| With soil contamination by heavy metals aggravating, the remediation of contaminated soil has become one of big concerns currently. Biochar is of high surface area, porosity and CEC, and can adsorb heavy metals, which might be as promising amendment to restore contaminated soil. To investigate sorption of biochars, pine wood was carbonized at temperature of 300, 500, 700℃(were referred to as P-300, P-500 and P-700). The samples were characterized via analysis for elemental, specific surface area, Fourier Transform Infrared Spectroscopy (FTIR), electron microscopy scanning and cation exchange capacity. Meanwhile, biochars were added to Lead and Cadmium contained soil and incubated over 60 days, The chemical forms, toxicity leaching amount and leachate properties of Pb, Cd in soils were measured. The relationship of structural characteristics and sorption mechanisms were discussed, and the feasibility of biochar to immobilize heavy metal was assessed. The primary results are as follows:①Biochars'surface area and structural characteristics were regulated by pyrolysis temperature, thereafter dominated sorption capacities and mechanisms. Specific surface area, porosity, CEC, oxygen-containing functional groups didn't show the linear relationship with increasing of pyrolytic temperature. Specific surface area increased dramatically between 300℃and 500℃. P-500 and P-700 were above to 287.16(379m~2/g), 343.25(453.19m~2/g) times as large as P-300(only 1.32m~2/g), respectively. P-500 and P-700 had porous structure. CEC of P-300 was least, and there were also not difference of that of 500℃and 700℃.The amount of acidic functional groups on P-300 was more than that of P-500 and P-700. All of samples had negative charge, which can absorb heavy metal ions with positive charge. The absorption capability of P-300 may be the highest.②The pH of contaminated soils rose to some extent with increasing pyrolysis temperature after biochars application. Both in a single contaminated soil or in combined contaminated soil, biochars can reduced acid extractable form concentrations of Pb and Cd, thus lessen bioavailability of heavy metal. Remediation of Pb in combined contaminated soil was better than single contaminated soil, but effect of Cd was contrary, The reason was that there was competition to adsorption sites between Pb and Cd in combined contaminated soil. Adsorption rate and adsorption capability of Pb on biochar were all larger than that of Cd. ③Compared with CK, amount of Pb and Cd in toxicity leaching decreased after 60d, and the range was 7.22%~36.90% and 9.88%~30.86%. P-700 was better than other two samples. Concentration of Pb and Cd in every toxicity leachate was lower than standard values of hazardous waste (Pb 5.0mg/L, Cd 1.0mg/L). The repaired soils were leached by simulated acid rain (pH4.5). All the pH of leachate was higher than 4.5, which showed all the repaired soils had buffer performance. Compared with CK, the Pb, Cd content of accumulated leachates decreased, and the treatment of P-700 had the best effect on reducing leaching amount of Pb, Cd from soils. The Pb content of leachates was progressively dropped with increasing leaching amount, The Cd content of leachates was firstly increased then decreased with the increase of leaching amount.④P-300 had very small surface area and porosity, so physical adsorption was weaker; but it had many surface polarity functional groups, so electrostatic forces between the polar groups and metal ions were stronger, so the remediation of heavy metal from P-300 depended mainly on electrostatic forces. With temperature increase, the content of surface polarity functional groups dropped, specific surface area and pore volume increased, so the sorption of heavy metal from P-500 and P-700 mainly relied on metal ion diffusion.
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