Distribution Of Heavy Metals In Superphosphate Production,effects On Soil And Reducing Heavy Metal Hazards

Based on the status quo of heavy metal pollution in soil and the development prospect of passive treatment,the paper choosed the traditional phosphate fertilizer-superphosphate(SSP)application as the goal,which studied the migration of heavy metals during the production of calcium in Guizhou Kaiyang phosphate area and the effects of heavy metals on the plant-soil system.Through the pot experiment and the appropriate passivating agent mixed with SSP,it explored effective ways and remediation methods that soil heavy metal pollution caused by phosphate fertilizer and provided scientific basis for the quality and safety of phosphate fertilizer in practical application.The main results were as follows:(1)In the process of calcium production,the content of heavy metals in the original phosphate rock and pulp is higher than the semi-finished products and the general calcium products.It indicated that the heavy metals in the phosphate rock are not all transferred to the finished products and some of absorbed the effluent.The migration and transformation of Pb,Cd and Cr is closely related to Ca5(PO4)3F and H2SO4.The contents of heavy metals Cd,Pb and Cr in this SSP plant are in accordance with GB/T 23349-2009.the greatest proportion of Pb and Cr in phosphate rock was residual fraction,and Cd was mainly oxidizable fraction.The proportion of the three heavy metals in the pulp had no significant change compared with the phosphate rock.Pb and Cr were dominated in the residue fraction,but the proportion of Pb and Cr in the water-soluble and weak acid-soluble was significantly higher than that in phosphate rock.Cd was mainly in the weak acid-soluble fraction.(2)After the application of SSP,the biomass of P.capitatum was higher than thatthe blank groups,and reached the highest under T4 treatment.Then the biomass of P.capitatum reduced with the increase of application rate.The increase rate was7.86% ~ 256.94%.The amount of SSP inhibited the growth of P.capitatum and affected its yield.Secondly,the roots of P.capitatum were also significantly affected,and the root length of P.capitatum was larger than blank group.The contents of three heavy metals in roots and shoots of P.capitatum were increased,and the root content was significantly higher than that of aboveground parts.The content of Cd in the roots of P·capitatum was in accordance with ISO 18664:2015 “Traditional Chinese Medicine--Determination of heavy metals in herbal medicines used in Traditional Chinese Medicine”.The contents of Cd in the shoots were higher than standard 1~2times;the root Pb content in the T4 treatment was the lowest 3.99 mg·kg-1,in line with safety standards;Blank group and other treatment of Pb content exceeded 1~2times.The upper part of the Pb content were not exceeded.(3)The correlation analysis showed that there was a significant negative correlation between the amount of SSP and pH value.And the pH value was positively correlated with soil available Cd.There was no significant positive correlation between the total and the available Cr content.It indicated the excessive application of SSP could lead to the increase of soil available Cr content.The amount of Pb in soil was significantly negatively correlated with Pb content,and the decrease of Pb in soil was 0.41% ~ 14.32% compared with blank group.The effect of fertilization on the accumulation of Pb in soil was not significant.When SSP was not applied,the biomass of P.capitatum increased with the increase of phosphate rock.The biomass of P.capitatum was increased under the middle level phosphate treatment.The biomass of P.capitatum was improved under the condition of medium-level phosphate treatment.The biomass of P.capitatum was significantly increased under lime treatment when SSP was medium and high level.The stem and leaf of P.capitatum was more developed under the condition of phosphate rock treatment,while the lime treatment of P.capitatum root more developed.(4)Soil pH value could be improved under phosphate and lime treatment.Without SSP,the pH value of the soil increased with the increase of the amount of phosphate rock,and the increase was 3.87%~7.27%.After adding SSP,the soil pH value was decreased,and the pH value increased to 5.65 when the content of SSP and phosphate rock was in the middle level.The pH value of the soil under the lime treatment was higher than that of the phosphate treatment.The pH value increased with the increase of the lime when the amount of SSP is fixed.Phosphate and lime treatment could significantly reduce the available contents of three heavy metals in the soil.The correlation analysis showed that the contents of Cd and Cr in soil were decreased negatively with the increase of pH value.There was no significant negative correlation between soil pH and available Pb content.The possible reason was that SSP was a phosphorus substance,and P and Pb formed stable compounds in the soil.The passivation effect was not significant after adding the passivating agent.