| With the rapid development of modern industry, the mining and application of heavy metals are more and more widely in recent years, large quantity of heavy metal wastewater is discharged into the environment without any standard treatment, which has produced a great threat to human health and ecological environment. Therefore, effective treatment of heavy metal wastewater has become important and urgent in today’s field of environmental protection. The traditional treatment process can only reduce the initial wastewater to a certain concentration, so heavy metals cannot be completely removed from the water. Adsorption is a promising heavy metal wastewater treatment method because of its high removal efficiency for low concentration heavy metal ions with wide range of adsorbents and can effectively achieve the emission standards. Activated carbon is effective in removing metal ions from aqueous solution, but its use is not suitable for widely application due to its high capital and operation costs. Currently, people are developing a variety of effective and low-cost adsorbents to solve the problem of expensive activated carbon.Sawdust is a kind of forestry waste, which is cheap and readily available with a huge production. It shows a broad application prospect in heavy metal wastewater treatment because of its porous surface structure and large quantity of hydroxyl functional group, but the adsorption capacity of raw sawdust is not high, modification is needed to improve the adsorption capacity. Modified sawdust was prepared by pyrolysis and NaOH soaking process, and the modification conditions of which was optimized in this paper. The change of surface structure characteristics was observed and evaluated through characterization. Through batch experiment in laboratory, the influential parameters such as pH, initial concentration, adsorption time, adsorbent dosage, particle size and ionic strength were determined. The adsorption performance of Cu2+ from aqueous solution by modified sawdust has been compared with raw sawdust under the same adsorption conditions. The adsorption isotherm, kinetics and thermodynamic parameters have been studied. Therefore, the adsorption process of modified sawdust for copper ion was further revealed, and the adsorption mechanism was explored.The result show that the best modification conditions is as follows: pyrolysis temperature was 240℃, the pyrolysis time was 45min, NaOH solution soaking time was 3h, NaOH solution immersion concentration was 0.4mol/L, NaOH solution soaking ratio was 1:20.The specific surface area of modified sawdust was larger and some obvious pores was appeared, the surface was smoother and more hydroxyl can be observed compared with the raw sawdust. The metal ion adsorption percentage on modified sawdust can reach 99% and the remaining concentration is less than the national requirements of a wastewater discharge standards (0.5mg/L) under the condition of initial concentration 20mg/L, adsorbent dosage 4g/L, pH 5.The adsorption equilibrium time of modified sawdust for Cu2+ shortened by 60 min, and the adsorption rate increased by 50% compared with the raw sawdust and activated carbon. The adsorption behavior of Cu2+ on modified sawdust fit well to the Langmuir models, and the maximum adsorption capacity could reach 31.25mg/g, which was 8 times and 3 times higher than that of unmodified sawdust and activated carbon, respectively. The pseudo-second-order kinetic model well fit the adsorption kinetics of Cu2+ on modified sawdust. Removal of copper ions from aqueous solution by modified sawdust is the result of chemical and physical adsorption interaction. |
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