Effect Of Immobilization Remediation Of Acidic Cd Contaminated Soil On Cd Accumulation In Leafy Vegetables

More than 16.7% of China's farmland soil is polluted by heavy metal,of which Cd is the most polluted heavy metal,which directly poses a threat to the quality and safety of agricultural products.Among the remediation methods for heavy metal pollution in farmland soil,in-situ immobilization remediation is widely used due to its low cost and remarkable effect.Most of the immobilization materials are calcium,phosphate and clay minerals,which are mainly used for the treatment of Cd pollution in acidic paddy fields in southern China,where rice is the model crop.However,the southern region is also an important producing area for vegetables,and there are relatively few researches on the remediation of vegetables as model crops.Vegetable cultivation mode is different from alternate dry and wet modes such as rice flooding and sun drying.At the same time,due to its short growth period,immobilization materials are required to function quickly in a short period of time,and the requirements for immobilization repairing are correspondingly higher.Therefore,the study used 5 kinds of leafy vegetables?pakchoi,lettuce and spinach?to carry out potted experiments,with the Cd content in leafy vegetables and the phytoavailable Cd content in the soil as the core indicators,to explore calcium,phosphorus or clay-containing minerals?including modified materials?immobilization effect.To provide scientific basis for the safe production of vegetable,the safe use of contaminated arable land and the restoration and treatment.The main results of this study are as follows:Experiments on immobilization effects of immobilization materials show:CaO,Ca?OH?₂,CaCO₃,calcium phosphate magnesium fertilizer,hydroxyapatite,calcium superphosphate,sepiolite,palygorskite,mercapto-grafted palygorskite,all materials can reduce the Cd content in three leafy vegetables and meet the food safety standards.And mercapto-grafted palygorskite treatment had a significantly effect.Compared with the control,the Cd content in pakchoi was reduced by68.37%-72.49%,the additive dosage of 0.2%and 0.3%in other pakchoi is reduced by 50.86%and48.63%,and the Cd content in lettuce was reduced by 58.82%-67.64%.Besides,mercapto-grafted palygorskite treatment also showed significantly effect in soil phytoavailable Cd content.Compared with the control,mercapto-grafted palygorskite decreased by 34.93%-46.07%,the other materials were smaller than the mercapto-grafted palygorskite.Limestone materials,phosphorus-containing materials and clay minerals all raised the soil pH value.The enrichment coefficient of pakchoi and lettuce decreased obviously,and reducing the Cd absorption of leafy vegetables.The enrichment of Cd in soil by pakchoi was relatively low,it indicated the pakchoi belong to the low accumulation variety of leafy vegetables.Experiments on the immobilization effect stability of immobilization materials show that immobilization materials had differences in the permanent stability.Cd content in edible parts of lettuce was higher than that of control group except mercapto-grafted palygorskite.Although Cd content in edible parts of spinach was lower than that of control group,only Cd content of mercapto-grafted palygorskite treatment reached the food safety standard.The soil phytoavailable Cd content of two leafy vegetables decreased significantly under the treatment of mercapto-grafted palygorskite,but the effect was not significant under other treatments.The pH value of the soil did not increase significantly in this experiment.The dynamic change experiment of the immobilization effect of immobilization materials show that palygorskite and mercapto-grafted palygorskite reduced the Cd content in the edible parts of the two leafy vegetables,and the Cd content decreased firstly,then increased and then gradually stabilized with the growth of leafy vegetables.And the immobilization effect of 0.5%dosage of palygorskite is better than that of 0.2%palygorskite,the mercapto-grafted palygorskite treatment is better than that of0.5%palygorskite.At the same time,mercapto-grafted palygorskite treatment significantly reduced the Cd content in stems and leaves of pakchoi and lettuce.Compared with the control,the Cd content in stems and leaves of pakchoi decreased by 75.6%and 78.9%,and the lettuce decreased by 49.5%and54.7%,respectively.Cd content in different leaves of leafy vegetables was decreased under treatment,and all leaves of mercapto-grafted palygorskite group were almost the same,and the leaves of palygorskite group gradually decreased from outside to inside.During the growth period of leafy vegetables,the soil phytoavailable Cd content showed a trend of decreasing firstly and then increasing,and the content of mercapto-grafted palygorskite group was the lowest.The pH of the soil added with immobilization materials gradually increased during the growth period of leafy vegetable,but the overall increase was small.From the research results,it can be found that mercapto-grafted palygorskite has the best immobilization effect and lasting stability on soil heavy metal Cd.Under low dosage,5 kinds of leafy vegetables can reach the national food safety standard at any growth stage,and under the condition of the soil pH value had no change,it also has the best immobilization effect on phytoavailable Cd in soil.