The Effects Of Additives On Adsorption Of Heavy Metals Onto Surficial Sediments

Surficial sediments are one of the most important components in the water system. The content of surficial sediments is weathering mineral with rock and soil fine particles, which suspended in the form of surface water runoff into rivers, lakes or the ocean and set in the underwater environment in the form of deposition conditions. Surficial sediments are the source and the fate of heavy metals in water. Adsorption is one of the most important reactions of heavy metals among the series of migration and transformation of heavy metals on sediment-water interface. Organic matters, hydrous metal oxides and clay minerals which are the content of surficial sediments would affect the absorption of heavy metals. The heavy metals absorbed on sediment would desorb with the tiny changes of environment around. Hence it is necessary to investigate the remediation of heavy metals in contaminative sediments.Many researches have been done to the remediation of heavy metals in contaminative sediments. Currently there are three kinds of remediation technologies including physical, chemical and bioremediation. However, the chemical remediation of heavy metals in sediments at home and abroad has been only researched in laboratory at the present time, and uses the remediation of heavy metals polluted soil for reference mostly. Four kinds of additive, including organic matters (humus, EDTA, tannin, citric acid and so on), Fe/Mn oxides (goethite, hematite, cryptomelane and so on), clay minerals (montmorillonite, illite, bentonite and so on) and phosphate (apatite, hydroxyapatite) additives would fix heavy metals in soils or surficial sediments. Organic matters, Fe/Mn oxides, clay minerals and hydroxyapatite additives were widely used on the immobilization and adsorption of heavy metals in soil or water. However, the effect of additives on heavy metals in surficial sediments is little investigated.In this research organic matters (humic acid, tannin and chitosan), Fe oxides (goethite, hematite, ferrihydrite, and powder iron ores), clay minerals (montmorillonite and illite) and hydroxyapatite additives were added respectively as additives to surficial sediments.Considering that the additives may affect the physiochemical property of surficial sediments, the components of surficial sediments (organic matters, Fe oxides, clay minerals and hydroxyapatite additives) were selected in this study. This research compared the effect of additives on the adsorption of copper and zinc onto surficial sediments, investigated the effect of additives on the desorption of copper, zinc, lead and cadmium absorbed on surficial sediments by the TCLP method, and finally compared the adsorption ratio of copper, zinc, lead and cadmium on surficial sediments after the adding of additives. This is in order to select additives which would fix heavy metals in surficial sediments effectively, and select additives for further research of heavy metals'immobilization in surficial sediments. It has been proved that:(1) The effect of organic additives on the adsorption quantity of copper and zinc onto surficial sediments is different with each other. Compare to copper, the addition of organic additives increase adsorbance more easily. The organic additives'dissolution may be one of the reasons that affect the adsorption quantity of heavy metals. All organic additives would increase the adsorption of zinc and cadmium on sediments. The effect of tannin on the adsorption of zinc and cadmium onto sediments may be the biggest in all of the organic additives. The three organic additives all would decrease copper, zinc and cadmium availability, and the availability decrease as long as the adding ratio rise. The effect of organic additives on the availability of copper, zinc and cadmium is tannin > humic acid> chitosan. Tannin, humic acid and chitosan added with a certain ratio would raises desorption efficiency of lead when pH is 4.93±0.05, and decrease desorption efficiency of lead when pH is 2.88±0.05. The four heavy metals availability all decrease, only when tannin were added with the ratio of 5%.(2) Only of powder iron ores added with a certain ratio would increase the adsorption of sediments to copper. Adsorption of zinc onto sediments increases after the addition of goethite or hematite. The mechanism of heavy metals absorbed onto Fe oxides additives may be the effect of ligand exchange shown in quoted passage. Fe oxides would all more or less increase the adsorption of cadmium onto sediments. Fe oxides additives would all decrease the availability of zinc and cadmium to different extent. In all of the Fe oxides additives powder iron ores decrease availability of zinc less than the other Fe oxides additives.(3) The addition of clay minerals additives would increase the adsorption of zinc onto sediments. The effect of hydroxyapatite to increase adsorption of heavy metals is bigger than the ability of clay minerals, the adsorbance of zinc and copper adsorbed onto sediment added with hydroxyapatite is 1.66 and 3.95 times to the adsorbance onto sediments alone. Clay minerals would decrease the adsorption of copper onto sediments. However, the addition of hydroxyapatite would increase the adsorption of copper, zinc, lead and cadmium, and the adsorption increase with the added ratio. In the two clay minerals selected in this research illite would only decrease the availability of lead to a certain extant, while montmorillonite would decrease the availability of zinc, lead and cadmium. The effect of hydroxyapatite to decrease the availability of heavy metals is bigger that clay minerals. It is clear that hydroxyapatite would decrease the availability of heavy metals largely.(4) Different additives have different effect on adsorption and desorption of different heavy metals. The order of difficulty about heavy metals'fixing is: zinc > cadmium > lead > copper. The order of effect about additives'fixing is: hydroxyapatite additives > organic matters additives > Fe oxides additives > clay minerals additives. The addition of organic matters, Fe oxides and clay minerals which have big absorption capacity to heavy metals, would not increase the adsorbance of all heavy metals. However, hydroxyapatite, the content of which is little in sediments, has the biggest capacity in increasing the adsorption and decreasing the availability of heavy metals in additives. It has been shown that hydroxyapatite has potential application value in amendment of heavy metals polluted sediments.