Remediation Of Cadmium-contaminated Paddy Soils By Washing With Calcium Chloride And Fulvate Acid

The contamination of heavy metal from soil has become one of the major problems affecting the sustainable development of agriculture in China.Because of its simple process and short cycle,the remediation effect is stable and the favorable remediation efficiency for high concentration contaminated soil,soil washing technology for soil remediation has been widely studied and applied in the remediation process of the heavy metal polluted soil.Cadmium polluted rice paddy soils from Changsha County in Hunan province were investigated in this thesis.Calcium chloride and fulvate were chosed as washing agents to explore the impact of various washing conditions on removal efficiency of Cd from soil;Furthermore,soil column leaching experiment was conducted by using calcium chloride as washing agent,and the migration behavior of Cd²⁺and Cl^-in the soil column was investigated in the experiment.Finally,preliminary exploration on the removal of Cd from soil by HA/Fe₃O₄/HGBs adsorbent capable of floating on the water was performed.The main results of this study are as follows:?1?The five kinds of paddy soil were collected as loam soil,sandy loam and silt loam,and Cd which was easy to be utilized by plants accounted for more than 40%of the total Cd,so it is suitable for the soil to be remediated by washing method.?2?The optimum conditions of calcium chloride and fulvate on the leaching of Cd in paddy soil:the concentration of calcium chloride and fulvate were 0.05 M and 4800 mg/L;the ratio of liquid to solid 4:1;the appropriate pH of calcium chloride and fulvate were respectively 5 and 7;leaching times were two;leaching time was 2 h.Under the optimal conditions,the removal of CaCl₂ on Cd from the soil was as high as 28.47%;the elution rate of Cd in the soil could up to 26.92%by fulvate.?3?Morphological analysis showed that water soluble and exchangeable Cd from paddy soil could be mainly removed by calcium chloride and fulvate.The leaching mechanism of calcium chloride on Cd from soil was mainly the exchange reaction between calcium ion and cadmium ion,and fulvic acid salt primarily removed Cd in the soil by chelation.The elution process of calcium chloride and fulvate on the removal of Cd in soil were suitable for pseudo-second order model,followed by double constant kinetic model.?4?The soil column leaching experiment which was performed with 0.05 M calcium chloride showed that it was very difficult to wash out the cadmium ion under natural conditions.Compared with soil column leaching,oscillating leaching of calcium chloride on the removal of cadmium in soil was more effective.In addition,the soil pH values after leaching of calcium chloride did not need special rectification.?5?The new adsorbent HA/Fe₃O₄/HGBs was synthesized by precipitation method,and the adsorbents were characterized by Fourier Transform Infrared spectroscopy?FT-IR??X-Ray Fluorescence?XRF?and Thermogravimetric Analysis?TGA?.HA/Fe₃O₄/HGBs adsorbent of 0.25 g was mixed with air dried soil of 5 g,and then was added with 25 mL deionized water,and oscillated 24 h at room temperature.The results showed that the adsorbent could remove the heavy metal Cd of paddy soil in the exchangeable state of 40%.