Regulation Of Cadmium Pollution In Water And Soil By Modified Biochar

With the rapid development of urbanization and industrialization,heavy metal pollution and other environmental problems are becoming more and more serious.Adsorbents are the most common heavy metal remediation materials and can effectively remove heavy metals from water and soil.Biochar,as a renewable and cheap adsorbent,has been widely used in the treatment of heavy metal wastewater.Traditional biochar has low adsorption removal efficiency for heavy metals and complex preparation process energy consumption,which limits its application in the field of environmental remediation.Therefore,the preparation of low-cost modified biochar with high adsorption performance to enhance its ability to remove heavy metal pollutants has become the main goal of this study.In this study,kiwi branch biochar（KB）and modified kiwi branch biochar were prepared and used as raw materials to study the adsorption effect and mechanism of cadmium in aqueous solution.The effects of concentration,adsorption time,solution ions and other factors on cadmium removal by modified biochar were discussed.The dynamic filtration removal of cadmium from kiwi branch biochar filled sand column and chitosan modified kiwi branch biochar filled sand column was studied.The effects of biochar filling method,quartz sand particle size,biochar addition ratio,flow rate and solution ions on cadmium removal were further explored.At the same time,a pot experiment was carried out with pakchoi as the research object to study the fixation effect of cadmium in contaminated soil and the accumulation effect of cadmium in plants with different biochar application ratio,which provided certain scientific and technical basis for the practical application of modified biochar.The main results are as follows:（1）The adsorption of Cd²⁺on Mg-Fe layered bimetallic hydroxide-functionalized kiwi branch biochar（KB/LDH）was studied.The adsorption kinetics and isotherms of KB/LDH for Cd²⁺,as well as the effects of ionic strength（IS）and coexisting ions on the adsorption capacity of Cd²⁺were studied.The maximum adsorption capacity of KB/LDH composite for Cd²⁺is 17.3 mg g^-1,while the maximum adsorption capacity of kiwi branch biochar is1.51 mg g^-1.The adsorption of Cd²⁺on KB/LDH composites increases with the decrease of IS,and the coexistence ions of K⁺,Cl^-and NO₃^-have little influence on the adsorption.（2）Production of cost-efficient composite materials from low-cost modified biochar for the removal of Cd²⁺from wastewater is much needed to meet the growing needs of industrial wastewater treatments.The studies were to screen the effective modified kiwi branch biochar and the novel chitosan-modified kiwi branch biochar（CHKB）was excellent as low-cost modified biochar for the removal of Cd²⁺from aqueous solution.Batch adsorption and characterization experiments indicated that the modification of kiwi branch biochar by chitosan remarkably improved its adsorption performance.The results show that the adsorption capacity of CHKB is mainly due to its abundant pore structure and a large number of surface functional groups（-OH,-NH,C=O,etc.）.The main mechanisms of CHKB adsorption of Cd²⁺are ion exchange,electrostatic interaction,surface complexation and precipitation.The adsorption isotherm of CHKB for Cd²⁺was consistent with Langmuir model,and the pseudo-second-order model could well fit the adsorption kinetics,which indicated that the adsorption process was a chemical monolayer adsorption process.The maximum adsorption capacity of CHKB on Cd²⁺was 128.21 mg g^-1,which was higher than that of KB and KB/LDH.Therefore,further in-depth study and analysis of CHKB with high adsorption performance can provide guidance for CHKB to repair cadmium polluted environment.（3）The biochar-packed sand columns with chitosan-modified kiwi branch biochar（CHKB）were used to filtrate Cd²⁺from wastewater,and the effect of biochar addition proportion,flow velocity,quartz sand particle size,solution ion and packing method on the transport of Cd²⁺were conducted in packed-column experiments.The results indicated an increasing amount of CHKB in column resulted that the recovery rate of Cd²⁺decreased.The recovery rate was influenced by the injection flow velocity for the time constrained the process of kinetic sorption.When injection flow velocity increased,the recovery rate of Cd²⁺in CHKB-sand columns increased.The decrease of quartz sand particle size and ionic valence state of solution in CHKB-sand columns resulted in the decrease of recovery rate.When mixed CHKB-sand packed column was applied,the recovery of Cd²⁺was lower than that of layered CHKB-sand packed column.As a result,the abundant surface functional groups improved the ability of CHKB to remove Cd²⁺,and CHKB-sand column performances were better for Cd²⁺removal than that of KB and pure sand column in each corresponding treatment.Larger amount of biochar,lower flow rate and ionic valence state of solution,smaller particle size of quartz sand and mixed filling are more conducive to Cd²⁺filtration and removal.Compared to the KB,CHKB was suggested as the low-cost and efficient adsorbent in filters and reactive barriers to remove Cd²⁺from wastewater.（4）Biochar plays an important role in soil remediation.However,there has been no clear understanding to the remediation efficiency of modified kiwi branch biochar on the Cd contaminated lou soil.The studies examine the Cd immobilization by KB and CHKB,and investigate the DTPA-Cd change in the lou soil and the impact on the Cd bioaccumulation in the pakchoi after KB and CHKB were used in Cd contaminated the lou soil.The results indicated that CHKB had a fantastic immobilization effect on Cd,compared to the control and KB.The application of CHKB,significantly enhanced the pakchoi yield and soil organic matter,but the content of DTPA-Cd in soil and the accumulation of Cd in pakchoi decreased significantly with the increase of KB and CHKB addition level,especially the application at 2%（w/w）.Moreover,application of 2%（w/w）CHKB in the soil(1,5 and 10mg kg^-1 for Cd)immensely reduced the Cd content in the leaves（15.12%,45.06%and65.44%）in comparison with that of the control and KB.It can be the reason that CHKB is an efficient biochar adsorbent with relatively stable structure,abundant micropores and functional groups in good contact with soil,which is beneficial to remedy the Cd contaminated soil.Biochar production can not only achieve the treatment and reuse of waste in the environment,but also produce absorption and stabilization effect of heavy metals in water and soil.The porosity,types of functional groups and internal structure of biochar jointly determine the adsorption and fixation performance of heavy metals.The conclusion of this study can lay a theoretical basis for the application of biochar in agricultural development and heavy metal polluted water and soil remediation.