六价铬在土壤中的吸附特性及风险评价研究

发布时间：2018-09-11 15:00

【摘要】：随着重金属污染的日益加剧，其对人类与动植物造成了巨大的危害。铬作为环境中常见的重金属，其危害性较强。铬在土壤的存在形式主要为Cr(III)与Cr(VI)，其中Cr(III)毒性较小，在土壤中的迁移能力较差，而Cr(VI)的毒性较大，其可通过食物链进入到动植物体内，在环境中的生物降解性较差，并且具有一定的致癌能力。六价铬在土壤中的迁移能力较强，其容易通过包气带进入到地下环境中。因此，研究土壤对六价铬的吸附性，并对其进入地下水的风险进行评估，具有重大的意义。本文通过静态吸附实验，研究了pH，有机质，铁铝氧化物，六价铬浓度，介质粒径对六价铬吸附的影响。通过动力学与热力学分析，对土壤吸附六价铬的特性进行了研究，阐明了土壤对六价铬的吸附机理。并利用多元回归分析研究了六价铬分配系数（Kd）与各影响因素之间的关系，提出了六价铬污染地下水的风险评估模型并进行风险评价。本文的研究内容和结论主要有：（1）不同铁氧化物（针铁矿、赤铁矿、水铁矿和磁铁矿）、氧化铝和腐殖酸均可对六价铬的吸附产生影响，且影响规律相似，即均随着pH的升高，各自吸附六价铬的能力降低。通过测定各自的电荷零点（PZC）后，发现在2pHPZC时，能够吸附六价铬，而当pHPZC时，基本不吸附六价铬。说明当2pHPZC时，土壤中上述物质含量越高，六价铬污染地下水的风险就越低；实验研究了不同pH、初始六价铬浓度和介质粒径对土壤吸附六价铬的影响，发现当2pH8时，随pH增加土壤吸附六价铬的能力降低，六价铬污染地下水的风险增强。初始六价铬浓度在10~100mg/L时，土壤对六价铬的吸附量随六价铬浓度的增大而增加，当六价铬浓度大于100mg/L时，随六价铬初始浓度的增大，土壤对六价铬的吸附量不再增加，说明随六价铬浓度的升高，可能会增加其对地下水污染的风险。随介质粒径的增大，其对六价铬的吸附能力减弱，导致六价铬污染地下水的风险升高；通过动力学与热力学分析了土壤对六价铬的吸附特性。由动力学分析结果可知，土壤对六价铬的吸附满足拟二级动力学方程，说明六价铬吸附未达到平衡时，其Kd较小对地下水的污染风险较高。由热力学分析结果可知，，土壤对六价铬的吸附能够自发进行（△G00），且为吸热反应（△H00），即温度升高，土壤对六价铬的吸附能力增加，六价铬污染地下水的风险降低；本文阐述了土壤吸附六价铬阴离子的两种机制，即专性吸附与非专性吸附，最终实验确定了土壤对六价铬的吸附主要为专性吸附（化学吸附），当六价铬存在形态为HCrO4-时，对地下水的污染风险低。（2）利用SPSS软件进行多元逐步回归分析，以pH、六价铬浓度、土壤有机质含量、氧化铁含量、氧化铝含量、粘粒含量为自变量，六价铬分配系数（Kd）为因变量，进行回归拟合后得到的回归方程为Kd=2.082+0.007C-0.204pH，R2=0.927，说明六价铬的吸附分配系数主要与六价铬浓度和pH条件有关。建立了六价铬对地下水污染风险的评估模型并进行风险评价，将六价铬对地下水的污染分为从轻微到极强的五种不同风险程度。
[Abstract]:As a common heavy metal in the environment, chromium is more harmful. The main forms of chromium in the soil are Cr (III) and Cr (VI), of which Cr (III) is less toxic, its migration ability in the soil is poor, and Cr (VI) is more toxic, which can be passed through food. Hexavalent chromium has a strong migration ability in the soil, and it is easy to enter the underground environment through aeration. Therefore, it is of great significance to study the adsorption of hexavalent chromium in the soil and evaluate the risk of its entry into groundwater. Righteousness.
In this paper, the effects of pH, organic matter, Fe-Al oxides, hexavalent chromium concentration and medium size on the adsorption of hexavalent chromium were studied by static adsorption experiments. The adsorption characteristics of hexavalent chromium in soils were studied by kinetic and thermodynamic analysis, and the adsorption mechanism of hexavalent chromium in soils was clarified. In this paper, a risk assessment model for groundwater contaminated by hexavalent chromium is proposed and the risk assessment is carried out.
(1) Different iron oxides (goethite, hematite, hydrous iron and magnetite), alumina and humic acid all have similar effects on the adsorption of hexavalent chromium, that is, their ability to adsorb hexavalent chromium decreases with the increase of pH. HPZC does not absorb hexavalent chromium basically. It shows that the higher the above-mentioned substance content in soil is, the lower the risk of hexavalent chromium contamination in groundwater is. The effects of different pH, initial hexavalent chromium concentration and medium size on the adsorption of hexavalent chromium by soil are studied experimentally. The risk of groundwater contamination increases. When the initial hexavalent chromium concentration is 10-100mg/L, the adsorption capacity of hexavalent chromium increases with the increase of hexavalent chromium concentration. When the hexavalent chromium concentration is higher than 100mg/L, the adsorption capacity of hexavalent chromium does not increase with the increase of the initial hexavalent chromium concentration. The risk of hexavalent chromium contamination in groundwater is analyzed by kinetics and thermodynamics. The results show that the adsorption of hexavalent chromium satisfies the quasi-second-order kinetic equation. The results of thermodynamic analysis show that the adsorption of hexavalent chromium can be spontaneously carried out (G00), and is an endothermic reaction (H00), that is, the adsorption capacity of hexavalent chromium increases with the increase of temperature, and the risk of hexavalent chromium contaminating groundwater decreases. In this paper, two mechanisms of adsorption of hexavalent chromium anion by soils, i.e. specific adsorption and non-specific adsorption, are described. Finally, it is determined that the adsorption of hexavalent chromium by soils is mainly specific adsorption (chemical adsorption). When hexavalent chromium exists in HCrO4-form, the risk of groundwater pollution is low.
(2) Multiple stepwise regression analysis was carried out with SPSS software. pH, hexavalent chromium concentration, soil organic matter content, ferric oxide content, alumina content, clay content as independent variables, and hexavalent chromium partition coefficient (Kd) as dependent variables. The regression equation was Kd=2.082+0.007C-0.204pH, R2=0.927, indicating the adsorption and distribution of hexavalent chromium. The coefficient is mainly related to the concentration of hexavalent chromium and pH condition.The risk assessment model of hexavalent chromium to groundwater pollution is established and the risk assessment is carried out.
【学位授予单位】：吉林大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：X53;X825

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