Mechanisms Of Heavy Metal Immobilization In The Matrix Of River Sediments

The acceleration of the industrialization process has caused a series of environmental problems besides improving the convenience of people's lives.Among them,heavy metal pollution has attracted more and more attention because of the toxic hazards and strong bioaccumulation.The wastewater received by the river usually contains a large amount of heavy metals,which will be deposited in the sediment of the river channel through physicochemical effects such as precipitation,adsorption and complexation,and gradually accumulate.When the external conditions change,the heavy metals deposited in the sediment are easily released,resulting in a series of environmental hazards.Therefore,it is particularly important to carry out green and efficient treatment and disposal of river sediment contaminated by heavy metals.Sediment dredging is an engineering measure that can completely remove contaminated sediment and effectively improve water quality.With further development of economy and society,the limitations of traditional dredging sediment treatment methods such as landfilling and composting have been becoming more and more obvious.Therefore,it is urgent to develop harmless and resource-based treatment methods for river dredging sediments.However,the large amount of heavy metal contained in the contaminated dredged sediment greatly limits the resource utilization of the sediment.Existing studies have shown that the oxdies of Al/Si/Fe and other inorganic components are mainly contained in the river dredging sediments,which can replace clay as a raw material for ceramic production.At the same time,the product phases such as aluminosilicate generated during ceramic sintering can also interact with heavy metals contained in the reaction matrices.Therefore,the preparation of ceramic products from river sediment as raw material by high-temperature sintering can effectively recycle the solid wastes while achieving a good heavy metal stabilizing effect.In this study,river dredging sediments were mixed with cadmium oxide,cadmium oxide,lead oxide,and nickel oxide according to a certain molar ratio and sintered to produce corresponding products.With the help of XRD,SEM-EDX,ICP-AES and other analytical testing methods,the structure and properties of the sintered products were fully characterized.The transformation behavior and stabilization mechanisms of the heavy metals in a complex sediment-based system were explicated in detail.Moreover,a prolonged leaching experiment was conducted on the sintered products,with the evaluation on the metal stability in the harsh acidic medium.During the experiment,the relevant parameters in the product preparation process were adjusted to verify the influence of external factors on the product structure and properties.The results show that whether in the monometallic system composed of sediment and cadmium or the multimetal system composed of sediment cadmium,lead and nickel,heavy metals can be effectively immobilized in the sintered products.From the perspective of migration and transformation of heavy metals,for monometallic system,the holding phase of cadmium is mainly oxide and silicates when the sintering temperature is low.After reaching certain temperature,cadmium will combine into aluminum silicon then become more stable.The ratios of raw material will have an effect on the end products.Specifically,when the content of cadmium is high,the cadmium-containing phase generated by the first reaction will further combine with cadmium oxide to form a series of cadmium-containing secondary products;while the primary products which contain cadmium will be the dominant phase when the the content of cadmium is low.For multimetal system,three kinds of heavy metals(cadmium,lead,nickel)can not only react with the components contained in the sediments to form aluminosilicate but also react with each other to generate spines which have a more stable structure.From the perspective of the sintered products' micromorphology,the sintering temperature has a significant effect on the micromorphology of the product in both two system.The products which are sintered in low temperature has a loose structure and poor compactness,while structure of the products which are sintered in the higher temperature is denser.Meanwhile,the high temperature will promote the solid-phase reaction so that the difference of surface contrast will be more obvious.From the perspective of the leaching behavior of elements,the produts which is sintered in high temperature has a lower leaching rate than that in low temperature.Moreover,there is competition for H+ between heavy metal elements and between heavy metal elements and other major elements in the river sediments.And the competition is dominated by the stability change of heavy metals.The leaching concentration of other elements change with the stability change of the heavy metals..