Preparation Of Modified Biochar And Its Application In Heavy Metal Pollution Remediation

In recent years,due to the rapid development of mining industry and agriculture,heavy metals have entered into the soil through atmospheric deposition,sewage irrigation,application of chemical fertilizers and pesticides and the storage of solid waste,which result in the heavy metal contamination in soil increasing seriously.Heavy metals in environment can cause long-term threats to human and ecosystems health.Therefore,it is imperative to prevent and remediate soil heavy metal pollution and improve soil quality.Biochar has unique physical and chemical properties,which can not only realize the resource utilization of organic waste,but also achieve the dual effects of remediating heavy metal contaminated soil and increasing crop yield.It is a soil heavy metal stabilization agent with great development and application prospects.To obtain the biochar with better heavy metal stabilization effect,the preparation and development of modified biochar materials has become the research hotspot.However,there are few related studies on the modification of biochar using a variety of chemical reagents.In this paper,wheat straw was used as raw material to prepare biochar（BC）,and three chemical reagents were used to chemically modify BC to prepare modified biochar（HNC）.The basic properties of HNC and its adsorption characteristics and adsorption mechanism for heavy metals were explored.Its stabilization effect on heavy metal-contaminated farmland soil and its impact on soil physicochemical and biological properties were also analyzed.In addition,the influence of BC and HNC on plant growth and the migration of heavy metals in soil-plant system were also analyzed.The potential health risk of heavy metals through the consumption of plant before and after the stabilization were evaluated.The results of present study can provide technical reference for the rational use of wheat biochar to remediate heavy metal contaminated farmland soil.The main conclusions are as follows:（1）Compared with BC,the number and types of functional groups of HNC increased significantly,such as the surface carboxyl group content increased significantly,from 1.251 mmol·g^-1 to 2.320 mmol·g^-1 and the emergence of-NH₂/-NO₂ groups in HNC.The p H of BC and HNC is 8.60 and 5.51,respectively.The BC surface has a typical porous structure,and the pores are arranged orderly.However,the surface of HNC is collapsed and broken,and the structure is relatively loose.Compared with BC,the specific surface area of HNC decreased from 3.176 m²/g to 2.703 m²/g,the total pore volume decreased from 0.01296 cm³/g to0.01025 cm³/g,and the micropore volume decreased from 7.243×10^-4 cm³/g to 4.308×10^-4 cm³/g.The average pore size increases from 10.20 nm to 13.46 nm.（2）Compared with BC,the adsorption effect of HNC on heavy metal ions was significantly improved under the same conditions.Compared with BC,the maximum adsorption capacity of HNC for Cu²⁺,Cd²⁺and Pb²⁺at 298.15 K increased by 73.17%,133.7%and 69.67%,respectively,and its maximum adsorption capacity for Cu²⁺,Cd²⁺and Pb²⁺was 31.06 mg·g^-1,58.48 mg·g^-1 and 161.3 mg·g^-1,respectively.In the multimetal conditions,the adsorption of BC and HNC showed that the competitive ability of Cu²⁺was stronger than that of Cd²⁺and Pb²⁺.The adsorption process of Cu²⁺and Cd²⁺by BC and HNC conformed to the pseudo-second-order kinetics and Langmuir adsorption model,indicating that the adsorption was monolayer chemisorption,while for Pb²⁺it was mainly monolayer physical adsorption.The complexation and co-precipitation involving oxygen-containing functional groups are the main mechanisms for the adsorption of Cu²⁺,Cd²⁺and Pb²⁺.（3）The application of BC and HNC reduced the bioavailable Pb,Ni,Cu,Cd and Zn in the soil,and the stabilization ability of HNC is higher than that of BC,and the stabilization effect on Cu was the best,with the stabilization rate of 21.98%.The application of BC and HNC can improve the soil fertility effectively.Compared with the BK,the content of soil organic matter and available potassium increased by 97.37%and216.6%,respectively,when BC dosage of 3%.In addition,the content of alkali-hydro nitrogen increased by110.8%when HNC dosage of 3%.In addition,the application of BC increased the soil p H by 0.05-0.16 units,whereas HNC decreased soil p H by 0.26-0.50 units.The application of BC enhanced the soil catalase and phosphatase,but inhibited the activity of the soil sucrase.Besides that,BC application also increased the abundance and diversity of microbial communities significantly.The application of HNC decreased the abundance and diversity of soil microbial communities,and also inhibited the activities of catalase,sucrase and phosphatase.Furthermore,both BC and HNC have nonsignificant effect on the activities of soil urease.（4）The application of both BC and HNC promoted the growth of spinach effectively.Compared with the BK,the fresh weight and dry weight of the aboveground and underground parts of spinach increased by101.9%,74.59%and 73.39%,50.19%,respectively,when the dosage of HNC of 2%.The application of BC can promote the extension of spinach root system,while HNC is more conducive to increase the biomass of spinach.The inhibitory effect of HNC on the accumulation of heavy metals in spinach tissue was better than that of BC.The contents of Pb,Ni,Cu,Cd and Zn in the aerial part of spinach with the HNC dosage of 3%decreased by 35.50%,33.25%,21.73%,30.31%and 19.09%,respectively,compared with the BK.Pb,Ni,Cu,Cd and Zn content in the underground part decreased by 71.35%,74.58%,56.77%,23.28%and 53.71%,respectively,compared with the BK.In addition,both BC and HNC could inhibit the migration of heavy metals from soil to spinach,and the inhibition effect on Pb was the most obvious.The enrichment coefficients of Cu,Zn,Cd and Ni in spinach leaves treated with 3%HNC were the lowest among the groups,decreased by 21.88%,19.04%,30.32%and 32.63%,respectively,compared with BK.The enrichment coefficient of Pb in spinach leaves decreased the most by 35.38%under 1%and 2%HNC treatments.The transfer coefficients of these five heavy metals are all less than 1,and they are easy to be accumulated in the underground part.Overall,the treatment effect is HNC>BC.（5）The potential health risk of heavy metals through the oral ingestion route of spinach was calculated.The results showed that the HQ values of Cu,Zn and Ni in spinach were all less than 1,with acceptable non-carcinogenic health risks to local residents.The HQ values of Cd and Pb were much higher than 1,indicate that their existence high non-carcinogenic health risks to local residents.After the application of 3%HNC,the HQ values of Pb and Cd were reduced from 1.579 and 25.20 to 1.048 and 17.56,respectively,but the risk of Cd was 16.76 times that of Pb and still much higher than the acceptable value（1.0）.In addition,the carcinogenic risk（CR）of Ni and Pb in spinach to local residents was 3.20×10^-4 and 4.33×10^-5,respectively,which were all within the acceptable range.However,the CR value of Cd in spinach was 107.6,which is much higher than the acceptable limit(10^-4).This result indicates that there is a serious carcinogenic risk to local residents.After 3%HNC treatment,the CR value of Cd was reduced from 107.6 to 74.99,but there is still a high risk of cancer.Therefore,although stabilization has a certain effect on heavy metals in highly contaminated farmland,the safety of these crops still unacceptable.The risk prevention measures,such as alternative planting,should be adopted to protect the health of local residents.