Extraction Of Pb And Cd From Soil By Magnetic Biochar And Its Mechanism

Due to the characteristics of non-degradation,high toxicity and easy accumulation in human body,pollution control of the heavy metals in soil has always been a research hotspot and difficulty domestic and overseas.At present,most of the remediation methods of heavy metals pollution in soil are fixation of heavy metals by passivation.However,the immobilized heavy metals might be reactivated.Biochar is a cheap and high-quality adsorption material.If the high adsorption capacity of biochar to heavy metals can be combined with the separation of magnetic substances（i.e.,to produce a kind of magnetic biochar,MBC）,and using MBC to extract the heavy metals from soil,then it would be possible that the permanent remediation will come true.So far,there is still a relevant research vacancy.Therefore,for the Pb and Cd pollution,MBCs with different pyrolysis temperatures（300-700℃）and ratios of iron oxides and biochar（1:2,1:1 and 2:1）were prepared in this paper.The adsorption of Pb²⁺and Cd²⁺in aqueous solution was studied,and the contribution of different Pb²⁺and Cd²⁺adsorption mechanisms by MBCs was quantitatively analyzed.Two kinds of contaminated soils were selected to investigate the changes of Pb and Cd concentration and their fractions before and after MBC extraction,to obtain the extractability of different fractions of Pb and Cd,and to the extraction mechanism of Pb and Cd in soil by MBC.The effects of MBC characteristics（such as adsorption capacity and magnetic property,etc.）and addition amount on the extraction of Pb and Cd were investigated,and the relationship between the Pb and Cd extraction rates and the characteristics as well as addition amounts of MBC were explored.On this basis,ryegrass（Lolium perenne L.）was selected to investigate the remediation effect by MBC.The main achievements of this work are as follows:（1）The characteristic of different kinds of MBC were represented and the adsorption of Pb²⁺and Cd²⁺in aqueous solution by different kinds of MBC was investigated.The results showed that with the increase of pyrolysis temperature,the p H value,ash content and specific surface area of MBC increased,while the number of surface functional groups and saturation magnetization（M_s）decreased.The M_s of MBC was enhanced by increasing the iron biochar ratio.With the increase of pyrolysis temperature,the adsorption capacity of MBC for Pb²⁺（142.3-266.9 mg/g）gradually increased,while that for Cd²⁺（7.51-31.68 mg/g）first decreased and then increased.With the increase of iron biochar ratio,the adsorption capacity of MBC for Pb²⁺and Cd²⁺first increased and then decreased,except for the MBC700 for Cd²⁺（its adsorption capacity gradually decreased）.The change of adsorption capacity was related to the ash content,specific surface area and number of functional groups of MBC,etc.（2）Quantitatively investigate the contribution of different adsorption mechanisms to the adsorption capacity of Pb²⁺and Cd²⁺by MBC and the effect of biochar-derived dissolved organic matter（BDOM）on the adsorption.The results showed that the adsorption mechanism of Pb²⁺by MBC was mainly precipitation（accounted for 45.9%-52.5%）,coordination withπelectrons（accounted for 19.5%-39.1%）and complexation（accounted for 7.6%-24.9%）.With the increase of pyrolysis temperature,the contribution of precipitation and coordination withπelectrons increased generally,while that of complexation decreased.The change of mineral content and aromatics of MBC with pyrolysis temperature was the main reason that affected the adsorption capacity of Pb²⁺.The main adsorption mechanisms of Cd²⁺by MBC were ion exchange（accounted for 48.8%-72.5%）and precipitation（accounted for 18.7%-46.3%）.The change of ion exchange points on the mineral surface in ash with pyrolysis temperature was the main reason that affected the adsorption capacity of Cd²⁺.Removing BDOM from the original biochar increased the total Pb²⁺and Cd²⁺adsorption capacities of MBC300 by 1.2 and 3.6 times,respectively.This was attributed to the increase of the adsorption capacity of ion exchange,precipitation and complexation for Pb²⁺（4.9,1.3and 0.7 times,respectively）,and the ion exchange and precipitation for Cd²⁺（3.3 and4.4 times,respectively）.In contrast,removal of BDOM reduced the total adsorption capacities of Pb²⁺and Cd²⁺by MBC500 and MBC700（up to 51.9%）.（3）MBC300 was added to two types of natural contaminated soils（collected from Henan and Hunan province）by 7.5%（w/w）addition amount to investigate the change of extraction amount of Pb and Cd in different kind of soils with time effect.During the experiment period（30 days）,the recovery rate of MBC300 in Henan and Hunan soil decreased to 74.2%and 62.0%,respectively.Meanwhile,the extraction rate of Pb and Cd in soil by MBC300 increased with time.On the 30th day,the extraction rates of Pb from Henan and Hunan soils by MBC300 were 42.1%and 26.2%,respectively,which were significantly higher than that of Cd（27.2%and 21.4%,respectively;p<0.05）.This was related to the adsorption capacity of MBC300 for Pb was significantly higher than that for Cd.After MBC300 extraction,the contents of different fractions of Pb and Cd in soils decreased.The highest extraction rate was exchangeable fraction（F1）,followed by carbonate-bound fraction（F2）,Fe-Mn oxides-bound fraction（F3）and organic-bound fraction（F4）.Therefore,the reason why the extraction rates of Pb and Cd in Henan soil were higher than those in Hunan soil was not only related to the recovery rate of MBC,but also affected by the fraction distribution of Pb and Cd in the soil.（4）The effects of the characteristics of MBC and its addition amount on the extraction of Pb and Cd from soil were investigated.The results showed that the recovery rate of MBC decreased with the increase of pyrolysis temperature,and increased with the increase of iron biochar ratio,which was significantly positively correlated with the M_s value of MBC.The effect of the addition amount（2.5%-15%,w/w）on the recovery rate of MBC was non-significant（p>0.05）.With the increase of pyrolysis temperature,the extraction rate of Pb increased（27.8%-37.3%）,while the extraction rate of Cd decreased first and then increased（23.9%-38.1%）.Correlation analysis showed that the Pb and Cd adsorption capacity of MBC was the main factor affecting the extraction rates of Pb and Cd in soil.However,the recovery rate of MBC was also an impact factor.Similarly,the change trend of the extraction rates of Pb and Cd in soil with the different iron biochar ratio was consistent with the trend of the Pb and Cd adsorption capacity by MBC.In other words,when the iron biochar ratio was1:1,MBC300 and MBC700 had the highest extraction rates of Pb and Cd,except that MBC700 for Cd（1:2 is the highest）.The extraction rates of Pb and Cd in the soil increased firstly with the increase of addition amount of MBC,there was no significant difference in extraction rate when the addition amount over 7.5%（w/w）（p<0.05）.（5）The growth situation and Pb/Cd enrichment of ryegrass in soil before and after MBC treatment were compared.The results showed that after using MBC to extract Pb and Cd from soil,the change of plant weight of ryegrass was not significant（p>0.05）,but the concentration of Pb and Cd in ryegrass decreased by 25.8%-30.9%and 63.0%-73.4%,respectively.In addition,the enrichment of Pb bioaccumulation factor（BCF）in ryegrass was not significantly changed（p>0.05）,but the BCF of Cd was significantly decreased by 50.6%-63.0%（p<0.05）.Therefore,using MBC to extract Pb and Cd from soil is a potential remediation method,especially for Cd with high mobility.