不同内源重金属生物炭对Cu和Cd吸附及其对老化作用的响应

Adsorption of Cu and Cd by biochars with various contents of endogenous heavy metals and their responses to aging

为明确老化作用对不同内源污染物生物炭吸附重金属稳定性的影响,该研究以不同污染程度(清洁、中度和重度污染)土壤种植的巨菌草秸秆制备3种不同内源Cu和Cd含量的生物炭RB、SB和JB,分析3种生物炭对Cu^2+和Cd^2+的吸附能力以及干湿和冻融老化对饱和吸附后生物炭中Cu和Cd的生物有效性的影响。结果表明:3种生物炭表面均分布丰富的孔隙结构,RB含有最高的pH值和灰分含量;生物炭对Cu^2+和Cd^2+的吸附符合Langmuir模型(R^2=0.951~0.998),且RB对Cu^2+和Cd^2+的吸附量最大,分别为54.3和37.3 mg/g;与此相同,饱和吸附后RB对Cu^2+和Cd^2+的固持量最大,分别为21.4和4.78 mg/g。与老化前相比,干湿老化较冻融老化更显著地降低了饱和吸附后生物炭中Cu的TCLP(Toxicity Characteristic Leaching Procedure)浸出含量,促进了Cu从酸溶态和残渣态向还原态和氧化态转化,降低了Cu的环境风险;但是干湿和冻融老化作用增加了饱和吸附后生物炭中Cd的TCLP浸出含量,促进了Cd从残渣态向酸溶态、还原态和氧化态转化,增加了Cd的环境风险。这可能是由于3种生物炭对Cu^2+的吸附主要以表面络合为主,对Cd^2+的吸附以化学沉淀机制为主。总体上,RB生物炭固持最高的Cu^2+和Cd^2+,但是干湿和冻融老化增加了饱和吸附后生物炭Cd环境风险,研究结果对于评估生物炭长期钝化修复稳定性具有一定的指导意义。

Biochar has gained increasing attention in recent years due to its potential use in environmental remediation.The application of biochar may adsorb heavy metals from wastewater and decrease the bioavailability of heavy metals in soil.The concentrations of endogenous heavy metals in biochar are significant higher than in its feedstock after pyrolysis treatment.However,limits for heavy metals in biochars are lacking in some countries,which may lead to potential environmental risk resulting from the large-scale application of biochars rich in heavy metals.Therefore,three kinds of biochars named RB,SB,and JB with various contents of Cu and Cd were prepared from the straws of Pennisetum sinese grew in clean soil,moderately-polluted,and highly-polluted soils by heavy metals,respectively.The physicochemical properties of three biochars were investigated by Scanning Electron Microscopy(SEM),Adsorption capacities of Cu^2+and Cd^2+for three biochars were evaluated by batch experiments.Finally,the effects of Dry-Wet(DW)and Freeze-Thaw(FT)aging on the stability of heavy metals adsorbed by three biochars with different contents of endogenous heavy metals were investigated.Results showed that large amount of micro-pores were distributed on the surface of biochars,and RB contained the highest p H value and ash content.The XPS analysis indicated that biochars had a lot of organic functional groups,such as C C/C–H,C–OH,C=O and O=C–OH.The adsorption data were better fitted by Langmuir isotherm model(R2=0.951-0.998)for three biochars.Adsorption capacities of Cu2+followed the order of RB>SB>JB,and the order of RB>JB>SB for adsorption capacities of Cd^2+.RB had the highest adsorption amounts of Cu^2+and Cd^2+with 54.3 and 37.3 mg/g among three biochars,respectively.Similarly,the highest concentrations of total Cu and Cd after saturated adsorption were found in RB with 21.4 and 4.78 mg/g,respectively.DW and FT aging significantly changed the bioavailability of Cu and Cd in three biochars after saturated adsorption.DW aging significantly reduced the TCLP-extractable Cu in biochar after saturated adsorption compared with that of FT aging.For instance,concentrations of TCLP-extractable Cu in SB-DW and SB-FT were decreased by 1.57 and 0.39 times than that of SB.Moreover,DW aging promoted the transformation of Cu from acid-soluble and residual fractions to reducible and oxidizable fractions,and reduced the environmental risk of Cu.However,DW and FT aging significantly increased the TCLP-extractable Cd in biochars after saturated adsorption,promoted the transformation of Cd from residual fraction to acid-soluble,reducible and oxidizable fractions,and increased the environmental risk of Cd.Especially for RB-DW and RB-FT,contents of acid-soluble Cd were increased by 6.55 and 7.99 times than that of RB.It may be due to the surface complexation and chemical precipitation played key roles for the adsorption of Cu and Cd,respectively.In short,RB retained the highest amount of Cu and Cd,but DW and FT aging increased the environmental risk of Cd in three biochars after saturated adsorption.The study is of great significance for evaluating the long-term remediation stabilization of biochar.