Hexavalent Chromium Adsorption On Iron Oxides And Iron Oxides-humics Complexes

As the growth of industry, the pollution caused by heavy metal is more and more serious. Among all of the heavy metal, hexavalent chromium is gaining a lot of attention because of its potential carcinogenicity and persistent effect on environment. When polluted by hexavalent chromium, its migration and transformation in water and soil is very important to the monitoring and assessment of the pollution. This paper synthesized two regular iron oxides(i.e., magnetite and hematite) via hydrothermal method. On one hand, these two iron oxides were acted as adsorbents of hexavalent chromium to investigate the influence factors of adsorption; on the other hand, magnetite and hematite became organic mineral complexes, which is very common in environment, after adsorbed humic acid or fulvic acid. These organic mineral complexes group are magnetite coated with humic acid(MH), magnetite coated with fulvic acid(MF), hematite coated with humic acid(HH) and hematite coated with fulvic acid(HF). The kinetics, isotherm, thermodynamics and the adsorption mechanism were utilized to study the interaction between the organic mineral complexes and hexavalent chromium. The main contents and results are summarized as follows:1. The morphology of magnetite and hematite is similar-globular particle with the diameter of about 10 nm. Most of pore is mesoporous and the difference in specific surface area and the distribution of micropore between the two iron oxides is evident. The specific surface area of hematite is higher than magnetite and the micropore area is much lower. The distinction in function group of humic acid and fulvic acid is apparent. Both humic substances exclusively contain some function group, but fulvic acid’s function group is much various.2. The pH of solution had the largest impact on the performance of hexavalent chromium adsorption. Although the difference of adsorption performance is small, the adsorption quantity of hematite is larger than magnetite as the growth of pH. Besides, two iron oxides still adsorbed a large amount of hexavalent chromium when the pH is higher than point of zero charge, indicating the key factors was not physical absorption. The formation of inner-sphere complexes may responsible for the small change of adsorption performance in different ion strength. As adsorbent dosage increased, the amount of hexavalent chromium adsorbed by magnetite and hematite was accelerate, while the utilization factor declined.3. In the preparation of organic mineral complexes, it was found that the adsorption quantity of humic acid or fulvic acid by hematite is higher than the one by magnetite, indicating the hematite is better than magnetite in adsorption once again. And the sorption of fulvic acid by magnetite or hematite is much bigger than humic acid, which may cause by the distinction in function group. The point of zero charge of four sorts of complexes was similar, much lower than pristine iron oxides, because of the surface coating by humics. And the existence of chemical adsorption was justified by their diverse performance of Cr(VI) sorption. In the end, variation trends of Cr(VI) adsorption by the two iron oxides before and after coated by humic substance.4. The kinetics data shows an excellent fit to pseudo-second-order model with high correlation coefficient, but poor fit to pseudo-first-order model. Langmuir equation, BET adsorption model and Freundlich isotherm can be used to fit the experimental data with good correlation coefficients, while the Temkin isotherm can not. By analyzing the parameters of the results fitted by above sorption model, it was found that all of adsorption process were spontaneous and occurred in the surface of sorbent. The chemisorptions and physisorptions may coexist in the sorption process by organic mineral complexes, although the chemiadsorption played a leading role. The adsorption by iron oxides and their complexes were all exothermic processes, apart from the hematite-fulvic acid complex.Compared to traditional method that employ pristine iron oxides as experimental subject, this study adopted four iron oxides-humics complexes as subjects, which represents extensive organic-inorganic complex in natural world. This study can mirror better the behaviors of transportation and transformation of hexavalent chromium in environment, and provide value information for control of hexavalent chrome.