Effect Of Biochar On The Migration Characteristics Of Chromium In Saline Alkali Soil

A certain electroplating plant in Anguang Town,Jilin Province discharged wastewater containing a large amount of chromium into the surrounding saline alkali soil,causing serious chromium pollution in the nearby soil,which has become an urgent problem to be solved.Biochar has strong stability,large specific surface area,high porosity,and strong adsorption capacity for heavy metals and other pollutants.It can effectively control its migration and transformation,and is widely used in the field of soil heavy metal remediation.The biochar added to the soil is prone to interact with various components in the soil,leading to changes in soil properties,changes in heavy metal adsorption and desorption ability,and the impact of passivation effect.To investigate the adsorption,desorption,and passivation relationships between biochar added saline alkali soil and heavy metals,this study used indoor simulation methods to culture saline alkali soil without biochar and saline alkali soil with 3%biochar added at room temperature,high temperature,and freeze-thaw cycling.The following results were obtained with the addition of fresh biochar as the control:Under different cultivation conditions,there was no significant difference in the adsorption and desorption of Cr（Ⅵ）by saline alkali soil without biochar added,and the influence of its cultivation conditions was ignored in subsequent experiments;The adsorption capacity and desorption rate of Cr（Ⅵ）on saline alkali soil with 3%biochar addition were significantly different.The adsorption capacity was as follows:high-temperature culture(SC-H,3504.781mg·kg^-1)>freeze-thaw culture(SC-F,3335.756 mg·kg^-1)>room temperature culture(SC-N,3200.536 mg·kg^-1)>addition of fresh biochar(SC-S,2862.486 mg·kg^-1),and the desorption rate was as follows:SC-S（60.22%）>SC-N（57.11%）>SC-F（56.59%）>SC-H（55.96%）;The number and types of functional groups in biochar increase under different cultivation conditions,and functional groups such as hydroxyl-OH,olefin C≡C,carboxyl-COO-,and aromatic C=C/C=O participate in the adsorption of Cr（Ⅵ）.The adsorption and desorption kinetics of Cr（Ⅵ）by different treatments of saline alkali soil can be fitted using Elovich equation and Pseudo-second-order dynamics equation.The adsorption and desorption thermodynamics can be fitted using Freundlich equation and Langmuir equation.The desorption capacity of Cr（Ⅵ）increases linearly with the increase of adsorption capacity,and the desorption rates of Cr（Ⅵ）are all greater than 50%.This indicates that the binding between Cr（Ⅵ）and the test sample is unstable,has strong mobility,and is prone to penetrating the soil aeration zone and polluting groundwater.At the same initial concentration,the desorption hysteresis coefficient of Cr（Ⅵ）is:HI（SC-S）>HI（SC-N）>HI（SC-F）>HI（SC-H）.Adding biochar and high-temperature cultivation of saline alkali soil can improve the fixation ability of Cr（Ⅵ）and inhibit desorption.The adsorption/desorption capacity of Cr（Ⅵ）on the test sample increases with the increase of temperature,ΔG<0、ΔH>0、ΔS>0 indicates that the adsorption/desorption of Cr（Ⅵ）by the test sample is a spontaneous,endothermic,and disordered process.Within the experimental p H range（3.0-11.0）,as the p H increases,the adsorption capacity of the test sample for Cr（Ⅵ）gradually decreases,while its desorption capacity gradually increases;At the same ion concentration,Na⁺and Mg²⁺have no significant effect on the adsorption capacity and desorption rate of Cr（Ⅵ）in saline alkali soil,while Al³⁺significantly increases the adsorption capacity and desorption rate of Cr（Ⅵ）in saline alkali soil.As the concentration of Al³⁺(0.01-0.5 mol·L^-1)increases,the adsorption/desorption capacity of Cr（Ⅵ）gradually increases,and its desorption rate continuously decreases.The basic physical and chemical properties of saline alkali soil with 3%biochar addition include an increase in available potassium,available phosphorus,alkali hydrolyzed nitrogen,and organic matter content.High temperature cultivation（MH1）has the greatest impact,increasing by 37.89%,53.21%,12.26%,and 21.53%compared to saline alkali soil without biochar addition（MH0）;The p H,exchangeable Na⁺,and salt content of saline-alkali soil with added biochar showed a decreasing trend;The content of soluble cations K⁺,Ca²⁺,and Mg²⁺increased,while the content of Na⁺decreased by 30.50%;The content of soluble anions CO₃^2-,HCO^-,Cl^-,and SO₄^2-decreased slightly.Under high-temperature cultivation conditions,the addition of 3%biochar had the best passivation effect on Cr（Ⅵ）.Compared with the 0th day,the amount of acid extractable Cr（Ⅵ）in the soil decreased by 18.74%,the amount of reducible Cr（Ⅵ）decreased by 25.11%,the amount of residual Cr（Ⅵ）increased by 16.39%,and the amount of oxidizable Cr（Ⅵ）increased by 28.39%.The results of redundancy analysis indicate that the content of oxidizable Cr（Ⅵ）is positively correlated with the content of alkaline nitrogen and available phosphorus in the soil,while negatively correlated with the p H,soluble salt content,and water-soluble SO₄^2-in the soil;The residual content of Cr（Ⅵ）is positively correlated with available potassium,available phosphorus,organic matter,and water-soluble K⁺,Ca²⁺,and Mg²⁺in the soil,while negatively correlated with soil p H,exchangeable Na⁺,and water-soluble CO₃^2-,HCO₃^-,and Na⁺.