Research On Removal Of Chromium From Wastewater By Pectin-stabilized Nanoscale Zero-valent Iron

In recent years,with the development of industrial,wasterwater pollution ofchromium is becoming more and more serious.Nanoscale zero-valent iron (Fe0)has received a great range of attention.Especially when hexvalent chromium(Cr(VI)) remediation by nanoscale zero-valent iron,many ways have occur.Nanoscale zero-valent iron(Fe0) measures particles smaller than100nanometre.The smaller particles,the larger specific surface area and the strong reduction.But these has also led to oxidation and agglomeration easily which diminishingparticle reactivity.According to these characteristics,stabilizers were added toavoid aggregation and to promote the dispersion of the nanoscale zero-valentiron (Fe0),thereby effectively improving nanoscale zero-valent iron (Fe0) for insitu reductive immobilization of Cr(VI) in water.In this paper is reported an efficient method of synthesizing Fe0with astabilizer,the pectin.It was observed by X-ray diffraction(XRD) andtransmission electron microscope(TEM).Hexvalent chromium (Cr(VI))remediation by nanoscale zero-valent iron were investigated by a series of batchexperiments in different reaction conditions. And the kinetics with mechanismof Cr(VI) removal is elucidated.The experimental results are as following:(1) Because the reaction of Cr (VI) by nanoscale zero-valent iron (Fe0)mainly occurs at the surface of iron particles,so the smaller nanoscale zero-valent iron (Fe0) particle,the larger specific surface area,the greater thecontact area,the faster the reaction rate.After addition of stabilizer,pectin,thesteric hindrance and electrostatic repulsion of Fe0can be enhanced.The bestdispersion,the more hard to reunite.In addition,when we use pectin-Fe0,thepectin can also passivate the highly reactive surface from reacting with thesurrounding medium,dissolved oxygen and water.The results show that,underthe experimental conditions,the optimal ratio of pectin: Fe0=5:1.(2) The effect factors of pectin-Fe0system on Cr (VI) removal ratereduction mainly contain:pectin addition,pectin-Fe0addition,initial Cr (VI)concentration,initial pH value,reaction temperature and Ca2+cationsconcentration.The experimental results show that:the higher of pectin-Fe0addition and reaction temperature could make the removal efficiency increase.The rate of Cr (VI) removal reduced with the higher of pH,initial Cr (VI)addition and Ca2+cations concentration.When Cr (VI) addition is20mg/L,thepectin-Fe0is0.12g/L,pH=5.5,T=25℃,the removal rate of Cr (VI) is98.3%after60min.(3) Reduction of Cr (VI) by pectin-Fe0was pseudo-first-order reaction.First-order rate coeffieientk obsincreased with increasing pectin-Fe0additionbut inversely with initial pH.And it increased with increasing nanoscalezero-valent iron addition and reaction temperature but inversely with Cr (VI)concentration and initial pH. In this research,according to the Arrhenius equation,we can obtain the superficial active energy E andk0.They are E=29.7KJ/molandk0=1991.6min-1.(4) The mechanism of Cr (VI) reduction by Fe0was as following,it has twotypes.H and Fe(II) were producted by Fe0corroding.On the one hand,Cr (VI)was removed directly by Fe0and Fe(II).On the other hand, Cr (VI) reacted withatomic H.The product was Cr(VI)&Fe(III) hydroxides. Then the Cr (VI) wasbeen removed.