| With the rapid development of economy,both industrial manufacture and human activities discharged lots of mercury into environment,therefore,mercury pollution has become a growing problem and attracted lots of attention worldwide.Adsorption method is recognized as an important approach in mercury treatment due to its unique features such as convenient operation,high effciency and low cost.Among many adsorbents,iron sulfides are considered as excellent adsorbents for mercury adsorption,because they contain lots of sulfur-containing groups,which have high affinity towards Hg(II).In this paper,a comparative evaluation of synthetic FeS and natural pyrite was performed.Afterwards,synthetic FeS was chosen for further study because of its superior performance in mercury adsorption.To overcome the instability of FeS,CMC-FeS,gelatin-FeS,starch-FeS and FeS/Al2O3 were synthesized successfully,which could highly inhibit the agglomeration of FeS and greatly improve the adsorption efficiency.Characterization techniques such as BET,SEM(EDS),TEM,XRD,FTIR and XPS were conducted to investigate the physicochemical properties of synthetic FeS and natural pyrite,and condition experiments of reaction time,initial mercury concentration,pH and competing ions were studied to explore the influence of environmental factors in Hg(II)removal.The characterization results confirmed that adsorption,precipitation,ion exchange and surface complexation were main reaction mechanisms in the mercury adsorption process by iron sulfides.Results of batch tests revealed that FeS had richer porous structure,smaller particle size,larger specific surface area,more active sites,and greater adsorption capacity compared to pyrite.The maximum adsorption capacities of FeS and pyrite were 769.2 mg/g and 9.9 mg/g,respectively.Findings from condition tests suggested that pyrite with low cost and stable structure can be applied as a long-term adsorbing material in the immobilization of low strength Hg(II)wastewater(<1 mg/L),while FeS can be considered as excellent candidate to high-concentration Hg(?)-containing wastewater(nearly 98%removal rate to 20 mg/L Hg(II)by 0.12 g/L FeS).Though synthetic FeS with high reactivity is suitable for adsorption of Hg(II)in water environment,the tiny size and high surface energy make it easy to agglomerate,which limits the full-scale application of FeS.In this work,biomaterial stabilization and physicochemical loading methods were applied to improve the adsorption properties of FeS,both of which could weaken the agglomeration effect of FeS particles and enhance the adsorption efficiency.Different stabilizers such as carboxymethyl cellulose(CMC),gelatin and starch were used to stabilize FeS in water for mercury adsorption.CMC-FeS,gelatin-FeS and starch-FeS were synthesized successfully,which could increase the adsorption efficiency of materials enormously.The removal process of the three materials obeyed pseudo-second-order kinetic model(R2 ? 0.9986)for Hg(II),implying that the rate-limiting step was the chemical sorption process rather than diffusion.Results demonstrated that CMC-FeS,gelatin-FeS and starch-FeS(mass ratio of stabilizer to FeS was 1:6)possessed better adsorption behavior than bare FeS,which showed 1726 mg/g,1939 mg/g and 1989 mg/g adsorption capacities for Hg(II),respectively.It could be seen in TEM images that stabilized-FeS nanoparticles(including CMC-FeS,gelatin-FeS and starch-FeS)were dispersed more uniformly than bare FeS.The influence of coexisting ions and natural organic matter tests showed that high concentration of cations(Cd2+,Pb2+,Cu+,and Ca2+)had little effect on Hg(II)removal efficiency,illustrating that the stabilized-FeS nanoparticles had high selective removal efficiency to Hg(II);Whereas,humic acid(HA)could inhibit the adsorption process obviously by complexation mechanism;The presence of Cl-could increase the ionic strength,enhance the charge transfer,and thus shorten the equilibrium distance.In addition,an Al2O3-supported nanoscale FeS,named FeS/Al2O3 was synthesized to optimize the adsorption performance of material.When the mass ratio of FeS to FeS/Al2O3 nanoparticles was 30%,the specific surface area of material was 142.7 m2/g.FeS could be distributed homogeneously on the surface of Al2O3,which suppressed the aggregation of FeS successfully.The results of adsorption isotherm and kinetics demonstrated that the maximum adsorption capacity of FeS/Al2O3 achieved 313 mg/g,namely 891 mg Hg/g FeS,and the adsorption process was fitted well by pseudo-second-order model(R2 =0.9994).Alkaline environment inhibited mercury uptake strongly,but over the pH range of 3?9,FeS/Al2O3 could maintain a high level(>97.5%)removal rate to 1 mg/L Hg(II).Al2O3 possesses strong moisture adsorption characteristics,which could keep the surrounding air dry,weakening the oxidation rate of FeS effectively.The study of long-term stability confirmed that FeS/Al2O3 could maintain over 95%removal efficiency for 1 mg/L Hg(II)solution when stored for 30 days,much higher than bare FeS(67.3%),which embodyed good long-term stability and prominent application potential of FeS/Al2O3. |
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