| Along with social economy development, heavy metals, such as Cr(VI), Cu(II),Pb(II), Zn(II), Cd(II), etc, are discharged into surface water increasingly. They arenondegradable and tend to accumulate along the food chain and cause harm to humanbody, finally. Pb(II), as one of most toxic heavy metals, exists in the wastewater ofmany industries. The compounds of Pb(II) can enter human body through therespiratory tract, the digestive tract and skin, and accumulate in the body. They maydamage the human nervous system, hematopoietic system, digestive system, immunesystem, kidney and cardiovascular, etc.MBFGA1is a kind of microbial flocculant harvested from the fermentationliquid of Paenibacillus polymyxa GA1. This paper used MBFGA1to treat leadwastewater, and the influences of the main factors of microbial flocculantsflocculating efficiency were investigated. On this basis, the adsorption behavior of theflocculation process and lead removal mechanism were studied.During the process of preliminary experiment, it could be found that theefficiency of MBFGA1handling of Pb(II) is not ideal with only a dosing. On the basisof further investigation, found that if using segmented addition, the efficiency can begreatly improved.210-3%(w/w) MBFGA1was added before rapid mixing (firststage) and410-3%(w/w) after two minutes’ slow mixing (second stage). The effectof CaCl2, pH, temperature and mixing time on the flocculation adsorption of Pb(II)ions onto MBFGA1was investigated. When the dose of CaCl2was410-3%(w/w),pH was8, mixing combinations were1minute rapid mixing at150rpm and30minutes slow mixing at40rpm, the removal efficiency of Pb(II) reached up to99.85%.Through the adsorption behavior study of Pb(II) removal using MBFGA1, theadsorption was mainly in slow mixing phase. The results indicated that Pb (II)adsorption could be described by the Langmuir adsorption model, and being themonolayer capacity negatively affected with an increase in temperature. Theadsorption process could be described by pseudo-second-order kinetic model,supported the assumption that the rate limiting step in adsorption of heavy metals arechemisorption involving valence forces through the sharing or exchange of electronsbetween adsorbent and metal ions. The mechanism investigation of Pb(II) removal using MBFGA1was by Fouriertransform-infrared spectra (FT-IR) analysis, environmental scanning electronmicroscope (ESEM) and energy spectrum (EDS) analysis, Zeta potential and ionicbond analysis. The results indicated that the main Pb(II) removal mechanism in thefirst stage was charge neutralization. MBFGA1had a large number of functionalgroups, which had strong capacity for removing Pb (II). It could seize OH-competitively, making the molecules stretch form a linear structure so that more ofthe active binding sites emerged. With the increase of pH, OH-in solution increased.The removal capacity was bigger at higher pH with more OH-provided. Ca2+reducedthe thickness of the diffuse double layer of adjacent colloids, thus reducing the interparticle distance and making MBFGA1attract more Pb (II) ions around its surface.The main Pb(II) removal mechanism in the second stage was adsorption, in whichMBFGA1acted like a bridging agent of many flocs adsorbed Ca2+. Pb(II) ionsadsorpted to MBFGA1by ion exchange of carboxyl, hudroxy, azyl, amide goroup,etc. The hydrogen bondings between the associated hydroxyls, made the flocculantsolubility increased and molecular chain at full stretch, thus facilitated bridgingformation, made small floc particles formed interconnected larger.In a word, MBFGA1was an microbial flocculant of ffective and safe, easydegradation, no second pollution, which has a far-reaching prospects on the treatmentof wastewater containing heavy metals. |
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