| Humic acid (HA) and iron oxides, as two important colloids on the earth, are widely distributed in natural environment. They can adsorb organic and inorganic pollutants through different mechanisms such as electrostatic adsorption, coordination, or ration bridging, thus affect the speciation, transformations, bioavailability and biotoxicity of pollutants. Therefore, the influence and mechanism of HA and iron oxides on the environmental behavior of pollutants have been a concern of environmental geochemistry.Hg and As, two heavy metals in the environment, are causing increasingly serious pollution to soil and water due to emission from industrial activities, atmospheric deposition, sewage irrigation as well as farmland fertilization. Pollution of Hg and As can affect not only the ecosystem structure and function of soil and water, but also the normal growth of plants. Besides, it may cause serious health issues for human beings due to enrichment and magnification of food chain. Coexisting in environmental media such as soil and water, HA and iron oxides interact with each other, which is bound to change their adsorption characteristics of heavy metals, thus differently impacting on the speciation, transformations, bioavailability and biotoxicity of heavy metals. So far, however, researches on the interaction between HA and iron oxides and the secondary adsorption of heavy metals are still weak. In this dissertation, three widely-distributed iron oxides (goethite, hematite, ferrihydrite) are chosen and HA-oxides complexes are prepared through batch experiments. On the basis of analyzing the adsorption characteristics of HA on the three iron oxides, the difference in Hg2+ and AsO43- adsorption kinetics and thermodynamics among three adsorbents are compared, as well as the influence of pH and ionic strength. Through XRD, SEM and coordination chemistry theory, the influence and mechanism of AsO43-and Hg2+ secondary adsorption on iron oxides complexed with HA are discovered, which will provide scientific basis for an in-depth understanding of the interaction between HA and iron oxides and their environmental impact, as well as for activity regulation and pollution remediation of Hg2+ and AsO43- and in soil and water environment. The results are as follows:1. The three iron oxides could strongly adsorb HA, with adsorption rate, strength and capacity being closely related to pH and ionic strength. HA adsorption on oxides changed the surface morphology and properties of oxides. (1) The HA adsorptions on the three iron oxides well fitted with Langmuir equation, and the maximum adsorption amounts (Qmax) by goethite (Gt), hematite (HeM) and ferrihydrite (FeH) were7.728μg/mg,2.252μg/mg and 3.143μg/mg, respectively; (2) Increasing pH decreased the adsorption at pH 2.5-10.0, and fast decrease occurred at pH range of 4-7, in which the HA adsorption amounts by Gt, HeM and FeM decreased by 39.4%,52.8% and 50.3%, respectively. (3) The HA adsorption increased with the increase of ionic strength from 0-0.01 mol/L, and then stabilized as ionic strength were high than 0.01mol/L. (4) The iron oxides had a high adsorption strength on the tested HA with a desorption rate lower than 2.5%at the tested pH value and ionic strength, and indicated that coordination between hydroxyl groups on iron oxides and HA was the main mechanism to the HA adsorption by the three oxides, which was supported by characterizations of the complex structures. (5) Complexation of HA on the oxides did not change the crystal structure of the oxides, but significantly alter their surface morphology. XRD images and SEM (scanning electronic microscopy) observation showed that the crystal structure of the oxides did not change after HA adsorption with a slight expansion in crystal lattice; characteristic peak remained the same before and after adsorption, but the surfaces became rough with HA adsorbed existing uniformly in spot form on the surfaces of oxides; pHpzc(Point of zero charge)of complexes correspondingly decreased with the sequence of PZC value remaining the same before and after adsorption; the specific surface area (SSA) of FeH-HA complex slightly decreased while the SSAs of Gt-HA and HeM-HA complexes increased, compared with the three bare oxides.2. HA could reduce Hg2+ and AsO43- to a certain degree, which could be described by second-order kinetic equation. The reduction rate of Hg2+ and AsO43-could reach up to 11.54% and 4.96% after 36h reaction at following conditions: 0.01mol/L ionic strength, pH7, HA concentration 4.0mg/L, Hg2+ 30 mg C/L, and 25±1℃ in darkness after instant heat sterilization, which indicates a higher reduction ability to Hg2+ than to AsO43-. During the concentration range of HA, Hg2+ and AsO43- in our adsorption and resolution experiments, coordinating chelation was the dominant mechanism compared with reduction.3. The adsorption characteristics of HA-oxides complexes varied from that of bare iron oxides, which meant a significant impact on the secondary adsorption of Hg2+:complexes had a higher adsorption rate and a bigger adsorption capacity of Hg2+ than bare iron oxides; pH and ionic strength affected Hg2+ adsorption on complexes consistently as on bare iron oxides. (1) The adsorption kinetics of Hg2+by three types adsorbents (HA, iron oxides as well as complexes) could be well described by Lagergren second-order reaction kinetics equation with the adsorption rate constants sequence being as iron oxides (Gt, HeM and FeM being 26.87、 156.03、13.36, respectively)<HA (30.21)< complexes (three complexes being 36.77、30.51、22.22 respectively); (2) Langmuir equation could well describe the characteristics of the three types of adsorbents on Hg2+ isothermal adsorption:QmAX of Hg2+ by HA being 1.523 mg/g; the Qmax of Hg2+ by oxides following the sequence of Gt (1.03 mg/g)>FeH (0.867 mg/g)> HeM (0.814 mg/g); the Qmax by complexes following the sequence of Gt complex (3.20mg/g)> FeM complex(1.96 mg/g)> HeM complex (1.75 mg/g). Generally, the sequence of the three types of adsorbents on Hg2+ adsorption was iron oxides< HA< complexes, which meant the Hg2+ adsorption could be increased after complexation while the adsorption amount was smaller than the sum of that by HA and iron oxides. The reason was that the complexation between HA and iron oxides caused the consumption of some binding sites. (3) pH value was the most influential factor of Hg2+ adsorption on iron oxides, HA and complexes but to a different extent, respectively. The adsorption capacity of bare iron oxides and their complexes increased with the increase of pH, the adsorption curve showing an "S" shape:adsorption rate increased slightly with the increase of pH when pH was lower than 5.5; fast increase occurred at pH range of 5.5～7.0, then adsorption rate stabilized at pH range of 7-10. Adsorption rate of Hg2+ on HA, reaching the peak at pH 5.5, was far less affected by pH than that on iron oxides and complexes. (4) Ionic strength could affect the Hg2+adsorption on the three adsorbents, adsorption rates increasing with ionic strength increasing, but to different extends:Hg2+ adsorption on HA was less affected by ionic strength than on iron oxides and complexes when ionic strength was lower than 0.01mol/L, while less influences on Hg2+ adsorption on iron oxides were found when ionic strength was higher than 0.02mol/L. (5) SEM observation showed that significant differences of surface morphology existed after adsorption of Hg2+ for all three oxides:the surfaces were getting blurred and folded, indicating a strong adsorption of Hg2+, and the accumulated Hg2+ ions on oxides surfaces caused high roughness of the surfaces. Most of the spots on the surfaces of complexes disappeared after Hg2+ adsorbed, and their structures were spilt, causing rough and uneven surfaces. The phenomenon was distinguished on Gt-HA complex and FeH-HA complex, and could support the occurrence of inner coordination between complexes and Hg2+ions.4. Complexation between HA and iron oxides could not only affect the secondary adsorption of Hg2+, but also affect the secondary adsorption of AsO43-: complexes also had a higher adsorption rate of AsO43- than bare iron oxides, but the adsorption capacity was decreased significantly; pH and ionic strength affected Hg2+ adsorption on complexes consistently as on bare iron oxides. (1) The adsorption kinetics of AsO43- ions on three types of adsorbents could be well described by Lagergren second-order reaction kinetics equation and the adsorption rate constants showed a sequence of iron oxides (Gt, HeM and FeH being 12.31,11.49,21.04, respectively)< HA (89.32)< complexes (three complexes being 121.3,102.4,198.5, respectively), which meant a higher AsO43- adsorption rate after complexation. (2) Langmuir equation could well describe the characteristics of the three types of adsorbents on AsO43- isothermal adsorption:Qmax of AsO43- adsorption on HA being 4.511 mg/g; the Qmax. sequence of AsO43- adsorption on oxides being as FeM(19.31 mg/g)> Gt(8.93 mg/g)> HeM(6.66 mg/g); the Qmax on complexes following the sequence of FeM complex(15.17 mg/g)> Gt complex(6.85 mg/g)> HeM complex (5.17 mg/g). Generally, the Qmax sequence of AsO43- adsorption on the three kinds of adsorbents was HA< complexes< oxides, which indicated that AsO43- adsorption amount on complexes was not only less than the sum amount on bare oxides and HA, but also less than that on bare HA. (3) The pH value, an influential factor, showed similar effects on AsO43- adsorption on the three adsorbents:the adsorption amount being large at the range of pH6-8 but decreasing at pH<5 or>8. Basically, AsO43-adsorption amount on a bare iron oxide was larger than that on its complex, which implied that complexation of HA with iron oxides reduced their AsO43- adsorption. (4) Ionic strength could also affect the AsO43- adsorption on the three adsorbents, but with much less impact than pH. AsO43- adsorption on iron oxides and complexes could increase with ionic strength increasing; AsO43- adsorption on HA followed the same rule when ionic strength was lower than 0.1mol/L then stabilized when the ionic strength was higher than 0.1mol/L. (5) SEM observation showed that most of the spots on the surfaces of complexes disappeared after AsO43- adsorption, and their structures were spilt, causing rough and uneven surfaces. The phenomenon could support the occurrence of inner coordination between complexes and AsO43- ions.5. The comparisons between Hg2+ and AsO43- secondary adsorption. Results showed:with regard to adsorption kinetics, the rate constants of AsO43- adsorption were larger than that of Hg2+ adsorption except on Gt and HeM, which indicated a faster speed of AsO43- adsorption by the three types of adsorbents; Qmax. of AsO43-adsorption was significantly larger than Qmax of Hg2+ adsorption, the former being several times larger than the latter, no matter what the absorbent was because of the differences between electrical property and charge of Hg2+ and AsO43- ions; complexes had a smaller adsorption amount on AsO43- while a larger adsorption amount on Hg2+than the corresponding oxides probably because of the difference of binding resistance caused by ionic radii. pH influence on Hg2+ adsorption by HA was similar to that on AsO43- adsorption while Hg2+ and AsO43- adsorption by bare oxides and complexes were affected by pH to different extend. Hg2+ adsorption on iron oxides or complexes was affected by pH to a significantly higher extend than AsO43- adsorption. |
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