热解温度对污泥基生物炭结构特性及对重金属吸附性能的影响

Effects of Pyrolysis Temperature on Structural Properties of Sludge-Based Biochar and Its Adsorption for Heavy Metals

鉴于污泥基生物炭作为重金属吸附剂的研究还缺乏足够的数据,为探讨不同热解温度对生物炭结构性质及其对水体重金属吸附能力的影响,在缺氧条件下于300~900℃范围内以城市污泥为原料制备生物炭,利用元素分析、比表面积测定、电位测定和红外光谱分析等方法对生物炭的理化性质和结构特征进行表征,并选用900℃生物炭进行了吸附重金属Pb、Cr和Cd的试验研究.结果表明:(1)300~900℃缺氧条件下制备的生物炭产率为44.39%~69.41%,污泥呈弱酸性(pH为6.35),热解后的生物炭呈碱性(pH为7.7~10.58).(2)900℃生物炭中w(H)、w(N)大幅降低,分别比干污泥中减少89.50%和77.16%,而w(C)降低29.22%,固碳作用显著.热解后生物炭比表面积明显增大,700和900℃生物炭比表面积分别达到58.48和87.55 m^2/g,最佳制备温度为700~900℃.(3)热解后的生物炭具有大量极性基团,热解温度越高,酸性基团越少,碱性基团含量增多.(4)热解作用使生物炭zeta电位升高,吸附能力增强.(5)900℃生物炭吸附Pb、Cr和Cd的最佳pH为7~8,对Pb、Cr和Cd的最大吸附量分别为2.38、2.48和1.16 mg/g.(6)各因素对生物炭吸附重金属的影响顺序,对于Pb和Cr表现为生物炭投加量>热解温度;对于Cd,表现为生物炭投加量>pH.研究显示,污泥基生物炭对Pb、Cr的吸附能力高于Cd,影响生物炭吸附行为的主导因子为生物炭投加量,影响Pb和Cr吸附的次要因子为生物炭热解温度,而影响Cd的次要因子为pH.生物炭吸附重金属的主要机理是离子交换吸附、络合反应、表面沉淀和竞争性抑制作用.

Sludge-based biochar could be a promising absorbent to remove heavy metals. However,relevant data are insufficient,especially in production and adsorption parameters,capacity and mechanism of adsorption. To investigate the influences of pyrolysis temperature on properties and heavy metal adsorption capacity of biochar,municipal sludge was pyrolysized to produce biochar at300-900 ℃under anoxic conditions. The physicochemical and structural properties of the biochar were characterized by elemental analysis,specific surface area determination,potential detecting,infrared analysis and others. Biochar at 900 ℃ was selected to adsorb Pb,Cr and Cd. The results indicated that：（ 1） At 300-900 ℃,the biochar yield was 44. 39%-69. 41%,the raw sludge was weakly acidic（ pH 6. 35）,and the biochar was transformed into alkaline（ pH 7. 70-10. 58） by pyrolysis.（ 2） During pyrolysis,elemental H and N were reduced rapidly by 89. 50% and 77. 16%,while C just decreased by 29. 22%,so carbon fixation was evident. The specific surface area of biochar increased apparently,with the values at 700 and 900 ℃ being 58. 48 and 87. 55 m^2/g respectively,larger than that of sludge（ 30. 0 m^2/g）. The optimal pyrolysis temperature was 700-900 ℃.（ 3） Pyrolysis generated plenty of polar groups in biochar including primary amide,primary amine,alcohol hydroxyl, carboxyl, aromatic sulfonamide, etc.Pyrolysis made acid groups decrease and basic groups increase.（ 4） Meanwhile, during the pyrolysis process, the zetapotential rose as the temperature increased,and so the adsorption capacity of biochar was enhanced.（ 5） The adsorption of Pb and Cr increased rapidly at pH 3-4,then reached the maximum at pH 4 and remained at equilibrium up to pH 9. The adsorption capacity of Cd increased at pH 3-7,then reached its peak at pH 7. The optimal pH for adsorption of three heavy metals by 900 ℃-biochar was 7-8. The maximum adsorption for Pb,Cr and Cd was 2. 38,2. 48 and 1. 16 mg/g,respectively.（ 6） The influences of various factors on biochar adsorption of heavy metals（ Pb and Cr） followed the order： biochar dosage temperature; and for Cd^2＋,biochar dosage pH. The conclusions showed that：（ 1） Sludge-based biochar adsorbed more Pb and Cr than Cd,and high hydration heat made Cd difficult to be adsorbed.（ 2） The primary factor that affected adsorbability was biochar dosage,while the secondary factor was pyrolysis temperature for Pb and Cr,but pH value for Cd.（ 3） The main mechanisms of Pb,Cr and Cd adsorbed by biochar involved surface precipitation,complexation,ion exchange adsorption and competitive inhibition.