The Adsorbability Of Alkali-modified Paulownia Leaf Powder On Pb²⁺ In Aqueous Solution

Issues concerning treatment of heavy metal wastewater have been widely given attention by scientists from all over the world, and serious threats caused to human health have also been gradually recognized. Traditional methods（such as chemical reduction method, ion exchange method, adsorption method and membrane separation method） for treatment of heavy metal wastewater are mainly applied to condition that concentration of heavy metal ion is high, when concentration of heavy metal ion is low, problems such as high operating cost, complex operation, low treatment efficiency and secondary pollution easily caused exist in case of treatment by traditional methods. While as to forestry and agricultural residues, because of low utilization rate, general treatment methods including open-air stacking and burning, may not only lead to tremendous energy waste, but also cause environmental pollution. However, forestry and agricultural residues possess some advantages such as short regenerative cycle, abundant production, biodegradation, extensive sources and low cost etc., besides, major components such as cellulose, hemicellulose and lignin contain abundant –OH,-COOH,-NH2 and other active groups with great specific surface area. It is indicated through study that forestry and agricultural residues can better remove heavy metal ions in wastewater after chemical modification. There are many methods for modification of forestry and agricultural residues, and it is found through lots of experiments that more adsorption sites of forestry and agricultural residues may be exposed through alkali treatment, thus reaching better adsorption effects. Paulownia is widely covered in the country, and there is a large planting amount, and it may produce lots of fallen leaf wastes in winter each year, not only possibly leading to environmental pollution, but also leading to huge resource water in case of being discarded under natural environment or open-air burning. Additionally, it is found through our research that removal rate of Pb²⁺ in waste water by paulownia leaf powder modified by Na OH- alcohol approaches to 100%, thus showing that paulownia leaf powder modified by alkali may generate good removal effect to Pb²⁺. However, production cost of Na OH is high, and processing may easily cause environmental pollution, not conforming to saving and environmental protection requirement. In contrast, as the natural material for production of Na OH, Ca（OH）2 possesses strong basicity and is low-cost and easily obtained. Therefore, paulownia leaf powder modified by Ca（OH）2 to produce efficient bio-adsorbent of heavy metal ions is of great scientific value and practical significance.In the paper, paulownia leaves were selected as object of study, and paulownia leaf powder was modified by Ca（OH）2 to adsorb low-concentration Pb²⁺ in the solution, additionally, influences of reaction time, adsorbent concentration, p H and initial concentration of heavy metal these four factors on absorption effect were discussed. Moreover, unmodified powder, modified powder and adsorbed powder are characterized through scanning electron microscope（SEM）, energy spectrum analysis（XPS） and X-fluorescence spectrum semi-quantitative analysis. Results indicated that adsorbing time, adsorbent amount, solution p H and initial concentration of metal ions may influence adsorbing effect. Adsorption equilibrium may be reached in case of adsorbent concentration of 0.8g/L, solution p H of 5 and reaction time of 60 min, and at that time adsorbing capacity reached 60.43 mg/g, removal rate reached 95.61%. In case that initial concentration of Pb²⁺ was within 15-100 mg· L-1, removal rate of Pb²⁺ through utilization of alkali-modified paulownia leaf powder adsorbent reached over 93.0%, and with increase of initial concentration of heavy metal, adsorbing capacity obviously increased. Adsorption process of Pb²⁺ may be described with pseudo-second-order adsorption kinetic model and isothermal adsorption model. It is found through scanning electron microscope（SEM） that the surface of unmodified powder is smooth, and each functional group is relatively dispersed which is appearing granular; while modified powder changes greatly, and the original structural form has been destroyed, and the surface becomes loose and rough, appearing laminar and flocculent; after adsorption of Pb²⁺, powder surface becomes aggregate again, and there are lots of particulate matters and meals which are closely linking among particles. It is found through energy spectrum analysis（XPS） and X-fluorescence spectrum semi-quantitative analysis that Pb²⁺ is adsorbed onto the surface of paulownia leaf powder adsorbent during adsorption process and cation exchange occurs with Ca²⁺, Mg²⁺, Si²⁺, Al²⁺, Fe²⁺ and other ions in the adsorbent.Preparation of modified paulownia leaf powder adsorbent by Ca（OH）2 instead of Na OH is not only energy-saving, environmental and low-cost, but it may also reach similar removal effect. Therefore, paulownia leaf powder modified by Ca（OH）2 acting as adsorbent to remove heavy metal ion is a method for heavy metal treatment which is worthy of intensive study and popularization.