Effect Of Biochar On Adsorption,immobilization,and Phytoavailability Of Cadmium In Loess Soil And Its Mechanisms

With the rapid development of urbanization and industrialization,heavy metal pollution of soil has become the growing environmental problems,which can not be ignored.Biochar is a carbon-rich material that is prouduced by pyrolysis of biomass under oxygen-limited conditions.Being rich in surface functional groups and developed pore structure,biochar has strong adsorption and immobilization capacities for heavy metals.Biochar,as an immobilizing material to remedy heavy metal contaminated soil,has become one of the hotspots in the field of soil remediation.In this paper,the research prospective and remediation technology for heavy metal contaminated soil were briefly introduced and the characteristics,mechanisms and application of biochar to adsorb of heavy metals in the remediation of heavy metal contaminated soil were also reviewed.By now,the relationship between the adsorption mechanism of heavy metals by biochar and the structure of biochar is not clear,as well as the relationship between the phytoavailability of cadmium in loess soil and the cadmium content in plant.In this research,biochars derived from corn straw,wheat straw,corn cob,and cattle manure were produced under different pyrolytic temperatures.The surface properties and structural characteristics of biochars were characterized by SEM-EDS,FTIR,XRD,XPS,specific surface area measurement and elemental analysis.The mechanisms on cadmium adsorption by biochars were discovered,and then the competitive adsorption of heavy metals in their binary and tenary systems onto biochars were also investigated.The mechanism on cadmium adsorption onto amino-modified biochar was also studied.Based on the results,the relationship between the adsorption capacity of cadmium by the loess soil amended with bochars and the properties of different biochars were illustrated.Finally,the relationship between cadmium speciation in loess soil and soil properties were discussed,and that between cadmium speciation in loess soil and cadmium content in plant were further studied.The results could provide a scientific basis for the control of heavy metal pollution in regional soil environment.The main results and innovations of the paper are as following.(1)The mechanism and structure-effect relationship of the interaction between biochar and cadmium was revealed.It was found that the adsorption was primarily attributed to the oxygen-containing functional groups of biochar,which was confirmed by batch adsorption experiments,mathematical modeling,and characterization of the Cd-laden biochar samples using SEM-EDS,XRD,FTIR and XPS.The XRD patterns of biochars are not obviously different before and after adsrorption of cadmium,indicating that there is no precipitation after adsorption of cadmium onto biochars,and the surface sedimentation is not the main mechanism of adsorption of cadmium by biochar.At the same time,the peaks of hydroxyl groups(—OH)and carbonyl(C=O)in the FTIR spectra before and after the adsorption of cadmium on biochars were shifted and the peak intensity decreased,indicating that the surface complexation of —OH and C=O with cadmium occurred.According to the XPS spectra of biochars,the binding energy of oxygen-containing functional groups in the process of adsorption of cadmium by biochar increased 0.2 eV,which indicated that the oxygen-containing functional groups on the surface of biochars react with cadmium and formed Cd—R(R represents —COO,—CO,etc.).In addition,the pH values of the equilibrium solution decreased from 5.4 to 5.1 after adsorption of cadmium by biochars,which further confirmed the formation of cadmium complexes on the surface of biochars.The adsorption behavior of Cd,Cu and Zn on biochars showed that the adsorption properties of biochars for Cd,Cu and Zn were related to the physicochemical properties of different metal elements.The adsorption mechanism of Cd on the amino-modified biochar is mainly due to the complexation of amino functional groups and oxygen-containing ones with cadmium,and the binding ability of amino groups with cadmium is stronger than that of oxygen-containing ones.(2)The relationship between the adsorption capacity of cadmium by the loess soil amended with bochar and biochar properties was illustrated.It was found that the physical and chemical properties of biochar had a significant effect on the adsorption of cadmium by the loess soil amended with bochar.The polarity of biochar is the most important factor affecting the adsorption of cadmium on the loess soil amended with bochar.The maximum adsorption capacity of the loess soil amended by biochar was increased with increasing biochar dose.The adsorption of cadmium on loess soils amended by biochar produced from different raw materials was also different,and the adsorption capacities of cadmium on loess soils amended with straw biochars was better than that of amended with maure and corncob biochars.At the same time,it was also found that the physical and chemical properties of biochars produced from different raw materials were also different,and the polar index(O+N)/C of biochar was positively correlated with the model KF of adsorption of cadmium on loess soil amended with bochar.There was no significant correlation between the specific surface area of biochar and the adsorption of cadmium by the loess soil amended with bochar.(3)The effects of biochar amendment on the cadmium speciation in loess soil and the relationship between the cadmium speciation and the content of cadmium in plant were studied.It was found that the application of biochar to cadmium contaminated loess soil could significantly reduce the content of phytoavailable cadmium.The mechanisms of application of biochar included the lime effect of biochar to change the occurrence of cadmium in loess soil,and the adsorption of cadmium by biochar to reduce the effective concentration of cadmium.The results showed that the application of biochar into cadmium contaminated loess soil could significantly result in enhancement of the pH value,cation exchange capacity(CEC)and organic matter(SOM)content of loess soil,and could lead to improvement of the content of Urease and alkaline phosphatase activity in the loess soil.The application of biochar to the cadmium contaminated loess soil causes the conversion of cadmium from unstable active states to stable residual ones.It was found that there was a significant(P < 0.01)positive relationship between the concentration of cadmium in maize roots and shoots and bioavailability of cadmium in loess soil,and a significant(P < 0.01)negative correlation between the concentration of cadmium in maize roots and shoots and bioavailability of cadmium in loess soil.The cadmium content in maize roots and shoots were also significantly decreased(P < 0.05)because of the application of biochar in the cadmium contaminated loess soil.