Thermal pretreatment of willow branches impacts yield and pore development of activated carbon in subsequent activation with ZnCl_(2) via modifying cellulose structure

Development of pore structures of activated carbon(AC)from activation of biomass with ZnCl_(2) relies on content and structure of cellulose/hemicellulose in the feedstock.Thermal pretreatment of biomass could induce dehydration and/or aromatization to change the structure of cellulose/hemicellulose.This might interfere with evolution of structures of AC,which was investigated herein via thermal pretreatment of willow branch(WB)from 200 to 360℃and the subsequent activation with ZnCl_(2) at 550℃.The results showed that thermal pretreatment at 360℃(WB-360)could lead to substantial pyrolysis to form biochar,with a yield of 31.9%,accompanying with nearly complete destruction of cellulose crystals and remarkably enhanced aromatic degree.However,cellulose residual in WB-360 could still be activated to form AC-360 with specific surface area of 1837.9 m~2·g^(-1),which was lower than that in AC from activation of untreated WB(AC-blank,2077.8 m~2·g^(-1)).Nonetheless,the AC-200 from activation of WB-200 had more developed pores(2113.9 m~2·g^(-1))and superior capability for adsorption of phenol,due to increased permeability of ZnCl_(2) to the largely intact cellulose structure in WB-200.The thermal pretreatment did increase diameters of micropores of AC but reduced the overall yield of AC(26.8%for AC-blank versus 18.0%for AC-360),resulting from accelerated cracking but reduced intensity of condensation.In-situ infrared characterization of the activation showed that ZnCl_(2) mainly catalyzed dehydration,dehydrogenation,condensation,and aromatization but not cracking,suppressing the formation of derivatives of cellulose and lignin in bio-oil.The thermal pretreatment formed phenolic-OH and C=O with higher chemical innerness,which changed the reaction network in activation,shifting morphology of fibrous structures in AC-blank to“melting surface”in AC-200 or AC-280.

化学老化对Mg改性生物炭矿物结构及Pb^(2+)吸附的影响

为了探究老化时间对金属改性生物炭的性质及其对重金属固定效果的影响,采用60℃及90℃恒温恒湿(湿度维持在40%)的非生物老化培养方法,对MgCl_(2)浸渍改性生物炭进行为期6个月的老化试验。FT-IR结果表明,老化易在生物炭表面引入—COOH、C O等含氧官能团,提高生物炭对Pb^(2+)的螯合能力;Zeta电位结果说明老化会因为—OH等官能团发生氧化而形成大量的—COOH,从而进一步提高生物炭的电负性。吸附等温曲线拟合结果表明,Mg改性及老化后的生物炭均符合Langmuir模型。Mg改性生物炭经60℃老化后,其比表面积是改性生物炭的22倍左右,这是因为老化过程中,矿物形态发生改变,形成具有多孔结构的镁氧化物。因此,500℃及700℃制备的Mg改性生物炭经60℃老化后,其对Pb^(2+)的吸附量分别提高了54.4%、67.4%,分别为73.83 mg·g^(-1)和92.59 mg·g^(-1),但经90℃老化后,Mg改性生物炭产生较多的甲基自由基,破坏了生物炭的碳结构,使得生物炭表面孔隙结构被破坏,比表面积下降,导致90℃老化后的生物炭对重金属的吸附能力(48.26 mg·g^(-1))低于对应的60℃老化生物炭的吸附能力(62.85 mg·g^(-1))。由此可见,虽然Mg改性生物炭在短时间(60℃培养)内会提高对重金属的固定能力,但随着老化的持续进行(90℃培养),具有微孔结构的镁氧矿物最终被转化为无定型矿物,从而显著降低对Pb^(2+)的固定效果。