Study On The Remediation Effect Of Nano-materials On The Cadmium Polluted Soil

In recent years,heavy metal cadmium(Cd)pollution has become more and more serious,which has seriously affected the yield and quality of crops and threatened human health.Passivation remediation is a simple,rapid and effective technique for the remediation of Cd contaminated soil.The key is to select passivation materials with wide source,high efficiency and environmental friendliness.In this study,three kinds of common nano-materials,such as nano-biological carbon,nano-hydroxylapatite and nano-zero-valent iron,were selected on this basis,pot experiments were carried out to explore the effects of nano-materials on the growth and quality of Brassica chinensis L.and the characteristics of rhizosphere soil.The effects of nano-materials on the vertical migration of Cd in Cd contaminated soil and the content of Cd in leaching solution were studied by soil column leaching test.The main findings are as follows:(1)The addition of nano-biological carbon,nano-hydroxylapatite and nano-zero-valent iron could increase soil p H and CEC at different culture time,and the effect was obvious with the increase of the addition amount,p H value increased by 0.07-0.76,0.11-0.71,0.04-0.53,CEC increased by 9.05%-44.32%,31.39%-62.87%,4.64%-35.70%.(2)The addition of nano-biological carbon,nano hydroxyapatite and nano-zero-valent iron alone can reduce the plant availability,leaching toxicity and bioavailable Cd content in soil,of which the reduction range for the plant availability Cd content is the largest,and the reduction ranges are 14.32%-51.05%,14.91%-69.05% and 7.90%-62.07% respectively after the culture is finished.The best addition ratios for reducing Cd availability to plants are 1%(nano-biological carbon),1%(nano-hydroxyapatite)and 0.5%(nano-zero-valent iron),respectively.(3)The effect of nano-materials on the form of Cd in soil is mainly manifested by the transformation of exchangeable Cd with high availability to form with low availability.After the cultivation,the reduction ranges of exchangeable Cd content of the three nano-materials are 22.89%-57.78%(nano-biological carbon),12.89%-73.33%(nano-hydroxyapatite)and33.33%-77.78%(nano-zero-valent iron),respectively.With the extension of culture time,the changes of all indexes were basically stable after 28 days.The optimal addition ratio of the three nano materials is 1%(nano-biological carbon),1%(nano-hydroxyapatite)and 0.5%(nano-zero-valent iron),respectively.(4)Under the condition of pot experiment,adding nano-biological carbon(1%),nano-hydroxyapatite(1%)and nano-zero-valent iron(0.5%)can significantly increase the biomass of Brassica chinensis L.,and the fresh weight is 1.87,2.28 and 1.66 times that of the control respectively.The content of Cd in the edible part of Brassica chinensis L.was reduced by 49.69%,68.10% and 57.06%,respectively.The content of Cd in the edible part of Brassica chinensis L.reached the limit of 0.2 mg/kg specified in the National Food Safety Standard(GB 2762-2017).The application of nano-materials increased the content of soluble sugar,soluble protein and vitamin in Brassica chinensis L.,and reduced the content of nitrite by65.84%,72.80% and 46.22% respectively,and the nitrite content did not exceed the safety standard of nitrite content(20 mg/kg)stipulated in the National Food Safety Standard(GB2762-2017),thus improving the nutritional quality and safety quality of Brassica chinensis L..(5)The addition of nano-materials increased the p H of Brassica chinensis L.rhizosphere soil and significantly reduced the available Cd content in the rhizosphere soil by 49.69%(nano-biological carbon),68.10%(nano-hydroxyapatite)and 57.06%(nano-zero-valent iron),respectively.At the same time,it promoted the transformation of exchangeable and carbonate-bound Cd content to the form of Cd that is difficult to be utilized by organisms,thus reducing the bioavailability.The activities of urease,catalase and sucrase in rhizosphere soil increased to different degrees after nano-materials were applied,thus improving the soil ecological environment.(6)Based on the results of soil column leaching tests,under leaching conditions,the addition of nano-biological carbon(1%),nano-hydroxyapatite(1%),nano zero-valent iron(0.5%)reduced the amount of Cd migration from 0-5,5-10,10-15,15-20 cm soil layers to the lower soil layers,resulting in a significant reduction of Cd content in 20-40 cm soil layers.Compared with no nano material added,Cd in 20-30 cm soil layer decreased by 47.06%,70.58% and 58.82% respectively,and in 30-40 cm soil layer decreased by 50.00%,83.33%and 67.00% respectively,thus reducing the risk of Cd migrating to deep soil layer.At the same time,the contents of available and exchangeable Cd in 0-20 cm soil layers are also significantly reduced after adding nano-materials,which indicates that adding nano-materials reduces the effectiveness of Cd,thus reducing the risk of vertical migration of Cd and the risk of pollution to the underlying soil.(7)With the increase of leaching times,the concentration of Cd in the soil leaching solution also gradually decreases.After leaching,the soil treated with nano-materials reduces the Cd content in the leaching solution by 53.28%(nano-biological carbon),72.71%(nano-hydroxyapatite),79.08%(nano-zero-valent iron),reaching the Class IV water standard(0.01 mg/kg)in the Groundwater Quality Standard(GB 14848-2017),thus reducing the risk of Cd pollution to groundwater.In conclusion,nano-biological carbon,nano-hydroxyapatite and nano-zero-valent iron have good passivation effect on soil heavy metal Cd,and can be used as efficient and environmentally friendly soil Cd pollution passivation repair materials for promotion.