The Research On Efficiency And Mechanism Of Trace Heavy Metals In Water Removed By Manganese Dioxide Formed In Situ

Heavy metal pollution accident happens frequently in recent years in China,heavy metal pollution of surface water become more sever. The heavy metalspresent in the aquatic environment are of great concern from an environmentalperspective because of their toxicity, accumulation in living beings andnon-biological degradation. The conventional water treatment process can notremove heavy metals in water efficiently, the treatments which are widely used inministry are not in point when the volume of solution is large and the concentrationof heavy metal is low, water quality is difficult to meet “Drinking Water HealthStandards”(GB5749-2006). Absorption has proven to be efficient method due to itssimplicity, easy scale-up and high efficency over a wide concentration range.However, the adsorbents usually suffer from some inherent problems such as lowadsorption capacity and selectivity. Therefore, there is an increasing demand fordeveloping more effective adsorbents to remove trace heavy metals from water. Inthis study, using manganese dioxide formed in situ as adsorbent, research influenceof removal efficiency by various factors, and the primary mechanisms has beendiscussed.In our study, selecting different types of oxidizing agents and reducing agents,to prepare manganese dioxide formed in situ(MOF) by reacting MnSO4, MnCl2,Na2S2O3with KMnO4andNaOCl. Through comparising the removal efficiency andanalyzing the characterization, it is seen that the best way to get MOF with highefficiency is reacting MnSO4with KMnO4.According to the performance analysis of heavy metal removal by MOF, theadsorption capacity of MOF is found to be more significant than several adsorbentsin literatures. The maximum adsorption capacity estimated with Langmuir model is625.00mg/g for Pb(Ⅱ),138.66mg/g for Cd(Ⅱ),104.21mg/g for Ni(Ⅱ),81.60mg/gfor Zn(Ⅱ) and240.49mg/g for Cu(Ⅱ). The uptake of heavy metals onto MOFapproach to equilibrium quickly. The removal of heavy metals increases withincreasing MOF dose and solution pH. The presence of Mg2+has no significantinfluence on heavy metal removal. The presence of Ca2+can reduce the removalefficiency within a limited extent. NO3-, Cl-andSO42-do not affect the adsorption, however the presence of CO32-, PO43-, SiO32-and B4O72-can promote the removalefficiency of MOF effectively. Different kinds of heavy metals do not have the sameaffinity with MOF. The results indicated that MOF has the highest selectivity onPb(Ⅱ). Finallly, the elimination of heavy metal in simulated water demonstratedthat this approach for heavy metal removal has a great potential application inenvironmental protection, especially in accidental pollution event.XRD analysis shows that there may be manganese dioxide crystals formed when MOFwas generated, the removal of heavy metal is the result of adsorption and coprecipitation.FTIR analysis shows that the characteristic peak of Mn-O has shifted after adsorption, thisphenomenon is related to inner-sphere complex formation or replacement between the targetand Mn, in the forrming of forming Pb-O, Cd-O, Ni-O,Zn-O and Cu-O. XPS analysisindicated that heavy metal uptake onto MOF can shift the characteristic peak ofheavy metal, which is also lead to inner-sphere complex formation.