Treatment Of Mercury Wastewater By Modified Agricultural Straw Active Carbon

Mercury wastewater is a kind of wastes with heavy metal discharged in the process of industrial products. It is known the harm of mercury wastewater and a variety of physical, chemical and biological treatments have been developed. However, there are still many disadvantages in these treatments to limit the engineering application. It is considered as one of the important sources of mercury pollution in environment. Along with the higher mercury emission threshold in both international and domestic policies, it is crucial that high-efficiency, low-consumption and low-cost treatments materials and technologies to be developed, to ensure the emission standard. Therefore, agricultural crops straw activated carbon was used as material in this research. After the modification, the characteristics and mechanisms of adsorption and the influence factors of modificated activated carbon had been studied, to find out the parameters in treatment of mercury wastewater. The aim is to provide scientific basis to develop low-cost, high-efficient and environmental friendly mercury adsorption materials. Results showed that:1. Activated carbon adsorption performance was influenced by modified conditions. There were remarkable differences in the Hg(Ⅱ) adsorption rate of modified activated carbon at different temperatures of 600℃, 800℃ and 1000℃. Activated carbon was modified at 1000℃ had the highest adsorption rate. After modified by sulfur, the higher content of sulfonium group in modified activated carbon increased the adsorption of Hg(Ⅱ). However, this adsorption was affected significantly by carbon-sulphur mass ratio, which the ratio of 1:3 had the highest effect in this research. Moreover, the size of carbon particle also has significant influence on the adsorption. No trend was observed along with the change of particle-size and medium size(150μm) of modified activated carbon had the best performance. There was no significant effect of the modification time of activated carbon on the adsorption.2. Compared with the unmodified activated carbon, the removal ratio of Hg(Ⅱ) in the solution of modified activated carbon was increased significantly. The adsorption capacity was related to the p H of solution, initial concentration and adsorbent dosage. The effective range of p H in modified activated carbon was smaller than unmodified activated carbon. The removal ratio of modified activated carbon for Hg(Ⅱ) reached maximum around p H=4, and the removal ratio of unmodified activated carbon was increasing with the increase of p H. Initial concentration of Hg(Ⅱ) also affected the adsorbent removal ratio. In the same initial concentration, modified activated carbon had greater removal ratio of Hg(Ⅱ) than unmodified activated carbon. Hg(Ⅱ) removal ratio increased with the increasing amount of activated carbon, but the adsorption capacity reduced. In the condition of initial concentration of Hg(Ⅱ) was 30 mg/L, activated carbon dosage was 3.0g/L, the removal ratio of modified activated carbon for Hg(Ⅱ) was 99.8%, whereas unmodified activated carbon was only 80%. In the adsorption equilibrium, adsorption capacity of modified activated carbon was 33.10mg/g, greater than the unmodified activated carbon of 11.67 mg/g. The activated carbon adsorption process was endothermic, but no significant difference was observed at different temperatures in the study. The adsorption progress of Hg(Ⅱ) by activated carbon complied with the Freundlich equation, the mechanism was multiple molecular layer adsorption, adsorption kinetics conformed to the Lagergren pseudo-second-order model, and chemical adsorption dominated the adsorption process.3. The effect of treatment of simulate mercury wastewater by modified activated carbon was affected by p H and contact time significantly. In the condition of activated carbon dosage was 4g/L, simulated wastewater concentration of Hg(Ⅱ) was 30mg/L, the optimum parameter were: p H=5, contact time= 5h, which removal ratio of Hg(Ⅱ) was 72%. It was noticed that there was a difference between process test results and the experimental results.Conclusion: after the modification of sulfur, the surface adsorption of straw activated carbon was improved significantly. The adsorption was mainly affected by p H, adsorbent dosage and initial concentration, without the affection of temperature. The process of activated carbon adsorption of Hg(Ⅱ) complied with the Freundlich equation, adsorption kinetics conformed to the Lagergren pseudo-second-order model, the adsorption mechanism was multiple molecular layer adsorption, adsorption type mainly chemical adsorption, chemical adsorption dominated the adsorption process, adsorption thermodynamics was endothermic reaction.