Migration Behavior And Driving Mechanism Of Heavy Metals In Soil After Biochar Stabilization

Ecological security and human health are closely related to heavy metal pollution.The remediation of heavy metal contaminated soil is an urgent task at present.Biochar has good stability,large surface area,and contains minerals?K,Ca,Mg,P?,which are basic characteristics.It has good adsorption effect on Cd,Pb and Cu.Therefore,as a new material,it is used in the soil ecosystem's carbon sequestration,reducing greenhouse gas emissions and soil pollution repair.However,the principle of biochar remediation of heavy metals in soil is to reduce the mobility of heavy metals in soils by precipitation and adsorption with heavy metals.The ecological harm of heavy metals is reduced.It is not the fundamental elimination of heavy metals.When environmental conditions change,whether heavy metals adsorbed by biochar will remigrate is an important content to objectively evaluate the stability of biochar for heavy metal remediation.Therefore,in this paper,the risk of re release of Pb and Cd adsorbed by biochar is evaluated by artificial aging simulation experiments under the condition of temperature and oxidation.The characteristics of the radial migration of biological carbon particles in quartz sand columns and the re migration behavior,distribution and key driving factors of Pb and Cd have been studied by biological carbon particles in quartz sand columns under single hydraulic conditions,hydraulics and interfacial chemistry.The first part of this study through the saturated adsorption Cd,Pb and straw biochar carried out three kinds of artificial accelerated aging experiment?chemical aging,physical aging and natural aging?,the biological carbon of different aging time to TCLP extraction,evaluation with the accumulation of time,the change of biological carbon adsorption of Cd and Pb in stability.The long term stability of biochar fixed Cd and Pb.When the aging time was 56 days,the TCLP-Cd of?RS+Cd?_CHEM??RS+Cd?_PHY and?RS+Cd?_SPON decreased by 49.23%,26.08%and 18.58%compared with the first day of aging,while the TCLP-Pb of?RS+Cd?_CHEM??RS+Cd?_PHY and?RS+Cd?_SPON decreased by 75.38%,37.81%and 18.74%compared with the first day.Three simulated aging experiments promoted the further stabilization of Cd and Pb by biological carbon,and the stabilization effect was CHEM aging>PHY aging>SPON aging.Aging can enhance the surface oxidation of biochar and increase the oxygen-containing functional groups.The broken biochar is conducive to the dissolution and release of P,Mg,Ca and other elements in biochar,providing more adsorption sites for Cd and Pb.The second part of this study is the Migration Experiment of biochar on quartz sand column under the condition of single hydraulics.The migration characteristics and distribution of biochar particles in sand column under different flow rates?2.18and 6.05 ml/min?were studied.The results show that:When the flow rate is2.18ml/min,the amount of Biocharcoal remaining in the quartz sand column is767.00g.When the flow rate is 6.05ml/min,the amount of Biocharcoal remaining in quartz sand column is 489.01g,which is 63.76%of the amount of retention at low speed.The increase of velocity makes the diffusivity of biological carbon particles in the quartz sand column.The increase of flow velocity causes the biological carbon to penetrate into the quartz sand column in advance.The slope of the curve of the penetration curve increases with the increase of velocity.The above three points indicate that the retention capacity of biological carbon in quartz sand columns gradually decreases with the increase of velocity.When the flow rate is 2.18ml/min,the biological carbon is distributed in a staircase form in the quartz sand column,and the maximum of the retention is at the depth of 0-0.5cm.When the flow rate is6.05ml/min,the depth is 0-0.5cm instead of the least.When the depth of the quartz sand column is 0.5-1cm,the maximum value of biochar is 31.56%,and the minimum is 0-0.5cm at the depth of 0-0.5cm.When the velocity of flow is increased from2.18ml/min to 6.05ml/min,the depth of the maximum retention of biological carbon has a downward migration of 0.5cm.The third part of this study is the Migration Experiment of biochar in quartz sand and quartz sand column under double action.The migration characteristics and driving mechanism of biological carbon in quartz sand column were studied under the conditions of different velocity?1.06 and 5.96 ml/min?and ionic strength?deionized water and 0.05M NaNO₃?.The results showed that under the three leaching conditions,the lowest mass of biochar migration was 0.5355g under column 1.The maximum mass transfer of biochar was 0.8691g under column 2.On the condition of column 1,the minimum radial migration distance is about 1.5-2CM.On the condition of column3,the migration distance of biochar radial migration is the largest,which is about3-3.5cm.At the interface of Chemistry under the effect of the increase of ionic strength,straw biochar and quartz sand particle surface zeta potential decreased between the two surfaces of the electrical double layer repulsion is reduced,inhibition of alkaline substances internal biological carbon release.The decrease of pH in the environmental solution leads to the decrease of the negative charge on the surface of the biological carbon and quartz sand particles,which makes the biological carbon more easily adsorbed on the surface of the quartz sand particles.With the increase of ionic strength in the solution,the electric double layer of biochar particles is compressed,the max of phi decreases accordingly,and the Min increases accordingly,and more biochar particles settle into the secondary potential at the small potential energy through active collision.Under the action of hydraulics,the increase of flow velocity makes the Zeta potential of the surface of biological carbon high,the stability of biological carbon is better,and it is not easy to adsorb on the surface of quartz sand.Moreover,the hydrodynamic shear force generated by the increase of pore water velocity leads to the deeper migration of biochar particles to quartz sand columns,which eventually leads to the migration of biochar particles to a deeper level.The fourth part of this study is the Migration Experiment of biochar saturated with Cd and Pb in saturated quartz sand under double action.The re migration behavior,distribution and key driving factors of biological charcoal and adsorbed Pb and Cd at different velocity?1.06 and 5.96 ml/min?and ionic strength?deionized water and 0.05M NaNO3?were studied.The results show that:Compared with the biological carbon that did not adsorb Cd and Pb,the increase was 73.35%,51.21%and 82.23%,respectively.In the three leaching conditions,as time goes on,biological carbon releases the adsorbed Cd and Pb,resulting in an increase in the electronegativity of the surface of the biological carbon.It can be seen from the Zeta potential that the Zeta potential of the adsorbed heavy metal bio carbon at different depths of the quartz sand column is higher than that of the unadsorbed biological carbon.The mass of the biological carbon that has been adsorbed on heavy metals has increased.The increase of ionic strength and flow velocity promoted the activation of Cd and Pb by biological carbon.The adsorption of Pb²⁺on straw biochar is mainly ion exchange.The adsorption of Cd²⁺is mainly cation-?action.The increase of flow rate leads to the increase of surface electronegativity of quartz sand particles.The retention of Pb2+and Cd2+in quartz sand columns increased.The fifth part of this study is mainly the graph column leaching experiment.By measuring the soil column Pb2+,Cd2+,and DOC in different depths of interstitial water changes.?With Rhizon Clearance Water Sampler?The remobilization and driving mechanism of Pb2+and Cd2+stabilized by biochar in the soil column under hydraulic and interface chemistry were studied.The results showed that as the flow rate increased,the adsorption capacity of soil particles on heavy metals increased.As the ionic strength increases,the adsorption capacity of Cd and Pb on biochar particles and soil particles decreases..And the effect of increasing ionic strength on Pb is greater than that of Cd.The increase of ionic strength inhibits the release of DOC from the soil and biochar.However,with the increase of the flow rate,the concentration of DOC in the interstitial water remains basically the same,and the release and migration of DOC are less affected.Because the stability of Pb in biochar particles and soil is mainly through ion exchange,the stability of Cd is mainly caused by cation?bond,resulting in different correlations of DOC concentrations in Cd and Pb concentrations at different depths of the soil column.