| Biochar has been widely applied in soil heavy metal remediation,because of its properties and structure characteristic.However,biochar exposed in the environment may cause the changes of its surface properties,and may alter the remediation efficiency of heavy metal.In this study,three kinds of biochars with different concentrations of endogenous Cu and Cd were produced from the stems of Pennisetum sp.that were planted in clean soil(Hongrang Station,HB)and polluted soils(Jiuniu Village,JB;Shuiquan,SB).After ageing using dry-wet cycles(DW)and freeze-thaw cycles(FT),the first part mostly studied surface properties of the three biochars were investigated using Boehm titration,FTIR(Fourier Transform infrared spectroscopy),SEM(scanning electron microscopy)and XPS(X-ray photoelectron spectroscopy),and the effect of ageing on the adsorption amounts of Cu and Cd by the three biochar was investigated;the second part mainly studied the effect of ageing on the stability of the three biochars adsorbed Cu and Cd;the last part largely studied the effect of ageing on the avilability of Cu and Cd in soil immobilized by the three biochars,in order to explore the environmental risks for long-term use.The main research contents are described as follwed:1.After ageing,pH values of the three biochars were significantly decreased by 0.05?0.93 units,respectively.Boehm titration results showed that the effect of ageing was increased the acidic functional groups on the surface of the three biochars,for example the effect of ageing significantly increased the content of lactone and phenolic hydroxyl groups in SB.The results of FTIR analysis also showed that the effect of ageing was increased the types and number of oxygen-containing functional groups on the surface of the three biochars.Meanwhile,the XPS analysis results showed that after ageing the three biochars produced more carboxyl functional groups and ester groups,which is consistent with the increase in the relative strength of COOH and C=O groups in the FTIR.The SEM results showed that after ageing the pores on the biochar surface did not significantly change,but the surface of the biochar had local fractures.The effect of ageing significantly promoted the adsorption of Cu by the three biochars.Compared with the fresh biochars,the adsorption amount of Cu by ageing biochar was significantly increased by 24.7%?94.3%.The effect of ageing has a certain inhibitory influence on the adsorption of Cd by the three biochars.For example,after DW and FT ageing the adsorption capacity of Cd by SB,JB and HB had been decreased by 25.8?34.4%?24.9%and 20.2%,respectively.2.The effect of ageing was significantly increased the number of acidic functional groups on the surface of the three biochars saturated with Cu and Cd.After ageing,the carboxyl groups on the three biochars increased by 16.6%?78.7%.The effect of DW and FT ageing significantly reduced the leachable contents of Cu in the three biochars by 43.2%?61.1%and 13.7%?28.2%?respectively.Moreover,all the ageing processes promoted the transformation of Cu from the acid-soluble and residual fractions to reducible and oxidizable fractions,and significantly reduced the values of risk assessment code(RAC)of Cu in the three biochars by 50%to 69%and 15%to 31%,respectively.The effect of ageing promotes the further stabilization of Cu in the three biochars.After ageing the risk level of Cu in the three biochars is:HB>SB>JB;However,the leachable contents of Cd in the three biochars after DW and FT ageing were significantly increased by 9.15%?206%and 34.1%?220%,respectively.Meanwhile,all the ageing processes promoted the transformation of Cd from the residual fraction to acid-soluble,reducible and oxidizable fractions,and increased the RAC values of Cd in the three biochars by 113%?661%and 223%?807%,respectively.The effect of ageing activated the Cd in the three biochars.The increasing risk of Cd in the three biochars affected by ageing is:HB>SB=JB.Evidently,the effect of ageing had promoted the further stability of the Cu adsorbed by the three biochars,but resulted in an increase in the availability of the adsorbed Cd.3.The three biochars were applied to contaminated soil at 2%,5%and 10%respectively.All of biochar could effectively increase soil pH,and the soil pH increases by 0.32?4.50 units,meanwhile,the amount of application of the three biochars was proportional to the pH values.The three biochars could significantly reduce the content of acid-soluble Cu by 18.6%?57.6%in the contaminated soil,respectively,and the amount of application of biochar was directly proportional to the effect of reduction.The effect of ageing had promoted the three biochars to reduce the contents of Cu of acid-soluble fraction in the soil.And compared with the control check(CK)group,Cu of the acid-soluble fraction significantly decreased by 26.1%?77.6%,and that of the reducible fraction significantly increased by 15.4%?75.0%;The application of biochar could lead to an increase in Cd of acid-soluble fraction,and the higher the amount of application,the more obvious the increase,especially for SB.And the contents of Cd of acid-soluble fraction in the 5%SB and 10%SB groups increased by 21.5%and 41.3%,respectively.The effect of ageing could reduce the content of available Cd in the soil.However,compared with the CK group,the aged SB and JB still activated Cd in the soil.Among them,the aged 5%SB?10%SB and 10%JB significantly increased acid-soluble fraction Cd by 24.1%?37.7%and 15.7%,respectively.But the aged 2%HB and 10%HB significantly decreased that of Cd by 19.9%and 13.3%.RAC analysis showed that compared with SB and JB biochar,HB biochar used for immobilization of Cu and Cd in soil had lower environmental risk,which might be related to its lower content of endogenous heavy metals.Figure[36]table[11]reference[188]... |
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