditions was systematically evaluated by using the model.4.To further improve the adsorption performance of MOF powder and promote its application in point-of-use devices and industrial water treatment,a dipping-drying strategy was developed to fabricate the thiol-laced MOF sponge monolith（TLMSM）by steadily loading Zr-MSA onto melamine networks.Molecular dynamic simulations suggested that the introduction of poly（vinylidene fluoride）has improved the interactions between Zr-MSA and substrate,which is crucial for the high stability and homogeneity of TLMSM.Benefiting from the more exposed active sites and interpenetrating networks,the TLMSM exhibited a superior adsorption rate for Hg（II）adsorption than Zr-MSA,and its adsorption capacity was increased by 1.6 times to954.7 mg g^-1.Moreover,the adsorbent showed superb selectivity toward Hg（II）(K_d～5.0×10⁷ m L g^-1),wide operating p H range（1.0-11.0）,good resistance to DOMs and coexisting cations/anions,and excellent cyclic reusability（removal efficiency>90.0after 25 cycles）.TLMSM also maintained satisfying Hg（II）removal ability in natural water and industrial wastewater scenarios.Continuous flow tests indicated that～1485 bed volumes of simulated Hg（II）-contaminated tap water(0.5 mg L^-1)can be effectively treated to meet drinking water standard,confirming the great potential of TLMSM in household devices.DFT calculations combined with advanced characterization tools systematically unveiled the formation of single-layer–S-Hg-Cl,and double-layer–S-Hg-O-Hg-Cl and–S-Hg-O-Hg-OH complexes during adsorption processes,which is the key to greatly enhance the adsorption capacity of TLMSM.