| Biochar made by natural fire or from artificial production under the condition of high temperature and oxygen restriction dissolves out some components after infiltrating and washing by soil moisture,rain or runoff,which are biochar extracts in soil.They are mainly composed of BDOM and inorganic components.BDOM have high reactivity and biochemical activity.They can not only be used as nutrient taken in and utilized by soil organisms and thus affecting their growth and reproduction,but also interact with heavy metals and organic pollutants influencing their migration and degradation in the environment.Meanwhile,the inorganic components in biochar extracts have made great contributions to the soil improvement of biochar.Current researches mainly focus on the composition and properties of BDOM,and their interaction with organic pollutants.In addition,there are few studies about the impact of BDOM on the removal of pollutants in soil systems.In addition,the study on the effect of inorganic components on the removal of pollutants in soil systems is extremely rare.Therefore,corn stover was used as raw material to prepare corn stover-derived biochar extracts to explore the effects of biochar extracts under different pyrolysis temperatures,concentrations and addition sequences on the adsorption of Rh B by montmorillonite.According to the adsorption results and the properties of biochar extracts,the role and related mechanism of components in biochar extracts were analyzed.There were significant differences in the composition and properties of BDOM prepared at different pyrolysis temperatures.Part of the biomass were carbonized at300℃.The degree of carbonization deepened at 500℃,while some of the C structure could be recombined into macro-molecular organic matter.At 700℃,biomass cracks into a large number of small molecules.Therefore,the order of BDOM fluorescent component content,humification degree,aromaticity and organic functional group was:500℃>300℃>700℃.700℃ BDOM had the lowest molecular weight and hydrophobicity.The inorganic component content and p H value in the biochar extracts increased significantly with the increment of pyrolysis temperature.The biochar extracts could inhibit the adsorption of Rh B by montmorillonite.The inhibition effect increased significantly with the increase of pyrolysis temperature and concentration of biochar extracts.After changing the addition order of biochar extracts,the adsorption capacity of montmorillonite for Rh B changed.The mechanism involved could be divided into two parts:the one was the complexation of Rh B by BDOM.Reduction of adsorption by complexation varied under different pyrolysis temperatures,and the order was:500℃>300℃>700℃.The complexation ability and the adsorption reduction induced by complexation were both related to the aromaticity and humification degree of BDOM.The other was that biochar extracts were alkaline.When they mixed with montmorillonite and Rh B,the p H value enhanced.Meanwhile,the adsorption capacity of montmorillonite for Rh B decreased with the increase of the p H value of the solution.The concentration of inorganic substances and alkalinity in biochar extracts augmented with increasing of pyrolysis temperature.Correspondingly,the order of contribution of alkalinity in biochar extracts under different pyrolysis temperature to inhibition of adsorption was:700℃>500℃>300℃.It is noteworthy that the contribution of alkalinity of biochar extracts to inhibiting of montmorillonite adsorption of Rh B was comparatively large.Under the p H condition of 700℃biochar extracts,the interaction of inorganic components with montmorillonite contributed more than 80%to inhibition of adsorption.Thereinto,complexation ability of BDOM to Rh B was related to BDOM aromaticity and humification degree.When the concentration of Rh B was 2～10 mg/L,the contribution of dynamical quenching to fluorescence quenching of 300℃ and500℃ BDOM and Rh B both gradually increased;while the contribution of dynamical quenching to fluorescence quenching of 700℃ BDOM and Rh B decreased first(<6mg/L)and then increased(>6 mg/L).This was probably because that the aromaticity and humification degree of BDOM pyrolysis under 300℃ and 500℃ were higher than that of 700℃.Combining contribution of dynamical quenching with the total quenching amount,the order of complexation ability of three BDOM to Rh B was:500℃BDOM>300℃BDOM>700℃BDOM.The FTIR results of montmorillonite after interacting with BDOM showed that montmorillonite could adsorb some biochar extracts,and its interlaminar substances changed.After 7 days of interaction between montmorillonite and biochar extracts,the concentration of[OH~-]in solution decreased significantly.It suggested that montmorillonite might adsorb alkaline components in biochar extracts,changing its surface properties and composition.However,montmorillonite could adsorb few BDOM.Therefore,there might be adsorption or ion exchange behavior between alkaline inorganic components in biochar extracts and montmorillonite.Whereafter,the adsorption capacity of montmorillonite for Rh B reduced.Hence,the experiment is significant and profound for the environmental benefits of BDOM and inorganic components in biochar extracts within soil systems,and provides an experimental case for the study on effect mechanism of BDOM on migration and removal of organic pollutants in soil systems.Additionally,the experiment analyzed the impact of biochar extracts on montmorillonite adsorption Rh B under the two conditions that there were biochar extracts in the soil originally,or biochar extracts and organic pollutants entered the soil simultaneously.It can provide research cases for the interaction between biochar extracts,minerals and organic pollutants,and help to explore the removal of organic pollutants in different soil environments. |
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