Effect Of Dissolved Organic Matter On Adsorption And Desorption Of Mercury By Soils And Sediment

As the natural organic ligand and adsorptive carrier in the environment,Dissolved organic matter (DOM)is an active chemical component in terrestrial and aquatic ecosystem.That widely exists in soils and could influence the transportation and environmental behavior of trace inorganic and organic pollutants. It was found that in DOM there were a great of active functions including carboxyl,carbonyl,amido and hydroxyl,which can caused a series of actions such as absorption,complexation,chelate,flocculation, deposit and redox and consequently impacting the transportation,transformation and destination of heavy metals cations.The agricultural application of organic fertilizer can bring about numerous organic matters in soil as well as the fallen litter,which may affect the chemical behaviors of heavy metals in environment. Soil,which is both the sources and sink,plays an important role in the biogeochemical cycle of mercury. When the mercury imported can be present in surface water and ground water by the precipitation and then most might be adsorbed by the suspended substance.Therefore,the transportation and transformation of mercury could greatly affected by the organic matter in sediments.Two kinds of soils,the Xanthi-Udic Ferralosols,Purpli-Udic Cambosols and pound sediment collected in Chongqing,China,were used for the present study.The effect of DOM on adsorption and desorption of Hg in materials above were investigated and the DOM were extracted from humus soils(DOM_H),rice straw(DOM_R)and compost of sewage sludge (DOM_S).It is important for the remediation of soil mercury pollution and understanding the environmental behavior in the sediment.The results showed the presence of DOM can reduce adsorption of mercury in both soils and sediment. The maximum adsorption of mercury was lowest by the addition of DOM_H,which were 392.16μg·g^-1, 414.94μg·g^-1and 454.55μg·g^-1in Xanthi-Udic Ferralosols,Purpli-Udic Cambosols and Pound sediment, respectively.It was indicated that DOM extracted from Humus soils most significantly inhibited the adsorption of mercury and the range of the mercury maximum adsorption decreased from 6.4%to 17.7%in Xanthi-Udic Ferralosols,and in Purpli-Udic Cambosols from 7.2%to 19.9%and in pound sediment from 7.4%to 20.2%.For the same material,the inhibition presented an order of DOM_H＞DOM_R＞DOM_S＞CK, while that was an order of pound sediment＞Purpli-Udic Cambosols＞Xanthi-Udic Ferralosols for the same DOM.In addition,the adsorption rate of mercury in both soils and sediment were reduced by the additions of all DOM in an order of DOM_H＞DOM_R＞DOM_S＞CK,showing that the maximum adsorption and adsorption rate of mercury depended on the types of tested material and additional DOM.The two dominative factors,the effect of pH buffer and competitive adsorption in soils and sediment,could affect the efficacy of DOM on mercury adsorption.Also,the results showed that DOM can promote the desorption of mercury,both the maximun desorption and desorption rate.When adding DOM_H,the maximum desorption of mercury reached 107.53μg·g^-1,109.89μg·g^-1and 76.34μg·g^-1in Xanthi-Udic Ferralosols,Purpli-Udic Cambosols and pound sediment,respectively.That showed DOM extracted from Humus soils most significantly enhanced the desorption of mercury,which is just the opposite of adsorption characteristics.The range of the maximum desorption of mercury increased from 5.12%to 16.31%in Xanthi-Udic Ferralosols,from 6.05%to 12.35% in Purpli-Udic Cambosols and in pound sediment from 2.06%to 5.36%when DOM_H,DOM_R and DOM_S were added.The maximum desorption of mercury was in an order of Purpli-Udic Cambosols＞Xanthi-Udic Ferralosols＞pound sediment when added the same DOM.On the other hand,for the same material the promotion action was different,the order for Xanthi-Udic Ferralosols was DOM_H＞DOM_R＞DOM_S＞CK and for the Purpli-Udic Cambosols and pound sediment the order was DOM_H＞DOM_S＞DOM_R＞CK.It was also found that desorption rate of mercury in Xanthi-Udic Ferralosols was enhanced in an order of DOM_H＞DOM_R＞DOM_S＞CK,while that was DOM_H＞DOM_S＞DOM_R＞CK in Purpli-Udic Cambosols and pound sediment.The influence of maximum desorption and desorptionrate of mercury varied with the types of tested materials and the kinds of DOM.The organic matter,pH and exchangeable cations of tested material may affect the adhesive capability between tested materials and mercury,and the molecular structure and hydrophobic component of DOM also have a different reaction on desorption of mercury.