| Phosphorus(P) is an important life element which plays a decisive role in the metabolism of all organisms, meanwhile, it is also a potential critical non renewable resource in the environment. P cycle in the environment is an important biogeochemical process which can strongly influence agricultural nonpoint source pollution, soil fertility, soil erosion, and water eutrophication, etc. Iron oxide(e.g., goethite, ferrihydrite) plays an important role in P immobilization and release due to its large surface area, variable-charge surface and high reactivity. Thus, many studies were carried out to explore the adsorption mechanisms of P on iron oxides and its influencing factors. However, iron oxides rarely occur alone in the environment; they are usually associated with organic matter(OM) including humic substances to form iron oxide-organic matter complexes through protonation, ligand group exchange or hydrogen bonding, electrostatic interaction, cation bridging, etc. The association of NOM with iron oxides will significantly change the binding characteristics of iron oxides on pollutants incluing P, and which is also a crucial mechanism for OM stabilization in soil. In addition, NOM can inhibit the crystallization progress of amorphous iron oxide due to the formation of iron oxide-OM complexes. Although iron oxide-OM complexes are important components of soil aggregates and play a significant role on the chemical behavior of pollutants in the environment, studies regarding to their effects on the transport and transformation of P are still insufficient. Previously, most studies only focused on the competitive adsorption of OM and P onto iron oxide surfaces, and the results were often not consistent. Meanwhile, dissolution of the iron oxide-OM complex tends to occur during the P adsorption process due to the variation of solution p H, resulting in the release of some absorbent components such as Fe ions and OM into the solution, which in turn would affect the speciation of P in solution. Actually, P speciation in solution directly determines the bioavilability of P, and have a direct impact on water eutrophication. Although it was confirmed that iron could dissolve from adsorbent when P adsored on goethite, there still was no study to focus on the relationship between P fraction distribution and iron dissolved in adsorption solution.The Three Gorges Reservoir(TGR) is a remarkable large water-regulating reservoir, which has continuously attracted worldwide attention. Water eutrophication has become one of the most important ecological environment problems threatening the water environment security in this area. In fact, the eutrophication and water bloom phenomenon have alreadly appeared in some tributaries of Daning River, Wujiang River, etc. to different degrees. Because P is a limiting nutrient element for eutrophication in aquatic ecosystem, many studies focused on the distribution and source, evaluation method of P in the water at the early stage of TGR, then carried out to discuss the distribution and release characteristic of P species and influenced by different environment factors in the water-level-fluctuating zone(WLFZ) of TGR in recent years. However, studies on the chemical behavior of P influenced by the important consitituents of OM, iron oxides and iron oxide-OM complexes in the water and soil environment are still considerable scarce, especially in the water system, therefore, it is very necessary to carry out the relevant study in the TGR.Consequently, lab simulation and field monitoring experiments were co-used to explore the adsoption behavior of P and the regulatation effect on P fraction in solution by iron oxide-humic acid complexes. Firstly, iron oxide-humic acid complexes were prepared in lab, and the adsorption and desorption characteristics of P by iron oxide-humic acid complexes were discussed under different p H and ionic strength(I), low molecular weight organic acids(LMWOAs) co-existed, then P fraction distribution and its mechanism in supernatant after P adsorption onto iron oxide-humic acid complexes were analyzed in this study using ultrafiltration method. On the other hand, P fraction distribution influenced by iron(oxide)-organic matter complex was discussed by ultrafiltration technology in the Jiangling River and Changshou Lake of TGR area. Meanwhile, the effect of UV irradiation on P fraction distribution was also carried out in the Jiangling River water in lab. Moreover, selective removal of organic matter or iron oxides from three typical soils(purple alluvial soil(KX), grey brown purple soil(FL) and yellow soil(FJ)) were explored to investigate its direct effect on P fraction and adsorption-desorption behavior in the WLFZ soil of TGR. The findlings of this study can further elucidade the relationship between organic matter, iron oxide and P in the environment, and can describe the coupling relationship between P cycle and carbon fixation to some degree, then can reveal the eutrophication driven by P fate in the context of global climate change. In addition, this study can provide a theoretical and practical significance for explantation the chemical behavior of P at the soild-water interface. The main findings are as follows:(1) The surface properties of iron oxide were changed after humic acid(HA) coated, then its adsorption characteristics for P was also significantly changed. HA coated on iron oxide forming iron oxide-HA complexes caused specific surface area and zero point of charge decreased, surface morphology changed as compared to pure iron oxide. The prepared iron oxide-humic acid complexes were fomed by ligand exchange between the-OH on the surface of iron oxide and the-COO- of HA. Meanwhile, iron oxide-HA complexes showed a decreased P adsorption capacity in comparison with pure iron oxide. Additionally, the inhibitory effect of HA on the adsorption capacity of P by amorphous ferrihydrite(FH) was greater than that by crystalline goethite(GE), however, amorphous FH-HA complex still showed a higher P adsorption capacity than crystalline GE-HA complex. The maximum adsorption capacity(Qmax) decreased in the order of FH(22.17 mg.g-1) > FH-HA(5.43 mg.g-1) > GE(4.67 mg.g-1) > GE-HA(3.27 mg.g-1).(2) LMWOAs, I and p H are the important factors to effect the adsorption of P by iron oxide-HA complex and pure iron oxide. P adsorption onto iron oxide-HA complex and pure iron oxide decreased with increasing initial p H and decreasing initial I. Then, LMWOAs co-existed in adsorption solution inhibited P adsorption onto pure iron oxide, however facilitated P adsorption onto iron oxide-HA complex. The inhibition effect of three kinds of LMWOAs on the adsorption of P by pure iron oxide were as follows: acetic acid < oxalic acid < citric acid, and the promotion effect of three kinds of LMWOAs on the adsorption of P by iron oxide-HA complex were as follows: acetic acid > oxalic acid > citric acid. Moreover, P desorption from iron oxide-HA complex and pure iron oxide were also influenced by LMWOAs co-existed in desorption solution. LMWOAs co-existed in desorption solution faciliated P desorption from pure iron oxide, and the order as follows: acetic acid < oxalic acid < citric acid. Notably, only oxalic acid or citric acid co-existed in desorption solution facilitated P desorption from iron oxide-HA complex, however, acetic acid co-existed inhibited P desorption. It was found that a pseudo-first order kinetics equation provided a good fit for P adsorption during the whole adsorption process, including rapid and slow phases. The values of rate constant during the rapid adsorption phase(krap) varied in the order: FH(8.1?10-3 min-1) > GE(2.1?10-3 min-1) > FH-HA(9.4?10-4 min-1) > GE-HA(9.2?10-4 min-1). The values of rate constant during the slow adsorption phase(kslow) were in the order: FH(4.1?10-4 min-1) > FH-HA(6.4?10-5 min-1) > GE-HA(2.6?10-5 min-1) > GE(1.5?10-5 min-1). The iron oxide component in iron oxide-HA complex was still the critical contributor for P adsorption, rather than the organic phase. HA coated on iron oxide may affect P adsorption by occupying the sites of Fe OH1/2- at iron oxide surface or electrostatic repulsion. Then, forming inner-sphere surface complex between iron oxide component of iron oxide-HA complex and P was probably the mian adsorption mechanism, and electrostatic adsorption also had partial contributions.(3) Material composition and p H, P fraction in supernatant have changed after P adsorption onto iron oxide-HA complex and pure iron oxide. With an initial P concentration of 20 mg·L-1(I=0.01 mol·L-1 and p H=7), ultrafiltration results shown that the 1 KDa~0.45 μm component of P was accounted for 10.6%, 11.6%, 6.5%, 4.0% of remaining total P concentration in supernatant after P adsorption onto FH, GE, FH-HA, GE-HA, respectively. However, the < 1 KDa component of P was still the predominant fraction in supernatant. Underestimate colloid P in supernatant was accounted for 2.1%, 31.2%, 35.2%, 27.7% of P adsorption onto solid surface of FH, FH-HA, GE and GE-HA, respectively. Thus, colloid P in supernatant could not be neglected. Meanwhile, it was found that iron dissolved from adsorbents, and colloidal iron component accounted for more than 95.0% of dissolved total iron in supernatant after P adsorption onto iron oxide-HA complexes and pure iron oxides. And, it was also found that organic matter dissolved from adsorbents, and colloidal dissolve organic matter component accounted for more than 77.0% of total dissolve organic matter in supernatant after P adsorption onto iron oxide-HA complexes. Notably, Fe3+ hydrolysis from adsorbents and further forming colloidal hydrous ferric oxide aggregation might be the main mechanism for interpreting the formation of colloid P in supernatant after P adsorption onto different adsorbents. And, colloidal adsorbent particle co-existed in supernatant was another important reason for it. Additionally, dissolve organic matter dissolved from iron oxide-HA complexes could occupy large adsorption sites of colloid iron in supernatant, which caused less colloid P(1 KDa~0.45 μm) in supernatant. We believe that the results can provide a new perspective for understanding the migration and transformation of P at the soild-liquid interface.(4) The simulation experiment in lab confirmed that iron oxide-organic matter complex had a significant influence on the distribution of P fraction in solution. Thus, the distribution of P fraction was discussed used ultrafiltration method in the Jiangling River and Changshou Lake of TGR area. Meanwhile, the effect of UV irradiation on P fraction distribution was also carried out in the Jiangling River water samples in lab. The results revealed that iron(oxide)-organic matter complex was the important driving factor to effect the distribution of P fraction in the Jiangling River and Changshou Lake. In the Jiangling River, the data showed that the predominant fraction of dissolved P was the component of 0.5 KDa~10 KDa, which was ranged from 48.0%~60.4% in all sample sites, then followed by the component of 10 KDa~0.22 μm, however, true dissolved P(<0.5 KDa) was the minimal component which could be directly used by organisms. After confluences of Qujiang River and Fujiang River, there was no significant change in different molecular weight P fractions in the Jialing River. Moreover, after UV irradiation, it was found that P in true dissloved fraction(<0.5 KDa) was obviously increased by 10.8%~29.1% in all sample sites, conversely, P decreased in both 10 KDa~0.22 μm and 0.5 KDa~10 KDa components. Thus, it was confirmed that UV irradiation was beneficial to increase bioavailable P in water system, then it was an important environmental factor to effect on P geochemical fate in aquatic system. Additionally, the formation mechanism of colloid P was probably caused by the presence of colloidal dissolve organic matter-metal(iron)-P complexes in all sample sites. On the other hand, the distribution of P fraction was difference in different functional zones in the Changshou Lake. The results presented that the component of <1 KDa dominated total dissolved P in the water sample of S2(ecological culture area and the mouth of the water inlet) at different depths, followed by the component of 10 KDa~0.45 μm and 1 KDa~10 KDa. However, the total dissolved P was dominated by the component of 10 KDa~0.45 μm in the water samples of S1, S3, S5(lake center and ecological tourist area) and S4(wetland reserve area) at different depths, then followed by the component of <1 KDa and 1 KDa~10 KDa. Formation of colloidal iron-P compounds might be a crucial process to cause P in colloid, and dissolve organic matter with higher molecular weight and strong aromatic strucrure could occupy adsorption sites of colloid Fe to decrease colloid P content in the water samples of Changshou Lake.(5) Besides, for further exploration the importance of lab findings in the real soil system, selective removal of organic matter or iron oxide from three typical soils were explored to investigate its direct effect on P fractions and adsorption-desorption behavior in the WLFZ of TGR. The data showed that readily oxidizable organic matter had a minor effect on P fractions and adsorption-desorption behavior in the three soils, however, free metal oxides were the key factor for that. Kinds of P fractions in three soils were not significant decreased followed with removal of readily oxidizable organic matter, meanwhile, there were no significant correlation between kinds of P fractions and organic matter. However, kinds of P fractions were significantly decreased with removal of free metal oxides in the three soils. Notably, different P fractions were both in the order as follows: Ca-P > OP > Fe/Al-P, before and after removal of organic matter or free metal oxides in the three soils. Moreover, after removal of organic matter, the adsorption capacity of FJ, KX, FL soil for P was only decreased by 0.5%, 2.3%, 6.5%, respectively, which indicated that P adsorbed on the three soils were little influenced by organic matter. In addition, after removal of free metal oxides, the adsorption capacity of FJ, KX, FL soil for P was significantly decreased by 45.6%, 51.7%, 43.9%, respectively, which revealed that P adsorbed on the three soils were dominated by free metal oxides. More importantly, the desorption capacity of three soils for P was increased after removal of free metal oxides, which presented that free metal oxides were also the predominant factor to control desorption behavior of freshly sorbed P, and had the important influence on the acitivity of P in soil. Then, the desorption capacity of FL for P was little decreased after removal of organic mater, and there were no distinction for it before and after removal of organic matter in KX and FJ soils, which showed that the desorption capacity of three soils for P were influenced by organic matter related to soil category. |
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