Risk Assessment And Potential Application Investigation Of Biochars Enriching Heavy Metals

To investigate the environmental risk and potential application of biochars which were rich in heavy metals(HMs),the spent biosorbent containing different heavy metals(Cu,Zn,Pb and Cd),common spent battery waste containing Zn and Mn,as well as the antibiotic residues containing Zn,were selected as the three kinds of raw materials to prepare the biochar by the pyrolysis.The characterization methods including SEM-EDX,BET,N2-adsorption-desorption,XPS,FTIR and TEM-SEAD diffraction were chosen to analyse the characters of the bichars.Digestion with HCl-HNO3-HClO4,BCR and leaching methods were used for measuring of HMs in the biochars.Environmental risk of metal-biochars was determined by the risk assessment code(RAC),Muller geo-accumulation index(Igeo)and potential ecological risk index(RI).The potential application of metal-biochars was also investigated in this study.The main results and conclusions are summarized below:(1)After the pyrolysis(350,550,750 ?),the pH value of the biosorbent biochars had significantly improved from 8.49,7.86,7.94,7.96(350?)up to 10.19,10.08,10.98,10.71(750?),respectively.EC value of the biochars moderately changed.With the rising pyrolysis temperature,the biochars production rate obviously decreased from 40%-55%(350?)to 27%-28%(750?)and the ash yield increased from 28%-30%(350?)to 48%-54%(750?).C/N ratio was found higher indicating the smaller mineralization rate,which was helpful for carbon sequestration in the soil.H/C,O/C ratio had decreased which indicated the stronger aromaticity of the biochars.The pyrolysis had magnified the SBET up to 208.41,183.38,230.20,241.33 m2 g-1(750?)and shrinked the average mesoporous pore size from 10-40 nm(350?)to 6-7 nm(750?)of the biochars.The FTIR analysis indicated less functional groups,but more and concentrated metallic oxide bonds were found in the biochars.XPS and TEM-SEAD diffraction analysis indicated significant transformation of Cu(II)oxide to Cu(I)and Cu(0),however,no significant transformation occurred in Zn(II),Pb(II)and Cd(II)oxide during the pyrolysis.The total content of HMs was concentrated,and the acid-soluble as well as reducible parts had gradually transformed into the oxidizable and residual fraction of the HMs.The RAC and RI value decreased markedly which was consistent with the fact that the stability of the HMs in the biochars had been enhanced and the environmental risk as well as the potential ecological risk had also been reduced.The increasing Igeo value revealed extensive geological accumulation risk of the HMs present in the biochars.The increasing germination index(GI)and average root length(AVrp)indicated a dramatic reduction in the phytotoxicity of the metal-biochars.The later applying the biochars enriching copper(Cu-biochars)into the algae solution had led to lower pH value and higher EC value.The pH value was stable in the range of 8-9 and the EC value was stable in the range of 2000-2100 ?S·cm-1.The Cu-biochars inhibited chlorophyll a content,which could indicate the reproductive growth of the alga had been retarded.(2)After the pyrolysis(550 ?),the pH value of the battery-sawdust composite biochars(BBC),added the the battery carbon powder with ratio of 10%,20%,30%(w/w)had obviously increased up 10.80,10.94,and 10.85.EC value decreased significantly to 1.67,2.08,2.30(mS·cm-1).The ash yield decreased to 21.34,29.89,37.26%.The content ratio(%)of C and N decreased,however,the C/N ratio had increased which indicated the smaller mineralization rate of the biochar in the soil.The specific surface area of the composite biochars had obviously decreased from 61.23 m2 g-1 to 37.27 m2 g-1,and the average pore radius(APR)increased from 7.13 nm to 12.65 nm.The relatively high content of Mn and Zn were found in the carbon composite which had a positive correlation with the addition ratio of the battery carbon powder.The leaching test had revealed that higher leaching risk of Mn and Zn existed in the strongly acidic environment.The GI and average root length(AVrp)affected by the composite biochars were all lower than that in the control test,which had determined that the application of composite biochars would have a potential risk to the ecological environment.The biochar,BBC 550-30%,was found having a better adsorption performance.Under the environmental condition with the temperature of 50 ?,pH 5.0,400 mg·L-1 Pb(II)solution,the maximum adsorbing capacity to Pb(II)of the BBC550-30% was up to 124.64 mg·L-1.Pb(II)adsorption was controlled mianly by surface adsorption,electrostatic attraction intraparticle diffusion and surface deposition process.(3)After the pyrolysis(550 ?),the pH value of the antibiotic residues biochars(B-ARs)obviously increased up to 9.85.EC value decreased to 1.82 mS·cm-1 and the ash yield only reduced by 6.5%.The content ratio of C and N,as well as C/N ratio decreased by 28.8%,44.0% and 21.2%,respectively.The mineralization rate of the B-ARs in the soil was higher than the biosorbent biochars and composite biochars.The Zn was significantly concentrated in the pyrolysis.The RAC as well as RI obviously decreased which was corresponding to the lower bioavailability,environmental risk and potential ecological risk.Applying the biochars enriching zinc(Zn-biochars)into the soil would cause certain toxicity to the plant growth.However,with the rising pyrolysis temperature,the toxicity of Zn in the biochars had been reduced significantly.Owing to the concentration of the Zn element in the biochars,the application of the Zn-biochars was worth for further research.