Experimental And Mechanism Research On The Adsorption And Fixation Of Soil Heavy Metal Lead By Biochar Under Aging

Heavy metals contaminated soil has characteristics of degradation-resistant,easy accumulation,and irreversibility,etc.,which can enter food chain through the ecosystem,and ultimately endangers human health.Chemical fixation/stabilization technology of using soil amendment is one of the most promising remediation methods for heavy metal contaminated soil.However,in practical application,the remediation effect of common soil conditioner on soil contaminated with heavy metals is influenced by types of soil amendment,types of contaminated heavy metals,soil properties,and implementation environment.Therefore,it is necessary to develop a novel soil amendment and explore its adsorption and fixation mechanism of heavy metals.Biochar is a soil amendment with great development and application prospects due to its characteristics of good stability,large specific surface area,excellent pore structure,and rich surface oxygen-containing functional groups,etc.In this paper,typical agroforestry biomass and artificial lead?Pb?contaminated soil were selected,and different biochars were prepared by using a fixed-bed reactor.The effects of biochar on the adsorption and stabilization of heavy metal?Pb?in soil under simulated aging effects were studied through soil cultivation and soil leaching experiments.The remediation mechanism and long-term stability of biochar on heavy metals?Pb?contaminated soil were discussed based on analyzing the physical and chemical properties of biochars.It provides a reference for large-scale application of biochar in in-situ remediation of heavy metal contaminated soil.The main work and conclusions of this paper are summarized as follows:Four typical agroforestry biomasses,involving pine chips?PC?,corn straw?CS?,wheat straw?WS?,and peanut shell?PS?,were selected as experimental materials,and the corresponding biochars were prepared under temperature of 450°C and holding time of 30 min in a fixed-bed reactor.The aging experiments of self-made biochars were carried out at conditions of natural aging,high temperature aging and freeze-thaw aging.The physiochemical properties of fresh biochars and aged biochars were characterized by using scanning electron microscope?SEM?,N₂ adsorption/desorption analyzer,X-ray diffractometer?XRD?,Fourier infrared spectrometer?FTIR?,and combined with industrial analysis,ultimate analysis and p H measurement.The results indicated that the volatile content of the four biochars decrease significantly,and the ash content and fixed carbon content increase significantly.The biochars are alkaline.Among the fresh biochars,CSC has the highest alkalinity,and PCC has the lowest alkalinity.Under the three aging effects,the alkalinity of the four biochars has decreased.The impact is minimal.The aging effect increase the content of O and reduced the content of C in biochar.After aging,the surface structure of the biochar is broken,and the pore walls of the tubular structure become thinner.The BET specific surface area,micropore volume,and microporosity of the CSC are all significantly improved,and the pore structure development is the best among the four biochars.Compared with fresh biochar,aging can effectively increase the content of oxygen-containing functional groups on the surface of biochar.The four types of biochars contain salt substances,and aging reduces the KCl content of CSC.An artificial Pb contaminated loess from Fufeng County,Shaanxi Province,China,were selected and blended with four fresh biochars of mass ratio of 5%to prepare soil samples.Soil aging cultivation was performed under simulated conditions of natural aging,high temperature aging and freeze-thaw aging.The Pb speciation in soil samples were measured by using a modified BCR continuous extraction procedure to study the aging effect of biochar on adsorption and stabilization of heavy metal?Pb?in soil.The mechanism of adsorption and fixation of heavy metals?especially in Pb?was discussed based on analyzing of the physicochemical characteristics of aging biochars.The results indicated that biochar could effectively reduce the content of available Pb in the soil under three aging conditions,and its remediation effect shows a trend of high temperature aging>freeze-thaw aging>natural aging.When the incubation time is larger than 60 d,the remediation effect of four biochars is ordered as CSC>WSC>PSC>PCC under the three aging methods.The rate of decrease in the content of acid-extractable Pb is different from the rate of increase in the content of Pb in the residue in different aging days and when different biochars were applied.PCC fixes Pb in soil mainly through the precipitation of metal carbonate formation;CSC and WSC fixes Pb in soil mainly through complexation and ion exchange;the main mechanisms of Pb adsorption and fixation of Pb in soil are precipitation,complexation and ion exchange.The soil samples cultured with different types of biochar at 60 days under three aging methods were used as research objects.The leaching experiment for simulating rainfall under natural field conditions was carried out to study the variation of p H value in soil leachate.The effects of aging method and biochar type on the release amount and release fraction of heavy metal?Pb?in soil were evaluated,and the long-term stability of Pb in soils with biochar aging culture was evaluated.The results showed that the application of biochar significantly increased the p H value of the leachate,which shows an overall upward trend during the leaching and is alkaline,and the high temperature aging soil has the highest alkalinity;soil with PSC under high temperature aging has the highest alkalinity.The p H value of charcoal soil-like leachate is the highest,and pine charcoal is the lowest.The single release and cumulative release of Pb in the leachate gradually decrease and stabilized with the process of leaching.Under the three aging methods,the cumulative release of Pb and the cumulative release fraction of Pb in the soil column shows a trend of CSC<WSC<PSC<PCC.The stability of heavy metal Pb in soil fixed by CSC is better.In the soil with the same kind of biochar,the three aging methods can effectively reduce the single release fraction and cumulative release fraction of Pb in the soil,and all shows the trend of high temperature aging<freeze-thaw aging<natural aging,that is,high temperature aging is more stability.Compared with the original soil sample,the release amount of Pb in the blank soil sample decreases slightly,and also shows a trend of NBS>FBS>HBS.