| With the rapid development of industry, agriculture and transportation, the heavy metals released through human activities have been entering into the environment, which may aggravate the harm to the ecological environment and human health. Heavy metals in the environment have persistence, high accumulation and high toxicity, and are difficult to be degraded by microorganisms, and can enter to human body through the food chains. Mercury and cadmium are two common heavy metal elements with great hazard. The excessive exploitation and abuse of mercury and cadmium resources not only lead to resource loss, but also increase the contents of mercury and cadmium in the environment and organisms. Hg(Ⅱ) and Cd(Ⅱ) in human body can cause the poisoning of human nervous system, hematopoietic system, digestive system and blood circulatory system, and even to death. “Minamata disease” and “Itai-Itai disease” occurred in Japan are two typical incidents of mercury and cadmium pollution.At present, the removal methods of Hg(Ⅱ) and Cd(Ⅱ) in wastewater mainly include chemical precipitation, ferrite method, ion exchange, solvent extraction, membrane separation, biological method, and etc. However, these methods have disadvantages of cumbersome operation, high cost and long-time treatment, and are easy to cause secondary pollution, which may limit their practical applications. Adsorption method has advantages of simple operation, high efficiency, low cost, environmental friendly and the reuse of adsorption material, which becomes current research hotspot. Sepiolite is a kind of purely natural, hydrous magnesium rich silicate layered clay minerals with huge specific surface area and good ion exchange capacity, and has been used as the adsorption material to remove dye, antibiotics, nitrogen, phosphorus, copper and lead in water phase. However, the surface of natural sepiolite is weak acid ic with small channel, and practical specific surface area is difficult to achieve the t heoretical value(800~900 m2/g), and need be modified to improve its adsorption ability. It was reported that 3-mercaptopropyltrimethoxysilane has been used to modify speiolite, and lead into sulfydryl functional group, and showed good adsorption effects for Pb(Ⅱ) and Cd(Ⅱ), but this method need expensive reagent and harsh conditions. Therefore, it is of great importance to explore a simple and low-cost modified method. This study modified sepiolite with thioglycolic acid, and characterized the modified material by various analytical techniques, and the adsorption characteristics and mechanism of modified sepiolite to Hg(Ⅱ) and Cd(Ⅱ) were investiagted. The main results were obtained as follows:1. The sulfydryl content in the modified sepiolite was 0.084% measured by indirect iodine method, and sepiolite before and after modification were characterized by scanning electron microscope(SEM), X-ray diffraction(XRD), surface area analysis, and Fourier transform infrared spectroscopy(FTIR). The results showed that the surface of the modified sepiolite becomes smooth with little impurity, and the gap is clean and smooth, and fiber bundle is more obvious, its structure is looser with more space. The modification treatment did not alter the structure of sepiolite, and the layer spacing increased from 1.203 nm to 1.213 nm, and the surface area decreased from 141.631 m2/g to 72.491 m2/g. When the p H in 2.94~9.64, sepiolite surface before and after modification were both negatively charged. The modified sepiolite had a strong absorption peak at 3400~3600 cm-1, belonging to stretching vibration peak of hydroxyl, and indicated the introduction of hydroxyl groups in the modification process. These results showed that the modification process may improve the adsorption potential of sepiolite.2. The influence factors of the adsorption of the modified sepiolite to Hg(Ⅱ) and Cd(Ⅱ) were studied by single factor experiments, and then orthogonal experiments were designed to investigate the effect of adsorption conditions on the adsorption capacity. The optimum adsorption conditions were obtained, the initial concentration of Hg(Ⅱ) was 5 mg/L, the initial p H was 6, the dosage of sulfydryl- modified sepiolite was 15 g/L, and the temperature was 30°C, and the removal rate of Hg(Ⅱ) in wastewater reached 93.67% under these conditions. The initial concentration of Cd(Ⅱ) was 5 mg/L, the initial pH was 6, the dosage of sulfydryl- modified sepiolite was 8 g/L, and the temperature was 25°C, and the removal rate of Cd(Ⅱ) in wastewater reached 96.44% under the optimal experimental conditions.3. The adsorption process of sepiolite to Hg(Ⅱ) and Cd(Ⅱ) can be described by pseudo second-order kinetic equation, and the adsorption equilibrium time s of the modified sepiolite to Hg(Ⅱ) and Cd(Ⅱ) shortened to 60 and 120 min, respectively. The initial adsorption rate constants of the modified sepiolite to Hg(Ⅱ) and Cd(Ⅱ) were 0.063 and 0.311 mg×g-1×min-1, respectively. These results showed that the modification can significantly improve the adsorption rates of sepiolite to Hg(Ⅱ) and Cd(Ⅱ), and shorten the adsorption equilibrium times. The adsorption thermodynamic characteristics can be well described by Langmuir isothermal adsorption model. The maximum adsorption capacities of modified sepiolite to Hg(Ⅱ) and Cd(Ⅱ) were obtained as 3.256 and 5.263 mg/g, respectively. The results indicated that the modification can significantly improve the adsorption abilities of sepiolite to the two heavy metals.4. Gibbs free energy changes(DG) of the adsorption of the two materials to Hg(Ⅱ) and Cd(Ⅱ) were negative, indicating that the adsorption process was spontaneous. Adsorption distribution cofficients(KD) increased with the increase of temperature, which showed that it was beneficial for the adsorption process at high temperature. Positive enthalpy changes(DH) indicated that the adsorption of the two materials to Hg(Ⅱ) and Cd(Ⅱ) were endothermic process, and further illustrated that high temperature can promote adsorption process. In addition, DH values of the adsorption of the two materials to Hg(Ⅱ) and Cd(Ⅱ) were less than 40 kJ/mol, and indicating that physical adsorption was the main adsorption form. Based on the fitted results of Langmuir adsorption isothermal equation, the adsorption of Hg(Ⅱ) and Cd(Ⅱ) may be single molecule layer adsorption. The modification process introduced sulfydryl groups, the adsorption process drived by the strong affinity of sulfydryl groups with Hg(Ⅱ) and Cd(Ⅱ) belonged to chemical adsorption. Therefore, the a dsorption process was the result of physical adsorption and chemical adsorption, and dominated by physical adsorption.5. HCl solution can be used to desorp the adsorbed Hg(Ⅱ) and Cd(Ⅱ), and the desorption percents increased with the increase of HCl concentration. Compared with the adsorbed Hg(Ⅱ), the adsorbed Cd(Ⅱ) was difficult to desorp, and indicating that the modified sepiolite has strong adsorption ability for Cd(Ⅱ). The desorption process is beneficial for the regeneration of adsorbent and recycling of heavy metals. |
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