Effect Of Aging On Adsorption And Stabilization Of Heavy Metal Cadmium By Biochar

With the acceleration of industrialization,Cd(Ⅱ)pollution in water bodies and soils has become an important environmental issue.Biochar as an adsorbent or for the treatment and treatment of Cd(Ⅱ)-containing wastewater and soil is economical,efficient and environmentally friendly.However,changes in the environment such as temperature,humidity,p H and other factors will cause the aging phenomenon of biochar,and then cause the structure and properties of biochar to change.In this study,biochar as adsorbent was prepared using Auricularia auricula dreg as a raw material.The physical,chemical,and biological aging methods were used to age the biochar.The effects of aging biochar on Cd(Ⅱ)in water and soil were investigated.Optimize optimal ads orption conditions through batch adsorption experiments;Scanning electron microscope(SEM),fourier infrared spectrometer(FTIR),x-ray diffraction(XRD),specific surface area and other methods were used to analyze the characteristics of biochar,and compare the surface structure,element content,functional group,and surface crystal changes before and after aging of biochar to reveal the aging and adsorption mechanism;Fit analysis of experimental data using kinetic,thermodynamic and isothermal models;Tessiver five-step continuous extraction method was used to extract five forms of Cd(Ⅱ)in soil mixed with aged biochar.Analysis of the effect of aging biochar on the stability of heavy metals in soil by the extracted heavy metal morphology.This study provided a theoretical basis for the treatment of wastewater and soil containing Cd(Ⅱ)by aged biochar.The main results were as follows:(1)The optimum conditions for Cd(Ⅱ)adsorption by biochar were:p H was 5.0,the initial concentration of Cd(Ⅱ)was 25 mg/L,the dosage of adsorbent was 1g/L,adsorption time was 120min and adsorption temperature was 30℃.Under this condition,the adsorption quantities of nature biochar(NB),physically aged biochar(FB),chemically aged biochar(AB),and bio-aging biochar(MB)were 24.20 mg/g,23.48 mg/g,22.70 mg/g,and 22.00 mg/g,respectively.With the increase of the aging cycle,the adsorption quantities of biochars for Cd(Ⅱ)became larger.After fifteen cycles of aging of biochar,the adsorption quantities of FB,AB and MB were 23.05 mg/g,23.02 mg/g,respectively.The adsorption process of Cd(Ⅱ)by NB was more in accordance with Langmuir model.When the biochar was treated by three aging methods in one cycle,the adsorption of Cd(Ⅱ)by FB conformed to Langmuir model,while that by AB and MB conformed to Freundlich model.According to the Freundlich model,it showed that the adsorption of Cd(Ⅱ)by physical aging biochar was monolayer adsorption,the adsorption of Cd(Ⅱ)by chemical aging biochar and bioaging biochar was multimolecular layer adsorption.After fifteen cycles of aging,the adsorption of Cd(Ⅱ)by FB and MB conformed to the Freundlich model,while the adsorption of Cd(Ⅱ)by AB conformed to the Langmuir model;The adsorption experiments of Cd(Ⅱ)on NB and aging biochar in one cycle could be better described by pseudo second-order kinetic model.In the initial stage,the adsorption speed was faster,and then gradually reached equilibrium state.It was proved that the adsorption of Cd(Ⅱ)on four kinds of biochar were chemical adsorption.However,when the aging cycle was fifteen,the adsorption of Cd(Ⅱ)by FB was more in line with the pseudo first-order kinetic model,which proved that physical adsorption played a major role in the adsorption process of Cd(Ⅱ).Through the study of adsorption thermodynamics,it was found that theΔG~0 of the adsorption of Cd(Ⅱ)by the four biochars were all negative,which indicated that the adsorption process was spontaneous.(2)Through the analysis of the element composition and content of biochar before and after aging,it was found that compared with NB,the content of C and H of MB was decreased,while the content of O and N increased.the hydrophilicity and polarity of biochar were enhanced.The change of element content in FB and AB was opposite to that of the MB,the content of O and N decreased,and the hydrophilicity of biochar decreased;It was observed by SEM that the surface structure and pore size of biochar treated by different aging methods were different.After physical and biological aging treatment of biochar,more irregular structures appeared on the surface,but after chemical aging treatment,the irregular structure of biochar surface became less.The results of FTIR and XRD analysis showed that after the physical and biological aging treatment of biochar,the strength of the active groups on the surface of biochar mainly changed;and after the chemical aging treatment of biochar,the number and types of crystal structures on the surface of biochar occured.From the BET results,it could be seen that the specific surface area of biochar became smaller after physical and chemical aging.On the contrary,the specific surface area of biochar became larger after biological aging.However,with the increase of aging cycle,the change trend of the specific surface area was very stable,and there was not much fluctuation;The number of acid oxygen-containing functional groups of MB increased with the increase of aging period,while that of FB did not change significantly with the increase of aging period.(3)Tessier’s five-step continuous extraction method was used to analyze the morphology of Cd in the soil.Compared with the control,the NB group,FB group,and MB group all significantly reduced the exchangeable Cd content by 40.27%,46.08%-66.23%,47.25%-63.68%.In addition,the carbonate-bound content was also reduced;the AB group increased the exchangeable content of Cd by 44.71%-71.77%,that is,the effective content of Cd in the soil increased.The p H,cation exchange capacity,organic matter content,and electrical conductivity of the soil were changed by adding different aging biochars.After adding NB,FB,and MB,the p H of the soil increased,but with the increase of the aging cycle,the change did not change significantly.After the addition of AB,the p H of the soil showed a downward trend.After the addition of NB,FB and MB,the soil cation exchange content also increased.With the increase of the aging cycle,the changes were6.06%,6.28%-11.19%and 7.74%-11.46%,respectively.After adding AB,the content of soil cation exchange decreased by about 10%;after adding NB,FB,and MB,the soil organic matter content increased.With the increase of the aging cycle,the changes were 17.46%and 18.53%-21.86%and 18.44%-25.65%.After the addition of AB,the soil organic matter content also increased,but the change was not significant compared with the other three types of biochar.After the addition of AB,the electrical conductivity of the soil changed most significantly.With the increase of the aging cycle,the change range was 4.28%-4.64%.