Study On The Characteristics And Mechanism Of Biochar-supported Ferro-phosphorus Nanomaterials On Lead And Cadmium In Aqueous Solution And Soil

Heavy metal pollution has been accumulated for a long time,which has seriously endangered human health and the ecological environment.Among them,cadmium and lead pollution is particularly serious.Solutions to the lead and cadmium pollution are hence important.Biochar,generally obtained by pyrolysis of waste biomass,is a low-cost environmental remediation material,which can be used as a heavy metal adsorbent because of its aromatic structure and oxygen-containing functional groups.Mineral-loaded biochar with porous structure can improve the interaction to heavy metals.Hydroxyapatite is kind of green and efficient material for heavy metal pollution treatment due to its strong affinity to divalent metal ions.Nano-hydroxyapatite has strong affinity to lead and cadmium because of its high specific surface area.Loading it onto biochar can relief agglomeration phenomenon and enhance the interaction between biochar and heavy metals.The biochar supported magnetic nano-hydroxyapatite can be separated and recycled after interacting with heavy metals,reducing the heavy metals concentrations in the environment.Based on the biochar-supported ferro-phosphorus nanomaterials,efficient nano-hydroxyapatite（n HAP）,magnetic nano-hydroxyapatite（Fe₃O₄/n HAP）,biochar supported nano-hydroxyapatite（n HAP@BC）and biochar supported magnetic nano-hydroxyapatite（Fe₃O₄/n HAP@BC）were prepared for the adsorption and stabilization of heavy metals in solutions and soil,respectively.Model equations were established for the adsorption mechanisms and the mechanism of Fe₃O₄/n HAP@BC on lead and cadmium in soil was proposed.The main conclusions are as follows:（1）The n HAP was loaded onto biochar at a mass ratio of 1:1 using a two-stage ultrasonic co-precipitation method to obtain the n HAP@BC with the highest specific surface of 301.33 m²/g.Fe₃O₄interacted with n HAP through chemical bonds due to the dehydration or van der Waals forces directly.n HAP is connected to the oxygen-containing functional group on the benzene ring through oxygen atoms,or directly connected to the aromatic ring by van der Waals force,electrostatic,π-electron interaction or hydrogen-bonding interaction.The hydroxyl and phosphate groups of n HAP were successfully introduced onto the composite material,with the original C=O,C=C and other functional groups of biochar,increasing the mineral composition and adsorption active sites.Fe₃O₄/n HAP@BC can be separated and recycled in aqueous solution and soil due to the addition of magnetic Fe₃O₄ to achieve the thorough removal of pollutants.（2）The adsorption capacity of BC on Pb²⁺and Cd²⁺was low,while n HAP and Fe₃O₄/n HAP have strong affinity to Pb²⁺and Cd²⁺with the maximum adsorption capacities of 1500 mg/g and 200 mg/g,respectively,indicating that they are the active ingredients.According to the characteristics and the concentration changes of Pb²⁺,Cd²⁺,Ca²⁺and p H during the adsorption process,the removal mechanisms of adsorption,complexation,ion exchange and dissolution-precipitation were confirmed.The interaction models were established through the reacting equation combined with the metal chemical fraction information.The quantitative analysis methods for the adsorption mechanisms of Pb²⁺and Cd²⁺were confirmed.The dynamic changes of the quantitative results of each mechanism of n HAP and Fe₃O₄/n HAP in the adsorption process,and the quantitative mechanism analysis results of adsorption in different concentrations of heavy metal solutions were deduced.Afterwards,the quantitative results on sorption mechanisms of Pb²⁺and Cd²⁺by Fe₃O₄/n HAP@BC was obtained combined with the existing quantitative method for the heavy metals adsorption mechanism by biochar.The results manifested that Pb²⁺was mainly removed through complexation and dissolution-precipitation by n HAP and Fe₃O₄/n HAP with the percentage of 2.5%and 97.5%,respectively.Complexation and ion exchange accounted for the largest proportion of 20%-30%and 60%-80%,respectively for the Cd²⁺adsorption.Cation-πinteraction,complexation,ion exchange and precipitation were the main mechanisms for the removal of Pb²⁺and Cd²⁺by biochar,accounting for3.65%,11.57%,65.28%and 19.49%for Pb²⁺,and 4.52%,29.27%,63%and 3.21%for Cd²⁺,respectively.The main effect of Fe₃O₄/n HAP@BC on Pb²⁺was dissolution-precipitation,followed by complexation,accounting for about 95%and 4.5%,respectively.The effect on Cd²⁺was mainly controlled by ion exchange and complexation,which related to the Fe₃O₄ ratio in the composite:the ion exchange and complexation for Cd²⁺removal accounted for 80%and 15%respectively using n HAP@BC and Fe₃O₄/n HAP,while about 50%and 45%respectively by Fe₃O₄/n HAP@BC.（3）After the application of materials to the polluted soil to stabilize Pb and Cd,stabilization could be achieved in a short time and Pb and Cd could still be stabilized after 120 days.Fe₃O₄ can improve the stabilization effect on soil Cd,which was mainly due to the complexation of the hydroxyl groups generated by Fe OOH around Fe₃O₄ with Cd.BC showed weak stabilization effect for Pb.TCLP-Cd could be re-leached after the BC addition due to the weak Cd-πinteraction.The residual Pb contents increased indicating the formation of stable lead phosphate,while the carbonate-bound Cd increased implying that the Cd stabilization was achieved through the formation of cadmium carbonate.In addition,both the material dosage and the soil initial p H obviously affected the stabilization of Pb and Cd.（4）Fe₃O₄/n HAP@BC reacted with Pb and Cd viaπbonds and hydroxyl or carboxyl complexation in soil.Because of the lower soil p H of Pb/Cd mixed soil,Pb was prone to react with materials and adsorb Cd afterwards,forming Pb⁺–O–Cd⁺compounds and achieving synergistic stabilization of Pb and Cd.Therefore,the stabilization effect of Pb and Cd was better than that in their respective single element contaminated soil.After magnetic stirring extraction operation,the interaction between materials and Pb which was adsorbed by physical adsorption（mainly exchangeable）turned weak.The minimum reacting amount of Pb and Cd can be calculated,which was at least 1056.2 and 1100.7 mg/kg of Pb in Pb soil and Pb/Cd mixed respectively and was at least59.8 and 69.1 mg/kg respectively for Cd in Cd soil and Pb/Cd mixed soil using F_1/17PC_0.5.（5）The short-term stabilization data of heavy metals in soil was analyzed.According to the different material dosages,material composition ratio and the stabilization rate of each heavy metal polluted soil,the suggestions of material dosage and composition ratio for soil remediation within the economical range were proposed,providing a strategy and reference for the engineering application of material to lead and cadmium contaminated soil remediation.