Adsorption And Desorption Of Vanadium In Different Media By Humic Acid

With the rapid economic growth, environmental pollution presents variety, large quantity and spreading to a wide range. Vanadium is widely distributed in nature, often associated in titanium magnetite ore, weathering residues, iron ore, phosphate ore, et al. Vanadium was classified as environment risk elements in the late 1980 s. Therefore, It is necessary to study the removal of vanadium pollutan from the environment. Humic acid（HA） is a natural polymers, and can form a stable complex with heavy metals by uniting the metal ions and the organic molecules. Humic acid molecular surface has pore structure, adsorption surface, and can be a good adsorption carrier.In this study, two different sources of HA were used to explore the adsorption and desorption charcteristic of vanadium by HA in different medium. HA（A） was purchased from the international association of humic acid. HA（B） was extracted from red soil and purified according to the method recommended by the international association of humic acid. The effects of some environmental parameters on the adsorption and desorption of vanadium by the humic acid were studied including initial concentration of pentavalent vanadium [V（V）], concentration of humic acid, time of adsorption, solution p H, temperature, ionic strength, species and concentration of electrolyte. The main results were as follows:1. Adsorption of vanadium（1） At 90.09 mg·L^-1 vanadium, with the increase of the concentrations of HA（A） and HA（B）, the adsorption quantity of vanadium increased first and then decreased. When the concentration of HA（A） was 1.09 g·L^-1, and the concentration of HA（B） was 0.727 g·L^-1, the adsorption quantity of vanadium by HA reached the maximum(47.58 mg·g^-1 and 21.49 mg·g^-1, respectively). The adsorption rate increased gradually with the increase of the concentration of HA. At 8.36 mg·L^-1 vanadium, with the increase of the concentrations of HA（A） and HA（B）, the adsorption capacity and adsorption rate of vanadium by the coexistence system of HA and SiO₂ rapidly increased and then leveled off.（2） With the increase of the concentration of vanadium, the adsorption quantity of vanadium by all absorb ants in this study gradually increased and then reached equilibrium. However, the adsorption rate by all adsorbents showed decrease trends. The Langmuir model could represent all adsorption process in this study, but the Freundlich model only fit the adsorption of vanadium by the coexistence system of HA（B） and SiO₂.（3） At 90.09 mg·L^-1 vanadium, with the increase of reaction time, the adsorption capacity and adsorption rate of vanadium by HA gradually increased and equilibrated at 660 min and 960 min for HA（A） and HA（B）, respectively; the adsorption capacity and adsorption rate of vanadium at both high and low concentration by SiO₂, and the coexistence system of HA and SiO₂ gradually increased and then leveled off; the secondary dynamic model can well describe the adsorption process of vanadium by HA（A）, indicating that the adsorption process was mainly controlled by the chemical adsorption mechanism. The pseudo-first-order kinetics and the secondary dynamic model can well describe the adsorption process of vanadium by SiO₂, indicating that the adsorption process was mainly controlled by the diffusion process and chemical adsorption mechanism. The pseudo-first-order kinetics and the double constant model can well describe the adsorption process of vanadium by the HA（B） and the coexistence system of HA（B） and SiO₂, indicating that the adsorption process was mainly controlled by the reaction rate and diffusion process comprehensively. The pseudo-first-order kinetics model can well represent the adsorption process of vanadium by the coexistence system of HA（A） and SiO₂, namely the adsorption process was mainly affected by diffusion reaction.（4） At 90.09 mg·L^-1 vanadium, with the increase of solution p H, the adsorption quantity of vanadium by HA gradually decreased; the adsorption quantity of high and low concentration of vanadium by SiO₂ and the coexistence system of HA（B） and SiO₂ gradually decreased; the adsorption quantity of high and low concentration of vanadium by the coexistence system of HA（A） and SiO₂ decreased firstly and then increased. Namely in acidic and alkaline conditions, the HA had better capacity of adsorbing vanadium.（5） At 90.09 mg·L^-1 vanadium, with the increase of reaction temperature, the adsorption quantity of vanadium by HA（A） gradually decreased, and there was no significant difference of the amout of vanadium adsorption by HA（B） between different reaction temperature; the adsorption quantity of high and low concentration of vanadium by the coexistence system of HA（A） and SiO₂ gradually decreased with the increase of reaction temperature and no significant difference was found by the coexistence system of HA（B） and SiO₂. These results indicate that too low or too high temperature was not conducive to the adsorption of vanadium by SiO₂.（6） There was no significant difference of the adsorption of vanadium by all adsorbents in certain kinds of supporting electrolyte, such as 0.005 mol·L^-1 NaNO₃, Na₂CO₃ and Na₂SO₄.（7） The coexisting ions(Fe²⁺, Al³⁺, K⁺, SO₄^2-, CO₃^2- and Cl^-) significantly affected the adsorption of vanadium by HA and SiO₂.（8） Infrared spectrum analysis of HA before and after the adsorption of vanadium showed that the main functional groups involved in the adsorption process were hydroxyl, methyl oxygen, carbon keys and silica keys, indicating that the adsorption of vanadium by HA（A） and HA（B） was mainly controlled by chemical adsorption.2. Desorption of vanadium（1） At 0.364 g·L^-1 HA（B）, the desorption quantity of vanadium absorbed by HA（B） reached the maximum of 2.601 mg·g^-1. The desorption rate gradually decreased and then leveled off with the increase of the concentration of HA（B）.（2） With the increase of the concentration of vanadium, the desorption quantity of vanadium absorbed by HA increased gradually.（3） The desorption progress of HA（B） occurred rapidly in the initial reaction stage（0 to 30 min） and then slowed down, and the desorption equilibrium was achieved at 720 min. The desorption progress of HA（B）+SiO₂ reached the desorption equilibrium at 120 min, and the desorption quantity was about 44.89 mg·kg^-1. The pseudo-second-order kinetics could characterize the desorption process of vanadium absorbed by HA（B）, SiO₂ and the coexistence system of HA（B） and SiO₂.（4） With the increase of solution p H, the desorption quantity and desorption rate of vanadium absorbed by HA（B） gradually decreased; the desorption quantity of vanadium absorbed by SiO₂ had a certain degree of increase; and the desorption quantity of vanadium absorbed by the coexistence system of HA（B） and SiO₂ increased gradually.（5） When the concentration of Na NO3 was 0.005 mol·L^-1 to1 mol·L^-1, no significant difference in the desorption quantity of vanadium absorbed by HA（B） was observed. Na NO3 would affect the desorption process of vanadium absorbed by SiO₂ and by the coexistence system of HA（B） and SiO₂.（6） With the increase of the reaction temperature, the desorption quantity and desorption rate of vanadium absorbed by HA（B） gradually increased; the desorption quantity of vanadium absorbed by SiO₂ increased firstly and then decreased; the desorption quantity reached the maximum about 57.99 mg·kg^-1 by the coexistence system of HA（B） and SiO₂.（7） When the concentration of NaNO₃, Na₂SO₄, Na Cl and NaNO₃ with Na₂SO₄ was 0.005 mol·L^-1, there was no significant effect of these electrodes on the desorption process.In general, the adsorption capacity and adsorption strength of vanadium by HA（A） were higher than thoes by HA（B）; the adsorption capacity and adsorption strength of vanadium by the coexistence system of HA（A） and SiO₂ were higher than those by HA（B） and SiO₂; and the adsorption capacity and adsorption strength of vanadium by the coexistence system were higher than those by the single absorbent. The desorption progress of vanadium by HA（B） and SiO₂ mainly controlled by the diffusion process and chemical desorption mechanism; while the desorption progress by the coexistence system of HA（B） and SiO₂ mainly controlled by the chemical desorption mechanism. The adsorption and desorption of vanadium by HA and SiO₂ were affected by many environmental factors, such as concentration of humic acid, time of adsorption, solution p H, temperature, ionic strength, species and concentration of electrolyte.In this study, we compared the adsorption and desorption characteristics of vanadium by different sources of humic acids. But the adsorption and desorption mechanisms were still not clear. Further study about the mechanism of vanadium adsorption by humic acids should be considered in the further. Moreover, in this study we just discuss the adsorption characteristics of vanadium by humic acids and SiO₂, coexistence metals should be taken into consideration in the future.