| The Eutrophication occurred in water bodies of China is becoming more aggravated, and several lakes such as Taihu, Caohu and Dianchi have been emphasized as the hot point of water control pollution. Recently many research results showed that water quality pollution caused by phosphorous pollution inlet, mainly account for agricultural non-point (ANP) pollution. Meanwhile, eutrophication has a significant relationship with phosphorous (P) losses from soils, for instance, water pollution caused by phosphorous losses from agricultural non-point highly accounted for 28%and 68%in Dianchi in Yunnan province and Nansihu in Shandong province respectively.The Three Gorges reservoir is an oversize reservoir with multifunction such as electricity generation, shipping and flood regulations. The environmental water security of the reservoir is so important that affects the country's entire society-economy development. The reservoir project inundated 632 km2 of land, as the water level fluctuation zones (WLFZ) created due to flooding, which cover an area with 30m vertical height and more than 400 km2 in the reservoir. After flooding or inundation occur, the water level fluctuation zones will be an important pool and source of nitrogen and phosphorous, which accompanied with periodic flooding and alternation of wet and dry conditions, furthermore exacerbated the releases of pollutants including nitrogen (N), P and heavy metals. In addition, in Chongqing, the most of geomorphic type belongs to lands and hills with low slope, which more than 60%of areas are slope land, and excess 70%of soil type is purple soil susceptible to erosion. The sediment load is1.45 x 102 million ton/year, which account for 26%of total load from upstream of Yangtze River. Due to the difficulty of control and preventment for ANP, the water fluctuation zones are inevitable to become the most serious source of nutrients, which would cause eutrophication in Three Gorge reservoir. From the preceding mentions, hence we know the releases and immobilizations of phosphorous from soils of WLFZS are so important for guarantee of water quality safety. However the most significant factor that impacts the adsorption and desorption of phosphorous in soils is soil colloids. Some reports had confirmed the P losses due to soil colloids bound P runoff was the one of the important mechanisms to induce eutrophication in corresponding water bodies. Despite of this broad consensus, it appears that a few researches about the effect of soil and corresponding colloids on adsorption/desorption of P, based on synchronous comparison. Additionally, P losses coupled with soil colloids losses is a vital way for slope soil P transferred into water, and the significant effect of soil/soil colloids in various conditions on P transformation and transfer also is crucial to understand the entire system, thus, it's theoretical significant to research the effect mentioned above. Especially in Three Gorges reservoir, where mainly is a slope lands areas, currently there is lack of systemic reports about the effect of typical soils/soil colloids from these areas (mainly originated from WLFZS) on the mechanism of P sorption, transformation and transfer.Although there are a few reports about P adsorption of typical soil in WLFZS, the simultaneously researches on P adsorption of soils and related soil colloids are still scarce. Meanwhile the effect of organic acids on P transfer occurred on the water-soil interface is rare to investigated. In previous studies on the mechanism of P adsorption in the conditions of alternation of wet and dry or the existence of organic acids, which mainly focused on the transformation of P speciation, the P uptake by crops and the P availability without giving enough attention on these mechanisms based on water source protection and environmental effect. In addition, there also exist some studies about the polyacrylamide (PAM) application in various farming regions and soil types. However in Three Gorge reservoir, due to the unique environmental conditions, we still don't understand the detailed results of the coordination between PAM and environment and the comprehensive application premises, which need more systemic and entire investigations. Thus, it's so significant to relocate our research attentions on the effect of different external conditions (including pH, low molecule organic acid, periodic flooding model) on P adsorption/desorption, which is helpful to understand the mechanism of P transfer and transformation, meanwhile more better explain the effect of external conditions and interactions with entire environmental system. Additionally, investigations on mechanism and effect of PAM on P losses in the conditions such as slope and rainfall intensity etc, will supply the necessary and comprehensive basic knowledge to figure out an effective and economical way to control of nutrients losses from steep slope lands. In this dissertation, lots of works had been finished for the investigation and discussion of the mitigation mechanism and control methods of P pollution in water. Its significance is building the systemic and extensive technology bases, which have the important practical and theoretic values for mitigation eutrophication, effectively direction and guidance of water pollution treatment and prevention works in reservoir areas, and secure the water quality safety.We conducted experiments to use typical soils in WLFZS of Three Gorges reservoir and purple soil from steep slope lands, as the research objectives for investigation of the characteristics of P adsorption in soil/soil colloid of WLFZS and the mechanism of P transfer in these areas under conditions of overlying water flooding, alternation of dry and wet and existence of organic acid. Meanwhile, this dissertation discussed the factors and mechanism, which impacted P transfer through simulated flooding experiment by adding artificial Yangtze River water. In addition, isothermal sorption, soil column and simulated rainfall experiments all were performed in attempt to identify the effect of PAM on P losses from purple soils in steep slope lands, and related factors also discussed in the following section below. In this way, the main results in our study went as follows.Through isothermal sorption experiment of alluvial soil, purple soil and yellowish soil and related soil colloids, it demonstrated the Langmiur equation was the best fitting tool to express the sorption process. For the maximum P adsorption, yellowish soil showed the highest value 1666.67 mg/kg, which was approximately 3.7 and 2.8 times than Alluvial soil and purple soil, respectively. The max P adsorption amount was consistent with FeOx and AlOx in soils, but had a no significant relationship with organic matter and clay. Thus the order of P desorption was alluvial soil> purple soil> yellowish soil. In addition, the P adsorption of the above three soil colloids all were far higher than the soils originated from the same sources, the maximum P adsorption amount were 4039.3,3155.9 and 3117.5 mg/kg, respectively, which approximately equaled 2.4,6.9 and 5.3 times of corresponding soils. More generally, the P adsorption reaction rates of soil colloids were higher than corresponding soils, which indicated the major reaction was chemical sorption, and second physical sorption in the process of P adsorption of soils colloids, in comparison with corresponding soils, which showed the contrary trend.The dynamic isothermal adsorption/desorption process of three soil colloids all were fitteds by. At 225 min, the rank of P cumulative adsorption amount was:yellowish soil colloid> purple soil colloid> alluvial soil colloid, in which the dynamic isothermal desorption rates of purple soil colloid and Alluvial soil colloid both were the same (about 22.7%), and higher than the desorption rate of yellowish soil (14.11%).In initial stage of simulated flooding, the rank of P release capacity and rate was alluvial soil> purple soil> yellowish soil. P releases from the above soils to overlying water increased with increasing external P addition rates and time duration. During the experimental flooding time (4-30 d), the relationship between P concentrations in overlying water of three soils and simulated flooding time, could be described by linear equation as Q= a+bt.In addition, after alternation of dry, water re-flooded in experiment to simulate the second flooding. Hence, both of the P releases amounts and rates were significantly lower than the situation in the first flooding.Among the various P concentration levels in overlying water, the P concentrations gradually decreased with the time duration. Initially (< 14 d), the P sorption rate of soil was high, in contrast to a lower sorption rate in later period (≥15 d). For indication of the relationship between P sorption amount of overlying water from three soils and flooding time duration, the equation Q= a-bin(t+c) could used to fit and express.The effect of pH in underlying water on soil P releases is evidentials, and the relationship between initial pH values in overlying water and P releases from soils could use non-linear equation QDp= a·xb to express. In greater acid or alkaline condition, the potential of P releases was higher. However the ability was the lowest in neural pH of water body. With the increasing time duration, the effect of initial pH on P releases is less significant.In the condition of three organic acids with various concentrations levels in overlying water, the relationship between P releases amount and flooding time duration was given by equation QDP= a x tb. Due to the impact of organic acid, the P release capacity of three soils followed as alluvial soil> grey alluvial soil> yellowish soil. At the same flooding time duration, the rank of three organic acids increasing P releases was oxalic acid> citric acid> tartaric acid. Meanwhile, our experiment suggested that the organic acids promoted the Fe and Mn ions releases, with more concentrations of organic acids, more release amounts. The both of order of organic acids increasing Fe and Mn ions releases from soils were: citric acid> oxalic acid> tarteric acid, and citric acid> tartaric acid> oxalic acid, respectively. Additionally, the P releases in overlying water had a significant relationship with Fe and Mn ions concentrations.The isothermal adsorption/desorption curve of soils treated by PAM indicated the relationship between P adsorption amount (Q), PAM application rate (x) could be fitted and expressed as Q= 95.5179X-0.3295. The P adsorption decreased by different extents when PAM added into. However the maximum buffer capacity (MBC) was promoted with PAM application rates increasing, in contrast to decreasing maximum adsorption amount (Xm), meanwhile the both of increasing P adsorption rate and reaction spontaneity were observed in our experiment. When PAM application rates were low (among 0.1%-0.2%, by weight/weight), no significant effect on P desorption was observed, conversely, the effect was so significant under greater PAM application rate (0.4%PAM, by w/w).The soil column experiment demonstrated the PAM applied in soil significantly inhibited the P losses in vertical directions in soil column, and the effect of application was much greater with increasing PAM application rate.In simulated rainfall condition, the total effect of PAM on runoff load produced in purple slope soil was hysteretic, which helped the generation of interflows. Meanwhile, the effect of PAM application on runoff from steeper slope lands, rather than gentle slope lands, is more significant. Building on this insight and experimental data, in the process of control of P losses from purple soil, the optimal application rate was 32 mg/kg when slope is 15°. In the two process of simulated rainfall, the total P losses and particle P from treatments were 24.48 mg and 3.49 mg, which only accounted for 11.8%and 2.4%of control group, respectively. Furthermore, the total runoff from 15°and 20°were 37.35 L和38.59 L, decreased by 11.0% and 8.1%in comparison with treatment of 8°, respectively, with the PAM application rate was 32 mg/kg. For the rainfall intensity (RI), the effect of PAM on control of P losses from runoff was greater under medium RI. However, the control of interflows by PAM is more significant under high RI, compared with medium and low RI. Moreover, the equation Q= a·tbcould be used to fit the relationship between total P cumulative losses from runoff and runoff time duration. IX... |
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