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工业废水中有毒重金属镉对水体的污染是一个世界性的环境问题。快速的工业化严重地促进了有毒重金属镉向河流的释放。与有机污染物不同,重金属镉不可生物降解,且可以持续地积聚在生物体内,对人体及其他物种造成严重的伤害,如众所周知的痛痛病。因此,必须在工业废水排出之前将水体中的镉移除[1-3]。我国钢铁工业水污染物排放标准(GB 13456-2012)规定,镉离子排放前浓度须低于0.1 mg·L−1[4]。截至目前,有多种物理化学技术被用来去除水中重金属离子,包括化学沉淀、离子交换、化学氧化/还原、反渗透、膜技术和吸附等。在这些技术中,吸附技术由于具有快速、高效、易于操作、成本低、吸附剂种类多和适用性强等特点,得到了广泛的应用[5-7]。其中农业和工业固体废物[8-11]、蒙脱土和高岭土[12]、壳聚糖[13]、高分子材料[3, 14]等吸附材料都显示了较好的从废水中吸附汞、铅、铜等重金属离子的能力,但对镉的去除往往表现不佳。如何更高效地去除重金属镉,开发新型的镉吸附剂,还需要深入的研究。
随着20世纪末纳米技术的发展,基于纳米材料吸附重金属离子的新技术被广泛应用[15-16]。纳米吸附剂具有比表面积大、特异性强、反应活性高、操作简单、无内扩散阻力等优点,是一种去除水溶液中重金属离子的高效吸附剂[17-18]。近年来,含硫化合物已被广泛用于去除重金属[19-20],因其原料来源广泛,廉价易得,且易于合成,在厌氧环境中处理效率高等优点在治理地下水和土壤重金属污染问题上引起广泛关注[21]。其中已有研究报道硫化亚铁可有效去除重金属[22],但它具有热力学稳定性差,容易遭受腐蚀并且对镉的吸附量较低等缺点[23]。而硫化锰(MnS)作为重要的金属硫化物,具有较强的还原能力,且光催化量子效率相比其他材料显著增大,是极具潜力的半导体材料[24-25]。然而,到目前为止,尽管对于纳米MnS的制备及其性质已有许多报道,但尝试使用MnS去除Cd2+的研究还鲜有报道。而且有研究显示Mn2+的存在会抑制水稻根部对于Cd2+的吸收[26]。因此,研究MnS对Cd2+的去除潜力是十分必要的。
本研究首先采用共沉淀法厌氧合成了MnS纳米颗粒,并对其进行表征,探讨了MnS纳米粒子对于水中重金属Cd的吸附性能,包括MnS对于水中重金属Cd的去除效率、吸附动力学和吸附等温线,最后探讨了MnS的除镉机制,为MnS在镉污染环境修复中的应用提供参考。
硫化锰纳米颗粒高效去除重金属镉
High-efficiency removal of heavy metal cadmium by manganese sulfide nanoparticles
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摘要: 中国镉污染问题日益严峻,开发高效的镉吸附剂,是解决环境镉污染问题的重要技术手段。采用共沉淀方法合成了硫化锰纳米颗粒,研究了其对重金属镉的吸附行为,并采用X射线衍射(XRD)、扫描电镜(SEM)、高分辨透射电镜(HR-TEM)、比表面积(BET)等技术手段探究了硫化锰纳米颗粒的形貌、化学组分以及镉的去除机制。结果表明,MnS纳米颗粒呈球状,平均粒径100 nm,比表面积30.56 m2·g−1。MnS纳米颗粒对Cd2+的吸附动力学数据较好地符合伪二级动力学模型;吸附等温线数据较好地符合Langmuir模型,说明MnS对Cd2+的吸附是以化学吸附为主的单分子层吸附。使用Langmuir拟合的MnS饱和最大镉吸附量为349.6 mg·g−1,在众多镉吸附材料中处于前列。对于模拟工厂重金属废水的处理,MnS纳米颗粒可以在5 h内使镉的浓度由60 mg·L−1降至国家规定排放线以下(<0.1 mg·L−1),且吸附过程中水体pH稳定,对水体干扰小。在多种重金属离子共存的情况下,仍可以达到接近100%的Cd2+去除率。硫化锰相对稳定,在空气中放置30 d仍有80%的镉去除率。较高的离子交换量形成CdS沉淀是MnS高效去除镉的主要原因。Abstract: The problem of cadmium pollution in China is becoming more and more serious. The development of high-efficiency cadmium adsorbent is an important technology to solve the problem of environmental cadmium pollution. Manganese sulfide nanoparticles were synthesized by coprecipitation, the adsorption behavior of manganese sulfide nanoparticles on cadmium was studied. The morphology, chemical composition and removal mechanism of cadmium were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), the surface area (BET) and other methods. The results show that MnS nanoparticles were spherical, with an average particle size of 100 nm and a specific surface area of 30.56 m2·g−1. The adsorption kinetics data of Cd2+ by MnS nanoparticles accorded well with the pseudo-second-order kinetics model; the adsorption data of Cd2+ by MnS nanoparticles was in good agreement with the Langmuir adsorption isotherm model, indicating that the above adsorption was a mainly monolayer chemical adsorption. The maximum cadmium adsorption capacity of MnS determined by Langmuir model was 349.6 mg·g−1, which was at the forefront among many cadmium adsorption materials. For the treatment of simulated wastewater, MnS nanoparticles could reduce the concentration of cadmium from 60 mg·L−1 to below the national discharge line (<0.1 mg·L−1) within 5 h, and the water pH during the adsorption process remained stable, and slight interference to water bodies occurred. In the solution of coexistence of various heavy metal ions, the Cd2+ removal rate still reached nearly 100%. MnS was a relatively stable absorbent with 80% the removal efficiency left after 30 days exposing in the air. The main reason for high-efficiency removal of cadmium was identified as CdS precipitate formation through the high ion exchange capacity.
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Key words:
- adsorbent /
- cadmium /
- heavy metal /
- metal sulfide /
- water treatment
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表 1 MnS纳米颗粒对Cd2+ 吸附动力学模型、Langmuir和Freundlich模型拟合参数
Table 1. Fitting parameters of adsorption kinetics model, Langmuir and Freundlich isotherm model for Cd2+ adsorption by MnS nanoparticles
伪一级动力学 伪二级动力学 Langmuir模型 Freundlich模型 k1 qe,exp R2 k2 qe R2 qm KL R2 KF n R2 1.319 248.16 0.578 0.010 258.90 0.595 349.6 0.024 0.991 77.39 4.2 0.903 表 2 多种镉离子吸附剂吸附量比较
Table 2. Comparison of Cd adsorption capacities of various adsorbents
吸附剂 吸附量qm/(mg·g−1) pH 吸附剂投加量/(g·L−1) 镉离子初始浓度/(mg·L−1) 来源 硫化锰纳米粒子 349.6 6 0.1 10~600 本研究 水合二氧化锰 111 6.05 0.2 1~60 [31] 铁-蒙脱石 25.7 5 4 20~200 [32] 小麦茎生物炭 11.6 5 4 11.2~134.4 [33] 磁性氧化铁纳米粒子 20.0 7 0.2 1.12~22.4 [34] EGTA-改性壳聚糖 83.2 4 2 1.12~1 120 [35] 磁性Fe3O4@生物炭纳米粒子 39.7 — 0.05 10~200 [36] 丙烯酸钠和丙烯酰胺共聚物/氧化石墨烯水凝胶 196.4 6 1 50~400 [37] 功能化MOF 88.7 6 1 0~200 [38] 二硫化钼 6.24 6 — 0~600 [39] 硫化亚铁 10 6 2.5 50~1 000 [40] 注:吸附量使用Langmuir和Freundlich模型拟合得出。 -
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