The Research Of Adsorption Characteristics Of Hexavalent Chromium In Soil And Risk Assessment

With heavy metal pollution is creasingly intensifying, more harmful effects onpeople, plants and animals have been caused. Chromium was one of common heavymetals in the environment, which has heavy harmfulness. The forms of chromium insoil mainly are Cr (III) and Cr (VI). Cr (III) has less toxicity and its migration abilitywas poor in soil. But Cr (VI) is toxic, and it could enter animals and plants throughthe food chain. Meanwhile it has poor biodegradability and certain ability of causingcancer. Usually, the migration ability of hexavalent chromium is strong in soil and it’seasy to through the vadose zone into the underground environment. Therefore, it isgreat significance to research adsorption of hexavalent chromium and evaluate therisk of the possible into groundwater.Through static adsorption experiments, effects of pH, organic matters, ironoxides, aluminum oxides, hexavalent chromium concentration and medium grain sizeon the adsorption of hexavalent chromium were studied in this paper. Cr (VI)adsorption properties of the soil were studied to clarify the mechanism of adsorptionof hexavalent chromium in soil by kinetics and thermodynamic analysis, andilluminate the adsorption mechanism of hexavalent chromium in soil. After that,analyzing the relationship between hexavalent chromium distribution coefficient (Kd)and the various factors by multiple regression, and proposing risk assessment methodof hexavalent chromium contamination of groundwater. The main research contentsand conclusions of this paper are as follows:(1) Different iron oxides (goethite, hematite, ferrihydrite and magnetite), aluminaand humic acid could have an impact on the adsorption of hexavalent chromium. Andeffect rules that their adsorption ability for hexavalent chromium reduced with theincreasing of pH are similar. By measuring their different point of zero charge (PZC),we found that, hexavalent chromium can be adsorbed at2<pH<PZC, and whilepH>PZC, hexavalent chromium is hardly adsorbed; It means that when the pH in theenvironment is more than2and less than their PZC, the higher the content of the eachof above-mentioned matters in the soil is, the lower the risk of hexavalent chromiumpollution of groundwater is; the influences of different pH, initial concentration of hexavalent chromium and medium diameter on adsorption of hexavalent chromium insoil were studied through experiments and found that when2<pH <8, with theincrease of pH value of soil adsorption ability of hexavalent chromium is reduced, therisk of hexavalent chromium pollution of groundwater is increased. When initialconcentration of hexavalent chromium was in10~100mg/L, the adsorption quantityof hexavalent chromium increased with the increase of concentration of hexavalentchromium. When the concentration of hexavalent chromium was more than100mg/L,with increasing of initial concentration of hexavalent chromium, the adsorption ofhexavalent chromium is no longer increased, which shows that elevatedconcentrations of hexavalent chromium may increase the risk of groundwatercontamination. With increasing of particle diameter, the adsorption ability ofhexavalent chromium gradually weakened. An increase in medium particle size leadsto the adsorption capacity of hexavalent chromium weakened, increasing risk ofhexavalent chromium pollution of groundwater. The adsorption characteristics ofhexavalent chromium in soil are studied by the thermodynamic and kinetic analysis.The kinetic analysis result shows that the adsorption of hexavalent chromium couldsatisfy secondary dynamics equation, which shows that when the adsorption not reachequilibrium, its Kdis smaller and risk of groundwater contamination is higher. Thethermodynamic analysis results show that the adsorption of hexavalent chromiumcould occur spontaneously (△G0<0), and it was an endothermic reaction (△H0>0).The amount of soil adsorption of hexavalent chromium increases with temperature,which helps reduce the risk of entering the groundwater; two anionic mechanismswere expounded in the study, namely specific adsorption and non-specific adsorption.Finally, the adsorption of hexavalent chromium in soil has been proven to be specificadsorption (chemical adsorption), when the form of hexavalent chromium is HCrO4-,its risk of contamination of groundwater is lower.(2) Using SPSS software for multiple regression analysis, with pH, concentrationof hexavalent chromium, soil organic matter content, iron content and aluminacontent, clay content as independent variables, and hexavalent chromium distributioncoefficient (Kd) as the dependent variable, and getting the regression equation:Kd=2.082+0.007C-0.204pH, R2=0.927, which shows that Cr (VI) adsorptiondistribution coefficient is mainly related to hexavalent chromium concentration andpH conditions. Risk assessment model is proposed in this paper for the risk assessment of hexavalent chromium pollution of groundwater. The risk assessmentmodel is established of hexavalent chromium pollution for groundwater and for riskassessment. Hexavalent chromium pollution to the groundwater can be divided intofive different levels of risk range from slight to great.