不同改性生物炭对农田土壤理化性质及铅、镉钝化的影响机制研究

EFFECT OF MODIFIED BIOCHAR ON PHYSICO-CHEMICAL PROPERTIES OF FARMLAND SOIL AND IMMOBILIZATION OF Pb AND Cd AND THE MECHANISMS

改性生物炭是良好的重金属钝化剂。但针对不同生物炭,联合多种方法进行改性后生物炭的吸附性能尚待深入研究,其对土壤理化性质和重金属铅(Pb)、镉(Cd)长期钝化效果的影响也有待研究。选取水稻秸秆、木屑和椰壳为生物炭材,经硝酸-高锰酸钾联合改性后进行表征,明确改性前后生物炭理化性质。开展室内培养实验,将改性生物炭按质量比为2.5%、5%和10%加入受试土壤,培养6个月后,测定土壤理化性质、Pb、Cd形态分布及钝化效率,探讨改性生物炭钝化土壤Pb、Cd的作用机制。结果表明:改性后,不同生物炭的比表面积、孔隙结构和含氧官能团数量均得到不同程度的改善,重金属吸附性能有效增强,以改性椰壳炭最为显著。添加改性生物炭能提高土壤pH并改善土壤结构,当其用量>5%时,土壤阳离子交换量和有机质含量分别提高了15.89 g/kg和5.28 cmol/kg,土壤自身对养分及重金属的固定能力得到了显著提升。改性生物炭-土壤体系主要通过离子交换、络合反应和共沉淀反应等促使土壤有效态Pb、Cd向其潜在活化形态和残渣态转化,转化程度与钝化培养时间和改性生物炭用量呈正相关。受元素特性和竞争吸附作用的影响,土壤Pb^(2+)可被优先吸附,并在2个月内逐渐达到吸附平衡。改性椰壳炭对土壤Pb(1000 mg/kg)、Cd(10 mg/kg)的钝化效果最佳,当用量为10%时,钝化率最高分别可达到59.72%和36.37%。此外,钝化培养过程中,生物炭“老化作用”促使土壤阳离子交换量和有机质含量持续升高,土壤Pb、Cd生物有效性不断降低,且无二次释放风险。相关研究结果表明:添加改性生物炭能改善土壤结构、提升土壤地力并能长期有效地固定土壤Pb、Cd,可在重金属污染土壤的修复中大面积推广应用。

Biochar,modified through acidification,alkalization or organification method,has been widely applied as an adsorbent for heavy metal immobilization.However,there is still a lack of information about the modification methods for improving the adsorption ability of biochar made from different raw materials.In addition,as a soil amendment,the influences and mechanisms of modified biochar in improving soil physico-chemical properties and stabilizing soil Pb and Cd also need further exploration.In this study,rice straw,sawdust and coconut shell were selected as the raw materials for the preparation of biochar.After modification by nitric acid and potassium permanganate,the surface characteristics of modified biochar such as specific surface area,pore structure and surface functional groups were measured.After then,the modified biochar was added into the tested soil(1000 mg/kg Pb,10 mg/kg Cd)with a mass ratio of 2.5%,5%and 10%respectively for 6 months of indoor immobilization test.Soil physico-chemical properties,speciation distribution of soil Pb and Cd and the relevant immobilization efficiency were measured at the end of the experiment.The results showed that,after modification,the specific surface area,micropore and oxygen-containing functional groups of different biochar were increased to varying degrees,which effectively enhanced the adsorption ability of biochar,especially for the modified coconut shell biochar.When the dosage of modified biochar was larger than 5%,the soil cation exchange capacity and organic matter content were observed increased by 15.89 g/kg and 5.28 cmol/kg respectively,which improved the fixation of soil nutrients and heavy metals.The modified biochar-soil system mainly promoted the transformation of soil available Pb and Cd to their potential activated and residual forms through ion exchange,complexation reaction and co-precipitation reaction.The degree of transformation was positively correlated with the immobilization time and dosage of modified biochar.Compared with Cd2+,Pb^(2+)in soil could be preferentially adsorbed and gradually reached adsorption equilibrium within 2 months due to the effect of competitive adsorption.The immobilization effect of modified coconut shell biochar on soil Pb and Cd was optimal with a dosage of 10%,and the highest immobilization rate was found as 59.72%and 36.37%respectively.In addition,continuous increases of soil cation exchange capacity and organic matter content were observed during the experiment,which might be caused by the“ageing effect”of biochar.Influenced by such effect,the bioavailability of Pb and Cd in soil kept decreasing and no secondary release of Pb^(2+)and Cd2+were detected.In conclusion,the addition of modified biochar can improve soil structure,enhance soil fertility,and effectively stabilize soil Pb and Cd over a long time,and could be used in remediation of heavy metal contaminated soil.