| With the rapid development of economy of China, water pollution has been becoming more and more serious. Heavy metal pollution in water environment has been already very common in our daily life. The cadmium ion has been identified as priority contaminants by World Health Organization, which also is a hot research object. Reference to our national environmental laws, cadmium is the first class of pollutants, which means that dilution cannot work as a necessary treatment. All of this reflects that researchers should pay much attention to the cadmium pollution on a global scale. The way to remove cadmium consists of chemical precipitation, physical and biological adsorption. The adsorption is one of the most common methods used to deal with the heavy metals in wastewater because of cheap, available material, simple operation and process. The adsorption fillers we commonly used include zeolite and activated carbon. A new type of adsorbent we studied is biomass charcoal. However, the adsorption capacity of the adsorbent is limited. We must improve the performance of the adsorbents, which could increase the cost and difficulty of the operation. On the contrary, if we combine many kinds of adsorbents together to improve the adsorption capacity, it could achieve high efficiency.The study chose activated carbon, zeolite and biomass carbon as adsorbents to compare their absorption capacity and to discuss the detail process of absorbing heavy metals in waste water by the static adsorption experiments. At the same time, we investigated the removal rates of cadmium ion of the three adsorbents at different times, p H, solid-liquid ratios, temperatures and initial concentrations of cadmium ion.The results indicated that the adsorption rates of activated carbon could reach 99% at the initial 20 min and the adsorption reached balance with the removal rate of 99.5% 1 hour later. The zeolite and biomass charcoal all spent 1 hour achieving their adsorption balance with the adsorption rate of 99.75% and 95.25%, respectively. The adsorption of zeolite was strongly influenced by low p H. When the p H was 3, the removal rate of cadmium ion was 83.57%. When the p H was 5~10, the adsorption could be described as parabola. When the p H was 7, the removal rate of cadmium ion reached the maximum with the value of 99.22%. However, the removal rate of the biomass charcoal was affected slightly at the p H between 3 and 6. The ability of adsorption remained stable at the p H between 8 and 10 with the removal rate of 95.23%. The p H less affected the absorption of activated carbon. When the solid-liquid ratio was 3, the adsorption of zeolite and activated carbon could achieve adsorption balance. The different solid-liquid ratios less affected the adsorption of biomass charcoal.Adsorption capacity of biomass charcoal and zeolite on the removal rate of cadmium ion did not changed a lot at different temperatures. When the temperature increased, the adsorption capacity of activated carbon increased at the first stage, and then decreased again. When the concentrations of cadmium ion increased, the maximum adsorption could reach 3.205mg/g. The adsorption of the three adsorbents could be described by quasi second order kinetics model and Longmuir adsorption isotherm with the correlation coefficient was 0.9872, 0.9422 and 0.9911 for zeolite, biomass charcoal, and activated carbon, respectively.Results showed that the combination of a variety of adsorbents could increase the removal rate of heavy metal ions in wastewater. Disposable adsorption played an important role in joint adsorption. In this experiment, the best combination of adsorption materials was zeolite-activated carbon-biomass carbon, whose removal rate could reach 85%. |
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