Effects Of Biochar From Co-pyrolysis Of Sludge And Attapulgite On Chemical Fraction Of Heavy Metals In Soil

Biochar is a carbon-rich substance which is pyrolyzed in anoxic environment.It has received more and more attention in agricultural soil improvement and environmental restoration.The preparation of biochar from municipal sludge is one of the approaches to realize the resource utilization of sludge,because the approach owns the advantages of low cost,no damage to the environment and waste utilization.The attapulgite is a cheap and easily available inorganic mineral material.Due to its special nano-scale crystal structure and fiber structure,it has strong adsorption capacity for heavy metal ions.After thermally modified at 300-400°C,the attapulgite exposed more internal surfaces and reactive groups to improve adsorption capacity.In this paper,the structure and properties of biochars prepared by co-pyrolysis of attapulgite with different proportions in sludge and the mechanism of their effects on heavy metals in soil are discussed,which can provide theoretical basis for the application of immobilized remediation of heavy metal pollution.The sludge samples and attapulgite of the study were from Lanzhou Qilihe Wastewater Treatment Plant and Banqiao Town,Linze County,Gansu Province.According to 5%,10%,15%,20%,25%and 30%of the dry matter quality,we added attapulgite powder into the sludge,and the sludge-attapulgite was co-pyrolysis of biochar in a vacuum tube pyrolysis furnace,and seven sludge-attapulgite rods from the control group were co-pyrolysis biochar,and the biological substance was gained.And basic properties and characterization of charcoal were also measured.The heavy metal contaminated soil passivation experiment was carried out,and 7 kinds of sludge-attapulgite co-pyrolysis biochar were prepared and poured into the soil according to the addition amount of 10 g/kg,respectively,passivation for 30 days,40 days,50 days,60 days.Heavy metals in the soil were measured at day and 70 days of treatment.By measuring the total amount of Cd,Cu,Zn,Ni and Cr in the passivation process and the morphological distribution of the BCR continuous extraction method,and analyzing the correlation of different heavy metals between the various forms.The main factors affecting the bioavailability of heavy metals and their mechanisms were analyzed from the aspects of soil physical and chemical properties,heavy metal complex pollution and exogenous elements.The main findings include:（1）The pH value of biochar prepared by adding different proportion of attapulgite was higher than that prepared by adding no attapulgite.The content of C and H in co-pyrolysis biochar decreased gradually,the content of O increased gradually,the content of N decreased slightly,the form of organic components changed,the aromaticity of biochar increased gradually,the hydrophilicity of biochar decreased gradually,and the number of oxygen-containing functional groups decreased gradually.Infrared spectrum analysis showed that the skeleton structure of biochar from sludge co-pyrolysis should be aromatic unsaturated skeleton and aldehyde or ketone polycondensation structure.The main groups were phenolic hydroxyl,carboxyl,methyl and amino groups,and their contents increased gradually with the content of attapulgite.XRD analysis shows that the inorganic ash in co-pyrolysis biochar is mainly composed of quartz（SiO₂）,potassium salt（KCl）,calcite（CaCO₃）and phosphorite（Ca,Mg）₃（PO₄）₂.Scanning electron microscopy（SEM）analysis showed that co-pyrolysis biochar may include acicular crystals,bundle aggregates and other attapulgite aggregates,which exhibited a relatively compact microstructure.（2）The chemical distribution of heavy metals in contaminated soil was determined by BCR continuous extraction method.The acid-soluble content of heavy metal Cd was the highest,followed by the reduced state and the oxidation state.The distribution of four morphological contents of heavy metal Cu is relatively uniform.The heavy metal Zn has the highest reduced content,followed by the residual state,the acid soluble state and the oxidation state.The content of residual state in heavy metal Ni is the highest,and the content of oxidation state is the highest in the non-residual state,and the content of acid soluble state is the least.The residue content of heavy metal Cr is the best,and the content of oxidation state is the highest in the non-residual state,and the content of reduced state is the least.（3）In the process of biochar passivation of contaminated soil,the morphological changes of heavy metals Cd,Cu,Zn,Ni and Cr in each treatment showed a trend of transformation from unstable to stable.Different ratio schemes for different treatments has different passivation processes due to different heavy metal elements,and B3（15%attapulgite addition in sludge）treatment has the best passivation effect on heavy metals Cd and Cr;B5(sludge The 25%attapulgite addition treatment has the best passivation effect on heavy metals Cu and Zn;B0（no attapulgite in the sludge）treatment has the best passivation effect on heavy metal Ni.（4）The bioavailability of heavy metals was studied by means of available forms and geochemical phases,activity coefficients of heavy metals,and risk assessment coding methods,and bioavailability of five heavy metals such as Cd,Cu,Zn,Ni and Cr.The evaluation results of sex and migration ability are basically the same.After biochar passivation of soil,the pollution risk of heavy metal activity coefficient in soil decreases continuously,among which Cd pollution is the main and Ni pollution is the smallest.（5）In the process of biochar passivation of contaminated soil,there are 8 treatments,and there are obvious antagonistic or synergistic effects among different heavy metals in different heavy metal forms.（6）Adding different sludge-attapulgite to co-pyrolysis of biochar and analyzing the physical and chemical properties of the contaminated soil used in the experiment,pH,conductivity,cation exchange amount,organic matter are increased to different degrees,and heavy metals are applied to the soil.The chemical form has a significant impact.（7）Using correlation and regression analysis,the effects of soil physical and chemical properties on the bioavailability of heavy metals and heavy metal complex pollution were studied.There are antagonistic or synergistic pollutions between heavy metals,and there is a certain composite pollution between various forms of heavy metals in soil and between heavy metals in various forms.Soil physical and chemical properties are important factors influencing the bioavailability of heavy metals in soils.