Study On Adsorption And Migration Of Cadmium,Copper,Lead In The Iron Oxide And Manganese Oxide

Soil and groundwater are the main carriers of pollutants in the ecological environment.Heavy metal pollution in soil and groundwater will directly affect environmental quality and human health.At present,in-situ treatment is considered to be an efficient,stable and sustainable method for heavy metal treatment,the characteristics of the reaction medium are the key and difficulty points for the treatment of target metal.Taken typical research area in this paper,through field survey,sampling test,preparation of iron oxide and manganese oxide,characteristic representation,single and multi-system adsorption experiments,migration experiment,hydrogeochemical simulation,the adsorption and migration behavior of Cd²⁺,Cu²⁺and Pb²⁺can be fully understood.This research is proved to have theoretical and practical value for the remediation of Cd²⁺,Cu²⁺and Pb²⁺in soil and groundwater.The main conclusions obtained are as follows:（1）Hydrochemical types of shallow groundwater are HCO₃-Ca and SO₄-Ca,about 91.7%of shallow groundwater samples are affected by NO₃^-,and more than80%are polluted by heavy metals.The contents of Cr,Cd,Pb,As,Zn,and Cu in the soil exceed the soil background value of Sichuan Province and the soil baseline value,the exceeded rates of Cr and Cd are higher,77%and 96%respectively.（2）Some adsorbates are prepared,including Na-montmorillonite,Acid-activated montmorillonite,bentonite,Acid-activated bentonite,corn straw biochar,K-Mn O₂,Na-Mn O₂,Mn O₂loaded on bentonite,Mn O₂loaded on biochar,Al-goethite,goethite,biochar loaded on goethite,ferrihydrite,hematite,magnetite,Fe-Mn,and so on.The phase composition and the micro-structure are detected by XRD,SEM,FTIR,and BET,the isoelectric point(p H_IEP)and zero point potential（PZC）of the adsorbates are determined.p H=3-10,the Zeta potential of the manganese oxides are negative.The PZC of K-Mn O₂is about 3.9.（3）The indoor leaching experiment can simulate the natural leaching to a certain extent.Using HYDRUS,one-dimensional solute transport model is established.Five adsorbents including Mn O₂loaded on bentonite,ferrihydrite,goethite,K-Mn O₂and Al-goethite are added into the soil,in the single-system leaching experiment,the fixation capacity for Cd,Cu,and Pb are:K-Mn O₂>Mn O₂loaded on bentonite>ferrihydrite,goethite,Al-goethite;goethite>Mn O₂loaded on bentonite>ferrihydrite,K-Mn O₂,Al-goethite;Mn O₂loaded on bentonite,K-Mn O₂,Al-goethite,ferrihydrite>goethite,respectively.In the multi-system leaching experiment,the concentration of Cd,Cu,and Pb showed a decreasing trend with the increase of soil depth.The migration ability are:Cd>Cu>Pb,the contents of Cd,Cu,and Pb are reduced by about 10-20 times.The fixation capacity for Cd is:Mn O₂loaded on bentonite,ferrihydrite,goethite>Al-goethite>K-Mn O₂,the fixation ability for Cu is not significant,and the fixation ability for Pb is:K-Mn O₂>Mn O₂loaded on bentonite>ferrihydrite,goethite,Al-goethite.The cumulative proportions of the upper,middle and lower layers of Cd are 59.52-87.30%,6.84-31.46%,5.86-15.11%;as for Cu are:37.27-45.67%,24.20-30.10%,26.10-36.68%;as for Pb are:34.41%-49.47%,25.08-34.70%,25.45-34.73%.（4）The absorption rate of Cd²⁺in single system are:K-Mn O₂,Na-Mn O₂（more than 70%）>montmorillonite,Mn O₂loaded on bentonite,Mn O₂loaded biochar,Fe-Mn,hematite,magnetite（about 40-60%）>Acid-activated montmorillonite,bentonite,Acid-activated bentonite,biochar（about 15-30%）.The absorption rate of Cu²⁺are:K-Mn O₂,Na-Mn O₂（70-95%）>montmorillonite,Mn O₂loaded bentonite,Mn O₂loaded biochar（about 40-80%）>Acid-activated montmorillonite,Acid-activated bentonite,biochar,Al-goethite,goethite,goethite loaded biochar（about 15-60%）>ferrihydrite,Fe-Mn,hematite,magnetite（about 5-20%）.The absorption rate of Pb²⁺are:K-Mn O₂,Na-Mn O₂,montmorillonite,Mn O₂loaded bentonite,Mn O₂loaded biochar,Al-goethite,goethite,goethite loaded biochar,ferrihydrite（approximately80-100%）>Acid-activatedmontmorillonite,Acid-activated bentonite,Fe-Mn,hematite and magnetite（about 5-40%）.Langmuir and Freundlich equations are used to fit the adsorption of Cd²⁺,Cu²⁺,Pb²⁺.In general,the adsorption rate of manganese oxide for Cd²⁺,Cu²⁺,Pb²⁺are higher than that of iron oxides,the composite materials have higher adsorption efficiency than that of single materials.The adsorption rates（adsorption capacity）of all adsorbates are:Pb²⁺>Cu²⁺>Cd²⁺.（5）As in the coexistence system of Cd²⁺,Cu²⁺,Pb²⁺,with the concentration increasing,the Cd²⁺absorption rates of K-Mn O₂and Na-Mn O₂decreases from70-90%to 20-30%,absorption rates of Cd²⁺by other materials decreased from10-40%to 1-5%;the Cu²⁺absorption rates of K-Mn O₂and Na-Mn O₂decreases from70-90%to 20-60%,absorption rates of Cu²⁺for other materials decrease from 15-70%to 5-15%;absorption rates of Pb²⁺for K-Mn O₂,Na-Mn O₂,Mn O₂loaded bentoniteand,Mn O₂loaded biochar are 45-90%,absorption rates of Pb²⁺for other materials are about 50%.The selectivity order of iron oxides and manganese oxides to Cd²⁺,Cu²⁺and Pb²⁺is Pb>Cu>Cd.（6）The changing range of the p H _balance/p H _initialfor the system of K-Mn O₂,Cd²⁺(20 mg·L^-1),and phosphate（6 m M or 0.6 m M）is 0.84-1.07,it almost keeps unchanged.The changing range of the p H _balance/p H _initialfor goethite（or ferrihydrite）,Cd²⁺(20 mg·L^-1),phosphate（6m M or 0.6 m M）is 0.99-1.71,there there has a large variety with 0.99-1.92 of p H=2-5.The adsorption process is increasing with adding the phosphate,the adsorption capacity of K-Mn O₂for Cd²⁺is close to 98%at p H=4,the adsorption rate of goethite and ferrihydrite for Cd²⁺are about 10-30%with pH=2-3.5,the adsorption rate suddenly reached over 85%at pH=6.