| Humic acid is widely distributed in soil and water, it is a group of organic polymer with complex structure in which many functional groups such as carboxyl and hydroxy is included, having profound influences on the transformation of organic and inorganic matters, biological activity as well. Iron oxides which are the most important mineral component of soil and sediment have huge surface area, surface redox and ligand complexing capacity. By different mechanisms, Humic acid and iron oxides can strongly adsorb metal ions, and thus having an important impact on the formation of soil structure, soil level differentiation, the occurrence forms of metal ions in the soil and water environment, bio-effectiveness, environmental toxicity and material recycling in environment. In the actual environment, the iron oxides and humic acid coexist in the soil and water and the interactions between them are complex, so the surface chemical properties is certain to be significantly different from the single system, which has great effect on heavy metal absorption. At present, there’s little research on the interaction and oxides and humic acid, specific response mechanism between iron oxide-humic acid and mercury is not clear.This paper studies the effect of pH, ionic strength on the adsorption of Hg by iron oxides and the complex and the adsorption-analytical characteristics of mercury before and after interaction between humic acid and iron oxide using adsorption-desorption thermodynamics and kinetics methods. By X-ray diffraction spectroscopy and scanning electron microscope, this article studies the difference of spectral characteristics before and after mercury is absorbed by the complex. The main findings are as follows:Different types of iron oxide show different adsorption capacity on humic acid. The adsorption capacity of goethite is stronger than that of hematite and ferrihydrite. Langmuir equation is most suitable for the isothermal adsorption of humic acid by the three iron oxides. The sequence of maximum adsorption capacity:goethite (7.73μg/g)> ferrihydrite (3.14μg/g)> hematite (2.25μg/g). Ionic strength and pH are two important factors which effect the interaction between humic acid and iron oxides. with the rise of pH, the adsorption quantity of humic acid decrease. It decreases sharply when pH is between4and7. With the increasing ionic strength, three iron oxides enhanced the adsorption capacity of humic acid, when the ionic strength is beyond0.01mol/L, the adsorption capacity is basically stable around2μg/mg. humic acid is difficult to desorption after it is adsorbed by iron oxide under different pH and ionic strength conditions, the desorption rate is0.8%-2%. Difficulty level of desorption (average desorption rate) at different ionic strength:ferrihydrite (1.26%)> hematite (1.04%), goethite (0.97%); at different pH:hematite (1.32%) and ferrihydrite (1.19%), goethite (1.06%).The adsorption capacity of the mercury by different types of iron oxides and iron oxides-complex is also quite different. The adsorption capacity of mercury by goethite is stronger than that of hematite and ferrihydrite; adsorption on mercury performance of iron oxides and humic acid complexes shows:goethite> ferrihydrite> hematite complexes. Mercury isothermal adsorption of hematite and ferrihydrite can better fit the Freundlich equation and Langmuir equation, but goethite fits the situation worse (R2=0.7948); complex on the mercury adsorption fits relatively better Freundlich equation; there is a big difference between the langmuir equation fitted the maximum adsorption capacity and the actual adsorption capacity. For different pH on the adsorption, the adsorption capacity increases with the increase in pH value, the adsorption curve shows a "S" shape, Adsorption of iron oxides and compounds of mercury are present a smaller pH range (5.5<pH<7). The adsorption capacity of goethite mercury increased from about65μg/g to330μg/g, while that of hematite and ferrihydrite respectively to200μg/g,230μg/g; and three complexes to226μg/g,132μg/g,175μg/g. The pH value mainly influences Hg2+in solution hydrolysis and oxidation of iron surface properties of iron oxides and their complexes to impact the adsorption of mercury. With the increase of ionic strength, iron oxides and their complexes on the mercury adsorption capacity gradually increased, the growth trend relatively cut down. Different mercury concentrations, mercury the size of the desorption rate as follows: ferrihydrite (12.51%), goethite (8.83%), hematite (8.41%); hematite complexes (35.28%)> ferrihydrate composite matter (21.92%)> goethite complexes (15.49%).Adsorption capacity of mercury by iron oxides is improved to some extent after they are combined with humic acid. The adsorption rate of mercury by goethite and hematite complexes is greater than the corresponding iron oxides:the adsorption rate increased by11.36%and6.97%respectively. Ferrihydrite compounds and ferrihydrite adsorption rate is roughly the same. The result shows that the adsorption rates of mercury by the complexes are higher than the corresponding iron oxides when pH is low and ionic strength is high. In the7<pH<9, average adsorption rates of the compound of goethite, hematite complexes, ferrihydrite complexes were higher than the corresponding iron oxide:14.00%,9.92%and15.51%. In the condition of the ionic strength greater than0.05mol/L, the average adsorption rates of the complexes are higher than the corresponding iron oxide:4.96%,5.18%and6.05%. Under different ionic strength conditions, the iron oxide-humic acid complexes of mercury desorption rates are much greater than the corresponding iron oxide. Among six different ionic strength, when the ionic strength is0mol/L, the desorption rates of iron oxides are quite different from that of the complexes and the rates are as follows:12.80%,34.12%and14.38%. At different pH values, complexes of hematite and ferrihydrite complex desorption rate is higher than its corresponding iron oxide, as pH=2.5, the desorption rates between iron oxides and their complexes shows the maximum:4.92%,10.13%; goethite and its complex desorption rate is roughly equivalent to six different pH conditions.The results of kinetic experiments show that:the isothermal adsorption of mercury by iron oxide and iron oxide-humic acid complex can be divided into two phases, adsorption capacity and adsorption rate of the first phase is much larger than that of the second phase, the adsorption quantity of the first stage covers more than85%of the equilibrium adsorption quantity. Parabolic equation and Elovich equation can describe the adsorption dynamics of mercury well,(R2values are greater than0.8), Elovich equation fits best. For the comparison’s sake, the adsorption capacity of mercury by the complexes should be greater than that by iron oxides. The sequence of time taken by90%adsorption equilibrium is as follows:goethite complexes<ferrihydrite complexes<ferrihydrite<hematite complexes<goethite<hematite, the equilibrium adsorption quantity:goethite complexes> ferrihydrite complexes> goethite> hematite complexes> ferrihydrite> hematite.After the three iron oxides adsorbing humic acid interaction, XRD patterns showed that the iron oxides surface adsorption of humic acid did not change its crystal structure, nor the formation of new lattice, only increased the lattice. Found by SEM mapping, after the interaction between iron oxides and humic acid, the surface of humic acid appears block structure; Comparing SEM spectra of iron oxides and their complexes before and after the Adsorption of Hg, we found that the adsorption of mercury, mineral and compound the material structure showed a certain degree of dissociation. The main role of Iron oxide and humic acid complexes adsorption the mercury is the combination between mercury and iron oxide and humic acid adsorbing bound mercury. pH regulation of the iron oxide surface charge is one of the mechanisms of complex adsorption of mercury.Under the experimental conditions, the reduction value of iron oxides and their complexes on the mercury is very poor, which little affects on the analysis of experimental data. Therefore, the experiment does not consider the reduction. |
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