| The treatment and restoration of heavy metal pollution of farmland soil has become one of the urgent tasks in Action Plan for Soil Pollution Prevention for the more and more serious affect on the safety of agricultural products and human health due to the heavy metal pollution of farmland soil.Recently,a great number of forestry soil heavy metal pollution repair technology have been developed while the in-situ repair of technology called Remediation As Production shows a wild and bright market application prospects.Among them,the leaching-curing in-situ repair technology is the most promising one due to its high repair efficiency,short repair time,and low secondary pollution.Taken the Cd pollution of agricultural land as focus,the heavy metal in the surface soil is activated and rinsed to the deep soil solidification,which reduces the absorption of heavy metals on the surface of the crops,guarantees the quality and safety of agricultural products,and prevents heavy metal Solidify the underlying soil and groundwater migration through the simulation test,according to the principle of leaching-solidification.The results provide theoretical basis and technical support for the in-situ remediation of Cd pollution in agricultural land.The screening of leaching agent and its compounding efficiency and mechanism are explored through batch test;Then,biochar-based solidified material and investigated its solidification effect on heavy metal Cd were prepared by adsorption-desorption test.Finally,a soil column simulation test was employed to explore the effectiveness of surface rinse-deep curing.The main results achieved are shown as following:（1）Eluent concentration screening test shows,among the six leaching agents,that are citric acid,tartaric acid,oxalic acid,malic acid,ferric chloride,and citric acid-sodium citrate,it’s exhibited that citric acid,tartaric acid,malic acid and citric acid-sodium citrate performs better performance with soil Cd leaching rates of 44.10%,41.52%,61.61%and 56.19%,respectively.From the compounding test,0.1 mol·L-1 citric acid-0.02 mol·L-1 ferric chloride（CA-Fe Cl3）,and 0.1 mol·L-1 lemon Acid-0.4 mol·L-1sodium citrate（CA-CAS）exhibits good performance with the leaching rates of 73.28%and 67.19%,respectively.（2）The leaching efficiency of the two compound leaching agents for Cd is relatively similar but their activation mechanisms are different.The main mechanism of CA-CAS activation of cadmium is the complexation of acid groups and functional groups,accounting for 74.64%while CA-Fe Cl3 is mainly acid-soluble,accounting for 69.42%.It’s shown that the maximum leaching rate for these two compound leaching agents were obtained at the solid-liquid ratio of 1:10,and the leaching rate of Cd increased rapidly from 0 to 60 minutes,and then the growth rate of the slowed down from 60 to 360 minutes and tends to be stable 360～1440min.The increase in the pH of the eluent significantly reduced the leaching rate of CA-Fe Cl3,while CA-CAS was less affected by pH.（3）Rice straw was chosen as raw material and solidified materials were prepared with the best ratio of bio-carbon loaded nano-hydroxyapatite（n HAP@BC）,nano-hydroxyapatite（n HAP）–biochar（BC）at 1:50.The results of BET,SEM,XRD,and FTIR showed that n HAP was successfully loaded on biochar without significant change of morphology,and the agglomeration weakened while the specific surface area increased.Under the conditions of neutral,alkaline and weakly acidic(<0.1mol·L-1),the material can be kept in a relatively stable state,which is positively correlated with pH and the adsorption capacity of Cd2+..There is a good adsorption effect on Cd2+.in solution for n HAP@BC with higher reaction rate in accordance with the quasi-second-order kinetic model and langmuir model.Under the conditions of 15℃,25℃and 35℃,the theoretical adsorption capacity of n HAP@BC for Cd2+in solution is 116.22,121.18,126.76 mg·g-1.When the added amount was 10g·L-1,the removal rate of cadmium in CA-Fe Cl3 leaching liquid by n HAP@BC reached 70.24%,and the removal rate of cadmium in CA-CAS leaching liquid reached 50.25%.（4）In combined remediation under CA-Fe Cl3 treatment,the total cadmium content in the plow layer in different pollution soil under the low,middle and high degree was reduced by 44.54%,60.13%and 47.71%respectively.And the total Cd content after curing layer interception decreased by 22.07%,30.82%and 58.28%respectively.Under CA-CAS treatment,the total cadmium content of the cultivated soil decreased by 39.83%,57.98%and 58.28%,and the total Cd content of the soil below the solidified layer decreased 15.77%,27.00%and 39.72%,respectively.After combined remediation,,the total Cd content in the topsoil in the polluted soils of low and medium-concentration is lower than the screening value in the standards of GB 15618-2018 and the concentrations meet Grade III and Grade IV standards of GBT 14848-2017.（5）There is drop of pH,CEC,organic matter,total phosphorus,and available phosphorus of the plough layer soil after leaching-solidification,and CA-Fe Cl3 leaching shows a more significant change on the physical and chemical properties of the soil than CA-CAS leaching.Hence,alkaline fertilizer is recommended to adjust the pH of the soil with after restoration to restore it to the pH before leaching and supply the nutrients that the soil may lose. |
