Remediation Effects And Mechanisms Of Phosphorus Or Oxidation Modified Biochar On Lead In Water And Soil

Biochar has been widely used for carbon sequestration and soil improvement due to it's rich stable carbon,rich surface microporous structure,a large specific surface area and a large number of nutrients.Facing the increasingly serious environmental heavy metal pollution,biochar also can be used as an adsorbent and passivator to remove heavy metals from water and immobilize heavy metals in soil,which is a win-win strategy for biomass waste recycling and environmental remediation.However,the immobilization capacity of the original biochar to heavy metals is limited,so it is a hot topic to find a good modification method to improve the immobilization capacity of biochar for heavy metals.Phosphorous materials are good to immobilize Pb in environment,but few studies have been done on Pb immobilization by biochar-phosphorous composite.In this article,new co-pyrolysis biochars were prepared by rape straw and phosphorus materials?phosphate rock,orthophosphates?.The properties of biochars were characterized by various technologies,such as specific surface area analysis,scanning electron microscope?SEM?,X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?,solid-state ¹³C nuclear magnetic resonance spectroscopy?NMR?,X-ray photoelectron spectroscopy?XPS?,and so on.The removal capacities and mechanisms of co-pyrolysis biochars for Pb in water were investigated using batch sorption,XRD,FTIR,scanning electron microscopy-energy dispersive spectrometer?SEM-EDS?,and so on.In addition,the immobilization effects and mechanisms of co-pyrolysis biochars on various heavy metals in natural contaminated soil were studied through laboratory simulated soil incubation experiments combined with chemical extraction,risk assessment and biochar recovery.Chemical oxidation is a simple and cost-effective method to improve the ability of biochars for metals removal,and the screening of oxidant is a key step for application of oxidized biochar.But there are too few studies on screening of high-efficiency oxidants and quantitative analysis of sorption mechanisms.In this study,rape straw biochars were oxidized by HNO₃,H₂O₂ and KMn O₄,respectively.The properties of oxidized biochars were characterized by FTIR,XRD,XPS,zeta potential and Boehm titration.The removal capacities of oxidized biochar for Pb and the quantitative analysis of sorption mechanisms were investigated through batch sorption experiments?isothermal sorption,kinetic sorption,p H sorption edge et al.?.The main contents and results are as follows:1. The original biochar and the co-pyrolysis biochars were prepared by co-pyrolyzing rape straw with phosphate rock in proportions of 1:0,5:1,2:1,and 1:1?w:w?,respectively.The results showed that phosphate rock might prevent the degradation of rape straw during pyrolysis.Compared with original biochar,the carbon retention of co-pyrolysis biochars increased by 8.6%-11.5%,but the aromaticity and graphitization of co-pyrolysis biochars reduced because the ratios of H/C and I_D/I_G?disordered peak/graphite peak?increased from 0.46 to 0.69 and from 1.59 to 1.82,respectively.Moreover,phosphate in both co-pyrolysis biochars could precipitate with Pb to form more stable pyromorphite,and low additive amount of phosphate rock?rape straw:phosphate rock=5:1?had positive effect on Pb removal capacity.While the high additive amount of phosphate rock?rape straw:phosphate rock<5:1?showed negative effect on Pb removal capacity.Therefore,the co-pyrolysis of RS with low additive amount of PR would be a promising method to improve carbon sequestration and enhance Pb removal.2. Rape straw biochar,rape straw-phosphate rock co-pyrolysis biochar,phosphate rock before and after calcination were mixed and incubated with heavy metals contaminated soil,respectively.The results showed that the application of phosphate rock and biochars could significantly increase the soil p H values and the content of soil available phosphorus,reduce the activity of heavy metals?Pb,Cd,Cu,Zn?in the soil,and promote the transformation of heavy metals from weak acid extraction and reducible states to oxidizable and residue states.Compared with the original rape straw biochar,the co-pyrolysis biochar had the same immobilization capacity on Cu,Pb and Cd in the soil,and better Zn immobilization capacity.Therefore,it is better to use co-pyrolysis biochar to remediate Pb,Cd,Cu and Zn contaminated soil.3. Here biochar-orthophosphate composites were prepared by co-pyrolyzing rape straw with orthophosphate?Ca?H₂PO₄?₂·H₂O/KH₂PO₄?,noted as WBC-Ca and WBC-K,respectively.The maximum sorption capacities of Pb by original biochar,WBC-Ca,and WBC-K were 184.1,566.3 and 1559 mmol/kg,respectively.The phosphorus in biochar played an important role to remove Pb by forming lead-precipitates.However,the species of lead-precipitates in three types of Pb-loaded biochars were different(Pb₅?PO₄?₃Cl,Pb₂P₂O₇,and Pb_n/2?PO₃?_n),individually,and which was because speciation of phosphorus had undergone significant thermochemical transformation during pyrolysis process.Orthophosphate in WBC-Ca was mainly transformed to pyrophosphate,while orthophosphate in WBC-K was transformed to both metaphosphate and pyrophosphate.And WBC-K had the highest Pb removal capacity mainly due to the presence of potassium metaphosphate.The present results warrant the promising application of co-pyrolysis biochar derived from rape straw and orthophosphate in removal of Pb from wastewater.4. Co-pyrolysis biochars derived from rape straw and Ca?H₂PO₄?₂·H₂O or KH₂PO₄were incubated with Pb,Cd,and Cu contaminated agricultural soil.Compared with the original biochar,co-pyrolysis biochar?especially BC-K?showed higher immobilization capacity for Pb Cd and Cu in soil.The addition of these co-pyrolysis biochars significantly decreased TCLP extracted concentrations?decreased by 5.9-81.7%?and ecological risks of heavy metals?Pb,Cd,and Cu?by transforming the metals from available speciation to stable speciation in soils.Co-pyrolysis biochars derived from rape straw and KH₂PO₄showed the highest immobilization capacities,and the immobilization capacities of biochars for the three metals were in the order of Pb>Cu>Cd.Co-pyrolysis biochars could precipitate and complex with heavy metals directly by the phosphate and-OH on their surface,and also could promote immobilization of heavy metals indirectly by increasing soil p H value and available P.Additionally,the content of carboxyl groups on biochars increased significantly and?-?*bonds decreased after incubation,which was beneficial to the further complexation of heavy metals.In summary,the application of co-pyrolysis biochar derived from rape straw and orthophosphate?especially for KH₂PO₄?could effectively reduce ecological risks of Pb,Cd,and Cu in contaminated soil.5. HNO₃,H₂O₂,and KMn O₄ oxidized rape straw biochar?BC-HNO₃,BC-H₂O₂,and BC-Mn?could effectively remove Pb from water.Compared with that of BC?175 mmol/kg?,the maximum Pb sorption capacities of BC-HNO₃and BC-H₂O₂increased to 526 and 917mmol/kg,in which contribution of surface complexation accounted for 55.1%and 39.0%,respectively.Due to the large surface area and abundant newly formed Mn O₂,BC-Mn showed the maximum Pb sorption capacity of 1343 mmol/kg,and its high removal efficiency appeared even at low p H value?p H=2?and high initial Pb concentration?1.0mol/L?.The Pb removal mechanisms of BC-HNO₃and BC-H₂O₂ mainly included ion exchange and surface complexation,while BC-Mn mainly removed Pb in water by ion exchange?97.4%?.The contribution of cation exchange accounted for 97.4%of the Pb sorption by BC-Mn.These results suggested BC-Mn had great potential for Pb removal from aqueous solution,and the quantitative analyses of sorption mechanisms revealed the contribution of each mechanism and provided a basis for evaluating application prospects of biochars.In summary,this paper systematically studied the immobilization effects and mechanisms of co-pyrolysis biochars derived from rape straw and phosphoru materials for Pb in water and heavy metals in contaminated soil.The results showed that co-pyrolysis biochar derived forme rape straw and orthophosphate?especially KH₂PO₄?could immobilize Pb in water and multiple heavy metas?Pb,Cd and Cu?in soil efficiently.BC-K is a promising material for remediation of Pb in environment.In addition,the removal capacities of Pb by the HNO₃,H₂O₂ and KMn O₄ oxidized rape straw biochars were compared,and the removal mechanisms were quantitative analysed.KMn O₄ oxidized biochar showed higher removal capacity than other oxidized biochar,and it could form stable innersphere complex with Pb and efficiently remove Pb from acidic water by ion exchange.It is of great significance for the screening of oxidant and Pb remediation in water.