The Adsorption Mechanisms Of Heavy Metals In Sediments And Pollution Ecology Of Heavy Metals In Baotou Section Of Yellow River

The Yellow River contains large amounts of sands, resulting in a peculiarhydraulic condition which is characterized by a magnificent interface between theparticulates and water. In addition, Baotou section of the Yellow River suffers fromsevere heavy metal pollution receiving the industrial discharges from the Baotou city.Therefore, this study was carried out on Baotou section of the Yellow River based onits representativeness on the mechanism of heavy metal adsorption on water-sedimentinterface, and on the heavy metal spatial distribution, mobilization, transformation,and accumulation effects in sediments.Because of their persistence in the environment, heavy metals often pose threatsto aquatic lives and plants in the environment. Based on the theories and methods ofenvironmental geochemistry, biogeochemistry, pollution ecology and modernsedimentology, the adsorption mechanisms and the geochemistry characters of theheavy metals in Baotou section of the Yellow River are researched in this paper. Tosay concretely, the adsorption-desorption mechanisms of the selected heavy metalsonto the sediments in the Yellow River are studied, which includes the impacts of thegrain sizes of the sediments and the usual ions in aquatic environment on theadsorption of the selected heavy metals, the competitive adsorption of the selectedheavy metals and the impact of the mineral composition of the sediments on thespecies distribution of the heavy metals. Meanwhile, spatial distribution, mobilization,transformation, and accumulation effects of the selected heavy metals in Baotousection of the Yellow River are discussed. In addition, it can be worth notice that the biological enrichment of heavy metals in coastal plants and natural fishes in Baotousection of the Yellow River is also analyzed for the first time, which may providevaluable database for the studies of biological availability, biogeochemical cycle andecological risk assessment of heavy metals in Baotou section of the Yellow River.The results are described as follows:1. The expression of surface fractal dimension (SFD) for size fractions of theYellow River sediment was deduced. Based on the expression, the SFD value ofdifferent size fractions of the sediment was calculated. The SFD value of thesediment in the Baotou section of the Yellow River is 1.91, and the SFD value of thesediment smaller than 63μm is 1.36. The variation extents of equilibrium adsorptioncapacity influenced by different grain size are ranked as Cu＞Pb＞Zn≈Cd. The Cu andPb present higher adsorption capacity at relatively large grain sizes; whereas, for Znand Cd, the adsorption capacity decreases with increasing grain size. The modifiedfractal model, Freundlich model and Langmuir model are used in this study, and themodified fractal model is found to fit the data of heavy metal (Cu, Pb, Zn and Cd)adsorption well compared to Freundlich model and Langmuir model.2. The heavy metals in the water system are affected by usual ions (such as Na⁺,K⁺, Mg²⁺, Ca²⁺, Cl^- and SO₄^2-) through different adsorption and desorptionmechanisms, which is caused by different dominant mechanism in different usual ionconcentration ranges. According to the results of the experiments, with the increasingof the ionic strength of the additional ions in the solutions, the adsorption capacitiesof the heavy metals increase when the compression of the electric double layerdominates on sediment-water interface, whereas decrease when the ionic activities ofthe heavy metals dominate; there is competitive adsorption between the heavy metalsand the additional cations when the structures or physicochemical properties of theadditional cations are similar to the heavy metals'; the complexations between theanions and the heavy metals lead to decrease of ionic activities and the valence statesof the heavy metals, and have influence on physical adsorption of the heavy metalswhich is caused by electrostatic attraction. 3. The competitive adsorption exists in different concentrations of heavy metalsolution and is not simply as occupying adsorption sites. The competitive adsorptionmechanisms are different in different concentration gradient of heavy metaladsorption reaction systems. And the competitive adsorption abilities of heavy metalson sediment-water interface in Baotou section of the Yellow River are ranked as:pb²⁺＞Cu²⁺＞Zn²⁺≥Cd²⁺.4. The illite and chlorite which are important minerals of carrying the heavymetals via particulates in Baotou section of the Yellow River have significantinfluence on species distribution of the heavy metals in secondary phase. And theregulation effect of calcium carbonate (calcite) on the concentration of CO₃^2- hasindirect influence on the species distributions of the heavy metals in sediment inaquatic environment, and further impact on the mobility of the heavy metals withother minerals. Furthermore, the quartz, orthoclase, plagioclase and riebeckite havesignificant influence on exchangeable form of the heavy metals.5. The sediments in the Yellow River have different functions as sink or source indifferent periods. The sediments can function as sink in low-water period andmedium water period, in which it is propitious of suspended matter deposition inwater system and heavy metal adsorption on sediment-water interface; whereas it canfunction as source in high-water period, in which the heavy metals adsorbed bysurface sediments would be transferred into the water phase again, then cause thesecondary pollution. The enrichment abilities of heavy metals in different periods areranked as: low-water period＞medium water period＞high-water period.6. The concentrating abilities of coastal plants and natural fishes to heavy metalsin Baotou section of the Yellow River are investigated for the first time. This papernot only lays foundations but also provides valuable firsthand database for the studiesof biological availability, biogeochemical cycle and the ecological risk evaluation.The concentrating abilities of coastal plants differ in different elements, samplingsites, plant species, and different nutritive organs. And the concentrating abilities ofnatural fishes differ in different elements, fish ages, fish species and different nutritive organs.