Effect And Stabilization Of Modified Biochar On Heavy Metals From Environment

With the increasing extraction and use of heavy metal resources,heavy metals such as lead（Pb）and cadmium（Cd）are discharged into water and soil environments in large quantities,posing a serious threat to the ecological environment and human health.Biochar（BC）and hydroxyapatite（HAP）materials,the common environmentally friendly materials for heavy metal treatment in environment,have stable porous structure and excellent adsorption capacity for heavy metals.HAP has good adsorption performance but is expensive,while BC material is cheaper but has poor adsorption performance.In this study,a HAP modified BC composite material was prepared which has a strong effect on lead and cadmium heavy metals,and significantly reduces the cost of materials.The biochar was pyrolyzed from rice straw in Nanjing and then modified by HAP through sol-gel method assisted with ultrasonic.The modified biochar had strong aromaticity,stable structure,weak hydrophilicity and low polarity through the characterization results;The specific surface area of modified biochar PC1at the mass ratio of BC:HAP=1:1 was 149.5 m²/g,which was three times that of BC.Meanwhile,BC provided more space for HAP which was more favorable for adsorption;Besides the intrinsic C=O,C=C and other functional groups on BC,the hydroxyl and phosphate were introduced through HAP,increasing the mineral composition of the material.The adsorption mechanism of Pb²⁺and Cd²⁺by HAP was obtained through the analysis of XPS,SEM,XRD,and FTIR for the modified biochar before and after adsorption as well as the simulation using Materials Studio software.The results showed that:The adsorption mechanism of Pb²⁺and Cd²⁺by modified biochar t was mainly the dissolution and precipitation of HAP,the co-precipitation of HAP/BC components,ion exchange,π-πinteractions,surface complexation,etc.Multiple mechanisms contributed the adsorption of Pb²⁺and Cd²⁺.The 24-hour lab-scale adsorption tests of Pb²⁺and Cd²⁺in aqueous solutions were conducted by the modified biochar.The results manifest that the most suitable p H range for Pb²⁺and Cd²⁺adsorption by the modified biochar was 5-6.Most adsorption completed within120 minutes（61.89%～100%）.The adsorption amount of Pb²⁺and Cd²⁺is significantly linearly correlated with the proportion of HAP component mass.Among the modified biochar materials,PC1 had the highest adsorption capacity for Pb²⁺and Cd²⁺of 774.87 and 158.80 mg/g,which was 6.44 and 8.19 times of BC respectively.From the analysis of TCLP leaching toxicity,the 24-hour adsorption of modified biochar materials in Pb/Cd contaminated soil shows that,the adsorption rate is rapid within 2 hours,and the pseudo-second-order kinetic model can be used to fit the adsorption characteristics of modified biochar materials for Pb and Cd in soil.The membrane diffusion,internal diffusion and chemical adsorption coexisted during the adsorption process,and the adsorption capacity of the material was determined by chemical adsorption.At the material addition ratio of 1%,after the adsorption of soil Pb and Cd by PC1 for 24h,the TCLP concentration decreased by7.44 and 15.78 times compared with that of BC treated soil sample respectively.The TCLP concentration is of the soil sample treated by PC5 and PC1 dropped from 19.5mg/L to 3.4mg/L and 1.5mg/L,both of which were lower than the limit of RCRA and GB5085.3-2007（5.0mg/L）,while the TCLP concentration of BC treated sample was still beyond the limit.The TCLP concentration of soil sample treated by PC1 at 8%dosage for 24h decreased by 1.10 and 4.22times of that at 1%dosage.When the dosage of PC1 increased to 8%,the TCLP concentration of Cd contaminated soil decreased from 4.3 mg/L to 0.3 mg/L,which was lower than the limit of 1.0 mg/L.A 90-day stabilization investigation of modified biochar materials on lead and cadmium contaminated soil was also carried out in this study.The results show that the stabilization rate of the modified materials used in contaminated soil was fast first and then gradually slowed to balance.The decrease of TCLP of PC1 treated Pb and Cd soils for one day accounted 97.95%and 97.35%of the 90-day period.The soil p H increased slightly with the addition of adsorbent material,which was within the environment p H range of 6.5～9.5 suitable for the survival of soil microorganisms.Meanwhile,the EC of soluble heavy metals in the soil decreased.After the treatment of Pb and Cd contaminated soil by PC1 for 90 days,the TCLP reduction was 6.9times and 24.9 times that by BC.The modified biochar material can promote the transformation of Pb and Cd in the soil from the acid extractable form to an inactive residual form,while presented little effect on the reducible and oxidizable form.The increments of residual Pb and Cd in PC1 treated soils were 12.54%and 12.84%respectively which were 5.1 and 33.8 times that of BC treated soil.Such results indicated that the modified biochar promoted the transformation of Pb and Cd to a stable form in the soil.By changing its metal form,the environmental risk of Pb and Cd is effectively reduced.This study has certain reference value and guiding significance for the preparation of modified biochar materials,the adsorption mechanism of Pb and Cd,and the engineering application of Pb and Cd contaminated site remediation.