Remediation And Utilization Of Slightly-moderately Cadmium Contaminated Farmland Soil

With the rapid development of economy and the continuous advancement of industrialization,more and more polluting enterprises involved in heavy metal processing are discharging waste water,waste residue and waste gas that are not up to standard,as well as the unreasonable use of pesticides and fertilizers with excessive heavy metal content,resulting in the gradual accumulation of cadmium?Cd?in farmland,causing serious soil Cd pollution problems.Consequently,how to repair and control farmland soil polluted by Cd and ensure production safety has become a hot topic in current research.In this paper,the current situation and distribution of Cd pollution in farmland soil in Zhuji City,Zhejiang Province were investigated.On the premise of mastering the Cd pollution level in this area,the bioremediation research of removing Cd from soil by hyperaccumulator combined with microorganisms was carried out.Via pot and field experiments,the effect of microorganism addition on improving the removal efficiency of hyperaccumulator was confirmed.Meanwhile,in-situ remediation technology research was conducted in the farmland soil of rice-wheat rotation in this region to explore the effects of different remediation methods on available cadmium content in soil,cadmium accumulation in grain and crop yield.The main conclusions are as follows:?1?Soil p H ranges from 4.5 to 8.5.The total cadmium content in the soil in the study area is 0.42-0.68 mg·kg^-1.Compared with the national standard value,it is found that the over-standard rate is relatively high,and the Nemerow index P_Cd value is 1.76>1,and the environmental quality grade is level 3,which belongs to the cadmium pollution over-standard soil.Meantime,it was also found that the available cadmium content in the soil accounts for more than half of the total cadmium content,with the highest proportion of 57.2%.The bioavailability of cadmium in the soil is relatively high and there is a risk of safe use.The results showed that the total content and availability of cadmium in the soil decreased with the increase of depth,indicating that the impact of exogenous human factors on pollution was greater than that of parent materials.The investigation results around farmland showed that the total cadmium content in surrounding factories,roads and other places was significantly higher than that in the test field,and there was the possibility of aggravating the Cd pollution in farmland soil.?2?The addition of biomass charcoal?C?,glutamic acid?G?and rhamnolipid?R?were employed to strengthen the Cd resistance bacterial strain,combined with indoor pot and outdoor field experiments,and the improvement the phytoremediation effect by the combination of hyperaccumulator plants and Cd resistance bacterial strain?M6?was studied.Pot experiment results showed that the extracted Cd of Ca Cl₂ and DTPA in soil increased by 21.14%-52.77%and 23.31%-95.08%respectively,and the p H value decreased,which significantly increased the bioavailability of Cd,increased the enrichment rate of hyperaccumulator,and also affected the microbial community structure and functional gene abundance in soil,among which biochar could well enhance the activity of the strain after entering soil.According to the field experiment,the treatment of M6,C and R significantly increased the Cd absorption in Sedum leaves,and the content of total Cd in soil decreased,which the R treatment reduced the soil total Cd content up to 16.47%.?3?Two years and four seasons of remediation and safe utilization research were carried out in farmland soil with rice-wheat rotation.The effective Cd content in soil and accumulation of Cd in grains under mineral application,physiological barrier agent spraying,mixed amendments application,alkaline substance application and different irrigation methods were investigated respectively.After remediation,the average content of available Cd in soil decreased from 0.266 mg·kg^-1 to 0.196mg·kg^-1,with an average decrease of 26.3%.The accumulation of Cd in crop grain also decreased obviously.The maximum decrease of Cd content in grain was 52.3%and 63.3%,respectively after applying bichar and lime.The effect of selenium based leaf surface barrier on reducing cadmium accumulation in grain was more obvious,but the proportion of the whole wheat season reaching the standard was lower.In2019,treatment with the high dosage of the mixed amendment makes the content of Cd in wheat and rice grain reach to 0.055 mg·kg^-1 and 0.189 mg·kg^-1 respectively,with the better effect than that of lime control;Meanwhile,it was found that during the whole growth duration,waterflooding decreased the Cd accumulation in grain by67.8%.