Abstract:

It is an urgent need for sustainable agricultural development to catch up with the trend of circular economy development, to which the resourceful utilization and harmless disposal of agricultural wastes could be an effective approach. Coix straw, the leftover remnant from coix seed production, was used as the feedstock for biochar adsorbent by rapid-pyrolysis for Hg(II) removal from aqueous solutions. The abundant availability of the low-cost coix straw waste makes it an ideal sustainable adsorbent for water treatment applications. This study investigated the impact of pyrolysis temperatures on physical characteristics of coix straw biochars (BCs) and their performances for aqueous uptake of Hg(II) ion. Physical and chemical characterizations of the prepared BCs were conducted by Scanning Electron Microscopy (SEM)/Energy Dispersive X-ray (EDX) spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) method, and X-ray Photoelectron Spectroscopy (XPS). The results showed the formation of a rich honeycomb-like porous carbon with oxygen-containing functional groups (e.g. -COOH, -OH) on the surface. Furthermore, as the pyrolysis temperature for BCs increased, the pore size increased, the number of surficial functional groups decreased, and both the specific surface area and pore volume firstly increased, then decreased. More than 92% of Hg(II) was removed from aqueous solution in the presence of BCs while the initial concentration of Hg(II) was no more than 100 mg∙L^-1. The Hg(II) removal rate increased rapidly within one hour and then showed a slow increasing trend until the equilibrium was reached. With higher dosage of biochar adsorbent applied, Hg(II) removal rate firstly increased and then decreased. The 2 g∙L^-1 biochar pyrolysed at 700 ℃applied treatment has the maximum removal rate in that case the maximal adsorption capacity of 235.3 mg∙g^-1 can be achieved. Adsorption equilibrium data were analyzed using Langmuir and Freundlich equilibrium models, and former isotherm was appropriate towards explain adsorption characteristics, that suggests the monolayer adsorption was happened on BCs. The pseudo-second order model was relevant to address adsorption behavior. The XPS and FTIR results indicated that the possible mechanisms of Hg(II)/BCs interaction in this process can be divided into coprecipitation and surface complexation supplemented by Hg-π non-covalent weak interactions.

Key words: coix straw, biochar, sewage treatment, adsorption, Hg(Ⅱ), removal

摘要：

农业废弃物资源化利用和无害化处理是实现农业可持续发展和发展循环经济的有效途径,对薏仁米（Semen Coicis）秸秆制备生物炭吸附剂,实现有机固体废弃物资源化利用,解决重金属废水处理难题,以薏仁米秸秆为原料,采用快速热解法制备生物炭。为探明不同温度下制备的薏仁米秸秆生物炭对重金属Hg²⁺的去除机制及机理,并用扫描电子镜-能谱分析法（SEM-EDS）、傅立叶变换红外光谱法（FT-IR）、氮吸附法（BET）、X射线光电子能谱法（XPS）脱附对制备的生物炭进行了表征,研究其对水中Hg²⁺的吸附特性及机制。通过结果表明,随裂解温度的升高,生物炭的孔径尺寸逐渐增大,表面极性官能团逐渐减少,比表面积、孔隙容积呈现先增加后减小的趋势。薏仁米秸秆生物炭具有丰富的蜂窝状孔结构和-COOH、-OH等表面活性基团。生物炭对质量浓度小于100 mg∙L^-1溶液中Hg²⁺的去除率大于92%,且生物炭对Hg²⁺的去除率主要发生在前1 h吸附时间内,然后趋于平衡;随添加量的增加,生物炭对Hg²⁺去除效率呈现先增加后减小的趋势,含量为2 g∙L^-1时生物炭对水中Hg²⁺的去除效率最高,且700 ℃制备的生物炭对Hg²⁺的去除效率最高,最大吸附量可达235.3 mg∙g^-1。吸附平衡等温线和吸附动力学结果表明,薏仁米秸秆生物炭对Hg²⁺的吸附过程符合Langmuir等温吸附模型和准二级动力学吸附模型,其对Hg²⁺的吸附为单层吸附;结合X射线光电子能谱和立叶变换红外光谱,吸附作用机制主要以共沉淀和表面络合为主,Hg-π非共价相互作用为辅的形式结合机理。

关键词: 薏仁米秸秆, 生物炭, 污水处理, 吸附, Hg(Ⅱ), 去除