| Because of the strong adsorption and immobilization ability to heavy metals by biochar, in recent years, the researches on biochar in the field of remediating soil contaminated by heavy metals have been paid much attention. When the source of biomasses, type and dosage of heavy metals and type of soils changed, obvious differences occur in the biochar immobilization mechanisms to heavy metals in contaminated soils. At present, most of the researches on the remediation of soils polluted by heavy metals using biochar have focused on slight acid soil, but those on the light sierozem soil were less. The influences of biochars on the fraction distribution and bioavailability of heavy metals in light sierozem have not been reported, which could reveal the existence, migration and other environmental effects of heavy metals, and could clarify the stabilization mechanisms of biochars to heavy metals in light sierozem. And for the researches on availability of heavy metals and changes of physiological traits of plant after applying biochar can preliminarily determine the range of suitable application amount of different biochars into the light sierozem contaminated by heavy metals, and can provide theoretical support to immobilization remediation of light sierozem contaminated by heavy metals.To explore different biochars’ effect and mechanism to immobilize heavy metal pollution in light sierozem soil, in this paper, the typical agricultural wastes in Gansu province, flax straw, rape straw and chicken manure were chosen. Pyrolysis of them at 600 ℃ and oxygen-limited condition was conducted. Then biochars chicken manure, rape straw and flax straw were obtained, labeled as CBC, RBC and FBC. The basic properties of the three kinds of biochars were measured and compared, and their differences in structure and content were characterized. The effects of biochars on light sierozem solely contaminated by Cd or Pb under different treatments in the soil-plant system were studied with the pot experiment. The influence mechanisms of biochars on morphology transformation and biological effectiveness of heavy metal were studied. At the same time, the suitable dosages of biochars were explored in light sierozem contaminated by heavy metal. Results could provide a reference for biochar immobilizing and repairing in light sierozem contaminated by heavy metals, The main condusions are following.The ash and carbonate contents of CBC were relatively higher, and its pH value reached 10.16, while those of RBC and FBC were 9.98 and 10.03, respectively. The contents of C in RBC and FBC both were above 60%, while the content of O in CBC was higher, with the value of 63.54%. The aromatization degree of CBC was the highest, and on its surface there were a lot of macropores’ with stronger polarity and hydrophilicity. In contrast, there were more mesopores on the surfaces of RBC and FBC, and their aromatization degree, polarity and hydrophilicity were lower. The specific surface area of RBC was the highest. Besides, the surfaces of these three kinds of biochars were all rich in carboxyls, lactones and hydroxyls measured by FTIR and Boehm titration. The total alkaline functional group content of CBC was 2.66 mmol/g, significantly higher than thoes of RBC and FBC, which was one of the factors causing its higher pH value.Applying the three kinds of biochars could significantly increase the pH value of soil. In the CBC processing, the increase of soil pH value was the largest, and the maximal increase were respectively found in the Cd – CBC and Pb – CBC systems, with the values of 11.54% and 11.36%.The contents of CEC and SOM of soil increased markedly with the amount of three kinds of biochar applied in soil. The varition of free iron oxide content and mechanical composition of soil was not obvious. The CBC improved significantly soil carbonate content, and the largest value of carbonates were found in the treatment of Cd – CBC and Pb – CBC, with the values of 6.7% and 12.04%. In contrast, the treatment of RBC and FBC could dilute soil carbonate content. However, the value of pH of soil were too excessive to suit for plant growth, so it was suggested that the applying amount of the CBC was less than 5% in light sierozem contaminated by heavy metal, and those of RBC and FBC were below 10%.The available contents of Cd and Pb were decreased after applying the three kinds of biochars, and the fractions of available heavy metals transformed into the steady ones. In the processing of the soils contaminated by Pb and Cd, with the treatment level of RBC within 10%, the immobilization for Pb and Cd was more obvious, when the applying level of FBC was greater than 10%, the immobilization for Pb was more obvious, when the applying level of CBC was greater than 10%, the immobilization for Cd was more obvious. The stepwise regression analysis results showed that after applying the three biochars into the light sierozem contaminated by Cd and Pb, the soil pH value was the main factor influencing the form transformation of heavy metals. In addition, after applying biochars into the light sierozem contaminated by Cd, the CEC, SOM, carbonate content and clay had significant correlation with distribution form of Cd; and after applying biochars into the light sierozem contaminated by Pb, the CEC, SOM and carbonate content were the important factors affecting the transformation of Pb.Cd and Pb were respectively enriched at the ground and underground part of the maize seedlings. The acid extractable fraction of heavy metals was the main factor affecting the biological effectiveness in the light sierozem contaminated by Cd and Pb. The effectiveness of Cd and Pb in the maize seedlings, could be reduced by the suitable application amount of RBC(5%) and FBC(5%). The antioxides enzyme activity was relatively low at this moment, and the growth of maize was well. The suitable amount of CBC applied in Cd contaminated light sierozem was 1%, of which the growth of maize was well. While in the Pb contaminated light sierozem that was between 1% and 5%. |
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